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Abstract Mass-loss influences stellar evolution, especially for massive stars with strong winds. Stellar wind bow shock nebulae driven by Galactic OB stars can be used to measure mass-loss rates ( $\dot{M}$). The standoff distance (R₀) between the star and the bow shock is set by momentum flux balance between the stellar wind and the surrounding interstellar medium (ISM). We created the Milky Way Project: mass-loss rates for OB Stars driving infrared bow shocks (MOBStIRS) using the online Zooniverse citizen science platform. We enlisted several hundred students to measureR₀and two other projected shape parameters for 764 cataloged infrared bow shocks. MOBStIRS incorporated 1528 JPEG cutout images produced from Spitzer GLIMPSE and MIPSGAL survey data. Measurements were aggregated to compute shape parameters for each bow shock image deemed high quality by participants. The average statistical uncertainty onR₀is 12.5% but varies from <5% to ∼40% among individual bow shocks, contributing significantly to the total error budget of $\dot{M}$. The derived nebular morphologies agree well with (magneto) hydrodynamic simulations of bow shocks driven by the winds of OB stars moving atV_a = 10–40 km s⁻¹with respect to the ambient ISM. A systematic correction toR₀to account for viewing angle appears unnecessary for computing $\dot{M}$. Slightly more than half of MOBStIRS bow shocks are asymmetric, which could indicate anisotropic stellar winds, ISM clumping on sub-pc scales, time-dependent instabilities, and/or misalignments between the local ISM magnetic field and the star-bow-shock axis. 

Abstract Stellar bow shock nebulae are arcuate shock fronts formed by the interaction of radiation-driven stellar winds and the relative motion of the ambient interstellar material. Stellar bow shock nebulae provide a promising means to measure wind-driven mass loss, independent of other established methods. In this work, we characterize the stellar sources at the center of bow shock nebulae drawn from all-sky catalogs of 24μm–selected nebulae. We obtain new, low-resolution blue optical spectra for 104 stars and measure stellar parameters temperatureT_eff, surface gravity $\log g$, and projected rotational broadening $v\sin i$. We perform additional photometric analysis to measure stellar radiusR_*, luminosityL_*, and visual-band extinctionA_V. All but one of our targets are O and early B stars, with temperatures ranging fromT= 16.5 to 46.8 kK, gravities from $\log g=$2.57 to 4.60, and $v\sin i$from <100 to 400 km s⁻¹. With the exception of rapid rotatorζOph, bow shock stars do not rotate at or near critical velocities. At least 60 of 103 (60%) OB bow shock stars are binaries, consistent with the multiplicity fraction of other OB samples. The sample shows a runaway fraction of 23%, with 19 stars havingv_2D≥ 25 km s⁻¹. Of the 19 runaways, at least 15 (≥79%) are binaries, favoring dynamical ejection over the binary supernova channel for producing runaways. We provide a comprehensive census of stellar parameters for bow shock stars, useful as a foundation for determining the mass-loss rates for OB-type stars—one of the single most critical factors in stellar evolution governing the production of neutron stars and black holes. 

Abstract We describe the discovery and characterization of TOI-7149 b, a 0.705 ± 0.075M_J, 1.18 ± 0.045R_Jgas giant on a ∼2.65 days period orbit transiting an M4V star with a mass of 0.344 ± 0.030M_⊙and an effective temperature of 3363 ± 59 K. The planet was first discovered using NASA’s TESS mission, which we confirmed using a combination of ground-based photometry, radial velocities, and speckle imaging. The planet has one of the deepest transits of all known main-sequence planet hosts at ∼12% (R_p/R_⋆∼ 0.33). Pushing the bounds of previous discoveries of giant exoplanets around M-dwarf stars (GEMS), TOI-7149 is one of the lowest mass M-dwarfs to host a transiting giant planet. We compare the sample of transiting GEMS to stars within 200 pc with a Gaia color–magnitude diagram and find that the GEMS hosts are likely to be high metallicity stars. We also analyze the sample of transiting giant planets using the nonparametricMRExoframework to compare the bulk density of warm Jupiters across stellar masses. We confirm our previous result that transiting Jupiters around early M-dwarfs have similar masses and densities to warm Jupiters around FGK stars, and extend this to mid M-dwarfs, thereby suggesting a potential commonality in their formation mechanisms. 

Abstract We present the discovery of TOI-6303b and TOI-6330b, two massive transiting super-Jupiters orbiting a M0 and a M2 dwarf star, respectively, as part of the Searching for Giant Exoplanets around M-dwarf Stars (GEMS) survey. These were detected by NASA’s Transiting Exoplanet Survey Satellite and then confirmed via ground-based photometry and radial velocity observations with the Habitable-zone Planet Finder. TOI-6303b has a mass of 7.84 ± 0.31M_J, a radius of 1.03 ± 0.06R_J, and an orbital period of 9.485 days. TOI-6330b has a mass of 10.00 ± 0.31M_J, a radius of 0.97 ± 0.03R_J, and an orbital period of 6.850 days. We put these planets in the context of super-Jupiters around M dwarfs discovered from radial-velocity surveys, as well as recent discoveries from astrometry. These planets have masses that can be attributed to two dominant planet formation mechanisms—gravitational instability and core accretion. Their masses necessitate massive protoplanetary disks that should either be gravitationally unstable, i.e., forming through gravitational instability, or be among the most massive protoplanetary disks known to date to form objects through core accretion. We also discuss their possible migration mechanisms via their eccentricity distribution. 

