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1. A Mass–Magnitude Relation for Low-mass Stars Based on Dynamical Measurements of Thousands of Binary Star Systems

   https://doi.org/10.3847/1538-3881/ac8cf7  

   Giovinazzi, Mark R. ; Blake, Cullen H. ( September 2022 , The Astronomical Journal)

   Stellar mass is a fundamental parameter that is key to our understanding of stellar formation and evolution, as well as the characterization of nearby exoplanet companions. Historically, stellar masses have been derived from long-term observations of visual or spectroscopic binary star systems. While advances in high-resolution imaging have enabled observations of systems with shorter orbital periods, measurements of stellar masses remain challenging, and relatively few have been precisely measured. We present a new statistical approach to measuring masses for populations of stars. Using Gaia astrometry, we analyze the relative orbital motion of >3800 wide binary systems comprising low-mass stars to establish a mass–magnitude relation in the GaiaG_RPband spanning the absolute magnitude range 14.5 >$M_{G_{\mathrm{RP}}}$> 4.0, corresponding to a mass range of 0.08M_⊙≲M≲ 1.0M_⊙. This relation is directly applicable to >30 million stars in the Gaia catalog. Based on comparison to existing mass–magnitude relations calibrated forK_smagnitudes from the Two Micron All Sky Survey, we estimate that the internal precision of our mass estimates is ∼10%. We use this relation to estimate masses for a volume-limited sample of ∼18,200 stars within 50 pc of the Sun and the present-day field mass function for stars withM≲ 1.0M_⊙, which wemore »find peaks at 0.16M_⊙. We investigate a volume-limited sample of wide binary systems with early-K dwarf primaries, complete for binary mass ratiosq> 0.2, and measure the distribution ofqat separations >100 au. We find that our distribution ofqis not uniform, rather decreasing towardq= 1.0.

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2. CSS1603+19: a low-mass polar near the cataclysmic variable period minimum

   https://doi.org/10.1093/mnras/stad1156  

   Liu, Yiqi ; Hwang, Hsiang-Chih ; Zakamska, Nadia L. ; Thorstensen, John R. ( April 2023 , Monthly Notices of the Royal Astronomical Society)

   CSS1603+19 is a cataclysmic variable (CV) with an orbital period of 81.96 min, near the minimal period of CVs. It is unusual in having a strong mid-infrared excess inconsistent with thermal emission from a brown dwarf companion. Here, we present time-resolved multiwavelength observations of this system. WISE photometry indicates that the mid-infrared excess displays a one-magnitude eclipsing-like variability during the orbit. We obtained near-infrared and optical spectroscopy using Gemini, MDM, and APO telescopes. Near-infrared spectra show possible cyclotron features indicating that the white dwarf has a magnetic field of about 5 MG. Optical and near-infrared spectra display double-peaked emission lines, with both components showing strong radial velocity variations during the orbital period and with the broad component leading the narrow component stably by about 0.2 of the orbital phase. We construct a physical model informed by existing observations of the system and determine that one component likely originates from the accretion column on to the magnetized white dwarf in synchronous rotation with the orbital motion and the other from the Roche overflow point. This allows us to constrain the masses of the binary components to be M1 > 0.24 M⊙ for the white dwarf accretor and M2 = 0.0644 ± 0.0074 M⊙ for themore »donor. We classify the system as an AM Herculis star, or a polar. It has likely completed its stint on the period gap, but has not yet gone through the period bounce.

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3. TOI-1075 b: A Dense, Massive, Ultra-short-period Hot Super-Earth Straddling the Radius Gap

   https://doi.org/10.3847/1538-3881/ac9c5b  

   Essack, Zahra ; Shporer, Avi ; Burt, Jennifer A. ; Seager, Sara ; Cambioni, Saverio ; Lin, Zifan ; Collins, Karen A. ; Mamajek, Eric E. ; Stassun, Keivan G. ; Ricker, George R. ; et al ( January 2023 , The Astronomical Journal)

