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1. Mass Transfer as an Explanation for the Lifetime Travel Time Discrepancy in IT Librae

   https://doi.org/10.3847/1538-3881/ac549d  

   Wysocki, Peter ; Gies, Douglas ; Shepard, Katherine ; Lester, Kathryn ; Orosz, Jerome ( March 2022 , The Astronomical Journal)

   The eclipsing binary IT Librae is an unusual system of two B-type stars that is situated about 1 kpc above the Galactic plane. The binary was probably ejected from its birthplace in the disk, but the implied time of flight to its current location exceeds the evolutionary lifetime of the primary star. Here we present a study of new high-dispersion spectroscopy and an exquisite light curve from the Kepler K2 mission in order to determine the system properties and resolve the timescale discrepancy. We derive a revised spectroscopic orbit from radial-velocity measurements and determine the component effective temperatures through comparison of reconstructed and model spectra (T₁= 23.8 ± 1.8 kK,T₂= 13.7 ± 2.5 kK). We use the Eclipsing Light Curve code to model the K2 light curve, and from the inclination of the fit we derive the component masses (M₁= 9.6 ± 0.6M_⊙,M₂= 4.2 ± 0.2M_⊙) and mean radii (R₁= 6.06 ± 0.16R_⊙,R₂= 5.38 ± 0.14R_⊙). The secondary star is overluminous for its mass and appears to fill its Roche lobe. This indicates that IT Librae is a post-mass-transfer system in which the current secondary was the mass donor star. The current primary star was rejuvenated by mass accretion,more »and its evolutionary age corresponds to the time since the mass transfer stage. Consequently, the true age of the binary is larger than the ejection time of flight, thus resolving the timescale discrepancy.

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2. Transit Hunt for Young and Maturing Exoplanets (THYME). VIII. A Pleiades-age Association Harboring Two Transiting Planetary Systems from Kepler

   https://doi.org/10.3847/1538-3881/ac7b28  

   Barber, Madyson G. ; Mann, Andrew W. ; Bush, Jonathan L. ; Tofflemire, Benjamin M. ; Kraus, Adam L. ; Krolikowski, Daniel M. ; Vanderburg, Andrew ; Fields, Matthew J. ; Newton, Elisabeth R. ; Owens, Dylan A. ; et al ( August 2022 , The Astronomical Journal)

   Young planets provide a window into the early stages and evolution of planetary systems. Ideal planets for such research are in coeval associations, where the parent population can precisely determine their ages. We describe a young association (MELANGE-3) in the Kepler field, which harbors two transiting planetary systems (KOI-3876 and Kepler-970). We identify MELANGE-3 by searching for kinematic and spatial overdensities around Kepler planet hosts with high levels of lithium. To determine the age and membership of MELANGE-3, we combine new high-resolution spectra with archival light curves, velocities, and astrometry of stars near KOI-3876 spatially and kinematically. We use the resulting rotation sequence, lithium levels, and color–magnitude diagram of candidate members to confirm the presence of a coeval 105 ± 10 Myr population. MELANGE-3 may be part of the recently identified Theia 316 stream. For the two exoplanet systems, we revise the stellar and planetary parameters, taking into account the newly determined age. Fitting the 4.5 yr Kepler light curves, we find that KOI-3876b is a 2.0 ± 0.1R_⊕planet on a 19.58 day orbit, while Kepler-970 b is a 2.8 ± 0.2R_⊕planet on a 16.73 day orbit. KOI-3876 was previously flagged as an eclipsing binary, which we rule outmore »using radial velocities from APOGEE and statistically validate the signal as planetary in origin. Given its overlap with the Kepler field, MELANGE-3 is valuable for studies of spot evolution on year timescales, and both planets contribute to the growing work on transiting planets in young stellar associations.

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3. Observationally Constraining the Starspot Properties of Magnetically Active M67 Sub-subgiant S1063

   https://doi.org/10.3847/1538-4357/ac3668  

   Gosnell, Natalie M. ; Gully-Santiago, Michael A. ; Leiner, Emily M. ; Tofflemire, Benjamin M. ( January 2022 , The Astrophysical Journal)

   Our understanding of the impact of magnetic activity on stellar evolution continues to unfold. This impact is seen in sub-subgiant stars, defined to be stars that sit below the subgiant branch and red of the main sequence in a cluster color–magnitude diagram. Here we focus on S1063, a prototypical sub-subgiant in open cluster M67. We use a novel technique combining a two-temperature spectral decomposition and light-curve analysis to constrain starspot properties over a multiyear time frame. Using a high-resolution near-infrared IGRINS spectrum and photometric data from K2 and ASAS-SN, we find a projected spot filling factor of 32% ± 7% with a spot temperature of 4000 ± 200 K. This value anchors the variability seen in the light curve, indicating the spot filling factor of S1063 ranged from 20% to 45% over a four-year time period with an average spot filling factor of 30%. These values are generally lower than those determined from photometric model comparisons but still indicate that S1063, and likely other sub-subgiants, are magnetically active spotted stars. We find observational and theoretical comparisons of spotted stars are nuanced due to the projected spot coverage impacting estimates of the surface-averaged effective temperature. The starspot properties found heremore »are similar to those found in RS CVn systems, supporting classifying sub-subgiants as another type of active giant star binary system. This technique opens the possibility of characterizing the surface conditions of many more spotted stars than previous methods, allowing for larger future studies to test theoretical models of magnetically active stars.

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4. Shock Breakout in Three-dimensional Red Supergiant Envelopes

   https://doi.org/10.3847/1538-4357/ac75e3  

   Goldberg, Jared A. ; Jiang, Yan-Fei ; Bildsten, Lars ( July 2022 , The Astrophysical Journal)

   UsingAthena++, we perform 3D radiation-hydrodynamic calculations of the radiative breakout of the shock wave in the outer envelope of a red supergiant (RSG) that has suffered core collapse and will become a Type IIP supernova. The intrinsically 3D structure of the fully convective RSG envelope yields key differences in the brightness and duration of the shock breakout (SBO) from that predicted in a 1D stellar model. First, the lower-density “halo” of material outside of the traditional photosphere in 3D models leads to a shock breakout at lower densities than 1D models. This would prolong the duration of the shock breakout flash at any given location on the surface to ≈1–2 hr. However, we find that the even larger impact is the intrinsically 3D effect associated with large-scale fluctuations in density that cause the shock to break out at different radii at different times. This substantially prolongs the SBO duration to ≈3–6 hr and implies a diversity of radiative temperatures, as different patches across the stellar surface are at different stages of their radiative breakout and cooling at any given time. These predicted durations are in better agreement with existing observations of SBO. The longer durations lower the predicted luminositiesmore »by a factor of 3–10 (L_bol∼ 10⁴⁴erg s⁻¹), and we derive the new scalings of brightness and duration with explosion energies and stellar properties. These intrinsically 3D properties eliminate the possibility of using observed rise times to measure the stellar radius via light-travel time effects.

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5. 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)

   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 precisemore »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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