# Three new late-type stellar companions to very dusty WISE debris disks
identified with VLT/SPHERE imaging

Elisabeth C. Matthews Observatoire de l’Université de Genève, Chemin Pegasi
51, 1290 Versoix, Switzerland  Department of Physics, and Kavli Institute for
Astrophysics and Space Research, Massachusetts Institute of Technology,
Cambridge, MA02139, USA  Sasha Hinkley  University of Exeter, School of
Physics & Astronomy, Stocker Road, Exeter, EX4 4QL, UK  Karl Stapelfeldt  Jet
Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove
drive, Pasadena CA 91109, USA  Arthur Vigan Aix Marseille Univ, CNRS, CNES,
LAM, Marseille, France  Dimitri Mawet  Cahill Center for Astronomy and
Astrophysics, California Institute of Technology, 1200 East California
Boulevard, MC 249-17,Pasadena, CA 91125, USA  Ian J. M. Crossfield  The
University of Kansas, Department of Physics and Astronomy, Malott Room 1082,
1251 Wescoe Hall Drive, Lawrence, KS, 66045,USA  Trevor J. David Center for
Computational Astrophysics, Flatiron Institute, New York, NY 10010, USA  Eric
Mamajek  Jet Propulsion Laboratory, California Institute of Technology, 4800
Oak Grove drive, Pasadena CA 91109, USA  Tiffany Meshkat IPAC, California
Institute of Technology, M/C 100-22, 1200 East California Boulevard, Pasadena,
CA 91125, USA  Farisa Morales Jet Propulsion Laboratory, California Institute
of Technology, 4800 Oak Grove drive, Pasadena CA 91109, USA  Deborah Padgett
Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove
drive, Pasadena CA 91109, USA

(Received …; Revised …; Accepted …)

###### Abstract

Debris disk stars are good targets for high contrast imaging searches for
planetary systems, since debris disks have been shown to have a tentative
correlation with giant planets. We selected 20 stars identified as debris disk
hosts by the WSIE mission, with particularly high levels of warm dust. We
observed these with the VLT/SPHERE high contrast imaging instrument with the
goal of finding planets and imaging the disks in scattered light. Our survey
reaches a median 5$\sigma$ sensitivity of 10.4 $\rm{M_{J}}$ at 25au and 5.9
$\rm{M_{J}}$ at 100au. We identified three new stellar companions (HD18378B,
HD19257B and HD133778B): two are mid-M type stars and one is late-K or early-M
star. Three additional stars have very widely separated stellar companions
(all at $>$2000au) identified in the Gaia catalog. The stars hosting the three
SPHERE-identified companions are all older ($\gtrsim$700Myr), with one having
recently left the main sequence and one a giant star. We infer that the high
volumes of dust observed around these stars might have been caused by a recent
collision between the planets and planetesimal belts in the system, although
for the most evolved star, mass loss could also be responsible for the
infrared excess. Future mid-IR spectroscopy or polarimetric imaging may allow
the positions and spatial extent of these dust belts to be constrained,
thereby providing evidence as to the true cause of the elevated levels of dust
around these old systems. None of the disks in this survey are resolved in
scattered light.

††facilities: VLT:Melipal,WISE,Gaia††software: astropy (Astropy Collaboration
et al., 2013), isochrones (Morton, 2015), matplotlib (Hunter, 2007), numpy
(van der Walt et al., 2011), pandas (Wes McKinney, 2010)

##  1 Introduction

Many nearby stars older than a few Myr host debris disks, the second-
generation belts of dust produced by collisional grinding of planetesimals in
the system (for reviews see Wyatt, 2008; Matthews et al., 2014). It has been
theorized that these disks are correlated with the presence of giant, wide-
separation planets. Small dust grains are rapidly removed by stellar winds,
and so the very existence of a debris belt implies that there are also larger
planetesimals in the system, that are experiencing ongoing collisions to
replenish the dust. Not only are these planetesimals the building blocks of
planets, but the presence of one or more substellar companions in such a
system would perturb the orbits of these planetesimals, enhancing the rate of
collisions and thus increasing the rate of dust production. To this end, a
link between debris disks and giant planets on long orbits has long been
theorized (e.g. Mustill & Wyatt, 2009) and observationally sought after (e.g.
Wahhaj et al., 2013; Rameau et al., 2013; Janson et al., 2013; Meshkat et al.,
2017).

Debris disks are typically inferred based on excess infrared emission above
the stellar photosphere of the target in question, with the temperature and
fractional luminosity of this infrared emission indicating the radius and
volume of the dust, respectively, although there are degeneracies between dust
radius and composition (e.g. Pawellek & Krivov, 2015). Several missions have
searched for debris disk stars based on their IR excess: in particular, the
all-sky WISE survey (Wright et al., 2010) surveyed the sky in four near- and
mid-IR filters. This has led to many new targets with high volumes of
circumstellar dust being identified (e.g. Padgett et al., 2011; Cotten & Song,
2016), which are well suited to direct imaging searches.

The relationship between debris disks and directly imageable planetary
companions is hard to quantify observationally, due to the inherent difficulty
in detecting these planetary companions with current instruments. Nonetheless,
Meshkat et al. (2017) demonstrated a tentative correlation between the two
populations: a statistically different occurrence rate was observed for
planets in debris disk systems versus those in a control sample, at the 88%
confidence level, with $6.3^{+3.5}_{-2.6}\%$ of debris disk systems hosting
wide separation giant planets, compared to just $0.7^{+1.1}_{-0.5}\%$ of those
without debris disks. To strengthen this relation requires additional deep
observations of nearby dusty systems, so as to increase the available sample
size of highly dusty systems with constraints on their giant planet
populations. Further, the theoretical and observational suggestions that
planets are more common in dusty systems makes these dusty systems an
excellent place to search for giant planets with direct imaging, since the
likelihood of detections is elevated by the higher occurrence rate.

Warm debris disks are an indicator of youth. These disks have temperatures of
a few tens to a few hundreds of kelvin, and can be detected via a 12$\mu$m or
22$\mu$m infrared excess. Wyatt et al. (2007b) predicted that disk luminosity
should fall as $t^{-1}$, and there is strong observational evidence that
debris disk occurrence rates decrease with age, a result that holds across
spectral type (e.g. Rieke et al., 2005; Su et al., 2006; Carpenter et al.,
2009; Plavchan et al., 2009; Thureau et al., 2014; Theissen & West, 2014).
Kennedy & Wyatt (2013) find that only 1 in 10,000 old stars ($>$Gyr) have a
12$\mu$m excess, whereas 1 in 100 young stars ($<$120Myr) possess such an
infrared excess. A survey targeting debris disk stars is therefore biased
towards young targets, even if no explicit age selection is performed. Young
planets are bright, due to the re-radiation of the energy liberated during
formation, and so are easier to detect than old planets at the same mass.
Debris disk hosts are prime targets for direct imaging: the occurrence rate of
giant, wide-separation planets in these systems is likely elevated, and these
targets are typically young and therefore host bright planets.

These targets are also potentially amenable to direct detection of the debris
disks in scattered light. In these cases, many opportunities to directly
characterize the disk are available: spectroscopic and photometric
measurements may reveal the composition of the disk, while the fractional
polarization and scattering phase function allow the grain structure and
porosity to be constrained (e.g Stark et al., 2014; Schneider et al., 2014;
Perrin et al., 2015; Milli et al., 2017). Dynamical modeling can also allow
additional constraints on the likely positions and masses of planets in the
system (e.g. Mouillet et al., 1997; Wyatt, 2005; Chiang et al., 2009; Nesvold
& Kuchner, 2015; Lee & Chiang, 2016; Shannon et al., 2016).

Dust is also occasionally detected around older stars, sometimes in remarkably
high quantities. Theissen & West (2017) identified extreme mid-infrared
excesses ($\textrm{L}_{\textrm{IR}}/\textrm{L}_{\star}$$>$0.01) around a
number of M-dwarfs, with ages $>$1Gyr. Typical dust dispersal timescales are
tens to hundreds of Myr, suggesting that this excess is not the remnants of
primordial dust but instead likely the result of dramatic collisions later in
the lifetime of the planetary system. Weinberger et al. (2011) studied in
detail the late-F spectroscopic binary BD+20 307, an old ($\gtrsim$1Gyr) star
that hosts an unusually large quantity of dust. The system lacks cold dust,
implying that the dust cannot be caused in a collisional cascade (Wyatt et
al., 2007a; Heng & Tremaine, 2010) or in a process akin to the late heavy
bombardment (Gomes et al., 2005; Strom et al., 2005). Instead, Weinberger et
al. (2011) infer that the dust was likely produced in a cataclysmic collision
when a terrestrial planet was destabilized from its orbit. They calculate that
such collisions are relatively common, with AFG-type stars hosting planetary
systems that undergo at least 0.2 impacts per main-sequence star lifetime.
Similarly the HD69830 system has significant dust quantities at an age of
$>$1Gyr (Beichman et al., 2005), and is orbited by at least three Neptune-
sized planets (Lovis et al., 2006). Perhaps those planets contributed to the
destabilization of a terrestrial planet, causing a collision and thereby an
elevated quantity of dust in the system. HD23514 (Rhee et al., 2008) and
HD169666 (Moór et al., 2009) also host more dust than is reasonable from a
collisional cascade, implying that the dust is instead produced in a
terrestrial planet collision. On the other hand, Bonsor et al. (2013) detect a
disk around the sub-giant star $\kappa$ CrB, a system also known to host at
least one planet (Johnson et al., 2008), and infer the disk likely survived
the entire $\sim$2.5Gyr lifetime of the system. These old systems with debris
disks allow a unique insight into the evolution and the longevity of planetary
systems. In particular, the subset of systems in which the observed dust is
the aftermath of a cataclysmic collision opens up opportunities to study the
dust properties, and thus infer the bulk compositions of the parent bodies
responsible for producing this dust.

In this paper, we present a survey of 20 highly dusty systems identified as
having a 22$\mu$m excess, indicating a corresponding belt of warm debris, with
the WISE mission (Wright et al., 2010). We chose targets with a high mid-IR
excess, that are early type and nearby. We generally avoided targets that had
previously been observed with high contrast imaging platforms, and avoided
observing known binaries. We did not carry out an explicit age selection, but
relied on the correlation between dust and youth to bias the sample towards
young stars. Each target was observed with the VLT/SPHERE high contrast
imager, with the aim of detecting planetary companions and debris disks in
scattered light. We detected late-type stellar companions to three of the
targets in this survey, and determined that several of the targets we studied
were in fact old stars. Indeed, all three stellar companions detected in this
work orbit old primary stars, with one having recently left the main sequence
and one being a giant star. We did not detect any substellar companions, or
resolve any of the debris disks in scattered light.

We describe our target selection process and the dusty nature of these targets
in Section 2. The observations and data processing are described in Sections 3
and 4 respectively. Section 5 describes our age determination, and
astrometric, spectroscopic and photometric analysis of candidate companions,
and in Section 6 we present the three new stellar companions identified with
VLT/SPHERE. Finally, Section 7 discusses these results in context.

