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1. An Exceptional Dimming Event for a Massive, Cool Supergiant in M51

   https://doi.org/10.3847/1538-4357/ac626c  

   Jencson, Jacob E. ; Sand, David J. ; Andrews, Jennifer E. ; Smith, Nathan ; Pearson, Jeniveve ; Strader, Jay ; Valenti, Stefano ; Beasor, Emma R. ; Rothberg, Barry ( May 2022 , The Astrophysical Journal)

   We present the discovery of an exceptional dimming event in a cool supergiant star in the Local Volume spiral M51. The star, dubbed M51-DS1, was found as part of a Hubble Space Telescope (HST) search for failed supernovae (SNe). The supergiant, which is plausibly associated with a very young (≲6 Myr) stellar population, showed clear variability (amplitude ΔF814W≈ 0.7 mag) in numerous HST images obtained between 1995 and 2016, before suddenly dimming by >2 mag inF814Wsometime between late 2017 and mid-2019. In follow-up data from 2021, the star rebrightened, ruling out a failed supernova. Prior to its near-disappearance, the star was luminous and red (M_F814W≲ − 7.6 mag,F606W−F814W= 1.9–2.2 mag). Modeling of the pre-dimming spectral energy distribution of the star favors a highly reddened, very luminous ($\log [L/L_{\odot }]=5.4$–5.7) star withT_eff≈ 3700–4700 K, indicative of a cool yellow or post-red supergiant (RSG) with an initial mass of ≈26–40M_⊙. However, the local interstellar extinction and circumstellar extinction are uncertain, and could be lower: the near-IR colors are consistent with an RSG, which would be cooler (T_eff≲ 3700 K) and slightly less luminous ($\log [L/L_{\odot }]=5.2$–5.3), giving an inferred initial mass of ≈19–22M_⊙. In either case, the dimming may be explained by a rare episode of enhanced mass loss that temporarily obscures the star, potentially a more extreme counterpart to the 2019–2020 “Great Dimming” of Betelgeuse. Given the emerging evidence that massive evolved stars commonly exhibit variability that can mimic a disappearing star, our work highlights a substantial challenge in identifying true failed SNe.

    

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2. Long Term Evolution of Surface Features on the Red Supergiant AZ Cyg

   https://doi.org/10.3847/1538-4357/ac0c7e  

   Norris, Ryan P. ; Baron, Fabien R. ; Monnier, John D. ; Paladini, Claudia ; Anderson, Matthew D. ; Martinez, Arturo O. ; Schaefer, Gail H. ; Che, Xiao ; Chiavassa, Andrea ; Connelley, Michael S. ; et al ( October 2021 , The Astrophysical Journal)

   We presentH-band interferometric observations of the red supergiant (RSG) AZ Cyg that were made with the Michigan Infra-Red Combiner (MIRC) at the six-telescope Center for High Angular Resolution Astronomy (CHARA) Array. The observations span 5 yr (2011–2016), which offers insight into the short and long-term evolution of surface features on RSGs. Using a spectrum of AZ Cyg obtained with SpeX on the NASA InfraRed Telescope Facility (IRTF) and synthetic spectra calculated from spherical MARCS, spherical PHOENIX, and SAtlas model atmospheres, we deriveT_effis between 3972 K and 4000 K and$\log g$between −0.50 and 0.00, depending on the stellar model used. Using fits to the squared visibility and GAIA parallaxes, we measure its average radius$R={911}_{-50}^{+57}{R}_{\odot }$. Reconstructions of the stellar surface using our model-independent imaging codes SQUEEZE and OITOOLS.jl show a complex surface with small bright features that appear to vary on a timescale of less than one year and larger features that persist for more than one year. The 1D power spectra of these images suggest a characteristic size of 0.52–0.69R_⋆for the larger, long lived features. This is close to the values of 0.51–0.53R_⋆that are derived from 3D RHD models of stellar surfaces. We conclude that interferometric imaging of this star is in line with predictions of 3D RHD models but that short-term imaging is needed to more stringently test predictions of convection in RSGs.

