VX Sagittarii

VX Sagittarii is an extreme asymptotic giant branch star located more than 1.5 kiloparsec away from the Sun in the constellation of Sagittarius. It is a pulsating variable star with an unusually large magnitude range. It is also one of the largest stars discovered so far, with a radius varying between 1,350 and 1,940 solar radii (940,000,000 and 1.35×109 km; 6.3 and 9.0 au). It is the most luminous known AGB star, at bolometric magnitude –8.6, which is even brighter than the theoretical limit at –8.0.[3]

VX Sagittarii

VX Sagittarii, circled
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Sagittarius
Right ascension 18h 08m 04.04831s[1]
Declination −22° 13′ 26.6327″[1]
Apparent magnitude (V) 6.5 - 14.0[2]
Characteristics
Evolutionary stage AGB, (possibly TŻO)[3]
Spectral type M4eIa - M10eIa[4]
Apparent magnitude (U) 11.72
Apparent magnitude (B) 9.41
Apparent magnitude (V) 6.52
Apparent magnitude (I) 2.11
Apparent magnitude (J) 1.23
Apparent magnitude (H) 0.13
Apparent magnitude (K) −0.50
Apparent magnitude (L) −1.61
Variable type SRc[2]
Astrometry
Radial velocity (Rv)+6.47±3.37[5] km/s
Proper motion (μ) RA: +0.36±0.76[5] mas/yr
Dec.: −2.92±0.78[5] mas/yr
Parallax (π)0.64 ± 0.04[5] mas
Distance5,100 ± 300 ly
(1,560 ± 100 pc)
Details
Mass12[6] M☉
Radius1,356,[3] between 1,120 and 1,550[5], 1,350–1,940 (pulsation)[7] R☉
Luminosity195000±62000[5], 110,000–190,000[7] L☉
Temperature2,900[8] (near min), 3,200-3,400 (near max)[6], 2,400–3,300[7] K
Other designations
VX Sgr, HIP 88838, BD−22°4575, CD−22°12589, HD 165674, 2MASS J18080404-2213266, AAVSO 1802-22
Database references
SIMBADdata

Observations

The star is classed as a cool semiregular variable of type SRc with a pulsational period of 732 days. The variations sometimes have an amplitude comparable to a long period variable, at other times they are much smaller. The spectral type varies between M4e around visual maximum and M9.8e at minimum light, and the luminosity class is Ia indicating a bright supergiant. The spectrum shows emission lines indicating that the star is losing mass through a strong stellar wind.[7]

The annual parallax of VX Sagittarii has been measured extremely accurately using VLBI and found to be 0.64±0.06 mas, indicating a distance of about 5,100 light years. This is compatible with the distance to Sagittarius OB1, the stellar association that VX Sagittarii is thought to belong to. Its radial velocity and proper motions are also consistent with other members of the association.[5]

Stellar characteristics

The effective temperature of VX Sagittarii is apparently highly variable from around 2,400 K at visual minimum to around 3,300 K near maximum. Such low temperatures are comparable to the very coolest AGB stars and unprecedented for a massive supergiant.[6][7] Its atmosphere is highly extended, irregular, and variable during the pulsations of the star, but the bolometric luminosity varies much less than the visual brightness and is calculated to be about 195,000 L☉. At an effective temperature of 3,300 K, the radius is expected to be somewhere between 1,120 R☉ and 1,550 R☉.[5] Older studies frequently calculated higher luminosities.[9][10]

The atmosphere of VX Sgr shows molecular water layers and SiO masers in the atmosphere, typical of an OH/IR star.[11] The masers have been used to derive an accurate distance of 1,590 parsecs.[12] The spectrum also indicates strong VO and CN. In many respects the atmosphere is similar to low mass AGB stars such as Mira variables, but a supergiant luminosity and size.[6]

References
  1. Van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653–664. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357. S2CID 18759600.
  2. 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.
  3. Tabernero, H. M.; Dorda, R.; Negueruela, I.; Marfil, E. (2020). "On the nature of VX Sagitarii: Is it a TZO, a RSG or a high-mass AGB star?". arXiv:2011.09184 [astro-ph.SR].
  4. Kiss, L. L.; Szabó, G. M.; Bedding, T. R. (2006). "Variability in red supergiant stars: Pulsations, long secondary periods and convection noise". Monthly Notices of the Royal Astronomical Society. 372 (4): 1721–1734. arXiv:astro-ph/0608438. Bibcode:2006MNRAS.372.1721K. doi:10.1111/j.1365-2966.2006.10973.x. S2CID 5203133.
  5. Xu, Shuangjing; Zhang, Bo; Reid, Mark J; Menten, Karl M; Zheng, Xingwu; Wang, Guangli (2018). "The Parallax of the Red Hypergiant VX Sgr with Accurate Tropospheric Delay Calibration". The Astrophysical Journal. 859 (1): 14. arXiv:1804.00894. Bibcode:2018ApJ...859...14X. doi:10.3847/1538-4357/aabba6. S2CID 55572194.
  6. Chiavassa; Lacour; Millour; Driebe; Wittkowski; Plez; Thiebeaut; Josselin; Freytag (2009). "VLTI/AMBER spectro-interferometric imaging of VX Sgr's inhomogenous outer atmosphere". Astronomy and Astrophysics. 511: A51. arXiv:0911.4422. Bibcode:2010A&A...511A..51C. doi:10.1051/0004-6361/200913288. S2CID 55877127.
  7. Lockwood, G.W.; Wing, R. F. (1982). "The light and spectrum variations of VX Sagittarii, an extremely cool supergiant". Monthly Notices of the Royal Astronomical Society. 198 (2): 385–404. Bibcode:1982MNRAS.198..385L. doi:10.1093/mnras/198.2.385.
  8. García-Hernández, D. A; García-Lario, P; Plez, B; Manchado, A; d'Antona, F; Lub, J; Habing, H (2007). "Lithium and zirconium abundances in massive Galactic O-rich AGB stars". Astronomy and Astrophysics. 462 (2): 711. arXiv:astro-ph/0609106. Bibcode:2007A&A...462..711G. doi:10.1051/0004-6361:20065785. S2CID 16016698.
  9. De Jager, C.; Nieuwenhuijzen, H.; Van Der Hucht, K. A. (1988). "Mass loss rates in the Hertzsprung-Russell diagram". Astronomy and Astrophysics Supplement Series. 72: 259. Bibcode:1988A&AS...72..259D.
  10. Nicolas Mauron; Eric Josselin (2010). "The mass-loss rates of red supergiants and the de Jager prescription". Astronomy & Astrophysics. 526: A156. arXiv:1010.5369v1. Bibcode:2011A&A...526A.156M. doi:10.1051/0004-6361/201013993. S2CID 119276502.
  11. Greenhill; et al. (1995). "The SiO Masers and Dust Shell of VX SGR". Astrophysics and Space Science. 224 (1–2): 1–9. Bibcode:1995Ap&SS.224..469G. doi:10.1007/BF00667909. S2CID 189849486.
  12. Chen, X.; Shen, Z. Q.; Xu, Y. (2007). "Measuring the Distance of VX Sagittarii with SiO Maser Proper Motions". Chinese Journal of Astronomy and Astrophysics. 7 (4): 531. Bibcode:2007ChJAA...7..531C. doi:10.1088/1009-9271/7/4/09.
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