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1. CHARA Near-infrared Imaging of the Yellow Hypergiant Star ρ Cassiopeiae: Convection Cells and Circumstellar Envelope

   https://doi.org/10.3847/1538-4357/ad6b2b  

   Anugu, Narsireddy; Baron, Fabien; Monnier, John D; Gies, Douglas R; Roettenbacher, Rachael M; Schaefer, Gail H; Montargès, Miguel; Kraus, Stefan; Le_Bouquin, Jean-Baptiste; Anderson, Matthew D; et al (October 2024, The Astrophysical Journal)

   Abstract Massive evolved stars such as red supergiants and hypergiants are potential progenitors of Type II supernovae, and they are known for ejecting substantial amounts of matter, up to half their initial mass, during their final evolutionary phases. The rate and mechanism of this mass loss play a crucial role in determining their ultimate fate and the likelihood of their progression to supernovae. However, the exact mechanisms driving this mass ejection have long been a subject of research. Recent observations, such as the Great Dimming of Betelgeuse, have suggested that the activity of large convective cells, combined with pulsation, could be a plausible explanation for such mass-loss events. In this context, we conducted interferometric observations of the famous yellow hypergiant,ρCassiopeiae using the CHARA Array inH-andK-band wavelengths.ρCas is well known for its recurrent eruptions, characterized by periods of visual dimming (∼1.5–2 mag) followed by recovery. From our observations, we derived the diameter of the limb-darkened disk and found that this star has a radius of 1.04 ± 0.01 mas, or 564–700R_⊙. We performed image reconstructions with three different image reconstruction software packages, and they unveiled the presence of giant hot and cold spots on the stellar surface. We interpret these prominent hot spots as giant convection cells, suggesting a possible connection to mass ejections from the star’s envelope. Furthermore, we detected spectral CO emission lines in theKband (λ= 2.31–2.38μm), and the image reconstructions in these spectral lines revealed an extended circumstellar envelope with a radius of 1.45 ± 0.10 mas. 

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2. Measuring the Spot Variability of T Tauri Stars Using Near-infrared Atomic Fe and Molecular OH Lines

   https://doi.org/10.3847/1538-4357/ad5e7f  

   Tang, Shih-Yun; Johns-Krull, Christopher M; Prato, L; Stahl, Asa G (September 2024, The Astrophysical Journal)

   Abstract As part of the Young Exoplanets Spectroscopic Survey, this study explores the spot variability of 13 T Tauri Stars (TTSs) in the near-infraredHband, using spectra from the Immersion GRating INfrared Spectrometer. By analyzing effective temperature (T_eff) sensitive lines of atomic Feiat ∼1.56259μm and ∼1.56362μm, and molecular OH at ∼1.56310 and ∼1.56317μm, we develop an empirical equivalent width ratio (EWR) relationship forT_effin the range of 3400–5000 K. This relationship allows for precise relativeT_effestimates to within tens of Kelvin and demonstrates compatibility with solar metallicity target models. However, discrepancies between observational data and model predictions limit the extension of theT_eff–EWR relationship to a broader parameter space. Our study reveals that both classical and weak-line TTSs can exhibitT_effvariations exceeding 150 K over a span of 2 yr. The detection of a quarter-phase delay between the EWR and radial velocity phase curves in TTSs indicates spot-driven signals. A phase delay of 0.06 ± 0.13 for CI Tau, however, suggests additional dynamics, potentially caused by planetary interaction, inferred from a posited 1:1 commensurability between the rotation period and orbital period. Moreover, a positive correlation betweenT_effvariation amplitude and stellar inclination angle supports the existence of high-latitude spots on TTSs, further enriching our understanding of stellar surface activity in young stars. 

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3. High-angular-resolution Imaging of the V892 Tau Binary System: A New Circumprimary Disk Detection and Updated Orbital Constraints

   https://doi.org/10.3847/1538-4357/acfda6  

   Vides, Christina L; Sallum, Steph; Eisner, Josh; Skemer, Andy; Murray-Clay, Ruth (November 2023, The Astrophysical Journal)

   Abstract We present a direct imaging study of V892 Tau, a young Herbig Ae/Be star with a close-in stellar companion and circumbinary disk. Our observations consist of images acquired via Keck II/NIRC2 with nonredundant masking and the pyramid wavefront sensor at $K\prime$band (2.12μm) and $L\prime$band (3.78μm). Sensitivity to low-mass accreting companions and cool disk material is high at $L\prime$band, while complimentary observations at $K\prime$band probe hotter material with higher angular resolution. These multiwavelength, multiepoch data allow us to differentiate the secondary stellar emission from disk emission and deeply probe the structure of the circumbinary disk at small angular separations. We constrain architectural properties of the system by fitting geometric disk and companion models to the $K\prime$- and $L\prime$-band data. From these models, we constrain the astrometric and photometric properties of the stellar binary and update the orbit, placing the tightest estimates to date on the V892 Tau orbital parameters. We also constrain the geometric structure of the circumbinary disk, and resolve a circumprimary disk for the first time. 

