# Search for Stellar Flybys in the Sco-Cen OB Association with the Gaia DR2

Yilun Ma (马逸伦) Department of Astronomy, University of California, Berkeley, CA
94720, USA  Robert J. De Rosa European Southern Observatory, Alonso de Córdova
3107, Vitacura, Santiago, Chile  Paul Kalas Department of Astronomy,
University of California, Berkeley, CA 94720, USA  SETI Institute, Carl Sagan
Center, 189 Bernardo Ave., Mountain View CA 94043, USA  Institute of
Astrophysics, FORTH, GR-71110 Heraklion, Greece

(Received June 21, 2021; Revised January 18, 2022; Accepted January 31, 2022)

###### Abstract

High-contrast imaging studies of debris disks have revealed a significant
diversity in their morphologies, including large-scale asymmetries. Theories
involving stellar flybys, an external source of gravitational disturbance,
have offered a plausible explanation for the origin of these morphological
variations. Our study is an experiment to gain empirical evidence that has
been lacking from such theories. We explore this paradigm by using astrometric
and radial velocity measurements from the Gaia DR2 and ground-based
observations to trace the trajectories of 625 stars in the Sco-Cen OB
Association from 5 Myr in the past to 2 Myr in the future. We identified 119
stars that had at least one past flyby event occurring within one Hill radius,
and 23 of these experienced flybys within 0.5 Hill radius. We found no
evidence of a significant correlation between the presence of flyby events and
infrared excess detections, although the sample is not uniformly sensitive to
infrared excess emission. Ten stars that had past flyby events host resolved
circumstellar disks that appear relatively symmetric in the existing data
except for the circumbinary disk surrounding HD 106906. We determined the
trajectory and relative velocity of each of these flyby events, and compared
these to the geometry of the spatially-resolved disks. Future work is needed
to measure the kinematics of lower mass stars and to improve sensitivity to
circumstellar disks for the entire sample.

Celestial mechanics (211), Close encounters (255), Debris disks (363),
Circumstellar disks (235)

††journal: AJ

##  1 Introduction

Our planetary system is mostly flat and symmetric with the terrestrial and
giant planets orbiting nearly in the same plane with low eccentricities.
However, highly eccentric orbits and misalignments in extrasolar planetary
systems (e.g., high mutual inclinations, disk-planet misalignments, disk
asymmetries, etc.) are not unusual (e.g., Kalas & Jewitt 1995; Tamuz et al.
2008; Triaud et al. 2010; Kennedy et al. 2012; De Rosa et al. 2020). One
mechanism to explain high eccentricities is planet-planet scattering, a form
of internal dynamical excitation (e.g., Rasio & Ford 1996; Raymond et al.
2010). How the scattering starts out in the first place can be attributed to
planet migration in gaseous circumstellar disks, thus disrupting the
primordial orbital configuration of the system (Kley & Nelson, 2012).

However, external stellar and sub-stellar perturbers can also reshape the
architectures of planetary systems. Such perturbers can be bound to the system
in cases of stellar multiplicity or distant planet-mass objects, or unbound in
the case of stellar or sub-stellar flybys. An example of the bound stellar
case is the RW Aur system. Cabrit et al. (2006) and Rodriguez et al. (2018)
suggested that periastron passages between RW Aur A and B could explain the
asymmetric features of the RW Aur B disk and the trailing material from the RW
Aur A disk. An example of the bound sub-stellar case is the 11 MJ mass
exoplanet HD 106906 b which orbits exterior to the distorted circumbinary
debris disk (Kalas et al., 2015) and which could explain the disk’s asymmetric
morphology (Nesvold et al., 2017; Rodet et al., 2017; Nguyen et al., 2021). If
the planet originally formed in the disk and evolved outward due to interior
instabilities, unbound stellar flybys may have raised its periastron distance
away from the central planetary region (Rodet et al., 2017; De Rosa & Kalas,
2019). Thus the HD 106906 system has evidence for both internal and external
perturbations at play.

Stellar flyby events have occurred during our own solar system’s history as
well. The existence of dynamically new, long-period comets entering the inner
solar system at random inclinations is consistent with a scenario where
primitive icy bodies were ejected to large aphelia by the giant planets, had
their perihelia raised and inclinations randomized by perturbations from
passing stars, and approach the Sun again at later epochs when their perihelia
are decreased by other passing stars (Oort, 1950; Hills, 1981; Duncan et al.,
1987). For example, Mamajek et al. (2015) found that the low-mass binary WISE
J072003.20084651.2—also known as “Scholz’s star”—had a flyby with our Sun at
0.25 pc roughly 70,000 years ago. This is the closest flyby event with our
solar system discovered to date. Using $Gaia$ DR2 data, Bailer-Jones et al.
(2018) calculate that Gl 710 (HIP 89825) will come within 0.09 pc of the Sun
$\sim$1.2 Myr in the future and estimate that roughly 20 stars per Myr pass
within 1 pc of the Sun.

Alternately, long-period comets may have been captured at early times (e.g.,
age $<$10 Myr) when the Sun still resided in its a birth cluster and small
bodies were ejected from their original formation sites around other stars due
to flybys (Levison et al., 2010). The high mutual inclination and eccentricity
of certain Kuiper Belt objects may be signatures of past stellar flybys of the
proto-Sun with other members of the Sun’s natal cluster after the gas giants
had formed (Ida et al., 2000; Kenyon & Bromley, 2004; Kobayashi et al., 2005;
Pfalzner et al., 2018).

Theoretical work has generally explored how the architectures of exoplanetary
systems could be modified in a young cluster environment by flybys. At the
earliest epochs the structure of protoplanetary disks may be altered, such as
by having disk outer radii truncated (Larwood, 1997; Rosotti et al., 2014;
Winter et al., 2018; Cuello et al., 2019). Close stellar encounters can also
create spiral structure, asymmetric rings, and vertically disturbed
populations of particles that may account for observed debris disk asymmetries
(Larwood & Kalas, 2001; Reche et al., 2009; Lestrade et al., 2011; Cuello et
al., 2020). Encounters between a planetary system and a flyby star could
excite the eccentricity of the outer planets and transfer the excitation of
eccentricity inward on a longer timescale (Zakamska & Tremaine, 2004). Flybys
could also completely eject outer planets from the system and leave the
surviving planetary system unstable (Hills, 1984; Malmberg et al., 2011;
Parker & Quanz, 2012).

In this paper, we extend the De Rosa & Kalas (2019) study of the HD 106906
system to the entire Sco-Cen OB association, identifying the possible stellar
flybys among its members using a combination of astrometry and radial
velocities from Gaia DR2 (Gaia Collaboration et al., 2018) and from various
other literature sources. In Section 2, we describe the characteristic of the
samples used in this study, such as mass estimation and sample completeness.
In Section 3, we present the procedure used to identify stellar flybys in the
samples. The results that comes out of those methods are presented in Sections
4.1 and 4.2. In Section 4.3, we discuss the correlation between the presence
of infrared excess (IR) and stellar close approaches. Section 4.4 then focuses
on close approaches involving stars hosting resolved debris disk and presents
the flyby geometry of those with determined disk orientations. Section 5
discusses potential encounters identified in the Upper Scorpius subgroup via
simulating precise RV measurements. Lastly, Section 6 summarizes the findings
and discusses potential future development.

##  2 Sample Properties

###  2.1 Sample Selection

Figure 1: Color-magnitude diagram of the two samples created using optical
photometry from Gaia DR2. Some stars that are not on the main sequence are not
plotted. Isochrones from MIST at the ages of the three subgroups are also
plotted for comparison. No extinction correction is applied to either the
sample or the isochrones. Figure 2: The mass distributions of the samples are
plotted. The full samples are plotted with solid lines; the mass distributions
of USCO stars with RV is plotted with the dotted line. The bin are drawn in
the logarithmic space.

This study used the three-dimensional positions and motions of the known
members of the nearby ($116-144\,\mathrm{pc}$, de Zeeuw et al. 1999) Scorpius-
Centaurus OB2 association to identify close stellar approaches. The positions
and motions were derived from precise Gaia Data Release 2 (DR2) astrometry
(Gaia Collaboration et al., 2018) and a combination of Gaia and ground-based
radial velocity (RV) measurements. The study focused on two samples of stars
from this association: the first based on a census of stars in the region from
the Hipparcos satellite (ESA, 1997), and the second based on a revised census
of the Upper Scorpius (US) subgroup using data from Gaia DR2.

The first sample, which we call the “Sco-Cen” sample, was a subset of a
catalog of Sco-Cen members constructed based upon Wright & Mamajek (2018) and
Song et al. (2012), which contains 539 stars from all three subgroups of the
association — Upper Scorpius (USCO), Upper Centaurus-Lupus (UCL), and Lower
Centaurus-Crux (LCC). The subset we used in this study contains 462 catalog
stars that have a radial velocity measurement from either Gaia or ground-based
instruments. When a target had a RV measurement from both Gaia and other
facilities, we opted to use the measurement with the lowest uncertainty. A
full listing of the Sco-Cen sample is given in Table A.

The second group, which we call the “USCO” sample, is derived from 1682 stars
in the Gaia DR2 catalog that are identified as USCO members in Table 4 of
Luhman & Esplin (2020). We excluded 120 stars missing parallax measurements
and 2 stars with negative parallax measurement and obtained a reduced sample
of 1560 stars. This sample included 56 stars which are in the Sco-Cen sample
described above. Among the stars in the USCO sample, 219 have RV from either
the Gaia catalog or ground-based measurements, and we adopt the same procedure
for targets with RV measurements from both Gaia and other facilities as with
the other sample. A full listing of the USCO sample is given in Table A. In
total the two samples consist of 625 unique targets.

###  2.2 Mass Estimation

To assess the relative significance of stellar flybys identified in this
study, we required an estimate of the mass of each star within both samples.
We generated color-magnitude diagrams (CMDs) of the two samples using optical
photometry from Gaia. Plotted in Fig. 1, these clearly demonstrate that the
Sco-Cen sample is restricted to more luminous stars ($M_{G}\lesssim 7$) due to
the sample being primarily derived from Hipparcos astrometry, whereas the USCO
sample includes stars that are as faint as $M_{G}\sim 15$. To assess the
completeness of the two samples, we derived the stellar masses of the two
samples based on the stars’ spectral types. We first interpolated a relation
between spectral type and effective temperature based on Table 5 of Pecaut &
Mamajek (2013). We then modelled the isochrones for the three subgroups with
the MESA Isochrones and Stellar Tracks (MIST) (Paxton et al., 2011, 2013;
Dotter, 2016; Choi et al., 2016), assuming the ages of USCO, UCL, and LCC to
be 10 Myr, 16 Myr, and 15 Myr, respectively (Pecaut & Mamajek, 2016), and the
metallicities of the subgroups to be solar for simplicity (Viana Almeida et
al., 2009). We obtained a relation between effective temperature and stellar
mass, and thus between spectral type to stellar mass for each subgroup
individually. For stars whose derived effective temperature is not covered by
the isochrones, we used a luminosity-mass relation from the MIST isochrones
and estimated their stellar masses using their G-band magnitudes in Gaia DR2.
We did not apply any extinction correction to the optical magnitudes as we do
not require precise masses, although the extinction towards many of the
sources in both samples is expected to be small (e.g. Pecaut & Mamajek, 2016).

We generated the mass distribution of the two samples using the derived
masses, which is shown in Fig. 2. For stars whose derived effective
temperatures are so low that they exceed the isochrones’ coverage, we assumed
the mass to be $0.1\,M_{\odot}$, which causes a slight over-abundance of stars
at the low-mass end of the mass distribution for the USCO sample. Based on the
mass distributions, we found that the USCO sample is indeed bottom-heavy and
matches the IMF quite well, while the Sco-Cen sample is deficient of lower-
mass stars. The Sco-Cen sample is complete to $\sim 1M_{\odot}$, and the USCO
sample indeed includes more low-mass stars and is complete down to $\sim
0.1M_{\odot}$. However, although the USCO sample has more stars with RV
measurements at the low-mass end than the Sco-Cen sample does, the RV
measurement is still incomplete for the star with $M\lesssim
0.3\,\mathrm{M_{\odot}}$.

Ideally, we would have a more complete sample for the LCC and UCL subgroups as
well in order to trace the dynamics of the subgroups more comprehensively.
However, since a thorough census of these two sub-groups had not yet been
performed using Gaia DR2 astrometry, we decided not to mix the two samples and
treat them separately to avoid bias against the two less-sampled subgroups.

##  3 Methods

###  3.1 Flyby Identification

The astrometry and radial velocities are used to trace the stellar motion of
the stars in both samples from 5 million years in the past to 2 million years
in the future. We assumed that the stars are gravitationally unbound with each
other and the time interval integrated over is short comparing to the galactic
orbital periods of the stars so that the effect of the galactic potential can
be neglected. Therefore, the trajectories of the stars can be approximated as
linear. In their analysis of stellar flybys of HD 106906, De Rosa & Kalas
(2019) showed that there is not a significant deviation in the result between
the linear approximation and an N-body simulation. We assumed that this is
true for all of the stars within both samples. Based on the linear
approximation, we computed the time of closes approach $t_{\mathrm{ca}}$
between any pair of stars in the sample as

$t_{\text{ca}}=-\frac{(\mathbf{r_{2}}-\mathbf{r_{1}})\cdot(\mathbf{v_{2}}-\mathbf{v_{1}})}{\left|\mathbf{v_{2}}-\mathbf{v_{1}}\right|^{2}}\,\,,$
(1)

where $\mathbf{r_{1}}$, $\mathbf{r_{2}}$ and $\mathbf{v_{1}}$,
$\mathbf{v_{2}}$ represent the pairs of positions and velocities of the two
stars. Subsequently, we are able to compute the distance of closest approach
$d_{\rm ca}$ of each pair of stars as

$d_{\text{ca}}=\left|(\mathbf{r_{2}}-\mathbf{r_{1}})+(\mathbf{v_{2}}-\mathbf{v_{1}})t_{\text{ca}}\right|\,\,.$
(2)

Uncertainties of the Gaia astrometry and the radial velocity (RV) measurement
are propagated with a Monte Carlo (MC) fashion. For each of the $10^{6}$
trials, we drew the position, parallax, and proper motion of both stars based
on a multivariate normal distribution using the correlation coefficients in
the Gaia DR2 catalog and drew the radial velocities from a separate Gaussian
distribution. Using Equation 1 and Equation 2, $t_{\text{ca}}$ and
$d_{\text{ca}}$ were computed for all $10^{6}$ trials. We used the median
values of $t_{\text{ca}}$ and $d_{\text{ca}}$ as the adopted values for these
parameters, and the 16th- and 84th-percentiles were used as the lower and
upper bounds, respectively.

We used the star’s Hill radius $R_{H}$ under the galactic potential as a mass-
weighted distance to classify the flybys. The Hill radius encodes the radial
boundary of a volume where a star has a greater gravitational influence on an
object than the galactic potential. The Hill radii were approximated using

$R_{H}\approx
a\left(\frac{m_{\star}}{3M_{\text{enc}}(a)}\right)^{\frac{1}{3}}\,\,,$ (3)

where $a$ is the distance from the galactic center to the star, $m_{\star}$ is
the derived stellar mass, and $M_{\text{enc}}(a)$ is the mass enclosed within
the star’s galactic orbit assuming a flat rotation curve for our Galaxy. If
$d_{\text{ca}}<0.5R_{H}$ for either star, it is categorized as a “close
encounter.” If $0.5R_{H}\leqslant d_{\text{ca}}\leqslant R_{H}$ (for either
star), then the event is called an “encounter.” For more generic references to
unbound interactions we use the term “flyby.” We do not use these terms to
refer to the periastron passages of bound objects even if the orbits are
highly eccentric and with large semi-major axes. Tables 3–5 have columns
$d_{ca}/R_{H,T}$ and $d_{ca}/R_{H,F}$ where the former lists how the closest
approach distance compares to the Hill radius of the target (T) and the latter
with respect to the Hill radius of the flyby star (F).

###  3.2 Flyby Geometry

For each flyby event that involve a resolved debris disk, we also derived the
geometry of the closest approaches. Assuming the disks are azimuthally
symmetrical centered around their host stars, we used four reference points
around the disk edge to carry out rotations that eventually leads to orienting
the disks in galactocentric coordinates, the coordinate system we used for
calculating the flyby timing and distances. These reference points are defined
as

$\mathbf{p_{1}}=\begin{bmatrix}0\\\ \frac{r}{d}\\\
0\end{bmatrix},\mathbf{p_{2}}=\begin{bmatrix}0\\\ -\frac{r}{d}\\\
0\end{bmatrix},\mathbf{p_{3}}=\begin{bmatrix}0\\\ 0\\\
\frac{r}{d}\end{bmatrix},\mathbf{p_{4}}=\begin{bmatrix}0\\\ 0\\\
-\frac{r}{d}\end{bmatrix},$ (4)

where $d$ is the distance to the host star and $r$ is a nominal radius of the
disk measured from the resolved images. The origin (0, 0, 0) is defined to be
the position of the host star, and the three axes are along the RA, DEC, and
line-of-sight directions, respectively. We carried out the first rotation
based on the position angle (PA) of the disk via the rotation matrix

$\mathbf{R_{\boldsymbol{\theta}}}=\begin{bmatrix}\cos(\text{PA})&\sin(\text{PA})&0\\\
-\sin(\text{PA})&\cos(\text{PA})&0\\\ 0&0&1\end{bmatrix}.$ (5)

To rotate the reference points according to the inclination of the disk, we
first defined the normal vector about which this second rotation was carried
out as

$\mathbf{\hat{n}}=\mathbf{R_{\boldsymbol{\theta}}}\left(\frac{\mathbf{p_{2}}-\mathbf{p_{1}}}{|\mathbf{p_{2}}-\mathbf{p_{1}}|}\right).$
(6)

Thus, we constructed our inclination-rotation matrix as

$\mathbf{R_{\boldsymbol{i}}}=\begin{bmatrix}(1-s)+n_{1}^{2}s&n_{1}n_{2}s-n_{2}c&n_{2}n_{3}s+n_{2}c\\\
n_{1}n_{2}s+n_{3}c&(1-s)+n_{2}^{2}s&n_{2}n_{3}s-n_{1}c\\\
n_{1}n_{3}s-n_{2}c&n_{2}n_{3}s+n_{1}c&(1-s)+n_{3}^{2}s\end{bmatrix}\,\,,$ (7)

where $s=1-\sin{i}$, $c=\cos{i}$ and $n_{1}$, $n_{2}$, and $n_{3}$ are the
three components of $\hat{\mathbf{n}}$, respectively. After correcting the
projection on the sky plane, we add the offset of the stars’ positions in
Cartesian ICRS coordinates to all four points to transform from the star frame
to the ICRS coordinate frame. We then transformed these four points from ICRS
to galactocentric coordinates using the procedures defined in
astropy.coordinates . In the galactocentric coordinates, we define the normal
vector of the debris disk as

$\hat{\mathbf{N}}=\frac{(\mathbf{p_{1}^{\prime}}-\mathbf{p_{2}^{\prime}})\times(\mathbf{p_{3}^{\prime}}-\mathbf{p_{4}^{\prime}})}{|(\mathbf{p_{1}^{\prime}}-\mathbf{p_{2}^{\prime}})\times(\mathbf{p_{3}^{\prime}}-\mathbf{p_{4}^{\prime}})|}\,\,,$
(8)

where the primed vectors are the galactocentric coordinates of the reference
points. We then calculated the closest approach angle $\theta_{\text{ca}}$,
the angle between the disk plane and the flyby star’s velocity vector
($\mathbf{v_{\text{rel}}}$) in the rest frame of the target star, as

$\theta_{\text{ca}}=\arcsin\left(\frac{\hat{\mathbf{N}}\cdot\mathbf{v_{\text{rel}}}}{|\mathbf{v_{\text{rel}}}|}\right)\,.$
(9)

The uncertainties on all properties were propagated in a Monte Carlo fashion.
The angle of closest approach defined previously is calculated for each of the
$10^{6}$ draws, where we drew disk position angles and inclinations from
Gaussian distributions using the literature measurement and uncertainty. We
validated our implementation of this algorithm by comparing to the results for
HD 106906 presented in De Rosa & Kalas (2019).