Abstract Brown dwarfs bridge the gap between stars and planets, providing valuable insight into both planetary and stellar-formation mechanisms. Yet the census of transiting brown-dwarf companions, in particular around M-dwarf stars, remains incomplete. We report the discovery of two transiting brown dwarfs around low-mass hosts using a combination of space- and ground-based photometry along with near-infrared radial velocities. We characterize TOI-5389Ab ( $68.0_{-2.2}^{+2.2}{M}_{\mathrm{J}}$) and TOI-5610b ( $40.4_{-1.0}^{+1.0}{M}_{\mathrm{J}}$), two moderately massive brown dwarfs orbiting early M-dwarf hosts (T_eff = 3569 ± 59 K and 3618 ± 59 K, respectively). For TOI-5389Ab, the best fitting parameters are periodP = 10.40046 ± 0.00002 days, radius $R_{\mathrm{BD}}=0.824_{-0.031}^{+0.033}$R_J, and low eccentricity $e=0.0962_{-0.0046}^{+0.0027}$. In particular, this constitutes one of the most extreme substellar-stellar companion-to-host mass ratios ofq= 0.150. For TOI-5610b, the best-fitting parameters are periodP = 7.95346 ± 0.00002 days, radius $R_{\mathrm{BD}}=0.887_{-0.031}^{+0.031}$R_J, and moderate eccentricity $e=0.354_{-0.012}^{+0.011}$. Both targets are expected to have shallow, but potentially observable, occultations: ≲500 ppm in the JohnsonKband. A statistical analysis of M-dwarf/BD systems reveals for the first time that those at short orbital periods (P < 13 days) exhibit a dearth of 13M_J < M_BD < 40M_Jcompanions (q < 0.1) compared to those at slightly wider separations. 

Abstract Contact binary star systems represent the long-lived penultimate phase of binary evolution. Population statistics of their physical parameters inform an understanding of binary evolutionary pathways and end products. We use light curves and new optical spectroscopy to conduct a pilot study of ten (near) contact systems in the long-period ( P > 0.5 days) tail of close binaries in the Kepler field. We use PHOEBE light-curve models to compute Bayesian probabilities on five principal system parameters. Mass ratios and third-light contributions measured from spectra agree well with those inferred from the light curves. Pilot study systems have extreme mass ratios q < 0.32. Most are triples. Analysis of the unbiased sample of 783 0.15 d < P < 2 days (near) contact binaries results in 178 probable contact systems, 114 probable detached systems, and 491 ambiguous systems for which we report best-fitting and 16th-/50th-/84th-percentile parameters. Contact systems are rare at periods P > 0.5 days, as are systems with q > 0.8. There exists an empirical mass ratio lower limit q min ( P ) ≈ 0.05–0.15 below which contact systems are absent, supporting a new set of theoretical predictions obtained by modeling the evolution of contact systems under the constraints of mass and angular momentum conservation. Premerger systems should lie at long periods and near this mass ratio lower limit, which rises from q = 0.044 for P = 0.74 days to q = 0.15 at P = 2.0 days. These findings support a scenario whereby nuclear evolution of the primary (more massive) star drives mass transfer to the primary, thus moving systems toward extreme q and larger P until the onset of the Darwin instability at q min precipitates a merger. 

Abstract We present the confirmation of TOI-5573 b, a Saturn-sized exoplanet on an 8.79 days orbit around an early M dwarf (3790 K, 0.59R_⊙, 0.61M_⊙, 12.30 Jmag). TOI-5573 b has a mass of $112_{-19}^{+18}$M_⊕(0.35 ± 0.06M_Jup) and a radius of 9.75 ± 0.47R_⊕(0.87 ± 0.04R_Jup), resulting in a density of $0.66_{-0.13}^{+0.16}$g cm⁻³, akin to that of Saturn. The planet was initially discovered by the Transiting Exoplanet Survey Satellite (TESS) and confirmed using a combination of 11 transits from four TESS Sectors (20, 21, 47, and 74), ground-based photometry from the Red Buttes Observatory, and high-precision radial velocity data from the Habitable-zone Planet Finder and NN-EXPLORE Exoplanet Investigations with Doppler spectrographs, achieving a 5σprecision on the planet’s mass. TOI-5573 b is one of the coolest Saturn-like exoplanets discovered around an M-dwarf, with an equilibrium temperature of only 528 ± 10 K, making it a valuable target for atmospheric characterization. Saturn-like exoplanets around M dwarfs likely form through core accretion, with increased disk opacity slowing gas accretion and limiting their mass. The host star’s supersolar metallicity supports core accretion, but uncertainties in M-dwarf metallicity estimates complicate definitive conclusions. Compared to other GEMS (Giant Exoplanets around M-dwarf Stars) orbiting metal-rich stars, TOI-5573 b aligns with the observed pattern that giant planets preferentially form around M-dwarfs with supersolar metallicity. Further high-resolution spectroscopic observations are needed to explore the role of stellar metallicity in shaping the formation and properties of giant exoplanets like TOI-5573 b. 

