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1. Fundamental Parameters for Central Stars of 103 Infrared Bow Shock Nebulae

   https://doi.org/10.3847/1538-4357/ade22f  

   Patten, Nikhil; Kobulnicky, Henry A; Povich, Matthew S; Whisnant, Angelica S; Andrews, Sydney; Boone, Alexandra; Dandu, Srujan; Jones, Naomi; Justice, S Nick; Hope, Dylan; et al (July 2025, The Astrophysical Journal)

   Abstract Stellar bow shock nebulae are arcuate shock fronts formed by the interaction of radiation-driven stellar winds and the relative motion of the ambient interstellar material. Stellar bow shock nebulae provide a promising means to measure wind-driven mass loss, independent of other established methods. In this work, we characterize the stellar sources at the center of bow shock nebulae drawn from all-sky catalogs of 24μm–selected nebulae. We obtain new, low-resolution blue optical spectra for 104 stars and measure stellar parameters temperatureT_eff, surface gravity $\log g$, and projected rotational broadening $v\sin i$. We perform additional photometric analysis to measure stellar radiusR_*, luminosityL_*, and visual-band extinctionA_V. All but one of our targets are O and early B stars, with temperatures ranging fromT= 16.5 to 46.8 kK, gravities from $\log g=$2.57 to 4.60, and $v\sin i$from <100 to 400 km s⁻¹. With the exception of rapid rotatorζOph, bow shock stars do not rotate at or near critical velocities. At least 60 of 103 (60%) OB bow shock stars are binaries, consistent with the multiplicity fraction of other OB samples. The sample shows a runaway fraction of 23%, with 19 stars havingv_2D≥ 25 km s⁻¹. Of the 19 runaways, at least 15 (≥79%) are binaries, favoring dynamical ejection over the binary supernova channel for producing runaways. We provide a comprehensive census of stellar parameters for bow shock stars, useful as a foundation for determining the mass-loss rates for OB-type stars—one of the single most critical factors in stellar evolution governing the production of neutron stars and black holes. 

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2. 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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3. Delayed Photons from Binary Evolution Help Reionize the Universe

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

   Secunda, Amy; Cen, Renyue; Kimm, Taysun; Götberg, Ylva; de Mink, Selma E. (September 2020, The Astrophysical Journal)

   Abstract High-resolution numerical simulations including feedback and aimed at calculating the escape fraction (f_esc) of hydrogen-ionizing photons often assume stellar radiation based on single-stellar population synthesis models. However, strong evidence suggests the binary fraction of massive stars is ≳70%. Moreover, simulations so far have yielded values off_escfalling only on the lower end of the ∼10%–20% range, the amount presumed necessary to reionize the universe. Analyzing a high-resolution (4 pc) cosmological radiation-hydrodynamic simulation, we study howf_escchanges when we include two different products of binary stellar evolution—stars stripped of their hydrogen envelopes and massive blue stragglers. Both produce significant amounts of ionizing photons 10–200 Myr after each starburst. We find the relative importance of these photons to be amplified with respect to escaped ionizing photons, because peaks in star formation rates (SFRs) andf_escare often out of phase by this 10–200 Myr. Additionally, low-mass, bursty galaxies emit Lyman continuum radiation primarily from binary products when SFRs are low. Observations of these galaxies by the James Webb Space Telescope could provide crucial information on the evolution of binary stars as a function of redshift. Overall, including stripped stars and massive blue stragglers increases our photon-weighted mean escape fraction () by ∼13% and ∼10%, respectively, resulting in. Our results emphasize that using updated stellar population synthesis models with binary stellar evolution provides a more sound physical basis for stellar reionization. 

