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An eccentric Brown Dwarf eclipsing an M dwarf

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An eccentric Brown Dwarf eclipsing an M dwarf

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We report the discovery of a $M=67\pm2 \mathrm{M_J}$ brown dwarf transiting the early M dwarf TOI-2119 on an eccentric orbit ($e=0.3362 \pm 0.0005$) at an orbital period of $7.200861 \pm 0.000005$ days. We confirm the brown dwarf nature of the transiting companion using a combination of ground-based and space-based photometry and high-precision velocimetry from the Habitable-zone Planet Finder. Detection of the secondary eclipse with TESS photometry enables a precise determination of the eccentricity and reveals the brown dwarf has a brightness temperature of $2100\pm80$ K, a value which is consistent with an early L dwarf. TOI-2119 is one of the most eccentric known brown dwarfs with $P<10$ days, possibly due to the long circularization timescales for an object orbiting an M dwarf. We assess the prospects for determining the obliquity of the host star to probe formation scenarios and the possibility of additional companions in the system using Gaia EDR3 and our radial velocities.
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An eccentric Brown Dwarf eclipsing an M dwarf
Caleb I. Cañas ,1, 2, Suvrath Mahadevan ,1, 2 Chad F. Bender ,3
Noah Isaac Salazar Rivera ,3Andrew Monson ,1Corey Beard ,4Jack Lubin ,4
Paul Robertson ,4Arvind F. Gupta ,1, 2 William D. Cochran ,5Connor Fredrick ,6, 7
Fred Hearty ,1, 2 Sinclaire Jones ,8Shubham Kanodia ,1, 2 Andrea S.J. Lin ,1, 2
Joe P. Ninan ,1, 2 Lawrence W. Ramsey ,1, 2 Christian Schwab ,9and
Guðmundur Stefánsson 8,
1Department of Astronomy & Astrophysics, The Pennsylvania State University, 525 Davey Laboratory, University
Park, PA 16802, USA
2Center for Exoplanets and Habitable Worlds, The Pennsylvania State University, 525 Davey Laboratory, University
Park, PA 16802, USA
3Steward Observatory, The University of Arizona, 933 N. Cherry Ave, Tucson, AZ 85721, USA
4Department of Physics & Astronomy, The University of California, Irvine, Irvine, CA 92697, USA
5McDonald Observatory and Center for Planetary Systems Habitability, The University of Texas at Austin, Austin,
TX 78730, USA
6Associate of the National Institute of Standards and Technology, 325 Broadway, Boulder, CO 80305, USA
7Department of Physics, University of Colorado, 2000 Colorado Avenue, Boulder, CO 80309, USA
8Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton, NJ 08540, USA
9Department of Physics and Astronomy, Macquarie University, Balaclava Road, North Ryde, NSW 2109, Australia
ABSTRACT
We report the discovery of a M= 67 ±2MJbrown dwarf transiting the early M
dwarf TOI-2119 on an eccentric orbit (e= 0.3362 ±0.0005) at an orbital period of
7.200861 ±0.000005 days. We confirm the brown dwarf nature of the transiting com-
panion using a combination of ground-based and space-based photometry and high-
precision velocimetry from the Habitable-zone Planet Finder. Detection of the sec-
ondary eclipse with TESS photometry enables a precise determination of the eccentric-
ity and reveals the brown dwarf has a brightness temperature of 2100 ±80 K, a value
which is consistent with an early L dwarf. TOI-2119 is one of the most eccentric known
brown dwarfs with P < 10 days, possibly due to the long circularization timescales for
an object orbiting an M dwarf. We assess the prospects for determining the obliquity of
the host star to probe formation scenarios and the possibility of additional companions
in the system using Gaia EDR3 and our radial velocities.
Keywords: Stars: Brown dwarfs, Eclipsing binary stars, Fundamental parameters of
stars
Corresponding author: Caleb I. Cañas
canas@psu.edu
NASA Earth and Space Science Fellow
Henry Norris Russell Fellow
arXiv:2112.03959v1 [astro-ph.SR] 7 Dec 2021
2Cañas et al.
1. INTRODUCTION
Brown dwarfs are objects with radii compara-
ble to Jupiter and masses between 13 80MJ
(see Chabrier & Baraffe 2000;Burrows et al.
2001), although this lower limit is not well de-
fined if these objects are classified on the ba-
sis of their formation mechanism (e.g., Chabrier
et al. 2014). As isolated objects, brown dwarfs
have traditionally been identified via photomet-
ric surveys by their colors (e.g., Pinfield et al.
2008;Zhang et al. 2009;Folkes et al. 2012;Reylé
2018) because their spectral energy distribution
peaks in the near-infrared. Brown dwarfs con-
tain complex spectral features that are difficult
to model and existing evolutionary models are
largely degenerate in age, radius, and metallic-
ity. This makes it difficult to determine fun-
damental properties, such as the mass and ra-
dius, for isolated brown dwarfs. When these
objects are eclipsing companions to a main-
sequence star, photometric and spectroscopic
observations yield a measurement of the mass
and radius.
Brown dwarfs are infrequent companions (.
1%) to main sequence stars (e.g., Vogt et al.
2002;Patel et al. 2007;Wittenmyer et al. 2009;
Sahlmann et al. 2011;Nielsen et al. 2019),
but previous radial velocity (RV) surveys have
facilitated their study (e.g., Campbell et al.
1988;Marcy & Butler 2000;Wittenmyer et al.
2009;Sahlmann et al. 2011;Bonfils et al. 2013).
These surveys have revealed the “brown dwarf
desert” (e.g., Marcy & Butler 2000;Grether
& Lineweaver 2006) or the apparent paucity
of brown dwarf companions to main sequence
stars within 3au. This feature has been at-
tributed to the different formation mechanisms
between low and high-mass companion brown
dwarfs (see Ma & Ge 2014;Chabrier et al. 2014)
where high-mass brown dwarfs (&43MJ) are
believed to form through molecular cloud frag-
mentation, similar to a binary stellar compan-
ion, while low-mass brown dwarfs form through
gravitational instabilities in the protoplanetary
disk.
Another factor that may sculpt the brown
dwarf desert is the orbital migration and tidal
in-spiral (e.g., Armitage & Bonnell 2002;Pät-
zold & Rauer 2002;Damiani & Díaz 2016) of
brown dwarf companions. Damiani & Díaz
(2016) note that tidal interactions with the
host star and angular momentum loss through
magnetic braking can lead to rapid in-spiral
of a brown dwarf companion orbiting main se-
quence dwarfs with outer convective envelopes.
The in-spiral time scale is a strong function of
stellar radius, such that early M dwarfs like
TOI-2119, have larger in-spiral timescales than
later main sequence dwarfs. This enables close
brown dwarf companions to exist for a longer
time despite the more efficient tidal dissipation
and magnetic braking mechanisms present in M
dwarfs. Carmichael et al. (2020) noted that
while there was no obvious trend in the fre-
quency of brown dwarfs with stellar host type,
a large percent of well-characterized (with mass
and radius measurements) brown dwarfs (6/23
or 26% of the then known brown dwarfs)
were found to transit M dwarfs. This appears
contrary to the occurrence of Jupiter-sized ex-
oplanets, in which the occurrence rate of these
planets decreases for M dwarf host stars (e.g.;
Endl et al. 2006;Johnson et al. 2010;Bon-
fils et al. 2013;Maldonado et al. 2020). Cur-
rently, there are 5 well-characterized Jupiter-
sized planets transiting M dwarfs and 8 tran-
siting M dwarf - brown dwarf systems (includ-
ing TOI-2119; Carmichael et al. 2020;Artigau
et al. 2021). More masses and radii for brown
dwarf companions are needed to probe any de-
pendence on stellar host type.
In this paper, we present a new M dwarf
system, TOI-2119, hosting a high-mass brown
dwarf on a 7.2-day eccentric orbit. We con-
firm the brown dwarf nature of TOI-2119.01 us-
An eccentric, eclipsing MD-BD system 3
ing space-based photometry from the Transiting
Exoplanet Satellite (TESS Ricker et al. 2015),
additional ground-based photometry, adaptive
optics (AO) imaging with the ShaneAO in-
strument (Srinath et al. 2014) on the 3 m
Shane Telescope at Lick Observatory, and near-
infrared (NIR) RVs with the northern spec-
trograph of the APO Galaxy Evolution Ex-
periment (APOGEE-2N; Majewski et al. 2017;
Zasowski et al. 2017) and the Habitable-zone
Planet Finder Spectrograph (HPF; Mahadevan
et al. 2012,2014).
