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HV 2112
	HV 2112 (circled in red)
Observation data; Epoch J2000.0 Equinox J2000.0
Constellation	Tucana
Right ascension	01h 10m 03.858s
Declination	−72° 36′ 52.62″
Apparent magnitude (V)	12.7 to below 16.7
Characteristics
Spectral type	M5.5 II (M3e – M7.5)
Apparent magnitude (J)	10.020
Apparent magnitude (H)	9.100
Apparent magnitude (K)	8.723
U−B color index	+0.33
B−V color index	+1.80
Variable type	Mira?
Astrometry
Radial velocity (Rv)	157 km/s
Proper motion (μ)	RA: 1.116±0.073 mas/yr ; Dec.: −1.291±0.067 mas/yr
Parallax (π)	−0.2029 ± 0.0446 mas
Absolute magnitude (MV)	−5.2
Details
Mass	~5 M☉
Radius	675 - 1,193 R☉
Luminosity	50,100 - 81,300 L☉
Surface gravity (log g)	0.0 cgs
Temperature	2,500 - 3,750 K
Metallicity [Fe/H]	-2.18 dex
Other designations
	HV 2112, 2MASS J01100385-7236526, SMC V2156, PMMR 187
Database references
SIMBAD	data

HV 2112 is a cool luminous variable star in the Small Magellanic Cloud. Until 2018, it was considered to be the most likely candidate for a Thorne–Żytkow object, but it is now thought to be an asymptotic giant branch star.

Discovery

HV 2112 was first reported as a variable star in 1908, by Henrietta Leavitt. At the time it was identified as Harvard no. 2112. No period was given, but it was reported to be "probably long". The magnitude range was given as 13.7 to fainter than 16.5, from photographic plates.^[12]

In 1966, analysis of Magellanic Cloud variable stars showed that HV 2112 had a photographic magnitude range from 13.0 to below 17.8. It was classified as a long-period variable, now known as a Mira variable, on the basis of its large amplitude and reasonably regular light variations, with a period of about 600 days.^[13]^[14]

Possible object types

A visual band light curve for HV 2112, plotted from ASAS-SN data^[15]

AGB star

HV 2112 had historically been treated as a very luminous asymptotic giant branch (AGB) star, a red giant that has exhausted its core helium and is in the last stages of its evolution. Large-amplitude class-M variables and stars with spectral types later than about M5 are almost always AGB stars rather than red supergiants. These stars have a theoretical maximum luminosity and, at the distance of the SMC, HV 2112 was typically calculated to be slightly more luminous than this limit at around 60,000 L_☉.^[4]

More modern calculations gave higher values for the luminosity of HV 2112 above 100,000 L_☉, which is unambiguously too luminous to be an AGB star. These calculations included an interstellar extinction value of 0.4 magnitudes which is higher than average for massive stars in the SMC. However, it is not exceptional for red supergiants, which are believed to show additional extinction due to circumstellar dust near the star.^[7]

Analysis of the proper motion of HV 2112 in 2016 reported that it is unusually large for an SMC star, although the radial velocity is consistent with other SMC objects. The proper motion of around 10 mas/year would indicate a space velocity of 3,100 km/sec at the distance of the SMC, well above its escape velocity. A more likely explanation of such a proper motion would be that HV 2112 lies about 3,000 parsecs away in our own galaxy. It would then be around 1,000 L_☉ rather than 100,000 L_☉ and so a typical AGB star. The over-abundance of heavy elements would then be explained as pollution from an unseen companion, producing an extrinsic S-type star.^[16] Other analyses of the proper motion show much smaller velocities, consistent with an object in the SMC^[17]^[18]

Thorne–Żytkow object

In 2014, HV 2112 was identified as a possible Thorne–Żytkow object (TZO) using the Magellan Clay telescope in Chile. To find candidate TZOs, Emily Levesque used the Apache Point Observatory to examine 24 red supergiant stars in the Milky Way, and the Magellan Clay telescope to look at 16 in the Large Magellanic Cloud and 22 in the Small Magellanic Cloud. The star was thought to contain unusually high levels of the elements lithium, molybdenum and rubidium that are expected only to be produced by TZOs.^[7]

