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1. Modeling Stellar Surface Features on a Subgiant Star with an M-dwarf Companion

   https://doi.org/10.3847/1538-3881/ac70ca  

   Schutte, Maria C.; Hebb, Leslie; Lowry, Simon; Wisniewski, John; Hawley, Suzanne L.; Mahadevan, Suvrath; Morris, Brett M.; Robertson, Paul; Rohn, Graeme; Stefansson, Gudmundur (June 2022, The Astronomical Journal)

   Abstract Understanding magnetic activity on the surface of stars other than the Sun is important for exoplanet analyses to properly characterize an exoplanet’s atmosphere and to further characterize stellar activity on a wide range of stars. Modeling stellar surface features of a variety of spectral types and rotation rates is key to understanding the magnetic activity of these stars. Using data from Kepler, we use the starspot modeling program STarSPot (STSP) to measure the position and size of spots for KOI-340, which is an eclipsing binary consisting of a subgiant star (T_eff= 5593 ± 27 K,R_⋆= 1.98 ± 0.05R_⊙) with an M-dwarf companion (M_⋆= 0.214 ± 0.006M_⊙).STSPuses a novel technique to measure the spot positions and radii by using the transiting secondary to study and model individual active regions on the stellar surface using high-precision photometry. We find that the average size of spot features on KOI-340's primary is ∼10% the radius of the star, i.e., two times larger than the mean size of solar-maximum sunspots. The spots on KOI-340 are present at every longitude and show possible signs of differential rotation. The minimum fractional spotted area of KOI-340's primary is $2_{-2}^{+12}\mathrm{\%}$, while the spotted area of the Sun is at most 0.2%. One transit of KOI-340 shows a signal in the transit consistent with a plage; this plage occurs right before a dark spot, indicating that the plage and spot might be colocated on the surface of the star. 

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2. Multiband light-curve analysis of the 40.5-min period eclipsing double-degenerate binary SDSS J082239.54+304857.19

   https://doi.org/10.1093/mnras/staa3571  

   Kosakowski, Alekzander; Kilic, Mukremin; Brown, Warren (December 2020, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT We present the Apache Point Observatory BG40 broad-band and simultaneous Gemini r-band and i-band high-speed follow-up photometry observations and analysis of the 40.5-min period eclipsing detached double-degenerate binary SDSS J082239.54+304857.19. Our APO data spans over 318 d and includes 13 primary eclipses, from which we precisely measure the system’s orbital period and improve the time of mid-eclipse measurement. We fit the light curves for each filter individually and show that this system contains a low-mass DA white dwarf with radius RA = 0.031 ± 0.006 R⊙ and a RB = 0.013 ± 0.005 R⊙ companion at an inclination of i = 87.7 ± 0.2○. We use the best-fitting eclipsing light curve model to estimate the temperature of the secondary star as Teff = 5200 ± 100 K. Finally, while we do not record significant offsets to the expected time of mid-eclipse caused by the emission of gravitational waves with our 1-yr baseline, we show that a 3σ significant measurement of the orbital decay due to gravitational waves will be possible in 2023, at which point the eclipse will occur about 8  s earlier than expected. 

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3. The EBLM project – IX. Five fully convective M-dwarfs, precisely measured with CHEOPS and TESS light curves

   https://doi.org/10.1093/mnras/stac2565  

   Sebastian, D.; Swayne, M. I.; Maxted, P. F. L.; Triaud, A. H. M. J.; Sousa, S. G.; Olofsson, G.; Beck, M.; Billot, N.; Hoyer, S.; Gill, S.; et al (September 2022, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Eclipsing binaries are important benchmark objects to test and calibrate stellar structure and evolution models. This is especially true for binaries with a fully convective M-dwarf component for which direct measurements of these stars’ masses and radii are difficult using other techniques. Within the potential of M-dwarfs to be exoplanet host stars, the accuracy of theoretical predictions of their radius and effective temperature as a function of their mass is an active topic of discussion. Not only the parameters of transiting exoplanets but also the success of future atmospheric characterization relies on accurate theoretical predictions. We present the analysis of five eclipsing binaries with low-mass stellar companions out of a subsample of 23, for which we obtained ultra-high-precision light curves using the CHEOPS satellite. The observation of their primary and secondary eclipses are combined with spectroscopic measurements to precisely model the primary parameters and derive the M-dwarfs mass, radius, surface gravity, and effective temperature estimates using the PYCHEOPS data analysis software. Combining these results to the same set of parameters derived from TESS light curves, we find very good agreement (better than 1 per cent for radius and better than 0.2 per cent for surface gravity). We also analyse the importance of precise orbits from radial velocity measurements and find them to be crucial to derive M-dwarf radii in a regime below 5 per cent accuracy. These results add five valuable data points to the mass–radius diagram of fully convective M-dwarfs. 

