arXiv subjects

David W. Latham

Publications and source records attributed to David W. Latham.

At least 37 records · Page 2Linked to original sources

arXiv2026

Ultraviolet spectroscopy reveals a hot and luminous companion to the Be star+black hole candidate MWC 656

The Galactic Be star binary MWC 656 was long considered the only known Be star+black hole (BH) system, making it a critical benchmark for models of massive binary evolution and for the expected X-ray emission of Be+BH binaries. However, recent dynamical measurements cast doubt on the presence of a BH companion. We present new multi-epoch ultraviolet spectroscopy from the Hubble Space Telescope, combined with high-resolution optical spectra, to reassess the nature of the companion. The far-ultraviolet spectra reveal high-ionisation features -- including prominent N v and He ii lines -- which are absent in the spectra of normal Be stars and are indicative of a hot, luminous companion. Spectral modelling shows that these features cannot originate from the Be star or from an accretion disc around a compact object. Instead, we find that the data are best explained by a hot ($T_\mathrm{eff} \approx 85$ kK), compact, hydrogen-deficient star with strong wind signatures, consistent with an intermediate-mass stripped star. Our revised orbital solution and composite spectroscopic modelling yield a companion mass of $M_2 = 1.48^{+0.55}_{-0.46}\,\mathrm{M}_\odot$, definitively ruling out a BH and disfavouring a white dwarf. MWC 656 thus joins the growing class of Be+stripped star binaries. The system's unusual properties -- including a high companion temperature and wind strength -- extend the known parameter space of such binaries. The continued absence of confirmed OBe+BH binaries in the Galaxy highlights a growing tension with population synthesis models.

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arXiv2026

A Planetary Illusion's Funeral: Non-detection of a Gaia DR3 Exoplanet Candidate, and the Role of Intermediate-precision Radial Velocities in Gaia Exoplanet Follow-up

The detection of exoplanets using astrometry has long been an area of interest, but is fraught with challenges. The Gaia mission is fundamentally reshaping this field thanks to its unprecedentedly precise all-sky astrometric observations. The 2022 release of Gaia DR3 brought the first exoplanets discovered from the Gaia astrometry, including a new candidate around the bright ($V=6.6$) solar-type star HD 12800. However, two years after announcement, the Gaia exoplanet candidate was retracted. In this work we report radial velocity observations of HD 12800 acquired with the TRES spectrograph, which we began immediately after the release of Gaia DR3. Our observations failed to detect the planet candidate; nonetheless, we emphasise that the originally proposed companion would have been easily detected in our radial velocity observations. We conclude with a discussion on the role of intermediate-precision ($\approx$10 m s$^{-1}$) RV spectrographs in the follow-up of Gaia astrometric exoplanet candidates, relevant to the forthcoming release of Gaia DR4. We argue that such observations may play an important role in planet confirmation for stars between approximately $8<G<12$, likely to represent a significant fraction of Gaia exoplanet discoveries.

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arXiv2026

Long-term Spectroscopic Survey of the Hyades Cluster: The Binary Population

We report the results of a radial velocity monitoring program in the Hyades region, carried out at the Center for Astrophysics over a period of more than 45 yr. Nearly 12,000 spectra were gathered for 625 stars brighter than $V \approx 14.5$, of which 55% are members or possible members of the cluster. New or updated spectroscopic orbital solutions are presented for more than 100 members and non-members, including several triple systems. In a few cases we incorporate available astrometry. The frequency of binaries in the Hyades with periods up to $10^4$ days is determined to be $40 \pm 5$%, after corrections for incompleteness. This is marginally higher than in other open clusters. The orbital period and eccentricity distributions are found to be similar to those of solar-type binaries in the field. The mass ratio distribution is essentially flat, or slightly rising toward mass ratios of unity. We revisit the determination of the tidal circularization period, obtaining a longer $P_{\rm circ}$ value of $5.9 \pm 1.1$ days compared to the previous estimate of 3.2 days, still somewhat short of the value expected if most or all of the action of tides happens during the pre-main-sequence phase. We estimate a line-of-sight velocity dispersion of $0.21 \pm 0.05$ km s$^{-1}$ within 5.5 pc of the cluster center (approximately the half-mass radius) and a larger dispersion beyond that distance. Our velocity measurements are accurate enough to clearly reveal the signatures of gravitational redshift and convective blueshift among the dwarfs and giants in the Hyades.