The paper is structured as follows: Section
2presents the observations used in this paper,
Section 3describes the method for spectroscopic
characterization and our best estimates of the
stellar parameters, and Section 4explains the
analysis of the photometric and RV data. A
discussion of the bulk properties of TOI-2119 in
context of other transiting brown dwarfs with
masses is presented in Section 5. We conclude
the paper in Section 6with a summary of our
key results.
2. OBSERVATIONS
2.1. TESS
TESS observed TOI-2119 (Gaia EDR3
1303675097215915264) in short-cadence mode
during Sectors 24 and 25 with data spanning
2020 Apr 16 through 2020 June 08. It has one
transiting candidate, TOI-2119.01, identified by
the TESS Science Processing Operations Cen-
ter pipeline (SPOC Jenkins et al. 2016) with an
orbital period of 7.2days. The “quick-look
pipeline” developed by Huang et al. (2020) also
detected TOI-2119.01 as a target of interest in
the full-frame image data of Sectors 24 and 25.
For this work, we use the pre-search data-
conditioned (PDCSAP; Jenkins et al. 2016)
light curves available at the Mikulski Archive
for Space Telescopes (MAST). The PDCSAP
photometry is corrected for instrumental sys-
tematics and dilution from other objects con-
tained within the aperture using algorithms
that were originally developed for the Kepler
mission (Stumpe et al. 2012;Smith et al. 2012).
Observations with non-zero data quality flags
that indicate anomalous data due to various
conditions, such as spacecraft events or cosmic
ray hits, are excluded from the analysis. The
quality flags are described in the TESS Science
Data Products Description Document (Table 28
in Tenenbaum & Jenkins 2018). Figure 1dis-
plays the photometry and the transits observed
by TESS. The TESS photometry reveals TOI-
2119 to be an active, flaring star and, to remove
the flares, we reject any median normalized ob-
servation larger than 1.01. We perform no addi-
tional outlier rejection beyond the data quality
flags and application of a threshold value.
2.2. TMMT
We observed one transit of TOI-2119.01 using
the robotic Three-hundred MilliMeter Telescope
(TMMT; Monson et al. 2017) at Las Campanas
Observatory (LCO) on the night of 2021 April
13. The observations were performed slightly
out of focus in the Bessell Ifilter (Bessell 1990)
with a point spread function FWHM of 400. We
obtained 107 frames using an exposure time of
100s while operating in a 1x1 binning mode.
In this mode, TMMT has a 13s readout time
between exposures, resulting in an effective ca-
dence of 113s and an observing efficiency of
90%. The observations began at airmass 5.29
and ended at airmass 1.76.
We processed the photometry using AstroIm-
ageJ (Collins et al. 2017) following the proce-
dures described in Stefánsson et al. (2017). The
light curve was extracted using simple aperture
photometry with an object aperture radius of
6 pixels (7.200), and inner and outer sky an-
nuli of 10 pixels (1200) and 15 pixels (17.900), re-
spectively, because these values minimized the
standard deviation in the residuals. Follow-
ing Stefánsson et al. (2017), we added the ex-
pected scintillation-noise errors in quadrature to
4Cañas et al.
Figure 1. (a) displays the PDCSAP photometry from TESS. The light curve reveals a periodic 5% signal
around a flaring, active M dwarf. The dashed line is the threshold of 1.01 which is used to flag data during
flares. (b) shows the transit after phasing the raw PDCSAP photometry from (a) to the period and ephemeris
determined by the SPOC pipeline. The large circles represent 10-min bins of the phase-folded data. (c) is
similar to (b) and presents the TESS occultation. (d) shows the phase-folded TMMT photometry.
the photometric error (including photon, read-
out, dark, sky background, and digitization
noise). Figure 1(d) displays the photometry
from TMMT.
2.3. Doppler Spectroscopy with APOGEE-2N
TOI-2119 was observed from the Apache
Point Observatory on 11 and 12 May 2018 using
APOGEE-2N, a multiplexed, high-resolution
(R22,500, NIR (λ1.51.7micron) fiber-
fed spectrograph (Wilson et al. 2012,2019) that
is mounted on the Sloan 2.5-meter telescope
(Gunn et al. 2006). The APOGEE-2N spec-
trograph was used as part of a survey in SDSS-
IV (Blanton et al. 2017) with the primary goal
of studying the galactic evolution of the Milky
Way through the chemical and dynamical anal-
ysis of various stellar populations and Galactic
regions.
For this work, we use the publicly available
DR16 (Jönsson et al. 2020) data of TOI-2119.
The APOGEE data pipeline (Nidever et al.
2015) performs sky subtraction, telluric and
barycentric correction, and wavelength and flux
calibration for each observation of TOI-2119.
The RVs were derived following the procedure
described in Cañas et al. (2019). Briefly, we
identified the best-fit synthetic spectrum by
cross-correlating the highest S/N spectra using
synthetic spectra generated from MARCs mod-
els (Gustafsson et al. 2008) that were specif-
ically generated for the APOGEE-2N survey
(see Mészáros et al. 2012;Zamora et al. 2015;
Holtzman et al. 2018). The synthetic spectrum
with the largest correlation was used in the fi-
nal cross-correlation to obtain the reported ra-
dial velocities in Table 1. The uncertainties for
each observation were calculated by following
the maximum-likelihood approach presented by
Zucker (2003). The derived RVs, the 1σun-
certainties, and the S/N per resolution element
(2pixels) are presented in Table 1.
An eccentric, eclipsing MD-BD system 5
2.4. High-resolution Doppler Spectroscopy with
HPF
We obtained eight 945-second visits of TOI-
2119 using HPF with a median signal-to-noise
ratio (S/N) per 1D extracted pixel of 168 at
1000 nm. HPF is a high-resolution (R
55,000), fiber-fed (Kanodia et al. 2018), NIR
(λ8080 12780 Å) spectrograph located
on the 10m Hobby-Eberly Telescope (HET) at
McDonald Observatory in Texas (Mahadevan
et al. 2012,2014) that achieves a long-term tem-
perature stability of 1mK (Stefánsson et al.
2016). The observations span 19 September
2020 through 26 May 2021 and were executed
in a queue by the HET resident astronomers
(Shetrone et al. 2007).
The HxRGproc tool was used to process the
raw HPF data and perform bias noise removal,
nonlinearity correction, cosmic-ray correction,
and slope/flux and variance image calculation
(Ninan et al. 2018). The one-dimensional spec-
tra were reduced using the procedures in Ninan
et al. (2018), Kaplan et al. (2019), and Met-
calf et al. (2019). The wavelength solution and
drift correction applied to the data was extrap-
olated from laser frequency comb (LFC) frames
that were taken as part of standard evening
and morning calibrations and from LFC cali-
bration frames obtained periodically through-
out the night. The extrapolation from LFC
frames enables precise wavelength calibration
on the order of <30 cm/s(Stefánsson et al.
2020), a value which is smaller than the pho-
ton noise for TOI-2119 (median RV uncertainty
of 48.5m/s).
We used a modified version (see Stefáns-
son et al. 2020) of the SERVAL code (SpEc-
trum Radial Velocity AnaLyser; Zechmeister
et al. 2018) to calculate RVs. SERVAL employs
the template-matching technique to derive RVs
(e.g., Anglada-Escudé & Butler 2012) and cre-
ates a master template from the observations
to determine the Doppler shift by minimizing
Table 1. RVs of TOI-2119.
