# The POKEMON Speckle Survey of Nearby M dwarfs. I. New Discoveries

Catherine A. Clark Northern Arizona University, 527 South Beaver Street,
Flagstaff, AZ 86011, USA  Lowell Observatory, 1400 West Mars Hill Road,
Flagstaff, AZ 86001, USA  Gerard T. van Belle Lowell Observatory, 1400 West
Mars Hill Road, Flagstaff, AZ 86001, USA  Elliott P. Horch Southern
Connecticut State University, 501 Crescent Street, New Haven, CT 06515, USA
Kaspar von Braun Lowell Observatory, 1400 West Mars Hill Road, Flagstaff, AZ
86001, USA  David R. Ciardi NASA Exoplanet Science Institute Caltech/IPAC,
Pasadena, CA 91125, USA  Jennifer G. Winters Center for Astrophysics $|$
Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA  Rocio Kiman
Kavli Institute for Theoretical Physics, University of California, Santa
Barbara, CA 93106, USA

(Received 2022 April 7; Revised 2022 May 5; Accepted 2022 May 24)

###### Abstract

M dwarfs are favorable targets for exoplanet detection with current
instrumentation, but stellar companions can induce false positives and inhibit
planet characterization. Knowledge of stellar companions is also critical to
our understanding of how binary stars form and evolve. We have therefore
conducted a survey of stellar companions around nearby M dwarfs, and here we
present our new discoveries. Using the DSSI speckle imager at the 4.3-meter
Lowell Discovery Telescope, and the similar NESSI instrument at the 3.5-meter
WIYN telescope, we carried out a volume-limited survey of M-dwarf multiplicity
to 15 parsecs, with a special emphasis on including the later M dwarfs that
were overlooked in previous surveys. Additional brighter targets at larger
distances were included for a total sample size of 1070 M dwarfs. Observations
of these 1070 targets revealed 26 new companions; 22 of these systems were
previously thought to be single. If all new discoveries are confirmed, then
the number of known multiples in the sample will increase by 7.6%. Using our
observed properties, as well as the parallaxes and 2MASS $K$ magnitudes for
these objects, we calculate the projected separation, and estimate the mass
ratio and component spectral types, for these systems. We report the discovery
of a new M-dwarf companion to the white dwarf Wolf 672 A, which hosts a known
M-dwarf companion as well, making the system trinary. We also examine the
possibility that the new companion to 2MASS J13092185-2330350 is a brown
dwarf. Finally, we discuss initial insights from the POKEMON survey.

stars: binaries: visual — stars: imaging — stars: low-mass — stars: statistics
— solar neighborhood

††journal: AJ††facilities: LDT(DSSI), WIYN(NESSI)††software: Astropy (Astropy
Collaboration et al., 2013), IPython (Pérez & Granger, 2007), Matplotlib
(Hunter, 2007), NumPy (Harris et al., 2020), Pandas (McKinney et al., 2010),
SciPy (Virtanen et al., 2020)

D

##  1 Introduction

Though they are the smallest and least luminous stars on the Main Sequence,
the M dwarfs occupy a captivating range of stellar parameter space. Their
masses span nearly a factor of ten (Baraffe & Chabrier, 1996), and the lowest-
mass M dwarfs have Main Sequence lifetimes of trillions of years (Laughlin et
al., 1997). Additionally, the M dwarfs dominate the galactic neighborhood,
accounting for approximately 75% of the stars in the Milky Way (Henry et al.,
2006). The M dwarfs also offer a significant opportunity for finding and
characterizing Earth-sized planets (e.g., López-Morales et al., 2019), due to
their relatively small star-to-planet mass and radius ratios, as well as their
sheer numbers.

However, one property that can inhibit the detection and characterization of
the planets that orbit M dwarfs is stellar multiplicity. “Third light”
contamination of light curves, which is caused by stellar companions, has been
shown to inhibit the detection of Earth-sized, transiting planets (Lester et
al., 2021), and has led to additional obstacles in planet characterization
including underestimated planet radii (Ciardi et al., 2015), skewed planet
radius distributions and occurrence rates (Hirsch et al., 2017; Teske et al.,
2018; Bouma et al., 2018), incorrect characterization of both stars’
properties (Furlan & Howell, 2020), and improper mean density and atmospheric
values (Howell, 2020). Additionally, close-in stellar companions can perturb
and truncate protoplanetary disks (Jang-Condell, 2015), gravitationally excite
planetesimals causing collisional destruction (Rafikov & Silsbee, 2015a, b),
and scatter and eject planets that have formed (Haghighipour & Raymond, 2007).
Furthermore, recent studies have suggested that even wide stellar companions
might affect the formation or orbital properties of giant planets (e.g.
Fontanive et al., 2019; Fontanive & Bardalez Gagliuffi, 2021; Hirsch et al.,
2021; Mustill et al., 2022; Su et al., 2021). As the M dwarfs have proven to
be such favorable targets for planet detection and characterization, measuring
their multiplicity is therefore crucial to understanding the planets that they
host.

Furthermore, work on Kepler (Borucki et al., 2011), K2 (Howell et al., 2014),
and now TESS (Ricker et al., 2015) suggests that the stellar companions to
exoplanet hosts have longer orbital periods than the companions to field stars
(e.g., Kraus et al., 2012; Bergfors et al., 2013; Wang et al., 2014; Kraus et
al., 2016; Ziegler et al., 2020; Hirsch et al., 2021; Howell et al., 2021;
Lester et al., 2021; Moe & Kratter, 2021; Clark et al., 2022). Multiplicity
measurements are therefore critical to understanding M-dwarf system
architectures and occurrence rates as well.

Because of the significance M dwarfs have for both stellar astrophysics and
exoplanet studies, many surveys have been carried out to determine the M-dwarf
multiplicity rate, especially in recent years. It has been shown that
multiplicity decreases with mass from O to M (e.g., Mason et al., 2009;
Raghavan et al., 2010; Duchêne & Kraus, 2013; Winters et al., 2019), but this
multiplicity has not been fully characterized for M dwarfs, particularly for
the later sub-types, due to their faintness and to resolution limits. This is
highlighted by the fact that a reasonably complete inventory of later M dwarfs
did not exist until recently (e.g., Kirkpatrick et al., 2014; Luhman &
Sheppard, 2014; Winters et al., 2021). Winters et al. (2019) produced the most
comprehensive M-dwarf multiplicity study to date: an all-sky, volume-limited
survey that extends to 25 pc. However, their study surveyed binaries with
separations larger than $2\arcsec$. The study presented here therefore
complements the Winters et al. (2019) survey by exploring the inner regions
around nearby M dwarfs with high-resolution imaging.

Here we present the 26 new discoveries detected throughout the Pervasive
Overview of ‘Kompanions’ of Every M dwarf in Our Neighborhood (POKEMON)
survey, which identified companions to nearby M dwarfs at large-telescope,
diffraction-limited resolution. The POKEMON survey is volume-limited through
M9, out to at least 15 pc, with additional brighter targets at larger
distances, resulting in a sample of 1070 nearby M dwarfs. A future paper will
present the full sample of M dwarfs we surveyed, the completeness of the
survey, and our updated M-dwarf multiplicity rate. Additionally, though Janson
et al. (2012) calculated M-dwarf multiplicity by spectral sub-type through M6,
our upcoming paper will establish the M-dwarf multiplicity rate by sub-type
through M9 for the first time. This initial paper in the series presents and
characterizes the new discoveries that have been revealed by the POKEMON
survey.

In Section 2, we describe our target selection process, observational routine,
and data reduction procedure. In Section 3, we note systems with both a new
discovery and a known companion, and we provide observed properties for the
systems with new discoveries. We also assess the likelihood that these new
discoveries are bound. Additionally, we estimate astrophysical properties for
the systems with new discoveries using the observed properties and parallaxes
from the literature. In Section 4, we discuss a system with both a known M
dwarf and a known white dwarf, a system with a potential brown dwarf
companion, and initial insights from the POKEMON survey. We summarize our
conclusions and discuss future work in Section 5.