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arXiv2026

Two warm sub-Saturn mass planets identified from the TESS Full Frame Images

Context. Characterization of warm giants is crucial to constrain giant planet formation and evolution. Measuring the mass and radius of these planets, combined with their moderated irradiation, allows us to estimate their planetary bulk composition, which is a key quantity to comprehend giant planet formation and structure. Aims. We present the discovery of two transiting warm giant planets orbiting solar-type stars from the Transiting Exoplanet Survey Satellite (TESS), which were characterized by further spectroscopic and photometric ground-based observations. Methods. We performed a joint analysis of photometric data with radial velocities to confirm and characterize TOI-883 b and TOI-899 b, two sub-Saturns orbiting solar-like stars. Results. TOI-883 b and TOI-899 b have masses of $0.123 \pm 0.012$ $M_J$ and $0.213 \pm 0.024$ $M_J$, radius of $0.604 \pm 0.028$ $R_J$ and $0.991 \pm 0.044$ $R_J$, periods of $10.06$ d and $12.85$ d and equilibrium temperature of $1086 \pm 19$ K and $1040 \pm 19$ K, respectively. Conclusions. While having similar masses, orbital periods and stellar host properties, these planets seem to have different internal compositions, which could point to distinct formation histories. Both planets are suitable targets for atmospheric studies to further constrain formation scenarios of planets in the Neptune-Saturn mass range

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arXiv2026

Gas-depleted planet formation occurred in the four-planet system around the red dwarf LHS 1903

Small exoplanet radii show two populations, referred to as super-Earths and sub-Neptunes, separated by a gap known as the radius valley. This may be produced by the removal of atmospheres due to stellar or internal heating, or lack of an initial envelope. We us transit photometry and radial velocity measurements to detect and characterize four planets orbiting LHS 1903, a red dwarf (M-dwarf) star in the Milky Way's thick disk. The planets have orbital periods between 2.2 and 29.3 days, and span the radius valley within a single planetary system. The derived densities indicate that LHS 1903 b is rocky, while LHS 1903 c and LHS 1903 d have extended atmospheres. Although the most distant planet from the host star, LHS 1903 e, has no gaseous envelope, indicating it formed from gas-depleted material.

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arXiv2026

Radial and Rotational Velocities of a Volume-Complete Sample of M Dwarfs with Masses 0.1-0.3 Msun within 15 parsecs

We present the results from a five-year campaign to gather multi-epoch, high-resolution spectra of a volume-complete sample of 413 M dwarfs with masses 10-30% that of the Sun that lie within 15 parsecs. We report weighted mean systemic radial velocities (RV) and rotational broadening measurements ($v \sin i$) for our targets. Our typical relative RV uncertainties are less than $50$ m/s for the isolated, slowly rotating targets in our sample, and increase but remain less than 1 km/s for more rapidly rotating stars. The majority of the single stars in our sample ($71\pm3$%) have rotational broadening below our detection limit of 3.4 km/s. When combined with astrometric data, our radial velocities allow us to calculate galactic space motions, which we use to assign thin or thick disk membership. We determine that 81% and 8% of our sample are highly probable thin and highly probable thick disk members, respectively. We report seven new multi-lined multiple systems and identify six additional targets with velocity variations indicative of long-period binaries, of which three are new detections. Finally, we find no significant difference in the stellar multiplicity rates of the thin disk ($22\pm2$%) and thick disk ($21\pm8$%) populations in our sample, implying that mid-M dwarfs are not significantly losing their companions at these relative ages. Our survey more than triples the number of these fully-convective stars with complete astrometric data and uniformly derived, multi-epoch, high-resolution RVs and rotational broadening measurements.