##  4 Results

###  4.1 Flyby Statistics

In the Sco-Cen sample, we identified seven past $close$ encounters and seven
future close encounters, each involving 13 stars, along with 37 past
encounters and 28 future encounters involving 63 and 45 stars, respectively.
There are 98 individual stars involved in all these events (including both
encounters and close encounters). Similarly, we carried out the same
calculation on 219 stars in the USCO sample where we identified 18 past close
encounters involving 33 stars, 7 future close encounters involving 13 stars,
60 past encounters involving 81 stars, and 39 future encounters involving 64
stars. In total, 116 individual stars are involved in all these events. The
time and distance of closest approach events of both samples are presented in
Figures 4 and 5. As the time approaches the present, more encounters and close
encounters are identified. We attribute this phenomenon to the increasing
uncertainties of $d_{\text{ca}}$ and $t_{\text{ca}}$ as we trace further back
to the past and into the future due to the non-zero uncertainties of the
astrometry and radial velocities, an effect also seen in studies of stellar
encounters with our own solar system (e.g., Bailer-Jones et al., 2018). The
full list of flyby events is presented in Table 4.

Figure 3: The spatial distribution of the two samples as well as the stars
with identified flyby events is plotted in the Galactic coordinates. The
dashed boxes mark the boundaries of USCO, UCL, and LCC identified by de Zeeuw
et al. (1999). The USCO sample is plotted within the inset.

The spatial distribution of flybys in both samples is presented in Fig. 3. We
observe that stars involved in flybys are distributed evenly in the sky,
except for a deficiency of events between $\ell=314^{\circ}$ and
$\ell=334^{\circ}$. This gap in the spatial distribution of flybys is
coincident with a lower density of stars seen on the western side of the UCL
subgroup. The lower density within this region is also seen in the spatial
distribution of fainter pre-main sequence candidate members identified from an
analysis of Gaia DR2 data (e.g., Damiani et al., 2019). The lower occurrence
of identified flybys within this region can simply be ascribed to a lower
three-dimensional density of stars within this part of the association.

The algorithm we used to identify flybys would naturally include some wide
binaries due to the fact that they share similar positions and velocities.
According to the Washington Double Star Catalog (WDS; Mason et al., 2020), in
the Sco-Cen sample, 22 out of the 98 Sco-Cen stars involved in flybys have
binary companions; 16 out of the 116 USCO stars involved in flybys have binary
companions as well; however, none of the identified flyby pairs is listed as
physically associated. Nonetheless, HD 143844 and HD 143215, which are
identified to have a flyby event, are identified as wide binaries in Hartman &
Lépine (2020). Four other flyby events that we identified (HD 144587 / HD
144175, HD 146897 / HD 147083, HD 137499 / HD 137432, and HD 121835 / HD
121190) are also catalogued to be wide binary systems by Jiménez-Esteban et
al. (2019). However, it is very difficult to determine whether a wide pair is
gravitationally bound given the very low relative orbital velocities and the
current radial velocity uncertainties for many of these stars. It is also
worth noting that when tracing the trajectories, the radial velocity
measurements of some stars that are in short-period binary systems may not be
representative of the systematic velocity, which would cause an incorrect
determination of the three-dimensional trajectories of these stars. Where
possible we used systemic velocities for spectroscopic binaries derived from
orbit fits presented in the literature (e.g., Levato et al., 1987).

Figure 4: Closest approach times and distances for the encounters and close
encounters identified in the Sco-Cen sample. Orange diamonds mark the close
encounters ($d_{\text{ca}}\leqslant 0.5R_{\text{H}}$); blue stars mark
encounters ($0.5R_{\text{H}}\leqslant d_{\text{ca}}\leqslant R_{\text{H}}$);
gray dots mark the pairs whose close approach distance is in neither star’s
Hill radius. The dotted horizontal lines mark the 1st-, 5th- and, 10th-
percentiles of the distribution of the separation of stars within the sample.
The median separation of stars in the Sco-Cen sample is 5.60 pc. Figure 5:
Closest approach times and distances for all the encounters and close
encounters identified in the USCO sample. Legends are the same as that in
Fig.4. The median separation of stars in this sample is 1.23 pc.

###  4.2 Field Star Flybys

We also ran our closest approach algorithm on a dataset combining all stars in
the Gaia DR2 that were not a part of either of the Sco-Cen samples that have
RV measurements from either Gaia or other ground-based observations reported
in SIMBAD. Among this dataset, we flagged the stars that are identified by
Damiani et al. (2019) as candidate Sco-Cen members based on their Gaia DR2
astrometry. Those that are not flagged are hereafter referred to as the field
sample or field stars. This field sample consists of approximately 7.5 million
stars, with greater completeness than the Sco-Cen sample to low-mass stars
(0.5–1 $M_{\odot}$) owing to the magnitude and effective temperature range of
stars with Gaia DR2 radial velocities.

We found that flybys involving field stars with $d_{\rm ca}<3$ pc are
significantly more frequent than those only involving members of the Sco-Cen
sample, in large part due to the difference in completeness between the two
samples. However, the two types of flyby (ScoCen-Field vs. ScoCen-ScoCen) have
different relative velocity distributions as shown in Fig.6. The median
relative velocity for encounters between Sco-Cen members and field stars is
$\sim 45\,\mathrm{km/s}$, whereas that for encounters between two Sco-Cen
members is $\sim 7\,\mathrm{km/s}$.

If we use the change in orbital speed as a measure for dynamical influence
(e.g., Kalas et al., 2001), we see this difference in relative velocity causes
a factor of $\sim 6$ difference between the two types of encounters.
Furthermore, it can be observed in Fig.7 that the events with $d\lesssim
1\,\mathrm{pc}$ and $v_{\mathrm{rel}}\lesssim 10\,\mathrm{km/s}$ are dominated
by encounters between members of the association. From this we infer that it
is the encounters between members of the association that are more dynamically
important than with field stars. Nevertheless, we report the details of
encounters with field stars, and with candidates from Damiani et al. (2019),
in Table 6.

Figure 6: The comparison of relative velocity distributions between ScoCen-
Field encounters (blue) and ScoCen-ScoCen encounters (orange) with
$d_{\mathrm{ca}}<3\,\mathrm{pc}$. Figure 7: The closest approach distance is
plotted against relative velocity for ScoCen-field encounters (blue) and
ScoCen-ScoCen encounters (orange).

###  4.3 Correlation with infrared excess

Various effects are possible when close stellar flybys interact with a star
surrounded by a disk of planetesimals and/or planet-mass objects. The flyby
can strip away outer material, truncating a circumstellar disk, but also
increasing the volume density of surviving material and dynamically heating it
(Pfalzner et al., 2005; Lestrade et al., 2011). This would increase the rate
of dust-producing collisions, manifesting as an infrared excess until the
entire system dynamically relaxes and dust grains are depleted by collisional
destruction, sublimation, or removal by radiation forces. The 5 Myr-old HD
141569 debris disk may be undergoing this process at the current epoch due to
the flyby of one or more M stars (Weinberger et al., 2000; Augereau &
Papaloizou, 2004; Ardila et al., 2005). To test for a possible correlation
between flybys and IR excesses, we crosschecked both of our samples with the
list of IR excess stars compiled in Cotten & Song (2016) and determined the
fraction of the sample stars with excess. In the Sco-Cen sample, we identified
126 stars with IR excess with the dust temperature between 50K and 575K; 33
out of 67 events involve 24 IR-excess stars. In the USCO sample, there are 42
IR-excess stars with the dust temperature ranging from 75 K to 450 K; 21 out
of 117 events involve 9 IR-excess stars.

As shown in Figure 8, the distributions of the close approach distance for
stars with and without IR excess overlap for the Sco-Cen sample. However,
since the Cotten & Song (2016) study does not have a uniform sensitivity for
the IR excess detection for all the stars and the sample itself is less
complete for the low mass stars, we would conservatively conclude that our
data do not provide evidence for a correlation between the presence of IR
excess and stellar flybys. For the same issue of sensitivity, only 42 out of
1560 stars in the USCO sample are included in Cotten & Song (2016). Most stars
in the USCO sample were too faint to be catalogued and have only been recently
identified with data from the Gaia satellite, which makes the sample bias more
significant. Consequently, we did not include USCO sample in this analysis.

Figure 8: The comparison of the close approach distances of the events
involving and not involving IR-excess stars for the Sco-Cen sample.

###  4.4 Flybys of stars with disks detected in scattered light or with ALMA

To search for morphological evidence of dynamical interactions between stellar
flybys and circumstellar material (e.g., radial asymmetries, spiral
structures, and vertical warps), we surveyed the literature for resolved
detections of debris disks around the stars identified as having flyby events
with other association members in the past. Table 4.4 and Fig. 9 present the
flyby parameters, including the current 3D separations between stars
($d_{0}$), the relative velocities of each flyby ($v_{\text{rel}}$), and the
relative angles between the disk midplanes and flyby trajectories
($\theta_{\text{ca}}$).

Out of all the flyby stars from both samples, five have debris disks resolved
in _scattered light_ at visible and/or near-infrared wavelengths: HD 106906,
HD 114082, HD 115600, HD 146897, and 2MASS J16042165-2130284 (RXJ1604.3-2130).
At thermal wavelengths, the Atacama Large Millimeter/Submillimeter Array
(ALMA) has been used to resolve the disks surrounding 2MASS J16064794-1841437
and HD 145718 (Ansdell et al., 2020). In the USCO sample, 2MASS
J16035767-2031055 hosts a circumstellar disk (Luhman & Mamajek, 2012) also
resolved with ALMA at 0.88 mm continuum and the CO $J=3-2$ line at 345.8 GHz
(Barenfeld et al., 2016). Circumstellar dust disks were weakly detected
(SNR$\sim$3–4) but unresolved at 1.24 mm by Lieman-Sifry et al. (2016) using
ALMA with $\sim$1” resolution around HD 113556 and HD 142315. We include these
two disks in our discussion to anticipate the availability of higher
resolution continuum maps in the near future. We discuss the details of all 10
cases below.

Table 1: Flyby events involving resolved or detected debris disks

Target star | Flyby star | $d_{0}$ | $d_{\text{ca}}$ | $t_{\text{ca}}$ | $\theta_{\text{ca}}$ | $v_{\text{rel}}$ | Ref
---|---|---|---|---|---|---|---
|  | (pc) | (pc) | (Myr) | (deg) | (km/s) |
HD 106906 | HD 106444 | $11.55_{-0.68}^{+0.69}$ | $0.65_{-0.40}^{+0.92}$ | $-3.49_{-1.75}^{+0.90}$ | $5.90_{-1.67}^{+1.77}$ | $3.22_{-1.09}^{+1.10}$ | 1
| HIP 59721 | $11.89_{-0.65}^{+0.64}$ | $0.71_{-0.11}^{+0.18}$ | $-2.19_{-1.03}^{+0.54}$ | $4.64_{-1.16}^{+1.00}$ | $5.30_{-1.69}^{+1.70}$ |
HD 113556 | TYC 8674-2317-1 | $7.03_{-0.46}^{+0.47}$ | $1.22_{-0.53}^{+0.88}$ | $-1.55_{-0.64}^{+0.46}$ |  $\cdots$ $\dagger$ | $4.34_{-1.45}^{+1.65}$ | 2
HD 114082 | HD 116402 | $26.09_{-0.86}^{+0.87}$ | $0.87_{-0.31}^{+0.36}$ | $-2.85_{-0.17}^{+0.16}$ | $-13.91_{-3.98}^{+3.96}$ | $8.95_{-0.40}^{+0.40}$ | 1
HD 115600 | TYC 8674-2317-1 | $2.37_{-0.54}^{+0.56}$ | $0.89_{-0.06}^{+0.09}$ | $0.62_{-0.26}^{+0.65}$ | $10.52_{-1.69}^{+1.79}$ | $3.35_{-1.79}^{+1.81}$ | 1
HD 142315 | HD 142705 | $2.21_{-0.53}^{+1.27}$ | $1.60_{-0.77}^{+0.77}$ | $-0.26_{-0.98}^{+0.62}$ |  $\cdots$ $\dagger$ | $2.70_{-1.34}^{+2.52}$ | 2
| UCAC4 336-078484 | $1.93_{-0.47}^{+1.10}$ | $1.36_{-0.45}^{+0.50}$ | $-0.17_{-0.79}^{+0.46}$ |  $\cdots$ $\dagger$ | $3.40_{-2.02}^{+3.47}$ |
| HD 142097 | $4.91_{-1.26}^{+1.35}$ | $1.55_{-0.12}^{+0.19}$ | $0.59_{-0.22}^{+0.35}$ |  $\cdots$ $\dagger$ | $7.77_{-2.59}^{+2.67}$ |
2M J16035767-2031055 | 2M J16053815-2039469 | $5.24_{-1.23}^{+1.24}$ | $0.52_{-0.16}^{+0.29}$ | $-0.43_{-0.39}^{+0.17}$ | $-11.52_{-27.92}^{+29.51}$ | $11.37_{-5.66}^{+5.81}$ | 3
| RX J1603.9-2031B | $9.05_{-4.91}^{+5.29}$ | $0.71_{-0.48}^{+2.06}$ | $0.96_{-1.23}^{+1.80}$ | $-11.65_{-29.84}^{+33.98}$ | $6.29_{-4.19}^{+5.68}$ |
2M J16042165-2130284 | EPIC 204548337 | $1.98_{-0.73}^{+1.35}$ | $1.06_{-0.69}^{+1.03}$ | $0.33_{-0.59}^{+0.56}$ | $-27.59_{-47.42}^{+86.39}$ | $2.86_{-1.18}^{+2.44}$ | 4
2M J16064794-1841437 | HD 144925 | $18.19_{-1.72}^{+1.72}$ | $0.73_{-0.24}^{+0.28}$ | $-1.32_{-0.40}^{+0.26}$ | $-30.90_{-0.83}^{+1.21}$ | $13.46_{-2.95}^{+2.96}$ | 5
HD 145718 | HD 145467 | $14.04_{-2.05}^{+2.11}$ | $1.16_{-0.60}^{+1.36}$ | $-2.31_{-1.03}^{+0.63}$ | $-20.56_{-1.84}^{+1.87}$ | $5.96_{-1.82}^{+1.86}$ | 5
| 2M J16120920-2247504 | $11.14_{-2.17}^{+2.21}$ | $1.04_{-0.33}^{+0.63}$ | $-1.96_{-1.25}^{+0.63}$ | $-18.17_{-1.81}^{+2.11}$ | $5.50_{-2.07}^{+2.08}$ |
| BD-21 4301 | $18.21_{-2.58}^{+2.57}$ | $1.30_{-0.79}^{+1.54}$ | $-1.20_{-0.73}^{+0.36}$ | $-19.15_{-1.40}^{+1.51}$ | $14.66_{-5.57}^{+5.61}$ |
| Gaia DR2 6242198746659039872 | $8.66_{-2.05}^{+2.10}$ | $1.12_{-0.48}^{+0.38}$ | $-1.03_{-1.26}^{+0.48}$ | $-21.61_{-2.68}^{+2.01}$ | $7.32_{-4.41}^{+4.78}$ |
| 2M J16125889-2245202 | $2.86_{-1.69}^{+2.20}$ | $0.93_{-0.18}^{+0.96}$ | $0.15_{-0.83}^{+0.69}$ | $8.91_{-21.88}^{+31.88}$ | $4.70_{-2.99}^{+4.93}$ |
HD 146897 | HD 146416 | $2.00_{-0.57}^{+1.16}$ | $0.65_{-0.43}^{+0.61}$ | $-0.44_{-0.33}^{+0.31}$ | $4.23_{-4.34}^{+4.37}$ | $4.08_{-0.32}^{+0.33}$ | 1
| HD 147083 | $1.82_{-1.03}^{+1.59}$ | $0.58_{-0.13}^{+0.46}$ | $0.51_{-1.09}^{+1.34}$ | $5.19_{-12.69}^{+10.26}$ | $1.88_{-1.21}^{+1.76}$ |
| HD 146743 | $6.62_{-1.27}^{+1.27}$ | $1.12_{-0.38}^{+0.53}$ | $1.63_{-0.39}^{+0.49}$ | $-2.26_{-4.20}^{+4.36}$ | $3.91_{-0.75}^{+0.75}$ |

Note. — $\dagger$ The target star disks have not been spatially resolved.