##  2 Observations

The POKEMON survey used speckle interferometry (Labeyrie, 1970), also known as
speckle imaging, to observe 1070 M dwarfs throughout the northern sky.

###  2.1 Target Selection

The initial basis for our target selection was the seminal Catalog of Nearby
Stars (CNS3; Gliese & Jahreiß 1991), with updates from recent nearby neighbor
discoveries from surveys such as RECONS (Henry et al., 2006; Winters et al.,
2015) and 2MASS (Skrutskie et al., 2006). A preliminary review of these
augmented CNS3 data indicated 522 objects out to 15 pc either identified
specifically as M dwarfs, or as potential M dwarfs from $H-K$ color and
absolute magnitude $M_{V}$. Out to 25 pc, this review resulted in
approximately 1,500 objects.

We expanded this list to include the AllWISE Motion Survey (Kirkpatrick et
al., 2014) with 3,525 new high proper motion objects, and the similar (but
non-overlapping) study by Luhman & Sheppard (2014) that identified 762 high
proper motion objects. An examination of the former’s new high proper motion
objects, using the spectral type-color relationships from Luhman & Sheppard
(2014), indicated that there were $\sim 350$ additional objects between the
spectral types of M4 and M9. Other surveys that were mined included the
Database of Ultracool Parallaxes based on the Hawaii Infrared Parallax Program
(Dupuy & Liu, 2012), the CARMENES and APOGEE input catalogs (Alonso-Floriano
et al., 2015; Deshpande et al., 2013), and the 2016 release of the Pan-STARRS
Parallax and Proper Motion Catalog (Waters et al., 2015), which already had a
specific focus on identifying nearby low-mass stars (Magnier et al., 2015).

Given our use of northern hemisphere facilities, a declination cut of
$\delta>-30^{o}$ was carried out. A cut at 15.5 in the $I$ band was also
necessary for most objects due to the faint limit of the instruments. These
cuts resulted in a sample consisting of 1070 objects.

We note that the parallax sources we used to create the original sample were
not perfect; newer astrometric data from Gaia (Gaia Collaboration, 2018,
2020a) have now provided us with more accurate and precise parallax
measurements that indicate some stars in our sample are at distances larger
than 15 pc. We chose to keep these stars in the sample despite being farther
than previously thought. Parallaxes for these objects are discussed further in
Section 3.3.

We present our sample in Figure 1, which is an Aitoff projection showing sky
locations of the single and multiple stars in the full sample, compared with
the sky locations of the 26 new companions.

Figure 1: The sky locations of the 1070 stars in the POKEMON sample. 290 of
these are known multiples. Single stars are marked with open black circles,
known multiples are marked with filled black circles, and the 26 systems with
new discoveries are marked with larger, red stars.

###  2.2 Observational Routine

We imaged the 1070 M dwarfs in the POKEMON sample over 50 nights between UT
2017 April 7 and UT 2020 February 10. The main instrument used in the survey
was the Differential Speckle Survey Instrument (DSSI; Horch et al., 2009),
which was resident at the 4.3-meter Lowell Discovery Telescope (LDT) in Happy
Jack, AZ, throughout the POKEMON survey. For brighter targets ($V<11$), we
used the NN-EXPLORE Exoplanet Stellar Speckle Imager (NESSI; Scott et al.,
2018), which was used on the 3.5-meter WIYN telescope at Kitt Peak National
Observatory outside Tucson, AZ.

DSSI and NESSI produce diffraction-limited images from speckle patterns
observed simultaneously at two wavelengths. Each instrument uses a dichroic
filter to split the collimated beam from the telescope at $\sim$700 nm into
two channels that are imaged on separate high-speed readout EMCCDs. The
standard filter arrangement is 692 and 880 nanometers (nm) for DSSI, and 562
and 832 nm for NESSI, each with 40, 44, or 50 nm bandpasses. The limiting
spatial resolution of these instruments at the telescopes we used is $\sim 40$
milliarcseconds (mas), which is comparable to the near-infrared adaptive
optics observing on the Keck II Telescope. These speckle cameras can therefore
identify stellar companions down to the $\sim 1$ au scale.

Bright objects ($V<11$) require only $\sim 1-2$ minutes of observing time,
during which the speckle camera obtains a single image cube of 1,000
40-millisecond speckle frames; these short exposures are necessary to “freeze”
out the atmosphere in our observations and to obtain good speckle contrast.
Fainter stars ($11<V<15.5$) require up to $\sim 10$ minutes of observing time
per target, during which the speckle camera obtains up to nine image cubes.
Standard observing also includes periodic observations of bright, unresolved,
single stars from the Bright Star Catalog (Hoffleit & Jaschek, 1982) to probe
the atmospheric conditions experienced by the target of interest. All data
cubes are stored as multi-extension FITS files.

The pixel scale and image orientation are empirically confirmed by observing
binaries with extremely well-known orbits (those listed as Grade 1 in the
Sixth Orbit Catalog; Hartkopf et al., 2001). Their ephemeris positions are
computed based on the orbital elements, and their scale and orientation are
derived from these results. We assume that the calibrations from Horch et al.
(2021) are also appropriate for this work, given that our targets were
observed on the same runs, besides those observed in February 2020. Figure 3
of Horch et al. (2021) shows the derived uncertainties from the known orbital
elements that we have incorporated into our analysis. However, Horch et al.
(2021) did not include data from February 2020. To reduce and analyze this
data, we used pixel scale values that were approximate, and that may be
updated in the future; the position angle and angular separation values from
this run may therefore be updated in the future as well.

It should be noted during our first observing run, from UT 2017 April 7 to UT
2017 April 17, the narrowband speckle filters were not installed within DSSI.
When observing without filters, more light reaches the detector, but there is
less contrast in the speckles. Furthermore, when filters are not used,
significant chromatic effects arise at higher airmasses. We did find that the
majority of our newly detected companions were first, or only, detected during
this observing run. However, we returned to using the speckle filters for all
other observing runs in order to increase the contrast in the speckles.
Nonetheless, we plan to investigate the possibility of observing without
speckle filters further in the future, both to confirm companions observed
only during a single epoch, and to probe new speckle imaging discovery space.
The most unique discovery from our filterless observations is discussed
further in Section 4.2.

Our observations of the new companions are summarized in Table 1. The 2MASS
ID, common name or identifier (if applicable), UT date, telescope, bandpass
($\lambda$), and bandpass width ($\Delta\lambda$) are listed. For the
filterless observations, instead of listing the bandpass and bandpass width of
the observation, we instead indicate whether the companion was detected in the
“blue” ($\lambda\lessapprox 700$ nm) image or the “red” ($\lambda\gtrapprox
700$ nm) image. We note that many of the new discoveries were only detected in
the “red” image, since the M dwarfs emit most strongly at near-infrared
wavelengths.