##  2 Target Selection

For this work the aim was to select targets with high volumes of circumstellar
dust from the all-sky WISE catalog of sources (Cutri et al., 2012). We
observed targets that Padgett et al. (2011) had identified as debris disk
stars. That work selected targets that have:

  1. 1.

A large W1-W4 excess (i.e. [3.6$\mu$m]-[24$\mu$m] color). Many of the targets
here have been subsequently observed with Herschel and/or Spitzer, allowing
for more detailed characterization of the infrared excess. These additional
measurements are not taken into account in our target selection. However, we
use fitted values of the disk $\textrm{L}_{\textrm{IR}}/\textrm{L}_{\star}$
and T${}_{\textrm{eff}}$ from Cotten & Song (2016), who do take into account
these additional infrared measurements when calculating the disk properties.

  2. 2.

A small offset of less that 0.2” between the Hipparcos and WISE source
positions when taking into account the proper motion of the target, so as to
ensure that the excess is indeed associated with the foreground star.

  3. 3.

Either a Hipparcos-measured distance within 120pc or, if the object was not in
Hipparcos, a large proper motion of $>$30mas/yr as a proxy for proximity of
the target. The distance cutoff was chosen so as to primarily select debris
disks rather than protoplanetary disks. There are several star-forming regions
at distances of $\sim$150pc, which include many protoplanetary disks with
large W1-W4 excesses. A secondary reason for the distance cut-off is to ensure
that targets will be bright and close enough that we are able to reach high
contrasts at small physical separations. The 30mas/yr proper motion
measurement is typical of stars at a distance of $\sim$120pc. This proper
motion cutoff also ensures that background stars and co-moving companions can
also easily be differentiated based on their relative motion when two
observations are collected a year apart. Padgett et al. (2011) also inspect
the observations of each candidate visually, and select only those targets
that are round and not nebulous. Several of the sources from Padgett et al.
(2011) were subsequently also identified as debris disk hosts in Cotten & Song
(2016), and a few were mentioned in Wu et al. (2013) and Patel et al. (2014),
both of which include a more severe distance cut.

We down-selected early-type stars that were visible from Cerro Paranal, where
SPHERE is located, from the list of Padgett et al. (2011) targets. We also
preferentially selected nearby targets. At the time of selection one target
(HD18378) had no parallax measurement, but was nonetheless selected based on
its large W4 excess and significant proper motion of 37.5mas/yr, suggesting
that the target is nearby. This target was subsequently determined by Gaia to
be at a distance of $520.7\pm 5.4$pc (Gaia Collaboration et al., 2018), which
is significantly further than we had anticipated. This target hosts one of the
stellar companions we detected, and is discussed in more detail in Section
6.3.

It is well known that searches for debris disks can be plagued with false
positives, such as background galaxies and blended late-type stars, when only
color and magnitude cuts are used to select targets (e.g Kuchner et al.,
2016). However, the visual inspection process carried out by Padgett et al.
(2011) and repeated in all the surveys mentioned here has been shown to reduce
the false positive rate to $\sim$7% (Silverberg et al., 2018). Note also that
this contamination rate decreases for brighter excesses due to the brightness
distribution of background galaxies, and the targets in this work should
therefore have an even lower contamination rate. As such we would expect at
most one or two of the targets in this survey to be contaminated, with the
remainder being bona fide debris disk stars.

The final target list is given in Table 1. In this table we also include the
WISE W1-W4 excess, representative of the quantity of dust around each object,
and list the $\textrm{L}_{\textrm{IR}}/\textrm{L}_{\star}$ value from (Cotten
& Song, 2016) for the 17 targets in that work. Cotten & Song (2016) includes
all available infrared photometry to fit the excess, meaning that Herschel and
Spitzer observations are used in addition to WISE measurements when available.
For this survey we observed 20 objects, with 12 of these subsequently re-
observed so as to search for common proper motion of the candidate companions.
All of the targets have infrared luminosity
$\textrm{L}_{\textrm{IR}}/\textrm{L}_{\star}>2\times 10^{-4}$, with the median
distance and stellar type of our sample being 104pc and A1 respectively. No
explicit selection was made based on the target ages, but we note that young
stars tend to have a larger infrared excess than old targets, as described
above, and so we expected that the survey would be biased towards young stars.
However, we found that several of the targets have older ages, with a few even
having left the main sequence. Section 5.1 includes a full discussion of the
age determination, and Section 7 describes our interpretation of these older
debris disk stars.

HD | HIP |  | SpT | SpT Ref | Distance[pc] | W1-W4[mag] |  $\textrm{L}_{\textrm{IR}}/\textrm{L}_{\star}$ [10-4] | Age[Myr] | Age Quality | Age Ref
---|---|---|---|---|---|---|---|---|---|---
18378 | — |  | K2III | HC75 | $520.7\pm 5.4$ | $1.27\pm 0.09$ | — | — | — | —
19257 | 14479 |  | F0V | K16 | $74.4\pm 0.3$ | $2.10\pm 0.08$ | 3.7 |  785$\pm$146 | 1 | This Work
24966 | 18437 |  | A0V | H82 | $114.9\pm 0.5$ | $1.73\pm 0.07$ | 10.9 |  128${}^{+99}_{-77}$ | 2 | N13
94893 | 53484 |  | F0V | H78 | $106.5\pm 0.4$ | $2.21\pm 0.04$ | 9.2 |  735${}^{+719}_{-313}$ | 2 | DH15
98363 | 55188 |  | A2V | HC75 | $138.6\pm 0.7$ | $2.73\pm 0.03$ | 1.8 |  17$\pm$8 | 3 | DZ99, H00, R11
113902 | 64053 |  | B8V | E66 | $100.4\pm 1.2$ | $0.81\pm 0.13$ | 10.5 |  16$\pm$8 | 3 | DZ99, H00, R11
119152 | 66837 |  | F0V | HS88 | $81.2\pm 0.3$ | $0.62\pm 0.08$ | 8.2 |  613${}^{+867}_{-297}$ | 2 | DH15
122802 | — |  | F3/5V | HS88 | $105.6\pm 0.9$ | $1.49\pm 0.05$ | — |  280$\pm$224 | 1 | This Work
123247 | 69011 |  | B9.5V | H78 | $98.8\pm 0.7$ | $2.08\pm 0.05$ | 21 |  16$\pm$8 | 3 | H00, R11
133778 | 73976 |  | F3/5V | HS88 | $163.6\pm 1.9$ | $0.89\pm 0.08$ | 7.2 |  1480${}^{+210}_{-180}$ | 1 | C11
138564 | 76234 |  | B9V | HS88 | $102.6\pm 0.7$ | $1.15\pm 0.08$ | 14.2 |  16$\pm$8 | 3 | H00, R11
138923 | 76395 |  | B8V | H70 | $134.2\pm 2.2$ | $1.32\pm 0.27$ | 6.6 |  16$\pm$8 | 3 | DZ99, H00, R11
151012 | 82069 |  | B9.5V | HS88 | $108.6\pm 0.6$ | $1.10\pm 0.08$ | 10.2 |  16$\pm$8 | 3 | R11
151029 | 81971 |  | A5V | AC00 | $109.5\pm 0.6$ | $0.86\pm 0.08$ | 13.3 |  97$\pm$9 | 1 | This Work
157728 | 85157 |  | A7V | AM95 | $42.8\pm 0.1$ | $1.16\pm 0.18$ | 5.1 |  807${}^{+301}_{-174}$ | 2 | DH15
158815 | — |  | G1V | HS88 | $144.3\pm 1.2$ | $1.39\pm 0.08$ | 2.7 |  1698$\pm$645 | 1 | This Work
176638 | 93542 |  | B9.5Vann | GG87 | $57.5\pm 1.2$ | $1.00\pm 0.21$ | 37.9 |  93${}^{+106}_{-35}$ | 2 | DH15
182681 | 95619 |  | B8.5V | H70 | $71.4\pm 0.7$ | $1.10\pm 0.17$ | 47 |  23$\pm$3 | 3 | G18
192544 | 99892 |  | A0III | HS88 | $81.6\pm 0.3$ | $0.82\pm 0.09$ | — |  48$\pm$20 | 1 | This Work
207204 | 107585 |  | A0V | HS99 | $95.3\pm 1.4$ | $0.90\pm 0.09$ | 10 |  54$\pm$30 | 1 | This Work
Table 1: Target stars observed in this survey. Distances are from Gaia data
release 2 (Gaia Collaboration et al., 2016a, 2018; Lindegren et al., 2018).
W1-W4 ([3.6$\mu$m]-[24$\mu$m]) excesses are calculated from the WISE all-sky
survey catalog (Cutri et al., 2012), and
$\textrm{L}_{\textrm{IR}}/\textrm{L}_{\star}$ values from (Cotten & Song,
2016) are given where available. We determine ages from the literature where
available, and using the isochrones package (Morton, 2015) otherwise (see
Section 5.1). References in the table indicate ages for individual targets
adopted in our analysis or moving group memberships; for Sco-Cen subgroups we
use the ages calculated in Pecaut & Mamajek (2016) and for the
$\beta$-Pictoris moving group we use the the age from Mamajek & Bell (2014).
To indicate the reliability of each age we additionally assign an age quality
flag between 1 and 3, with 3 being the most reliable. The age determinations
and quality flag designations are described in detail in Section 5.1. Spectral
Type References are AC00: Abt & Corbally 2000; AM95: Abt & Morrell 1995; E66:
Evans 1966; GG87: Gray & Garrison 1987; H70: Hube 1970; H78: Houk 1978; H82:
Houk 1982; HC75: Houk & Cowley 1975; HS88: Houk & Smith-Moore 1988; HS99: Houk
& Swift 1999; K16: Kuchner et al. 2016. Age References are DH15: David &
Hillenbrand 2015; DZ99: de Zeeuw et al. 1999; G18: Gagné et al. 2018; H00:
Hoogerwerf 2000; N13: Nielsen et al. 2013; R11: Rizzuto et al. 2011.

##  3 Observations

We observed each target using the SPHERE instrument at the VLT (Beuzit et al.,
2019). We used the IRDIFS configuration, which allows simultaneous imaging
with both the IRDIS and IFS subsystems (see Dohlen et al. 2008, Claudi et al.
2008). The IRDIS subsystem provides imaging over a relatively large field of
view (11”$\times$12.5”), while the IFS instrument provides spectra for a
smaller field of view (1.73”$\times$1.73”, R$\sim$50). IRDIS was used in dual-
band imaging mode (Vigan et al., 2010) with the H23 filter pair ($\lambda$ =
1588.8 nm, $\Delta\lambda$ = 53.1 nm and $\lambda$ = 1667.1 nm,
$\Delta\lambda$ = 55.6 nm), while the IFS was used in the YJ mode, which
covers $\lambda$=0.95-1.35$\mu$m and has 39 distinct wavelength channels
(Zurlo et al., 2014; Mesa et al., 2015). Our data were collected with the
apodized pupil Lyot coronagraph (Carbillet et al., 2011; Guerri et al., 2011)
in its N_ALC_YJH_S configuration optimized for the H band, allowing for an
inner working angle of $\sim$0.15′′.