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4. TOI-2084 b and TOI-4184 b: Two new sub-Neptunes around M dwarf stars

   https://doi.org/10.1051/0004-6361/202346838  

   Barkaoui, K; Timmermans, M; Soubkiou, A; Rackham, B V; Burgasser, A J; Chouqar, J; Pozuelos, F J; Collins, K A; Howell, S B; Simcoe, R; et al (September 2023, Astronomy & Astrophysics)

   We present the discovery and validation of two TESS exoplanets orbiting nearby M dwarfs: TOI-2084 b, and TOI-4184b. We characterized the host stars by combining spectra fromShane/Kast andMagellan/FIRE, spectral energy distribution analysis, and stellar evolutionary models. In addition, we used Gemini-South/Zorro & -North/Alopeke high-resolution imaging, archival science images, and statistical validation packages to support the planetary interpretation. We performed a global analysis of multi-colour photometric data from TESS and ground-based facilities in order to derive the stellar and planetary physical parameters for each system. We find that TOI-2084 band TOI-4184 bare sub-Neptune-sized planets with radii ofR_p= 2.47 ± 0.13R_⊕andR_p= 2.43 ± 0.21R_⊕, respectively. TOI-2084 b completes an orbit around its host star every 6.08 days, has an equilibrium temperature ofT_eq= 527 ± 8 K and an irradiation ofS_p= 12.8 ± 0.8S_⊕. Its host star is a dwarf of spectral M2.0 ± 0.5 at a distance of 114 pc with an effective temperature ofT_eff= 3550 ± 50 K, and has a wide, co-moving M8 companion at a projected separation of 1400 au. TOI-4184 b orbits around an M5.0 ± 0.5 type dwarf star (K_mag= 11.87) each 4.9 days, and has an equilibrium temperature ofT_eq= 412 ± 8 K and an irradiation ofS_p= 4.8 ± 0.4S_⊕. TOI-4184 is a metal poor star ([Fe/H] = −0.27 ± 0.09 dex) at a distance of 69 pc with an effective temperature ofT_eff= 3225 ± 75 K. Both planets are located at the edge of the sub-Jovian desert in the radius-period plane. The combination of the small size and the large infrared brightness of their host stars make these new planets promising targets for future atmospheric exploration with JWST. 

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5. TOI-1135 b: A young hot Saturn-size planet orbiting a solar-type star

   https://doi.org/10.1051/0004-6361/202349016  

   Mallorquín, M; Lodieu, N; Béjar, V_J S; Zapatero_Osorio, M R; Sanz-Forcada, J; Alarcon, M R; Tabernero, H M; Nagel, E; Collins, K A; Ciardi, D R; et al (May 2024, Astronomy & Astrophysics)

   Despite the thousands of planets in orbit around stars known to date, the mechanisms of planetary formation, migration, and atmospheric loss remain unresolved. In this work, we confirm the planetary nature of a young Saturn-size planet transiting a solar-type star every 8.03 d, TOI-1135 b. The age of the parent star is estimated to be in the interval of 125-1000 Myr based on various activity and age indicators, including its stellar rotation period of 5.13 ± 0.27 days and the intensity of photospheric lithium. We obtained follow-up photometry and spectroscopy, including precise radial velocity measurements using the CARMENES spectrograph, which together with the TESS data allowed us to fully characterise the parent star and its planet. As expected for its youth, the star is rather active and shows strong photometric and spectroscopic variability correlating with its rotation period. We modelled the stellar variability using Gaussian process regression. We measured the planetary radius at 9.02 ± 0.23R_⊕(0.81 ± 0.02R_Jup) and determined a 3σupper limit of < 51.4M_⊕(< 0.16M_Jup) on the planetary mass by adopting a circular orbit. Our results indicate that TOI-1135 b is an inflated planet less massive than Saturn or Jupiter but with a similar radius, which could be in the process of losing its atmosphere by photoevaporation. This new young planet occupies a region of the mass-radius diagram where older planets are scarse, and it could be very helpful to understanding the lower frequency of planets with sizes between Neptune and Saturn. 

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