A 2018 paper re-appraising the properties of HV 2112 found no evidence for unusual chemical abundances and a luminosity that is lower than previously thought. This suggests that the star is unlikely to be a TZO, and is much more likely an intermediate mass AGB star.^[9]

Binary star

HV 2112 is listed in the OGLE catalogue as an unresolved multiple star. The proper motions and radial velocity are consistent with other SMC objects, while the parallax is negative but acceptably close to the expected value for such a distant object.^[19]

References

^ ^a ^b ^c ^d ^e Cutri, Roc M.; Skrutskie, Michael F.; Van Dyk, Schuyler D.; Beichman, Charles A.; Carpenter, John M.; Chester, Thomas; Cambresy, Laurent; Evans, Tracey E.; Fowler, John W.; Gizis, John E.; Howard, Elizabeth V.; Huchra, John P.; Jarrett, Thomas H.; Kopan, Eugene L.; Kirkpatrick, J. Davy; Light, Robert M.; Marsh, Kenneth A.; McCallon, Howard L.; Schneider, Stephen E.; Stiening, Rae; Sykes, Matthew J.; Weinberg, Martin D.; Wheaton, William A.; Wheelock, Sherry L.; Zacarias, N. (2003). "VizieR Online Data Catalog: 2MASS All-Sky Catalog of Point Sources (Cutri+ 2003)". CDS/ADC Collection of Electronic Catalogues. 2246: II/246. Bibcode:2003yCat.2246....0C.
^ Kochanek, C. S.; Shappee, B. J.; Stanek, K. Z.; Holoien, T. W.-S.; Thompson, Todd A.; Prieto, J.-L.; Dong, Subo; Shields, J. V.; Will, D.; et al. (2017). "The All-Sky Automated Survey for Supernovae (ASAS-SN) Light Curve Server v1.0". Publications of the Astronomical Society of the Pacific. 129 (980): 104502. arXiv:1706.07060. Bibcode:2017PASP..129j4502K. doi:10.1088/1538-3873/aa80d9. S2CID 118995322.
^ Shappee, B. J.; Prieto, J. L.; Grupe, D.; Kochanek, C. S.; Stanek, K. Z.; De Rosa, G.; Mathur, S.; Zu, Y.; Peterson, B. M.; et al. (2014). "The Man behind the Curtain: X-Rays Drive the UV through NIR Variability in the 2013 Active Galactic Nucleus Outburst in NGC 2617". The Astrophysical Journal. 788 (1): 48. arXiv:1310.2241. Bibcode:2014ApJ...788...48S. doi:10.1088/0004-637X/788/1/48. S2CID 119113433.
^ ^a ^b Wood, P. R.; Bessell, M. S.; Fox, M. W. (1983). "Long-period variables in the Magellanic Clouds – Supergiants, AGB stars, supernova precursors, planetary nebula precursors, and enrichment of the interstellar medium". Astrophysical Journal. 272: 99. Bibcode:1983ApJ...272...99W. doi:10.1086/161265.
^ ^a ^b Boyer, Martha L.; Srinivasan, Sundar; Van Loon, Jacco Th.; McDonald, Iain; Meixner, Margaret; Zaritsky, Dennis; Gordon, Karl D.; Kemper, F.; Babler, Brian; et al. (2011). "Surveying the Agents of Galaxy Evolution in the Tidally Stripped, Low Metallicity Small Magellanic Cloud (SAGE-SMC). II. Cool Evolved Stars". The Astronomical Journal. 142 (4): 103. arXiv:1106.5026. Bibcode:2011AJ....142..103B. doi:10.1088/0004-6256/142/4/103. S2CID 119268816.
^ ^a ^b Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007–2013)". VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S. 1. Bibcode:2009yCat....102025S.
^ ^a ^b ^c ^d ^e Levesque, Emily (1 September 2014). "Discovery of a Thorne-̇Żytkow object candidate in the Small Magellanic Cloud". Monthly Notices of the Royal Astronomical Society: Letters. 443: L94–L98. arXiv:1406.0001. Bibcode:2014MNRAS.443L..94L. doi:10.1093/mnrasl/slu080. S2CID 119192926.
^ ^a ^b ^c Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.