BJDTDB RV σS/N
( m/s) ( m/s)
APOGEE-2N:
2458249.80011 18688 213 226a
2458250.84530 11816 209 163
HPF:
2459111.58596 17407 15 108b
2459117.57945 12063 17 93
2459231.02335 1908 10 164
2459232.02920 4353 9 175
2459238.00846 319 9 169
2459247.00784 10639 8 197
2459300.84858 16666 13 124
2459360.90067 2871 7 226
aThe APOGEE-2N S/N is the median value
per resolution element (2pixels). For
comparison, HPF has a resolution 2.53
times that of APOGEE-2N with a resolu-
tion element and PSF that each span 3
pixels.
bThe HPF S/N is the median value per 1D
extracted pixel at 1000 nm. All exposure
times for HPF are 945 s.
the χ2statistic. The master template was gen-
erated from all observed spectra while mask-
ing telluric regions identified using a synthetic
telluric-line mask generated from telfit (Gul-
likson et al. 2014). The barycentric correction
for each epoch was calculated using barycorrpy
(Kanodia & Wright 2018) which implements the
algorithms from Wright & Eastman (2014). The
derived HPF RVs, the 1σuncertainties, and the
S/N per pixel at 1000 nm for TOI-2119 are pre-
sented in Table 1.
2.5. Adaptive Optics Imaging
TOI-2119 was observed in the Ksband with
the ShARCS camera on the Shane 3m telescope
6Cañas et al.
at Lick Observatory (Srinath et al. 2014). It was
observed under natural guide star mode using
the 5-point dither process described in Furlan
et al. (2017). The seeing was 200 during the
observations. The data were reduced using a
custom AO pipeline developed that rejects all
overexposed or underexposed images and ex-
cludes data that are flagged as erroneous (e.g.,
due to lost guiding, shutters closed early due to
weather, etc.). The pipeline applies standard
dark correction, flat correction, and a sigma
clipping process to all images. A master sky
image is produced from the 5-point dither pro-
cess which is subtracted from each image. The
final image is produced by interpolating all im-
ages onto a single centroid. A 5σcontrast curve
is generated from the final image using the algo-
rithms described in Espinoza et al. (2016) and
is presented in Figure 2. The poor seeing dur-
ing the night TOI-2119 was observed prevents
any constraints <0.8300 (marked as the hatched
region in Figure 2). There are no bright com-
panions (Ks<4) that could be a source of
contamination in the photometry at separations
of 0.8300 6.500 from TOI-2119.
2.6. Speckle Imaging
We used NESSI (Scott et al. 2018) on the
3.5 m WIYN Telescope at KPNO to perform
speckle imaging on 2021 October 26. TOI-
2119 was observed in two narrowband filters
centered at 562nm and 832nm. The images in
each filter were reconstructed following the pro-
cedures outlined in Howell et al. (2011). The
NESSI contrast curves in both filters are shown
in Figure 3, along with an inset of the image in
the 832nm narrowband filter. The NESSI data
show no evidence of blending from a bright com-
panion ∆Mag <4at separations of 0.15 1.200.
2.7. Non-detection of Spectroscopic
Companions within 2”
The fibers for APOGEE-2N have a field-of-
view of 200 (see Wilson et al. 2019) and we
use the H-band APOGEE-2N spectra to search
for light from secondary stars around TOI-2119.
We employ the software binspec_plus, which
is based on binspec (El-Badry et al. 2018b,a),
to search for the faint spectrum of a second star
by modeling the observed spectrum as the sum
of two model spectra. binspec was designed
to fit both single and double-lined spectra, but
its sensitivity is limited to the detection of
moderate-mass ratio binaries (0.4.q.0.85)
because of the limited temperature regime of
the spectral models. binspec_plus1extends
the spectral model to include redder dwarfs and
giants. We fit the APOGEE-2N spectrum of the
TOI-2119 with a neural network spectral model
(see Ting et al. 2019) trained on the Kurucz stel-
lar library (Kurucz 1979) that is valid for slowly
rotating (vsin i < 45 km s1) main-sequence
stars in the regime of 3000K < Te<7000 K,
4.0<log g < 5.0, and 1<[Fe/H] <0.5.
The model selection criterion from Table B1 of
El-Badry et al. (2018b) relies on two values,
χ2=χ2
single χ2
binary, which quantifies how
better a fit is obtained by the binary model,
and the improvement fraction, fimp, which de-
scribes how better the binary model fit is rel-
ative to how different it is from a single-star
model. TOI-2119 is classified as a single-lined
spectroscopic binary (SB1) because both χ2
and fimp are improved relative to a single-star
model, but a binary component fit is disfa-
vored and shows negligible improvement in the
fit when compared to the SB1 fit (χ2<3000
and fimp <0). This analysis of the APOGEE-
2N data reveals no evidence for secondary light
within 200 from companion or background dwarf
stars with 3000K < Te<7000 K.
3. STELLAR PARAMETERS
3.1. Spectroscopic Parameters
1https://github.com/tingyuansen/binspec_plus/
An eccentric, eclipsing MD-BD system 7
Figure 2. Above is the 5σcontrast curve obtained using the ShARCS camera in the Ksfilter. ShaneAO
cannot place constraints within the hatched region (<0.8300) due to poor seeing (200) during the observation.
The data show there are no bright companions (Ks<4) at separations of 0.8300 6.500 from the host star.
The inset image is a cutout centered on TOI-2119 where the scale bar reflects 100.
The spectroscopic stellar parameters (Te,
log g, and [Fe/H]) for TOI-2119 were calcu-
lated using the HPF-SpecMatch package (Ste-
fánsson et al. 2020, S. Jones et. al. 2021),
which employs the empirical template match-
ing methodology discussed in Yee et al. (2017).
HPF-SpecMatch derives the stellar properties by
comparing the highest S/N observed spectra to
a library of high quality (S/N>100) HPF
stellar spectra with well-determined properties
(values adopted from Yee et al. 2017). It iden-
tifies the best-matching library spectrum using
χ2minimization, creates a composite spectrum
from a weighted, linear combination of the five
best-matching library spectra, and derives the
stellar properties using these weights. The re-
ported uncertainty for each stellar parameter
is the standard deviation of the residuals from
a leave-one-out cross-validation procedure ap-
plied to the entire HPF library in the chosen
spectral order.
The library contains 166 stars and spans the
following parameter space: 3000 K < Te<
5500 K,4.4<log g < 5.2, and 0.5<[Fe/H] <
0.5. We used HPF order index 17 (spanning
10460 10570 Å) for the spectral matching of
TOI-2119 because it has little to no telluric con-
tamination. Table 2lists the derived spectro-
scopic parameters with their uncertainties.
8Cañas et al.
Figure 3. The 5σcontrast curve obtained using NESSI in the narrowband filters centered at 562nm and
832nm showing no bright companions (∆Mag <4) between 0.15 1.200 from the host star. NESSI cannot
place constraints within the hatched region (<0.0500) The inset image is the 832nm narrowband filter centered
on TOI-2119 where the scale bar reflects 100.
Table 2. Summary of Stellar Parameters.
Parameter Description Value Reference
Main identifiers:
TIC ··· 236387002 TIC
Gaia EDR3 ··· 1303675097215915264 Gaia EDR3
Equatorial Coordinates, Proper Motion, Distance, and Maximum Extinction:
αJ2016 Right Ascension (RA) 16:17:43.17 Gaia EDR3
δJ2016 Declination (Dec) 26:18:15.16 Gaia EDR3
µαProper motion (RA, mas/yr)29.27 ±0.02 Gaia EDR3
µδProper motion (Dec, mas/yr)6.86 ±0.03 Gaia EDR3
dDistance in pca31.46 ±0.03 Bailer-Jones
Table 2 continued
An eccentric, eclipsing MD-BD system 9
Table 2 (continued)
Parameter Description Value Reference
AV,max Maximum visual extinction 0.01 Green
Optical and near-infrared magnitudes:
BJohnson Bmag 13.86 ±0.03 APASS
VJohnson Vmag 12.37 ±0.04 APASS
g0Sloan g0mag 13.07 ±0.02 APASS
r0Sloan r0mag 11.76 ±0.02 APASS
i0Sloan i0mag 10.75 ±0.02 APASS
J J mag 8.98 ±0.02 2MASS
H H mag 8.39 ±0.03 2MASS
KsKsmag 8.14 ±0.02 2MASS
W1WISE1 mag 8.05 ±0.02 WISE
W2WISE2 mag 7.97 ±0.02 WISE
W3WISE3 mag 7.88 ±0.02 WISE
W4WISE4 mag 7.8±0.2WISE
Spectroscopic Parametersb:
TeEffective temperature in K 3553 ±67 This work
[Fe/H] Metallicity in dex 0.1±0.1This work
log(g)Surface gravity in cgs units 4.74 ±0.04 This work
Model-Dependent Stellar SED and Isochrone fit Parametersc:
M?Mass in M0.53 ±0.02 This work
R?Radius in R0.51 ±0.01 This work
ρ?Density in g/cm35.7±0.4This work
AvVisual extinction in mag 0.005 ±0.003 This work
Other Stellar Parameters:
Prot Rotation period in days 13.2±0.2This work
vsin i?Rotational broadening in km/s <2This work
Age Age in Gyrs 0.75.1This work
RV Systemic radial velocity in km/s 15.72 ±0.02 This work
References are: TIC (Stassun et al. 2019), Gaia EDR3 (Gaia Collaboration et al. 2021), Bailer-
Jones (Bailer-Jones et al. 2021), Green (Green et al. 2019), APASS (Henden et al. 2018),
2MASS (Cutri et al. 2003), WISE (Wright et al. 2010)
aGeometric distance from Bailer-Jones et al. (2021).
bDerived using the HPF-SpecMatch algorithm.
cEXOFASTv2 derived values using MIST isochrones.
