# Speckle Interferometry at SOAR in 2021

Andrei Tokovinin  Cerro Tololo Inter-American Observatory — NFSs NOIRLab
Casilla 603, La Serena, Chile<EMAIL_ADDRESS>Brian D. Mason
U.S. Naval Observatory, 3450 Massachusetts Ave., Washington, DC, USA
<EMAIL_ADDRESS>Rene A. Mendez  Universidad de Chile, Casilla
36-D, Santiago, Chile<EMAIL_ADDRESS>Edgardo Costa  Universidad de Chile,
Casilla 36-D, Santiago, Chile

###### Abstract

The speckle interferometry program at the the 4.1 m Southern Astrophysical
Research Telescope (SOAR), started in 2008, now accumulated over 30,300
individual observations of 12,700 distinct targets. Its main goal is to
monitor orbital motion of close binaries, including members of high-order
hierarchies and low-mass dwarfs in the solar neighborhood. The results from
2021 are published here, totaling 2,623 measurements of 2,123 resolved pairs
and non-resolutions of 763 targets. The median measured separation is
0$\farcs$21, and 75 pairs were closer than 30 mas. The calibration of scale
and orientation is based on the observations of 103 wide pairs with well-
modeled motion. These calibrators are compared to the latest Gaia data
release, and minor (0.5%) systematic errors were rectified, resulting in
accurate relative positions with typical errors on the order of 1 mas. Using
these new measurements, orbits of 282 binaries are determined here (54 first
determinations and 228 corrections). We resolved for the first time 50 new
pairs, including subsystems in known binaries. A list of 94 likely spurious
pairs unresolved at SOAR (mostly close Hipparcos binaries) is also given.

Visual Binary Stars; Orbital Elements; Multiple Stars

##  1 Introduction

This paper continues the series of double-star measurements made at the 4.1 m
Southern Astrophysical Research Telescope (SOAR) since 2008 with the speckle
camera, HRCam. Previous results were published by Tokovinin, Mason, & Hartkopf
(2010a, hereafter TMH10) and in (Tokovinin et al., 2010b; Hartkopf et al.,
2012; Tokovinin, 2012; Tokovinin et al., 2014, 2015, 2016, 2018a, 2019, 2020,
2021). Observations reported here were made during 2021.

The structure and content of this paper are similar to other paper of this
series. Section 2 reviews all speckle programs that contributed to this paper,
the observing procedure, and the data reduction. The results are presented in
Section 3 in the form of electronic tables archived by the journal. We also
discuss new resolutions, present new orbits resulting from this data set, and
give a list of likely spurious unresolved pairs. A short summary and an
outlook of further work in Section 4 close the paper.

##  2 Observations

###  2.1 Observing Programs

As in previous years, HRCam (see Section 2.2) was used during 2021 to execute
several observing programs, some with common targets. Table 1 gives an
overview of these programs and indicates which observations are published in
the present paper. The numbers of observations are approximate. Overall, 5,138
observations were made during 2021. Here is a brief description of the main
programs.

Orbits of resolved binaries. New measurements contribute to the steady
improvement of the quantity and quality of orbits in the Sixth Catalog of
Visual Binary Star Orbits (Hartkopf, Mason & Worley, 2001). See Anguita-Aguero
et al. (2022); Gómez et al. (2022) as recent examples of this work. We provide
a large table of updated and first-time orbits in Section 3.3.

Hierarchical systems of stars are of special interest because their
architecture is relevant to star formation, while dynamical evolution of these
hierarchies increases chances of stellar interactions and mergers (Tokovinin,
2021b). Orbital motions of several triple systems are monitored at SOAR and
these data are used for the orbit determinations (Tokovinin & Latham, 2020;
Tokovinin, 2021a).

Hipparcos binaries within 200 pc are monitored to measure masses of stars and
to test stellar evolutionary models, as outlined by, e.g., Horch et al. (2015,
2017, 2019). The southern part of this sample is addressed at SOAR (Mendez et
al., 2017). This program overlaps with the general work on visual orbits.

Table 1: Observing programs  Program | PI | $N$ | Publ.aaThis columns indicates whether the results are published here (Yes) or deferred to future papers (No).
---|---|---|---
Orbits | Mason, Tokovinin | 1092 | Yes
Hierarchical systems | Tokovinin | 149 | Yes
Hipparcos binaries | Mendez, Horch | 451 | Yes
Neglected binaries | R. Gould, Mason | 624 | Yes
Nearby M dwarfs | E. Vrijmoet | 323 | No
TESS follow-up | C. Ziegler | 724 | No
Acceleration stars | K. Franson | 388 | No

Neglected close binaries from the Washington Double Star Catalog, WDS (Mason
et al., 2001),111See the latest online WDS version. were observed as a
‘filler’ at low priority. In some cases, we resolved new inner subsystems,
thus converting classical visual pairs into hierarchical triples. Some WDS
pairs are moving fast near periastron, allowing calculation of their first
orbits after several observations at SOAR. Another result of this effort is a
list of spurious pairs that should be removed from the WDS.

Nearby K and M dwarfs are being observed at SOAR since 2018 following the
initiative of T. Henry and E. Vrijmoet. The goal is to assemble statistical
data on orbital elements, focusing on short periods. The sample includes known
and suspected binaries detected by astrometric monitoring, Gaia, etc. First
results on M dwarfs are published by Vrijmoet et al. (2022). In 2021, we
continued to monitor these pairs, some of them with fast orbital motion.

TESS follow-up continues the program executed in 2018–2020. Its results are
published in (Ziegler et al., 2020, 2021). All speckle observations of TESS
targets of interest are promptly posted on the EXOFOP web site. These data are
used in the growing number of papers on TESS exoplanets, mostly as limits on
close companions to exohosts.

Accelerating stars were observed as potential targets of high-contrast imaging
of exoplanets in a program led by K. Franson and B. Bowler. A substantial
fraction of nearby stars with small accelerations, detected by comparing Gaia
and Hipparcos astrometry (Brandt, 2021), are just binaries with stellar
companions. Their resolution at SOAR serves to clean the target list for high-
contrast imaging of the remaining stars (possible exohosts) and presents an
independent interest for future orbit calculation and multiplicity statistics.

Observations of young stars belonging to moving groups were published in our
previous paper (Tokovinin et al., 2021). In 2021, a modest number of
additional targets from this program were observed.

If observations of a given star were requested by several programs, they are
published here even when the other program still continues. We also publish
here the measurements of previously known pairs resolved during surveys, for
example in the TESS follow-up.

The observations were grouped into 12 observing runs lasting from 0.5 to 3
nights each. The time allocated in 2021 through NOIRLab and Chilean TACs
amounted to 10 nights, two more nights were contributed by the SOAR partners
for TESS follow-up, and some engineering time (morning hours of 8 nights
equivalent to 3 full nights) was also used for speckle observations.

###  2.2 Instrument and Observing Procedure

The observations reported here were obtained with the high-resolution camera
(HRCam) — a fast imager designed to work at the 4.1 m SOAR telescope
(Tokovinin, 2018). The instrument and observing procedure are described in the
previous papers of these series (e.g. Tokovinin et al., 2020), so only the
basic facts are reminded here. HRCam receives light through the SOAR Adaptive
Module (SAM) which provides correction of the atmospheric dispersion. We used
mostly the near-infrared $I$ filter (824/170 nm) and the Strömgren $y$ filter
(543/22 nm), with four observations made in the $V$ filter; the transmission
curves of HRCam filters are given in the instrument manual. In the standard
observing mode, two series of 400 200$\times$200 pixel images (image cubes)
are recorded. The pixel scale is 0$\farcs$01575, so the field of view is
3$\farcs$15; the exposure time is normally 24 ms. For survey programs such as
TESS follow-up, we use the $I$ filter and a 2$\times$2 binning, doubling the
field. Pairs wider than $\sim$1$\farcs$4 are observed with a 400$\times$400
pixel field, and the widest pairs are sometimes recorded with the full field
of 1024 pixels (16″) and a 2$\times$2 binning.