Table 1: Summary of observations for targets with a new companion  2MASS ID | Name | UT Date | Telescope | $\lambda$ | $\Delta\lambda$
---|---|---|---|---|---
|  | (YYYY-MM-DD) |  | (nm) | (nm)
$03104962-1549408$ | LP 772-11 | 2017-10-20 | LDT | 880 | 50
$03542561-0909316$ |  | 2017-10-21 | LDT | 880 | 50
$07011725+1348085$ | G 110-29 | 2017-04-15 | LDT | red |
$07411976+6718444$ | LP 58-260 | 2017-04-17 | LDT | red |
$09510964-1219478$ | GJ 369 | 2017-05-07 | LDT | 880 | 50
$11030845+1517518$ | L 1258-55 | 2017-04-17 | LDT | blue |
|  | 2017-04-17 | LDT | red |
|  | 2018-01-31 | LDT | 692 | 40
|  | 2018-01-31 | LDT | 880 | 50
$12190600+3150433$ | LTT 13435 | 2017-04-17 | LDT | blue |
|  | 2017-04-17 | LDT | red |
|  | 2018-01-31 | LDT | 692 | 40
|  | 2018-01-31 | LDT | 880 | 50
$12435889-1614351$ | LP 796-1 | 2017-04-15 | LDT | red |
$13092185-2330350$ | CE 303 | 2017-04-16 | LDT | blue |
|  | 2017-04-16 | LDT | red |
$14235017-1646116$ |  | 2017-04-17 | LDT | red |
|  | 2018-01-31 | LDT | 880 | 50
$15020759+7527526$ | LP 22-174 | 2017-04-13 | LDT | red |
|  | 2019-09-14 | LDT | 880 | 50
$15085332+4934062$ |  | 2017-04-13 | LDT | red |
|  | 2020-02-09 | LDT | 880 | 50
$15211607+3945164$ | LP 222-70 | 2017-04-07 | LDT | red |
$15263317+5522206$ | LP 135-316 | 2017-04-07 | LDT | red |
$15434848+2552376$ | G 167-54 | 2017-04-12 | LDT | blue |
|  | 2017-04-12 | LDT | red |
$15471513+0149218$ | LP 623-40 | 2017-04-15 | LDT | red |
$16041322+2331386$ |  | 2017-04-12 | LDT | blue |
|  | 2017-04-12 | LDT | red |
|  | 2017-05-04 | LDT | red |
$17183572+0156433$ | Wolf 672 B | 2017-04-09 | LDT | blue |
|  | 2017-04-09 | LDT | red |
$17335314+1655129$ |  | 2017-04-17 | LDT | red |
|  | 2019-09-13 | LDT | 880 | 50
$18191622-0734518$ |  | 2017-04-11 | LDT | red |
$18523373+4538317$ | LHS 3420 | 2017-04-12 | LDT | red |
|  | 2019-09-13 | LDT | 880 | 50
$20081786+3318122$ | GJ 1250 | 2019-09-14 | LDT | 880 | 50
$21011610+3314328$ | L 1504-143 | 2017-10-21 | LDT | 692 | 40
|  | 2017-10-21 | LDT | 880 | 50
|  | 2018-08-02 | WIYN | 832 | 40
$21134479+3634517$ |  | 2018-08-29 | WIYN | 832 | 40
|  | 2019-09-13 | LDT | 880 | 50
$22520522-1532511$ | LP 821-27 | 2019-09-14 | LDT | 880 | 50
$23024353+7505591$ | LP 49-357 | 2019-01-20 | WIYN | 562 | 44
|  | 2019-01-20 | WIYN | 832 | 40

Figure 2: The newly detected companion to 2MASS J11030845+1517518, shown in
the reconstructed image on the left and in the detection plot on the right.
These data were taken on UT 2017 April 17. The companion is at a separation of
$0.39\arcsec$ (6.8 au), and its delta magnitude is measured to be 1.5.

###  2.3 Data Reduction

The data are reduced with a version of the bispectrum speckle reduction code
described in Horch et al. (2009, 2011a, 2011b), which uses bispectral analysis
(Lohmann et al., 1983) to compute a reconstructed image. This code uses the
fact that binary systems produce a fringe pattern in the Fourier plane. A 2-D
autocorrelation function is calculated for each speckle frame and summed over
all frames. The Fourier transform of the autocorrelation function is found and
squared to obtain the power spectrum, which is normalized. After dividing by
the power spectrum of a point source, the residual 2-D power spectrum appears
as a set of fringes for each pair of stars in the field. This is fit using a
cosine-squared function to determine the relative astrometry and photometry
(position angle, angular separation, and delta magnitude) of any pairs of
stars in the field. Reconstructed images are constructed from the object’s
modulus (the square-root of the power spectrum), and the phase estimate is
obtained from the bispectrum.

By examining annuli in the reconstructed image that are centered on the
primary star, we determine all local maxima and minima in the annulus, and
derive their mean value and standard deviation. We then estimate the detection
limit as the mean value of the maxima plus five times the average sigma of the
maxima and minima. In doing so, the data reduction pipeline produces a curve
of this detection limit as a function of separation. Example data
demonstrating the reconstructed image and contrast curve are shown in Figure
2.

##  3 Results

Our observations of 1070 nearby M dwarfs revealed 145 known multiples and 26
previously undetected stellar companions. A future paper will present our
detections of known multiples; here we present the new discoveries. In this
section, we discuss systems that host both a new discovery and a known
companion. We also report observed and astrophysical properties for the new
discoveries; the observed properties were measured via our speckle
observations, and the astrophysical properties were estimated using the
observed properties as well as parallaxes from the literature.

###  3.1 Known Companions

Four of the 26 systems with new discoveries host known companions from the
literature as well. This means that these systems are now trinary, or in the
case of 2MASS J21011610+3314328, quaternary, as the B component of the system
(2MASS J21012062+3314280) hosts a close-in stellar companion as well (Janson
et al., 2014). Properties for the four systems with known companions are
included in Table 2.

In the case of 2MASS J17183572+0156433, the known companion is a white dwarf.
This system is discussed in detail in Section 4.1.

Table 2: Properties for systems with a previously known companion  2MASS ID | $\theta$ | $\rho$ |  $\Delta$m | Reference
---|---|---|---|---
| (∘) | ($\arcsec$) |  |
$12190600+3150433$ | 222 | 1.7 | 2.9 | Lamman et al. (2020)
$17183572+0156433$ | 139 | 13 | 0.1 | Luyten (1997)
$21011610+3314328$ | 95 | 57 | 0.9 | Luyten (1997)
$23024353+7505591$ | 209 | 4.0 | 1.3 | Luyten (1997)

###  3.2 New Discoveries

The observed properties for the new companions are reported in Table 3, where
we have included the 2MASS ID of the primary, date of observation (measured in
Besselian years), seeing, position angle ($\theta$), angular separation
($\rho$), delta magnitude ($\Delta$m), and bandpass ($\lambda$). Again, for
filterless observations, instead of listing the bandpass of the observation,
we instead indicate whether the companion was detected in the “blue”
($\lambda\lessapprox 700$ nm) image or the “red” ($\lambda\gtrapprox 700$ nm)
image. We also include entries for the known companion to 2MASS
J12190600+3150433, since it was detected in our observations as well. The
angular separation and delta magnitude distributions for the new discoveries
are shown in Figure 3. A scatter plot of delta magnitude versus angular
separation is shown in Figure 4.