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arXiv2026

TOI-6692b: An eccentric 130 day period giant planet with a single transit from TESS

We report the discovery and characterization of TOI-6692 b, an eccentric (e~0.54) Jupiter on a 130-day orbit. TOI-6692 b was first detected as a community TESS Object of Interest (cTOI) by the Visual Survey Group and the Planet Hunters group as a single transit candidate via TESS observation. The period was subsequently confirmed via radial velocity monitoring from the Planet Finder Spectrograph on the 6.5m Magellan telescope. Additional radial velocities were acquired with the CHIRON, FEROS, and CORALIE spectrographs. LCOGT ground-based photometric follow-up was conducted over 2 weeks to detect another transit and refine the period. Although we did not detect an ingress or egress of the 11.04 hr transit, we did detect a possible in-transit signal in the multi-night data and provide an updated ephemeris for future monitoring. TOI-6692 b is one of few planets with orbital periods longer than 100 days that have a secure mass, radius, and eccentricity detection. As with most giant planets at these orbital periods, the eccentricity of TOI-6692 b is lower than that expected of planets undergoing high-eccentricity tidal migration, but is more consistent with the expectations of planet-planet scattering outcomes. A long-term radial velocity trend was detected and further monitoring is warranted to determine the outer companion period. TOI-6692 b is also one of few TESS single transit targets that have its period eventually confirmed via follow-up photometric campaigns timed to capture transits despite the relatively large ephemeris uncertainties. Such efforts highlight the capabilities of night-to-night stability on ground-based photometric facilities today.

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arXiv2026

TOI-4495: A Pair of Aligned, Near-Resonant Sub-Neptunes that Likely Experienced Overstable Migration

We report the discovery of a sub-Neptune and a Neptune-like planet ($R_b = 2.48^{+0.14}_{-0.10}\,R_\oplus$, $R_c = 4.03^{+0.23}_{-0.15}\,R_\oplus$) orbiting the F-type star TOI-4495. The planets have orbital periods of 2.567 days and 5.185 days, lying close to a 2:1 mean-motion resonance (MMR). Our photodynamical analysis of the TESS light curves constrains the planetary masses to $M_b = 7.7 \pm 1.4\,M_\oplus$ and $M_c = 23.2 \pm 4.7\,M_\oplus$. The measured masses and radii indicate the presence of volatile-rich gaseous envelopes on both planets. The Rossiter-McLaughlin effect and the Doppler shadow of TOI-4495 c reveal a well-aligned orbit with a projected stellar obliquity of $λ= -2.3^{+8.3}_{-7.8}\,\mathrm{deg}$. Combined with the low mutual inclination constrained by the photodynamical analysis ($ΔI < 8.7\,\mathrm{deg}$), the planetary orbits are likely coplanar and aligned with the host star's spin axis. We show that the planets are near, but not in, the 2:1 MMR, with a circulating resonant angle. We also find substantial free eccentricity for the inner planet, TOI-4495 b ($e_b = 0.078^{+0.020}_{-0.013}$). Given the observed proximity to the 2:1 resonance and the more massive outer planet, TOI-4495 b and c are particularly susceptible to resonant overstability, which can convert resonantly excited eccentricity into free eccentricity. However, additional mechanisms (e.g., planetesimal scattering) may be required to further excite the eccentricity by $\sim 4\%$. To prevent tidal damping from reducing the eccentricity below the observed level over the star's lifetime (1.9 Gyr), the reduced tidal quality factor of TOI-4495 b must be $Q' \gtrsim 10^5$, consistent with the presence of a thick envelope on the planet.