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4. A Near-infrared Search for Helium in the Superluminous Supernova SN 2024ahr

   https://doi.org/10.3847/1538-4357/addc67  

   Kumar, Harsh; Berger, Edo; Blanchard, Peter K; Gomez, Sebastian; Hiramatsu, Daichi; Andrews, Moira; Bostroem, K Azalee; Dong, Yize; Farah, Joseph; Padilla_Gonzalez, Estefania; et al (July 2025, The Astrophysical Journal)

   Abstract We present a detailed study of SN 2024ahr, a hydrogen-poor superluminous supernova (SLSN-I), for which we determine a redshift ofz= 0.0861. SN 2024ahr has a peak absolute magnitude ofM_g≈M_r≈ −21 mag, rest-frame rise and decline times (50% of peak) of about 40 and 80 days, respectively, and typical spectroscopic evolution in the optical band. Similarly, modeling of the UV/optical light curves with a magnetar spin-down engine leads to typical parameters: an initial spin period of ≈3.3 ms, a magnetic field strength of ≈6 × 10¹³G, and an ejecta mass of ≈9.5M_⊙. Due to its relatively low redshift, we obtained a high signal-to-noise ratio near-IR (NIR) spectrum about 43 rest-frame days postpeak to search for the presence of helium. We do not detect any significant feature at the location of the Heiλ2.058μm feature and place a conservative upper limit of ∼0.05M_⊙on the mass of helium in the outer ejecta. We detect broad features of Mgiλ1.575μm and Mgiiλ2.136μm, which are typical of Type Ic SNe, but with higher velocities. Examining the sample of SLSNe-I with NIR spectroscopy, we find that, unlike SN 2024ahr, these events are generally peculiar. This highlights the need for a large sample of prototypical SLSNe-I with NIR spectroscopy to constrain the fraction of progenitors with helium (Ib-like) and without helium (Ic-like) at the time of explosion, and hence the evolutionary path(s) leading to the rare outcome of SLSNe-I. 

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5. Characterising the orbit and circumstellar environment of the high-mass binary MWC 166 A

   https://doi.org/10.1051/0004-6361/202243957  

   Zarrilli, Sebastian A.; Kraus, Stefan; Kreplin, Alexander; Monnier, John D.; Gardner, Tyler; Mérand, Antoine; Morrell, Sam; Davies, Claire L.; Labdon, Aaron; Ennis, Jacob; et al (September 2022, Astronomy & Astrophysics)

   Context. Stellar evolution models are highly dependent on accurate mass estimates, especially for highly massive stars in the early stages of stellar evolution. The most direct method for obtaining model-independent stellar masses is derivation from the orbit of close binaries. Aims. Our aim was to derive the first astrometric plus radial velocity orbit solution for the single-lined spectroscopic binary star MWC 166 A, based on near-infrared interferometry over multiple epochs and ∼100 archival radial velocity measurements, and to derive fundamental stellar parameters from this orbit. A supplementary aim was to model the circumstellar activity in the system from K band spectral lines. Methods. The data used include interferometric observations from the VLTI instruments GRAVITY and PIONIER, as well as the MIRC-X instrument at the CHARA Array. We geometrically modelled the dust continuum to derive relative astrometry at 13 epochs, determine the orbital elements, and constrain individual stellar parameters at five different age estimates. We used the continuum models as a base to examine differential phases, visibilities, and closure phases over the Br γ and He  I emission lines in order to characterise the nature of the circumstellar emission. Results. Our orbit solution suggests a period of P  = 367.7 ± 0.1 d, approximately twice as long as found with previous radial velocity orbit fits. We derive a semi-major axis of 2.61 ± 0.04 au at d  = 990 ± 50 pc, an eccentricity of 0.498 ± 0.001, and an orbital inclination of 53.6 ± 0.3°. This allowed the component masses to be constrained to M 1  = 12.2 ± 2.2  M ⊙ and M 2  = 4.9 ± 0.5  M ⊙ . The line-emitting gas was found to be localised around the primary and is spatially resolved on scales of ∼11 stellar radii, where the spatial displacement between the line wings is consistent with a rotating disc. Conclusions. The large spatial extent and stable rotation axis orientation measured for the Br γ and He  I line emission are inconsistent with an origin in magnetospheric accretion or boundary-layer accretion, but indicate an ionised inner gas disc around this Herbig Be star. We observe line variability that could be explained either with generic line variability in a Herbig star disc or V/R variations in a decretion disc scenario. We have also constrained the age of the system, with relative flux ratios suggesting an age of ∼(7 ± 2)×10 5 yr, consistent with the system being composed of a main-sequence primary and a secondary still contracting towards the main-sequence stage. 

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