10 Cañas et al.
3.2. Spectral Energy Distribution Fitting
We modeled the spectral energy distribu-
tion (SED) to derive model-dependent stel-
lar parameters using the EXOFASTv2 analysis
package (Eastman et al. 2019). Our analy-
sis uses the MIST model grid (Dotter 2016;
Choi et al. 2016) which is based on the AT-
LAS12/SYNTHE stellar atmospheres (Kurucz
1970,1993). EXOFASTv2 calculates the bolo-
metric corrections for the SED fit by linearly
interpolating the precomputed bolometric cor-
rections provided by the MIST team in a grid
of log g,Teff , [Fe/H], and AV2.
The fit uses Gaussian priors on the (i) 2MASS
JHK magnitudes, Sloan g0, r0, i0magnitudes
and Johnson BV magnitudes from Henden et al.
(2018), and Wide-field Infrared Survey Explorer
magnitudes (Wright et al. 2010); (ii) host star
surface gravity, temperature, and metallicity
derived from HPF-SpecMatch; and (iii) the ge-
ometric distance calculated from Bailer-Jones
et al. (2021). We apply a uniform prior for
the visual extinction in which the upper limit
is determined from estimates of Galactic dust
(Green et al. 2019) calculated at the distance
determined by Bailer-Jones et al. (2021). The
Rv= 3.1reddening law from Fitzpatrick (1999)
is used by EXOFASTv2 to convert the extinction
determined by Green et al. (2019) to a visual
magnitude extinction. Table 2contains the stel-
lar priors and derived stellar parameters with
their uncertainties. The model-dependent mass
and radius for TOI-2119 are 0.53±0.02 Mand
0.51 ±0.01 R, respectively.
3.3. Rotation Period
TOI-2119 is a flaring star exhibiting pho-
tometric modulations of <1% that persist
throughout the TESS photometry (see Figure
1(a)). It is chromospherically active as the cal-
2http://waps.cfa.harvard.edu/MIST/model_grids.
html#bolometric
cium II infrared triplet lines are observed to be
in emission in each HPF observation. We there-
fore attribute the variability in the light curve to
activity-induced photometric modulation and
attempt to constrain the rotation period of TOI-
2119 using TESS photometry. For this search,
we excise all transit events in the TESS data
within a window of 0.107 days (1.25 times the
transit duration) from the expected mid-transit.
We analyzed the TESS data using the general-
ized Lomb-Scargle periodogram (Zechmeister &
Kürster 2009), the wavelet power spectra (e.g.,
Bravo et al. 2014), and the auto-correlation
function (e.g., McQuillan et al. 2013a,b), and
estimated the rotation period to be in the range
of 5-20 days.
To further constrain the rotation period, we
used publicly available data from the (i) Zwicky
Transient Facility (ZTF; Masci et al. 2019)
in the zg band, (ii) All-Sky Automated Sur-
vey for SuperNovae (ASAS-SN; Shappee et al.
2014;Kochanek et al. 2017) in the Vand g0
bands, and (iii) SuperWASP (Butters et al.
2010). We modeled the ground-based photome-
try using the juliet analysis package (Espinoza
et al. 2019), which performs the parameter es-
timation using dynesty (Speagle 2020), a dy-
namic nested-sampling algorithm. The photo-
metric model is a Gaussian process that uses
the approximate quasi-periodic covariance func-
tion from the celerite package (Equation 56
in Foreman-Mackey et al. 2017) because it has
been used to reliably infer stellar rotation rates
(e.g., Angus et al. 2018;Robertson et al. 2020).
We used our constraints from the TESS pho-
tometry to place a uniform prior on the rotation
period of 520 days. The fit yields a rotation
period of 13.2±0.2days and is included in Ta-
ble 2. Figure 4displays the ground-based pho-
tometry used for this analysis and the posterior
distribution on the rotation period.
With a period of 13.2±0.2days, TOI-2119
has intermediate rotation period based on the
An eccentric, eclipsing MD-BD system 11
Figure 4. (a) is the phased ground-based photometry that was modeled with a Gaussian process. The large
black points represent 1-day bins of the phased photometry. (b) presents the posterior distribution of the
rotation period of the Gaussian process model. We derive a rotation period of 13.2±0.2days.
classification scheme of Newton et al. (2016).
Newton et al. (2016) could not provide an age
range for M dwarfs with rotation periods span-
ning 10–70 days but showed that M dwarfs with
Prot <10 days have a mean age of 0.7±0.3Gyr
while those with Prot >70 days have mean ages
of 5.1+4.2
2.6Gyr. TOI-2119 most probably has an
age between 0.75.1Gyr. This is age range is
also consistent with the rotation period and age
relationship from Engle & Guinan (2018).
4. PHOTOMETRIC AND RV MODELING
We use allesfitter (Günther & Daylan
2021) to jointly model the photometry and RVs.
allesfitter calculates the transit and RV
models using the ellc package (Maxted 2016)
and performs the parameter estimation using
dynesty. The RV model is a standard Keple-
rian model while the photometric model is the
sum of a transit model, an occultation model,
and the same Gaussian process noise model de-
12 Cañas et al.
scribed in Section 3.3 to account for correlated
noise in the TESS photometry. The transit
model adopts a quadratic limb-darkening law
where the limb-darkening coefficients are pa-
rameterized following Kipping (2013a) while the
occultation model assumes uniform limb dark-
ening. Both the photometric and RV models
also include a simple white-noise model in the
form of a jitter term that is added in quadrature
to the error bars of each instrument.
Table 3provides a summary of the inferred
system parameters and respective confidence in-
tervals and Figure 5displays the model poste-
riors. The data reveal a high-mass brown dwarf
(M2= 67 ±2MJand R2= 1.11 ±0.03RJ)
orbiting TOI-2119 on an eccentric orbit (e=
0.3362 ±0.0005) with a period of 7.200861 ±
0.000005 days. The eccentricity of the orbit is
determined with exquisite precision because of
the powerful constraint on eand ωfrom the
presence of both a transit and occultation (see
Winn 2010).