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arXiv2025

Validation of a Third Earth-sized Planet in the TOI-2267 Binary System

We report the validation of a third terrestrial exoplanet in the nearby (22 pc) TOI-2267 system. TOI-2267 is a binary system with stellar components TOI-2267A (M5, 3030 K) and TOI-2267B (M6, 2930 K), with an on-sky separation of 0.$^{\prime\prime}$384 (8 au projected separation). TOI-2267 hosts two Earth-sized planets (TOI-2267 b, $1.00\pm0.11 R_{\oplus}$, and TOI-2267 c, $1.14\pm0.13 R_{\oplus}$, if orbiting the primary star; or $1.22\pm0.29 R_{\oplus}$ and $1.36\pm0.33 R_{\oplus}$, respectively, if orbiting the secondary star) with orbital periods of 2.3 and 3.5 days. This system also contains a third Earth-sized planet candidate with an orbital period of 2.0 days that was previously identified as a likely planet with a low false-positive probability, but could not be firmly validated due to the lack of independent observations beyond TESS data. We combine two new transit observations from the 5.1m Hale Telescope at Palomar Observatory with archival TESS data and high-resolution imaging to statistically validate the planetary nature of TOI-2267 d ($0.98\pm0.09 R_{\oplus}$ if orbiting the primary star, or $1.77\pm0.43 R_{\oplus}$ if orbiting the secondary star) using the updated TRICERATOPS+ pipeline. We attempt to determine the host star for TOI-2267 d using transit shape stellar density analysis, but are unable to conclusively assign a host. Our validation of TOI-2267 d suggests that TOI-2267 is either the first known double transiting M dwarf binary system, or hosts three planets in an extremely compact orbital configuration.

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arXiv2025

An Ancient Brown Dwarf Transiting a Metal-Poor Thick Disk Star

We report the discovery of TOI-7019b, the first transiting brown dwarf (BD) known to orbit a star that is part of the Milky Way's ancient thick disk, as defined chemically ([Fe/H] $= -0.79 \pm 0.05$ dex, [$α$/Fe] $= +0.26 \pm 0.05$ dex, [M/H] $= -0.59 \pm 0.06$ dex) and kinematically ($v_{\perp} \approx 150 \pm 1$ km s$^{-1}$). We estimate a system age $τ= 12 \pm 2$ Gyr by fitting the host star's spectrum and spectral energy distribution to alpha-enhanced isochrones, and independently using the age-metallicity relation of the thick disk. This makes TOI-7019 by far the most metal-poor and ancient BD host known to date. We measure a BD mass of $61.3 \pm 2.1$ $M_{\rm J}$ and radius of $0.82 \pm 0.02$ $R_{\rm J}$ from a joint analysis of transit photometry and radial velocity measurements, along with an orbital period of $48.2592 \pm 0.0001$ days and an orbital eccentricity of $0.403 \pm 0.002$. The measured radius appears $12.3\% \pm 2.8\%$ larger than predicted relative to standard evolutionary models for old, metal-poor brown dwarfs, hinting at missing physics like the magnetic inhibition of convection. TOI-7019b lowers the probed metallicity regime for transiting BDs by over a factor of two, making it a benchmark system to test evolutionary models in the low-metallicity regime. Future measurements of TOI-7019b's atmosphere will test whether a brown dwarf's atmospheric composition tracks its host star's abundances, as expected for binary-like co-formation.