The speckle power spectra are calculated and displayed immediately after
acquisition for quick evaluation of the results. Observations of close pairs
are accompanied by observations of single (reference) stars to account for
such instrumental effects as telescope vibration or aberrations. Bright stars
can be resolved and measured below the formal diffraction limit by fitting a
model to the power spectrum and using the reference. The resolution and
contrast limits of HRCam are further discussed in TMH10 and in the previous
papers of this series. The standard magnitude limit is $I\approx 12$ mag under
typical seeing; pairs as faint as $I\approx 16$ mag were measured under
exceptionally good seeing, albeit with reduced accuracy and resolution.

Custom software helps to optimize observations by selecting targets, pointing
the telescope, and logging. Typically, about 300 targets are covered on a
clear night. The observing programs are executed in an optimized way,
depending on the target visibility, atmospheric conditions, and priorities,
while minimizing the telescope slews. Reference stars and calibrator binaries
are observed alongside the main targets as needed; their observations are
published here as well.

###  2.3 Data Processing

The data processing is described in TMH10 and Tokovinin (2018). We use the
standard speckle interferometry technique based on the calculation of the
power spectrum and the speckle autocorrelation function (ACF). Companions are
detected as secondary peaks in the ACF and/or as fringes in the power
spectrum. Parameters of the binary and triple stars (separation $\rho$,
position angle $\theta$, and magnitude difference $\Delta m$) are determined
by modeling (fitting) the observed power spectrum. The true quadrant is found
from the shift-and-add (SAA) images whenever possible because the standard
speckle interferometry determines position angles modulo 180°. The resolution
and detection limits are estimated for each observation.

###  2.4 Calibration of Scale and Orientation

Since 2014, the pixel scale and angular offset of HRCam are inferred from
observations of several relatively wide (from 0$\farcs$5 to 3″) calibration
binaries, called calibrators for brevity. Their motion is accurately modeled
based on previous observations at SOAR (Tokovinin et al., 2015). Before 2014,
the HRCam calibration was derived from special experiments described in TMH10
and from the comparison of HRCam with the wide-field imager, SAMI, attached to
another port of SAM. The scale and orientation of both imagers were mutually
related by mapping the motion of a point source located at the SAM focal plane
and mounted on the translation stages. The orientation of SAMI was determined
from the astrometric solutions of sky images.

We revised here the list of calibrators and their models using all SOAR
observations from 2007.7 to 2021.75. After removing faint stars and binaries
with known or suspected subsystems, the list counts 103 calibrators. The
motion of 88 calibrators is modeled by their visual orbits, specially adjusted
to accurately represent the HRCam measurements (many of those orbits are of
low grade owing to the insufficient coverage); the remaining 15 pairs are
modeled by linear functions of the separation $\rho$ and position angle
$\theta$ vs. time.

Figure 1: Dependence of the rms residuals of calibrators on the separation.
Plus signs and squares correspond to radial $\Delta\rho$ and tangential
$\rho\Delta\theta$ residuals, respectively, and the dashed line is a linear
fit to both.

Using the new models of the calibrators, small corrections of the angular
offset and scale for each observing run were recomputed and applied. Figure 1
plots the rms residuals $\sigma$ between observations and models in the radial
and tangential directions vs. separation $\rho$. The residuals in both
directions are similar, and they can be jointly approximated by the linear
formula

$\sigma\approx 0.81+1.15\rho\;\;\;{\rm mas}.$ (1)

Thus, a 1″ binary is measured by HRCam with a typical accuracy of 2 mas. A
linear increase of measurement errors with separation is expected from the
physics of light propagation through the atmosphere (differential tilts).
Equation 18 from Kenyon et al. (2006) with parameters $C_{1}=500$ arcsec rad-1
m2/3 s1/2, typical for Cerro Pachón, binary separation of 1″, exposure time of
8 s, and baseline of 2 m (half telescope diameter) predicts the atmospheric
error of 0.5 mas, less than 2 mas according to (1) and roughly matching the
lower envelope of points in Figure 1. Note, however, that the calibrator’s
model errors and the residual calibration errors of individual runs also
contribute to $\sigma$ and these contributions increase linearly with $\rho$.
Calibrators with a large $\Delta m$ have larger residuals to models.

Figure 2: Dependence of the difference between SOAR and Gaia positions in
separation (top) and angle (bottom) on the separation. In the top plot, the
dashed line is a linear fit and the solid line is a scale factor.

The set of calibrators assures a good consistency between the observing runs,
but might contain small global offsets in angle and scale, being based
entirely on the HRCam data. An external comparison is needed to control the
systematics. Here we compare the updated calibrator models with the relative
positions of these pairs derived from the Gaia Early Data Release 3 (EDR3)
astrometry (Gaia collaboration, 2021) and referring to the nominal epoch
2016.0. For some closer pairs, the Gaia positions strongly deviate from the
2016.0 positions inferred from the models, suggesting errors in the Gaia
reductions. In most such pairs with bad or suspicious Gaia positons, there are
no parallaxes and proper motions (PMs) for one or both components. The
remaining 58 pairs demonstrate good consistency between Gaia positions and
models, allowing us to look for systematic differences.

The comparison between the good Gaia EDR3 calibrators and their models is
shown in Figure 2. A simple linear fit to the separation differences gives

$\rho_{\rm SOAR}-\rho_{\rm EDR3}\approx 4.8+3.11\rho\;\;\;{\rm mas},$ (2)

as shown by the dashed line in the top plot. The existence of an offset at
zero separation is not expected; it could result from the errors of the fit.
If real, this is probably an artifact of the Gaia measurement algorithm
related to blended sources (a similar offset was found in the Gaia DR2
positions). If only the 26 calibrators wider that 1$\farcs$5 are selected, the
mean scale factor is $\rho_{\rm SOAR}/\rho_{\rm EDR3}=1.0053$, and its rms
scatter is 0.0009. The formal error of the mean scale factor is thus
$10^{-4}$. The bottom panel of Figure 2 clearly shows an offset between the
position angles of Gaia EDR3 and SOAR. The mean offset is $\theta_{\rm
SOAR}-\theta_{\rm EDR3}=-0\fdg 21$, the rms scatter is 0$\fdg$095, and the
formal error of the mean is 0$\fdg$02.

In the light of the comparison of calibrators with Gaia, the separations
measured by HRCam so far must be divided by the scale factor of 1.0053
(decrease by half a percent), and all position angles must be increased by
0$\fdg$2\. These corrections have a negligibly small effect on the orbits of
close binaries derived from our data, but are relevant when a small wobble in
the motion of wide pairs is studied and when the HRCam data are used jointly
with the Gaia relative positions. These corrections are applied to the models
of the calibrators and to the measurements published here.

Our strategy of observing wide pairs with slow motion for retroactive
calibration, adopted from the beginning of the SOAR speckle program, has
proven to be correct. Although the orbits of most such binaries are not of
high quality, they can accurately represent the observed motion and serve for
comparison with Gaia. The alternative strategy of using good-quality orbits of
fast and close pairs, adopted by some other speckle programs (e.g. Horch et
al., 2021), delivers a substantially inferior calibration precision. For
example, a 100 mas pair measured with a 1 mas accuracy cannot constrain the
calibration to better than 1%, even if its orbit were known exactly.

##  3 Results

###  3.1 Data Tables

The results (measures of resolved pairs and non-resolutions) are presented in
exactly the same format as in Tokovinin et al. (2021). The long tables are
published in machine-readable format; here we describe their content.