References. — (1) Esposito et al., 2020; (2) Lieman-Sifry et al., 2016; (3)
Barenfeld et al., 2016, 2017; (4) Mayama et al., 2012; (5) Ansdell et al.,2020

####  4.4.1 HD 106906

We confirm the De Rosa & Kalas (2019) result that HD 106906 (F5V) has
experienced two flyby events: one close encounter with HD 106444 (F5V) at
$d_{\text{ca}}=0.65_{-0.40}^{+0.92}\,\text{pc}$ and
$t_{\text{ca}}=-3.49_{-1.75}^{+0.90}\,\text{Myr}$ and an encounter with HIP
59721 (G9V) at $d_{\text{ca}}=0.71_{-0.11}^{+0.18}\,\text{pc}$ and
$t_{\text{ca}}=-2.19_{-1.03}^{+0.54}\,\text{Myr}$. The two candidate
perturbers likely form a wide binary system (Mason et al., 2001). The two
events are only separated by $\sim 1.5\,\text{Myr}$ and are nearly co-planar,
the closest approach angles are $5.90_{-1.67}^{+1.77}$ deg and
$4.64_{-1.16}^{+1.00}$ deg, respectively.

An asymmetric inner disk is observed in $H$-band scattered light (Kalas et
al., 2015; Lagrange et al., 2016). Moreover, Kalas et al. (2015) detected a
significant asymmetry of the outer disk ($>$200 au projected radius) via
Hubble Space Telescope/Advanced Camera for Surveys (HST/ACS), with a truncated
fan structure on the southeast side and an extended linear morphology to the
northwest. The outer giant planet HD 106906 b is a likely source of the
perturbation, with the flybys playing a dynamical role in raising its
periastron distance away from the central region close to the binary (Rodet et
al., 2017; Nesvold et al., 2017; Nguyen et al., 2021).

####  4.4.2 HD 113556

We found an encounter between HD 113556 (F2V) and TYC 8674-2317-1 (K4) with
$d_{\text{ca}}=1.22_{-0.53}^{+0.88}\,\text{pc}$ at
$t_{\mathrm{ca}}=-1.55_{-0.64}^{+0.46}\,\mathrm{Myr}$. HD 113556 has Spitzer-
detected 24 and 70 $\mu$m infrared excesses (Chen et al., 2005, 2011a) in
addition to the ALMA detection at 1.24 mm (Lieman-Sifry et al., 2016), which
is unresolved. The flyby star, TYC 8674-2317-1, does not have reported
thermal-IR excess from its WISE photometry, although it has not been targeted
with either Spitzer, Herschel, or ALMA, all of which are more sensitive to
cold dust at wider separations.

####  4.4.3 HD 114082

HD 114082 (F3V) had one encounter $2.85_{-0.17}^{+0.16}$ Myr ago with
$d_{\text{ca}}=0.87_{-0.31}^{+0.36}$ pc. The interaction has
$\theta_{\text{ca}}=-13.91^{+3.96}_{-3.98}\,\text{deg}$ and a high relative
velocity ($v_{\text{rel}}=8.95\pm 0.40\,\text{km/s}$). Wahhaj et al. (2016)
first resolved the HD 114082 disk in $H$-band scattered light as a ring with
inner radius $\sim$28 au and $i\sim 83\arcdeg$. Esposito et al. (2020)
confirmed the ring structure in both $H$-band total intensity and polarized
intensity images. They noted that dust scattered light was 1.5$-$1.8 times
brighter on the west side of the disk compared to the east out to a radius of
1.2”. Deep optical images obtained with HST/STIS have detected the disk ansae
to $\sim$5.4” (520 au) radius from the star but a significant asymmetry in
brightness or structure between the two ansae is not evident (T. Esposito,
priv. comm.). The flyby star, HD 116402 (G3V), does not have a measured
infrared excess, and there are no published high-contrast imaging data
searching for a debris disk around this star.

####  4.4.4 HD 115600

We calculated that HD 115600 (F2IV/V) will encounter TYC 8674-2317-1 with
$t_{\text{ca}}=0.62_{-0.26}^{+0.65}\,\text{Myr}$ and
$d_{\text{ca}}=0.89_{-0.06}^{+0.09}\,\text{pc}$. Though $t_{\text{ca}}$ is in
the future, a closest approach at the present epoch is within the 3-$\sigma$
uncertainty. The closest approach angle is
$\theta_{\text{ca}}=10.52_{-1.69}^{+1.79}\,\text{deg}$. Previous GPI (Currie
et al., 2015) and SPHERE (Gibbs et al., 2019) detections of the HD 115600 disk
show a ring of dust at roughly 46 au radius. Currie et al. (2015) report a
possible (2$\sigma$) stellocentric offset that could be attributed to an
eccentric planet (Thilliez & Maddison, 2017), but the offset was not confirmed
by Gibbs et al. (2019). An optical image with HST/STIS over a wider field of
view does not detect the disk beyond $\sim$1.5” (164 au) radius (T. Esposito,
priv. comm.) and shows no ring or spiral structures that would be evidence for
a dynamical perturbation.

####  4.4.5 HD 142315

HD 142315 (B9V) had very recent encounters with HD 142705 (A0V) at
$d_{\mathrm{ca}}=1.60\pm 0.77\,\mathrm{pc}$ and
$t_{\mathrm{ca}}=-0.26_{-0.98}^{+0.62}\,\mathrm{Myr}$ and UCAC4 336-078484
(M0.5) at $d_{\mathrm{ca}}=1.36_{-0.45}^{+0.50}\,\mathrm{pc}$ and
$t_{\mathrm{ca}}=-0.17_{-0.79}^{+0.46}\,\mathrm{Myr}$; given the
uncertainties, these two flyby events are at their closest distances at the
present epoch. Figure 9 (panel e) shows that HD 142705 and UCAC4 336-078484
have very similar posterior distributions in the $d_{\mathrm{ca}}$–
$t_{\mathrm{ca}}$ space, suggesting that the two stars share similar space
velocities and may be a gravitationally-bound wide binary. These two stars are
separated on the sky by 0.20 deg, and have a three-dimensional separation of
$1.40_{-0.75}^{+1.36}\,\mathrm{pc}$. At this separation, the escape velocity
for the two stars assuming a circular orbit is $0.11_{-0.03}^{+0.05}$ km s-1.
From their astrometry and radial velocities, we measured a relative velocity
of $3.28_{-2.26}^{+3.52}$ km s-1. When considering the uncertainties, this is
not significantly different from the predicted escape velocity. More precise
radial velocity measurements of the two stars are required in order to refine
the estimate of their relative velocities. HD 142315 is also predicted to
encounter HD 142097 (A5V) in the future at
$d_{\mathrm{ca}}=1.55_{-0.12}^{+0.19}\,\mathrm{pc}$ and
$t_{\mathrm{ca}}=0.59_{-0.22}^{+0.35}\,\mathrm{Myr}$.

####  4.4.6 2MASS J16035767-2031055 (J1603-2031)

We calculated that the K0 star 2MASS J16035767-2031055 had a very recent close
approach with 2MASS J16053815-2039469 (M0) at
$t_{\mathrm{ca}}=-0.43_{-0.39}^{+0.17}\,\mathrm{Myr}$ with a close approach
distance at $d_{\mathrm{ca}}=0.52_{-0.16}^{+0.29}\,\mathrm{pc}$. We also
identified that the K-star will have another close approach with RX
J1603.9-2031B at $t_{\mathrm{ca}}=0.96_{-1.23}^{+1.80}\,\mathrm{Myr}$ with a
close approach distance of
$d_{\mathrm{ca}}=0.71_{-0.48}^{+2.06}\,\mathrm{pc}$. Incorporating the
position angle and inclination derived by Barenfeld et al. (2017) based on the
ALMA continuum detection, we computed the flyby angle of the two events to be
$\theta_{\mathrm{ca}}=-11.52_{-27.92}^{+29.51}\,\mathrm{deg}$ and
$\theta_{\mathrm{ca}}=-11.65_{-29.84}^{+33.98}\,\mathrm{deg}$, respectively.
The flyby angle is basically unconstrained due to the the large ($\sim 30\%$)
uncertainties in the fitted disk orientation. Additionally, Garufi et al.
(2020) carries out observations on this star in $H$-band using SPHERE but
reports a non-detection of any disk in scattered light.

J1603-2031 (also catalogued as RX J1603.9-2031A) and RX J1603.9-2031B were
originally paired by Köhler et al. (2000), but their relative velocities are
not consistent with them being gravitationally-bound. RX J1603.9-2031B is
itself known to be a binary of two stars with similar magnitudes on a 53-year
orbit (Tokovinin & Briceño, 2018). The RV semi-amplitude of their assumed
orbit is approximately 3.6 km s-1. While this is larger than the uncertainties
on the radial velocity measurement used for the analysis presented here, the
measured velocity of the blended spectrum may not be significantly affected by
binary motion due to the similar magnitudes of both components. A joint fit of
the astrometry presented in Tokovinin & Briceño (2018) and radial velocity
measurements where the two stars are spectrally resolved will be needed to
determine the systemic velocity, refining the trajectory of this binary
relative to J1603-2031.

####  4.4.7 2MASS J16042165-2130284 (J1604-2130)

This K2 star will encounter the M1 star EPIC 204548337 (2MASS
J16032787-2153155) with $d_{\text{ca}}=1.22_{-0.53}^{+0.88}\,\text{pc}$ at
$t_{\mathrm{ca}}=0.33_{-0.59}^{+0.56}\,\mathrm{Myr}$. Given the uncertainties
in $t_{\mathrm{ca}}$ the closest approach epoch is near the present time and
the current separation between the two stars is shown in Table 4.4. J1604-2130
hosts one of the largest disks in Upper Scorpius subgroup, appearing as a
face-on dust and gas ring between roughly 15 and 300 au radius in thermal
emission (Mathews et al., 2012; Zhang et al., 2014; Barenfeld et al., 2016;
Dong et al., 2017) and scattered light (Mayama et al., 2012; Pinilla et al.,
2015, 2018). The median close approach angle is calculated to be
$-27.59_{-47.42}^{+86.39}$ (Table 4.4). However, due to the bimodality of the
posterior distribution of the close approach angle as displayed in panel (g)
of Fig.9, we divided the distribution at $\theta_{\mathrm{ca}}=0^{\circ}$ and
calculated the median of each section to be
$52.46_{-30.30}^{+15.75}\,\mathrm{deg}$ and
$-64.89_{-14.05}^{+34.18}\,\mathrm{deg}$, which suggests that the flyby of
EPIC 204548337 is nearly perpendicular to the disk plane. The azimuthal
structure of the dust ring has a rotating deficit of scattered light that can
be explained as shadowing by optically thick clumps orbiting within an inner
dust disk. The existence of a clumpy inner disk misaligned with respect to the
outer disk is supported by the observation of aperiodic photometric dips
(Ansdell et al., 2016). Future work is needed to investigate if EPIC 204548337
plays a dynamical role in misaligning the outer disk relative to the inner
disk (e.g., Kraus et al. 2020; Ginski et al. 2021).

####  4.4.8 2MASS J16064794-1841437 (J1606-1841)

J1606-1841 (M0.0e) has an ALMA-resolved disk with $i=55.5^{+0.1}_{-0.1}$ deg
and PA=$20^{+1}_{-1}$ deg (Ansdell et al., 2020). The bi-lobed thermal
emission map suggests a narrow ring-like structure. The flyby star HD 144295
(A0V) came within 0.7 $R_{H}$ (Table 5) at
$t_{\mathrm{ca}}=-1.32_{-0.40}^{+0.26}\,\mathrm{Myr}$. The high relatively
velocity $v_{\mathrm{rel}}$ = 13.46 km/s (Table 4.4) makes it unlikely that
the disk was significantly perturbed by the encounter. On the other hand, HD
144295 (HIP 79124) is a triple stellar system (Asensio-Torres et al., 2019;
Ruane et al., 2019) that needs future characterization to improve the
determination of the systemic velocity.

####  4.4.9 HD 145718

In Table 4.4 this A5 star it is distinguished by having a total of five
flybys, all within $\sim$2.5 million years. The flyby of 2MASS
J16125889-2245202 (M0) has the smallest $d_{\mathrm{ca}}$ (0.93 pc),
corresponding to 0.65 $R_{H}$ (Table 5). Ansdell et al. (2020) report that HD
145718 has an ALMA-resolved disk with PA=$1^{+1}_{-1}$ deg and
$i=70.4^{+1.2}_{-1.2}$ deg. The inner circumstellar disk was resolved using
VLTI GRAVITY in $K$-band, giving PA=$2_{+2}^{-2}$ deg and
$i=71.9^{+1.2}_{-1.2}$ (Gravity Collaboration et al., 2019). It was also
resolved with VLTI PIONEER in $H$-band (Kluska et al., 2020), giving
PA=$-3^{+4}_{-4}$ deg and $i=48^{+3}_{-3}$ deg. The main discrepancy is in the
determination of disk inclination. For the purpose of calculating the
encounter geometry, we adopt the values given by Ansdell et al. (2020). We
note that the posterior distribution of the close approach angle with 2MASS
J16125889-2245202 is bimodal—this is likely due to the fact that both stars’
RV measurements are not certain about the direction of the line-of-sight
motion. Therefore, we divided the distributions at
$\theta_{\mathrm{ca}}=18^{\circ}$ into two sections and calculated the median
of each section to be $33.64_{-7.98}^{+22.34}\,\text{deg}$ and
$-10.22_{-4.23}^{+10.67}\,\text{deg}$, respectively.

####  4.4.10 HD 146897

We find three flybys for HD 146897 (F2/F3V; HIP 79977), two in the recent past
or present, and one in the future. HD 146897 had a close encounter with HD
146416 (B9V) at $t_{\text{ca}}=-0.44_{-0.33}^{+0.31}\,\text{Myr}$ and
$d_{\text{ca}}=0.65_{-0.43}^{+0.61}$ pc. The flyby is nearly coplanar with the
debris disk ($\theta_{\text{ca}}=4.23_{-4.34}^{+4.37}\,\text{deg}$) which was
first resolved in scattered light by Thalmann et al. (2013). Goebel et al.
(2018) measured a brightness asymmetry along the midplane in images obtained
with Subaru SCExAO/CHARIS. This asymmetry is also seen in GPI images (Esposito
et al., 2020) and extends to $>$6” (789 au) radius in optical HST/STIS images
(Kalas et al., 2020).

HD 146897 also has a close encounter with HD 147083 (A7III/IV) in the future
or present with $t_{\text{ca}}=0.51_{-1.09}^{+1.34}\,\text{Myr}$,
$d_{\text{ca}}=0.58_{-0.13}^{+0.46}\,\text{pc}$. The closest approach angle is
$\theta_{\text{ca}}=5.19_{-12.69}^{+10.26}\,\text{deg}$. The error bar on
$\theta_{\text{ca}}$ is large compared to the other flyby events because of
the large uncertainty in the radial velocity of the flyby star.

Lastly, HD 146897 will possibly encounter HD 146743 (F3V) at
$t_{\text{ca}}=1.63_{-0.39}^{+0.50}\,\text{Myr}$ and
$d_{\text{ca}}=1.12_{-0.38}^{+0.53}\,\text{pc}$. However, it is unlikely this
encounter will affect the current disk morphology given the large closest
approach distance.

Figure 9: Closest approach distance as a function of closest approach time and
closest approach angle for each of the flyby events involving detected debris
disks. The crosses represent the median values of MC outputs; if the posterior
distribution is bimodal, the median about each mode is marked by a red cross.
The contours represent the $1\sigma$ and $2\sigma$ credible regions.

##  5 Potential Encounters in Upper Scorpius Subgroup

Figure 10: The potential close approaches that would have happened within the
USCO subgroup by simulating the effect of perfectly knowing the RV of stars
without RV measurement. The dotted lines mark the 1st-, 5th-, 10th-, and 50th-
percentiles of the distribution of the separation of stars within the sample.

There are $1341$ stars in the USCO sample that do not have RV measurement. We
also attempted to identify potential close approaches between these stars and
those that have RV measurements. While we cannot confidently determine which
of these stars will have experienced a flyby event in the past, we can
identify candidate encounters to motivate future radial velocity measurements
of these stars to better understand their kinematics. For each pair of stars
(let the one with RV measurement be star A and the other be star B), we
sampled the 6D space position and motion of star A derived from Gaia
astrometry and radial velocities, and the five known astrometric parameters of
star B with $10^{5}$ trials using the method in Section 3.1. For star B the
radial velocity was fixed at a constant value, allowing us to identify
candidate flyby events assuming this velocity. We repeated this exercise for
radial velocities consistent with the distribution of other Upper Sco members
($-5\pm 4$ km s-1; (Gagné et al., 2018)), changing the radial velocity from
$-21$ km s-1 to $11$ km s-1 ($-4\sigma$ to $+4\sigma$) in steps of 0.01 km
s-1.

Flyby events identified from this analysis that had $t_{\rm CA}<0$ Myr and
where the radial velocity of star B was within the $1\sigma$ range of the
distribution of Upper Sco members were selected as candidate flyby events
within the USCO sample. Overall, we identified 30 potential close encounters
and 224 potential encounters. The distribution of these candidate events in
terms of closest approach time and distance is shown in Figure 10, and a
complete listing is given in Table 7.

Of these candidate encounters, four involve three stars with a measured IR
excess in Cotten & Song (2016); HD 144587, HD 146069, and HD 145631 that has
two candidate encounters. The paucity of excess stars within this sample is
not surprising; many of these stars have only recently been identified as
members of the Upper Sco subgroup. Of these three, only HD 144587 has been
observed by a high-contrast imager to search for polarized emission from
circumstellar material, but no detection was reported (Uyama et al., 2017).
All of the stars have been the subject of adaptive optics searches for stellar
companions. HD 145631 was resolved as a hierarchical triple system consisting
of an F-type primary and at 191 au two K-type companions with a projected
separation of 7.3 au (Lafrenière et al., 2014). It is not known what effect
these companions are having on the morphology of the disk as it has not yet
been spatially resolved.