   Populating the exoplanet mass–radius diagram in order to identify the underlying relationship that governs planet composition is driving an interdisciplinary effort within the exoplanet community. The discovery of hot super-Earths—a high-temperature, short-period subset of the super-Earth planet population—has presented many unresolved questions concerning the formation, evolution, and composition of rocky planets. We report the discovery of a transiting, ultra-short-period hot super-Earth orbitingTOI-1075(TIC351601843), a nearby (d= 61.4 pc) late-K/early-M-dwarf star, using data from the Transiting Exoplanet Survey Satellite. The newly discovered planet has a radius of 1.791${}_{-0.081}^{+0.116}$R_⊕and an orbital period of 0.605 day (14.5 hr). We precisely measure the planet mass to be 9.95${}_{-1.30}^{+1.36}$M_⊕using radial velocity measurements obtained with the Planet Finder Spectrograph mounted on the Magellan II telescope. Our radial velocity data also show a long-term trend, suggesting an additional planet in the system. While TOI-1075 b is expected to have a substantial H/He atmosphere given its size relative to the radius gap, its high density (${9.32}_{-1.85}^{+2.05}$g cm⁻³) is likely inconsistent with this possibility. We explore TOI-1075 b’s location relative to the M-dwarf radius valley, evaluate the planet’s prospects for atmospheric characterization, andmore »discuss potential planet formation mechanisms. Studying the TOI-1075 system in the broader context of ultra-short-period planetary systems is necessary for testing planet formation and evolution theories and density-enhancing mechanisms and for future atmospheric and surface characterization studies via emission spectroscopy with the JWST.

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4. Magnetic braking saturates: evidence from the orbital period distribution of low-mass detached eclipsing binaries from ZTF

   https://doi.org/10.1093/mnras/stac2945  

   El-Badry, Kareem ; Conroy, Charlie ; Fuller, Jim ; Kiman, Rocio ; van Roestel, Jan ; Rodriguez, Antonio C. ; Burdge, Kevin B. ( October 2022 , Monthly Notices of the Royal Astronomical Society)

   We constrain the orbital period (Porb) distribution of low-mass detached main-sequence eclipsing binaries (EBs) with light-curves from the Zwicky Transient Facility (ZTF), which provides a well-understood selection function and sensitivity to faint stars. At short periods (Porb ≲ 2 d), binaries are predicted to evolve significantly due to magnetic braking (MB), which shrinks orbits and ultimately brings detached binaries into contact. The period distribution is thus a sensitive probe of MB. We find that the intrinsic period distribution of low-mass (0.1 ≲ M1/M⊙ < 0.9) binaries is basically flat (${\rm d}N/{\rm d}P_{\rm orb} \propto P_{\rm orb}^0$) from Porb = 10 d down to the contact limit. This is strongly inconsistent with predictions of classical MB models based on the Skumanich relation, which are widely used in binary evolution calculations and predict ${\rm d}N/{\rm d}P_{\rm orb} \propto P_{\rm orb}^{7/3}$ at short periods. The observed distributions are best reproduced by models in which the magnetic field saturates at short periods with a MB torque that scales roughly as $\dot{J}\propto P_{\rm orb}^{-1}$, as opposed to $\dot{J} \propto P_{\rm orb}^{-3}$ in the standard Skumanich law. We also find no significant difference between the period distributions of binaries containing fully and partially convective stars. Our results confirmmore »that a saturated MB law, which was previously found to describe the spin-down of rapidly rotating isolated M dwarfs, also operates in tidally locked binaries. We advocate using saturated MB models in binary evolution calculations. Our work supports previous suggestions that MB in cataclysmic variables (CVs) is much weaker than assumed in the standard evolutionary model, unless mass transfer leads to significant additional angular momentum loss in CVs.

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5. The ELM Survey. IX. A Complete Sample of Low-mass White Dwarf Binaries in the SDSS Footprint

   https://doi.org/10.3847/1538-4357/ac72ac  

   Brown, Warren R. ; Kilic, Mukremin ; Kosakowski, Alekzander ; Gianninas, A. ( July 2022 , The Astrophysical Journal)

   We present the discovery of 17 double white dwarf (WD) binaries from our ongoing search for extremely low mass (ELM) < 0.3M_⊙WDs, objects that form from binary evolution. Gaia parallax provides a new means of target selection that we use to evaluate our original ELM Survey selection criteria. Cross-matching the Gaia and Sloan Digital Sky Survey (SDSS) catalogs, we identify an additional 36 ELM WD candidates with 17 <g< 19 mag and within the 3σuncertainties of our original color selection. The resulting discoveries imply the ELM Survey sample was 90% complete in the color range −0.4 < (g−r)₀< −0.1 mag (approximately 9000 K <T_eff< 22,000 K). Our observations complete the sample in the SDSS footprint. Two newly discovered binaries, J123950.370−204142.28 and J232208.733+210352.81, have orbital periods of 22.5 and 32 minutes, respectively, and are future Laser Interferometer Space Antenna gravitational-wave sources.