We observed each target for approximately 2200s in a series of short
individual exposures with duration between 8s and 64s. Exact exposure times
and number of images for each observation are given in Table 2. These images
were collected in an angular differential imaging sequence (ADI, Marois et al.
2006), by using the instrument in pupil-stabilized mode. The multi-wavelength
nature of both subsystems means the images also use the spectral differential
imaging technique (SDI, Racine et al. 1999), and the angular and spectral
variation of speckles are both used in our analysis, as described in Section
4. The average field rotation was 31∘ for IRDIS data and 33∘ for IFS data. We
collected flux calibration frames, where the position of the target is offset
from the coronagraph, and star position calibration frames, where a sinusoidal
deformation to the deformable mirror creates four offset copies of the star
that can together be used to determine the position of the star behind the
coronagraph. These flux and position calibration frames were collected either
immediately before or immediately after the science sequence. We additionally
used daytime calibrations for flat-field correction.

For 12 of the targets we collected a second epoch of data, so that astrometric
monitoring could be used to differentiate between co-moving companions and
stationary background objects. These observation sequences were typically
shorter, with a median total integration time of 1024s, and had
correspondingly smaller field rotations. Exposure times and field rotation for
these follow-up observations are also given in Table 2.

##  4 Data Reduction

Data reduction was performed following Matthews et al. (2018) and Vigan et al.
(2015). We first performed basic calibrations using the ESO data reduction and
handling pipeline (DRH, Pavlov et al. 2008) and a custom IDL pipeline (now
available in a python implementation in the vlt-sphere111see
https://github.com/avigan/SPHERE package, see Vigan 2020). We created master
dark and flat frames, and calibrated the star position behind the coronagraph.
Frames were then normalized by integration time. In the case of IFS data, we
additionally created an IFU flat field, and performed a cross-talk correction
and a wavelength calibration correction as described in Vigan et al. (2015).
Frames were then realigned based on the star center position calibration and
dither position for each frame. Finally, the data were converted from spectra
into (x,y,$\lambda$) cubes using the DRH.

We then carried out a principal component analysis (PCA, see e.g. Soummer et
al., 2012; Amara & Quanz, 2012)) to remove stellar speckle noise, for both the
IRDIS and IFS datacubes. We used a full-frame PCA treatment, where we
simultaneously exploit the angular and spectral variation of the speckle noise
across the sequence of images (Marois et al., 2006; Racine et al., 1999). We
first corrected for the anamorphic distortion of the instrument optics and
applied the epsilon correction detailed in the ESO SPHERE User
Manual222https://www.eso.org/sci/facilities/paranal/instruments/sphere/doc.html
which accounts for a slight mis-synchronization between the telescope rotation
angle and the header values for each file. We then rescaled the images in
proportion to their wavelength, such that the characteristic scale of speckles
(proportional to $\lambda/D$, Racine et al. 1999) was equivalent between
images, and on-sky candidate companion signals were physically displaced
between images at different wavelengths. The nature of the ADI observing
sequence means that stellar speckles are also aligned between timesteps in the
original image orientation, while on-sky candidates move their position
between frames due to the apparent rotation of the sky. We then used a custom
PCA code to find principal components that can be subtracted off: this method
selects for similar features between images, and so allows the majority of
speckle noise to be captured and removed. For this process we considered the
rotational and spectral variation simultaneously. Finally, the images were co-
added across time and optionally also across wavelength. Our final data
products are both a single broadband image with optimal sensitivity to
companions, and a set of narrow-band images where the relative H2 and H3
magnitudes can be measured in the case of IRDIS data, and spectral extraction
can be performed in the case of IFS data.

In this process we aimed to optimize the balance of removing sufficient
starlight without subtracting too much planetary flux. To do this, we
calculated and stored reductions with a range of between 0 and $\sim$1/3 of
the available principal components removed. We searched for candidates in
several reductions with different numbers of components removed, and when
calculating contrast limits took the entire range of reductions into account,
as described in Section 4.1.

|  | IRDIS | IFS
---|---|---|---
Target | UT Date | $N_{\textrm{im}}$ |  T${}_{\textrm{exp}}$[s] |  |  Rot[∘] | $N_{\textrm{im}}$ |  T${}_{\textrm{exp}}$[s] |  |  Rot[∘]
HD 18378 | 2016 Sep 15 | 80 | 32 |  | 15.7 | 40 | 64 |  | 16.5
| 2017 Aug 29 | 32 | 64 |  | 12.6 | 32 | 64 |  | 13.4
HD 19257 | 2015 Aug 26 | 48 | 32 |  | 8.0 | 49 | 32 |  | 8.4
| 2017 Sep 01 | 32 | 16 |  | 2.7 | 34 | 16 |  | 3.0
HD 24966 | 2016 Sep 19 | 70 | 32 |  | 32.6 | 70 | 32 |  | 34.0
HD 94893 | 2015 Apr 11 | 48 | 32 |  | 16.9 | 49 | 32 |  | 17.5
| 2018 Jan 07 | 16 | 16 |  | 3.1 | 17 | 16 |  | 3.4
HD 98363 | 2015 Apr 11 | 48 | 32 |  | 11.5 | 45 | 32 |  | 11.0
| 2018 Mar 24 | 16 | 64 |  | 6.3 | 16 | 64 |  | 6.7
HD 113902 | 2016 Jun 06 | 80 | 32 |  | 19.4 | 80 | 32 |  | 21.0
| 2017 Jun 23 | 64 | 32 |  | 15.9 | 32 | 64 |  | 16.9
HD 119152 | 2016 May 01 | 144 | 16 |  | 48.2 | 72 | 32 |  | 53.8
HD 122802 | 2016 Jul 25 | 288 | 8 |  | 9.1 | 144 | 16 |  | 10.6
| 2018 May 15 | 192 | 8 |  | 26.8 | 180 | 8 |  | 27.3
HD 123247 | 2015 Apr 06 | 48 | 32 |  | 17.5 | 49 | 32 |  | 18.1
| 2018 Mar 22 | 32 | 32 |  | 9.6 | 16 | 64 |  | 10.2
HD 133778 | 2016 May 16 | 320 | 8 |  | 91.3 | 160 | 16 |  | 97.8
| 2017 Jun 22 | 256 | 2 |  | 28.5 | 136 | 4 |  | 30.7
HD 138564 | 2016 May 07 | 144 | 16 |  | 34.0 | 72 | 32 |  | 35.6
| 2018 Mar 17 | 64 | 16 |  | 13.5 | 32 | 32 |  | 14.4
HD 138923 | 2016 May 03 | 144 | 16 |  | 53.0 | 72 | 32 |  | 56.1
HD 151012 | 2016 Jun 04 | 320 | 8 |  | 21.0 | 160 | 16 |  | 25.5
| 2017 Jul 15 | 512 | 2 |  | 14.0 | 396 | 2 |  | 15.5
HD 151029 | 2016 May 17 | 80 | 32 |  | 20.9 | 80 | 32 |  | 22.5
HD 157728 | 2016 May 01 | 160 | 16 |  | 13.3 | 44 | 64 |  | 17.7
HD 158815 | 2016 May 03 | 240 | 8 |  | 20.7 | 263 | 8 |  | 20.0
| 2017 Jun 29 | 256 | 8 |  | 37.3 | 248 | 8 |  | 39.4
HD 176638 | 2016 Jun 04 | 80 | 32 |  | 31.3 | 40 | 64 |  | 32.9
| 2017 May 28 | 128 | 16 |  | 26.0 | 32 | 64 |  | 26.9
HD 182681 | 2016 May 15 | 288 | 8 |  | 80.0 | 144 | 16 |  | 85.0
HD 192544 | 2016 May 16 | 144 | 16 |  | 50.5 | 72 | 32 |  | 53.8
HD 207204 | 2016 Jun 30 | 80 | 32 |  | 26.5 | 81 | 32 |  | 29.0
Table 2: Observation details for each of our VLT/SPHERE sequences. In each
case, the listed exposure time refers to each individual science image.
Initial observations totaled $\sim$35-45 minutes of on-sky integration, and
follow-up observations were often shorter depending on the magnitude and
separation of the candidates identified in the initial observations.

###  4.1 Survey Sensitivity

We used a fake planet injection technique to calculate sensitivity as a
function of radius. Fake planets were first inserted into each cleaned
datacube at a variety of position angles, separations and magnitudes. We
injected several fake planets into each datacube, but ensured a minimum
separation of 100mas between planets so as to avoid cross contamination. For
each set of fake planets, we repeated the injection with five different
position angles. A PCA algorithm was applied to each of these injected
datacubes as described in Section 4, and the signal to noise of each fake
planet in the processed data calculated. We interpolated fake planets at the
same position across magnitude, so as to find the precise magnitude that
corresponds to a 5$\sigma$ detection. This process was repeated several times
with up to 1/3 of the available principal components removed, so as to
determine which reduction is the most sensitive to companions as a function of
radius. Finally, the contrast correction term described in Mawet et al. (2014)
was applied to account for the small number of resolution elements at small
inner working angles.

In this survey we reach typical contrasts of 13.8 mag at $0.5\arcsec$ with
both subsystems, and 15.4 mag in the wide field at a separation of $4\arcsec$
with IRDIS. A plot of the median contrast of our survey is presented in Figure
1: for this plot we only include the deepest observation of each target, which
is the first epoch for the majority of targets since typically follow-up
observations had shorter exposure times. The only target where the second
epoch outperformed the first is HD 98363, for which the first observation was
taken in poor weather conditions. Contrast curves of individual observations
are available on DIVA, the Direct Imaging Virtual Archive (Vigan et al.,
2017), and as machine readable files provided with this article. Each file
includes the measured contrast, and the 1-$\sigma$ upper and lower bounds, for
regularly spaced radial separations between 0.1” and the edge of the field of
view.

Figure 1: Median and best contrasts for initial observations in this survey,
with the performance of the SPHERE/IRDIS subsystem given in orange and that of
SPHERE/IFS in blue. In each case, the contrast shown is the ASDI contrast,
i.e. the contrast curve when both spectral and angular diversity are used to
subtract speckles, and where the images are co-added across wavelength.
Although the IFS slightly outperforms IRDIS in the very near-field, the
instruments perform similarly beyond $\sim 0.5\arcsec$. We reach typical
contrasts of 13.8 magnitudes with both systems at $0.5\arcsec$

SPHERE/IRDIS contrast limits are converted to approximate mass limits using
the COND models (Baraffe et al., 2003) for temperatures below 1700K and the
DUSTY models (Chabrier et al., 2000) otherwise. This represents a somewhat
optimistic scenario where planets formed following a ‘hot-start’ process, with
high initial entropy and effective temperature at formation. For this
conversion we also use the age determined for each target, as discussed below.
Since no age could be confirmed for HD18378, we did not calculate a mass
sensitivity for this target. Contrasts and mass limits for each individual
target except HD18378 are shown in Figure 2. For HD19257 the contrast is
reduced due to the presence of a very bright companion close to the target
star (see Section 6.2). We are least sensitive to low mass companions for
HD158815 and HD133778, which are the oldest two targets in the survey, and for
HD19257, which is also relatively old and has a bright companion reducing the
contrast sensitivity. The 7 targets with the best mass sensitivity are all
members of nearby moving groups, and are therefore all very young targets. The
median survey sensitivity is 10.4 $\rm{M_{J}}$ at 25au and 5.9 $\rm{M_{J}}$ at
100au.