    

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3. 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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4. COol Companions ON Ultrawide orbiTS (COCONUTS). III. A Very Red L6 Benchmark Brown Dwarf around a Young M5 Dwarf

   https://doi.org/10.3847/1538-4357/ac7ce9  

   Zhang, Zhoujian ; Liu, Michael C. ; Morley, Caroline V. ; Magnier, Eugene A. ; Tucker, Michael A. ; Vanderbosch, Zachary P. ; Do, Aaron ; Shappee, Benjamin J. ( August 2022 , The Astrophysical Journal)

   Abstract We present the third discovery from the COol Companions ON Ultrawide orbiTS (COCONUTS) program, the COCONUTS-3 system, composed of the young M5 primary star UCAC4 374−046899 and the very red L6 dwarf WISEA J081322.19−152203.2. These two objects have a projected separation of 61 ′ ′ (1891 au) and are physically associated given their common proper motions and estimated distances. The primary star, COCONUTS-3A, has a mass of 0.123 ± 0.006 M ⊙ , and we estimate its age as 100 Myr to 1 Gyr based on its stellar activity (via H α and X-ray emission), kinematics, and spectrophotometric properties. We derive its bulk metallicity as 0.21 ± 0.07 dex using empirical calibrations established by older and higher-gravity M dwarfs and find that this [Fe/H] could be slightly underestimated according to PHOENIX models given COCONUTS-3A’s younger age. The companion, COCONUTS-3B, has a near-infrared spectral type of L6 ± 1 int-g , and we infer physical properties of T eff = 1362 − 73 + 48 K, log ( g ) = 4.96 − 0.34 + 0.15 dex, R = 1.03 − 0.06 + 0.12 R Jup , and M = 39 − 18 + 11 M Jup using its bolometric luminosity, its host star’s age, and hot-start evolution models. We construct cloudy atmospheric model spectra at the evolution-based physical parameters and compare them to COCONUTS-3B’s spectrophotometry. We find that this companion possesses ample condensate clouds in its photosphere ( f sed = 1) with the data–model discrepancies likely due to the models using an older version of the opacity database. Compared to field-age L6 dwarfs, COCONUTS-3B has fainter absolute magnitudes and a 120 K cooler T eff . Also, the J − K color of this companion is among the reddest for ultracool benchmarks with ages older than a few hundred megayears. COCONUTS-3 likely formed in the same fashion as stellar binaries given the companion-to-host mass ratio of 0.3 and represents a valuable benchmark to quantify the systematics of substellar model atmospheres. 

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5. Spectroscopic and evolutionary analyses of the binary system AzV 14 outline paths toward the WR stage at low metallicity

   https://doi.org/10.1051/0004-6361/202345881  

   Pauli, D. ; Oskinova, L. M. ; Hamann, W.-R. ; Bowman, D. M. ; Todt, H. ; Shenar, T. ; Sander, A. A. ; Erba, C. ; Gómez-González, V. M. ; Kehrig, C. ; et al ( May 2023 , Astronomy & Astrophysics)

   Context. The origin of the observed population of Wolf-Rayet (WR) stars in low-metallicity galaxies, such as the Small Magellanic Cloud (SMC), is not yet understood. Standard, single-star evolutionary models predict that WR stars should stem from very massive O-type star progenitors, but these are very rare. On the other hand, binary evolutionary models predict that WR stars could originate from primary stars in close binaries. Aims. We conduct an analysis of the massive O star, AzV 14, to spectroscopically determine its fundamental and stellar wind parameters, which are then used to investigate evolutionary paths from the O-type to the WR stage with stellar evolutionary models. Methods. Multi-epoch UV and optical spectra of AzV 14 are analyzed using the non-local thermodynamic equilibrium (LTE) stellar atmosphere code PoWR. An optical TESS light curve was extracted and analyzed using the PHOEBE code. The obtained parameters are put into an evolutionary context, using the MESA code. Results. AzV 14 is a close binary system with a period of P  = 3.7058 ± 0.0013 d. The binary consists of two similar main sequence stars with masses of M 1, 2  ≈ 32  M ⊙ . Both stars have weak stellar winds with mass-loss rates of log Ṁ /( M ⊙ yr −1 ) = −7.7 ± 0.2. Binary evolutionary models can explain the empirically derived stellar and orbital parameters, including the position of the AzV 14 components on the Hertzsprung-Russell diagram, revealing its current age of 3.3 Myr. The model predicts that the primary will evolve into a WR star with T eff  ≈ 100 kK, while the secondary, which will accrete significant amounts of mass during the first mass transfer phase, will become a cooler WR star with T eff  ≈ 50 kK. Furthermore, WR stars that descend from binary components that have accreted significant amount of mass are predicted to have increased oxygen abundances compared to other WR stars. This model prediction is supported by a spectroscopic analysis of a WR star in the SMC. Conclusions. Inspired by the binary evolutionary models, we hypothesize that the populations of WR stars in low-metallicity galaxies may have bimodal temperature distributions. Hotter WR stars might originate from primary stars, while cooler WR stars are the evolutionary descendants of the secondary stars if they accreted a significant amount of mass. These results may have wide-ranging implications for our understanding of massive star feedback and binary evolution channels at low metallicity. 

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