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arXiv2025

Searching for Exoplanets Born Outside the Milky Way: VOYAGERS Survey Design

Observations over the past few decades have found that planets are common around nearby stars in our Galaxy, but little is known about planets that formed outside the Milky Way. We describe the design and early implementation of a survey to test whether planets also exist orbiting the remnant stars of ancient dwarf galaxies that merged with the Milky Way, and if so, how they differ from their Milky Way counterparts. VOYAGERS (Views Of Yore - Ancient Gaia-enceladus Exoplanet Revealing Survey) is a radial velocity (RV) search using precision spectrographs to discover exoplanets orbiting very low metallicity ($-2.8 < [\mathrm{Fe/H}] \leq -0.8$) stars born in the dwarf galaxy Enceladus, which merged with the Milky Way galaxy about 10 Gyr ago. A sample of 22 candidates have been screened from a catalog of Gaia-Enceladus-Sausage (GES) members using a combination of stellar properties and reconnaissance observations from the TRES spectrograph. Precision RV measurements have been initiated using the NEID, HARPS-N, and CARMENES spectrographs. We plan to focus most upcoming observations on 10 main sequence targets. Data collection is well underway, with 778 observations on 22 candidates (385 of which are 10 focus targets), but far from complete. This survey is designed to be sensitive to sub-Neptune mass planets with periods up to hundreds of days. We note that the RV analysis gives mass multiplied by $\sin (inclination)$ or the minimum mass for exoplanets. The expected survey yield is three planets, assuming that occurrence rates are similar to those in the Milky Way and taking into account the degeneracy with inclination in our yield models. Our survey is designed to detect at least one exoplanet if occurrence rates are similar to known Milky Way exoplanets or, if no exoplanets are discovered, to rule out a Milky Way-like planet population in GES with 95% confidence level.

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arXiv2025

Metallicities from High-Resolution TRES Spectra with uberMS: Performance Benchmarks and Literature Comparison

As the field of exoplanetary astronomy has matured, demand has grown for precise stellar abundances to probe subtle correlations between stellar compositions and planetary demographics. However, drawing population-level conclusions from the disparate measurements in the literature is challenging, with various groups measuring metallicities using bespoke codes with differing line lists, radiative transfer models, and other assumptions. Homogeneous analyses are thus critical. Here we use the neural-net framework uberMS to measure iron abundances and alpha enrichments from high-resolution optical spectra observed by the Tillinghast Reflector Echelle Spectrograph (TRES), a key resource used for the follow-up of candidate exoplanet hosts. To contextualize these measurements and benchmark our method's performance, we compare to external constraints on metallicity using the Hyades cluster, wide binaries, and asteroids, to external constraints on $T_{\rm eff}$ and $\log g$ using stars with interferometric radii, and to the results of other abundance measurement methods using overlap samples with the APOGEE and SPOCS catalogs, as well as by applying the SPC method directly to TRES spectra. We find that TRES-uberMS provides reliable parameter estimates with errors of roughly 100 K in $T_{\rm eff}$, 0.09 dex in $\log g$, and 0.04 dex in [Fe/H] for many nearby dwarf stars, although [Fe/H] performance is poorer for mid-to-late K dwarfs, with the bias worsening with decreasing $T_{\rm eff}$. Performance is also worse for evolved stars. For [$α$/Fe], our error may be as good as 0.03 dex for dwarfs based on external benchmarks, despite sizable and variable systematic differences when comparing with specific alpha-element abundances from other catalogs.

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arXiv2025

THYME XIII: Two young Neptunes orbiting a 75-Myr star in the Alpha Persei Cluster

Young planets with mass measurements are particularly valuable in studying atmospheric mass-loss processes, but these planets are rare and their masses difficult to measure due to stellar activity. We report the discovery of a planetary system around TOI-6109, a young, 75 Myr-old Sun-like star in the Alpha Persei cluster. It hosts at least two transiting Neptune-like planets. Using three TESS sectors, 30 CHEOPS orbits, and photometric follow-up observations from the ground, we confirm the signals of the two planets. TOI-6109 b has an orbital period of P=$5.6904^{+0.0004}_{-0.0004}$ days and a radius of R=$4.87^{+0.16}_{-0.12}$ R$_\oplus$. The outer planet, TOI-6109 c has an orbital period of P=$8.5388^{+0.0006}_{-0.0005}$ days and a radius of R=$4.83^{+0.07}_{-0.06}$ R$_\oplus$. These planets orbit just outside a 3:2 mean motion resonance. The near-resonant configuration presents the opportunity to measure the planet's mass via TTV measurements and to bypass difficult RV measurements. Measuring the masses of the planets in this system will allow us to test theoretical models of atmospheric mass loss.