Figure 2: Left: Contrasts for each individual observation. The contrast for
HD19257 is markedly less than for the other observations, due to the presence
of a bright companion 1.3” from the star. Right: Mass sensitivity of the
survey, as a function of projected separation. For this conversion we use the
COND models (Baraffe et al., 2003) for objects below 1700K and the DUSTY
models (Chabrier et al., 2000) otherwise. Contrasts are only shown out to 4”
and 200au since sensitivity is largely flat beyond these separations; our
field or view extends to $\sim$6” and between $\sim$250au and $\sim$850au
depending on the stellar distance. The median survey sensitivity is 10.4
$\rm{M_{J}}$ at 25au and 5.9 $\rm{M_{J}}$ at 100au

##  5 Methods & Results

###  5.1 Age Determination

Determining accurate ages is vital in high contrast imaging, since the
relationship between brown dwarf/planet mass and luminosity is dependent on
the age of the object. The target age therefore plays a role in determining
both the contrast limits that the survey reaches, and in the case of low mass
companions, the properties of those companions. Targets are not selected
explicitly for youth, as described above, but we nonetheless expected a bias
towards young stars since the occurrence of extrasolar dust is much higher for
young stars Kennedy & Wyatt (2012).

To determine ages for the sample, we first turned to the literature. The
targets in our survey have anywhere between 0 and 8 independent age
determinations in the literature, which are usually but not always consistent
for the same target, as demonstrated in Figure 3. These literature ages are
calculated by two classes of methods. First, several of the targets in this
work are members of moving groups, with well constrained ages. Many of the
targets have previously been identified as members of the Scorpius-Centaurus
association, which has a well defined and well studied age spread (see e.g.
Pecaut & Mamajek, 2016). We also searched for previously unidentified moving
group members using the BANYAN-$\Sigma$ tool (Gagné et al., 2018), which
compares galactic position and space velocity to known nearby, young
associations. We found that HD182681 has a 90.2% likelihood of being a member
of the $\beta$-Pictoris moving group. A number of other works also quote young
ages for this target (see Figure 3), confirming that this object is indeed a
moving group member and not aligned with the group by chance.

Secondly, many of the targets have had ages determined by comparison to
isochrones. However, this approach can lead to biased ages, since it does not
account for the non-uniform distribution of masses or the fact that time is
non-linear on an HR diagram (Pont & Eyer, 2004; Takeda et al., 2007). Both
Nielsen et al. (2013) and David & Hillenbrand (2015) therefore use a more
sophisticated Bayesian inference technique to extract age designations from
isochrones. In some cases no isochronal ages have been previously determined,
and so we fit ages using the isochrones tool (Morton, 2015), using the Gaia
parallax and BVGJHK photometry from Tycho, 2MASS and Gaia (Høg et al., 2000;
Skrutskie et al., 2006; Gaia Collaboration et al., 2016b). To verify these
values, we repeat the fits using only the BVG, and then only the JHK
photometry, and check for consistency.

In Table 1, we report our final choice of age for each target, and a quality
flag with value between 1 and 3, indicating how reliable this age is, with 3
being the most reliable. We assign ages by the following procedure:

  * •

If the object is in a moving group, we use the age of that moving group. These
objects are assigned an age quality flag 3.

  * •

If a Bayesian isochronal age exists in the literature, we use this age
designation. We use the ages from David & Hillenbrand (2015) where possible,
and the ages from Nielsen et al. (2013) otherwise. These objects are assigned
an age quality flag 2.

  * •

For the remaining targets, there are at most one literature age reference, and
we determine independent ages for each of these objects using isochrones. We
use the Casagrande et al. (2011) age for HD133778, which is discussed further
below, and our own ages for the other targets. These objects are assigned an
age quality flag 1. We show our estimated isochrones ages for all of the
targets in this work in Figure 3 to demonstrate their reliability. The
majority of our age estimates are consistent with other literature estimates
for these targets. Of the seven targets in moving groups, our estimates are
consistent to within 1$\sigma$ of the moving group age for five targets, and
consistent to within 1.5$\sigma$ for the remaining two.

  * •

HD18378 is an evolved star, and so it is not possible to extract a useful age
determination using isochrones (see Section 6.3 for a detailed discussion).
This is the most evolved star in our sample, and in this case the excess
infrared flux might be due to mass loss rather than the presence of a debris
disk.

Figure 3: Literature ages for candidates in this survey, from a variety of
sources. In most but not all cases, the literature age determinations are
consistent. We also independently generate ages for each target with the
isochrones package (Morton, 2015) using BVGJHK photometry and Gaia parallaxes.
These ages broadly consistent with literature values, and are only used for
targets that are not moving group members and that have one or fewer
literature age references. The age used in analysis for each object is listed
in Table 1. References are Moór et al. (2006); Rhee et al. (2007); Casagrande
et al. (2011); Tetzlaff et al. (2011); Zorec & Royer (2012); Nielsen et al.
(2013); Chen et al. (2014); David & Hillenbrand (2015).

The age analysis for HD 133778 warrants special discussion, both since there
is contradictory literature and since the object has a co-moving companion
(see Section 6.1). The star is listed as a Sco-Cen member in de Geus et al.
(1990), but not in the more recent works such as de Zeeuw et al. (1999);
Rizzuto et al. (2011). Casagrande et al. (2011) find the star to have an age
$\sim$1.5 Gyr and a mass $\sim 1.73\,\rm{M}_{\odot}$ using isochronal
analysis. Since the target RV ($-37\,\mbox{km\,s${}^{-1}$}$, Nordström et al.
2004, Torres et al. 2006) is inconsistent with the values expected for a Sco-
Cen member (typically +5 km s-1, de Bruijne 1999) and the star is lithium poor
(Torres et al., 2006), we use the older age as listed in Casagrande et al.
(2011). This suggests that the object was a late A type star, but has recently
left the main sequence.

###  5.2 Candidate Companions

We identified candidate companions by visually inspecting each PCA-processed
image. To ensure an optimum balance between removal of stellar signal and
preservation of companion signal, we visually inspected data products with 1,
6 and 20 principal components removed. Once candidates are flagged, relative
astrometry and photometry can be extracted from the IRDIS images. To determine
the relative astrometry, we measured the companion positions by fitting
Gaussians to the sources. The stellar position behind the coronagraph is
inferred using the star center calibration frames. We assume that the
candidate companion positions can be determined to an accuracy of 0.1pix, and
that the position of the star behind the coronagraph can be determined to
0.2pix by using the star center calibration frames. This is in line with other
SPHERE analyses; in particular Maire et al. (2016) determined that the star
position is accurate to better than 0.1pix ($\sim$1.2mas). We converted
detector positions to astrophysical separations following the ESO SPHERE User
Manual333https://www.eso.org/sci/facilities/paranal/instruments/sphere/doc.html:
we used a platescale of 12.255$\pm$0.021mas/pix and a true north correction of
-1.700$\pm$0.076∘ for data before 2015 December, and -1.75$\pm$0.08∘ for data
after 2016 February. The uncertainties from the point-finding and from the
detector properties are similar in magnitude, with the uncertainty on the
point-finding dominant for close candidates, and the uncertainty on detector
properties dominant for more distant sources.

We determined relative photometry of the candidate companions by measuring the
flux within a small aperture centered on the candidate positions, and
comparing this value to the measured brightness of the host stars, measured
using the same aperture, in the flux calibration frames. We accounted for
self-subtraction by injecting several fake planets into the data at varying
separations and known magnitudes. These were then extracted using the same
aperture photometry procedure, and we used the measured magnitude to calculate
a self-subtraction fraction as a function of radius, which can be applied to
the real candidate companions.

For HD19257, the candidate companion (later confirmed to be co-moving, see
Section 6.2) is sufficiently bright that it is clearly visible in the flux
calibration frames, and the signal from the companion is severely distorted in
the PCA processed images due to extensive self-subtraction. In this case we
instead measured the photometry and astrometry directly from the cleaned and
pre-processed flux calibration frames to avoid biases and self-subtraction.
For the HD18378B and HD133778B companions, which are also confirmed to be co-
moving (see Sections 6.3 and 6.1, we refine the photometry by performing
negative fake planet subtraction and iterating with a Levenberg-Marquardt
least-squares minimization. This process is identical to that used for the IFS
spectral extraction, as described in Section 5.3. This procedure shows results
in good agreement with the simpler approach (i.e. aperture photometry with a
correction to account for self-subtraction) and is significantly more
computationally expensive, and so is only used for these two co-moving
companions and not for the unconfirmed candidate companions or background
stars.

In this survey we identified a total of 162 candidates, with 98 of these
candidates associated with the target HD 158815 which is very close to the
galactic plane (galactic latitude b=+6.7∘). The median number of candidate
companions per target in this survey is 1, while the mean is 8. Of the 162
candidates identified, 3 are confirmed to be co-moving companions in the
stellar mass regime (namely HD 18378 B, HD 19257 B and HD 133778 B; see
Sections 6.1-6.3).

####  5.2.1 Common Proper Motion Analysis

In Figure 4 we present combined common proper motion plots for each target
where we detect candidates. Host star proper motions are taken from the Gaia
DR2 catalog (Gaia Collaboration et al., 2018). In each case we plot a point
indicating the final astrometric position of each individual candidate
relative to its initial position, i.e., we show the vector position change of
the companion between epochs. We also show the predicted path and final
position for a stationary background object relative to the target star.
Candidates are color-coded to indicate their designation as companion or
background (but note that the 31 candidates with only one astrometric
measurement are not indicated in this plot). In most cases, the final
positions of candidates are clustered around the predicted position for a
background object, and we conclude that these objects are not associated with
the target star.

Figure 4: Combined common proper motion plots for those companions for which
we have two astrometric measurements. In each case the epoch 1 positions are
aligned to [0,0] and plotted with a black circle, and the change in relative
astrometry (i.e., the vector motion) between the epochs is indicated in red
for background objects and blue for companions. The black line and black cross
indicate the expected motion of a background star, and the expected position
of background stars at the second epoch, with the dominant error being the
accuracy of our astrometry. Note that this position is the same for all
background objects, since it is derived from the proper motion and parallax of
the foreground object. The HD158815 points are partially transparent for
clarity. Some systematic errors are seen, which are discussed in the text.