Table 3. System Parameters for TOI-2119
Parameter Units Prior Value
Photometric Parameters TESS TMMT
Linear Limb-darkening Coefficienta. . . . q1.......................... U(0,1) 0.18+0.05
0.03 0.6±0.1
Quadratic Limb-darkening Coefficienta.q2.......................... U(0,1) 0.8+0.1
0.20.06+0.06
0.04
Photometric Jitter . . . . . . . . . . . . . . . . . . . . . σphot (ppm) . . . . . . . . . . . . . . . J(104,106) 0.004+0.769
0.004 10 ±10
RV Parameters APOGEE-2N HPF
RV Oset.............................. γ(km/s)................... U(20,10) 14.5±0.111.27 ±0.01
RV Jitter.............................. σRV (m/s)................. J(103,103) 1 ±1 10+20
10
Orbital Parameters:
Orbital Period . . . . . . . . . . . . . . . . . . . . . . . . . P(days) . . . . . . . . . . . . . . . . . . N(7.2,0.1) 7.200861 ±0.000005
Time of Conjunction . . . . . . . . . . . . . . . . . . . TC(BJDTDB )............. N(2458958.6,0.1) 2458958.67756 ±0.00006
ecos ω............................... ............................ U(1,1) 0.5798 ±0.0004
esin ω............................... ............................ U(1,1) 0.009 ±0.003
Semi-amplitude velocity . . . . . . . . . . . . . . . K(km/s) ................. U(1,20) 10.59 ±0.02
Scaled Radius . . . . . . . . . . . . . . . . . . . . . . . . . R2/R?.................... U(0,1) 0.226 ±0.001
(R?+R2)/a .......................... ............................ U(0,1) 0.0454 ±0.0003
cos i................................... ............................ U(0,1) 0.0259 ±0.0005
Surface brightness ratio . . . . . . . . . . . . . . . . J.......................... U(0,1) 0.0270.003
+0.004
TESS Gaussian Process Hyperparameters:
B...................................... Amplitude (106ppm). . . . . J(106,106) 6+2
1
C...................................... Additive Factor ........... J(106,106) 90 ±90
L. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Length scale (days) . . . . . . . . J(106,106) 1.5+0.5
0.3
PGP ................................... Period (days) .............. J(1,100) 20+30
10
Derived Parameters:
Scaled Semi-major Axis. . . . . . . . . . . . . . . . a/R?...................... ··· 27.0±0.2
Impact Parameter . . . . . . . . . . . . . . . . . . . . . b........................... ··· 0.623 ±0.009
Impact Parameter of occultation . . . . . . . bs.......................... ··· 0.62 ±0.01
Time of Pericenter . . . . . . . . . . . . . . . . . . . . . Tp(BJDTDB ).............. ··· 2458963.790 ±0.002
Table 3 continued
An eccentric, eclipsing MD-BD system 13
Figure 5. Joint Photometry and RV fit for TOI-2119. (a) displays the detrended TESS photometry phased
to the derived ephemeris in Table 3in the top panel a with the residuals to the best fit transit model in
the bottom panel. (b) is identical to (a) but shows the occultation. (c) is similar to (a) and presents the
transit fit to the TMMT data. Panels (a)-(c) are plotted in units of orbital phase (from 0-1) where the black
points represent 20-min bins of the phase-folded data. (d) shows the phased RVs plotted with the best fit
RV model and the corresponding residuals. For all panels, the best-fitting model is plotted as a dashed line
while the shaded regions denote the 1σextent of the derived posterior solution.
Table 3 (continued)
Parameter Units Prior Value
Time of Occultation . . . . . . . . . . . . . . . . . . . Ts(BJDTDB ).............. ··· 2458963.790 ±0.002
Eccentricity. . . . . . . . . . . . . . . . . . . . . . . . . . . . e.......................... ··· 0.3362 ±0.0005
Argument of Periastron . . . . . . . . . . . . . . . . ω(degrees) . . . . . . . . . . . . . . . . ·· · −0.9±0.3
Orbital Inclination . . . . . . . . . . . . . . . . . . . . . i(degrees) . . . . . . . . . . . . . . . . . ··· 88.51 ±0.03
Transit Duration . . . . . . . . . . . . . . . . . . . . . . . T14 (hours) . . . . . . . . . . . . . . . . ··· 2.039 ±0.007
Occultation Duration . . . . . . . . . . . . . . . . . . T14 (hours) . . . . . . . . . . . . . . . . ··· 2.025 ±0.009
Mass .......................... ........ M2(MJ) .................. ··· 67 ±2
Mass ratio............................. q=M2/M?............... ··· 0.12 ±0.02
Radius................................. R2(RJ) .................. ··· 1.11 ±0.03
Surface Gravity . . . . . . . . . . . . . . . . . . . . . . . . log g2(cgs) ................ ··· 5.158 ±0.008
Density................................ ρ2(g/cm3) ................ ··· 60 ±5
Semi-major Axis . . . . . . . . . . . . . . . . . . . . . . . a(au) ..................... ··· 0.064 ±0.002
Transit Depth (TESS bandpass) . . . . . . . (%) . . . . . . . . . . . . . . . . . . . . . . . . ··· 5.09 ±0.05
Occultation Depth (TESS bandpass) . . . ppm (×106).............. ··· 1400 ±200
T2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Brightness Temperature (K) ··· 2100 ±80
aUsing the q1and q2parameterization from Kipping (2013a).
14 Cañas et al.
5. DISCUSSION
5.1. Brightness Temperature
The depth of the secondary eclipse observed
in TESS can be modeled as a function of vari-
ous fundamental properties (e.g., Charbonneau
et al. 2005;Esteves et al. 2013;Shporer 2017):
Depth = R2
R?2Rτ(λ)F2(λ, T2)
Rτ(λ)F?,ν (λ, Te)+AgR2
a2
,
(1)
where τ(λ)is the TESS transmission function3,
Teand F?,ν (λ, Te)are the effective temperature
and flux of the host star, T2and F2(λ, T2)
is the brightness temperature and flux of TOI-
2119.01, and Agis the geometric albedo. The
value of (R2/a)2= 70 ppm and, even if the geo-
metric albedo were unity, this is a small fraction
of the eclipse depth (1400 ±200 ppm). Marley
et al. (1999) provide a more realistic geometric
albedo of Ag0.1for a massive brown dwarf
transiting an early M dwarf. This value of Ag
would limit the contribution from the second
term of Equation 1to 7ppm. For TOI-2119,
we ignore any contribution to the eclipse depth
from reflected light because the contribution
from these terms are .10 ppm, a value which is
below the precision of TESS. The contribution
from ellipsoidal variations (e.g., Shporer 2017)
is similarly negligible and would have an ampli-
tude .10 ppm. To solve for the brightness tem-
perature of TOI-2119.01, we use our posterior
distribution of the eclipse depth and estimate
the fluxes of TOI-2119 and its brown dwarf com-
panion using the BT-Settl models (Allard et al.
2012a,b) based on the Caffau et al. (2011) solar
abundances. To account for the uncertainties in
the stellar parameters, we calculate the poste-
rior distribution of the brightness temperature
with a Monte Carlo sampling method where
we use the stellar parameters derived from our
HPF-SpecMatch analysis as Gaussian priors and
3https://heasarc.gsfc.nasa.gov/docs/tess/
the-tess-space-telescope.html#bandpass
allow them to vary while deriving the tempera-
ture.
This analysis yields a temperature of T2=
2100 ±80 K for TOI-2119.01. The measured
temperature of TOI-2119.01 spans the M-L
transition (between 2000-2500 K; Allard et al.
2013) and is comparable to those of early L-
dwarfs from field (e.g., Helling & Casewell 2014;
Zhang et al. 2017) and astrometric discoveries
(Dupuy & Liu 2017). Future observations of
TESS in Sectors 51 and 52 will provide addi-
tional eclipses to further improve the eclipse
depth and temperature determination. Addi-
tional observations of the secondary eclipse in
different bandpasses (e.g., Beatty et al. 2014;
Croll et al. 2015;Beatty et al. 2017) are nec-
essary to probe the dayside eclipse spectrum of
TOI-2119.01 and compare it to theoretical at-
mospheric models (e.g., Beatty et al. 2020).
5.2. Comparison to the existing brown dwarf
population
An analysis of the Kepler (Santerne et al.
2016) and CoRoT (Csizmadia & CoRot Team
2016) samples of short-period transiting brown
dwarfs revealed that they are rare with an oc-
currence rate of 0.20.3% around Sun-
like stars. Including TOI-2119.01, there are 48
known transiting brown dwarfs, 9 of which have
M dwarf host stars. Figure 6shows the loca-
tion of TOI-2119.01 on a mass-radius diagram
of transiting brown dwarfs compiled from the
literature (Carmichael et al. 2020;Mireles et al.
2020;Casewell et al. 2020;Grieves et al. 2021).
For comparison, we include the cloudless, solar
metallicity evolutionary models by Marley et al.
(2021) and the evolutionary models by Phillips
et al. (2020), each calculated in chemical equi-
librium, at age of 0.5, 5 and 10 Gyr.
TOI-2119.01 is located in a cluster of other
high-mass brown dwarfs and appears to be con-
sistent (within 2σ) with brown dwarf models for
ages <1Gyr. This age is within the range
of 0.75.1Gyr determined from the rotation
An eccentric, eclipsing MD-BD system 15
period (see Section 3.3). The small (<2σ) dis-
crepancy between the observed mass and radius
and the predicted values for models between
0.75.1Gyrs is also seen with different evolu-
tionary tracks, such as the models from Baraffe
et al. (2003) and Saumon & Marley (2008).