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arXiv2025

Migration and Evolution of giant ExoPlanets (MEEP) II: Super-Jupiters and Lithium-rich Host Stars

Although hot Jupiters were the first exoplanets discovered orbiting main sequence stars, the dominant mechanisms through which they form and evolve are not known. To address the questions surrounding their origins, the Migration and Evolution of giant ExoPlanets (MEEP) survey aims to create a complete, magnitude-limited ($G<$12.5) sample of hot Jupiters that can be used to constrain the frequency of different migration pathways. NASA's Transiting Exoplanet Survey Satellite provides the unique combination of sky-coverage and photometric precision to achieve this goal, which will likely be a key result of the mission. In this second installment of the MEEP survey, we reanalyze one benchmark hot Jupiter system, TOI-4138, and discover four additional super-Jupiters which are each more than five times as massive as Jupiter: TOI-4773 b, TOI-5261 b, TOI-5350 b, and TOI-6420 b. One of these planets, TOI-5261 b, is 11.49 times the mass of Jupiter, nearly massive enough to ignite deuterium fusion, and has an eccentric ($e = 0.1585$) orbit. TOI-4138, TOI-4773, TOI-5350, and TOI-6420 each have lithium absorption features in their spectra. TOI-4138 is an F-type subgiant with a lithium equivalent width of $120. \pm 13$ mÅ, which is $\sim 4.5σ$ larger than the median lithium equivalent width of a control sample of 1381 similar stars, making TOI-4138 a compelling candidate for planetary engulfment.

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arXiv2025

The HD 60779 Planetary System: A Transiting Sub-Neptune on a 30-day Orbit and a More Massive Outer World

We present the discovery of the planetary system orbiting the bright (V = 7.2), nearby (35 pc), Sun-like star HD 60779, which has a mass of 1.050 +/- 0.044 solar masses and a radius of 1.129 +/- 0.013 solar radii. We report two TESS transits and a subsequent CHEOPS transit of HD 60779 b, a sub-Neptune with a radius of 3.250 (+0.100 / -0.098) Earth radii on a 29.986175 (+0.000030 / -0.000033) day orbit. Additionally, 286 HARPS-N radial velocity measurements reveal the mass of planet b (14.7 +1.1 / -1.0 Earth masses) and the presence of an outer planet, HD 60779 c, with an orbital period of 104.25 (+0.30 / -0.29) days and a minimum mass (m sin i) of 27.7 +/- 1.6 Earth masses. Both planets' orbits are consistent with being circular, suggesting that they have a dynamically quiet history. The data are not sufficient to determine whether planet c transits. HD 60779's uniquely high systemic radial velocity (129.75 +/- 0.12 km/s) allows its Lyman-alpha emission to avoid absorption by the interstellar medium, making it a prime candidate for probing atmospheric escape from HD 60779 b. HD 60779 is also the third-brightest host of a sub-Neptune with orbital period greater than 25 days and with both mass and radius measured, distinguishing it in terms of accessibility to spectroscopic characterization.

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arXiv2025

HIP 15429: A newborn Be star on an eccentric binary orbit

We identified a new post-interaction binary, HIP 15429, consisting of a stripped star and a recently formed, rapidly rotating Be star companion ($v \sin i \approx 270$ km/s) sharing many similarities with recently identified bloated stripped stars. From orbital fitting of multi-epoch radial velocities we find a 221-day period. We also find an eccentricity of $e=0.52$, which is unexpectedly high as tides are expected to have circularised the orbit efficiently during the presumed recent mass transfer. The formation of a circumbinary disk during the mass transfer phase or the presence of an unseen tertiary companion might explain the orbit's high eccentricity. We determined physical parameters for both stars by fitting the spectra of the disentangled binary components and multi-band photometry. The stripped nature of the donor star is affirmed by its high luminosity at a low inferred mass ($\lesssim 1 \mathrm{M}_\odot$) and imprints of CNO-processed material in the surface abundances. The donor's relatively large radius and cool temperature ($T_{\mathrm{eff}} = 13.5 \pm 0.5$ kK) suggest that it has only recently ceased mass transfer. Evolutionary models assuming a 5-6 $\mathrm{M}_\odot$ progenitor can reproduce these parameters and imply that the binary is currently evolving towards a stage where the donor becomes a subdwarf orbiting a Be star. The remarkably high eccentricity of HIP 15429 challenges standard tidal evolution models, suggesting either inefficient tidal dissipation or external influences, such as a tertiary companion or circumbinary disk. This underscores the need to identify and characterise more post-mass transfer binaries to benchmark and refine theoretical models of binary evolution.

