Cool, Luminous, and Highly Variable Stars in the Magellanic Clouds from ASAS-SN: Implications for Thorne-Żytkow Objects and Super-Asymptotic Giant Branch Stars

Abstract

Stars with unusual properties can provide a wealth of information about rare stages of stellar evolution and exotic physics. However, determining the true nature of peculiar stars is often difficult. In this work, we conduct a systematic search for cool and luminous stars in the Magellanic Clouds with extreme variability, motivated by the properties of the unusual SMC star and Thorne-Żytkow Object (TŻO) candidate HV2112. Using light curves from ASAS-SN we identify 38 stars with surface temperatures T $<$ 4800K, luminosities $\log$(L/L${}_{\odot }$) $>$ 4.3, variability periods $>$ 400 days, and variability amplitudes $\Delta$V $>$ 2.5 mag. Eleven of these stars possess the distinctive double-peaked light curve morphology of HV2112. We use the pulsation properties and derived occurrence rates for these 12 objects to constrain their nature. From comparisons to stellar populations and models, we find that one star may be a red supergiant with large amplitude pulsations. For the other 11 stars we derive current masses of $\sim$5-10 M${}_{\odot }$, below the theoretical minimum mass of $\sim$15 M${}_{\odot }$ for TŻOs to be stable, casting doubt on this interpretation. Instead, we find that the temperatures, luminosities, mass-loss rates, and periods of these stars are consistent with predictions for super-Asymptotic Giant Branch (s-AGB) stars that have begun carbon burning but have not reached the superwind phase. We infer lifetimes in this phase of $\sim ($1$-$7) $\times$ 10${}^4$ years, also consistent with an s-AGB interpretation. If confirmed, these objects would represent the first identified population of s-AGB stars, illuminating the transition between low- and high-mass stellar evolution.

arXiv comment29 pages, 14 figures, 4 tables, plus Appendix. Accepted for publication in the Astrophysical Journal

References in corpus (84)