Abstract We report the discovery of a close-in (P_orb= 3.349 days) warm Neptune with clear transit timing variations (TTVs) orbiting the nearby (d= 47.3 pc) active M4 star, TOI-2015. We characterize the planet's properties using Transiting Exoplanet Survey Satellite (TESS) photometry, precise near-infrared radial velocities (RVs) with the Habitable-zone Planet Finder Spectrograph, ground-based photometry, and high-contrast imaging. A joint photometry and RV fit yields a radius $R_p={3.37}_{-0.20}^{+0.15}{R}_{\oplus }$, mass $m_p={16.4}_{-4.1}^{+4.1}{M}_{\oplus }$, and density ${\rho }_p={2.32}_{-0.37}^{+0.38}\mathrm{g}{\mathrm{cm}}^{-3}$for TOI-2015 b, suggesting a likely volatile-rich planet. The young, active host star has a rotation period ofP_rot= 8.7 ± 0.9 days and associated rotation-based age estimate of 1.1 ± 0.1 Gyr. Though no other transiting planets are seen in the TESS data, the system shows clear TTVs of super-period $P_{\sup }\approx 430\mathrm{days}$and amplitude ∼100 minutes. After considering multiple likely period-ratio models, we show an outer planet candidate near a 2:1 resonance can explain the observed TTVs while offering a dynamically stable solution. However, other possible two-planet solutions—including 3:2 and 4:3 resonances—cannot be conclusively excluded without further observations. Assuming a 2:1 resonance in the joint TTV-RV modeling suggests a mass of $m_b={13.3}_{-4.5}^{+4.7}{M}_{\oplus }$for TOI-2015 b and $m_c={6.8}_{-2.3}^{+3.5}{M}_{\oplus }$for the outer candidate. Additional transit and RV observations will be beneficial to explicitly identify the resonance and further characterize the properties of the system. 

Abstract We confirm the planetary nature of TOI-5344 b as a transiting giant exoplanet around an M0-dwarf star. TOI-5344 b was discovered with the Transiting Exoplanet Survey Satellite photometry and confirmed with ground-based photometry (the Red Buttes Observatory 0.6 m telescope), radial velocity (the Habitable-zone Planet Finder), and speckle imaging (the NN-Explore Exoplanet Stellar Speckle Imager). TOI-5344 b is a Saturn-like giant planet (ρ= 0.80 ${}_{-0.15}^{+0.17}$g cm⁻³) with a planetary radius of 9.7 ± 0.5R_⊕(0.87 ± 0.04R_Jup) and a planetary mass of ${135}_{-18}^{+17}{M}_{\oplus }$(0.42 ${}_{-0.06}^{+0.05}{M}_{\mathrm{Jup}}$). It has an orbital period of ${3.792622}_{-0.000010}^{+0.000010}$days and an orbital eccentricity of ${0.06}_{-0.04}^{+0.07}$. We measure a high metallicity for TOI-5344 of [Fe/H] = 0.48 ± 0.12, where the high metallicity is consistent with expectations from formation through core accretion. We compare the metallicity of the M-dwarf hosts of giant exoplanets to that of M-dwarf hosts of nongiants (≲8R_⊕). While the two populations appear to show different metallicity distributions, quantitative tests are prohibited by various sample caveats. 

Abstract We report on the discovery of a transiting giant planet around the 3500 K M3-dwarf star TOI-6383A located 172 pc from Earth. It was detected by the Transiting Exoplanet Survey Satellite and confirmed by a combination of ground-based follow-up photometry and precise radial velocity measurements. This planet has an orbital period of ∼1.791 days, a mass of 1.040 ± 0.094M_J, and a radius of ${1.008}_{-0.033}^{+0.036}{R}_J$, resulting in a mean bulk density of ${1.26}_{-0.17}^{+0.18}$g cm⁻³. TOI-6383A has an M dwarf companion star, TOI-6383B, which has a stellar effective temperature ofT_eff∼ 3100 K and a projected orbital separation of 3126 au. TOI-6383A is a low-mass dwarf star hosting a giant planet and is an intriguing object for planetary evolution studies due to its high planet-to-star mass ratio. This discovery is part of the Searching for Giant Exoplanets around M-dwarf Stars (GEMS) Survey, intending to provide robust and accurate estimates of the occurrence of GEMS and the statistics on their physical and orbital parameters. This paper presents an interesting addition to the small number of confirmed GEMS, particularly notable since its formation necessitates massive, dust-rich protoplanetary discs and high accretion efficiency (>10%).