For a few of the sources, the astrometry does not show strong agreement with
the background hypothesis, either because there is significant scatter between
the candidate companions (such as HD151012), or because the median candidate
companion motion vector is offset from the predicted motion vector (such as
HD94893). For HD151012, this is likely an issue in the centering of individual
point sources: many of the points or only marginally above the detection
limit, and are redetected at below a conventional 5$\sigma$ detection
threshold in the second epoch, where the sensitivity is marginally lower than
the first epoch. Indeed, the four points that are most divergent from the
background hypothesis are also the four faintest points that are redetected.
This indicates that the 0.1px accuracy in location is too optimistic for the
faintest of point sources targets. For HD94893, the median candidate vector is
offset by 28mas, or slightly over 2 pixels, from the background hypothesis. A
similar effect is seen for HD98363 and HD123247, where the offsets are 26mas
and 21mas. The average position error for these sources are 13mas, 11mas and 9
mas respectively. This indicates that we are seeing a systematic error at the
2$\sigma$ level in each case. The source of this systematic error is unclear.
It is likely associated with either the stellar position drifting behind the
coronagraph, or a small misalignment of the north angle. The median candidate
vector is offset from the common proper motion hypothesis by 58mas, 110mas and
65mas for the three targets, and so in each case the background hypothesis
remains a much better fit than the common proper motion hypothesis.

Archival high resolution data exists for HD24966, HD157728, HD176638 and
HD182681 (Nielsen et al., 2013; Wahhaj et al., 2013; Galicher et al., 2016),
with the first two publications reporting on the same data. For both HD24966
and HD157728, we did not detect any candidate companions, a result in
agreement with Wahhaj et al. (2013), who report no candidates for HD24966 and
a single wide candidate to HD157728 at 11.4”, well outside our field of view.
For HD176638 and HD182681, all of the archival candidates in our field of view
have previously been reported as backgrounds, and our astrometry shows good
agreement with the archival measurements, as demonstrated in Figure 5. We see
a small offset between our measurements of the three HD176638 candidate
companions and the stationary background object hypothesis: such small
systematic errors are common when comparing different instruments with their
own systematics. For HD182681, our measurements show good agreement with the
archival astrometry.

Based on multi-epoch astrometry and comparisons to archival astrometry, we are
able to confirm 119 of the candidate companions as background sources.

Figure 5: Common proper motion plots for those candidates with archival
astrometry. The black line indicates the predicted path for a stationary
background object, which is measured relative to the first archival epoch for
HD176638, and relative to the second epoch for HD182681 since the first epoch
is inconsistent with other early measurements. In each case the orange, green
and blue points indicate astrometry from Wahhaj et al. (2013), Galicher et al.
(2016) and this work respectively, with darker points being measurements and
lighter points being the predicted background position at each epoch. For
HD176638 we see a small offset between the astrometry reported in this work
and that from Wahhaj et al. (2013), common in comparative astrometry between
different instruments with their own systematics. For HD182681 we see broadly
good agreement between all epochs, albeit with significant scatter between the
earlier measurements. Our astrometry is consistent with previous works, and
with each of these candidates being a background source.

####  5.2.2 Color-Magnitude Analysis

Our second approach to differentiating companions and background stars is
based on the colors of candidates. We calculated the H2-H3 color of each
candidate, and the absolute magnitude in H2 under the assumption that the
candidate is at the distance of the host star. The SPHERE H3 filter lies on a
methane absorption band, and so for sufficiently cold objects (spectral type T
or later) we expect to see a negative H2-H3 color, while earlier type objects
have a color of $\sim$0\. We can therefore use this color to differentiate
faint background stars from faint companions, since for faint objects a color
$\sim$0 indicates that the object is not at the distance of the host, and is
instead a background object of earlier spectral type, with no methane
absorption. In Figure 6, we show the CMD for candidates in our survey. We show
the large number of candidate companions to HD158815 in a separate panel for
clarity. Those objects without CPM data, i.e. those for which the color-
magnitude analysis is particularly important, are highlighted as triangles in
the figure. For reference, we also plot CMD positions of late type stars and
field brown dwarfs from the SpeX Prism Library444
http://pono.ucsd.edu/~adam/browndwarfs/spexprism. We selected all available
objects with high resolution spectral data, and integrated the flux of each
object across the SPHERE H2 and H3 filters. Objects of spectral type M and L
have color $\sim$0 in these filters, and there is a sharp turn-off at L-T
transition with T objects showing negative H2-H3 colors due to methane
absorption in the atmosphere. Based on the positions of the SpeX objects on
this CMD, we assume all candidates in our survey with M${}_{\textrm{H2}}>$ 15
and H2-H3 $>$ (15-M${}_{\textrm{H2}}$)/1.5 are background objects. Finally,
the three confirmed co-moving companions detected in this survey are
highlighted with stars in the plot: based on H2 magnitude alone, HD 133778 B
and HD 18378 B have spectral type $\sim$M4, while HD 19257 B is $\sim$M0.
These companions and their properties are discussed further in Sections
6.1-6.3.

Using this CMD approach, we are able to confirm an additional 16 candidate
companions as background stars.

Figure 6: Color-Magnitude Diagrams for candidate companions in this survey.
For each candidate we plot the absolute magnitude if the object were at the
distance of the host star against the H2-H3 color. Black points indicate the
expected trend for late type stars: objects of spectral type T0 or later start
to have significant methane absorption, and a correspondingly negative H2-H3
color. Faint candidates with H2-H3$\sim$0 are inconsistent with being at the
distance of the host star, and are therefore presumed to be background objects
rather than co-moving companions. Specifically, we assume all candidates with
M${}_{\textrm{H2}}>$ 15 and H2-H3 $>$ (15-M${}_{\textrm{H2}}$)/1.5 are
background objects. Those candidates without common-proper motion data, for
which the color-magnitude analysis is particularly important, are indicated
with triangular points. The three large stars indicate the location of the
three co-moving companions on the CMD, which are discussed further in Sections
6.3-6.2.

####  5.2.3 Unconfirmed Candidate Companions

Using the combination of astrometric and color-magnitude analysis, we confirm
144 of the 162 candidate companions as background objects. Three candidates
are confirmed to be co-moving companions. The remaining 15 objects are likely
background stars since all are widely separated from their host stars, but
more data would be required to measure astrometry relative to the host and
thus confirm this conclusion. Twelve of these unconfirmed objects are
candidate companions to HD158815, and are highly likely to be backgrounds, due
to the crowded background field of this target. The closest separation of an
unconfirmed candidate companion to its host star is 2.1”, and the unconfirmed
candidates follow the approximately radial distribution that would be expected
for background stars. In Appendix A we include additional notes for each
individual target explaining the final designations of each candidate as a
companion, a background object or a likely background object. Individual
candidate astrometry is available in Table 3 and in the online DIVA archive
(Vigan et al., 2017).

Target | Date | N | PA[deg] | PA_err | Sep[mas] | Sep_err | RA[mas] | RA_err | Dec[mas] | Dec_err | Status
---|---|---|---|---|---|---|---|---|---|---|---
HD18378 | 2016-09-15 | 1 | 279.35 | 0.40 | 418.2 | 2.8 | -412.6 | 2.8 | 68.0 | 2.9 | C
HD18378 | 2017-08-29 | 1 | 279.15 | 0.40 | 419.2 | 2.8 | -413.8 | 2.8 | 66.7 | 2.9 |
HD19257 | 2015-08-26 | 1 | 127.70 | 0.16 | 1292.9 | 2.9 | 1022.9 | 3.2 | -790.7 | 3.3 | C
HD19257 | 2017-09-01 | 1 | 127.28 | 0.16 | 1291.0 | 2.9 | 1027.2 | 3.2 | -782.0 | 3.3 |
HIP94893 | 2015-04-11 | 1 | 275.69 | 0.15 | 2547.5 | 5.2 | -2534.9 | 5.2 | 252.5 | 6.5 | BG_cpm
HIP94893 | 2018-01-07 | 1 | 276.66 | 0.15 | 2497.6 | 5.1 | -2480.8 | 5.1 | 289.6 | 6.4 |
HIP94893 | 2015-04-11 | 2 | 218.04 | 0.14 | 3647.9 | 6.9 | -2247.6 | 8.2 | -2873.1 | 7.7 | BG_cpm
HIP94893 | 2018-01-07 | 2 | 217.27 | 0.14 | 3598.4 | 6.8 | -2179.3 | 8.2 | -2863.4 | 7.7 |
Table 3: Astrometry for each individual candidate at each epoch. The full
table is available in machine readable form. In the status column, we indicate
companions with C, candidates confirmed as backgrounds based on their proper
motion as BG_cpm, those confirmed as backgrounds based on their CMD position
as BG_color and finally those candidates that are unconfirmed as ? (although
note that these are all likely to be background stars as detailed in Section
5.2.3).

###  5.3 IFS spectral extraction

For companions within the IFS field of view, we perform a dedicated analysis
to extract a spectrum that can be compared to empirical or theoretical
templates. To avoid biases usually induced by spectral differential imaging
(Racine et al., 1999) in high-contrast spectroimaging or spectroscopic data
(e.g. Thatte et al., 2007; Vigan et al., 2012), we perform an ADI analysis
channel-by-channel, that is considering each wavelength in the IFS data
independently. For the speckle subtraction we use a full-frame PCA with
typically only a few modes subtracted.

At each wavelength, the precise astrometry and photometry of the candidate
companions are estimated using negative fake planet subtraction in the pre-
processed data cube (e.g. Marois et al., 2010). A rough estimation of the
object position and contrast is first performed using a 2D Gaussian fit. These
initial guesses are then used as a starting point for a Levenberg-Marquardt
least-squares minimization, where the position and contrast of the negative
fake companion are varied to minimize the residual noise after ADI processing
in a circular aperture of radius $\lambda/D$ that is centered on the position
of the companion. When a minimum is reached, the position and contrast of the
fake companion are taken as the optimal values for the astrometry and
photometry. This approach has already been used reliably for various
companions over a wide range of contrast (Bonnefoy et al., 2011; Zurlo et al.,
2016; Vigan et al., 2016).

To estimate the error bars on the astrophotometry, we inject fake positive
companions at the same separation but different position angles than the real
candidate, with the optimal photometry. Then, the same minimization procedure
as described previously is applied to these fake positive companions to derive
their best fit photometry and astrometry. This operation is repeated at 100
different position angles, each time with only a single positive fake
companion injected to avoid any bias in the results due to the presence of
multiple companions at the same separation. The error bars on the astrometry
and photometry of the real candidate are then estimated using the 1$\sigma$
deviations measured on the determination of the astrophotometry of the
positive fake companions. Although this procedure cannot capture the
uncertainties related to the variation of the observing conditions, it enables
a reliable estimation of the uncertainties related to the image processing and
signal extraction.

Only the photometry from this spectral extraction process is used in our
analysis, although we note that the astrometry is consistent with that derived
from the IRDIS data.