Figure 6shows that the brown dwarf models
evolve quickly for objects <5Gyr, emphasizing
the need for precise ages to accurately discrim-
inate between these models. A small discrep-
ancy is not surprising because of the approxi-
mate age determination and the complexity of
model atmospheres and the equation of state
for these ultracool objects (see Burrows et al.
2011;Chabrier et al. 2019;Phillips et al. 2020;
Marley et al. 2021). Some of the observed infla-
tion in radii for very low-mass stars and brown
dwarfs may be attributed to strong magnetic ac-
tivity, which would inhibit efficient convection
(e.g., López-Morales 2007;Torres et al. 2010;
Stassun et al. 2012;MacDonald & Mullan 2017)
and result in a larger radius when compared to
evolutionary models. We have also ignored the
role of the stellar insolation for the shortest pe-
riod brown dwarfs. Additional well character-
ized transiting brown dwarfs, particularly those
with known ages, are necessary to further im-
prove evolutionary tracks of brown dwarfs.
From a statistical analysis of the brown dwarf
population, Ma&Ge(2014) postulated the
brown dwarf sample is comprised of two popu-
lations: (i) a low-mass group with M < 42.5MJ
with an eccentricity distribution comparable to
gas giants and (ii) a high-mass group with an
eccentricity distribution comparable to binary
stars. Figure 7compares TOI-2119.01 on the
period-eccentricity diagrams with the transit-
ing brown dwarfs described above and from the
catalogue compiled by Ma & Ge (2014). TOI-
2119.01 is the most eccentric high-mass brown
dwarf with a period <10 days. The period and
eccentricity of TOI-2119 is inconsistent with the
10 day circularization period (plotted as a
dashed line in Figure 7) observed in M dwarf bi-
naries (e.g., Udry et al. 2000;Mayor et al. 2001)
and the 1012 day circularization period ob-
served in sun-like binaries (e.g., Duquennoy &
Mayor 1991;Meibom & Mathieu 2005;Ragha-
van et al. 2010). We note that two systems in
young clusters with ages <1Gyr, the 2M0535-
05 (Stassun et al. 2006) and AD 3116 (Gillen
et al. 2017) systems, host massive brown dwarfs
with non-zero eccentricities with P < 10 days,
but this is not surprising given their young ages.
TOI-2119.01 is consistent with the shorter cir-
cularization period of 35days that has
been suggested for the giant exoplanet popu-
lation (e.g., Halbwachs et al. 2005;Pont et al.
2011;Bonomo et al. 2017).
5.3. Potential Formation Mechanisms
There are various mechanisms of formation
for brown dwarfs (see Whitworth et al. 2007;
Chabrier et al. 2014;Whitworth 2018) including
gravitational instability and turbulent fragmen-
tation of a molecular cloud (Padoan & Nord-
lund 2002,2004;Hennebelle & Chabrier 2008),
disk instability and migration (e.g., Helled et al.
2014;Kratter & Lodato 2016;Nayakshin 2017;
Müller et al. 2018), and core accretion for low-
mass brown dwarfs (e.g., Lambrechts & Jo-
hansen 2012;Mollière & Mordasini 2012). The
mass of TOI-2119.01 means it may have been
formed through gravitational instability in a
disk, and simulations by Forgan et al. (2018)
show that dynamical interactions and scattering
between fragments in a gravitationally unstable
disk can readily form brown dwarf systems in a
variety of configurations. Subsequent fragment-
fragment interactions during formation can lead
to inward scattering and produce a population
of low semi-major axis, high-eccentricity ob-
jects. As such, the observed eccentricity of TOI-
2119.01 may be an imprint of high-eccentricity
migration.
16 Cañas et al.
Figure 6. The brown dwarf mass-radius diagram showing TOI-2119.01 and all substellar companions from
the literature with masses between 10 90MJand radii <2RJ. Grey circles are systems with non-M dwarf
hosts while red diamonds are brown dwarfs transiting M dwarfs. TOI-2119 is plotted as the blue star. Brown
dwarfs with radii >2RJare found in young clusters with ages 1Gyr. The solid lines (Sonora) are the
cloudless, solar metallicity evolutionary tracks from Marley et al. (2021) and the dash-dotted lines (ATMO)
are the evolutionary tracks from Phillips et al. (2020) at ages 0.5, 5, and 10 Gyr. Contour lines of fixed log g
values are included for reference.
5.4. Astrometric Constraints on Additional
Companions
The high eccentricity of TOI-2119.01 may be
the result of dynamical interactions with a long-
period companion. We use Gaia EDR3 to probe
the existence of a possible tertiary compan-
ion in the system. TOI-2119 is not listed as
a likely wide binary from the study of proper
motions by El-Badry et al. (2021). Despite
no clear bound companion among other Gaia
sources, the precision of Gaia EDR3 allows us
to probe binarity using the re-normalized unit
weight error (RUWE), which is the square root
of the reduced χ2statistic that has been cor-
rected for calibration errors, and the excess as-
trometric noise (), a measure of the additional
noise required to explain the scatter from the
derived astrometric solution (Lindegren et al.
2018,2021). Lindegren et al. (2021) note that
these values are sensitive to the photocentric
motions of unresolved objects.
For orbital periods much shorter than the
baseline of observations, the astrometric wobble
of the primary star around the center of mass
may appear as noise when adopting a single-star
astrometric solution (e.g., Kervella et al. 2019;
Kiefer et al. 2019). A large excess astromet-
ric noise with a significance value, D > 2, or
a large RUWE has been shown to be a likely
indicator of unresolved companions in recent
studies (e.g., Belokurov et al. 2020;Penoyre
et al. 2020;Gandhi et al. 2020;Stassun & Torres
2021). TOI-2119 has an RUWE of 1.9314 and
= 0.2644 mas with a significance of D= 161.3
in Gaia EDR3. Penoyre et al. (2020) note that
An eccentric, eclipsing MD-BD system 17
Figure 7. The eccentricity as a function of the period for brown dwarfs from the catalogues of Ma & Ge
(2014), Carmichael et al. (2020), Mireles et al. (2020), Casewell et al. (2020), and Grieves et al. (2021).
Triangles denote the brown dwarfs with masses smaller than 42.5 MJwhile the squares are larger than this
mass. Systems in young clusters: 2M0535-05A/B (Stassun et al. 2006), AD 3116 (Gillen et al. 2017), RIK 72
(David et al. 2019), and 2M1510Aa/b (Triaud et al. 2020) are denoted with yellow markers. The dashed line
indicates the maximum eccentricity for systems unaffected by tides when adopting a circularization period
of 10 days. TOI-2119.01, the blue square, is most eccentric high-mass brown dwarf discovered with a period
<10 days.
an RUWE >1.4could reliably be used to iden-
tify binary systems in an analysis of mock and
real data of short period binaries. We use the
analytical expression in Equation 17 of Penoyre
et al. (2020) to calculate the projected astromet-
ric scatter from a single-body astrometric fit, δθ,
assuming that many periods of the binary are
observed, as
δθ =$a|ql|
(1 + q) (1 + l)s1sin2i
23 + sin2i(cos2ω2)
4e2,
(2)
where ais the semi-major axis in au, q=
M2/M?is the mass ratio, l=L2/L?is the lu-
minosity ratio, eis the eccentricity, iis the in-
clination, ωis the argument of pericenter, and
$is the parallax in mas. Equation 2should be
a suitable approximation for the expected scat-
ter due to TOI-2119.01 because Gaia EDR3’s
temporal baseline (34 months) is much larger
than the orbital period (7.2 days). The lu-
minosity ratio is estimated as l0.00065 in
the Gaia bandpass4using the BT-Settl mod-
els from Section 5.1. The expected astrometric
noise from all observations contained in Gaia
EDR3 is δθ = 0.1420 mas. Even if we use
the Gaia Observation Scheduling Tool5to ob-
tain the timestamps for the Gaia EDR3 obser-
vations (79 different scans) and calculate the
two-dimensional deviations at each Gaia obser-
vation following Appendix B from Penoyre et al.