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4. The Solar Neighborhood L: Spectroscopic Discovery of K Dwarfs Younger Than 1 Gyr and New Binaries within 30 pc

   https://doi.org/10.3847/1538-3881/ac8d6a  

   Hubbard-James, Hodari-Sadiki; Lesley, D Xavier; Henry, Todd J; Paredes, Leonardo A; Nisak, Azmain H (October 2022, The Astronomical Journal)

   Abstract As part of a comprehensive effort to characterize the nearest stars, the CHIRON echelle spectrograph on the CTIO/SMARTS 1.5 m telescope is being used to acquire high-resolution (R= 80,000) spectra of K dwarfs within 50 pc. This paper provides spectral details about 35 K dwarfs from five benchmark sets with estimated ages spanning 20 Myr–5.7 Gyr. Four spectral age and activity indicators are tested, three of which aligned with the estimated ages of the benchmark groups—the Naidoublet (5889.95 and 5895.92 Å), the Hαline (6562.8 Å), and the Liiresonance line (6707.8 Å). The benchmark stars are then used to evaluate seven field K dwarfs exhibiting variable radial velocities for which initial CHIRON data did not show obvious companions. Two of these stars are estimated to be younger than 700 Myr, while one exhibits stellar activity unusual for older K-dwarf field stars and is possibly young. The four remaining stars turn out to be spectroscopic binaries, two of which are being reported here for the first time with orbital periods found using CHIRON data. Spectral analysis of the combined sample of 42 benchmark and variable radial velocity stars indicates temperatures ranging from 3900 to 5300 K and metallicities from −0.4 < [Fe/H] < +0.2. We also determine $\log g=4.5–4.7$for main-sequence K dwarfs. Ultimately, this study will target several thousand of the nearest K dwarfs and provide results that will serve present and future studies of stellar astrophysics and exoplanet habitability. 

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5. The Peak of the Fallback Rate from Tidal Disruption Events: Dependence on Stellar Type

   https://doi.org/10.3847/2041-8213/ad0388  

   Bandopadhyay, Ananya; Fancher, Julia; Athian, Aluel; Indelicato, Valentino; Kapalanga, Sarah; Kumah, Angela; Paradiso, Daniel A; Todd, Matthew; Coughlin, Eric R; Nixon, C J (January 2024, The Astrophysical Journal Letters)

   Abstract A star completely destroyed in a tidal disruption event (TDE) ignites a luminous flare that is powered by the fallback of tidally stripped debris to a supermassive black hole (SMBH) of massM_•. We analyze two estimates for the peak fallback rate in a TDE, one being the “frozen-in” model, which predicts a strong dependence of the time to peak fallback rate,t_peak, on both stellar mass and age, with 15 days ≲t_peak≲ 10 yr for main sequence stars with masses 0.2 ≤M_⋆/M_⊙≤ 5 andM_•= 10⁶M_⊙. The second estimate, which postulates that the star is completely destroyed when tides dominate the maximum stellar self-gravity, predicts thatt_peakis very weakly dependent on stellar type, with $t_{\mathrm{peak}}=\left(23.2\pm 4.0\mathrm{days}\right){\left(M_{•}/{10}^6{M}_{\odot }\right)}^{1/2}$for 0.2 ≤M_⋆/M_⊙≤ 5, while $t_{\mathrm{peak}}=\left(29.8\pm 3.6\mathrm{days}\right){\left(M_{•}/{10}^6{M}_{\odot }\right)}^{1/2}$for a Kroupa initial mass function truncated at 1.5M_⊙. This second estimate also agrees closely with hydrodynamical simulations, while the frozen-in model is discrepant by orders of magnitude. We conclude that (1) the time to peak luminosity in complete TDEs is almost exclusively determined by SMBH mass, and (2) massive-star TDEs power the largest accretion luminosities. Consequently, (a) decades-long extra-galactic outbursts cannot be powered by complete TDEs, including massive-star disruptions, and (b) the most highly super-Eddington TDEs are powered by the complete disruption of massive stars, which—if responsible for producing jetted TDEs—would explain the rarity of jetted TDEs and their preference for young and star-forming host galaxies. 

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