VX Sagittarii

VX Sagittarii

A visual band light curve for VX Sagittarii, plotted from AAVSO data [1]
Observation data Epoch J2000.0        Equinox J2000.0 (ICRS)
Constellation	Sagittarius
Right ascension	18h 08m 04.04831s [2]
Declination	−22° 13′ 26.6327″ [2]
Apparent magnitude  (V)	6.5 - 14.0 [3]
Characteristics
Evolutionary stage	OH/IR (ERSG/RHG, [4] AGB/Super-AGB, [5] or TŻO ) [5]
Spectral type	M4eIa - M10eIa [6]
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 [3]
Astrometry
Radial velocity (Rv)	+6.47±3.37 [4] km/s
Proper motion (μ)	RA: +0.36±0.76 [4] mas/yr Dec.: −2.92±0.78 [4] mas/yr
Parallax (π)	0.64±0.04  mas [4]
Distance	5,100 ± 300  ly (1,560 ± 100  pc)
Details
Mass	10 to 12 [5]   M☉
Radius	1,360+250 −230, [5] between 1,120 and 1,550 [4] , 1,350–1,940 (pulsation) [7] , 1,480 [8]   R☉
Luminosity	195,000±62,000 [4]   L☉
Temperature	2,900 [9] (near min), 3,200-3,400 (near max) [10] , 2,400–3,300 [7]   K
Other designations
VX Sgr, AAVSO 1802-22, BD−22°4575, CD−22°12589, HD 165674, HIP 88838, 2MASS J18080404-2213266
Database references
SIMBAD	data

VX Sagittarii (abbreviated to VX Sgr) is a luminous cool OH/IR pulsating variable star with an unusually large magnitude range located in the constellation of Sagittarius and more than one kiloparsec away from the Sun. Although it is often treated as an unusually cool red supergiant (or hypergiant), it may be instead either an extremely large asymptotic giant branch star or a possible but unlikely Thorne–Żytkow object. ^[5] Nonetheless, it is one of the largest stars discovered and also one of the most luminous and massive cool stars in the Milky Way, with a radius pulsating between 1,350 and 1,940 solar radii (940,000,000 and 1.35×10⁹  km ; 6.3 and 9.0  au ).

Assuming it is an AGB star, VX Sgr would be the most luminous known of its kind, exceeding the theoretical limit for the bolometric magnitude at −8.0. ^[5]

Observations

In 1904, it was announced that Henrietta Leavitt had discovered that the star, then known as BD −22°4575, is a variable star. ^[11] It was given its variable star designation, VX Sagittarii, in 1911. ^[12] 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 as 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. ^[4]

Stellar characteristics

The effective temperature of VX Sagittarii is 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. ^{[10] [7]} Its atmosphere is extended, irregular, and variable during the pulsations of the star, but the bolometric luminosity varies 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_☉. ^[4] Older studies frequently calculated higher luminosities. ^{[13] [14]}

The atmosphere of VX Sgr shows molecular water layers and SiO masers in the atmosphere, typical of an OH/IR star. ^[15] The SiO masers have been used to derive a distance of 1.57±0.27 kiloparsecs. ^[16] 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 with a supergiant's luminosity and size. ^[10]

Another paper from May 2018 suggests that VX Sagitarii may be a hypergiant. ^[4] This would make it one of the very rare red hypergiant stars. However, a 2021 paper concludes that VX Sagittarii is a massive AGB star, rather than a red supergiant or hypergiant. Because it displays rubidium in its spectrum and has a high mass loss and luminosity, it may be a type of AGB star known as a super-AGB star, a type of star with masses in between low-mass stars and high-mass stars. ^[5]

See also

Other late-type red supergiants

• S Persei
• MY Cephei
• NML Cygni
• VY Canis Majoris
• IRC −10414
• AH Scorpii

Other Thorne–Żytkow object candidates

• HV 2112 – A former supergiant TŻO candidate
• HV 11417 – Another possible supergiant TŻO candidate

