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

O'Grady, Anna J. G. and Drout, Maria R. and Shappee, B. J. and Bauer, Evan B. and Fuller, Jim and Kochanek, C. S. and Jayasinghe, T. and Gaensler, B. M. and Stanek, K. Z. and Holoien, Thomas W.-S. and Prieto, J. L. and Thompson, Todd A. (2020) Cool, Luminous, and Highly Variable Stars in the Magellanic Clouds from ASAS-SN: Implications for Thorne-Żytkow Objects and Super-Asymptotic Giant Branch Stars. Astrophysical Journal, 901 (2). Art. No. 135. ISSN 1538-4357. doi:10.3847/1538-4357/abafad.

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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 Small Magellanic Cloud star and Thorne–Żytkow Object (TŻO) candidate HV 2112. Using light curves from ASAS-SN, we identify 38 stars with surface temperatures T < 4800 K, luminosities log(L/L_⊙) > 4.3, variability periods >400 days, and variability amplitudes ΔV > 2.5 mag. Eleven of these stars possess the distinctive double-peaked light-curve morphology of HV 2112. 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 ~5–10 M_⊙, below the theoretical minimum mass of ~15 M_⊙ for TŻOs to be stable, casting doubt on this interpretation. Instead, we find that the temperatures, luminosities, mass-loss rates (MLRs), 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 ~(1–7) × 10⁴ yr, 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.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
O'Grady, Anna J. G.0000-0002-7296-6547
Drout, Maria R.0000-0001-7081-0082
Shappee, B. J.0000-0003-4631-1149
Bauer, Evan B.0000-0002-4791-6724
Fuller, Jim0000-0002-4544-0750
Kochanek, C. S.0000-0001-6017-2961
Jayasinghe, T.0000-0002-6244-477X
Gaensler, B. M.0000-0002-3382-9558
Holoien, Thomas W.-S.0000-0001-9206-3460
Prieto, J. L.0000-0003-1072-2712
Thompson, Todd A.0000-0003-2377-9574
Additional Information:© 2020 The American Astronomical Society. Received 2020 May 20; revised 2020 August 14; accepted 2020 August 14; published 2020 September 30. The authors thank John Percy, Emily Levesque, Carolyn Doherty, Martha Boyer, Marten van Kerkwijk, Dae-Sik Moon, Katie Breivik, and Dan Huber for helpful conversations, and Tyler Downey and Miranda Herman for helpful edits. The authors thank the anonymous reviewer for a helpful and constructive referee report. The authors at the University of Toronto acknowledge that the land on which the University of Toronto is built is the traditional territory of the Huron-Wendat, the Seneca, and most recently, the Mississaugas of the Credit River. They are grateful to have the opportunity to work in the community, on this territory. The Dunlap Institute is funded through an endowment established by the David Dunlap family and the University of Toronto. A.O. acknowledges support from the Queen Elizabeth II Graduate Scholarship in Science and Technology, Lachlan Gilchrist Fellowship Fund, and the Walter C. Sumner Memorial Fellowship. M.R.D. acknowledges support from the Dunlap Institute at the University of Toronto and the Canadian Institute for Advanced Research (CIFAR). Part of this work was supported through the Hubble Fellowship Grant NSG-HF2-51373 to M.R.D., awarded through the Space Telescope Science Institute, which is operated by the Association of the Universities for Research in Astronomy Inc., for NASA, under contract NAS5-26555. B.J.S., K.Z.S., and C.S.K. are supported by NSF grants AST-1515927, AST-1814440, and AST-1908570. B.J.S. is also supported by NSF grants AST-1920392 and AST-1911074. B.M.G. acknowledges the support of the Natural Sciences and Engineering Research Council of Canada (NSERC) through grant RGPIN-2015-05948, and of the Canada Research Chairs program. Support for J.L.P. is provided in part by FONDECYT through the grant 1191038 and by the Ministry of Economy, Development, and Tourism's Millennium Science Initiative through grant IC120009, awarded to The Millennium Institute of Astrophysics, MAS. T.A.T. is supported in part by NASA grant 80NSSC20K0531. This research was supported in part by the National Science Foundation under grant No. NSF PHY-1748958. This research benefited from interactions made possible by the Gordon and Betty Moore Foundation through grant GBMF5076. We thank the Las Cumbres Observatory and its staff for its continuing support of the ASAS-SN project. ASAS-SN is supported by the Gordon and Betty Moore Foundation through grant GBMF5490 to the Ohio State University and NSF grant AST-1515927. Development of ASAS-SN has been supported by NSF grant AST-0908816, the Mt. Cuba Astronomical Foundation, the Center for Cosmology and AstroParticle Physics at the Ohio State University, the Chinese Academy of Sciences South America Center for Astronomy (CASSACA), the Villum Foundation, and George Skestos. This research has made use of the SVO Filter Profile Service ( supported from the Spanish MINECO through grant AYA2017-84089 (Rodrigo et al. 2012; Rodrigo & Solano 2013); the SIMBAD database, operated at CDS, Strasbourg, France (Wenger et al. 2000); and the NASA/IPAC Infrared Science Archive, which is funded by the National Aeronautics and Space Administration and operated by the California Institute of Technology. This research was enabled in part by support provided by Compute Canada ( Software: astropy (Astropy Collaboration et al. 2018), IRAF (Tody 1986, 1993), ISIS (Alard & Lupton 1998; Alard 2000), TOPCAT (Taylor 2005), MARCS (Gustafsson et al. 2008), DUSTY (Nenkova et al. 2000).
Group:Astronomy Department, TAPIR
Funding AgencyGrant Number
David Dunlap FamilyUNSPECIFIED
University of TorontoUNSPECIFIED
Ontario Ministry of Colleges and UniversitiesUNSPECIFIED
Canadian Institute for Advanced Research (CIFAR)UNSPECIFIED
NASA Hubble FellowshipNSG-HF2-51373
Natural Sciences and Engineering Research Council of Canada (NSERC)RGPIN-2015-05948
Canada Research Chairs ProgramUNSPECIFIED
Fondo Nacional de Desarrollo Científico y Tecnológico (FONDECYT)1191038
Iniciativa Científica Milenio del Ministerio de Economía, Fomento y TurismoIC120009
Gordon and Betty Moore FoundationGBMF5076
Gordon and Betty Moore FoundationGBMF5490
Mt. Cuba Astronomical FoundationUNSPECIFIED
Center for Cosmology and AstroParticle Physics (CCAPP)UNSPECIFIED
Chinese Academy of Sciences South America Center for Astronomy (CASSACA)UNSPECIFIED
Villum FoundationUNSPECIFIED
Ministerio de Economía, Industria y Competitividad (MINECO)AyA2017-84089
Subject Keywords:Massive stars ; Asymptotic giant branch stars ; Variable stars ; Chemically peculiar stars ; Light curves ; Photometry
Issue or Number:2
Classification Code:Unified Astronomy Thesaurus concepts: Massive stars (732); Asymptotic giant branch stars (2100); Variable stars (1761); Chemically peculiar stars (226); Light curves (918); Photometry (1234)
Record Number:CaltechAUTHORS:20200916-113000011
Persistent URL:
Official Citation:Anna J. G. O'Grady et al 2020 ApJ 901 135
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:105431
Deposited By: George Porter
Deposited On:16 Sep 2020 20:30
Last Modified:16 Nov 2021 18:43

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