##  6 Confirmed Companions

Three stellar companions are identified with VLT/SPHERE in this survey, and we
find evidence in the Gaia catalog of three additional very-wide stellar
companions, which are outside of the field of view of the SPHERE instrument.
Table 4 presents an overview of the systems with stellar companions, and we
also discuss these companions individually below.

| HD18378 | HD19257 | HD133778 | HD98363 | HD138564 | HD151012
---|---|---|---|---|---|---
Detected by | SPHERE | SPHERE+Gaia | SPHERE | Gaia | Gaia | Gaia
Host SpT | K2III | A5V | G2V | A2V | B9V | B9.5V
Companion SpT |  M4$\pm$2 | early M? |  M4$\pm$1 | — | — | —
T${}_{\textrm{eff}}$ Comp [K] | — | n/a | — | 4360 | 3590 | 4100
SPHERE Epoch 1 | 2016-09-15 | 2015-08-26 | 2016-05-16 |  |  |
Separation [arcsec] |  0.418$\pm$0.003 |  1.293$\pm$0.003 |  0.989$\pm$0.003 |  |  |
Projected Sep [au] |  218$\pm$3 |  96.2$\pm$0.5 |  162$\pm$2 |  |  |
Position Angle [∘] |  279.4$\pm$0.4 |  127.70$\pm$0.16 |  88.3$\pm$0.2 |  |  |
SPHERE Epoch 2 | 2017-08-29 | 2017-09-01 | 2017-06-22 |  |  |
Separation [arcsec] |  0.418$\pm$0.003 |  1.291$\pm$0.003 |  0.990$\pm$0.003 |  |  |
Projected Sep [au] |  218$\pm$3 |  96.1$\pm$0.5 |  162$\pm$2 |  |  |
Position Angle [∘] |  279.2$\pm$0.4 |  127.28$\pm$0.16 |  88.4$\pm$0.2 |  |  |
Gaia Epoch |  | Gaia DR2 |  | Gaia DR2 | Gaia DR2 | Gaia DR2
Separation [arcsec] |  |  1.2537$\pm$0.0012 |  | 49.65 | 22.59 | 34.09
Projected Sep [au] |  |  93.1$\pm$0.4 |  |  6860$\pm$70 |  2312$\pm$15 |  3670$\pm$20
Position Angle [∘] |  | 130.6 |  | 304.0 | 49.6 | 144.6
$\Delta$H2 [mag] |  10.76$\pm$0.05 |  3.28$\pm$0.04 |  8.16$\pm$0.03 |  |  |
$\Delta$H3 [mag] |  10.61$\pm$0.05 |  3.16$\pm$0.04 |  8.06$\pm$0.03 |  |  |
$\Delta$G [mag] |  |  4.53$\pm$0.03 |  |  3.625$\pm$0.002 |  6.7315$\pm$0.0005 |  7.1179$\pm$0.0006
Est. Orbital Period [yr]a | 2600 | 630 | 1400 | 350,000 | 73,000 | 150,000
Est. Orbital Motion [mas/yr] a | 1 | 13 | 4 | 1 | 2 | 1
Table 4: Summary of the stellar companions to stars in this survey. The
companion to HD98363 is commonly known as Wray-15 788; companions to HD19257,
HD138564 and HD151012 have only Gaia names and are Gaia DR2
123185851097644416, Gaia DR2 6003218283764194304 and Gaia DR2
6034203483517407616 respectively. The quoted H-band contrast is in the SPHERE
H2 filter for SPHERE targets, and is not listed for those objects we do not
detect with SPHERE; the $\Delta$G values are the contrast in the main Gaia
filter. T${}_{\textrm{eff}}$ values are the Gaia temperature fits, where
available. Full epoch astrometry, including the relevant dates, are also
listed for both the SPHERE observations and the Gaia DR2 measurements.
a: the estimated orbital period and motion values rely on many assumptions,
most notably that the projected separation is the real separation, and the
orbit is circular and the projected. These values are provided only to give a
sense of the parameter space of these companions.

###  6.1 HD 133778 B: a mid-M companion to an star that recently left the main
sequence

We identify a companion to HD 133778 for the first time (Figure 7), which
shows clear evidence for common proper motion with the host (see Figure 4
above). The companion is at a projected separation of 989$\pm$3mas
(=162$\pm$2au at the host distance) and a position angle of 88.3$\pm$0.2∘, and
we measure a movement of 2.7mas relative to the host between our two datasets,
a value smaller than the astrometric precision of SPHERE. As such, we do not
attempt to fit an orbit at this point.

The companion falls within the IFS field of view for a subset of frames, and
is sufficiently bright that a low resolution spectrum (R$\sim$35) can be
extracted even from just these few data frames. Since the total integration
time is significantly shorter in the second observation, we only use the data
from 2016-05-16 for the spectral and photometric analysis. The extracted
spectrum, and the H2/H3 band photometry, are shown in Figure 8 with M1-M7
reference spectra from Cushing et al. (2005) also plotted. Visually, HD 133778
B matches the M3 and M4 spectra best, and the absolute magnitude is a good
match for an M4 star (see Figure 6 above) so we class this object as M4$\pm$1.

HD 133778 is an old star, as described in Section 5.1. The object was
previously a late A-type star, but has recently left the main sequence. It is
unusual for such an old star to host a massive debris disk. The disk could
either have been formed in a recent massive collision between small planets in
the system, or between a planet and a planetesimal belt. Alternatively, the
disk might have lasted the entire lifetime of the system. We discuss these
scenarios in more detail in Section 7

Figure 7: Discovery images of HD 133778B. The left and right panels show data
from 2016-05-16 and 2017-06-22 respectively. In each case, the SPHERE/IRDIS
data is reduced using a PCA algorithm to remove stellar speckles, and the H2
and H3 data are coadded. The host star is in the center behind the
coronagraphic mask, and the companion is clearly visible to the left hand side
of the image. Note that the bright circular ring at $\sim$0.9” is the ring of
speckle noise at the edge of the AO control radius, smeared due to the
rotation between images, and is not related to the debris disk in this system.
Figure 8: Spectrum for HD 133778B. Both the IFS (YJ) and IRDIS (H) bands are
shown. HD133778 shows a good match to spectral types $\sim$M3-M4, which is in
agreement with the absolute magnitude of this object. Reference spectra are
from Cushing et al. (2005), and the stars used as reference objects for M1 to
M7 respectively are GL229A, GL411, GL388, GL213, GL51, GL406 and VB8.

###  6.2 HD 19257B: a newly identified stellar companion to an old early-F
star

We detected a bright companion to HD 19257, at a separation of
1.293$\pm$0.003” and a position angle of 127.70$\pm$0.16∘. The object is
clearly identifiable even in the flux calibration frames, and both the
contrast and the astrometry of this candidate were measured from the flux
calibration frames directly. This avoids the uncertainty in stellar flux and
position introduced by the coronagraph, and the self subtraction introduced by
the PCA algorithm, which is a significant effect for a companion as bright as
this and would bias both the astrometry and the photometry. We find contrast
ratios of $\Delta\textrm{H2}$=3.28$\pm$0.04 and
$\Delta\textrm{H3}$=3.16$\pm$0.04 for the object.

The relative astrometry of the candidate is in agreement with common proper
motion with the host star, as shown in Figure 4, confirming that this is
indeed a bound companion. The position of the candidate changes by 10mas
relative to the host star, with the two measurements consistent at the
2$\sigma$ level. The slight change in measured position could correspond to a
marginal detection of orbital motion (which could cause an on-sky motion of up
to $\sim$13mas/yr, see Table 4), but we do not attempt to fit the orbit with
only two datapoints.

The companion is sufficiently bright that it also appears in the Gaia DR2
catalog, with a G magnitude 11.53$\pm$0.03 (Gaia Collaboration et al., 2018).
However, the companion is only marginally detected by Gaia, and no parallax or
proper motion are quoted in the catalog. Indeed, the companion is at the limit
of the contrast sensitivity of Gaia: at the measured Gaia separation of
1.253”, Brandeker & Cataldi (2019) find a 50% detection fraction at a contrast
of 4.63 and a 90% detection fraction of 4.27; the contrast of HD 19257B is
4.53$\pm$0.03mag in the G band. Given that this is a marginal detection, the
companion flux should perhaps be interpreted with caution.

Assuming that the Gaia flux is correctly measured, we derive stellar
properties for the companion using isochrones (Morton, 2015). We input the
Gaia parallax of the host star HD19257 A and the G and H band apparent
magnitudes of the companion. The median sample is a star with a mass of 0.54M⊙
and radius 0.49R⊙. We repeat the procedure using only the parallax and H band
magnitude, and find a somewhat larger star with mass 0.69M⊙ and radius 0.67R⊙.
When using only the H band, the median sample has a much bluer color than we
measure (2.6 compared to 1.9), perhaps indicating an incorrectly measured flux
in one of the bands. The isochrones package also predicts that this object is
a very old star: whether or not the G-band magnitude is included, the median
age of the samples is $\sim$7Gyr, which is significantly older than the age
extracted for the host star of $\sim$800Myr, but likely unreliable due to the
limited number of photometric measurements and the discrepancy between the
predicted and measured G-H color of the object.

Figure 9: The discovery image of HD 19257B. This is a flux calibration image
taken with SPHERE/IRDIS in the H2 filter, and without the coronagraph present.
Bad-pixels have been removed and North is oriented up. HD 19257A is visible in
the center and HD 19257B to the lower left.

Frémat et al. (2007) recorded HD 19257 as a suspected binary due to discrepant
radial velocity measurements, but the star is not identified as a
spectroscopic binary in that work and the authors instead conclude that it is
a variable star. That work quotes an effective temperature of 7367$\pm$195K
and a logg of 4.23$\pm$0.06. The target is listed as spectral type A5 by
Cannon & Pickering (1993), and as an A9III by Frémat et al. (2007). Most
recently, Kuchner et al. (2016) determined a spectral type of F0V. This is
consistent with McDonald et al. (2012) and Kennedy & Wyatt (2013) which do not
quote spectral types but find effective temperatures of 7227 K and 7458 K
respectively, and with Gaia which finds a temperature of 7420$\pm$120K. Since
the Gaia catalog resolves the companion and finds a very similar temperature
to Frémat et al. (2007), we conclude that these temperature fits are not
affected by the presence of the companion.

We determine an age of 790$\pm$150Myr for this target. This is older than the
majority of dust-hosting stars: the system is somewhat reminiscent of HD
133778, in that both are older stars, with stellar mass companions and large
quantities of dust. Is it possible that these stellar companions are stirring
planetesimals and maintaining the debris disk until much older than the
typical age of highly dusty systems? Alternatively, perhaps planetary
components of these systems are migrating due to the widely separated
companion, and this migration has recently caused a ring of planetesimals to
be disturbed, and elevated the dust volumes in these systems to detectable
levels.

###  6.3 HD 18378 B: a mid-M companion to a giant star

We identify a companion to HD 18378 for the first time (Figure 10), at a
separation of 418$\pm$3mas from the host, a position angle of 279.4$\pm$0.4∘
and a contrast of $\Delta$H2=10.76$\pm$0.05mag. The object shares common
proper motion with the host (see Figure 4), with the relative astrometry
consistent between epochs. For a face-on, circular orbit, the orbital period
of this companion would be $\sim$2600 years, corresponding to a movement of
just $\sim$1mas.