References

1. "Download Data". aavso.org. AAVSO. Retrieved 1 October 2021.
2. 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.
3. 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.
4. 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.
5. Tabernero, H. M.; Dorda, R.; Negueruela, I.; Marfil, E. (2021). "On the nature of VX Sagitarii. Is it a TZO, a RSG or a high-mass AGB star?". Astronomy & Astrophysics. 646: A98. arXiv: 2011.09184 . doi:10.1051/0004-6361/202039236. S2CID   227013580.
6. 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.
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 . ISSN   0035-8711.
8. Wallstrom, S. H. J.; Danilovich, T.; Muller, H. S. P.; Gottlieb, C. A.; Maes, S.; Van de Sande, M.; Decin, L.; Richards, A. M. S.; Baudry, A.; Bolte, J.; Ceulemans, T.; De Ceuster, F.; de Koter, A.; Mellah, I. El; Esseldeurs, M. (7 December 2023). "ATOMIUM: Molecular inventory of 17 oxygen-rich evolved stars observed with ALMA". Astronomy & Astrophysics. 681: A50. arXiv: 2312.03467 . doi:10.1051/0004-6361/202347632. ISSN   0004-6361.
9. 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.
10. Chiavassa, A.; Lacour, S.; Millour, F.; Driebe, T.; Wittkowski, M.; Plez, B.; Thiébaut, E.; Josselin, E.; Freytag, B.; Scholz, M.; Haubois, X. (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.
11. Leavitt, Henrietta; Pickering, Edward C (August 1904). "Sixteen New Variable Stars in Sagittarius". Harvard College Observatory Circular. 91: 1. Bibcode:1904HarCi..91....1L . Retrieved 22 December 2024.
12. Dunér; Hartwig; Müller (December 1911). "Benennung von neu entdeckten veränderlichen Sternen". Astronomische Nachrichten. 190 (4): 57–72. Bibcode:1911AN....190...57.. doi:10.1002/asna.19111900402 . Retrieved 22 December 2024.
13. 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.
14. Mauron, N.; Josselin, E. (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. ISSN   0004-6361. S2CID   119276502.
15. Greenhill, L. J.; et al. (1995). "The sio masers and dust shell of VX sgr". Astrophysics and Space Science. 224 (1–2): 469–470. Bibcode:1995Ap&SS.224..469G. doi:10.1007/BF00667909. ISSN   0004-640X. S2CID   189849486.
16. 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 .