Table 3: Observed properties for systems with a new companion  2MASS ID | Date | Seeing | $\theta$ | $\rho$ |  $\Delta$m | $\lambda$
---|---|---|---|---|---|---
| (2000+) | ($\arcsec$) | (∘) | ($\arcsec$) |  | (nm)
$03104962-1549408$ | 17.8028 | 0.86 | 281.3 | 0.3577 | 1.68 | 880
$03542561-0909316$ | 17.8055 | 1.23 | 39.5 | 2.9997 | 3.04 | 880
$07011725+1348085$ | 17.2874 | 1.1 | 93.7 | 1.2858 | 1.67 | red
$07411976+6718444$ | 17.2929 | 0.86 | 200.7 | 0.2085 | 0.70 | red
$09510964-1219478$ | 17.3477 | 1.73 | 38.5 | 0.4497 | 3.30 | 880
$11030845+1517518$ | 17.2931 | 0.72 | 48.8 | 0.3912 | 1.42 | blue
| 17.2931 | 0.81 | 48.1 | 0.3908 | 1.54 | red
| 18.0851 | 0.70 | 44.4 | 0.3694 | 1.53 | 692
| 18.0851 | 0.68 | 43.9 | 0.3680 | 1.00 | 880
$12190600+3150433$ | 17.2931 | 0.66 | 331.8 | 0.1458 | 0.18 | blue
| 17.2931 | 0.72 | 331.3 | 0.1415 | 0.09 | red
| 18.0851 | 0.61 | 297.3 | 0.1287 | 0.01 | 692
| 18.0851 | 0.57 | 298.5 | 0.1253 | 0.01 | 880
| 17.2931 | 0.66 | 218.8 | 1.7697 | 5.54 | blue
| 17.2931 | 0.72 | 222.9 | 1.6789 | 2.3 | red
| 18.0851 | 0.61 | 216.5 | 1.6852 | 2.64 | 692
| 18.0851 | 0.57 | 218.7 | 1.6814 | 3.72 | 880
$12435889-1614351$ | 17.2876 | 0.91 | 284.7 | 0.3420 | 0.45 | red
$13092185-2330350$ | 17.2906 | 0.63 | 105.1 | 1.1947 | 0.99 | blue
| 17.2906 | 1.03 | 102.5 | 1.1752 | 1.03 | red
$14235017-1646116$ | 17.2935 | 0.78 | 18.7 | 0.6604 | 3.29 | red
| 18.0852 | 0.63 | 289.2 | 0.6039 | 2.57 | 880
$15020759+7527526$ | 17.2825 | 0.56 | 130.5 | 0.6188 | 2.39 | red
| 19.7023 | 0.73 | 135.7 | 0.6256 | 0.56 | 880
$15085332+4934062$ | 17.2826 | 0.56 | 310.2 | 0.1574 | 1.50 | red
| 20.1085 | 1.34 | 336.5 | 0.2655 | 1.66 | 880
$15211607+3945164$ | 17.2662 | 0.86 | 90.6 | 0.2378 | 0.78 | red
$15263317+5522206$ | 17.2664 | 0.73 | 218.3 | 0.2280 | 1.13 | red
$15434848+2552376$ | 17.2799 | 0.71 | 262.4 | 0.2448 | 0.31 | blue
| 17.2799 | 0.75 | 262.0 | 0.2447 | 0.16 | red
$15471513+0149218$ | 17.2881 | 0.73 | 287.0 | 0.2650 | 1.72 | red
$16041322+2331386$ | 17.2800 | 0.63 | 109.0 | 0.1065 | 0.85 | blue
| 17.2800 | 0.61 | 107.7 | 0.0982 | 0.37 | red
| 17.3401 | 0.75 | 112.5 | 0.1018 | 0.45 | red
$17183572+0156433$ | 17.2719 | 1.32 | 55.6 | 0.5307 | 1.05 | blue
| 17.2719 | 1.28 | 55.0 | 0.5325 | 1.11 | red
$17335314+1655129$ | 17.2938 | 0.69 | 101.0 | 0.1443 | 1.06 | red
| 19.6995 | 1.05 | 62.6 | 0.3639 | 1.08 | 880
$18191622-0734518$ | 17.2775 | 0.74 | 132.9 | 0.5168 | 1.44 | red
$18523373+4538317$ | 17.2802 | 0.81 | 37.7 | 0.4920 | 0.54 | red
| 19.6995 | 0.99 | 28.7 | 0.4684 | 0.81 | 880
$20081786+3318122$ | 19.7026 | 0.69 | 237.6 | 0.4166 | 0.48 | 880
$21011610+3314328$ | 17.8048 | 1.39 | 348.7 | 0.2649 | 2.07 | 692
| 17.8048 | 1.19 | 347.8 | 0.2561 | 2.01 | 880
| 18.5853 | 1.15 | 355.2 | 0.2869 | 1.65 | 832
$21134479+3634517$ | 18.6600 | 0.65 | 10.6 | 0.3868 | 0.31 | 832
| 19.6995 | 1.15 | 5.5 | 0.3596 | 0.14 | 880
$22520522-1532511$ | 19.7027 | 0.60 | 252.4 | 0.7280 | 0.55 | 880
$23024353+7505591$ | 19.0532 | 0.81 | 3.0 | 0.4862 | 3.30 | 562
| 19.0532 | 0.63 | 3.7 | 0.4883 | 2.38 | 832

Figure 3: Angular separation (left) and I-band or “red” delta magnitude
(right) distributions for the 26 new discoveries revealed by the POKEMON
survey. These values were obtained from our DSSI and NESSI speckle
observations, and have not been corrected for anisoplanatism. Figure 4:
Scatter plot of delta magnitude versus angular separation for the 26 new
discoveries. Most new discoveries are measured to be within $\sim 0.7\arcsec$
of their primaries, though they span a range of delta magnitudes.

We calculate residuals by computing the difference between the observed
properties in each channel. The angular separation residuals have an average
value of 3.4 mas, with a standard deviation of 6.5 mas. The position angle
residuals have an average value of 0.6 degrees, with a standard deviation of
1.0 degrees. The delta magnitude residuals have an average value of 0.06, with
a standard deviation of 0.23. The residuals result from the subtraction of two
independent measurements with presumably the same uncertainty, so the
subtraction has an uncertainty that is $\sqrt{2}$ larger than the uncertainty
of either individual measure. This means that the uncertainty in a single
angular separation measure is given by 6.5 mas divided by $\sqrt{2}$, or 4.6
mas. The average uncertainty in position angle is then 0.7 degrees, and the
average uncertainty in delta magnitude is 0.16. When we take these values and
add them in quadrature to the calibration uncertainties from Horch et al.
(2021), we obtain values of 5.1 mas for the average angular separation
uncertainty, 2 degrees for the average position angle uncertainty, and 0.21
for the average delta magnitude uncertainty. These uncertainties were
propagated throughout our analysis described in Section 3.4. These values are
slightly larger than those derived in Horch et al. (2017), Colton et al.
(2021), or Horch et al. (2021); this is likely due to the faintless of our
targets, and the filterless observations that took place during the April 2017
observing run. In general, filterless observations reduce the precision of our
astrometry, but allow us to observe fainter companions.

###  3.3 Likelihood That the New Discoveries Are Bound

We note that it is highly unlikely these new discoveries are background stars
due to the small fields-of-view of the instruments. We find that the number of
stars brighter than $I=15.5$ within a $2\arcsec$ area on the sky is $\sim
0.0008$. This means that for our 26 detections, there is a $\sim 2.1\%$ chance
that even a single detection is a random background star. Nonetheless, we
investigate whether the new discoveries are bound in various ways.

First, Winters et al. (2019) list two of our targets found to host a new
companion as suspected multiples in their work. In the case of the first
suspected multiple, 2MASS J09510964-1219478, no astrometric parameters could
be determined in the Double and Multiple Systems Annex (Lindegren et al.,
1997), which could indicate that the star is actually a short-period
astrometric binary. In the case of the second suspected multiple, Winters et
al. (2019) used photometry from October 2013 to determine that 2MASS
J20081786+3318122 is overluminous. We detected a companion to 2MASS
J20081786+3318122 in 2019; there are therefore six years between the epochs of
observation, which indicates that the object inducing this overluminosity is a
common proper motion companion rather than a background contaminant.

We also assessed the boundedness of the new discoveries using second epoch
observations. Because the targets in the POKEMON sample are nearby, they have
high proper motions. Therefore, a second epoch observation can be taken to
test the new discoveries for common proper motion with the target star. Ten of
the 26 new discoveries have second epoch observations.

Additionally, we used the 2MASS identifiers of the targets found to have a new
companion to find their Early Data Release 3 (EDR3; Gaia Collaboration, 2020a)
identifiers, or in one case their Data Release 2 (DR2; Gaia Collaboration,
2018) identifier, on the Gaia archive111https://gea.esac.esa.int/archive. If a
secondary object was listed within $5\arcsec$ of the target, we calculated its
angular separation from the target using its coordinates in Gaia. If the
angular separation agreed with the separation we measured from our speckle
observations, then we examined whether the parallaxes and proper motions of
the secondary agreed with those of the primary to within $3\sigma$. We
determined that three of our new discoveries are bound using this methodology.

Moreover, nine of our 26 systems do not have parallaxes or proper motions
listed in Gaia. This often occurs for close-in binary systems, as Gaia is
unable to resolve the system, and the relative motion of the pair disrupts the
parallax measurements. The absence of Gaia parallaxes or proper motions for
these targets therefore suggests that the new discoveries we detected in these
nine systems are indeed real.