##  6 Conclusion

Using astrometry from Gaia and radial velocities from a combination of
literature sources, we searched for close stellar flybys within the Scorpius-
Centaurus OB2 association. We calculated the closest approach distance and the
time at which the close approaches take place for two samples of stars. The
first consisted of 462 stars spanning all three sub-groups of Sco-Cen whose
membership were assessed primarily from Hipparcos astrometry (e.g., de Zeeuw
et al., 1999), while the second contained 219 stars within the Upper Sco
subgroup identified from a more recent analysis of Gaia DR2 astrometry (Luhman
& Esplin, 2020). We identified 12 past close encounters (defined as one star
entering the half hill sphere radius of another), and 12 future close
encounters amongst the first sample, and 37 more distant encounters. Amongst
the Upper Sco sample, we identified 36 past close encounters, 18 future close
encounters, and 106 more distant encounters. We also searched for candidate
flyby events involving the Upper Sco sample members that do not yet have
radial velocity measurements. We identified 41 candidate close encounters
given simulated radial velocities consistent with the measured distribution of
other Upper Sco members (Gagné et al., 2018).

Motivated by theoretical work exploring the interaction between stellar flybys
and the evolution of exoplanetary system architectures, we cross-matched the
flyby events identified within this work with catalogues of stars within Sco-
Cen that have measured infrared-excesses (Cotten & Song, 2016), indicative of
the presence of circumstellar material. For the subset of stars with an
infrared excess we searched the literature for spatially resolved detections
of these disks from which their geometry can be measured. A total of 10 stars
with at least one identified flyby event have debris disks that have been
resolved either in scattered light from ground or space-based instrumentation,
or detected at sub-millimeter wavelengths with ALMA. In one case, for HD
106906, the disk exhibits a strong brightness asymmetry (Kalas et al., 2015),
but it is likely that this is caused by the orbiting planetary-mass companion
(Bailey et al., 2014; Nguyen et al., 2021) rather than a direct interaction
between the disk and the two flyby stars. For the remaining targets the
existing evidence does not show significantly disturbed morphologies that
could be caused by a flyby of another star.

Our incomplete census of the full Sco-Cen association limited the analysis
presented here to the more massive stars ($>1$ M⊙) for two of the three
subgroups. A more exhaustive search for flyby events can be undertaken when
the membership of the three subgroups are re-assessed using data from current
and future Gaia data releases. Another significant limitation is the precision
of the radial velocity measurements for many of the stars within our sample; a
large uncertainty in the radial velocity at the current epoch significantly
hampers our ability to predict the relative positions and velocities of pairs
of stars within the cluster at earlier times. Future Gaia data releases will
contain radial velocity measurements for many more of the cluster members than
used in this study, but at a relatively low precision ($\sim$1 km s-1). In
order to accurately trace back the motions of the cluster members to
confidently determine which pairs of stars experienced a close encounter in
the past, precision radial velocities are needed in conjunction with the
precision astrometry provided by Gaia. With these data, we will be able
investigate more interesting and stronger dynamical interactions between flyby
stars and debris disks to provide empirical evidence of the impact of stellar
flybys on the evolution of planetary systems.

We thank the anonymous referees for their comments that helped to improve the
quality of this manuscript. The first author also thanks Eugene Chiang,
Fanghui Wan, and Tianqi Wei for the valuable discussions and/or support
regarding the work. This work is supported by NSF AST-1518332, NASA NNX15AC89G
and NNX15AD95G/NEXSS. This work benefited from NASA’s Nexus for Exoplanet
System Science (NEXSS) research coordination network sponsored by NASA’s
Science Mission Directorate. This research has made use of the SIMBAD database
and the VizieR catalogue access tool, both operated at the CDS, Strasbourg,
France. This work has made use of data from the European Space Agency (ESA)
mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data
Processing and Analysis Consortium (DPAC,
https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has
been provided by national institutions, in particular the institutions
participating in the Gaia Multilateral Agreement.

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##  Appendix A Tables of the two samples and all identified events

Table 2: RV measurements of Sco-Cen sample

Target |  Gaia DR2 identifier | SpT | Mass | Subgroup |  Gaia RV |  Gaia RV error | RV | RV error | ref.
---|---|---|---|---|---|---|---|---|---
|  |  | (M⊙) |  | (km/s) | (km/s) | (km/s) | (km/s) |
HD 103823 | 3463980463840622848 | G2/3V | 1.256 | LCC | $19.3$ | 0.28 | $19.5$ | 2.3 | 14
⋮ | ⋮ | ⋮ | ⋮ | ⋮ | ⋮ | ⋮ | ⋮ | ⋮ | ⋮

References. — (1) Thackeray & Hutchings, 1965; (2)Kaufmann & Klippel, 1973;
(3) Evans, 1967; (4) Levato et al., 1987; (5) Mathias et al., 1991; (6) Levato
et al., 1996; (7) Stickland et al., 1996; (8) Torres et al., 2006; (9)
Gontcharov, 2006; (10) Kharchenko et al., 2007; (11) Chen et al., 2011b; (12)
Song et al., 2012; (13) Dahm et al., 2012; (14) Kordopatis et al., 2013; (15)
Desidera et al., 2015; (16) Budding et al., 2015; (17) Riedel et al., 2017;
(18) Tokovinin, 2018; (19) Gaia Collaboration et al., 2018; (20) De Rosa &
Kalas, 2019.

Note. — The full table will be available in the online version of the paper
and on Vizier.

Table 3: RV measurements of USCO sample

Target |  Gaia DR2 identifier | SpT | Mass |  Gaia RV |  Gaia RV error | RV | RV error | ref.
---|---|---|---|---|---|---|---|---
|  |  | (M⊙) | (km/s) | (km/s) | (km/s) | (km/s) |
2MASS J15401546-1620577 | 6261607772589226496 |  $\cdots$ | 12.971 | -12.12 | 4.24 | -12.12 | 4.24 | 8
⋮ | ⋮ | ⋮ | ⋮ | ⋮ | ⋮ | ⋮ | ⋮ | ⋮

References. — (1) Levato et al., 1987; (2) Mathias et al., 1991; (3) Duflot et
al. (1995); (4) Gontcharov, 2006; (5) Kharchenko et al., 2007; (6) Chen et
al., 2011b; (7) Dahm et al., 2012; (8) Gaia Collaboration et al., 2018

Note. — The full table will be available in the online version of the paper
and on Vizier

Table 4: Events identified from Sco-Cen sample

Star 1 | Star 2 |  $t_{\text{ca}}$ (Myr) |  $d_{\text{ca}}$ (pc) | $d_{\text{ca}}/R_{\text{H,T}}$ | $d_{\text{ca}}/R_{\text{H,F}}$ | Category | IR Excess?
---|---|---|---|---|---|---|---
2MASS J12143187-5110156 | CD-50 6815 | $0.02_{-0.30}^{+0.30}$ | $0.18_{-0.14}^{+0.40}$ | $0.17_{-0.12}^{+0.36}$ | $0.14_{-0.11}^{+0.31}$ | TF0.5 |  $\cdots$
CD-48 8201 | HD 116650 | $-0.21_{-0.09}^{+0.07}$ | $0.51_{-0.28}^{+0.24}$ | $0.42_{-0.23}^{+0.20}$ | $0.39_{-0.22}^{+0.19}$ | TF0.5 |  $\cdots$
HD 104231 | HD 104125 | $-0.61_{-0.32}^{+0.26}$ | $0.24_{-0.11}^{+0.28}$ | $0.18_{-0.09}^{+0.21}$ | $0.16_{-0.08}^{+0.18}$ | TF0.5 | T
HD 106444 | HIP 59721 | $-0.09_{-0.37}^{+0.32}$ | $0.08_{-0.05}^{+0.16}$ | $0.06_{-0.04}^{+0.12}$ | $0.06_{-0.04}^{+0.13}$ | TF0.5 |  $\cdots$
HD 106906 | HD 106444 | $-3.49_{-1.75}^{+0.90}$ | $0.65_{-0.40}^{+0.92}$ | $0.48_{-0.30}^{+0.68}$ | $0.48_{-0.30}^{+0.69}$ | TF0.5 | T
HD 107821 | TYC 8983-854-1 | $0.42_{-0.16}^{+0.22}$ | $0.56_{-0.11}^{+0.20}$ | $0.37_{-0.07}^{+0.13}$ | $0.45_{-0.09}^{+0.16}$ | TF0.5 |  $\cdots$
HD 110634 | 2MASS J12454884-5410583 | $-0.94_{-0.19}^{+0.14}$ | $0.62_{-0.17}^{+0.14}$ | $0.45_{-0.12}^{+0.10}$ | $0.49_{-0.13}^{+0.11}$ | TF0.5 | T
HD 145631 | HD 145502 | $0.34_{-0.48}^{+0.49}$ | $0.64_{-0.20}^{+0.46}$ | $0.39_{-0.12}^{+0.28}$ | $0.27_{-0.09}^{+0.20}$ | TF0.5 | T
HD 146897 | HD 146416 | $-0.44_{-0.33}^{+0.31}$ | $0.65_{-0.43}^{+0.61}$ | $0.45_{-0.30}^{+0.42}$ | $0.40_{-0.27}^{+0.37}$ | TF0.5 | T
HD 146897 | HD 147083 | $0.51_{-1.09}^{+1.34}$ | $0.58_{-0.13}^{+0.46}$ | $0.40_{-0.09}^{+0.32}$ | $0.39_{-0.09}^{+0.31}$ | TF0.5 | T
HD 147010 | HD 147104 | $0.19_{-0.14}^{+0.27}$ | $0.32_{-0.06}^{+0.43}$ | $0.19_{-0.04}^{+0.27}$ | $0.20_{-0.04}^{+0.27}$ | TF0.5 |  $\cdots$
TYC 7344-788-1 | 2MASS J16265700-3032232 | $-0.01_{-0.31}^{+0.31}$ | $0.37_{-0.27}^{+0.63}$ | $0.31_{-0.23}^{+0.53}$ | $0.32_{-0.24}^{+0.55}$ | TF0.5 |  $\cdots$
2MASS J15334052-3917467 | HD 138204 | $0.01_{-0.08}^{+0.13}$ | $0.72_{-0.02}^{+0.02}$ | $0.93_{-0.02}^{+0.03}$ | $0.48_{-0.01}^{+0.01}$ | F0.5 |  $\cdots$
HD 145554 | HD 145502 | $0.06_{-0.46}^{+0.45}$ | $0.92_{-0.64}^{+1.38}$ | $0.56_{-0.39}^{+0.85}$ | $0.39_{-0.28}^{+0.60}$ | F0.5 | T
CD-62 657 | TYC 8983-564-1 | $-1.06_{-0.88}^{+0.41}$ | $1.18_{-0.33}^{+0.27}$ | $0.92_{-0.26}^{+0.21}$ | $0.94_{-0.26}^{+0.21}$ | TF |  $\cdots$
CPD-64 1859 | 2MASS J12205449-6457242 | $0.11_{-0.42}^{+0.35}$ | $0.84_{-0.58}^{+1.08}$ | $0.68_{-0.46}^{+0.87}$ | $0.69_{-0.48}^{+0.89}$ | TF |  $\cdots$
HD 104021 | CPD-52 5025 | $0.15_{-0.15}^{+0.16}$ | $0.90_{-0.11}^{+0.08}$ | $0.63_{-0.07}^{+0.06}$ | $0.75_{-0.09}^{+0.07}$ | TF |  $\cdots$
HD 105613 | HD 104900 | $-0.24_{-0.15}^{+0.08}$ | $1.49_{-0.35}^{+0.20}$ | $1.00_{-0.23}^{+0.13}$ | $0.91_{-0.21}^{+0.12}$ | TF | T
HD 105857 | HD 106218 | $-0.13_{-0.06}^{+0.05}$ | $1.18_{-0.02}^{+0.02}$ | $0.79_{-0.01}^{+0.01}$ | $0.87_{-0.01}^{+0.01}$ | TF | T
HD 106036 | HD 106538 | $-1.10_{-0.34}^{+0.22}$ | $0.76_{-0.14}^{+0.17}$ | $0.50_{-0.09}^{+0.11}$ | $0.58_{-0.10}^{+0.13}$ | TF | T
HD 106473 | [SZB2012] 9 | $-0.61_{-0.19}^{+0.20}$ | $1.08_{-0.48}^{+1.24}$ | $0.75_{-0.33}^{+0.86}$ | $0.88_{-0.39}^{+1.01}$ | TF |  $\cdots$
HD 106906 | HIP 59721 | $-2.19_{-1.03}^{+0.54}$ | $0.71_{-0.11}^{+0.18}$ | $0.53_{-0.08}^{+0.13}$ | $0.55_{-0.09}^{+0.14}$ | TF | T
HD 107696 | HD 106490 | $0.61_{-0.28}^{+0.28}$ | $1.72_{-0.61}^{+0.90}$ | $0.91_{-0.32}^{+0.48}$ | $0.73_{-0.26}^{+0.38}$ | TF |  $\cdots$
HD 108501 | TYC 8983-854-1 | $-0.17_{-0.35}^{+0.29}$ | $1.04_{-0.06}^{+0.24}$ | $0.66_{-0.04}^{+0.15}$ | $0.85_{-0.05}^{+0.20}$ | TF |  $\cdots$
HD 109197 | CPD-53 5235 | $1.80_{-0.32}^{+0.46}$ | $0.83_{-0.40}^{+0.66}$ | $0.55_{-0.26}^{+0.44}$ | $0.66_{-0.31}^{+0.52}$ | TF |  $\cdots$
HD 109808 | HD 110461 | $-0.01_{-0.10}^{+0.10}$ | $1.42_{-0.12}^{+0.49}$ | $0.93_{-0.08}^{+0.32}$ | $0.87_{-0.07}^{+0.30}$ | TF |  $\cdots$
HD 110634 | HD 110697 | $-0.59_{-0.12}^{+0.11}$ | $1.22_{-0.07}^{+0.05}$ | $0.90_{-0.05}^{+0.04}$ | $0.89_{-0.05}^{+0.04}$ | TF | T
HD 113556 | TYC 8674-2317-1 | $-1.55_{-0.64}^{+0.46}$ | $1.22_{-0.53}^{+0.88}$ | $0.90_{-0.39}^{+0.65}$ | $0.99_{-0.43}^{+0.71}$ | TF | T
HD 113901 | HD 114365 | $0.08_{-0.13}^{+0.17}$ | $1.24_{-0.03}^{+0.12}$ | $0.92_{-0.02}^{+0.09}$ | $0.71_{-0.02}^{+0.07}$ | TF |  $\cdots$
HD 114319 | HD 108904 | $-0.98_{-0.05}^{+0.05}$ | $1.13_{-0.21}^{+0.49}$ | $0.81_{-0.15}^{+0.35}$ | $0.85_{-0.16}^{+0.37}$ | TF |  $\cdots$
HD 115470 | HD 115988 | $-0.63_{-0.23}^{+0.18}$ | $1.57_{-0.17}^{+0.30}$ | $0.97_{-0.11}^{+0.19}$ | $0.96_{-0.10}^{+0.18}$ | TF |  $\cdots$
HD 115600 | TYC 8674-2317-1 | $0.62_{-0.26}^{+0.65}$ | $0.89_{-0.06}^{+0.09}$ | $0.65_{-0.05}^{+0.07}$ | $0.72_{-0.05}^{+0.07}$ | TF | T
HD 116402 | HD 114082 | $-2.85_{-0.17}^{+0.16}$ | $0.87_{-0.31}^{+0.36}$ | $0.67_{-0.24}^{+0.28}$ | $0.64_{-0.23}^{+0.26}$ | TF | TF
HD 119718 | HD 116794 | $-1.78_{-0.14}^{+0.12}$ | $1.27_{-0.36}^{+0.39}$ | $0.94_{-0.27}^{+0.29}$ | $0.95_{-0.27}^{+0.29}$ | TF | T
HD 143215 | HD 143844 | $-0.44_{-0.07}^{+0.07}$ | $1.27_{-0.16}^{+0.17}$ | $0.98_{-0.13}^{+0.13}$ | $0.97_{-0.12}^{+0.13}$ | TF |  $\cdots$
HD 143637 | HD 146285 | $-2.91_{-0.29}^{+0.24}$ | $1.35_{-0.77}^{+1.26}$ | $0.98_{-0.56}^{+0.92}$ | $0.77_{-0.44}^{+0.72}$ | TF |  $\cdots$
HD 144587 | HD 144844 | $0.23_{-0.30}^{+0.35}$ | $1.05_{-0.26}^{+1.16}$ | $0.74_{-0.18}^{+0.82}$ | $0.65_{-0.16}^{+0.71}$ | TF | T
HD 144981 | HD 144925 | $-0.93_{-0.27}^{+0.20}$ | $0.84_{-0.14}^{+0.19}$ | $0.54_{-0.09}^{+0.12}$ | $0.54_{-0.09}^{+0.12}$ | TF | T
HD 144981 | HD 145631 | $-1.51_{-1.71}^{+0.69}$ | $1.27_{-0.44}^{+1.99}$ | $0.82_{-0.28}^{+1.28}$ | $0.78_{-0.27}^{+1.22}$ | TF | TF
HD 145189 | HD 144254 | $0.76_{-0.14}^{+0.17}$ | $1.02_{-0.28}^{+0.45}$ | $0.69_{-0.19}^{+0.31}$ | $0.66_{-0.18}^{+0.29}$ | TF | T
HD 145467 | HD 145718 | $-2.31_{-1.03}^{+0.63}$ | $1.15_{-0.60}^{+1.36}$ | $0.80_{-0.42}^{+0.94}$ | $0.78_{-0.41}^{+0.92}$ | TF |  $\cdots$
HD 146743 | HD 147083 | $1.42_{-0.46}^{+0.83}$ | $1.01_{-0.34}^{+0.70}$ | $0.70_{-0.23}^{+0.49}$ | $0.69_{-0.23}^{+0.48}$ | TF |  $\cdots$
HD 146897 | HD 146743 | $1.63_{-0.39}^{+0.49}$ | $1.12_{-0.38}^{+0.53}$ | $0.78_{-0.26}^{+0.37}$ | $0.78_{-0.26}^{+0.37}$ | TF | T
HD 147220 | HD 146416 | $0.63_{-0.04}^{+0.04}$ | $0.88_{-0.30}^{+0.33}$ | $0.62_{-0.21}^{+0.23}$ | $0.54_{-0.18}^{+0.20}$ | TF |  $\cdots$
HD 150742 | HD 150591 | $0.38_{-0.27}^{+0.33}$ | $1.34_{-0.32}^{+0.67}$ | $0.64_{-0.16}^{+0.32}$ | $0.71_{-0.17}^{+0.35}$ | TF | T
HD 151721 | HD 152057 | $0.32_{-0.04}^{+0.04}$ | $1.16_{-0.11}^{+0.14}$ | $0.78_{-0.07}^{+0.10}$ | $0.80_{-0.07}^{+0.10}$ | TF | T
TYC 8983-854-1 | TYC 8983-564-1 | $-0.78_{-0.58}^{+0.38}$ | $0.69_{-0.24}^{+0.53}$ | $0.56_{-0.20}^{+0.43}$ | $0.55_{-0.19}^{+0.42}$ | TF |  $\cdots$
HD 106797 | 2MASS J12205449-6457242 | $0.07_{-0.19}^{+0.18}$ | $1.56_{-0.03}^{+0.02}$ | $0.99_{-0.02}^{+0.01}$ | $1.29_{-0.02}^{+0.01}$ | T | T
HD 106797 | CPD-64 1859 | $-0.07_{-0.26}^{+0.22}$ | $1.52_{-0.11}^{+0.19}$ | $0.97_{-0.07}^{+0.12}$ | $1.23_{-0.09}^{+0.15}$ | T | T
HD 107821 | CD-62 657 | $0.02_{-0.11}^{+0.11}$ | $1.41_{-0.05}^{+0.10}$ | $0.95_{-0.03}^{+0.06}$ | $1.10_{-0.04}^{+0.07}$ | T |  $\cdots$
HD 108483 | CD-49 7027 | $-1.20_{-1.44}^{+1.85}$ | $2.27_{-1.12}^{+4.77}$ | $0.96_{-0.47}^{+2.02}$ | $1.78_{-0.88}^{+3.74}$ | T |  $\cdots$
HD 108501 | 2MASS J12205449-6457242 | $-0.30_{-0.56}^{+0.21}$ | $1.55_{-0.51}^{+0.29}$ | $0.99_{-0.33}^{+0.18}$ | $1.28_{-0.42}^{+0.24}$ | T |  $\cdots$
HD 108501 | TYC 8983-564-1 | $-0.05_{-0.37}^{+0.39}$ | $1.55_{-0.16}^{+0.44}$ | $0.99_{-0.10}^{+0.28}$ | $1.23_{-0.12}^{+0.35}$ | T |  $\cdots$
HD 110634 | 2MASS J12474824-5431308 | $-2.17_{-0.44}^{+0.36}$ | $1.33_{-0.61}^{+0.85}$ | $0.98_{-0.45}^{+0.63}$ | $1.23_{-0.56}^{+0.79}$ | T | T
HD 120642 | CD-51 7721 | $1.31_{-0.27}^{+0.44}$ | $1.51_{-0.32}^{+0.31}$ | $0.85_{-0.18}^{+0.18}$ | $1.22_{-0.26}^{+0.25}$ | T |  $\cdots$
HD 142315 | HD 142097 | $0.59_{-0.22}^{+0.35}$ | $1.55_{-0.12}^{+0.19}$ | $0.95_{-0.08}^{+0.12}$ | $1.05_{-0.08}^{+0.13}$ | T | T
HD 142315 | HD 142705 | $-0.26_{-0.98}^{+0.62}$ | $1.60_{-0.77}^{+0.77}$ | $0.98_{-0.48}^{+0.48}$ | $1.03_{-0.50}^{+0.50}$ | T | TF
HD 148184 | HD 147010 | $-0.85_{-0.49}^{+0.41}$ | $2.28_{-1.18}^{+1.80}$ | $0.98_{-0.51}^{+0.77}$ | $1.40_{-0.73}^{+1.11}$ | T |  $\cdots$
HD 150742 | HD 150287 | $0.04_{-0.24}^{+0.24}$ | $1.95_{-0.08}^{+0.06}$ | $0.94_{-0.04}^{+0.03}$ | $1.38_{-0.06}^{+0.04}$ | T | T
HD 113524 | HD 113703 | $1.10_{-0.52}^{+0.82}$ | $1.95_{-0.53}^{+2.22}$ | $1.47_{-0.40}^{+1.67}$ | $0.94_{-0.26}^{+1.08}$ | F | T
HD 121835 | HD 121190 | $-0.13_{-0.34}^{+0.26}$ | $1.35_{-0.22}^{+0.06}$ | $1.02_{-0.17}^{+0.04}$ | $0.83_{-0.14}^{+0.04}$ | F |  $\cdots$
HD 137499 | HD 137432 | $-0.09_{-0.97}^{+0.97}$ | $1.62_{-0.28}^{+1.06}$ | $1.19_{-0.21}^{+0.78}$ | $0.81_{-0.14}^{+0.53}$ | F | T
HD 144254 | HD 144470 | $-0.20_{-0.43}^{+0.35}$ | $2.01_{-1.27}^{+2.26}$ | $1.29_{-0.82}^{+1.45}$ | $0.72_{-0.46}^{+0.81}$ | F |  $\cdots$
HD 144587 | HD 144175 | $0.08_{-0.87}^{+1.02}$ | $1.46_{-0.08}^{+0.33}$ | $1.03_{-0.06}^{+0.23}$ | $0.90_{-0.05}^{+0.20}$ | F | T
HD 144587 | HD 144661 | $0.32_{-0.59}^{+0.47}$ | $1.58_{-0.90}^{+1.76}$ | $1.11_{-0.64}^{+1.24}$ | $0.90_{-0.51}^{+1.00}$ | F | T
HD 144844 | HD 144661 | $0.29_{-0.28}^{+0.30}$ | $1.70_{-0.58}^{+1.47}$ | $1.04_{-0.36}^{+0.90}$ | $0.97_{-0.33}^{+0.84}$ | F |  $\cdots$
HD 144925 | HD 145502 | $0.04_{-0.30}^{+0.28}$ | $2.09_{-0.38}^{+0.29}$ | $1.34_{-0.25}^{+0.18}$ | $0.90_{-0.17}^{+0.12}$ | F |  $\cdots$