^ ^a ^b ^c ^d ^e Beasor, Emma; Davies, Ben; Cabrera-Ziri, Ivan; Hurst, Georgia (2 July 2018). "A critical re-evaluation of the Thorne-Żytkow object candidate HV 2112". Monthly Notices of the Royal Astronomical Society. 479 (3): 3101. arXiv:1806.07399. Bibcode:2018MNRAS.479.3101B. doi:10.1093/mnras/sty1744. S2CID 67766043.
^ Anders, F.; Khalatyan, A.; Chiappini, C.; Queiroz, A. B.; Santiago, B. X.; Jordi, C.; Girardi, L.; Brown, A. G. A.; Matijevič, G.; Monari, G.; Cantat-Gaudin, T.; Weiler, M.; Khan, S.; Miglio, A.; Carrillo, I.; Romero-Gómez, M.; Minchev, I.; De Jong, R. S.; Antoja, T.; Ramos, P.; Steinmetz, M.; Enke, H. (2019). "Photo-astrometric distances, extinctions, and astrophysical parameters for Gaia DR2 stars brighter than G = 18". Astronomy and Astrophysics. 628: A94. arXiv:1904.11302. Bibcode:2019A&A...628A..94A. doi:10.1051/0004-6361/201935765. S2CID 131780028.
^ "HV 2112". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2021-05-05.
^ Leavitt, Henrietta S. (1908). "1777 variables in the Magellanic Clouds". Annals of Harvard College Observatory. 60: 87. Bibcode:1907AnHar..60...87L.
^ Payne-Gaposchkin, Cecilia; Gaposchkin, Sergei (1966). "Variable Stars in the Small Magellanic Cloud". Smithsonian Contributions to Astrophysics. 9: 1–205. Bibcode:1966SCoA....9....1P. doi:10.5479/si.00810231.9.1.
^ O’Grady, Anna J. G.; Drout, Maria R.; Shappee, B. J.; Bauer, Evan B.; Fuller, Jim; Kochanek, C. S.; Jayasinghe, T.; Gaensler, B. M.; Stanek, K. Z.; Holoien, Thomas W. S.; Prieto, J. L.; Thompson, Todd A. (October 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". The Astrophysical Journal. 901 (2): 135. arXiv:2008.06563. Bibcode:2020ApJ...901..135O. doi:10.3847/1538-4357/abafad.
^ "ASAS-SN Variable Stars Database". ASAS-SN Variable Stars Database. ASAS-SN. Retrieved 6 January 2022.
^ MacCarone, Thomas J.; De Mink, Selma E. (2016). "Large proper motion of the Thorne-Żytkow object candidate HV 2112 reveals its likely nature as foreground Galactic S-star". Monthly Notices of the Royal Astronomical Society: Letters. 458 (1): L1. arXiv:1601.05455. Bibcode:2016MNRAS.458L...1M. doi:10.1093/mnrasl/slw004. S2CID 119306614.
^ Worley, C. Clare; Irwin, Mike. J.; Tout, Christopher A.; Żytkow, Anna N.; Fraser, Morgan; Izzard, Robert G. (2016). "The proper motion of HV2112: A TŻO candidate in the SMC". Monthly Notices of the Royal Astronomical Society: Letters. 459 (1): L31–L35. arXiv:1602.08479. Bibcode:2016MNRAS.459L..31W. doi:10.1093/mnrasl/slw034. S2CID 118463486.
^ McMillan, Paul J; Church, Ross P (2018). "Gaia DR2 confirms that candidate Thorne-Żytkow object HV 2112 is in the SMC". Research Notes of the AAS. 2 (2): 18. arXiv:1804.10192v1. Bibcode:2018RNAAS...2...18M. doi:10.3847/2515-5172/aac0fb. S2CID 125376171.
^ González-Fernández, Carlos; Dorda, Ricardo; Negueruela, Ignacio; Marco, Amparo (2015). "A new survey of cool supergiants in the Magellanic Clouds". Astronomy & Astrophysics. 578: A3. arXiv:1504.00003. Bibcode:2015A&A...578A...3G. doi:10.1051/0004-6361/201425362. S2CID 55959019.

Notes

^ Calculated from effective temperature and luminosity figures, with reference to the nominal solar temperature of 5,772 K: ${\sqrt {(5772/3750)^{4}*81,283}}=675.4\ R\odot$ ${\sqrt {(5772/2500)^{4}*50,119}}=1,193.4\ R\odot$