Furthermore, Vrijmoet et al. (2020) notes that three out of four unresolved
multi-star red dwarf systems within 25 pc in Gaia DR2 have a parallax error
larger than or equal to 0.32 mas, and parallaxes more than $\sim 10\%$
different than the long-term RECONS results. Thus, the large Gaia parallax
errors for five of the targets found to have a new companion could be caused
by the previously unresolved companions that we detected.

Finally, we examined the Gaia EDR3 re-normalized unit weight error (RUWE)
values associated with the targets found to have a new companion. The Gaia
RUWE metric acts like a reduced chi-squared, where large values can indicate a
poor model fit to the astrometry, assuming that the star is single. Single
sources typically have RUWE values of $\sim 1$, while sources with RUWE values
$>1.4$ are likely non-single or otherwise extended (Ziegler et al., 2020; Gaia
Collaboration et al., 2021). Following Vrijmoet et al. (2020), which surveyed
M dwarfs specifically, we use RUWE $>2$ to distinguish single and
(potentially) non-single sources, and find eight targets with elevated RUWE
values.

In Table 4, we list the parallaxes and proper motions, as well as their errors
and sources, for all targets found to have a new companion. We also include
entries for the three new discoveries that appear in Gaia. 17 of the targets
found to have a new companion have Gaia parallax estimates either from DR2 or
EDR3. Parallaxes for the other nine primaries were obtained using the Fourth
Edition of the General Catalogue of Trigonometric Stellar Parallaxes (YPC; van
Altena et al., 1995), the MEarth survey (Dittmann et al., 2014), and the URAT
Parallax Catalog (Finch et al., 2018). Proper motions were obtained either
from Gaia or the fourth US Naval Observatory CCD Astrograph Catalog (UCAC4;
Zacharias et al., 2012). We use $*$ to note suspected multiples from Winters
et al. (2019). We use † to note targets that have a second epoch observation.
We use ‡ to note the new discoveries that appear in Gaia. We use $\S$ to note
targets that do not have parallaxes or proper motions in Gaia EDR3. We use
$\|$ to note targets with a Gaia parallax error $>0.32$ mas. Finally, we use #
to note targets with RUWE $>2$.

Table 4: Parallaxes and proper motions

2MASS ID | Parallax | Parallax Error | Parallax Source | Proper Motion | Proper Motion Error | Proper Motion Source | RUWE
---|---|---|---|---|---|---|---
| (mas) | (mas) |  | (mas/yr) | (mas/yr) |  |
$03104962-1549408$ $\|$, $\\!$#$\|$, $\\!$#footnotemark:  | 23.2 | 0.7 | Gaia EDR3 | 325.0, 0.9 | 0.6, 0.7 | Gaia EDR3 | 12.8
$03542561-0909316$ ‡‡footnotemark:  | 47.41 | 0.02 | Gaia EDR3 | -95.41, 111.01 | 0.02, 0.02 | Gaia EDR3 | 1.3
| 47.54 | 0.03 | Gaia EDR3 | -96.52, 99.07 | 0.03, 0.03 | Gaia EDR3 | 1.9
$07011725+1348085$ ‡‡footnotemark:  | 24.52 | 0.04 | Gaia EDR3 | 415.50, -97.40 | 0.05, 0.04 | Gaia EDR3 | 1.1
| 24.1 | 0.1 | Gaia EDR3 | 407.4, -96.4 | 0.2, 0.1 | Gaia EDR3 | 2.8
$07411976+6718444$ $\\!$#$\\!$#footnotemark:  | 41.5 | 0.3 | Gaia EDR3 | -110.3, -273.4 | 0.1, 0.2 | Gaia EDR3 | 9.1
$09510964-1219478$ ** Winters et al. (2019) suspected multiple | 76.16 | 0.02 | Gaia EDR3 | 1137.72, -1455.38 | 0.02, 0.01 | Gaia EDR3 | 1.1
$11030845+1517518$ †, §†, §footnotemark:  | 55 | 8 | URAT | -419, -84 | 8, 8 | UCAC4 |
$12190600+3150433$ ††footnotemark:  | 35.20 | 0.08 | Gaia EDR3 | -295.61, 5.13 | 0.07, 0.08 | Gaia EDR3 | 1.2
$12435889-1614351$ §§ No Gaia EDR3 parallax or proper motions | 51 | 3 | URAT | -422, 74 | 8, 8 | UCAC4 |
$13092185-2330350$ | 66.60 | 0.1 | Gaia EDR3 | 15.7, -383.77 | 0.1, 0.07 | Gaia EDR3 | 1.0
$14235017-1646116$ ††footnotemark:  | 16.93 | 0.03 | Gaia EDR3 | -108.81, -85.83 | 0.03, 0.02 | Gaia EDR3 | 1.5
$15020759+7527526$ †, §†, §footnotemark:  | 58 | 3 | MEarth | -132, 160 | 8, 8 | UCAC4 |
$15085332+4934062$ †, $\\!$#†, $\\!$#footnotemark:  | 25.2 | 0.3 | Gaia EDR3 | -104.9, -13.3 | 0.3, 0.4 | Gaia EDR3 | 27
$15211607+3945164$ §§ No Gaia EDR3 parallax or proper motions | 45 | 1 | MEarth | -436, 178 | 8, 8 | UCAC4 |
$15263317+5522206$ §§ No Gaia EDR3 parallax or proper motions | 42 | 4 | MEarth | -111, 235 | 8, 8 | UCAC4 |
$15434848+2552376$ §§ No Gaia EDR3 parallax or proper motions | 45 | 2 | MEarth | -171, 317 | 8, 8 | UCAC4 |
$15471513+0149218$ $\|$, $\\!$#$\|$, $\\!$#footnotemark:  | 56.4 | 0.4 | Gaia EDR3 | -213.4, -65.4 | 0.4, 0.3 | Gaia EDR3 | 10
$16041322+2331386$ †, $\\!$#†, $\\!$#footnotemark:  | 47.7 | 0.2 | Gaia EDR3 | -162.3, 16.9 | 0.1, 0.2 | Gaia EDR3 | 9.8
$17183572+0156433$ $\|$, $\\!$#$\|$, $\\!$#footnotemark:  | 27.8 | 0.4 | Gaia EDR3 | -447.5, -283.4 | 0.4, 0.3 | Gaia EDR3 | 17
$17335314+1655129$ †, $\|$, $\\!$#†, $\|$, $\\!$#footnotemark:  | 60.9 | 0.5 | Gaia EDR3 | -135.0, -130.5 | 0.5, 0.4 | Gaia EDR3 | 16
$18191622-0734518$ §§ No Gaia EDR3 parallax or proper motions | 10.4 | 0.3 | Gaia DR2 | -175.1, -213.5 | 0.6, 0.5 | Gaia DR2 |
$18523373+4538317$ †, §†, §footnotemark:  | 46 | 2 | MEarth | 206, 465 | 8, 8 | UCAC4 |
$20081786+3318122$ *, §*, §footnotemark:  | 46 | 5 | YPC | 340, 375 | 8, 8 | UCAC4 |
$21011610+3314328$ †, $\|$, $\\!$#†, $\|$, $\\!$#footnotemark:  | 49.8 | 0.7 | Gaia EDR3 | 325.6, -164.2 | 0.6, 0.7 | Gaia EDR3 | 37
$21134479+3634517$ †, §†, §footnotemark:  | 51 | 2 | URAT | -15.7, -92.3 | 6.5, 6.9 | UCAC4 |
$22520522-1532511$ | 37.64 | 0.07 | Gaia EDR3 | 331.41, 14.47 | 0.08, 0.07 | Gaia EDR3 | 1.1
$23024353+7505591$ ‡‡footnotemark:  | 18.88 | 0.01 | Gaia EDR3 | 285.23, 22.88 | 0.02, 0.02 | Gaia EDR3 | 1.1
| 18.91 | 0.01 | Gaia EDR3 | 286.33, 17.36 | 0.01, 0.02 | Gaia EDR3 | 1.2

$\textdagger$$\textdagger$footnotetext:  Second epoch observation

$\textdaggerdbl$$\textdaggerdbl$footnotetext:  New discovery in Gaia

∥∥footnotetext:  Gaia parallax error $>0.32$ mas

##footnotetext:  RUWE $>2$

The only stars that do not have an indicator that they are non-single are
2MASS J13092185-2330350 and 2MASS J22520522-1532511. 2MASS J13092185-2330350
is a unique case where we obtained 46 data cubes on the same target because of
its faintness, which could explain why its small and faint companion did not
induce one of these indicators; this system is discussed further in Section
4.2. In the case of 2MASS J22520522-1532511, a secondary object does appear in
Gaia EDR3, but it does not have a parallax or proper motions listed; this
secondary object could potentially be the new discovery we detected.