Note. — The letters “T” and “F” in the table stand for “target star” and
“flyby star”, respectively. In the “Category” column, “T” and “F” denote
encounters with $0.5<d_{\text{ca}}/R_{\text{H,T}}<1$ and
$0.5<d_{\text{ca}}/R_{\text{H,F}}<1$, respectively; “TF” denotes encounters
satisfying both previous conditions. Those ending with “0.5” denenote the fact
that $d_{\text{ca}}/R_{\text{H}}<=0.5$, where $R_{\text{H}}$ is the Hill
radius of target star (T), flyby star (F), or both (TF). Similarly, in the “IR
Excess” column, “T” or “F” denotes that the target star or the flyby star has
IR excess detected, respectively; “TF” denotes that both stars have IR excess
detected, according to Cotten & Song 2016.

Table 5: Events identified from USCO sample

Star 1 | Star 2 |  $t_{\text{ca}}$ (Myr) |  $d_{\text{ca}}$ (pc) | $d_{\text{ca}}/R_{\text{H,T}}$ | $d_{\text{ca}}/R_{\text{H,F}}$ | Category | IR Excess?
---|---|---|---|---|---|---|---
Gaia DR2 6041813096094711424 | Gaia DR2 6041813100399783552 | $0.09_{-0.27}^{+0.29}$ | $0.16_{-0.12}^{+0.41}$ | $0.17_{-0.12}^{+0.43}$ | $0.15_{-0.12}^{+0.40}$ | TF0.5 |  $\cdots$
CD-26 11326 |  $[$SMH2016$]$ NV6 | $-0.08_{-0.35}^{+0.30}$ | $0.54_{-0.16}^{+0.31}$ | $0.39_{-0.12}^{+0.22}$ | $0.48_{-0.14}^{+0.28}$ | TF0.5 |  $\cdots$
2MASS J16210010-2536031 | 2MASS J16211379-2533314 | $-0.03_{-0.16}^{+0.16}$ | $0.30_{-0.14}^{+0.49}$ | $0.24_{-0.11}^{+0.39}$ | $0.25_{-0.12}^{+0.41}$ | TF0.5 |  $\cdots$
HD 146001 | 2MASS J16145546-2527597 | $0.25_{-0.43}^{+0.55}$ | $0.46_{-0.32}^{+0.80}$ | $0.26_{-0.18}^{+0.45}$ | $0.34_{-0.23}^{+0.59}$ | TF0.5 |  $\cdots$
HD 147196 | 2MASS J16211450-2342200 | $-0.04_{-0.31}^{+0.30}$ | $0.43_{-0.30}^{+0.77}$ | $0.23_{-0.16}^{+0.41}$ | $0.38_{-0.26}^{+0.67}$ | TF0.5 |  $\cdots$
2MASS J16053815-2039469 | 2MASS J16035767-2031055 | $-0.43_{-0.39}^{+0.17}$ | $0.52_{-0.16}^{+0.29}$ | $0.49_{-0.14}^{+0.27}$ | $0.43_{-0.13}^{+0.24}$ | TF0.5 |  $\cdots$
UCAC4 354-076591 | 2MASS J16111742-1918285 | $0.19_{-0.98}^{+1.11}$ | $0.47_{-0.10}^{+0.25}$ | $0.44_{-0.09}^{+0.24}$ | $0.45_{-0.10}^{+0.24}$ | TF0.5 |  $\cdots$
UCAC4 354-076591 | Wa Oph 2 | $-0.05_{-0.14}^{+0.15}$ | $0.46_{-0.18}^{+0.50}$ | $0.43_{-0.17}^{+0.47}$ | $0.36_{-0.14}^{+0.39}$ | TF0.5 |  $\cdots$
2MASS J16053153-1945435 | ScoPMS 28 | $0.11_{-0.23}^{+0.27}$ | $0.32_{-0.07}^{+0.62}$ | $0.29_{-0.07}^{+0.58}$ | $0.32_{-0.07}^{+0.64}$ | TF0.5 |  $\cdots$
2MASS J16051687-1948374 | ScoPMS 28 | $-0.12_{-0.28}^{+0.26}$ | $0.36_{-0.22}^{+0.77}$ | $0.32_{-0.19}^{+0.68}$ | $0.37_{-0.23}^{+0.80}$ | TF0.5 |  $\cdots$
ASAS J160811-1904.9 | HD 144925 | $-0.35_{-0.25}^{+0.20}$ | $0.37_{-0.10}^{+0.27}$ | $0.29_{-0.08}^{+0.21}$ | $0.24_{-0.07}^{+0.17}$ | TF0.5 |  $\cdots$
b Sco | CD-25 11133 | $0.23_{-1.39}^{+1.25}$ | $1.10_{-0.81}^{+3.15}$ | $0.42_{-0.31}^{+1.20}$ | $0.80_{-0.58}^{+2.27}$ | T0.5 |  $\cdots$
b Sco | HD 141618 | $-0.01_{-1.72}^{+1.73}$ | $0.95_{-0.51}^{+2.80}$ | $0.36_{-0.19}^{+1.07}$ | $0.69_{-0.37}^{+2.05}$ | T0.5 |  $\cdots$
CD-23 12602 | UCAC4 334-078461 | $-0.16_{-0.64}^{+0.56}$ | $0.60_{-0.14}^{+0.31}$ | $0.44_{-0.10}^{+0.23}$ | $0.54_{-0.12}^{+0.27}$ | T0.5 | T
HD 142705 | UCAC4 336-078484 | $-0.01_{-0.64}^{+0.58}$ | $0.52_{-0.09}^{+0.17}$ | $0.34_{-0.06}^{+0.11}$ | $0.51_{-0.08}^{+0.17}$ | T0.5 | T
HD 146366 | 2MASS J16172297-2121119 | $-0.03_{-0.05}^{+0.05}$ | $0.66_{-0.08}^{+0.10}$ | $0.48_{-0.06}^{+0.07}$ | $0.68_{-0.08}^{+0.10}$ | T0.5 |  $\cdots$
$\omega$ Sco | 2MASS J16053815-2039469 | $-0.29_{-0.74}^{+0.54}$ | $1.21_{-0.51}^{+1.38}$ | $0.44_{-0.18}^{+0.50}$ | $1.13_{-0.48}^{+1.29}$ | T0.5 |  $\cdots$
$\nu$ Sco A | UCAC4 354-076591 | $0.12_{-0.56}^{+0.54}$ | $1.15_{-0.66}^{+0.78}$ | $0.49_{-0.28}^{+0.34}$ | $1.07_{-0.61}^{+0.73}$ | T0.5 |  $\cdots$
HD 144925 | 2MASS J16064794-1841437 | $-1.32_{-0.40}^{+0.26}$ | $0.73_{-0.24}^{+0.28}$ | $0.47_{-0.15}^{+0.18}$ | $0.68_{-0.22}^{+0.27}$ | T0.5 |  $\cdots$
HD 144586 | 2MASS J16083138-1802414 | $-1.01_{-0.54}^{+0.34}$ | $0.73_{-0.16}^{+0.20}$ | $0.45_{-0.10}^{+0.12}$ | $0.68_{-0.15}^{+0.19}$ | T0.5 | T
Gaia DR2 6045624866694553856 | HD 147648 | $-0.29_{-0.03}^{+0.03}$ | $0.80_{-0.12}^{+0.21}$ | $0.67_{-0.10}^{+0.18}$ | $0.45_{-0.07}^{+0.12}$ | F0.5 |  $\cdots$
EPIC 203390805 | HD 147648 | $-0.32_{-0.04}^{+0.04}$ | $0.60_{-0.10}^{+0.15}$ | $0.56_{-0.09}^{+0.14}$ | $0.34_{-0.05}^{+0.08}$ | F0.5 |  $\cdots$
ROXs 12 | HD 147701 | $-0.29_{-0.31}^{+0.18}$ | $0.92_{-0.54}^{+1.03}$ | $0.86_{-0.51}^{+0.96}$ | $0.46_{-0.27}^{+0.52}$ | F0.5 |  $\cdots$
EPIC 204184435 | HD 145333 | $-0.20_{-0.19}^{+0.16}$ | $0.65_{-0.29}^{+0.35}$ | $0.51_{-0.23}^{+0.27}$ | $0.45_{-0.20}^{+0.24}$ | F0.5 |  $\cdots$
$[$PGZ2001$]$ J161031.9-191305 | HD 144925 | $0.25_{-0.35}^{+0.48}$ | $0.75_{-0.33}^{+0.91}$ | $0.65_{-0.29}^{+0.80}$ | $0.48_{-0.21}^{+0.59}$ | F0.5 |  $\cdots$
TYC 6804-148-1 | HD 149598 | $-0.18_{-0.30}^{+0.30}$ | $0.95_{-0.52}^{+1.15}$ | $0.69_{-0.38}^{+0.83}$ | $0.70_{-0.38}^{+0.84}$ | TF |  $\cdots$
2MASS J16332693-2616274 | DoAr 61 | $0.43_{-1.88}^{+1.26}$ | $0.85_{-0.14}^{+0.23}$ | $0.75_{-0.13}^{+0.20}$ | $0.69_{-0.12}^{+0.19}$ | TF |  $\cdots$
HD 147255 | 2MASS J16161720-2609101 | $-0.62_{-0.09}^{+0.08}$ | $1.06_{-0.20}^{+0.22}$ | $0.77_{-0.15}^{+0.16}$ | $0.86_{-0.17}^{+0.18}$ | TF |  $\cdots$
2MASS J16161720-2609101 | HD 146331 | $0.15_{-0.74}^{+0.87}$ | $1.03_{-0.63}^{+1.25}$ | $0.83_{-0.51}^{+1.01}$ | $0.63_{-0.39}^{+0.77}$ | TF |  $\cdots$
2MASS J16212918-2405190 | HD 147196 | $-0.02_{-0.17}^{+0.18}$ | $1.06_{-0.13}^{+0.57}$ | $0.90_{-0.11}^{+0.48}$ | $0.57_{-0.07}^{+0.31}$ | TF |  $\cdots$
Gaia DR2 6050024970485018624 | Gaia DR2 6050047785351251712 | $-0.05_{-1.13}^{+1.37}$ | $0.99_{-0.36}^{+0.52}$ | $0.84_{-0.30}^{+0.44}$ | $0.84_{-0.30}^{+0.44}$ | TF |  $\cdots$
Gaia DR2 6050047785351251712 | CD-23 12840A | $-0.01_{-0.29}^{+0.33}$ | $1.04_{-0.50}^{+1.15}$ | $0.88_{-0.43}^{+0.98}$ | $0.77_{-0.37}^{+0.86}$ | TF |  $\cdots$
RX J1620.7-2348 | HD 147196 | $-0.27_{-0.94}^{+1.55}$ | $0.95_{-0.67}^{+1.51}$ | $0.76_{-0.54}^{+1.21}$ | $0.51_{-0.36}^{+0.81}$ | TF |  $\cdots$
2MASS J16151239-2318453 | 2MASS J16132929-2311075 | $-0.29_{-0.57}^{+0.58}$ | $1.05_{-0.05}^{+0.44}$ | $0.89_{-0.05}^{+0.38}$ | $0.81_{-0.04}^{+0.34}$ | TF |  $\cdots$
HD 142933 | UCAC4 334-078461 | $-2.16_{-0.71}^{+0.48}$ | $0.99_{-0.42}^{+0.80}$ | $0.72_{-0.31}^{+0.58}$ | $0.89_{-0.38}^{+0.72}$ | TF |  $\cdots$
RX J1602.8-2401B | EPIC 204063024 | $-0.11_{-0.58}^{+0.73}$ | $0.70_{-0.13}^{+0.13}$ | $0.56_{-0.11}^{+0.11}$ | $0.77_{-0.15}^{+0.15}$ | TF |  $\cdots$
CD-25 11133 | HD 141618 | $0.39_{-1.17}^{+0.94}$ | $0.97_{-0.71}^{+1.91}$ | $0.70_{-0.52}^{+1.38}$ | $0.71_{-0.52}^{+1.40}$ | TF |  $\cdots$
UCAC4 334-078461 | HD 142249 | $-2.85_{-1.45}^{+0.77}$ | $0.85_{-0.47}^{+1.34}$ | $0.76_{-0.42}^{+1.20}$ | $0.61_{-0.34}^{+0.97}$ | TF |  $\cdots$
UCAC4 334-078461 | UCAC4 336-078484 | $0.06_{-0.67}^{+0.73}$ | $0.99_{-0.10}^{+0.35}$ | $0.89_{-0.09}^{+0.32}$ | $0.96_{-0.10}^{+0.34}$ | TF |  $\cdots$
HD 142705 | CD-23 12602 | $0.25_{-0.74}^{+1.12}$ | $1.07_{-0.30}^{+0.53}$ | $0.69_{-0.19}^{+0.34}$ | $0.79_{-0.22}^{+0.39}$ | TF | TF
HD 142705 | UCAC4 334-078461 | $-0.03_{-0.86}^{+1.06}$ | $0.87_{-0.41}^{+0.71}$ | $0.56_{-0.26}^{+0.46}$ | $0.78_{-0.37}^{+0.64}$ | TF | T
2MASS J16110479-2333166 | UCAC4 333-083075 | $-0.48_{-1.48}^{+2.27}$ | $0.73_{-0.18}^{+0.72}$ | $0.68_{-0.17}^{+0.67}$ | $0.68_{-0.17}^{+0.67}$ | TF |  $\cdots$
2MASS J16110479-2333166 | HD 145333 | $0.01_{-0.15}^{+0.20}$ | $0.97_{-0.09}^{+0.23}$ | $0.90_{-0.08}^{+0.22}$ | $0.67_{-0.06}^{+0.16}$ | TF |  $\cdots$
Gaia DR2 6242198746659039872 | 2MASS J16120920-2247504 | $0.27_{-1.31}^{+0.76}$ | $0.90_{-0.33}^{+0.87}$ | $0.93_{-0.34}^{+0.90}$ | $0.84_{-0.30}^{+0.81}$ | TF |  $\cdots$
2MASS J16125889-2245202 | HD 145718 | $0.15_{-0.83}^{+0.69}$ | $0.93_{-0.18}^{+0.96}$ | $0.87_{-0.17}^{+0.90}$ | $0.65_{-0.13}^{+0.68}$ | TF |  $\cdots$
2MASS J16120920-2247504 | HD 145718 | $-1.96_{-1.25}^{+0.63}$ | $1.04_{-0.33}^{+0.63}$ | $0.97_{-0.31}^{+0.59}$ | $0.73_{-0.23}^{+0.44}$ | TF |  $\cdots$
2MASS J16120920-2247504 | BD-21 4301 | $-0.71_{-0.70}^{+0.32}$ | $0.97_{-0.26}^{+1.13}$ | $0.91_{-0.24}^{+1.06}$ | $0.74_{-0.20}^{+0.86}$ | TF |  $\cdots$
HD 145468 | BD-21 4301 | $-0.21_{-0.22}^{+0.15}$ | $1.04_{-0.16}^{+0.07}$ | $0.70_{-0.11}^{+0.05}$ | $0.79_{-0.12}^{+0.05}$ | TF |  $\cdots$
HD 145718 | BD-21 4301 | $-1.20_{-0.73}^{+0.36}$ | $1.30_{-0.79}^{+1.54}$ | $0.92_{-0.55}^{+1.08}$ | $0.99_{-0.60}^{+1.17}$ | TF |  $\cdots$
HD 145718 | HD 145467 | $-2.31_{-1.03}^{+0.63}$ | $1.16_{-0.60}^{+1.36}$ | $0.81_{-0.42}^{+0.95}$ | $0.80_{-0.42}^{+0.94}$ | TF |  $\cdots$
EPIC 204554162 | HD 146366 | $0.59_{-0.07}^{+0.07}$ | $0.77_{-0.24}^{+0.29}$ | $0.72_{-0.23}^{+0.27}$ | $0.56_{-0.18}^{+0.21}$ | TF |  $\cdots$
HD 146366 | HD 146743 | $0.09_{-0.05}^{+0.05}$ | $1.00_{-0.01}^{+0.03}$ | $0.73_{-0.01}^{+0.02}$ | $0.73_{-0.01}^{+0.02}$ | TF |  $\cdots$
CD-22 11315 | HD 143600 | $-0.36_{-1.11}^{+1.13}$ | $0.82_{-0.25}^{+1.14}$ | $0.62_{-0.19}^{+0.86}$ | $0.51_{-0.16}^{+0.71}$ | TF |  $\cdots$