Table 2: Measurements of Double Stars at SOAR  Col. | Label | Format | Description, units
---|---|---|---
1 | WDS | A10 | WDS code (J2000)
2 | Discov. | A16 | Discoverer code
3 | Other | A12 | Alternative name
4 | R.A. | F8.4 | R.A. J2000 (deg)
5 | Decl. | F8.4 | Declination J2000 (deg)
6 | Epoch | F9.4 | Julian year (yr)
7 | Filt. | A2 | Filter
8 | $N$ | I2 | Number of averaged cubes
9 | $\theta$ | F8.1 | Position angle (deg)
10 | $\rho\sigma_{\theta}$ | F5.1 | Tangential error (mas)
11 | $\rho$ | F8.4 | Separation (arcsec)
12 | $\sigma_{\rho}$ | F5.1 | Radial error (mas)
13 | $\Delta m$ | F7.1 | Magnitude difference (mag)
14 | Flag | A1 | Flag of magnitude differenceaaFlags: q – the quadrant is determined; * – $\Delta m$ and quadrant from average image; : – noisy data or tentative measures.
15 | (O$-$C)θ | F8.1 | Residual in angle (deg)
16 | (O$-$C)ρ | F8.3 | Residual in separation (arcsec)
17 | Ref | A9 | Orbit referencebbReferences are provided at https://www.astro.gsu.edu/wds/orb6/wdsref.txt
Table 3: Unresolved Stars  Col. | Label | Format | Description, units
---|---|---|---
1 | WDS | A10 | WDS code (J2000)
2 | Discov. | A16 | Discoverer code
3 | Other | A12 | Alternative name
4 | R.A. | F8.4 | R.A. J2000 (deg)
5 | Decl. | F8.4 | Declination J2000 (deg)
6 | Epoch | F9.4 | Julian year (yr)
7 | Filt. | A2 | Filter
8 | $N$ | I2 | Number of averaged cubes
9 | $\rho_{\rm min}$ | F7.3 | Angular resolution (arcsec)
10 |  $\Delta m$(0.15) | F7.2 | Max. $\Delta m$ at 0$\farcs$15 (mag)
11 |  $\Delta m$(1) | F7.2 | Max. $\Delta m$ at 1″(mag)
12 | Flag | A1 | : marks noisy data
Table 4: Notes  WDS | Comment
---|---
02346$-$4210 | This is WDSS 0234367$-$421019 (HIP 11989), see 2020ApJS..247…66H
02434$-$6643 | B is fainter than A in $y$ filter, but brighter in $I$.
02415$-$7128 | HIP 12548 is a quadruple system, orbits in 2022AJ….163..161T
03260$-$3558 | B 1449BC is a triple in a young (40 Myr) cluster, first 41 yr orbit here.
03284+2248 | The subsystem BAG 2Aa,Ab is not detected in 2018-2021, spurious?
03478$-$1854 | This is WDSS 0347502$-$185407 (TIC 121088959), see 2020ApJS..247…66H
04293$-$3124 | SIG 4 is a brown dwarf. Noisy measures, poor accuracy. Orbit $P=55$ yr.
04375+1509 | CHR 153 contains a subsystem, orbits in 2021AJ….161..144T
04404+1631 | CHR 154 is a triple in the Hyades, orbits in 2021AJ….161..144T
05303$-$6653 | A small $\Delta I=-0.06$ mag an does not match $\Delta G=0.93$ mag, variable?
05321$-$0305 | The resolution of V1311Ori Ba,Bb is reported in 2022AJ….163..127T
05351$-$0249 | A and B components of SKF2259 (Haro 5-2, a PMS star) were resolved
| into subsystems by Bo Reipurth (2022, in preparation).
06035+1941 | MCA 24 is a B8III triple with a 14-day subsystem. The outer orbit is updated here.
06314+0749 | Triple system, orbits in 2020AJ….160…69T
07498$-$0317 | This is WDSS 0347502$-$185407, see 2018MNRAS.480.4884E
08240$-$1548 | HIP 41171 is a quadruple system, see 2019AJ….158..222T
11100$-$6645 | The resolution is reported by Powell et al. 2021AJ….162..299P
| This is a TESS object with enigmatic eclipses, apparently by dust.
11221$-$2447 | This is a PMS quadrule HD 98800, orbits in 2021A&A…655A..15Z
13175+2024 | The subsystem in YSC 129 (HIP 64386) belongs to A, not to B.
| Preliminary orbits indicate periods of 34 and 5 yr for AB and Aa,Ab,
respectively.
15234$-$5919 | B is red, fainter than A in the $V$ band but brighter in $I$.
16243$-$5921 | The measured pair (0$\farcs$99, $\Delta I=4.2$) is found in Gaia EDR3 at the same position.
| The parallax is 0.17 mas, so it cannot be the 0$\farcs$3 pair HDS2317Aa,Ab
17289$-$3244 | AB (2$\farcs$36) is WDSS 1728556$-$324356, BC is a new pair, see the text.
19250$-$3205 | AB is WDSS 1924594$-$320432 (TIC 11247221), see 2020ApJS..247…66H;
| The 0$\farcs$08 pair Aa,Ab is new.
21145$-$7403 | This is WDSS 2114277$-$74024 (HIP 104556), see 2021MNRAS.506.2269E

Table 2 lists 2,623 measures of 2,123 resolved pairs and subsystems, including
new discoveries. The pairs are identified by their WDS-style codes based on
the J2000 coordinates and discoverer designations adopted in the WDS catalog
(Mason et al., 2001), as well as by alternative names in column (3), mostly
from the Hipparcos catalog. Equatorial coordinates for the epoch J2000 in
degrees are given in columns (4) and (5) to facilitate matching with other
catalogs and databases. In the case of resolved multiple systems, the position
measurements and their errors (columns 9–12) and magnitude differences (column
13) refer to the individual pairings between components, not to their
photocenters. As in the previous papers of this series, we list the internal
errors derived from the power spectrum model and from the difference between
the measures obtained from two data cubes. The real errors are usually larger,
especially for difficult pairs with substantial $\Delta m$ and/or with small
separations. Residuals from orbits and from the models of calibrators,
typically between 1 and 5 mas rms, characterize the external errors of the
HRcam astrometry (Figure 1).

The flags in column (14) indicate the cases where the true quadrant is
determined (otherwise the position angle is measured modulo 180°), when the
relative photometry of wide pairs is derived from the long-exposure images
(this reduces the bias caused by speckle anisoplanatism), and when the data
are noisy or the resolutions are tentative (see TMH10). For binary stars with
known orbits, the residuals to the latest orbit and its reference are provided
in columns (15)–(17). Residuals close to 180° mean that the orbit swaps the
brighter (A) and fainter (B) stars. However, in some binaries the secondary is
fainter in one filter and brighter in other (e.g. 15234$-$5919). In these
cases, it is better to keep the historical identification of the components in
agreement with the orbit and to give a negative magnitude difference $\Delta
m$.

Nonresolutions are reported in Table 3. Its first columns (1) to (8) have the
same meaning and format as in Table 2. Column (9) gives the minimum resolvable
separation when pairs with $\Delta m<1$ mag are detectable. It is computed
from the maximum spatial frequency of the useful signal in the power spectrum
and is normally close to the formal diffraction limit $\lambda/D$. The
following columns (10) and (11) provide the indicative dynamic range, i.e. the
maximum magnitude difference at separations of 0$\farcs$15 and 1″,
respectively, at $5\sigma$ detection level. The last column (12) marks noisy
data by the flag “:”. In Table 4 we provide short notes on some pairs, mostly
the alternative designations and relevant references.

###  3.2 New Pairs

Figure 3: Fragments of speckle ACFs of some newly resolved triple stars. The
intensity and spatial scales are chosen for best representation of each
system. North is up, east left. Blue letters mark the ACF peaks corresponding
to the component’s location (as opposed to other symmetric peaks), O marks the
ACF center. The outer and inner separations are indicated.