(2020), we obtain a time averaged δθ = 0.1406
mas. This value is more than half of the ob-
served and we have ignored other sources of
error, such that that the observed may be
completely consistent with the presence of TOI-
2119.01 (q= 0.12±0.02). The larger may also
be a potential indicator of an additional, long-
distance companion, but we note that other
scenarios, such as a close (within 0.1500), faint
4https://www.cosmos.esa.int/web/gaia/edr3-passbands
5https://gaia.esac.esa.int/gost/
18 Cañas et al.
on-sky companion (e.g., Ziegler et al. 2020;Be-
lokurov et al. 2020) could enhance the value of
. Future releases from Gaia will allow a proper
motion analysis (e.g., Kervella et al. 2019) of
TOI-2119 to evaluate the possibility of a third
body in the system.
5.5. RV Constraints on Additional
Companions
We use thejoker (Price-Whelan et al. 2017)
to perform a rejection sampling analysis on the
residuals of our fit to the HPF RVs to constrain
the potential mass of a third companion in the
TOI-2119 system. The HPF data have a tem-
poral baseline of 249 days and, for the rejection
sampling we consider any orbits with periods
P < 10000 days. The analysis with thejoker
uses a log-uniform prior for the period (between
8< P < 10000 days), the Beta distribution
from Kipping (2013b) as a prior for the eccen-
tricity, and a uniform prior for the argument of
pericenter and the orbital phase. Out of the
>×108(227) samples analyzed with thejoker,
a total of 56653 samples survived (0.05% ac-
ceptance rate). The masses from the surviving
samples let us reject the presence of any addi-
tional low-inclination (sin i1) brown dwarfs
(M < 11MJ) within 7.4 au of TOI-2119. Any
tertiary companion, even on an inclined orbit,
would need to have a period short enough for
Gaia to detect binary motion (100 years, see
Penoyre et al. 2020).
5.6. Tidal Evolution of TOI-2119
TOI-2119.01 is on a short-period orbit with
non-zero eccentricity and we may expect that
tides have affected its orbit (e.g., Mazeh 2008;
Damiani & Díaz 2016). Changes to the orbit
may be due to tidal torques that emerge either
from the deformation of the brown dwarf by the
host star or from the deformation of the host
star by a brown dwarf (Hut 1981). Tidal dis-
sipation and magnetic braking from the spin-
down of TOI-2119 should cause orbital decay of
the brown dwarf (e.g., Damiani & Lanza 2015).
To estimate the timescales for circularization
and inspiral, we adopt the tidal model presented
in Equations 1 and 2 of Jackson et al. (2008)
and, similar to Persson et al. (2019), we define
the following timescales for inspiral, τa, and cir-
cularization, τe:
1
τa
=1
τa,?
+1
τa,BD
(3)
1
τe
=1
τe,?
+1
τe,BD
(4)
1
τa,?
=a13/2
BD
9
2rG
M?
R5
?MBD
Q?
,(5)
1
τa,BD
=a13/2
BD
63
2pGM3
?
R5
BDe2
BD
QBDMBD
,(6)
1
τe,?
=a13/2
BD
171
16 rG
M?
R5
?MBD
Q?
,(7)
1
τe,BD
=a13/2
BD
63
4pGM3
?
R5
BD
QBDMBD
(8)
where τe,? and τe,BD represent the contribu-
tions to the circularization timescale and τa,?
and τa,BD are the contributions to the inspiral
timescale due to tides raised on the star and
the BD, respectively. For the tidal quality fac-
tors, we assume the brown dwarf is compara-
ble to Jupiter and adopt a value of QBD = 105
(see Goldreich & Soter 1966;Lainey et al. 2009;
Lainey 2016). We adopt a nominal value of
Q?= 107for TOI-2119 based on the model-
ing of Gallet et al. (2017) and, for simplicity,
assume the tidal dissipation factors remain con-
stant. We note that Qwill change as the star
or brown dwarf evolve (e.g., Barker & Ogilvie
2009;Gallet et al. 2017). Using the system pa-
rameters from Table 3, we estimate a timescale
for circularization of 56 Gyr and a timescale
for in-spiral of 221 Gyr. In each case, the
tides raised on the star by the brown dwarf
(τ?) dominate the timescales. The circulariza-
tion timescale is 180 Gyr even with a second
order expansion to account for the moderate ec-
centricity of the orbit (Equation 2 in Adams &
An eccentric, eclipsing MD-BD system 19
Laughlin 2006). The orbit of TOI-2119 is not
evolving due to tides and should remain unper-
turbed for the main sequence lifetime of TOI-
2119.
Equations 5-8have a large dependency on the
radii of each object (R5) and the timescales
should change as both the host star and brown
dwarf evolve. We use the evolutionary models of
Baraffe et al. (2015) to obtain the radii of TOI-
2119 and TOI-2119.01 as a function of time to
explore the evolution of the circularization and
inspiral timescales. Figure 8shows that for the
lifetime of TOI-2119, the ages for circulariza-
tion and inspiral are orders of magnitude larger
than the system age, except for ages of a few
Myr when the system age is comparable to the
timescale for circularization. If the observed ec-
centricity is primordial, TOI-2119.01 may have
briefly begun circularization early in the system
but then stopped as the host star contracted.
The circularization timescale is only compara-
ble to the system age for a few million years and
the eccentricity of the system has probably not
changed significantly due to tides.
5.7. Potential for Measuring the True
Spin-Orbit Alignment
The projected spin-orbit angle (λ), or the pro-
jected angle between the stellar rotation axis
and the normal to the planet of the orbit, can
shed light on the dynamical and formation his-
tory of a system (e.g., Winn & Fabrycky 2015;
Dawson & Johnson 2018). Measurements of λ
for massive (>3MJ), hot (Te>6000K) plan-
ets and brown dwarfs have revealed that most
of these systems are less likely to be retrograde
and have lower values of |λ|(e.g., Hébrard et al.
2010,2011;Triaud 2018;Zhou et al. 2019).
There are only a few measurements of λfor
transiting objects in the mass range 10 80 MJ:
HAT-P-2 b Loeillet et al. (2008), CoRoT-3 b
(Triaud et al. 2009), XO-3 b (Hirano et al.
2011), KELT-1 b (Siverd et al. 2012), WASP-18
b (Albrecht et al. 2012), and HATS-70 b (Zhou
et al. 2019). These objects orbit hot stars above
the Kraft break (Kraft 1967), the region where
stars become fully-radiative (Te6100 K) and
are observed to have low projected stellar obliq-
uities |λ|(see Zhou et al. 2019). The lack of
high |λ|for these massive substellar companions
is thought to be a result of tidal realignment, as
the realignment timescale is dependent on the
mass ratio q2(e.g., Barker & Ogilvie 2009;
Dawson 2014;Triaud 2018).
Unlike the current set of brown dwarfs orbit-
ing FGK dwarfs with obliquity measurements,
TOI-2119 orbits a much cooler M dwarf and
any primordial misalignment should still be
present because the expected timescale for spin-
orbit alignment τi>1012 years (e.g., Barker
& Ogilvie 2009) if we adopt Q?= 107and
QBD = 105. With only an upper limit of
vsin i?<2km/s, we have no constraint on the
stellar inclination and we recover a uniform dis-
tribution for cos i?when using the formalism of
Masuda & Winn (2020) to estimate the stellar
inclination. If the stellar equator is well-aligned
with a viewer such that sin i?= 1, we expect
a rotational velocity of vsin i?= 1.95 ±0.05
km/s from our measured rotation period and
stellar radius. TOI-2119 is an early M dwarf
with a peak in its SED at around 0.91mi-
crons which makes it possible to determine the
value of vsin iusing any precise optical spec-
trograph with a higher resolution than HPF
(R55000), such as MAROON-X (Seifahrt
et al. 2016), EXPRES (Jurgenson et al. 2016),
CARMENES (Quirrenbach et al. 2014,2018),
or NEID (Schwab et al. 2016).