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arXiv2025

Validation of TESS Planet Candidates with Multi-Color Transit Photometry and TRICERATOPS+

We present an upgraded version of TRICERATOPS, a software package designed to calculate false positive probabilities for planet candidates identified by the Transiting Exoplanet Survey Satellite (TESS). This enhanced framework now incorporates ground-based light curves in separate bandpasses, which are routinely obtained as part of the candidate vetting process. We apply this upgraded framework to explore the planetary nature of 14 TESS planet candidates, combining primarily J band light curves acquired with the 200-inch Hale Telescope at Palomar Observatory with complementary archival observations from the Las Cumbres Observatory Global Telescope (LCOGT), the Fred Lawrence Whipple Observatory (FLWO), and the Teide Observatory, along with existing TESS data and contrast curves from high-resolution imaging. As a result of this analysis we statistically validate (False Positive Probability < 1.5% and Nearby False Positive Probability < 0.1%) six new planets in five systems: TOI-1346 b, TOI-1346 c, TOI-2719 b, TOI-4155 b, TOI-6000 b, and TOI-6324 b. For these systems, we provide updated estimates of their stellar and planetary properties derived from the TESS and ground-based observations. These new systems contain planets with radii between 0.9-6 Re and orbital periods between 0.3-5.5 days. Finally, we use our upgraded version of TRICERATOPS to quantify the relative importance of multi-wavelength transit photometry and high-resolution imaging for exoplanet candidate validation, and discuss which kinds of candidates typically benefit the most from ground-based multi-color transit observations.

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arXiv2025

A New Brown Dwarf Orbiting an M star and An Investigation on the Eccentricity Distribution of Transiting Long-Period Brown Dwarfs

The orbital eccentricities of brown dwarfs encode valuable information of their formation and evolution history, providing insights into whether they resemble giant planets or stellar binaries. Here, we report the discovery of TOI-5575b, a long-period, massive brown dwarf orbiting a low-mass M5V star ($\rm 0.21\pm0.02\,M_\odot$) delivered by the TESS mission. The companion has a mass and radius of $\rm 72.4\pm4.1\,M_J$ and $\rm 0.84\pm0.07\,R_J$ on a 32-day moderately eccentric orbit ($e=0.187\pm0.002$), making it the third highest-mass-ratio transiting brown dwarf system known to date. Building on this discovery, we investigate the eccentricity distributions of a sample of transiting long-period ($10\leq P\lesssim 1000$ days, $\sim$0.1-1.5 AU) giant planets, brown dwarfs and low-mass stars. We find that brown dwarfs exhibit an eccentricity behavior nearly identical to that of giant planets: a preference for circular orbits with a long tail toward high eccentricities. Such a trend contrasts sharply with direct imaging findings, where cold (5-100 AU) brown dwarfs and giant planets display distinct eccentricity distributions. Our results suggest that transiting long-period brown dwarfs and giant planets probably 1) form in different routes at exterior orbits but undergo analogous dynamical evolution processes and migrate inwards; or 2) both contain two sub-groups, one with widely spread eccentricities while the other has circular orbits, that jointly sculpt the eccentricity distributions. The low-mass-star systems appear to be a distinctive population, showing a peak eccentricity at about 0.3, akin to more massive stellar binaries.

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