2MASS J16022461-2200248 | EPIC 204548337 | $-0.07_{-0.31}^{+0.31}$ | $0.85_{-0.19}^{+0.48}$ | $0.82_{-0.18}^{+0.47}$ | $0.87_{-0.19}^{+0.50}$ | TF |  $\cdots$
HD 143567 | TYC 6212-1023-1 | $-0.13_{-0.85}^{+0.71}$ | $0.88_{-0.09}^{+0.17}$ | $0.54_{-0.06}^{+0.11}$ | $0.74_{-0.08}^{+0.15}$ | TF |  $\cdots$
2MASS J16085673-2033460 | 1RXS J160801.7-202755 | $-0.06_{-0.36}^{+0.34}$ | $0.81_{-0.41}^{+0.94}$ | $0.67_{-0.34}^{+0.78}$ | $0.76_{-0.39}^{+0.88}$ | TF |  $\cdots$
2MASS J16053815-2039469 | 2MASS J16053153-1945435 | $-0.80_{-0.52}^{+0.27}$ | $0.69_{-0.40}^{+0.63}$ | $0.64_{-0.37}^{+0.59}$ | $0.64_{-0.37}^{+0.59}$ | TF |  $\cdots$
RX J1603.9-2031B | 2MASS J16035767-2031055 | $0.96_{-1.23}^{+1.80}$ | $0.71_{-0.48}^{+2.06}$ | $0.66_{-0.45}^{+1.92}$ | $0.58_{-0.40}^{+1.69}$ | TF |  $\cdots$
HD 145554 | UCAC4 354-076591 | $0.01_{-0.43}^{+0.70}$ | $0.85_{-0.31}^{+0.51}$ | $0.52_{-0.19}^{+0.31}$ | $0.80_{-0.29}^{+0.47}$ | TF | T
HD 145554 | 2MASS J16111742-1918285 | $0.01_{-0.42}^{+0.59}$ | $0.92_{-0.25}^{+0.41}$ | $0.57_{-0.15}^{+0.25}$ | $0.87_{-0.24}^{+0.39}$ | TF | T
HD 145554 | Wa Oph 2 | $-0.07_{-0.27}^{+0.26}$ | $1.20_{-0.19}^{+0.51}$ | $0.74_{-0.12}^{+0.31}$ | $0.92_{-0.15}^{+0.39}$ | TF | T
UCAC4 354-076591 | BD-18 4244 | $-0.34_{-0.33}^{+0.53}$ | $0.98_{-0.71}^{+1.48}$ | $0.91_{-0.66}^{+1.38}$ | $0.82_{-0.60}^{+1.24}$ | TF |  $\cdots$
2MASS J16111742-1918285 |  $[$PGZ2001$]$ J161031.9-191305 | $-0.22_{-0.61}^{+0.86}$ | $0.70_{-0.27}^{+1.20}$ | $0.66_{-0.25}^{+1.14}$ | $0.61_{-0.23}^{+1.05}$ | TF |  $\cdots$
2MASS J16111742-1918285 | Wa Oph 2 | $0.02_{-0.18}^{+0.17}$ | $0.71_{-0.11}^{+0.47}$ | $0.67_{-0.11}^{+0.45}$ | $0.55_{-0.09}^{+0.37}$ | TF |  $\cdots$
2MASS J16111742-1918285 | BD-18 4244 | $-0.34_{-0.37}^{+0.34}$ | $0.91_{-0.27}^{+1.17}$ | $0.86_{-0.26}^{+1.11}$ | $0.77_{-0.23}^{+0.99}$ | TF |  $\cdots$
HD 143956 | 2MASS J16051687-1948374 | $0.12_{-0.43}^{+0.44}$ | $0.89_{-0.14}^{+0.48}$ | $0.55_{-0.08}^{+0.30}$ | $0.78_{-0.12}^{+0.42}$ | TF | T
HD 143956 | ScoPMS 28 | $-0.09_{-0.21}^{+0.15}$ | $0.96_{-0.02}^{+0.04}$ | $0.59_{-0.01}^{+0.02}$ | $0.99_{-0.02}^{+0.04}$ | TF | T
2MASS J16052157-1821412 | 2MASS J16052076-1821367 | $0.16_{-0.49}^{+0.36}$ | $0.76_{-0.54}^{+1.44}$ | $0.61_{-0.43}^{+1.16}$ | $0.85_{-0.60}^{+1.60}$ | TF |  $\cdots$
HD 144586 | ASAS J160811-1904.9 | $-1.64_{-1.29}^{+0.65}$ | $1.19_{-0.64}^{+0.97}$ | $0.73_{-0.39}^{+0.60}$ | $0.94_{-0.50}^{+0.76}$ | TF | T
$\chi$ Oph | 2MASS J16171596-1947041 | $-1.68_{-0.67}^{+0.52}$ | $2.28_{-1.04}^{+2.05}$ | $0.98_{-0.45}^{+0.88}$ | $1.92_{-0.88}^{+1.73}$ | T |  $\cdots$
HD 146001 | 2MASS J16130234-2501460 | $0.47_{-1.27}^{+1.31}$ | $1.65_{-0.29}^{+1.12}$ | $0.94_{-0.16}^{+0.64}$ | $1.84_{-0.32}^{+1.25}$ | T |  $\cdots$
HD 146285 | 2MASS J16173031-2438390 | $-0.02_{-0.10}^{+0.09}$ | $1.14_{-0.10}^{+0.05}$ | $0.65_{-0.06}^{+0.03}$ | $1.07_{-0.09}^{+0.05}$ | T |  $\cdots$
HD 147196 | 2MASS J16212844-2312110 | $0.05_{-0.20}^{+0.24}$ | $1.24_{-0.07}^{+0.19}$ | $0.66_{-0.04}^{+0.10}$ | $1.08_{-0.06}^{+0.17}$ | T |  $\cdots$
HD 146706 | EPIC 204184435 | $1.59_{-0.60}^{+0.90}$ | $1.31_{-0.69}^{+3.00}$ | $0.81_{-0.42}^{+1.85}$ | $1.03_{-0.54}^{+2.36}$ | T |  $\cdots$
HD 146367 | 2MASS J16151239-2318453 | $-0.97_{-0.61}^{+0.28}$ | $1.27_{-0.18}^{+0.32}$ | $0.89_{-0.13}^{+0.22}$ | $1.07_{-0.15}^{+0.27}$ | T |  $\cdots$
HD 143472 | 2MASS J16001330-2418106 | $-0.99_{-0.82}^{+0.43}$ | $1.26_{-0.57}^{+3.67}$ | $0.85_{-0.38}^{+2.46}$ | $1.22_{-0.55}^{+3.54}$ | T |  $\cdots$
HD 142705 | UCAC4 337-077055 | $-0.14_{-1.22}^{+0.67}$ | $1.40_{-0.40}^{+0.44}$ | $0.90_{-0.26}^{+0.29}$ | $1.56_{-0.45}^{+0.49}$ | T | T
UCAC4 336-078484 | UCAC4 337-077055 | $-0.02_{-0.70}^{+0.45}$ | $0.93_{-0.18}^{+0.27}$ | $0.89_{-0.17}^{+0.26}$ | $1.03_{-0.20}^{+0.30}$ | T |  $\cdots$
HD 142315 | UCAC4 336-078484 | $-0.17_{-0.79}^{+0.46}$ | $1.36_{-0.45}^{+0.50}$ | $0.83_{-0.28}^{+0.30}$ | $1.31_{-0.43}^{+0.48}$ | T | T
HD 144661 | HD 144844 | $0.29_{-0.28}^{+0.30}$ | $1.70_{-0.58}^{+1.47}$ | $0.97_{-0.33}^{+0.84}$ | $1.04_{-0.36}^{+0.90}$ | T |  $\cdots$
HD 144822 | 2MASS J16053815-2039469 | $0.56_{-0.18}^{+0.30}$ | $1.41_{-0.13}^{+0.14}$ | $0.98_{-0.09}^{+0.10}$ | $1.32_{-0.12}^{+0.13}$ | T |  $\cdots$
$\omega$ Sco | 1RXS J160801.7-202755 | $0.16_{-1.08}^{+0.98}$ | $2.32_{-1.62}^{+4.07}$ | $0.84_{-0.58}^{+1.47}$ | $2.17_{-1.51}^{+3.80}$ | T |  $\cdots$
HD 145554 |  $[$PGZ2001$]$ J161031.9-191305 | $0.03_{-0.78}^{+0.71}$ | $1.38_{-0.07}^{+0.41}$ | $0.84_{-0.04}^{+0.25}$ | $1.20_{-0.06}^{+0.36}$ | T | T
HD 145554 | BD-18 4244 | $-0.18_{-0.64}^{+0.48}$ | $1.43_{-0.65}^{+0.97}$ | $0.88_{-0.40}^{+0.59}$ | $1.21_{-0.55}^{+0.82}$ | T | T
HD 145631 | 2MASS J16093909-1955023 | $-0.78_{-0.84}^{+0.37}$ | $1.45_{-0.48}^{+0.40}$ | $0.89_{-0.30}^{+0.25}$ | $1.30_{-0.43}^{+0.36}$ | T | T
HD 145631 | UCAC4 354-076591 | $0.45_{-1.16}^{+0.51}$ | $1.62_{-0.63}^{+1.29}$ | $1.00_{-0.39}^{+0.79}$ | $1.52_{-0.59}^{+1.21}$ | T | T
$\nu$ Sco A | 2MASS J16111742-1918285 | $0.10_{-0.63}^{+0.62}$ | $1.38_{-0.88}^{+1.02}$ | $0.59_{-0.38}^{+0.44}$ | $1.30_{-0.83}^{+0.96}$ | T |  $\cdots$
$\nu$ Sco A |  $[$PGZ2001$]$ J161031.9-191305 | $-0.16_{-0.48}^{+0.42}$ | $1.40_{-0.71}^{+2.24}$ | $0.60_{-0.31}^{+0.96}$ | $1.22_{-0.62}^{+1.96}$ | T |  $\cdots$
$\nu$ Sco A | Wa Oph 2 | $0.06_{-0.39}^{+0.38}$ | $1.71_{-0.94}^{+1.53}$ | $0.74_{-0.41}^{+0.66}$ | $1.32_{-0.73}^{+1.18}$ | T |  $\cdots$
$\nu$ Sco A | BD-18 4244 | $-0.08_{-0.44}^{+0.43}$ | $1.65_{-0.87}^{+1.34}$ | $0.71_{-0.37}^{+0.58}$ | $1.39_{-0.73}^{+1.13}$ | T |  $\cdots$
$\nu$ Sco A | HD 144925 | $0.04_{-0.30}^{+0.28}$ | $2.09_{-0.38}^{+0.29}$ | $0.90_{-0.16}^{+0.12}$ | $1.34_{-0.25}^{+0.18}$ | T |  $\cdots$
HD 144981 | 2MASS J16093909-1955023 | $0.80_{-4.07}^{+3.17}$ | $1.44_{-0.47}^{+1.84}$ | $0.93_{-0.30}^{+1.19}$ | $1.29_{-0.42}^{+1.65}$ | T | T
HD 144981 | ASAS J160811-1904.9 | $-2.02_{-2.80}^{+1.28}$ | $1.48_{-0.66}^{+2.14}$ | $0.95_{-0.42}^{+1.38}$ | $1.16_{-0.52}^{+1.69}$ | T | T
Wa Oph 2 | BD-18 4244 | $0.03_{-0.40}^{+0.28}$ | $1.26_{-0.74}^{+1.11}$ | $0.97_{-0.57}^{+0.85}$ | $1.06_{-0.62}^{+0.93}$ | T |  $\cdots$
HD 143956 | 2MASS J16053815-2039469 | $-0.92_{-0.46}^{+0.25}$ | $1.46_{-0.45}^{+0.28}$ | $0.90_{-0.28}^{+0.17}$ | $1.36_{-0.42}^{+0.26}$ | T | T
HD 144569 | ScoPMS 45 | $-1.37_{-0.55}^{+0.37}$ | $1.46_{-0.64}^{+0.63}$ | $0.91_{-0.40}^{+0.39}$ | $1.20_{-0.53}^{+0.52}$ | T | T
$[$SMH2016$]$ NV6 | HD 147592 | $0.02_{-0.45}^{+0.47}$ | $1.52_{-0.03}^{+0.09}$ | $1.36_{-0.03}^{+0.08}$ | $0.98_{-0.02}^{+0.06}$ | F |  $\cdots$
2MASS J16153587-2529008 | HD 146285 | $0.07_{-0.10}^{+0.11}$ | $1.38_{-0.08}^{+0.12}$ | $1.13_{-0.07}^{+0.10}$ | $0.78_{-0.05}^{+0.07}$ | F |  $\cdots$
ROXs 12 | EPIC 203725441 | $0.05_{-0.24}^{+0.12}$ | $1.34_{-0.45}^{+0.56}$ | $1.25_{-0.42}^{+0.52}$ | $0.98_{-0.33}^{+0.41}$ | F |  $\cdots$
EPIC 203725441 | HD 147701 | $-0.30_{-1.28}^{+1.93}$ | $1.64_{-0.72}^{+1.64}$ | $1.20_{-0.53}^{+1.20}$ | $0.82_{-0.36}^{+0.82}$ | F |  $\cdots$
Gaia DR2 6049726075120913152 | HD 145792A | $-1.29_{-0.96}^{+0.56}$ | $1.44_{-0.37}^{+0.73}$ | $1.22_{-0.31}^{+0.61}$ | $0.76_{-0.19}^{+0.38}$ | F |  $\cdots$
EPIC 203893268 | HD 144844 | $0.55_{-0.32}^{+0.62}$ | $1.31_{-0.67}^{+0.85}$ | $1.22_{-0.62}^{+0.80}$ | $0.80_{-0.41}^{+0.52}$ | F |  $\cdots$
2MASS J16212844-2312110 | HD 146706 | $-0.40_{-0.51}^{+0.28}$ | $1.24_{-0.35}^{+1.17}$ | $1.08_{-0.31}^{+1.03}$ | $0.76_{-0.22}^{+0.72}$ | F |  $\cdots$
EPIC 204063024 | HD 144175 | $-0.19_{-0.68}^{+0.45}$ | $1.01_{-0.35}^{+0.62}$ | $1.12_{-0.39}^{+0.69}$ | $0.62_{-0.21}^{+0.38}$ | F |  $\cdots$
2MASS J16110479-2333166 | EPIC 204184435 | $-0.02_{-0.21}^{+0.23}$ | $1.26_{-0.02}^{+0.13}$ | $1.18_{-0.02}^{+0.12}$ | $0.99_{-0.01}^{+0.10}$ | F |  $\cdots$
EPIC 204184435 | BD-21 4301 | $-1.77_{-1.70}^{+0.70}$ | $1.32_{-0.69}^{+4.24}$ | $1.04_{-0.54}^{+3.34}$ | $1.00_{-0.52}^{+3.22}$ | F |  $\cdots$
Gaia DR2 6242198746659039872 | HD 145718 | $-1.03_{-1.26}^{+0.48}$ | $1.12_{-0.48}^{+0.38}$ | $1.15_{-0.49}^{+0.39}$ | $0.79_{-0.34}^{+0.27}$ | F |  $\cdots$
Gaia DR2 6242198746659039872 | BD-21 4301 | $-0.95_{-1.07}^{+0.56}$ | $1.29_{-0.71}^{+4.94}$ | $1.33_{-0.74}^{+5.09}$ | $0.98_{-0.54}^{+3.75}$ | F |  $\cdots$
2MASS J16022461-2200248 | 2MASS J16004056-2200322 | $0.05_{-0.33}^{+0.32}$ | $1.16_{-0.13}^{+0.38}$ | $1.12_{-0.12}^{+0.36}$ | $0.94_{-0.10}^{+0.30}$ | F |  $\cdots$
EPIC 204548337 | 2MASS J16042165-2130284 | $0.33_{-0.59}^{+0.56}$ | $1.06_{-0.69}^{+1.03}$ | $1.09_{-0.71}^{+1.06}$ | $0.82_{-0.53}^{+0.79}$ | F |  $\cdots$
V1150 Sco | HD 143567 | $-0.09_{-0.54}^{+0.71}$ | $1.46_{-0.09}^{+0.48}$ | $1.50_{-0.10}^{+0.49}$ | $0.89_{-0.06}^{+0.29}$ | F |  $\cdots$
EPIC 204787127 |  $\omega$ Sco | $-0.05_{-1.12}^{+0.90}$ | $2.41_{-1.56}^{+3.81}$ | $2.68_{-1.74}^{+4.24}$ | $0.87_{-0.56}^{+1.37}$ | F |  $\cdots$
2MASS J16111742-1918285 | HD 144925 | $-0.01_{-0.14}^{+0.14}$ | $1.48_{-0.10}^{+0.09}$ | $1.40_{-0.09}^{+0.08}$ | $0.95_{-0.06}^{+0.06}$ | F |  $\cdots$