Observations with many of these precise spec-
trographs would also enable a detection of the
Rossiter-McLaughlin effect (RM effect; Winn
2010;Triaud 2018) to measure λ. For TOI-2119,
the possibility of independent measurements of
vsin i?,λ, and Prot would allow for an estimate
of the spin-orbit angle, ψ.ψis one of a few
fundamental orbital parameters and can serve
20 Cañas et al.
Figure 8. Above are the timescales for inspiral, τa, and circularization, τe, derived using Equations 3and 4,
adopting Q?= 107and QBD = 105, and using the radii for the M dwarf and brown dwarf from evolutionary
models by Baraffe et al. (2015). The dashed line is plotted for reference and corresponds where the time
scale is equal to the system’s age.
as a potential diagnostic of theories of migra-
tion (e.g., Fabrycky & Winn 2009). A first or-
der estimate for the amplitude of the RM effect
is V= 2/3 (R1/R?)2vsin i?1b2(Equa-
tion 1, Triaud 2018). We estimate an ampli-
tude of 50 m/s for TOI-2119.01, if we assume
vsin i?= 1.95 km/s, and this precision can
be achieved with current spectrographs (e.g.,
MAROON-X, EXPRES, CARMENES, NEID).
A measurement of ψin the TOI-2119 system
will enable a complete dynamical characteriza-
tion to inform us how this system could have
formed.
6. SUMMARY
We report the discovery of a brown dwarf
(M2= 67 ±2MJand R2= 1.11 ±0.03RJ) on
an eccentric (e=0.3362 ±0.0005), short period
orbit (P= 7.200861±0.000005 days) transiting
and occulting the M dwarf TOI-2119. The rota-
tion period of 13.2±0.2days suggests the system
probably has an age between 0.75.1Gyrs while
evolutionary models for brown dwarfs favor ages
<1Gyr. The difficulty in constraining the age
of TOI-2119 limits our ability to use the brown
dwarf companion to further constrain evolution-
ary models. The secondary eclipses observed
by TESS reveal a temperature of 2100 ±80 K
for TOI-2119.01, which is consistent with mea-
sured temperatures of L0-L2 dwarfs. The high
eccentricity and excess astrometric noise from
Gaia EDR3 are suggestive of an additional com-
panion in this system, but we can only exclude
the existence of massive brown dwarfs on low-
inclination (sin i1) orbits with our RVs. The
precision of the orbital parameters of TOI-2119
will enable detailed astrometric analysis of fu-
ture Gaia releases to confirm the existence of a
An eccentric, eclipsing MD-BD system 21
distant, tertiary companion. The precise deter-
mination of the orbital inclination and rotation
period make this system amenable to a mea-
surement of the true spin-orbit angle with obser-
vations from high resolution spectrometers. A
measurement of ψwould be the first for a mas-
sive substellar companion around a cool host
star and will further our understanding of the
dynamical history of TOI-2119.
ACKNOWLEDGMENTS
We thank the anonymous referee for a
thoughtful reading of the manuscript, and for
useful suggestions and comments which made
for a clearer manuscript. This work was sup-
ported by NASA Headquarters under the NASA
Earth and Space Science Fellowship Program
through grant 80NSSC18K1114 and by the Al-
fred P. Sloan Foundation’s Minority Ph.D. Pro-
gram through grant G-2016-20166039. The
Center for Exoplanets and Habitable Worlds is
supported by the Pennsylvania State University
and the Eberly College of Science.
These results are based on observations ob-
tained with the Habitable-zone Planet Finder
Spectrograph on the HET. We acknowledge
support from NSF grants AST 1006676, AST
1126413, AST 1310875, AST 1310885, AST
2009889, AST 2108512 and the NASA Astro-
biology Institute (NNA09DA76A) in our pur-
suit of precision radial velocities in the NIR.
We acknowledge support from the Heising-
Simons Foundation via grant 2017-0494. The
Hobby-Eberly Telescope is a joint project of
the University of Texas at Austin, the Penn-
sylvania State University, Ludwig-Maximilians-
Universität München, and Georg-August Uni-
versität Gottingen. The HET is named in honor
of its principal benefactors, William P. Hobby
and Robert E. Eberly. The HET collaboration
acknowledges the support and resources from
the Texas Advanced Computing Center. We
are grateful to the HET Resident Astronomers
and Telescope Operators for their valuable as-
sistance in gathering our HPF data. We would
like to acknowledge that the HET is built on In-
digenous land. Moreover, we would like to ac-
knowledge and pay our respects to the Carrizo
& Comecrudo, Coahuiltecan, Caddo, Tonkawa,
Comanche, Lipan Apache, Alabama-Coushatta,
Kickapoo, Tigua Pueblo, and all the American
Indian and Indigenous Peoples and communities
who have been or have become a part of these
lands and territories in Texas, here on Turtle
Island.
Computations for this research were per-
formed on the Pennsylvania State University’s
Institute for Computational and Data Sci-
ences’ Roar supercomputer, including the Cy-
berLAMP cluster supported by NSF grant
MRI-1626251.
Some of the data presented in this paper were
obtained from from the Mikulski Archive for
Space Telescopes (MAST) at the Space Tele-
scope Science Institute. The specific observa-
tions analyzed can be accessed via 10.17909/t9-
v3f8-w427. Support for MAST for non-HST
data is provided by the NASA Office of Space
Science via grant NNX09AF08G and by other
grants and contracts. This work includes data
collected by the TESS mission, which are pub-
licly available from MAST. Funding for the
TESS mission is provided by the NASA Sci-
ence Mission directorate. This research made
use of the NASA Exoplanet Archive, which
is operated by Caltech, under contract with
NASA under the Exoplanet Exploration Pro-
gram. This research has made use of the SIM-
BAD database, operated at CDS, Strasbourg,
France, and NASA’s Astrophysics Data Sys-
tem Bibliographic Services. 2MASS is a joint
project of the University of Massachusetts and
IPAC at Caltech, funded by NASA and the
NSF.
These results are based on observations ob-
tained with the 3 m Shane Telescope at Lick Ob-
servatory. We acknowledge support from NSF
22 Cañas et al.
grant AST 1910954. The authors thank the
Shane telescope operators, AO operators, and
laser operators for their assistance in obtaining
these data.
Some of the observations in this paper made
use of the NN-EXPLORE Exoplanet and Stellar
Speckle Imager (NESSI). NESSI was funded by
the NASA Exoplanet Exploration Program and
the NASA Ames Research Center. NESSI was
built at the Ames Research Center by Steve B.
Howell, Nic Scott, Elliott P. Horch, and Emmett
Quigley. The authors thank Mark E. Everett for
assistance in obtaining these data.
These results are based on observations ob-
tained with the Samuel Oschin Telescope 48-
inch and the 60-inch Telescope at the Palo-
mar Observatory as part of the Zwicky Tran-
sient Facility project. ZTF is supported by
the National Science Foundation under Grant
No. AST-2034437 and a collaboration including
Caltech, IPAC, the Weizmann Institute for Sci-
ence, the Oskar Klein Center at Stockholm Uni-
versity, the University of Maryland, Deutsches
Elektronen-Synchrotron and Humboldt Univer-
sity, the TANGO Consortium of Taiwan, the
University of Wisconsin at Milwaukee, Trinity
College Dublin, Lawrence Livermore National
Laboratories, and IN2P3, France. Operations
are conducted by COO, IPAC, and UW. This
work makes use of data from the first public re-
lease of the WASP data (Butters et al. 2010) as
provided by the WASP consortium and services
at the NASA Exoplanet Archive, which is oper-
ated by the California Institute of Technology,
under contract with NASA under the Exoplanet
Exploration Program.
This work has made use of data from
the European Space Agency (ESA) mission
Gaia (https://www.cosmos.esa.int/gaia), pro-
cessed by the Gaia Data Processing and Anal-
ysis 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 par-
ticipating in the Gaia Multilateral Agreement.
Facilities: ASAS, Gaia, HET (HPF),
PO:1.2m (ZTF), PO:1.5m (ZTF), Shane, Su-
perWASP, TESS, WIYN (NESSI)
Software: allesfitter (Günther &
Daylan 2019), astroquery (Ginsburg et al.
2019), astropy (Astropy Collaboration et al.
2018), barycorrpy (Kanodia & Wright 2018),
dynesty (Speagle 2020), ellc (Maxted 2016),
EXOFASTv2 (Eastman et al. 2019), HPF-SERVAL,
HPF-SpecMatch,juliet (Espinoza et al. 2019),
matplotlib (Hunter 2007), numpy (van der
Walt et al. 2011), pandas (McKinney 2010),
scipy (Virtanen et al. 2020), telfit (Gullik-
son et al. 2014), thejoker (Price-Whelan et al.
2017)
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