###  3.4 Estimated Astrophysical Properties

Here we estimate projected separations, component masses and system mass
ratios, and component spectral types for the 26 systems with new discoveries,
assuming that there are no additional unknown components in each system (e.g.,
spectroscopic components).

We calculated projected separations for these 26 systems using our measured
angular separations and the parallax estimates given in Table 4.

We then estimated mass ratios for these 26 systems using the empirical
relationship between mass and luminosity (Delfosse et al., 2000; Benedict et
al., 2016). This relationship was recently calibrated for late-type stars by
Mann et al. (2019). Their code, which is publicly available on $\tt
github$222https://github.com/awmann/M_-M_K-, calculates a mass and its
uncertainty given a user-provided distance and apparent $K$ magnitude (and
uncertainties). We calculated our distances using the parallaxes from Table 4,
and obtained our $K$ photometry from 2MASS (Skrutskie et al., 2006). However,
the resolution for 2MASS is estimated to be 5”, and all of the new companions
detected throughout this survey are within 5” of their primary stars. This
means that the 2MASS $K$ magnitudes for these systems convolve the flux from
both the primary and secondary stars. In order to separate the 2MASS $K$
magnitudes into their component parts, we first estimated a $K$-band delta
magnitude using the method outlined by Lamman et al. (2020). For this we used
our measured speckle $I$-band delta magnitude, or the “red” delta magnitude in
the case of the filterless observations, and the PARSEC theoretical stellar
evolution models (Marigo et al., 2017).

We then derived two equations to solve for the $K$ magnitude of the primary
and secondary stars. For this we used the $K$-band delta magnitude and the
2MASS $K$ magnitude of the primary star.

The first equation we derived is simply the difference between the secondary
$K$ magnitude and the primary $K$ magnitude:

$\Delta K=K_{2}-K_{1}$ (1)

The second equation originated from the equation for apparent magnitude, which
is defined as a logarithmic luminosity ratio of a body to some standard.

$m=-2.5\log_{10}(\frac{L}{L_{0}})$ (2)

The combined apparent magnitude of the binary system can then be written as

$m_{binary}=-2.5\log_{10}(\frac{L_{binary}}{L_{0}})$ (3)

where

$\frac{L_{binary}}{L_{0}}=\frac{L_{1}}{L_{0}}+\frac{L_{2}}{L_{0}}=10^{-m_{1}/2.5}+10^{-m_{2}/2.5}$
(4)

Our second equation is therefore

$K_{binary}=-2.5\log_{10}(10^{-K_{1}/2.5}+10^{-K_{2}/2.5})$ (5)

We then used the $\tt scipy.optimize$ subpackage (Virtanen et al., 2020) to
solve Equations 1 and 5 for our two unknowns: the $K$ magnitude of the primary
and the $K$ magnitude of the secondary.

When observing 2MASS J12190600+3150433, we detected the companion known to the
literature in addition to the new discovery. Therefore, to calculate the
component $K$ magnitudes for this system, we used an additional equation to
calculate the $K$-band delta magnitude for the tertiary in the system, and
added a $10^{-K_{3}/2.5}$ term to Equation 5.

Once we obtained a $K$ magnitude for each component in a given system, we were
able to estimate the component masses and calculate a mass ratio for the
system. We note that the relationship between mass and luminosity only behaves
for Main Sequence stars; in the case of pre-Main Sequence stars, their ages
must be known and accounted for as well. The M dwarfs, and in particular the
later spectral sub-types, have extensive pre-Main Sequence phases. Therefore,
the masses for young members of our sample could be estimated incorrectly if
their ages are unknown. Winters et al. (2019) finds 17 known young members in
their sample of 1120 nearby M dwarfs. Six of these are in our sample as well;
however, none of these young stars host a new discovery.

We also determined component spectral types using the component masses we
obtained, as well as reference stellar masses and spectral types333“A Modern
Mean Dwarf Stellar Color and Effective Temperature Sequence”,
http://www.pas.rochester.edu/~emamajek/EEM_dwarf_UBVIJHK_colors_Teff.txt
(Pecaut et al., 2012; Pecaut & Mamajek, 2013). We note that this is a coarse
method for estimating spectral types; we are therefore currently using the
Titan Monitor (TiMo) facility at Lowell Observatory to obtain homogeneous
spectral types for the entire POKEMON sample.

The projected separations, mass ratios, and component spectral types for these
26 systems are reported in Table 5. We also include an entry for the known
companion to 2MASS J12190600+3150433, since it was detected in our
observations as well.

We note that for some of the 26 stars with new companions, multiplying their
seeing value by their separation value from Table 3 produces a value larger
than 0.6 arcseconds squared. As shown in previous LDT speckle papers (Horch et
al., 2015, 2020), a value larger than 0.6 arcseconds squared could indicate
that the photometry has a systematic error, which could make the values listed
in Table 5 unreliable. We note these systems with an asterisk.