Table 5: New Double Stars  WDS | Name | $\rho$ | $\Delta m$ | Programaa HIP – Hipparcos suspected binary; MSC – multiple system; REF – reference star; YMG – young moving groups; NEG – neglected pair; TESS – TESS followup.
---|---|---|---|---
J2000 |  | (arcsec) | (mag) |
00272$-$7324 | TOK 908 | 0.24 | 1.7 | HIP
00438$-$3911 | TOK 909 | 0.66 | 3.6 | HIP
01333$-$5142 | TOK 910 | 0.36 | 2.6 | HIP
02035$-$3005 | RST 2270 Aa,Ab | 0.22 | 3.1 | MSC
03230$-$7047 | HEI 630 BC | 0.12 | 0.8 | MSC
03385$+$1336 | YR 10 Ab1,Ab2 | 0.04 | 0.7 | MSCbbSee comments in the text.
05053$-$4208 | BRT 668 Aa,Ab | 0.15 | 2.2 | TESS
05192$-$0304 | A 53 Ba,Bb | 0.37 | 1.9 | MSCbbSee comments in the text.
05427$-$7316 | HEI 667 AC | 1.35 | 4.4 | NEG
06005$-$2753 | HDS 819 Ba,Bb | 0.09 | 1.6 | NEG
06159$-$5126 | RST 179 BC | 0.07 | 1.0 | MSC
06223$-$2021 | TDS 3781 BC | 0.12 | 1.4 | MSC
07018$-$1118 | HU 112 Ba,Bb | 0.04 | 0.1 | MSCbbSee comments in the text.
08321$-$2730 | HDS 1220 BC | 0.04 | 0.2 | NEG
08420$-$0109 | A 1750 Ba,Bb | 0.12 | 2.7 | NEGbbSee comments in the text.
08471$-$4504 | TOK 911 | 0.04 | 1.0 | REF
09046$-$4104 | RSS 189 Aa,Ab | 0.06 | 0.3 | HIPbbSee comments in the text.
09343$-$3223 | RST 2637 Aa,Ab | 0.22 | 3.3 | MSC
10350$-$5247 | B 1685 BC | 0.40 | 2.4 | TESS
11247$-$6139 | BSO 5 Aa,Ab | 0.25 | 2.8 | MSC
12059$+$2628 | HDS 1707 Ba,Bb | 0.07 | 1.5 | HIP
12118$-$5306 | HDS 1722 AB | 0.83 | 5.6 | WDS
12282$+$2502 | TOK 912 | 0.03 | 0.7 | HIP
13514$+$2620 | SKF 260 Ba,Bb | 0.25 | 0.2 | MSCbbSee comments in the text.
13592$-$4528 | TOK 913 | 1.20 | 4.6 | REF
17131$-$8552 | JNN 320 Ba,Bb | 0.06 | 0.1 | YMGbbSee comments in the text.
17289$-$3244 | TOK 914 BC | 0.32 | 2.0 | YMGbbSee comments in the text.
17373$-$4300 | SEE 510 Aa,Ab | 0.24 | 3.7 | WDS
17374$-$3544 | HDS 2488 AC | 2.24 | 5.3 | NEG
18153$-$4928 | TOK 915 | 0.07 | 0.0 | YMG
18248$-$0621 | TOK 320 Ba,Bb | 0.13 | 1.3 | MSC
18285$-$4129 | BRT 1058 Aa,Ab | 0.17 | 0.2 | YMGbbSee comments in the text.
18436$-$5138 | RST 5451 Ba,Bb | 0.18 | 0.3 | NEG
18483$-$3710 | JSP 787 Aa,Ab | 0.59 | 3.3 | MSC
18581$-$2953 | SKF 1507 Aa,Ab | 0.07 | 1.6 | YMG
18581$-$2953 | SKF 1507 Ba,Bb | 0.37 | 0.0 | YMGbbSee comments in the text.
19250$-$3205 | TOK 916 Aa,Ab | 0.08 | 0.9 | YMG
20008$-$2411 | TOK 917 | 0.70 | 2.6 | HIP
20446$-$6630 | TOK 918 | 0.08 | 0.1 | HIP
20557$-$6609 | HDS 2981 AC | 1.32 | 6.3 | WDS
21433$-$4327 | HDS 3095 Ba,Bb | 0.11 | 3.1 | HIP
21510$+$2911 | A 889 Aa,Ab | 0.03 | 0.4 | MSC
22039$-$2451 | SEE 465 Aa,Ab | 0.65 | 1.7 | MSCbbSee comments in the text.
22039$-$2451 | SEE 465 Ba,Bb | 0.15 | 1.9 | MSCbbSee comments in the text.
22146$-$2142 | TOK 919 | 0.09 | 1.1 | MSC
22538$-$3559 | HDS 3255 Ba,Bb | 0.19 | 2.1 | NEG
23214$-$1340 | HDS 3326 Aa,Ab | 0.05 | 0.0 | NEG
23224$-$5857 | TOK 920 | 0.07 | 0.1 | HIP
23293$-$5135 | TOK 921 | 0.25 | 2.7 | HIP
23465$-$4135 | TOK 922 | 0.74 | 3.4 | HIP

Table 5 highlights the 50 pairs resolved in 2021 or resolved earlier but not
yet published. All measurements of these pairs are found in Table 2. The pairs
are identified by the WDS-style codes and the discovery codes or other names.
The following columns contain the separation $\rho$, the magnitude difference
$\Delta m$, and the observing program. Two pairs were found independently by
others, but we keep them in the table for consistency because they are not yet
featured in the WDS or its supplement, WDSS. The majority of newly discovered
binaries belong to hierarchical systems with three or more components.
Typically, new close subsystems in known visual binaries were discovered by
HRCam. Comments on some multiple systems are given below, and the speckle ACFs
of selected triples are shown in Figure 3.

03385+1336 (HIP 16991, F8) is a quadruple system at 95 pc distance. The outer
pair A,B has a separation of 77″, the intermediate pair Aa,Ab (YR 10) is at
0$\farcs$27, and the newly discovered subsystem Ab1,Ab2 is at 0$\farcs$04\.
The estimated period of Ab1,Ab2 is $\sim$10 yr, and it should leave an imprint
on the observed motion of Aa,Ab.

05192$-$0304 (HIP 28419, K3V). The subsystem Ba,Bb in a 1$\farcs$5 binary A 53
has been discovered by Hirsch et al. (2021) using adaptive optics. They
measured it at 243$\fdg$4 and 0$\farcs$520 in 2015.410 and determined the
magnitude difference of $\Delta K_{\rm Ba,Bb}=1.2$ mag; another observation
was made in 2017.931. Our measurement in 2021 shows that Ba,Bb has retrograde
motion and is closing down; its estimated period is 25 yr. The outer pair A,B
had a separation of 4$\farcs$9 in 1900, and now it closed down to 1$\farcs$5\.
This low-mass triple system is located at 15.7 pc from the Sun.

07018$-$1118 (HIP 33868, B2V). The main star A is an eclipsing binary GU CMa
with a close tertiary companion discovered by eclipse timing. The resolution
of Ba,Bb at 0$\farcs$04 (AB is at 0$\farcs$65) makes this a quintuple
hierarchical system. The estimated period of Ba,Bb is 25 yr. More observations
are needed to confirm the close pair Ba,Bb and to follow its motion. It cannot
be excluded that the eclipsing pair belongs to star B, in which case its
tertiary component is identical to Bb and the system is quadruple rather than
quintuple.

08420$-$0109 (HD 74113, A6V) is a strange case. The orbit of the main pair A,B
(A 1750) with a period of $P=250$ yr and a semimajor axis of $a=0\farcs 205$
is determined here. The new faint star Bb with a separation of 0$\farcs$12
from Ba would make the system dynamically unstable, unless its true separation
is larger than $\sim$0$\farcs$6\. In such case, Bb is an outer companion, and
Ba,Bb looks close only in projection. The detection of Ba,Bb is securely
confirmed in three observing runs. This subsystem is also apparent in our 2018
observation, although it was overlooked at the time because the ACF was
distorted by telescope vibration. Gaia does not give the parallax. The sky is
not crowded, making a random superposition of unrelated stars at such close
separation extremely unlikely. A similar 0$\farcs$48 pair ADS 6941
(BD$-$00°2045), located at 137″ from AB, also lacks Gaia parallax and PM, and
its relation to this triple is presently unknown.

