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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)

   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. Determination of the Starspot Covering Fraction as a function of Stellar Age from Observational Data

   https://doi.org/10.1093/mnras/stz3197  

   Nichols-Fleming, Fiona ; Blackman, Eric G ( November 2020 , Monthly Notices of the Royal Astronomical Society)

   Abstract The association of starspots with magnetic fields leads to an expectation that quantities which correlate with magnetic field strength may also correlate with starspot coverage. Since younger stars spin faster and are more magnetically active, assessing whether starspot coverage correlates with shorter rotation periods and stellar youth tests these principles. Here we analyze the starspot covering fraction versus stellar age for M-, G-, K-, and F-type stars based on previously determined variability and rotation periods of over 30,000 Kepler main-sequence stars. We determine the correlation between age and variability using single and dual power law best fits. We find that starspot coverage does indeed decrease with age. Only when the data are binned in an effort to remove the effects of activity cycles of individual stars, do statistically significant power law fits emerge for each stellar type. Using bin averages, we then find that the starspot covering fraction scales with the X-ray to bolometric ratio to the power λ with 0.22 ± 0.03 < λ < 0.32 ± 0.09 for G-type stars of rotation period below 15 days and for the full range of F- and M-type stars. For K-type stars, we find two branches of λ separated by variability bins, with the lower branch showing nearly constant starspot coverage and the upper branch λ ∼ 0.35 ± 0.04. G-type stars with periods longer than 15 days exhibit a transition to steeper power law of λ ∼ 2.4 ± 1.0. The potential connection to previous rotation-age measurements suggesting a magnetic breaking transition at the solar age, corresponding to period of 24.5 is also of interest. 

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3. Searching for flaring star–planet interactions in AU Mic TESS observations

   https://doi.org/10.1093/mnras/stac1232  

   Ilin, E. ; Poppenhaeger, K. ( May 2022 , Monthly Notices of the Royal Astronomical Society)

   Planets that closely orbit magnetically active stars are thought to be able to interact with their magnetic fields in a way that modulates stellar activity. This modulation in phase with the planetary orbit, such as enhanced X-ray activity, chromospheric spots, radio emission, or flares, is considered the clearest sign of magnetic star–planet interaction (SPI). However, the magnitude of this interaction is poorly constrained, and the intermittent nature of the interaction is a challenge for observers. AU Mic is an early M dwarf, and the most actively flaring planet host detected to date. Its innermost companion, AU Mic b, is a promising target for magnetic SPI observations. We used optical light curves of AU Mic obtained by the Transiting Exoplanet Survey Satellite to search for signs of flaring SPI with AU Mic b using a customized Anderson–Darling test. In the about 50 d of observations, the flare distributions with orbital, rotational, and synodic periods were generally consistent with intrinsic stellar flaring. We found the strongest deviation (p = 0.07, n = 71) from intrinsic flaring with the orbital period of AU Mic b, in the high-energy half of our sample (ED > 1 s). If it reflects the true SPI signal from AU Mic b, extending the observing time by a factor of 2–3 will yield a >3σ detection. Continued monitoring of AU Mic may therefore reveal flaring SPI with orbital phase, while rotational modulation will smear out due to the star’s strong differential rotation.

    

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4. 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 here 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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5. Magnetic and Rotational Evolution of ρ CrB from Asteroseismology with TESS

   Metcalfe, Travis S. ; van Saders, Jennifer L. ; Basu, Sarbani ; Buzasi, Derek ; Drake, Jeremy J. ; Egeland, Ricky ; Huber, Daniel ; Saar, Steven H. ; Stassun, Keivan G. ; Ball, Warrick H. ; et al ( October 2021 , The Astrophysical journal)

   null (Ed.)

   During the first half of main-sequence lifetimes, the evolution of rotation and magnetic activity in solar-type stars appears to be strongly coupled. Recent observations suggest that rotation rates evolve much more slowly beyond middle-age, while stellar activity continues to decline. We aim to characterize this mid-life transition by combining archival stellar activity data from the Mount Wilson Observatory with asteroseismology from the Transiting Exoplanet Survey Satellite (TESS). For two stars on opposite sides of the transition (88 Leo and ρ CrB), we independently assess the mean activity levels and rotation periods previously reported in the literature. For the less active star (ρ CrB), we detect solar-like oscillations from TESS photometry, and we obtain precise stellar properties from asteroseismic modeling. We derive updated X-ray luminosities for both stars to estimate their mass-loss rates, and we use previously published constraints on magnetic morphology to model the evolutionary change in magnetic braking torque. We then attempt to match the observations with rotational evolution models, assuming either standard spin-down or weakened magnetic braking. We conclude that the asteroseismic age of ρ CrB is consistent with the expected evolution of its mean activity level, and that weakened braking models can more readily explain its relatively fast rotation rate. Future spectropolarimetric observations across a range of spectral types promise to further characterize the shift in magnetic morphology that apparently drives this mid-life transition in solar-type stars. 

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