Table 5: Estimated astrophysical properties for systems with a new companion  2MASS ID | Projected Separation | Primary Mass | Companion Mass | Mass Ratio | Primary Spectral Type | Companion Spectral Type
---|---|---|---|---|---|---
| (au) | ($M_{\odot}$) | ($M_{\odot}$) |  |  |
$03104962-1549408$ | $15.4\pm 0.6$ | $0.29\pm 0.03$ | $0.15\pm 0.01$ | $0.53\pm 0.09$ | M3 | M5
$03542561-0909316$**Seeing $\times\rho>0.6$ arcseconds squared | $63.28\pm 0.04$ | $0.54\pm 0.03$ | $0.19\pm 0.02$ | $0.35\pm 0.05$ | M0.5 | M4
$07011725+1348085$**Seeing $\times\rho>0.6$ arcseconds squared | $52.4\pm 0.1$ | $0.30\pm 0.03$ | $0.16\pm 0.01$ | $0.53\pm 0.09$ | M3 | M5
$07411976+6718444$ | $5.0\pm 0.2$ | $0.15\pm 0.01$ | $0.120\pm 0.008$ | $0.8\pm 0.1$ | M5 | M5.5
$09510964-1219478$**Seeing $\times\rho>0.6$ arcseconds squared | $5.90\pm 0.08$ | $0.50\pm 0.03$ | $0.16\pm 0.01$ | $0.31\pm 0.04$ | M0.5 | M5
$11030845+1517518$ | $7\pm 1$ | $0.28\pm 0.05$ | $0.17\pm 0.03$ | $0.6\pm 0.2$ | M3 | M4.5
$12190600+3150433$ | $3.8\pm 0.3$ | $0.46\pm 0.03$ | $0.45\pm 0.03$ | $1.0\pm 0.1$ | M1.5 | M1.5
**Seeing $\times\rho>0.6$ arcseconds squared | $47.7\pm 0.1$ | $0.46\pm 0.03$ | $0.15\pm 0.01$ | $0.33\pm 0.05$ | M1.5 | M5
$12435889-1614351$ | $6.7\pm 0.5$ | $0.17\pm 0.02$ | $0.15\pm 0.02$ | $0.86\pm 0.2$ | M4.5 | M5
$13092185-2330350$**Seeing $\times\rho>0.6$ arcseconds squared | $17.65\pm 0.06$ | $0.082\pm 0.004$ | $0.076\pm 0.003$ | $0.93\pm 0.08$ | M8 | L1
$14235017-1646116$ | $37.3\pm 0.1$ | $0.59\pm 0.03$ | $0.23\pm 0.02$ | $0.39\pm 0.05$ | K8 | M3.5
$15020759+7527526$ | $10.7\pm 0.5$ | $0.114\pm 0.007$ | $0.086\pm 0.004$ | $0.76\pm 0.08$ | M5.5 | M7.5
$15085332+4934062$ | $8.4\pm 0.3$ | $0.51\pm 0.03$ | $0.31\pm 0.03$ | $0.60\pm 0.08$ | M0.5 | M3
$15211607+3945164$ | $5.3\pm 0.3$ | $0.14\pm 0.01$ | $0.113\pm 0.007$ | $0.8\pm 0.1$ | M5 | M5.5
$15263317+5522206$ | $5.5\pm 0.7$ | $0.15\pm 0.02$ | $0.11\pm 0.01$ | $0.7\pm 0.2$ | M5 | M5.5
$15434848+2552376$ | $5.5\pm 0.4$ | $0.16\pm 0.01$ | $0.16\pm 0.01$ | $0.9\pm 0.2$ | M4.5 | M5
$15471513+0149218$ | $4.9\pm 0.2$ | $0.130\pm 0.008$ | $0.091\pm 0.004$ | $0.70\pm 0.08$ | M5 | M6.5
$16041322+2331386$ | $2.1\pm 0.4$ | $0.17\pm 0.01$ | $0.15\pm 0.01$ | $0.9\pm 0.1$ | M4.5 | M5
$17183572+0156433$**Seeing $\times\rho>0.6$ arcseconds squared | $19.1\pm 0.3$ | $0.35\pm 0.03$ | $0.22\pm 0.02$ | $0.6\pm 0.1$ | M3 | M3.5
$17335314+1655129$ | $4.2\pm 0.2$ | $0.25\pm 0.02$ | $0.16\pm 0.01$ | $0.7\pm 0.1$ | M3.5 | M4.5
$18191622-0734518$ | $50\pm 1$ | $0.52\pm 0.03$ | $0.33\pm 0.03$ | $0.64\pm 0.09$ | M0.5 | M3
$18523373+4538317$ | $10.4\pm 0.6$ | $0.14\pm 0.01$ | $0.115\pm 0.008$ | $0.8\pm 0.1$ | M5 | M5.5
$20081786+3318122$ | $9\pm 1$ | $0.18\pm 0.03$ | $0.15\pm 0.02$ | $0.8\pm 0.2$ | M4.5 | M5
$21011610+3314328$ | $5.1\pm 0.2$ | $0.40\pm 0.03$ | $0.18\pm 0.02$ | $0.46\pm 0.08$ | M2.5 | M4.5
$21134479+3634517$ | $7.1\pm 0.3$ | $0.18\pm 0.02$ | $0.17\pm 0.02$ | $0.9\pm 0.2$ | M4.5 | M4.5
$22520522-1532511$ | $19.34\pm 0.09$ | $0.20\pm 0.02$ | $0.16\pm 0.01$ | $0.8\pm 0.1$ | M4 | M5
$23024353+7505591$ | $25.87\pm 0.09$ | $0.67\pm 0.02$ | $0.41\pm 0.02$ | $0.62\pm 0.05$ | K5 | M2

We note that our spectral type estimates for 2MASS J14235017-1646116 and 2MASS
J23024353+7505591 are K8 and K5, respectively. Kirkpatrick et al. (2016) lists
2MASS J14235017-1646116 as a M0.5 star, but 2MASS J23024353+7505591 does not
have a spectral type listed in the literature. In order to confirm our
spectral type estimates, one would need to obtain low-resolution spectroscopy
of these objects, which is outside the scope of this work. However, we are in
the process of following up POKEMON targets with the TiMo facility at Lowell
Observatory.

##  4 Discussion

Here we discuss a new companion in a system with both a known M dwarf and a
known white dwarf, a potential brown dwarf binary, and initial insights from
the POKEMON survey.

###  4.1 A Triple System with a Known White Dwarf

As noted in Section 3.2, four of the 26 systems with new discoveries also host
companions known to the literature. The companion to 2MASS J17183572+0156433,
called Wolf 672 A, is a known white dwarf (Gianninas et al., 2011).

This system was excluded from the final POKEMON sample, since all systems with
primaries more massive than M dwarfs have now been removed. We also do not
include this system in our calculation of the M-dwarf multiplicity or
companion rates. Nonetheless, we note here the discovery of a new M-dwarf
companion in this system, which makes the system trinary.

Systems that include both an M dwarf and a white dwarf are critical to our
understanding of M dwarfs, as our low-mass neighbors are notoriously difficult
to model, whereas the white dwarfs have a well-defined chain of models
(Fontaine et al., 2001) that can be used to estimate the age of their Main
Sequence companion (Garcés et al., 2011; Kiman et al., 2021). This estimation
of ages using white dwarfs can have uncertainties as low as 10-20% (Fouesneau
et al., 2019). White dwarf/M dwarf systems can also be used to study mass loss
(Debes, 2006). This system is therefore of particular interest for
understanding the formation and evolution of the M-dwarf companions.

Wolf 672 A is included in the Montreal White Dwarf
Database444https://www.montrealwhitedwarfdatabase.org, where its effective
temperature and log $g$ are listed as $12461\pm 113$ and $7.89\pm 0.01$,
respectively. Using these parameters, the fact that the Montreal White Dwarf
Database lists Wolf 672 A as a dA white dwarf, and the $\tt wdwarfdate$ code
(Dotter, 2016; Choi et al., 2016; Cummings et al., 2018; Bédard et al., 2020,
Kiman et al. in prep) that is publicly available on $\tt
github$555https://github.com/rkiman/wdwarfdate, we estimate an age for the
white dwarf Wolf 672 A, and thus its companions, the M dwarf Wolf 672 B and
the new discovery we detected (Figure 5). We find an age for this system of
$8.89^{+3.45}_{-3.62}$ Gyr, demonstrating the utility of observing white
dwarf/M dwarf systems.

Figure 5: Using the $\tt wdwarfdate$ code, we estimate an age for the white
dwarf Wolf 672 A, and thus its companions, the M dwarf Wolf 672 B and the new
discovery we detected. The panels (from left to right) show the Main Sequence
age (the age of the progenitor), the cooling age, the total age, the initial
mass (the mass of the progenitor), and the final mass of the white dwarf. We
find a cooling age of $0.30^{+0.01}_{-0.01}$ Gyr, and a final mass of
$0.55^{+0.01}_{-0.01}M_{\odot}$, which are in complete agreement with the
Montreal White Dwarf Database values for Wolf 672 A of 0.29 Gyr and 0.55
$M_{\odot}$.

###  4.2 A Potential Brown Dwarf Companion

As noted in Section 2.2, during our observing run in April 2017, the
narrowband speckle filters were not installed within DSSI. Although these
filters produce more contrast in the speckles, a large portion of the photons
gathered by the telescope remain unused. Filterless observing could thus be
beneficial for probing new speckle imaging discovery space, such as in the
case of 2MASS J13092185-2330350.

2MASS J13092185-2330350 is listed as an M7 star (Gizis, 2002), and has a
magnitude of 18.0 in the R band (Reid et al., 2007). On UT April 16 2017, we
obtained 46 data cubes for 2MASS J13092185-2330350, rather than the standard
nine, using DSSI at the 4.3-meter LDT. These observations were taken at the
end of the night, which was an opportunity to test the possibility of
observing systems beyond the faint limit of DSSI. As shown in Section 3.2,
these observations revealed a companion in both channels, at $1.1947\arcsec$
and with a delta magnitude of 0.99 at 692 nm, and at $1.1752\arcsec$ and with
a delta magnitude of 1.03 at 880 nm. According to our analysis described in
Section 3.4, the projected separation for this new companion is 17.64 au. The
mass of the primary is 0.082 $M_{\odot}$, and the mass of the secondary is
0.076 $M_{\odot}$, corresponding to a mass ratio of 0.926. We estimate that
the spectral type of the primary is M8, and that the spectral type of the
secondary is L1, making the secondary a potential brown dwarf. In order to
confirm these spectral types, one would need to obtain low-resolution
spectroscopy of these objects; this is outside the scope of this work, though
we are currently following up POKEMON targets with the TiMo facility at Lowell
Observatory. In any case, this system provides an argument for further
investigation into filterless speckle observations, as well as systems beyond
the faint limit of the instruments.