09046$-$4104 (HIP 44550, G1V) is a three-tier hierarchical system. The inner
pair is an eclipsing binary V405 Vel with a 10 day period. It is orbited by
star Ab at 0$\farcs$06 discovered here with an estimated period of 20 yr. This
subsystem explains the large astrometric noise in Gaia. The outermost
component B, at 9$\farcs$5 separation, has common PM and parallax.

13514+2620 (HIP 6723, K6V) is a low-mass 2+2 quadruple at 46 pc distance.
Aa,Ab is a known 0$\farcs$3 pair YSC 50, and Ba,Bb at 0$\farcs$25 is
discovered here. Both inner pairs have estimated periods of $\sim$50 yr and
large mass ratios. The outer pair A,B at 3$\farcs$3 has a period of the order
of 1 kyr. Gaia did not measure the parallaxes of A and B because they are
close binaries.

17131$-$8552 (GSC 09526$-$00895, M0V) is an interesting triple system where
the estimated periods of the outer 0$\farcs$26 and inner 0$\farcs$06 pairs are
25 and 3 yr, respectively. All three stars have comparable magnitudes. Gaia
gives no parallax for this star.

17289-3244 (HD 317617, TYC 7379-279-1, K3V). This triple system has been
resolved in 2015.41 by Bonavita et al. (2021) in a survey of nearby young
stars. They measured BC at 318$\fdg$2, 0$\farcs$416 with $\Delta H_{\rm
BC}=1.9$ mag.

17373$-$4300 (HIP 86228, $\theta$ Sco, F1III) was observed on request by R.
Argyle because it was suspected to be a close pair in Hipparcos. The newly
resolved high-contrast 0$\farcs$24 pair is different from the known 6$\farcs$2
pair SEE 510. The latter in fact is spurious, because it was not seen in the
wide-field images taken with HRCam, and the secondary is not found in Gaia.
The Hipparcos measurement of this pair at 6$\farcs$5 separation is spurious,
probably caused by the wrongly interpreted signal from the close pair measured
here.

18248$-$0621 (HIP 90246, K7V) is another nearby (42 pc) 2+2 quadruple system
discovered here. The outer binary is at 55″ separation. Its components A and B
are revealed to be close pairs with separations of 0$\farcs$31 and 0$\farcs$13
and estimated periods of 40 yr and 15 yr, respectively. The Gaia astrometry of
A and B has large noise owing to their binary nature, but the Gaia parallaxes
and PMs of A and B confirm that they form a physical system.

18285$-$4129 (TYC 7909-2501-1) is a physical triple system. The WDS pair BRT
1058 (5$\farcs$2, 178°, $\Delta m=2$ mag) is not found in Gaia, but a much
fainter ($G=15.48$ mag) Gaia star at 6$\farcs$1, 205$\fdg$0 from A is a
physical companion with common PM and parallax. Star A is resolved here into a
0$\farcs$17 pair, although its designation as BRT 1058Aa,Ab appears
misleading.

18581$-$2953 (TYC 6872-1011-1, M0V) is another nearby 2+2 quadruple discovered
here by resolving both components of the 28″ pair into 0$\farcs$07 and
0$\farcs$37 subsystems. The closer one, Aa,Ab, is expected to have a short
period of $\sim$10 yr. The system belongs to a young moving group. Star A was
observed at Keck by Ruane et al. (2019), but this close pair was hidden behind
the coronagraphic mask of 0$\farcs$25 radius, preventing its detection.

22039$-$2451 (HIP 108923, G6V) contains five components in a hierarchical
configuration and is at 57 pc distance from the Sun. The outermost component C
is at 122″ from the 3$\farcs$4 pair AB (SEE 465 AB). We resolved here both
components of SEE 465 into close pairs Aa,Ab (0$\farcs$66) and Ba,Bb
(0$\farcs$15) with estimated periods of 130 yr and 20 yr, respectively, and
substantial magnitude differences. Gaia measured matching parallaxes of A, B,
and C, while their PMs differ owing to the motion in the inner subsystems.
Although Gaia did not resolve Aa,Ab explicitly, it is marked as having double
transits, indicating the detection of Ab.

Some wide pairs resolved here are also found in the Gaia EDR3. When parallaxes
and PMs of both components are given, the wide pair can be classified as
physical (e.g. 13592$-$4528) or optical (chance projection), as
17374$-$3544AC.

###  3.3 New and Updated Orbits

Orbits of visual binary stars is a traditional battleground of human ingenuity
against inaccurate data and insufficient coverage. Availability of computing
power added to various historical methods developed during two centuries such
approaches as statistical sampling (e.g. Anguita-Aguero et al., 2022) and a
brute-force exploration of the multidimensional parameter space (Blunt et al.,
2020). Space astrometry and radial velocities (RVs) are also used nowadays for
orbit calculation (Brandt et al., 2021). This field is rejuvenated by recent
imaging of substellar and planetary companions and the need to infer their
orbits from the short observed arcs. The situation will be further exacerbated
by millions of binaries expected in the future data releases of Gaia, e.g. one
million binaries published by El-Badry et al. (2021), and lacking coverage of
their orbits.

Positional measurements provided by the SOAR speckle program contain a rich
material for calculation of new visual orbits and improvement of the known
ones. Some orbits resulting from this program are published in (Anguita-Aguero
et al., 2022; Gómez et al., 2022; Tokovinin, 2021a) and in the circulars of
the IAU Commission G1. The quality of these orbits ranges from tentative and
preliminary (grades 5 and 4) to good, reliable, and excellent (grades 3 to 1).
Orbits are graded using the code and methodology described in ORB6 (Hartkopf,
Mason & Worley, 2001).

Table 6 lists elements of 174 new and corrected orbits determined from the
observations made at SOAR in 2021\. The complementary Table 7 contains
additional 108 preliminary orbits where no reliable estimates of the element’s
errors could be derived. Most preliminary orbits have grades 4 and 5. The
constraints on these orbits are insufficient for meaningful estimates of the
elements’ errors, and some of these orbits will be improved or even
dramatically revised in the future as new data are accumulated. Calculation
and improvement of orbits is a never ending process where orbital solutions
progressively become more reliable and accurate, while new low-grade
preliminary orbits are being added to the catalog. Note however that some
grade 4 orbits are placed in Table 6 because the errors are well defined.
Conversely, some orbits of official grade 3 are still considered preliminary
and placed in Table 7. A one-dimensional grading system cannot grasp all
aspects of the orbit quality; it is useful for a first-order evaluation. The
errors of the elements and the plots provided in the ORB6 catalog give a more
comprehensive picture.

Most orbits were fitted by the weighted least-squares method using the IDL
program ORBIT (Tokovinin, 2016c). The weights are proportional to
$\sigma^{-2}$, and the measurement errors $\sigma$ are assigned depending on
the data source: 2 to 5 mas for speckle interferometry with telescopes of 4 m
class and Gaia, 10 mas for Hipparcos, 50 mas and larger for visual micrometer
measures. Some outlying visual measures are ignored. We also use the radial
velocities where available. Some orbits were fitted using the grid search
algorithm (Hartkopf, McAlister & Franz, 1989). Different systems of weights
are adopted by other orbit computers; usually more weight is given to the
historical micrometer data at the expense of a worse fit to the accurate
speckle positions.

The orbital elements and their errors (determined by the least-squares
fitting) are given in Table 6 in the standard notation. The last columns
contain the grade and the reference codes to previously published orbits, when
available. The full bibliographic references are provided in the ORB6 online
catalog.