The low-resolution spectrum of this object in the YJ band, and the two H band
photometric points, are shown in Figure 11. Spectra were extracted for both
epochs of observation, and are plotted against each-other. There is a clear
absolute flux offset between the two epochs. This is likely a calibration
offset due to weather changes within an observation: since the host star and
the companion are observed concurrently and not simultaneously, slight changes
in transmission of the atmosphere between the flux calibration and the science
observation can cause such offsets. This calibration offset should not affect
the shape of the spectra. For both epochs, the spectrum shows a relatively
flat slope between 0.9-1.2$\mu$m, and a small peak between 1.2-1.4$\mu$m,
which is significantly more pronounced in the second epoch.

In the second panel of Figure 11, the median of the two epochs is plotted
against reference spectra for M1-M7 field dwarfs from Cushing et al. (2005).
Although the spectrum is relatively noisy, the slope short of 1.2$\mu$m
matches that of the reference spectra between M1 and M5, while the peak at
1.3$\mu$m is visually a closer match to spectral types M5-M6. None of the
reference spectra accurately capture both the slope of the relatively flat
1.0-1.2$\mu$m region of the spectrum and the peak in emission between
1.25-1.3$\mu$m. The absolute flux of the object in the H band is similar to
that of M4 field dwarfs (see Figure 6). We therefore type this object as
M4$\pm$2, with more data required for a more precise spectral classification.

Very limited literature exists for this target with the only references
mentioning the host star being Houk & Cowley (1975) and Olsen (1994), who both
list it as a K2III star. Gaia DR2 quotes the star as having
T${}_{\textrm{eff}}=4590\pm 80$K and R=11.9$\pm$0.4R⊙ (Gaia Collaboration et
al., 2018) which appears to be in agreement with the classification of this
star as a giant.

There are two potential interpretations of this system. This star would have
been an early A object while on the main sequence, and perhaps represents the
later stages of evolution of some of the other stars in this survey. It could
have maintained a debris disk throughout its lifetime, or perhaps a recent
massive collision between planetesimals or even small planets in the system
has elevated the rate of dust production to a detectable level. Alternatively,
and perhaps more likely, it might be that we are simply observing infrared
excess due to mass loss from this giant star.

Figure 10: Discovery images of HD 18378 B. Both IRDIS and IFS data are reduced
using a PCA algorithm to remove stellar speckles, and only the central portion
of the data is shown. The companion is clearly visible to the right of the
star, which is behind an occulting mask in the center of the image. The point
to the north of the star in the 2016-09-15 IFS data is due to flux persistence
from a previous observation, and is not astrophysical in nature. Figure 11:
Upper: Spectra for HD18378B as observed on 2016-09-14 (blue) and 2017-08-29
(red). Both the IFS (YJ) and IRDIS (H) bands are shown. The spectra show
broadly similar shapes with an absolute offset, likely due to the flux
calibration as detailed in the main text. In both cases, a peak is seen at
$\sim$1.3$\mu$m, but this is much more pronounced in the second (red)
spectrum. Lower: The median of the two spectra is plotted against seven
reference spectra. Reference stars are from Cushing et al. (2005) and are the
same stars as those in Figure 8. Visually, the closest match is an early to
mid M star, and using both the spectral and absolute magnitude information we
type the object as M4$\pm$2.

###  6.4 Wide binaries identified in Gaia

####  6.4.1 HD98363B (Wray 15-788)

HD98363 has previously been identified as being in a wide stellar binary with
Wray 15-788, with the pair at a projected separation of $\sim$50”
($\sim$6900au) (Bohn et al., 2019). Wray 15-788 hosts a disk, with H$\alpha$
emission suggesting that the disk is protoplanetary (Pecaut & Mamajek, 2016).
Bohn et al. (2019) detected the disk in scattered light for the first time and
discovered a large inner cavity, indicating that the disk is likely in the
transition stage. This system is fascinating, since the HD98363 disk appears
to be gas-poor and as such in the more evolutionarily advanced debris disk
stage (Chen et al., 2012; Moór et al., 2017). Although resolving this disk
would be highly interesting, it is somewhat unsurprising given the relatively
low $\textrm{L}_{\textrm{IR}}/\textrm{L}_{\star}$(see Discussion section) that
we were not able to directly detect the HD98363 disk in this work. The disk
has recently been resolved by Hom et al. (2020), using the GPI polarimetry
mode (Perrin et al., 2010), and that work discusses the architecture of the
system in more detail.

####  6.4.2 Other bound companions from Gaia

We also search the Gaia catalog for companion stars outside of the SPHERE
field of view. For each star, we search for companions out to a projected
separation of 10,000 au at the distance of the target. We select as potential
companions those sources which have both parallaxes consistent with the host
to 3$\sigma$, and proper motions in both RA and Dec consistent with the host
to 8$\sigma$. This relatively wide constraint on proper motion is used to
ensure that any differential motion between the two targets due to their
mutual orbit does not exclude a bound pair from the selection. We recover
three potential binaries: the HD98363+Wray 15-788 system discussed above, and
anonymous field star companions to HD138564 and HD151012. These companions
have Gaia source IDs 6003218283764194304 and 6034203483517407616 respectively
in the DR2 catalog.

To check that each pair is indeed a bound stellar binary, we estimate an
approximate mass for each component, and calculate an approximate orbital
velocity of the two stars relative to each other. This includes several coarse
assumptions: we assume a circular orbit, and acknowledge that our estimates of
secondary mass are very approximate, given that they are based only on the
Gaia photometric fits. These values can nonetheless be compared to the
observed differential velocity of the two systems: a significantly higher
differential velocity would indicate an unbound pair. A differential velocity
below this threshold does not confirm that the pair is bound, since motion
could be occurring along the line of sight and since we are using very
approximate orbital values, but the combination of small separation, similar
parallax and a low differential velocity is a good indicator that the system
is a stellar binary. Both the HD98363 and HD151012 companions show
differential velocities smaller than the expected orbital velocity; HD138564
and its companion shows a differential velocity marginally higher than the
expected orbital velocity (2.24$\pm 0.25$mas/yr compared to an expected value
$\sim$1.95mas/yr). Given that the differential velocity is only marginally
above this threshold, which is somewhat approximate, we conclude that all
three pairs are likely stellar binaries.

The properties of all three of these companions are listed in Table 4. All
three are in a similar parameter regime: the companions are at projected
separations of approximately 6900au, 2300au and 3700au for HD98363, HD138564
and HD151012 respectively. Further, all three primary stars are members of the
Scorpius-Centaurus association, and are therefore young. These are in a
markedly different regime than the companions detected with SPHERE in this
work, which are all within a few hundred au of their hosts and orbit older
stars.

##  7 Discussion

We identified three late-type stellar companions to highly dusty stars
identified by WISE, one of which was also resolved by Gaia. We used Gaia to
confirm that three additional stars in the survey have equidistant and co-
moving stellar companions at several thousand au, one of which had been
previously identified by Bohn et al. (2019). The fraction of wide binaries
($\sim$30%) appears to be in broad agreement with previous works: Rodriguez &
Zuckerman (2012) find that $25\pm 4\%$ of debris disks are in binary or triple
star systems while Thureau et al. (2014) found that the rates of debris disk
occurrence around A-stars with and without stellar companions are similar
($26\pm 7\%$ and $24\pm 6\%$ respectively). These works both find a lack of
binaries with intermediate separations between $1-100$ au, and the three
SPHERE binaries presented here are consistent with that finding, with
projected separations of 96.2$\pm$0.5au, 162$\pm$2au, and 218$\pm$3au. The
actual separations in each case are likely higher, but cannot be accurately
determined without additional astrometric monitoring of the candidates to
determine their orbits. In each case, even the projected separation is
significantly wider than the predicted dust radius, as is expected for non-
destructive interference.

It is striking that all three objects with intermediate-separation stellar
companions are relatively old: one is $\sim$790Myr, one has recently left the
main sequence and one is a giant star. The post main sequence evolution of the
star through the sub-giant and giant phases is relatively short lived, and as
such it is less common to observe objects at this stage of evolution. For this
survey to have detected multiple such stars with high dust volumes hints that
it might be relatively common for these evolving systems to generate bright,
but short-lived, debris disks. Even further, it seems that the combination of
an older star with both a stellar companion and a bright debris disk might be
common.

One mechanism to explain these older systems with debris disk is the active
migration of a planetary system, perhaps in the vicinity of a planetesimal
belt. Kratter & Perets (2012) studied the effect of post main sequence stellar
evolution on a planet in a binary star system, and found that stellar
evolution can cause the planet orbit to evolve, possibly causing ejections,
collisions, or star-hopping (meaning a planet orbiting an evolving primary
moves into a chaotic orbit and then stabilizes in orbit around the secondary
star). Although that work only studies single planets, we suggest that this
complex and sometimes chaotic orbital evolution could also lead to collisions
between a giant planet and additional planets in the system, or between a
planet and a previously stable planetesimal belt. In either case, the
collisions this process causes would lead to an elevated level of dust in the
system for a short time, and a collision with a planetesimal belt in
particular could trigger a collisional cascade that quickly generates large
volumes of dust. Alternatively, chaotic orbital evolution to a very small
separation from the star could lead to a tidal disruption event, which would
also eject matter into the system and potentially trigger further collisions
and thus dust production. Additional constraints on the composition and
location of the disk could provide clues as to the true origin of this dust.
Mid-IR spectroscopy may allow the radial location(s) of this dust belt to be
constrained, and if the disks can be resolved in polarized light this would
allow the dust location(s) and structure to be analyzed. Together, these
measurements would provide clues as to whether this dust is indeed the product
of a giant collision or tidal disruption event (see e.g. Weinberger et al.,
2011), or is instead a debris disk that has survived the entire lifetime of
the system (similar to $\kappa$ CrB, Bonsor et al., 2013). A ring of dust with
a small radial extent, and a lack of cold dust at wider separations from the
star, would be strong evidence that the dust in these systems was created in a
recent dramatic collision, since in this case the dust could not have been
replenished from a cold reservoir. Note however that the presence of cold dust
does not preclude the possibility of the warm dust having been created in a
recent collision.

No planets were detected in this survey, although this result is in line with
the relatively low occurrence rate of planets accessible to high contrast
imaging (e.g. Meshkat et al., 2017; Nielsen et al., 2019; Vigan et al., 2020).
To assess the occurrence rates of planets in dusty systems will require a much
larger sample size, likely only achievable through a meta-study of several
campaigns that have prioritized highly dusty systems (e.g. Janson et al.,
2013; Rameau et al., 2013; Wahhaj et al., 2013; Meshkat et al., 2015; Matthews
et al., 2018; Launhardt et al., 2020).