###  4.3 Initial Insights from the POKEMON Survey

Though the full analysis of the POKEMON sample has yet to be published, we
discuss here initial insights that can be made about the POKEMON sample based
on the 26 newly discovered companions.

As shown in Figure 3, almost all of the angular separations we measured are
within $\sim 0.7\arcsec$, and the smallest angular separation was measured to
be $0.0982\arcsec$. There are also new companions with delta magnitudes as
large as 3.30. We note that these distributions have not been completeness-
corrected. Therefore close companions at large delta magnitudes would be
preferentially missed, since the sensitivity of the instruments decreases with
decreasing separation. Additionally, wide companions would be preferentially
missed due to the small field-of-views of the instruments, or preferentially
excluded since many wide companions have been detected by previous surveys.
Nonetheless, the small angular separations and the large range of delta
magnitudes of the new discoveries discussed here explain why speckle imaging
was needed to detect these 26 new companions.

In Figure 6, we show the mass ratio and projected separation distributions for
the 26 new companions. The mass ratios follow a relatively flat distribution,
while the logarithms of the projected separations follow a roughly Gaussian
distribution. The peak of our projected separation distribution is at 7.8 au
with a standard deviation of $\sigma_{\log a}=0.4$. The sample that generated
these distributions is of course small; the statistics on these distributions
will become more robust once all detections from the full POKEMON survey are
included. Nonetheless, our peak is comparable to that of Winters et al.
(2019), which found a peak at 20 au for their sample of M dwarfs within 25 pc,
and at 4 au for M dwarfs within 10 pc. In contrast, Raghavan et al. (2010)
found a peak at 51 au for solar-type stars within 25 pc. It is telling that
though we used a different technique than Winters et al. (2019) to probe
M-dwarf multiplicity, our projected separation distributions are consistent
with one another, and are not consistent with the distribution from Raghavan
et al. (2010); these results demonstrate the differences between the stellar
companions that M dwarfs host, and those that solar-type stars host.

Figure 6: The mass ratio (left) and projected separation (right) distributions
for the 26 previously undetected companions discovered throughout the POKEMON
survey. These values were estimated using our observed properties, as well as
the parallaxes and 2MASS $K$ magnitudes for these objects. The mass ratios
follow a relatively flat distribution, while the projected separations follow
a roughly Gaussian distribution, as expected. The peak of our projected
separation distribution is at 7.8 au, with a standard deviation of
$\sigma_{\log a}=0.4$.

There are 1070 M dwarfs in the POKEMON sample; 290 of these are currently
known to host at least one stellar companion. We discovered 26 new companions
throughout this survey; 22 of these were discovered in systems that were
previously thought to be single. These 22 new discoveries therefore increase
the number of M dwarfs in the POKEMON sample with a known companion by 7.6%.
Though we have not yet performed sufficient analysis to determine an updated
M-dwarf multiplicity rate, these new discoveries increase the companion
fraction from 27.1%, which is in agreement with Winters et al. (2019), to
29.2% — a 2.1% percentage units increase. This increase demonstrates that it
is critical to survey our M-dwarf neighbors with high-resolution imaging in
order to search for stellar companions within $2\arcsec$, and to fully
understand M-dwarf multiplicity.

##  5 Conclusions and Future Work

We have carried out the POKEMON speckle survey of nearby M dwarfs, which is
volume-limited through M9, out to at least 15 pc, with additional brighter
targets at larger distances. The POKEMON survey has resulted in observations
of 1070 M dwarfs at large-telescope, diffraction-limited resolution. These
observations have revealed 26 new companions to these objects.

We used our observed properties, parallaxes from the literature, and the
targets’ 2MASS $K$ magnitudes to estimate astrophysical properties for these
objects including projected separation, mass ratio, and component spectral
types.

We report the discovery of a new companion in a system with both a known M
dwarf and a known white dwarf, and discuss a new companion that is a potential
brown dwarf. We also explore initial insights from the POKEMON survey.

In a forthcoming publication, we will present the full POKEMON survey, as well
as an updated M-dwarf multiplicity rate, calculated by spectral sub-type
through M9 for the first time. We are also currently carrying out follow-up of
the targets in the POKEMON sample in various ways. First, we are using the
Titan Monitor facility at Lowell Observatory to obtain homogeneous spectral
types for the entire POKEMON sample. Additional follow-up observing is being
pursued with the Quad-camera Wavefront-sensing Six-channel Speckle
Interferometer (QWSSI; Clark et al., 2020), which was commissioned at the LDT
in July 2020. While DSSI and NESSI image at two visible wavelengths, QWSSI
observes at four channels in the optical, as well as two in the near-infrared,
and also includes simultaneous wavefront sensing. Because the M dwarfs emit
most strongly in the near-infrared, QWSSI will aid us in measuring the
multiplicity of the late-type M dwarfs that were unable to be explored by the
previous-generation speckle imagers. This new capability will allow us to
build on the POKEMON survey and develop a more complete picture of M-dwarf
multiplicity for the later sub-types, and potentially even the L- and
T-dwarfs.

We thank our anonymous reviewer for their thoughtful assessment. We also thank
Frederick Hahne, Zachary Hartman, and Joe Llama for their contributions to and
feedback on this manuscript. Finally, we thank the army of TOs at the LDT and
the WIYN Telescope for all of their help and insight during our 50 nights of
observing. This research was supported by NSF Grant No. AST-1616084 and JPL
RSA No. 1610345. These results made use of the Lowell Discovery Telescope at
Lowell Observatory. Lowell is a private, non-profit institution dedicated to
astrophysical research and public appreciation of astronomy and operates the
LDT in partnership with Boston University, the University of Maryland, the
University of Toledo, Northern Arizona University and Yale University. Lowell
Observatory sits at the base of mountains sacred to tribes throughout the
region. We honor their past, present, and future generations, who have lived
here for millennia and will forever call this place home. These results are
also based on observations from Kitt Peak National Observatory, the NSF’s
National Optical-Infrared Astronomy Research Laboratory (NOIRLab Prop. ID:
2018B-0126; PI: C. Clark), which is operated by the Association of
Universities for Research in Astronomy (AURA) under a cooperative agreement
with the National Science Foundation. Data presented herein were obtained at
the WIYN Observatory from telescope time allocated to NN-EXPLORE through the
scientific partnership of the National Aeronautics and Space Administration,
the National Science Foundation, and the NSF’s National Optical-Infrared
Astronomy Research Laboratory. This work presents results from the European
Space Agency (ESA) space mission Gaia (Gaia Collaboration, 2020b). Gaia data
are being processed by the Gaia Data Processing and Analysis Consortium
(DPAC). Funding for the DPAC is provided by national institutions, in
particular the institutions participating in the Gaia MultiLateral Agreement
(MLA). The Gaia mission website is https://www.cosmos.esa.int/gaia. The Gaia
archive website is https://archives.esac.esa.int/gaia. This work has used data
products from the Two Micron All Sky Survey (Skrutskie et al., 2019), which is
a joint project of the University of Massachusetts and the Infrared Processing
and Analysis Center at the California Institute of Technology, funded by NASA
and NSF. Information was collected from several additional large database
efforts: the Simbad database and the VizieR catalogue access tool, operated at
CDS, Strasbourg, France; NASA’s Astrophysics Data System; the Washington
Double Star Catalog maintained at the US Naval Observatory; and the fourth US
Naval Observatory CCD Astrograph Catalog (Zacharias et al., 2020).

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