• Gaia Data Release 2. Summary of the contents and survey properties
• The Wide-field Infrared Survey Explorer (WISE): Mission Description and Initial On-orbit Performance
• Measuring Reddening with SDSS Stellar Spectra and Recalibrating SFD
• Modules for Experiments in Stellar Astrophysics (MESA)
• Binary companions of evolved stars in APOGEE DR14: Search method and catalog of ~5,000 companions
• Modules for Experiments in Stellar Astrophysics (MESA): Binaries, Pulsations, and Explosions
• MESA Isochrones and Stellar Tracks (MIST). I: Solar-Scaled Models
• A grid of MARCS model atmospheres for late-type stars I. Methods and general properties
• Evolutionary tracks and isochrones for low- and intermediate-mass stars: from 0.15 to 7 M_sun, and from Z=0.0004 to 0.03
• The SIMBAD astronomical database
• Modules for Experiments in Stellar Astrophysics (MESA): Convective Boundaries, Element Diffusion, and Massive Star Explosions
• MESA Isochrones and Stellar Tracks (MIST) 0: Methods for the construction of stellar isochrones
• The Man Behind the Curtain: X-rays Drive the UV through NIR Variability in the 2013 AGN Outburst in NGC 2617
• Modules for Experiments in Stellar Astrophysics (MESA): Pulsating Variable Stars, Rotation, Convective Boundaries, and Energy Conservation
• A method for optimal image subtraction
• A Quantitative Comparison of SMC, LMC, and Milky Way UV to NIR Extinction Curves
• The All-Sky Automated Survey for Supernovae (ASAS-SN) Light Curve Server v1.0
• Astrometry.net: Blind astrometric calibration of arbitrary astronomical images
• Preliminary Results from NEOWISE: An Enhancement to the Wide-field Infrared Survey Explorer for Solar System Science
• Spitzer Survey of the Large Magellanic Cloud, Surveying the Agents of a Galaxy's Evolution (SAGE) I: Overview and Initial Results
• An eclipsing binary distance to the Large Magellanic Cloud accurate to 2 per cent
• The Dawes Review 2: Nucleosynthesis and stellar yields of low and intermediate-mass single stars
• Gaia Data Release 2: Kinematics of globular clusters and dwarf galaxies around the Milky Way
• The MACHO Project LMC Microlensing Results from the First Two Years and the Nature of the Galactic Dark Halo
• GYRE: An open-source stellar oscillation code based on a new Magnus Multiple Shooting Scheme
• The Star Formation History of the Large Magellanic Cloud
• The ASAS-SN Catalog of Variable Stars I: The Serendipitous Survey
• The Magellanic Clouds Photometric Survey: The Large Magellanic Cloud Stellar Catalog and Extinction Map
• The Star Formation History of the Small Magellanic Cloud
• An empirical formula for the mass-loss rates of dust-enshrouded red supergiants and oxygen-rich Asymptotic Giant Branch stars
• The Magellanic Clouds Photometric Survey: The Small Magellanic Cloud Stellar Catalog and Extinction Map
• Slowing the Spins of Stellar Cores
• Spectral synthesis including massive binaries
• The Supernova Channel of Super-AGB Stars
• The search for failed supernovae with the Large Binocular Telescope: confirmation of a disappearing star
• Super and massive AGB stars - IV. Final fates - Initial to final mass relation
• Evolution of massive stars with pulsation-driven superwinds during the RSG phase
• The mass-loss rates of red supergiants and the de Jager prescription
• A New Class of Luminous Transients and A First Census of Their Massive Stellar Progenitors
• Advanced burning stages and fate of 8-10 Mo stars
• The ASAS-SN Catalog of Variable Stars II: Uniform Classification of 412,000 Known Variables
• Super and Massive AGB Stars: II - Nucleosynthesis and Yields - Z=0.02, 0.008 and 0.004
• Asymptotic Giant Branch evolution at varying surface C/O ratio: effects of changes in molecular opacities
• The Evolution of Massive Stars. I. Red Supergiants in the Magellanic Clouds
• Luminosities and mass-loss rates of SMC and LMC AGB stars and Red Supergiants
• Surveying the Agents of Galaxy Evolution in the Tidally-Stripped, Low Metallicity Small Magellanic Cloud (SAGE-SMC) II. Cool Evolved Stars
• The ASAS-SN Catalog of Variable Stars III: Variables in the Southern TESS Continuous Viewing Zone
• Super-AGB Stars and their role as Electron Capture Supernova progenitors
• The temperatures of Red Supergiants
• The Dynamics and Outcomes of Rapid Infall onto Neutron Stars
• AGB stars in the Magellanic Clouds. II. The rate of star formation across the LMC
• High-Mass X-ray binaries in the Small Magellanic Cloud
• The supernova rate and delay time distribution in the Magellanic Clouds
• Super and massive AGB stars - III. Nucleosynthesis in metal-poor and very metal-poor stars - Z=0.001 and 0.0001
• The Search for Failed Supernovae with the Large Binocular Telescope: Constraints from 7 Years of Data
• The luminosities of cool supergiants in the Magellanic Clouds, and the Humphreys-Davidson limit revisited
• Yellow and Red Supergiants in the Large Magellanic Cloud
• Star-formation history and X-ray binary populations: the case of the Large Magellanic Cloud
• Late-Type Red Supergiants: Too Cool for the Magellanic Clouds?
• Molecular opacities for low-mass metal-poor AGB stars undergoing the Third Dredge Up
• The ASAS-SN Catalog of Variable Stars V: Variables in the Southern Hemisphere
• Photometric metallicity map of the Large Magellanic Cloud
• Almost Gone: SN~2008S and NGC~300 2008OT-1 are Fainter than their Progenitors
• The Period-Luminosity Relation of Red Supergiant Stars in the Small Magellanic Cloud
• Discovery of a Thorne-Zytkow object candidate in the Small Magellanic Cloud
• Spitzer Infrared Spectrographic point source classification in the Small Magellanic Cloud
• Red Supergiant Stars in the Large Magellanic Cloud: I. The Period-Luminosity Relation
• The Link Between the Formation Rates of Clusters and Stars in Galaxies
• Variable Evolved Stars and YSOs Discovered in the Large Magellanic Cloud using the SAGE Survey
• Photometric metallicity map of the Small Magellanic Cloud
• The ASAS-SN Catalog of Variable Stars VI: An All-Sky Sample of $\delta$ Scuti Stars
• Variability of Red Supergiants in M31 from the Palomar Transient Factory
• A catalogue of bright (K < 9) M dwarfs
• Evolutionary models of red supergiants: Evidence for a metallicity-dependent mixing length and implications for Type IIP supernova progenitors
• The ASAS-SN Catalog of Variable Stars VII: Contact Binaries are Different Above and Below the Kraft Break
• Identification of a Class of Low-Mass Asymptotic Giant Branch Stars Struggling to Become Carbon Stars in the Magellanic Clouds
• The ASAS-SN Catalog of Variable Stars IV: Periodic Variables in the APOGEE Survey
• HV2112, a Thorne-Zytkow Object or a Super Asymptotic Giant Branch Star
• A critical re-evaluation of the Thorne-Żytkow object candidate HV 2112
• Large proper motion of the Thorne-Żytkow object candidate HV 2112 reveals its likely nature as foreground Galactic S-star
• Beyond Gaia: Asteroseismic Distances of M giants using Ground-Based Transient Surveys
• The shapes of light curves of Mira-type variables
• An All-Sky Search For R Coronae Borealis Stars in ASAS-SN
• Luminosities of AGB Variables

Cited by in corpus (17)

• Final Targeting Strategy for the SDSS-IV APOGEE-2S Survey
• SN2023ixf in Messier 101: A Variable Red Supergiant as the Progenitor Candidate to a Type II Supernova
• The Time-Averaged Mass-Loss Rates of Red Supergiants As Revealed by their Luminosity Functions in M31 and M33
• An Exceptional Dimming Event for a Massive, Cool Supergiant in M51
• Observational predictions for Thorne-Żytkow objects
• The VVV survey: Long-period variable stars I. Photometric catalog of ten VVV/OGLE tiles
• Prospects for Multimessenger Observations of Thorne-Żytkow Objects
• The first catalog of candidate white dwarf-main sequence binaries in open star clusters: A new window into common envelope evolution
• Cool, Luminous, and Highly Variable Stars in the Magellanic Clouds. II: Spectroscopic and Environmental Analysis of Thorne-Żytkow Object and Super-AGB Star Candidates
• Rethinking Thorne-Żytkow Object Formation: The Fate of X-ray Binary LMC X-4 and Implications for Ultra-long Gamma-ray Bursts
• Wide binary pulsars from electron-capture supernovae
• On the nature of VX Sagitarii: Is it a TZO, a RSG or a high-mass AGB star?
• Properties of Thorne-Zytkow object explosions
• Thorne-Żytkow Objects
• Population synthesis of Thorne-Żytkow objects: Rejuvenated donors and unexplored progenitors in the common envelope formation channel
• Constraints on Weak Supernova Kicks from Observed Pulsar Velocities
• A Survey for C II Emission-Line Stars in the Large Magellanic Cloud. II. Final Results and the Origin of C II Emission in [WC] Spectra