Table 6: Visual Orbits with Element’s Errors [Fragment]  WDS | Discov. | $P$ | $T$ | $e$ | $a$ | $\Omega$ | $\omega$ | $i$ | Grade | Ref.aaReferences to previous orbits are provided in wdsref.txt. Additional references are: Circ204 – Circular of IAU Commission G1, No. 204, 2021; Gomez2022 – Gómez et al. (2022); Horch2021 – Horch et al. (2021);
---|---|---|---|---|---|---|---|---|---|---
HIP |  | (yr) | (yr) |  | (arcsec) | (deg) | (deg) | (deg) |  |
00219$-$2300 | RST5493 BC | 18.441 | 2019.527 | 0.831 | 0.1338 | 91.3 | 213.6 | 92.1 | 3 | Tok2021c
|  |  $\pm$0.555 |  $\pm$0.447 |  $\pm$0.034 |  $\pm$0.0204 |  $\pm$1.5 |  $\pm$16.0 |  $\pm$1.7 |  |
00277$-$1625 | YR 1 Aa,Ab | 6.586 | 2014.311 | 0.762 | 0.0675 | 167.2 | -0.9 | 18.5 | 2 | Tok2020e
|  |  $\pm$0.027 |  $\pm$0.035 |  $\pm$0.015 |  $\pm$0.0012 |  $\pm$29.9 |  $\pm$31.5 |  $\pm$9.0 |  |
00324$+$0657 | MCA 1 Aa,Ab | 27.506 | 1989.001 | 0.810 | 0.1593 | 105.8 | 14.3 | 110.8 | 2 | Jte2018
|  |  $\pm$0.051 |  $\pm$0.105 |  $\pm$0.025 |  $\pm$0.0022 |  $\pm$0.8 |  $\pm$2.3 |  $\pm$2.3 |  |
01077$-$1557 | HDS 148 | 15.493 | 2019.120 | 0.980 | 0.0738 | 11.0 | 213.0 | 66.8 | 3 | Tok2021c
|  |  $\pm$0.755 |  $\pm$0.207 | fixed |  $\pm$0.0152 |  $\pm$7.1 |  $\pm$20.3 |  $\pm$11.1 |  |

Table 7: Preliminary Visual Orbits [Fragment]  WDS | Discov. | $P$ | $T$ | $e$ | $a$ | $\Omega$ | $\omega$ | $i$ | Grade | Ref.aaSee Notes to Table 6
---|---|---|---|---|---|---|---|---|---|---
|  | (yr) | (yr) |  | (arcsec) | (deg) | (deg) | (deg) |  |
00258$+$1025 | HDS 57 | 69.179 | 2020.847 | 0.471 | 0.1413 | 43.6 | 59.2 | 137.0 | 4 | Tok2021c
00304$-$6236 | JNN 296 Aa,Ab | 60.644 | 2016.792 | 0.664 | 0.3132 | 82.1 | 24.6 | 130.1 | 4 | first
00348$-$5853 | I 439 | 350.000 | 2020.615 | 0.960 | 0.7021 | 119.4 | 146.2 | 48.9 | 4 | Tok2021c
01114$+$1526 | BEU 2 | 62.568 | 1993.862 | 0.000 | 0.4676 | 120.5 | 0.0 | 51.4 | 4 | Tok2021c
01117$+$0835 | HDS 158 | 47.008 | 2023.553 | 0.853 | 0.1985 | 132.0 | 172.6 | 63.8 | 4 | first

Figure 4: Updated orbits of two calibrators, STF 1196 AB (top) and STF 2262 AB
(bottom). Accurate speckle measurements are plotted as squares (red color
marks the 2021 observations published here), the less accurate micrometric
observations as crosses. Dotted lines connect the positions to the locations
on the orbit (ellipse). The scale is in arcseconds, and the main component is
at coordinate origin.

Figure 4 shows updated orbits of two calibrator binaries. Although the orbit
of STF 1196 ($\zeta$ Cnc AB) has been determined many times in the past, its
latest update by Izmailov (2019) shows appreciable systematic residuals to the
accurate SOAR and Gaia positions. This pair has been measured more than 1000
times since its discovery by W. Struve in 1825, mostly using visual
micrometers. However, fitting the orbit to all available data, suitably
weighted, does not lead to a good result because the numerous but
intrinsically inaccurate visual measurements pull the solution in the wrong
direction. Here we use only the micrometer measurements made by W. and O.
Struve in the 19th century and ignore all other measurements except speckle
interferometry with telescopes of 1.8 m aperture or larger. This pair has been
frequently observed by speckle interferometry since 1975 as a calibrator, and
the speckle data now cover most of its 60 yr orbit. Our new accurate orbit
makes this binary a first class calibrator. It is bright, has a small
magnitude difference, and is accessible from both hemispheres. Incidentally,
the pair belongs to the nearby (25 pc) quintuple multiple system $\zeta$ Cnc,
and the orbit of its subsystem Ca,Cb with a period of 17.3 yr is also updated
here. With an accurate parallax from future Gaia data releases, both orbits
will yield accurate masses.

The bottom panel of Figure 4 shows a more typical calibrator where accurate
measurements cover only a small arc of the long-period orbit. The elements
fitted with appropriate weights accurately represent this arc. The 18
calibrated SOAR measurements have rms residuals of 3.6 mas and agree very well
with the Gaia EDR3 position.

Figure 5: Four orbits computed here for the first time but, nevertheless,
relatively well constrained. See the caption to Figure 4.

Figure 5 illustrates some orbits determined here for the first time. The
bright ($V=3.0$ mag) star $\zeta$ CMa (HIP 30122, HR 2282, 06203$-$3005) has
been resolved at SOAR in 2019.95 and is designated as TOK 821 in the WDS. A
spectroscopic orbit with a period of 675 days has been published by Colacevic
(1941). Fixing the period to its spectroscopic value, the remaining visual
elements are fitted to the 7 measurements made at SOAR so far. Using the
Hipparcos parallax of 9.0$\pm$0.13 mas, the orbit gives a mass sum of 12.2
${\cal M}_{\odot}$ for this pair of spectral type B2.5V.

The K5V dwarf HIP 40724 (08187$-$1512) has been resolved at SOAR in 2018.25
into a binary with a substantial magnitude difference $\Delta I\approx 2.5$
mag. Its orbit with $P=2.5$ yr uses the unpublished measure in 2016.04
communicated by T. Henry (2021, private communication). The pair passed
through the periastron in 2021, and its measurements at close separations are
tentative. With the parallax of 28.85$\pm$0.56 mas, the orbit corresponds to
the mass sum of 1.4 ${\cal M}_{\odot}$ . The classical visual binary B 227
(12216$-$2716, HIP 60281, F5V) was discovered by van den Bos (1928) in 1926.
Its 92 yr orbit uses the micrometer measurements (with suitable quadrant
adjustment, considering the small $\Delta m$), two published speckle-
interferometric measurements in the 1980s, and two measurements made at SOAR
in 2017 and 2021. Finally, the faint pair of M-type dwarfs CRC 73 (16488+1039)
discovered in 2014 by Cortes-Contreras et al. (2017) and observed at SOAR
three times in 2018–2021 has a quasi-circular orbit with $P=10.7\pm 1.2$ yr.
Despite the grade 4, the orbit is well constrained, and the errors of its
elements are modest.