Note. — The notations in this table are the same as in Table 4. The target and
flyby stars are presented using the Gaia DR2 identifiers. Only Gaia DR2
6237422850406493952 (HD 142315) has been observed by Esposito et al. (2020)
but no disk was detected.

Table 6: Events involving candidates and field stars with $d_{\text{ca}}<1$ pc
and $v_{\mathrm{rel}}<10$ km s-1

Star 1 | Star 2 |  $t_{\text{ca}}$ (Myr) |  $d_{\text{ca}}$ (pc) |  $v_{\mathrm{rel}}$ (km s-1) | Category
---|---|---|---|---|---
Gaia DR2 3463980463840622848 | Gaia DR2 3463980459544094208 | $0.01_{-0.34}^{+0.33}$ | $0.39_{-0.28}^{+0.46}$ | $0.80_{-0.10}^{+0.24}$ | F
Gaia DR2 3481965141177021568 | Gaia DR2 3481965995873045888 | $-0.19_{-0.36}^{+0.53}$ | $0.25_{-0.18}^{+0.59}$ | $4.80_{-3.15}^{+4.83}$ | F
Gaia DR2 4126469681670065408 | Gaia DR2 4126469685970383360 | $-0.53_{-0.31}^{+0.27}$ | $0.42_{-0.20}^{+0.25}$ | $3.96_{-0.82}^{+0.83}$ | C
Gaia DR2 5334680613863027200 | Gaia DR2 5334681571583520512 | $-0.43_{-0.63}^{+0.51}$ | $0.61_{-0.32}^{+0.89}$ | $7.66_{-2.28}^{+2.29}$ | C
Gaia DR2 5341470132513014272 | Gaia DR2 5341856262911551872 | $0.29_{-1.36}^{+0.57}$ | $0.81_{-0.31}^{+0.28}$ | $1.91_{-1.25}^{+1.98}$ | C
Gaia DR2 5343524947923387520 | Gaia DR2 5342085785963213184 | $-1.10_{-0.75}^{+0.37}$ | $0.97_{-0.30}^{+0.30}$ | $5.93_{-2.33}^{+2.33}$ | C
Gaia DR2 5343610331876174336 | Gaia DR2 5343603288120259072 | $-0.18_{-0.26}^{+0.32}$ | $0.79_{-0.22}^{+0.92}$ | $4.39_{-1.90}^{+3.71}$ | C
Gaia DR2 5359955053246566144 | Gaia DR2 5347989239999386240 | $0.41_{-0.12}^{+0.23}$ | $0.76_{-0.04}^{+0.08}$ | $3.08_{-1.15}^{+1.16}$ | F
Gaia DR2 5855799303791243776 | Gaia DR2 5855802430512817152 | $0.08_{-0.05}^{+0.06}$ | $0.31_{-0.01}^{+0.02}$ | $9.04_{-2.64}^{+2.64}$ | C
Gaia DR2 5855799303791243776 | Gaia DR2 5855802808484620288 | $0.09_{-0.05}^{+0.07}$ | $0.21_{-0.02}^{+0.02}$ | $8.27_{-2.60}^{+2.60}$ | C
Gaia DR2 5862028930545927936 | Gaia DR2 5862081981988722048 | $-0.53_{-1.09}^{+1.79}$ | $0.79_{-0.39}^{+1.59}$ | $5.66_{-3.81}^{+5.97}$ | C
Gaia DR2 5907308051965769216 | Gaia DR2 5907311870203075456 | $0.03_{-0.39}^{+0.33}$ | $0.41_{-0.23}^{+0.62}$ | $5.27_{-3.34}^{+5.46}$ | C
Gaia DR2 5964856323071807360 | Gaia DR2 5967459073270375936 | $-2.43_{-0.83}^{+0.53}$ | $0.81_{-0.36}^{+0.50}$ | $5.41_{-1.32}^{+1.33}$ | C
Gaia DR2 5969081333984323584 | Gaia DR2 5969095215321931008 | $0.33_{-0.54}^{+0.69}$ | $0.63_{-0.06}^{+0.21}$ | $3.38_{-1.42}^{+1.45}$ | C
Gaia DR2 5997082081177906048 | Gaia DR2 5997022192152593664 | $1.78_{-0.78}^{+1.65}$ | $0.81_{-0.33}^{+1.59}$ | $3.88_{-2.03}^{+2.13}$ | C
Gaia DR2 5997082081177906048 | Gaia DR2 5997034252420531712 | $0.37_{-0.32}^{+0.38}$ | $0.50_{-0.23}^{+0.47}$ | $8.27_{-2.27}^{+2.31}$ | C
Gaia DR2 5997082081177906048 | Gaia DR2 5997035317572422912 | $0.31_{-0.94}^{+1.09}$ | $0.40_{-0.26}^{+1.14}$ | $5.29_{-2.44}^{+2.51}$ | C
Gaia DR2 5997082081177906048 | Gaia DR2 5997081909379214080 | $-0.05_{-0.24}^{+0.23}$ | $0.30_{-0.22}^{+0.40}$ | $9.87_{-2.57}^{+2.60}$ | F
Gaia DR2 5997082081177906048 | Gaia DR2 5997082046818384768 | $0.38_{-1.03}^{+1.25}$ | $0.30_{-0.22}^{+0.85}$ | $3.15_{-1.92}^{+2.22}$ | C
Gaia DR2 5997082081177906048 | Gaia DR2 5997082390415552768 | $0.34_{-0.48}^{+0.65}$ | $0.23_{-0.10}^{+0.35}$ | $4.35_{-2.27}^{+2.33}$ | C
Gaia DR2 5997082081177906048 | Gaia DR2 5997082768372892160 | $0.35_{-0.59}^{+0.81}$ | $0.15_{-0.07}^{+0.39}$ | $4.92_{-2.91}^{+3.11}$ | C
Gaia DR2 5997082081177906048 | Gaia DR2 5997082871452217344 | $0.72_{-0.60}^{+0.97}$ | $0.28_{-0.14}^{+0.42}$ | $4.14_{-2.16}^{+2.23}$ | C
Gaia DR2 5997082081177906048 | Gaia DR2 5997409254599602688 | $0.59_{-0.46}^{+0.57}$ | $0.78_{-0.12}^{+0.21}$ | $6.89_{-2.04}^{+2.04}$ | C
Gaia DR2 5997082081177906048 | Gaia DR2 5997410491550194816 | $-0.17_{-0.42}^{+0.39}$ | $0.44_{-0.25}^{+0.59}$ | $5.82_{-2.37}^{+2.41}$ | C
Gaia DR2 5997082081177906048 | Gaia DR2 5997457736191421184 | $-0.25_{-0.37}^{+0.33}$ | $0.61_{-0.35}^{+0.61}$ | $7.93_{-2.40}^{+2.42}$ | F
Gaia DR2 5997082081177906048 | Gaia DR2 5997457736191421440 | $0.11_{-0.32}^{+0.33}$ | $0.28_{-0.19}^{+0.39}$ | $8.05_{-1.73}^{+1.74}$ | C
Gaia DR2 5997082081177906048 | Gaia DR2 5997490206145065088 | $1.38_{-0.68}^{+1.11}$ | $0.92_{-0.29}^{+1.41}$ | $3.32_{-1.57}^{+1.87}$ | C
Gaia DR2 5997082115537645696 | Gaia DR2 5997410491550194816 | $-0.07_{-0.29}^{+0.31}$ | $0.99_{-0.64}^{+1.20}$ | $6.81_{-1.89}^{+2.15}$ | C
Gaia DR2 5997082115537645696 | Gaia DR2 5997457736191421440 | $0.13_{-0.28}^{+0.31}$ | $0.88_{-0.60}^{+0.94}$ | $8.89_{-1.52}^{+1.58}$ | C
Gaia DR2 5997809579916196608 | Gaia DR2 5997809579916196480 | $0.05_{-0.30}^{+0.26}$ | $0.83_{-0.60}^{+1.30}$ | $6.68_{-3.06}^{+5.99}$ | C
Gaia DR2 6002639803206476672 | Gaia DR2 6002610666147516544 | $-0.04_{-0.45}^{+0.27}$ | $0.65_{-0.25}^{+0.37}$ | $3.13_{-1.77}^{+2.98}$ | C
Gaia DR2 6007264520906861824 | Gaia DR2 6007264516610101376 | $-0.20_{-0.82}^{+0.76}$ | $0.33_{-0.28}^{+1.30}$ | $7.57_{-5.04}^{+6.68}$ | C
Gaia DR2 6009052258106701312 | Gaia DR2 6009033291530588928 | $-0.49_{-0.45}^{+0.24}$ | $0.52_{-0.22}^{+0.80}$ | $8.30_{-4.19}^{+4.35}$ | C
Gaia DR2 6010721900869155584 | Gaia DR2 6010721900869155328 | $0.07_{-0.23}^{+0.23}$ | $0.16_{-0.11}^{+0.23}$ | $2.52_{-0.96}^{+1.02}$ | F
Gaia DR2 6013074512147971072 | Gaia DR2 6013065853493820416 | $0.10_{-2.28}^{+2.01}$ | $0.68_{-0.34}^{+0.48}$ | $0.71_{-0.36}^{+0.64}$ | C
Gaia DR2 6013398451460692992 | Gaia DR2 6013399830146943104 | $0.20_{-0.42}^{+0.51}$ | $0.45_{-0.16}^{+0.49}$ | $4.93_{-2.06}^{+2.15}$ | C
Gaia DR2 6013398451460692992 | Gaia DR2 6013399894569703040 | $-0.09_{-0.16}^{+0.15}$ | $0.37_{-0.14}^{+0.24}$ | $9.98_{-2.32}^{+2.35}$ | C
Gaia DR2 6021142934659068288 | Gaia DR2 6021156025719468800 | $-0.02_{-0.29}^{+0.32}$ | $0.66_{-0.37}^{+0.80}$ | $5.43_{-2.33}^{+2.80}$ | C
Gaia DR2 6042459132190170496 | Gaia DR2 6042459132197205504 | $0.00_{-0.16}^{+0.16}$ | $0.14_{-0.11}^{+0.37}$ | $8.79_{-5.84}^{+9.34}$ | C
Gaia DR2 6045505569684851456 | Gaia DR2 6045517286362149888 | $0.02_{-0.20}^{+0.17}$ | $0.82_{-0.31}^{+0.66}$ | $8.53_{-4.70}^{+7.06}$ | C
Gaia DR2 6046055943965657856 | Gaia DR2 6049110039371711232 | $0.49_{-0.20}^{+0.20}$ | $0.84_{-0.43}^{+0.87}$ | $7.44_{-1.51}^{+1.69}$ | C
Gaia DR2 6050496459111584384 | Gaia DR2 6051735162041951616 | $-0.51_{-0.23}^{+0.19}$ | $0.88_{-0.34}^{+1.32}$ | $7.78_{-2.59}^{+3.02}$ | C
Gaia DR2 6053025507724358784 | Gaia DR2 5861241504060387328 | $0.13_{-0.79}^{+0.56}$ | $0.31_{-0.02}^{+0.12}$ | $4.15_{-2.88}^{+4.47}$ | C
Gaia DR2 6053025507724358784 | Gaia DR2 5861249509876340096 | $-0.23_{-0.37}^{+0.43}$ | $0.54_{-0.13}^{+0.69}$ | $2.79_{-1.40}^{+2.46}$ | C
Gaia DR2 6053028978057952256 | Gaia DR2 5861209416444946816 | $0.67_{-0.28}^{+0.57}$ | $0.88_{-0.48}^{+0.84}$ | $9.14_{-4.56}^{+4.85}$ | C
Gaia DR2 6053028978057952256 | Gaia DR2 5861249509876340096 | $0.12_{-0.58}^{+0.45}$ | $0.83_{-0.38}^{+0.57}$ | $2.42_{-0.99}^{+1.57}$ | C
Gaia DR2 6053693083047384576 | Gaia DR2 6053774103316625664 | $0.73_{-0.26}^{+0.62}$ | $0.66_{-0.10}^{+0.51}$ | $6.91_{-3.38}^{+3.45}$ | C
Gaia DR2 6054195353727048960 | Gaia DR2 6054163296087784960 | $-1.53_{-0.69}^{+0.38}$ | $0.38_{-0.11}^{+0.16}$ | $4.33_{-1.32}^{+1.32}$ | C
Gaia DR2 6054195353727048960 | Gaia DR2 6058916136904260096 | $1.60_{-0.28}^{+0.37}$ | $0.87_{-0.55}^{+1.08}$ | $7.96_{-1.54}^{+1.62}$ | F
Gaia DR2 6054752668651631872 | Gaia DR2 6054691748830295168 | $-0.42_{-0.19}^{+0.12}$ | $0.72_{-0.06}^{+0.05}$ | $7.14_{-1.97}^{+1.97}$ | C
Gaia DR2 6055724499479516544 | Gaia DR2 6055724499479516800 | $0.00_{-0.16}^{+0.16}$ | $0.02_{-0.01}^{+0.05}$ | $2.20_{-1.52}^{+2.38}$ | F
Gaia DR2 6057088443681190272 | Gaia DR2 6057093112266125696 | $0.60_{-1.93}^{+1.41}$ | $0.67_{-0.48}^{+2.39}$ | $4.45_{-2.87}^{+4.58}$ | C
Gaia DR2 6061991269115435136 | Gaia DR2 6061962578725632640 | $-0.06_{-0.25}^{+0.31}$ | $0.54_{-0.01}^{+0.06}$ | $4.92_{-3.40}^{+5.27}$ | C
Gaia DR2 6070080754075553792 | Gaia DR2 6068230310365894400 | $-1.04_{-0.43}^{+0.27}$ | $0.86_{-0.43}^{+0.43}$ | $6.24_{-1.71}^{+1.75}$ | C
Gaia DR2 6071348491003883136 | Gaia DR2 6071087597518919040 | $-0.30_{-0.51}^{+0.29}$ | $0.95_{-0.06}^{+0.32}$ | $3.46_{-2.17}^{+3.15}$ | C
Gaia DR2 6071348491003883136 | Gaia DR2 6071307396762179200 | $-1.46_{-1.89}^{+0.65}$ | $0.60_{-0.19}^{+0.42}$ | $3.07_{-1.95}^{+2.18}$ | C
Gaia DR2 6073430622416839296 | Gaia DR2 6073227208454695424 | $0.30_{-0.14}^{+0.18}$ | $0.78_{-0.19}^{+0.32}$ | $6.38_{-1.99}^{+2.03}$ | C
Gaia DR2 6074563978391541888 | Gaia DR2 6074563978391541504 | $-0.24_{-0.73}^{+0.74}$ | $0.14_{-0.11}^{+0.45}$ | $1.55_{-0.99}^{+1.47}$ | C
Gaia DR2 6074796250199371648 | Gaia DR2 6074796245904371968 | $0.09_{-0.45}^{+0.34}$ | $0.19_{-0.14}^{+0.49}$ | $5.90_{-4.00}^{+6.31}$ | C
Gaia DR2 6075374902576831616 | Gaia DR2 6061305448711222400 | $1.38_{-0.01}^{+0.01}$ | $0.85_{-0.58}^{+0.79}$ | $8.83_{-0.16}^{+0.16}$ | F
Gaia DR2 6075815841096386816 | Gaia DR2 6075816592695096576 | $-0.21_{-0.21}^{+0.17}$ | $0.31_{-0.19}^{+0.45}$ | $3.65_{-2.04}^{+2.69}$ | C
Gaia DR2 6078777306935807616 | Gaia DR2 6078074169250372736 | $-1.82_{-0.39}^{+0.30}$ | $0.84_{-0.43}^{+0.65}$ | $9.62_{-1.47}^{+1.48}$ | C
Gaia DR2 6080080090775083392 | Gaia DR2 6068961863556280832 | $0.73_{-0.02}^{+0.01}$ | $0.80_{-0.36}^{+0.72}$ | $8.66_{-0.18}^{+0.18}$ | F
Gaia DR2 6080080090775083392 | Gaia DR2 6080072286810323968 | $0.25_{-0.18}^{+0.23}$ | $0.49_{-0.10}^{+0.44}$ | $3.96_{-1.78}^{+2.43}$ | C
Gaia DR2 6084425360726815360 | Gaia DR2 6081530720270030208 | $2.05_{-0.61}^{+1.32}$ | $0.91_{-0.60}^{+1.81}$ | $8.54_{-3.47}^{+3.52}$ | C
Gaia DR2 6086467149517306368 | Gaia DR2 6134516941522596352 | $-0.07_{-0.19}^{+0.20}$ | $0.81_{-0.28}^{+0.34}$ | $4.27_{-0.79}^{+0.81}$ | C
Gaia DR2 6088264477373916032 | Gaia DR2 6088154010814701568 | $0.65_{-0.26}^{+0.52}$ | $0.84_{-0.30}^{+0.30}$ | $7.90_{-3.38}^{+3.43}$ | C
Gaia DR2 6090072894822497280 | Gaia DR2 6090088769021601792 | $1.26_{-0.56}^{+1.51}$ | $0.76_{-0.23}^{+0.97}$ | $8.01_{-4.83}^{+5.16}$ | C
Gaia DR2 6106992866945163904 | Gaia DR2 6107169755177631872 | $0.56_{-1.38}^{+1.04}$ | $0.92_{-0.40}^{+2.39}$ | $5.65_{-3.46}^{+5.68}$ | C
Gaia DR2 6108518340549366656 | Gaia DR2 6108424740322313600 | $0.39_{-1.47}^{+1.29}$ | $0.84_{-0.68}^{+2.35}$ | $2.79_{-1.58}^{+2.42}$ | C
Gaia DR2 6124952431621117056 | Gaia DR2 6124951400828926976 | $-0.50_{-1.45}^{+2.28}$ | $0.49_{-0.27}^{+1.11}$ | $5.48_{-3.79}^{+5.89}$ | C
Gaia DR2 6125358151409872256 | Gaia DR2 6125358151409872512 | $-0.23_{-0.13}^{+0.12}$ | $0.21_{-0.12}^{+0.17}$ | $7.57_{-1.31}^{+1.31}$ | C
Gaia DR2 6130345536147985024 | Gaia DR2 6131956458118782208 | $-2.28_{-0.90}^{+0.53}$ | $0.93_{-0.60}^{+0.89}$ | $8.16_{-2.31}^{+2.35}$ | C
Gaia DR2 6203184676791371520 | Gaia DR2 6203189762032648448 | $-0.51_{-1.08}^{+1.23}$ | $0.90_{-0.61}^{+2.04}$ | $3.46_{-2.05}^{+3.25}$ | C
Gaia DR2 6204835284262018688 | Gaia DR2 6204835795359050752 | $-0.58_{-0.65}^{+0.39}$ | $0.48_{-0.34}^{+0.93}$ | $3.06_{-1.83}^{+2.34}$ | C
Gaia DR2 6237422850406493952 | Gaia DR2 6240333803140170880 | $-0.80_{-0.61}^{+0.34}$ | $0.93_{-0.56}^{+0.98}$ | $6.71_{-3.27}^{+3.70}$ | C