Further reading

• Chiavassa, A.; et al. (February 2022), "The extended atmosphere and circumstellar environment of the cool evolved star VX Sagittarii as seen by MATISSE", Astronomy & Astrophysics, 658: A185, arXiv: 2112.10695 , Bibcode:2022A&A...658A.185C, doi:10.1051/0004-6361/202142514, S2CID   245335318, A185.
• Su, Jiang-Bo; et al. (December 2020), "Observational characteristics of SiO masers around VX Sgr", Research in Astronomy and Astrophysics, 20 (12): 192, Bibcode:2020RAA....20..192S, doi:10.1088/1674-4527/20/12/192, S2CID   221408165, 192.
• Su, J. B.; et al. (January 2018), "Dynamics of SiO Masers around VX Sgr", The Astrophysical Journal, 853 (1): 42, Bibcode:2018ApJ...853...42S, doi: 10.3847/1538-4357/aaa12b , S2CID   125500678, 42.
• Su, J. B.; et al. (September 2014), "Outward Motions of SiO Masers around VX Sgr", Journal of Astrophysics and Astronomy, 35 (3): 539–540, Bibcode:2014JApA...35..539S, doi:10.1007/s12036-014-9280-2, S2CID   255487708.
• Su, J. B.; et al. (July 2012), "High-resolution VLBA Observations of Three 7 mm SiO Masers toward VX Sgr at Five Epochs", The Astrophysical Journal, 754 (1): 47, Bibcode:2012ApJ...754...47S, doi: 10.1088/0004-637X/754/1/47 , S2CID   120825137, 47.
• Su, J. B.; et al. (June 2011), "Pumping Mechanisms for SiO Masers around VX Sgr", Journal of Astrophysics and Astronomy, 32 (1–2): 261–264, Bibcode:2011JApA...32..261S, doi:10.1007/s12036-011-9002-y, S2CID   121796389.
• Lu, R. -S.; et al. (2010), "High-frequency VLBI Imaging of Sgr A* and VX Sgr", Proceedings of the 10th European VLBI Network Symposium and EVN Users Meeting: VLBI and the new generation of radio arrays. September 20-24, 2010. Manchester, UK, vol. 10, p. 087, Bibcode:2010evn..confE..87L, doi: 10.22323/1.125.0087 , 87.
• Chen, Xi; et al. (April 2006), "Inward Motions of the Compact SiO Masers around VX Sagittarii", The Astrophysical Journal, 640 (2): 982–994, arXiv: astro-ph/0512011 , Bibcode:2006ApJ...640..982C, doi:10.1086/500168, S2CID   14240593.
• Kamohara, Ryuichi; et al. (April 2005), "Time Variation of SiO Masers in VX Sagittarii over an Optically Quiescent Phase", Publications of the Astronomical Society of Japan, 57 (2): 341–345, Bibcode:2005PASJ...57..341K, doi:10.1093/pasj/57.2.341.
• Murakawa, K.; et al. (September 2003), "The radially expanding molecular outflow of VX Sagittarii", Monthly Notices of the Royal Astronomical Society, 344 (1): 112, Bibcode:2003MNRAS.344....1M, doi: 10.1046/j.1365-8711.2003.06626.x .
• Yates, J. A.; et al. (2002), Mineese, Victor; Reid, Mark (eds.), "The H2O maser proper motions of RT Vir and VX Sgr", Cosmic Masers: From Proto-Stars to Black Holes, IAU Symposium #206, held 5–10 March 2001 in Angra dos Reis, Rio de Janeiro, Brazil, vol. 206, San Francisco: Astronomical Society of the Pacific, p. 298, Bibcode:2002IAUS..206..298Y.
• Szymczak, M.; et al. (June 2001), "Magnetic field structure in the outer OH maser envelope of VX Sagittarii", Astronomy and Astrophysics, 371 (3): 1012–1017, Bibcode:2001A&A...371.1012S, doi: 10.1051/0004-6361:20010451 , S2CID   123021236.
• Pashchenko, M. I.; Rudnitskii, G. M. (May 1999), "Observations of late-type variable stars in the water-vapor radio line. The supergiant VX Sagittarii", Astronomy Reports, 43 (5): 311–324, Bibcode:1999ARep...43..311P.
• Berulis, I. I.; et al. (1999), "H2O maser emission of the M-type supergiant VX Sgr", Astronomical and Astrophysical Transactions, 18 (1): 77–82, Bibcode:1999A&AT...18...77B, doi:10.1080/10556799908203038.
• Trigilio 7, C.; et al. (June 1998), "The circumstellar magnetic field of VX Sagittarii", Monthly Notices of the Royal Astronomical Society, 297 (2): 497–501, Bibcode:1998MNRAS.297..497T, doi: 10.1046/j.1365-8711.1998.01538.x .
• Doeleman, S. S.; et al. (1998), Zensus, J. A.; Taylor, G. B.; Wrobel, J. M. (eds.), "86 GHz VLBI Imaging of the SiO Masers in VX SGR", Radio Emission from Galactic and Extragalactic Compact Sources, IAU Colloquium 164, ASP Conference Series, vol. 144, p. 363, Bibcode:1998ASPC..144..363D.
• Zell, Philip J.; Fix, John D. (July 1996), "The Spatial Distribution of Circularly Polarized 1612 MHz OH Maser Emission From VX SGR", Astronomical Journal, 112: 252, Bibcode:1996AJ....112..252Z, doi: 10.1086/118007 .
• Trigilio, C.; et al. (1996), Pallavicini, Roberto; Dupree, Andrea K. (eds.), "Magnetic field measurements in the circumstellar envelope of VX SGR", Cool stars, stellar systems, and the sun: Proceedings of the 9th Cambridge workshop; held 3–6 October 1995 in Florence, Italy, Astronomical Society of the Pacific Conference Series, vol. 109, San Francisco: Astronomical Society of the Pacific, p. 549, Bibcode:1996ASPC..109..549T.
• Greenhill, Lincoln J.; et al. (June 1994), Breckinridge, James B. (ed.), "Circumstellar envelope of VX SGR resolved with long-baseline interferometry at millimeter and midinfrared wavelengths", Conference Proceedings of SPIE: Amplitude and Intensity Spatial Interferometry II, Amplitude and Intensity Spatial Interferometry II, vol. 2200, pp. 304–315, Bibcode:1994SPIE.2200..304G, doi:10.1117/12.177248, S2CID   120318207.
• Greenhill, L. J.; et al. (May 1993), "The Circumstellar Envelope of VX SGR", Bulletin of the American Astronomical Society, 25: 824, Bibcode:1993AAS...182.2203G.
• Yu, Zhiyao (June 1992), "The Circumstellar Envelope of VX-Sagittarii", Astrophysics and Space Science, 192 (1): 53–62, Bibcode:1992Ap&SS.192...53Y, doi:10.1007/BF00653259, S2CID   119015723.
• Pijpers, F. P. (November 1990), "A model for the wind of the M supergiant VX Sagittarii", Astronomy and Astrophysics, 238: 256, Bibcode:1990A&A...238..256P.
• Netzer, Nathan (July 1989), "Interaction of Dust and Radiation in Circumstellar Envelopes. I. The Expansion Velocity Gradient in the Envelope of VX Sagittarii", The Astrophysical Journal, 342: 1068, Bibcode:1989ApJ...342.1068N, doi:10.1086/167663.
• Chapman, J. M.; Cohen, R. J. (May 1986), "MERLIN observations of the circumstellar envelope of VX Sagittarius", Monthly Notices of the Royal Astronomical Society, 220 (3): 513–528, Bibcode:1986MNRAS.220..513C, doi: 10.1093/mnras/220.3.513 .
• Smith, Horace A. (February 1978), "Beat Phenomena in the Light Curve of VX Sgr", The Journal of the American Association of Variable Star Observers, 6 (2): 63, Bibcode:1978JAVSO...6...63S.
• Dinerstein, Harriet (1973), "VX Sagittarii: A Variable at Many Wavelengths", Journal of the American Association of Variable Star Observers, 2 (2): 52–59, Bibcode:1973JAVSO...2...52D.
• Humphreys, Roberta M.; Lockwood, G. W. (March 1972), "Spectroscopic and Photometric Changes in the Peculiar Infrared Star VX Sagittarius", The Astrophysical Journal, 172: L59, Bibcode:1972ApJ...172L..59H, doi:10.1086/180891.
• Robinson, B. J.; et al. (March 1971), "Similarity of the OH Emissions from VX Sagittarii and VY Canis Majoris", Astrophysical Letters, 8: 171, Bibcode:1971ApL.....8..171R.