###  3.4 Spurious Pairs

Table 8: Likely Spurious Pairs  WDS | Discoverer | Resolved | UnresolvedaaAdditional indications of the spurious nature of visual pairs: R – no excess noise in Gaia EDR3, RUWE$<$2; L – long estimated period;
---|---|---|---
00077$-$5615 | HDS 15 | 0$\farcs$1 HIP | 2021, L, R
00259$-$3112 | HDS 58 | 0$\farcs$2 HIP | 2021, L, R
00292$-$3755 | HDS 65 | 0$\farcs$2 HIP | 2021, L, R
00374$-$3904 | WHI 1 | 0$\farcs$3 Sp 1987 | 2021, R
00386$-$3903 | WHI 2 | 0$\farcs$8 Sp 1987 | 2021, L, R
00537$-$7910 | HDS 116 | 0$\farcs$1 HIP | 2021, L, R
00546$-$8240 | HDS 120 | 0$\farcs$2 HIP | 2021, L, R
01475$-$0753 | HDS 240 | 0$\farcs$1 HIP | 2021, L, R
01511$-$7832 | HDS 251 | 0$\farcs$2 HIP | 2021, L, R
01519$-$3120 | HDS 254 | 0$\farcs$2 HIP | 2021, L, R
01571$-$5031 | VOU 20 | 0$\farcs$4 Vis 1929 | 2021, L, R
02088$-$6126 | HDS 286 | 0$\farcs$1 HIP | 2021, R
02109$-$7558 | HDS 293 | 0$\farcs$3 HIP | 2014-21
02376$-$3659 | B 678AB | 0$\farcs$7 Vis 1927 | 2021
03023$-$7154 | FIN 360 | 0$\farcs$1 Vis 1961 | 2021, R
03031$-$2339 | DAM 1296 | 0$\farcs$1 2015 | 2021
03047$-$3410 | WHI 4 | 0$\farcs$2 Sp 1987 | 2021, L, R
03096$-$0100 | HDS 405 | 0$\farcs$3 HIP | 2021, L, R
04112+1538 | CHR 202 | 0$\farcs$1 Sp 1985-86 | 2018-21, R
04118$-$2444 | DAM 312 Aa,Ab | 0$\farcs$1 | 2021, R
04129$-$1107 | HDS 534 | 0$\farcs$2 HIP | 2021, L, R
04416$-$4302 | WHI 6 | 0$\farcs$4 Sp 1987 | 2021
04513$-$6804 | FIN 362 | 0$\farcs$1 Vis 1961 | 2010-21
04562$-$0331 | HDS 640 | 0$\farcs$2 HIP | 2021, L, R
05093$-$3813 | HDS 677 | 0$\farcs$2 HIP | 2021, L, R
05343$-$3626 | HDS 737 | 0$\farcs$1 HIP | 2021,L, R
05397$-$0042 | BAL 677 Aa,Ab | 1$\farcs$0 2003 | 2021, L, R
05488$-$2507 | B 90 | 0$\farcs$2 Vis 1925 | 2021, R
06113$-$1635 | HDS 844 | 0$\farcs$1 HIP | 2021, L
06125$-$3515 | HDS 849 | 0$\farcs$2 HIP | 2021, L, R
06226$-$6238 | HDS 872 | 0$\farcs$1 HIP | 2021, L, R
06358$-$1028 | HDS 901 | 0$\farcs$2 HIP | 2021, R
06447$-$3213 | PRO 27 | 1$\farcs$1 Vis 1911 | 2021, L
06596$-$2754 | SEE 73 | 0$\farcs$3 Vis 1897 | 2021, R
07374$-$3330 | HDS 1079 | 0$\farcs$5 HIP | 2021, L, R
08227$-$1555 | HDS 1194 | 0$\farcs$2 HIP | 2021, L, R
08542$-$6142 | RST 4901 | 0$\farcs$3 Vis 1942-69 | 2018-21, L,R
15219$-$3445 | HDS 2162 | 0$\farcs$1 HIP | 2021, L, R
15596$-$5612 | HDS 2251 | 0$\farcs$1 HIP | 2021, L, R
16015$-$5416 | HDS 2258 | 0$\farcs$2 HIP | 2021, L, R
16048$-$6902 | HDS 2271 | 0$\farcs$2 HIP | 2021, L, R
16118$-$5532 | HDS 2290 | 0$\farcs$2 HIP | 2021, L, R
16243$-$5921 | HDS 2317Aa,Ab | 0$\farcs$3 HIP | 2021, L, R
16398$-$4706 | HDS 2363 | 0$\farcs$1 HIP | 2018-21, L, R
17001$-$1639 | HDS 2406 | 0$\farcs$2 HIP | 2021, R
17026$-$5504 | HDS 2411 Aa,Ab | 0$\farcs$2 HIP | 2021, L, R
17105$-$1343 | HDS 2425 | 0$\farcs$2 HIP | 2021, R
17118$-$2626 | HDS 2429 | 0$\farcs$2 HIP | 2021, L, R
17238$-$2138 | HDS 2456 | 0$\farcs$1 HIP | 2021, R
17253$-$5600 | HDS 2459 | 0$\farcs$2 HIP | 2021, L, R
17254$-$1643 | HDS 2461 | 0$\farcs$2 HIP | 2017-21, L, R
17317$-$0959 | HDS 2474 | 0$\farcs$1 HIP | 2021, R
17419$-$0944 | HDS 2503 | 0$\farcs$1 HIP | 2021, R
17482$-$2801 | HDS 2514 | 0$\farcs$1 HIP | 2021, L, R
17492$-$3315 | HDS 2515 | 0$\farcs$2 HIP | 2021, L, R
17582$-$1916 | CHR 66 | 0$\farcs$4 Sp 1983 | 2021, L, R
17583$-$5010 | HDS 2533 | 0$\farcs$2 HIP | 2021, L
17587$-$1152 | HDS 2535 | 0$\farcs$1 HIP | 2021, R
18052$-$3921 | HDS 2549 | 0$\farcs$3 HIP | 2021, L, R
18174$-$2730 | HDS 2585 | 0$\farcs$2 HIP | 2021, L, R
18197$-$4542 | CHR 148 | 0$\farcs$3 Sp 1989 | 2008-21
18218$-$5526 | HDS 2597 | 0$\farcs$1 HIP | 2021, L, R
18272$+$0012 | STF2316Aa,Ab | 0$\farcs$1 vis 1951 | 2009-21
18320$-$0607 | HDS 2629 | 0$\farcs$1 HIP | 2021, L, R
18351$-$1659 | MCA 52 | 0$\farcs$2 Sp 1980 | 2017-21, L, R
18387$-$1429 | HDS 2641 AB | 0$\farcs$1 HIP | 2014-21
18423$-$0720 | HDS 2649 | 0$\farcs$1 HIP | 2008-21, R
18501$-$0823 | HDS 2670 | 0$\farcs$1 HIP | 2013-21, L, R
19020$-$1705 | OCC 9061 | 0$\farcs$1 Occ 2014 | 2021, R
19024$-$2541 | HDS 2699 | 0$\farcs$2 HIP | 2021, R
19044$-$0541 | HDS 2704 | 0$\farcs$2 HIP | 2021, R
19205$-$0525 | ISO 10 Aa,Ab | 0$\farcs$5 Sp 1987 | 2021
19223$-$2226 | I 1399 | 0$\farcs$4 Vis 1925 | 2021
19230$-$0144 | HDS 2743 | 0$\farcs$1 HIP | 2021, L, R
19293$-$6833 | HDS 2769 | 0$\farcs$2 HIP | 2021, L, R
19317$-$3153 | VOU 85 | 0$\farcs$1 Vis 1937 | 2021, L, R
19383$-$1527 | ARU 16 | 0$\farcs$7 Sp 1984 | 2021, L, R
19420$-$2223 | HDS 2793 | 0$\farcs$1 HIP | 2021, L, R
19425$-$1607 | BU 1288 | 0$\farcs$2 Vis 1889 | 2021, R
19460$-$0735 | HDS 2808 | 0$\farcs$1 HIP | 2021, L, R
19492+0817 | ARU 17 | 0$\farcs$4 Sp 1984 | 2021, R
20007$-$1757 | ARU 18 | 0$\farcs$4 Sp 1984 | 2021, L, R
20020$-$1754 | ARU 19 | 0$\farcs$4 Sp 1984 | 2021, L, R
20029$-$8455 | HDS 2859 | 0$\farcs$2 HIP | 2021, L, R
20109$-$0637 | ARU 20 | 0$\farcs$5 Sp 1984 | 2021, L, R
21104$-$4114 | ARU 3 | 0$\farcs$2 Sp 1984 | 2021, R
21415$-$7723 | BLM 6 | 0$\farcs$1 Sp 1976 | 2021
21420$-$2940 | HDS 3090 | 0$\farcs$1 HIP | 2021, L, R
22456$-$8529 | JSP 836 | 0$\farcs$7 Vis 1929 | 2021, R
22461$-$1210 | ARU 5 | 0$\farcs$3 Sp 1982 | 2014-21
22473$-$3410 | CHR 189 AB | 0$\farcs$3 Sp 1992 | 2015-21, R
22532$-$1102 | HDS 3249 | 0$\farcs$2 HIP | 2021, L, R
23114$-$4259 | B 594 | 0$\farcs$2 vis 1925-63 | 2008-21
23256$-$2556 | HDS 3332 | 0$\farcs$2 HIP | 2021, L, R