Note. — Star 1 are stars from the Sco-Cen sample and Star 2 are stars from the
candidate and field star population. All identifiers are from Gaia DR2. The
category column uses “C” and “F” to represent ScoCen-candidate encounters and
ScoCen-field encounters.

Table 7: Simulated Potential Flyby Events from USCO Sample

Target Star | Flyby Star | $\mathrm{RV}_{\text{sim}}$ | $t_{\text{ca}}$ | $d_{\text{ca}}$ | $d_{\text{ca}}/R_{\text{H,T}}$ | $d_{\text{ca}}/R_{\text{H,F}}$ | Category | IR Excess?
---|---|---|---|---|---|---|---|---
|  | (km/s) | (Myr) | (pc) |  |  |  |
HD 146285 | 2MASS J16172615-2450592 | $-3.76$ | $-0.27_{-0.11}^{+0.10}$ | $0.30_{-0.09}^{+0.18}$ | $0.17_{-0.05}^{+0.10}$ | $0.34_{-0.10}^{+0.20}$ | TF0.5 |  $\cdots$
HD 145333 | 2MASS J16140211-2301021 | $-1.57$ | $-0.65_{-0.29}^{+0.28}$ | $0.63_{-0.38}^{+0.60}$ | $0.44_{-0.26}^{+0.41}$ | $0.47_{-0.28}^{+0.44}$ | TF0.5 |  $\cdots$
HD 145333 | 2MASS J16140159-2258462 | $-3.08$ | $-0.86_{-0.26}^{+0.23}$ | $0.44_{-0.27}^{+0.42}$ | $0.31_{-0.19}^{+0.29}$ | $0.46_{-0.28}^{+0.44}$ | TF0.5 |  $\cdots$
2MASS J16053815-2039469 |  $[$PGZ2001$]$ J160517.9-202420 | $-7.30$ | $-0.26_{-0.28}^{+0.16}$ | $0.33_{-0.18}^{+0.27}$ | $0.31_{-0.17}^{+0.25}$ | $0.40_{-0.22}^{+0.33}$ | TF0.5 |  $\cdots$
2MASS J16111742-1918285 | K2-33 | $-1.23$ | $-0.60_{-0.49}^{+0.43}$ | $0.36_{-0.19}^{+0.22}$ | $0.34_{-0.18}^{+0.21}$ | $0.43_{-0.23}^{+0.27}$ | TF0.5 |  $\cdots$
2MASS J16051687-1948374 | HD 144273 | $-1.32$ | $-0.58_{-0.78}^{+0.50}$ | $0.33_{-0.12}^{+0.23}$ | $0.29_{-0.11}^{+0.20}$ | $0.20_{-0.08}^{+0.14}$ | TF0.5 |  $\cdots$
UCAC4 355-076729 | 2MASS J16033829-1854076 | $-6.00$ | $-0.24_{-0.34}^{+0.15}$ | $0.40_{-0.20}^{+0.32}$ | $0.41_{-0.21}^{+0.33}$ | $0.44_{-0.23}^{+0.35}$ | TF0.5 |  $\cdots$
HD 144925 |  $[$PGZ2001$]$ J160900.7-190852 | $-2.38$ | $-0.04_{-0.12}^{+0.12}$ | $0.35_{-0.05}^{+0.05}$ | $0.22_{-0.03}^{+0.03}$ | $0.36_{-0.06}^{+0.06}$ | TF0.5 |  $\cdots$
HD 147701 | WSB 32 | $-5.92$ | $-0.33_{-0.23}^{+0.21}$ | $0.72_{-0.50}^{+0.73}$ | $0.36_{-0.25}^{+0.37}$ | $0.68_{-0.46}^{+0.68}$ | T0.5 |  $\cdots$
HD 146285 | REP 38A | $-4.47$ | $-1.18_{-0.15}^{+0.13}$ | $0.68_{-0.33}^{+0.56}$ | $0.39_{-0.19}^{+0.32}$ | $0.82_{-0.40}^{+0.67}$ | T0.5 |  $\cdots$
HD 146285 | 2MASS J16231145-2517357 | $-5.56$ | $-0.90_{-0.13}^{+0.12}$ | $0.81_{-0.25}^{+0.44}$ | $0.46_{-0.14}^{+0.25}$ | $0.98_{-0.31}^{+0.53}$ | T0.5 |  $\cdots$
HD 146285 | 2MASS J16245302-2505561 | $-1.79$ | $-1.20_{-0.16}^{+0.14}$ | $0.51_{-0.28}^{+0.47}$ | $0.29_{-0.16}^{+0.27}$ | $0.53_{-0.29}^{+0.48}$ | T0.5 |  $\cdots$
HD 146285 | BKLT J162326-244822 | $-7.95$ | $-1.00_{-0.12}^{+0.11}$ | $0.62_{-0.27}^{+0.37}$ | $0.35_{-0.15}^{+0.21}$ | $0.89_{-0.38}^{+0.53}$ | T0.5 |  $\cdots$
HD 145792A | 2MASS J16121495-2440292 | $-8.27$ | $-0.38_{-0.40}^{+0.35}$ | $0.79_{-0.19}^{+0.42}$ | $0.42_{-0.10}^{+0.22}$ | $1.19_{-0.29}^{+0.64}$ | T0.5 |  $\cdots$
HD 146706 | EPIC 204206497 | $-4.71$ | $-0.70_{-0.77}^{+0.52}$ | $0.76_{-0.22}^{+0.44}$ | $0.47_{-0.13}^{+0.27}$ | $0.87_{-0.25}^{+0.50}$ | T0.5 |  $\cdots$
HD 146706 | 2MASS J16171698-2259541 | $-7.34$ | $-0.22_{-0.34}^{+0.31}$ | $0.75_{-0.28}^{+0.41}$ | $0.46_{-0.17}^{+0.25}$ | $1.08_{-0.40}^{+0.59}$ | T0.5 |  $\cdots$
HD 146367 | 2MASS J16151519-2252537 | $-5.27$ | $-0.58_{-0.39}^{+0.21}$ | $0.50_{-0.23}^{+0.38}$ | $0.35_{-0.16}^{+0.27}$ | $0.87_{-0.40}^{+0.67}$ | T0.5 |  $\cdots$
2MASS J16261964-2137207 | 2MASS J16240942-2134075 | $-3.58$ | $-1.10_{-0.92}^{+0.51}$ | $0.56_{-0.27}^{+0.41}$ | $0.59_{-0.28}^{+0.43}$ | $0.77_{-0.36}^{+0.56}$ | T0.5 |  $\cdots$
2MASS J16261964-2137207 | 2MASS J16220658-2127089 | $-2.24$ | $-1.28_{-1.09}^{+0.76}$ | $1.37_{-0.70}^{+0.86}$ | $1.44_{-0.73}^{+0.90}$ | $1.87_{-0.95}^{+1.17}$ | T0.5 |  $\cdots$
EPIC 204637674 | 2MASS J16220658-2127089 | $-7.00$ | $-0.58_{-0.72}^{+0.45}$ | $0.58_{-0.28}^{+0.37}$ | $0.49_{-0.24}^{+0.31}$ | $0.80_{-0.38}^{+0.51}$ | T0.5 |  $\cdots$
UCAC4 334-078461 | EPIC 204190866 | $-4.53$ | $-0.59_{-0.90}^{+0.65}$ | $0.50_{-0.19}^{+0.27}$ | $0.45_{-0.17}^{+0.25}$ | $0.64_{-0.24}^{+0.35}$ | T0.5 |  $\cdots$
HD 144587 | 2MASS J16084894-2400045 | $-3.14$ | $-0.21_{-0.62}^{+0.53}$ | $0.70_{-0.26}^{+0.36}$ | $0.49_{-0.18}^{+0.26}$ | $0.98_{-0.36}^{+0.51}$ | T0.5 | T
HD 145333 | 2MASS J16181445-2319251 | $-8.99$ | $-1.25_{-0.19}^{+0.17}$ | $0.61_{-0.32}^{+0.48}$ | $0.42_{-0.22}^{+0.33}$ | $0.96_{-0.51}^{+0.76}$ | T0.5 |  $\cdots$
HD 145333 | 2MASS J16183027-2245451 | $-7.45$ | $-1.45_{-0.22}^{+0.20}$ | $0.64_{-0.36}^{+0.60}$ | $0.45_{-0.25}^{+0.42}$ | $1.02_{-0.57}^{+0.95}$ | T0.5 |  $\cdots$
HD 145468 | 2MASS J16114530-2254329 | $-6.17$ | $-0.28_{-0.59}^{+0.51}$ | $0.59_{-0.30}^{+0.82}$ | $0.40_{-0.20}^{+0.56}$ | $1.05_{-0.54}^{+1.47}$ | T0.5 |  $\cdots$
2MASS J16042165-2130284 | 2MASS J16071199-2123472 | $-1.13$ | $-0.82_{-0.45}^{+0.37}$ | $0.58_{-0.34}^{+0.55}$ | $0.45_{-0.26}^{+0.42}$ | $0.83_{-0.48}^{+0.78}$ | T0.5 |  $\cdots$
HD 146743 | 2MASS J16174810-2140322 | $-8.63$ | $-0.77_{-0.85}^{+0.68}$ | $0.43_{-0.14}^{+0.29}$ | $0.31_{-0.10}^{+0.21}$ | $0.61_{-0.20}^{+0.42}$ | T0.5 |  $\cdots$
HD 145631 | EPIC 205019241 | $-1.34$ | $-0.49_{-0.53}^{+0.34}$ | $0.79_{-0.31}^{+0.11}$ | $0.49_{-0.19}^{+0.07}$ | $1.30_{-0.51}^{+0.18}$ | T0.5 | T
HD 145631 |  $[$PBB2002$]$ USco J161052.4-193734 | $-1.47$ | $-0.50_{-0.55}^{+0.36}$ | $0.75_{-0.37}^{+0.34}$ | $0.46_{-0.23}^{+0.21}$ | $0.91_{-0.45}^{+0.42}$ | T0.5 | T
HD 144925 | 2MASS J16033777-1845083 | $-8.11$ | $-1.43_{-0.36}^{+0.27}$ | $0.67_{-0.33}^{+0.52}$ | $0.43_{-0.21}^{+0.33}$ | $0.75_{-0.37}^{+0.58}$ | T0.5 |  $\cdots$
HD 144925 | 2MASS J16044966-1850465 | $-4.33$ | $-0.81_{-0.24}^{+0.21}$ | $0.55_{-0.32}^{+0.50}$ | $0.36_{-0.20}^{+0.32}$ | $0.91_{-0.53}^{+0.82}$ | T0.5 |  $\cdots$
HD 144925 | 2MASS J16040975-1833367 | $-1.27$ | $-1.10_{-0.20}^{+0.17}$ | $0.59_{-0.29}^{+0.37}$ | $0.38_{-0.19}^{+0.24}$ | $0.80_{-0.40}^{+0.51}$ | T0.5 |  $\cdots$
UCAC2 21567782 | Gaia DR2 6048725691338492800 | $-8.58$ | $-0.82_{-0.49}^{+0.47}$ | $1.15_{-0.66}^{+1.08}$ | $0.91_{-0.52}^{+0.85}$ | $1.21_{-0.70}^{+1.13}$ | F0.5 |  $\cdots$
HD 147196 | 2MASS J16223000-2411228 | $-4.11$ | $-0.15_{-0.17}^{+0.20}$ | $1.28_{-0.43}^{+0.78}$ | $0.68_{-0.23}^{+0.42}$ | $1.34_{-0.45}^{+0.82}$ | F0.5 |  $\cdots$
HD 146069 | 2MASS J16140159-2258462 | $-5.04$ | $-1.65_{-1.63}^{+0.74}$ | $1.18_{-0.63}^{+0.76}$ | $0.82_{-0.44}^{+0.53}$ | $1.23_{-0.66}^{+0.79}$ | F0.5 | T
EPIC 204637674 | 2MASS J16220587-2121556 | $-6.77$ | $-0.15_{-0.50}^{+0.35}$ | $0.92_{-0.18}^{+0.25}$ | $0.77_{-0.15}^{+0.21}$ | $0.96_{-0.19}^{+0.26}$ | F0.5 |  $\cdots$
HD 145333 | 2MASS J16223000-2411228 | $-4.53$ | $-1.76_{-0.26}^{+0.23}$ | $1.37_{-0.41}^{+0.70}$ | $0.95_{-0.28}^{+0.49}$ | $1.44_{-0.43}^{+0.73}$ | F0.5 |  $\cdots$
BD-21 4301 | 2MASS J16140159-2258462 | $-3.10$ | $-1.16_{-0.88}^{+0.38}$ | $1.33_{-0.28}^{+0.34}$ | $1.01_{-0.21}^{+0.26}$ | $1.40_{-0.29}^{+0.36}$ | F0.5 |  $\cdots$
2MASS J16085673-2033460 | 2MASS J16050978-1942146 | $-2.29$ | $-1.70_{-0.61}^{+0.44}$ | $1.21_{-0.38}^{+0.40}$ | $1.00_{-0.31}^{+0.33}$ | $1.32_{-0.42}^{+0.43}$ | F0.5 |  $\cdots$
2MASS J16172297-2121119 | 2MASS J16132054-2229159 | $-1.39$ | $-2.00_{-0.68}^{+0.48}$ | $1.26_{-0.46}^{+0.80}$ | $1.30_{-0.48}^{+0.82}$ | $1.38_{-0.50}^{+0.87}$ | F0.5 |  $\cdots$
ScoPMS 45 |  $[$T64$]$ 6 | $-6.83$ | $-1.35_{-0.90}^{+0.90}$ | $1.15_{-0.43}^{+0.54}$ | $0.94_{-0.36}^{+0.45}$ | $1.20_{-0.46}^{+0.57}$ | F0.5 |  $\cdots$

Note. — The notations in this table are the same as in Table 4.
$\mathrm{RV}_{\text{sim}}$ represents the RV value of the flyby star, which do
not have RV measurement, that outputs the minimum $d_{\text{ca}}$.