None of the disks in this survey were detected in scattered light. This is
unsurprising given the small total sample size, and the typical temperatures
and fractional luminosities of each disk. To demonstrate this point, we
calculate the expected radii for each debris disk, based on the best-fit
temperatures from Cotten & Song (2016), for the 17 targets from our survey in
that work. Temperatures are converted to radii following the process described
in Pawellek & Krivov (2015), which takes into account the composition of dust
grains, and we use the 50% astrosilicate + 50% ice composition. Temperature
and radius values are given in Table 5.The dust radii and fractional infrared
luminosities are plotted in Figure 12, in both physical and angular
separations. For comparison, we also show disks that have been imaged in
scattered light with SPHERE/IRDIS in the H-band. Some of these had previously
been imaged in scattered light with other platforms, while others were
resolved for the first time with SPHERE. These disks are typically brighter
than those in our survey, but several targets in our survey are of comparable
brightness to some of the disks that have been imaged in scattered light. The
GPIES disk survey (Esposito et al., 2020) found that only disks with excesses
greater than $\textrm{L}_{\textrm{IR}}/\textrm{L}_{\star}$=$10^{-4}$ were
directly detected.

Although the disks in this survey are consistently above the
$\textrm{L}_{\textrm{IR}}/\textrm{L}_{\star}$=$10^{-4}$ threshold, several are
within the inner working angle of the instrument. Some of those beyond this
threshold are still within the parameter space where detections are hard, and
very dependent on factors such as the alignment of the disk and disk
composition. Of all the disks in the survey, HD176638 and HD182681 host the
disks that are best suited to scattered light imaging: these systems have
bright disks, widely separated from their stars and as such are in an ideal
parameter space to be imaged in scattered light. Perhaps these disks are face
on or close to face on, a configuration in which the integrated surface
brightness is lower and in which the process of removing stellar speckles also
strongly removes disk signal. If the disks are indeed face-on, polarimetric
imaging might allow for them to be directly imaged.

Figure 12: Fractional luminosities and predicted radii of the debris disks in this survey (purple), and of those disks that have been imaged in scattered light with SPHERE in the H band (green). For each disk we use the blackbody temperature from Cotten & Song (2016) and convert the temperature to a predicted radius using the relations in Pawellek & Krivov (2015). Dust radii are shown in physical units on the left, and projected separation on the right, and for two-temperature disks we indicate both predicted radii, joined by a line. The inner working angle of VLT/SPHERE is additionally illustrated on the right with gray shading. Although most disks are outside the inner working angle of SPHERE, many are close and therefore likely obscured by the stellar speckles that are dominant at close separations. Many of the targets here are brighter than the faintest disks that have been imaged in scattered light with SPHERE, but are close to their stars and sufficiently faint that detection is very dependent on actual disk radius and orientation. We include only reference disks that have been imaged with VLT/SPHERE in the H-band using IRDIS, and note that some of these disks had previously identified with other platforms. For these disks, we also plot the radius (or radii) as measured from the resolved images using dark green diamonds. Reference data for resolved disks are from Olofsson et al. (2016); Lagrange et al. (2016); Wahhaj et al. (2016); Perrot et al. (2016); Milli et al. (2017); Choquet et al. (2017); Matthews et al. (2017); Bonnefoy et al. (2017); Feldt et al. (2017); Engler et al. (2019); Gibbs et al. (2019). Target |  T1 [K] |  R1 [au] |  T2 [K] |  R2 [au]
---|---|---|---|---
HD19257 | 235 | 10 | — | —
HD24966 | 170 | 22 | 80 | 100
HD94893 | 135 | 29 | — | —
HD98363 | 350 | 5 | 130 | 34
HD113902 | 230 | 14 | — | —
HD119152 | 140 | 27 | — | —
HD123247 | 130 | 39 | — | —
HD133778 | 190 | 17 | — | —
HD138564 | 175 | 23 | — | —
HD138923 | 275 | 10 | — | —
HD151029 | 140 | 28 | — | —
HD151012 | 150 | 28 | — | —
HD157728 | 200 | 14 | 90 | 69
HD158815 | 170 | 15 | — | —
HD176638 | 115 | 55 | — | —
HD182681 | 90 | 83 | — | —
HD207204 | 170 | 21 | — | —
Table 5: Predicted dust temperatures and radii. We quote temperatures from
(Cotten & Song, 2016), and list here only the 17 targets from our survey that
are also in that work. Temperatures are converted to radii following the
relation from Pawellek & Krivov (2015), and we use their “50%
astrosilicate+50% ice” grain composition. Where the best fit from (Cotten &
Song, 2016) suggests the presence of two dust belts, we list both temperatures
and the corresponding radii for each.

##  8 Conclusions

We have presented a survey of 20 systems with mid-infrared excesses as
detected by WISE. All of these stars have been carefully vetted to confirm
that the excess is real and associated with the star, rather than being
contamination from a background source. 19 of these are nearby, early-type,
debris disk hosting stars, while one of the objects is a giant star, and so
the excess could instead be due to mass loss.

We detected three stellar companions in our VLT/SPHERE observations, and found
evidence for three additional stellar companion in Gaia, which were outside
the field of view of the current observations. We collected spectroscopy of
two of these stellar companions in the YJ band, and by comparing these to the
spectra of field stars found that both are $\sim$M4 type. We present
preliminary estimates of the likely orbital parameters of these systems, based
on the projected separations and companion spectral types, but do not attempt
orbit fitting given that we have only two astrometric measurements of each
over a relatively short time baseline. Further astrometric monitoring would
allow the true separation, eccentricity and orbital period of these companions
to be extracted. We also detected three more widely separated companions in
the Gaia catalog, a result that seems broadly in line with the expected
stellar companion rate for debris disk stars.

The systems with intermediate-separation stellar companions are all old. We
suggest that these are similar to other late-type stars with large volumes of
dust, such as BD+20 307, where Weinberger et al. (2011) inferred that the dust
was likely a result of a terrestrial planet being destabilized from its orbit
and causing a massive collision. Mid-IR spectroscopy or polarimetric imaging
of the post-main sequence dusty systems presented here could confirm the disk
location and spatial extent, and thus demonstrate whether the dust is indeed
the result of a recent collision, or if it is instead an unusually long-lived
dust disk.

Due to the small sample size of this project, we do not perform a statistical
analysis of planet occurrence rates in debris systems. A future work should
combine results from several surveys studying debris disk excess stars to
confirm with high statistical confidence whether exoplanet companions are
indeed more common in the dustiest systems, as has been hinted at in Meshkat
et al. (2017).

We thank the anonymous referee for their thorough reading of this work, which
improved the quality of the manuscript. AV acknowledges funding from the
European Research Council (ERC) under the European Union’s Horizon 2020
research and innovation programme (grant agreement No. 757561). Part of this
research was carried out at the Jet Propulsion Laboratory, California
Institute of Technology, under a contract with the National Aeronautics and
Space Administration. Based on observations collected at the European
Organisation for Astronomical Research in the Southern Hemisphere under ESO
programmes 095.C-0426(A), 097.C-1042(A) and 099.C-0734(A). This research has
made use of the VizieR catalogue access tool, CDS, Strasbourg, France. The
original description of the VizieR service was published in A&AS 143, 23. This
research has made use of the SIMBAD database, operated at CDS, Strasbourg,
France.

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##  Appendix A Target Notes

This appendix describes the candidate companions identified for each
individual target within this survey, and our determination of each candidate
as a companion or a background star.

HD 18378: We detect a co-moving companion (see Section 6.3) and no other
candidates.

HD 19257: A single, bright co-moving companion is identified, as discussed in
Section 6.2.

HD 24966: No candidate companions are detected for this target.

HD 94893: Five candidates are detected. The median movement of the candidates
between our two data epochs is a little less than expected, perhaps indicating
a small mis-calibration of the stellar position in one of the two
observations. Nonetheless, each object moves significantly between the two
epochs, and we conclude that they are all background objects.

HD 98363: This target was observed in poor weather in 2015, and six candidates
detected. A follow-up observation in significantly better weather conditions
was carried out in 2018. The six candidates from the 2015 observation were all
recovered and confirmed to be backgrounds based on their proper motion, and an
additional 13 candidates were detected. Eleven of these are faint
(M${}_{H2}<$15) and have H2-H3$\sim$0, and we conclude that these are also
background stars. Of the remaining 2, one is too bright for H2-H3 color to
differentiate companions and background stars, and one is offscreen in the H2
filter. Both are likely background objects given their separation from the
host, but more data would be required to confirm this conclusion.

HD 113902: Eight candidates are detected, and all show good agreement with the
background hypothesis between our two epochs of data.

HD 119152: No candidate companions are detected for this target.

HD 112802: One wide separation candidate is marginally detected on the very
edge of the detector, at a separation of $6.3\arcsec$ from the host star. Both
astrometry and photometry are biased this far from the target star, and so we
do not include this candidate in our work.

HD 123247: Six candidates are identified in two epochs of data, and common
proper motion testing confirms that all of these candidates are background
objects.

HD 133778: Two candidates are detected, at $0.99\arcsec$ and $5.2\arcsec$. The
close candidate is confirmed to be co-moving, and is discussed in detail in
Section 6.1, while the object at 5.2$\arcsec$ does not share common proper
motion with the target and is a background object.

HD 138564: Four candidates are detected, all of which show consistent motion
with background objects.

HD 138923: A single candidate is detected in one epoch of data. The candidate
is sufficiently bright that CMD analysis does not allow companions and
background stars to be differentiated. Further work is required to confirm if
this object is a companion, although at a projected separation of
3.98$\arcsec$ (534 au), it seems unlikely that the object is bound.

HD 151012: This target is observed twice: twelve candidates are found in the
first epoch and nine are recovered in a second epoch of data and confirmed to
be background objects. The remaining three objects are confirmed to be
background stars based on their CMD position.

HD 151029: No candidate companions are detected for this target.

HD 157728: No candidate companions are detected for this target.

HD 158815: This object is observed twice and 98 candidates are detected: 86 in
both epochs, 10 only in the first epoch and 2 only in the second epoch. All of
the candidates that are detected twice are clear background objects (see
Figure 4). A further 3 objects are likely backgrounds given their position on
a CMD (see Section 5.2.2). Finally, seven candidates are sufficiently bright
that the H2-H3 color does not distinguish companions and background stars, and
two candidates are only in the field of view in the H3 images, and so we
cannot calculate an H2 magnitude or an H2-H3 color. Many of these candidates
for which color is not a determinant are very close to the conservative
threshold that we use to confirm background position from CMD position alone,
and are still likely to be background objects. These ambiguous objects are
also all separated from the host by at least 2”, with all but two separated by
more than 5”, and the field is particularly crowded, so we suggest that these
are all highly likely to be background sources.

HD 176638: Three candidate companions are detected, and all are confirmed to
be background objects based on astrometry at two epochs of data.

HD 182681: One candidate is detected, and the target is only observed once so
we cannot test for common proper motion. The candidate is too bright for the
H2-H3 color to indicate whether this object is likely to be a genuine
companion.

HD 192544: No candidate companions are detected for this target.

HD 207204: No candidate companions are detected for this target.