In 2021, we observed a substantial number of “neglected” pairs that have only
one documented resolution and no further confirming observations. Our
measurements provide such confirmation and in several cases reveal additional
close subsystems (section 3.2). Some pairs were not resolved because orbital
motion decreased their separation below the detection limit of HRCam. Such
binaries are candidates for future observations and, eventually, orbit
calculation. However, the majority of unresolved double stars are spurious
discoveries. A list of spurious pairs was presented in (Tokovinin et al.,
2019); here we continue this effort and list another 94 apparently spurious
pairs in Table 8. Its columns contain WDS code, discoverer designation,
separation in arcseconds, observing method (HIP — Hipparcos, Sp — speckle
interferometry, Vis — visual micrometer, Occ — occultation), and years when
the pair was resolved. The last column gives the years when the pair was not
resolved at SOAR (some targets have multiple observations) and additional
indications from Gaia that the star is single, explained below. Pairs in Table
8 have an ‘X’ code added to their entry in the WDS, indicating the pair is not
real.

Gaia EDR3 does not contain measurements of close double stars, but gives
several clues concerning their reality. Pairs with separations from
0$\farcs$15 to 0$\farcs$7 and modest magnitude difference have no parallaxes
and PMs in EDR3. Gaia astrometric solutions with excessive noise caused by
companions are distinguished by the reduced unit weight error (RUWE)
parameter, which is less than 1.4 for point sources and can be larger for
binaries. Furthermore, the Gaia parallaxes allow us to evaluate orbital
periods from projected separations. Take for example the first entry in Table
8, 00077$-$5615 (HDS 15). Hipparcos measured in 1991.25 a separation of
0$\farcs$142 that corresponds to 153 au, given the parallax of 0.93 mas from
Gaia EDR3. The orbital semimajor axis cannot be less than half of the
separation, so the minimum period computed by the third Kepler’s law is 481 yr
(assuming a mass sum of 2 ${\cal M}_{\odot}$ ). The real period is likely
longer, so the hypothesis that this pair has closed down in 30 yr between the
Hipparcos and SOAR observations cannot be true. HDS 15 is thus definitely
spurious, and RUWE=1.0 confirms this. The letters L (long-period) and R (small
RUWE) in the last column of Table 8 mark similar cases. An opposite example is
11557$-$1438 (RST 3761). This pair has been consistently unresolved at SOAR
between 2018 and 2021. However, it was measured several times between 1937 and
1989, has a RUWE=2.3, and the parallax of 4.5 mas suggests an orbital period
on the order of 200 yr. So, RST 3761 is a real binary, it just closed down at
present. In our future work, we will check neglected pairs using Gaia to avoid
wasting telescope time for their observation.

Most entries in Table 8 are Hipparcos pairs with separations of 0$\farcs$2 or
less. This is below the diffraction limit of the 30 cm Hipparcos aperture. The
majority of Hipparcos pairs with comparable separations are real, and a small
number of spurious resolutions could be caused by problems in the data
reduction. Spurious binaries discovered by visual observers and never
confirmed are also found in our list. The third group of spurious pairs
results from speckle interferometry. Several false discoveries have been made
by HRCam and later retracted (see Tokovinin et al., 2019). The discovery codes
ARU (Argue et al., 1985), WHI (White et al., 1991), and ISO (Isobe, 1991)
correspond to early speckle programs that produced a number spurious pairs
owing to lack of experience with this technique.

##  4 Summary and outlook

The total number of observations made with HRCam to date is about 30,300. This
paper documents the observations made in 2021 and their use for improving the
orbits. It reports 50 newly resolved pairs (the majority of which are members
of triple and higher-order systems) and gives a list of likely spurious pairs
checked at SOAR.

Speckle interferometry was invented half a century ago and now it has
completely replaced the visual micrometer measurements. The time span of
speckle data becomes comparable to the 100–200 yr of visual coverage, and in
some cases the latter loses its scientific value, as illustrated by the first
orbit in Figure 4. Furthermore, the historic visual double stars, still a
majority of the WDS entries, are complemented by the new Hipparcos and speckle
pairs. Millions of Gaia pairs will soon completely dominate over the present-
day sample of double stars. So, will the current speckle programs be viable in
the future, or will they progressively lose their importance, retaining only a
historic value, like the visual micrometer measurements or the Ptolemeus
astrometric catalog? We believe that in the near term, the demand for new
speckle data will actually increase, being driven by Gaia and other missions.

The Gaia catalog already contains accurate relative positions, motions, and
parallaxes of many thousands of double stars wider than $\sim$0$\farcs$7 and
gives hints on the closer pairs (missing parallaxes or a large RUWE). Accurate
parallaxes add value to the visual orbits, enabling good measurement of
stellar masses. Although the current Gaia parallaxes of visual binaries are
often biased by orbital motion, this caveat will be corrected in the next data
release 3 which will account for acceleration. Unbiased parallaxes of single
components in visual triples, available now in the EDR3, can also serve for
the mass measurent of their inner orbital pairs.

The huge future impact of Gaia on the binary star research can be gleaned by
looking back at the Hipparcos mission, which revealed thousands of close pairs
missed by the historic visual surveys. Follow-up speckle monitoring of the
Hipparcos binaries (including the work reported here) leads to the
determination of their orbits and masses. The Gaia binaries will require a
similar follow-up, but their large number will necessitate a selective
strategy. Determination of all potentially accessible orbits (the default
approach until now) will be replaced by the astrophysically motivated samples.
Obvious candidates are low-mass nearby stars (see e.g. Vrijmoet et al., 2022),
high-mass stars, pre-main-sequence stars, binaries hosting exoplanets, and
hierarchical systems. So, the demand for new observations of close visual
binaries will likely increase in the near future.

We are grateful to the anonymous referee for a thorough check of the data. We
thank the SOAR operators for efficient support of this program, and the SOAR
director J. Elias for allocating some technical time. This work is based in
part on observations carried out under CNTAC programs CN2019A-2, CN2019B-13,
CN2020A-19, CN2020B-10, and CN2021B-17. R.A.M. and E.C. acknowledge support
from the FONDECYT/CONICYT grant No. 1190038. The research of A.T. is supported
by the NSFs NOIRLab. This work used the SIMBAD service operated by Centre des
Données Stellaires (Strasbourg, France), bibliographic references from the
Astrophysics Data System maintained by SAO/NASA, and the Washington Double
Star Catalog maintained at USNO. This work has made use of data from the
European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia)
processed by the Gaia Data Processing and Analysis Consortium (DPAC,
https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has
been provided by national institutions, in particular the institutions
participating in the Gaia Multilateral Agreement. SOAR.

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