Upsilon Sagittarii
Location of υ Sgr (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Sagittarius
Right ascension 19h 21m 43.62284s[1]
Declination −15° 57′ 18.0625″[1]
Apparent magnitude (V) 4.61[2]
Characteristics
Spectral type A2 Ia + B2 Vpe[3]
U−B color index −0.53[2]
B−V color index +0.10[2]
Variable type PV Tel[4] (β Lyr?[5])
Astrometry
Radial velocity (Rv)8.9[6] km/s
Proper motion (μ) RA: 1.34[1] mas/yr
Dec.: −6.25[1] mas/yr
Parallax (π)1.83 ± 0.23 mas[1]
Distanceapprox. 1,800 ly
(approx. 550 pc)
Absolute magnitude (MV)−4.73 / −1.14[7]
Orbit[8]
Period (P)137.9 days
Semi-major axis (a)270.8 R☉
Eccentricity (e)0
Inclination (i)50[9]°
Semi-amplitude (K1)
(primary)		49.6[10] km/s
Semi-amplitude (K2)
(secondary)		29.7[10] km/s
Details
"visible"
Mass2.5 + 4[11] M☉
Radius23[12] R☉
Luminosity4,700[12] L☉
Surface gravity (log g)1.2[12] cgs
Temperature12,300[3] K
Metallicity [Fe/H]−0.2[3] dex
Rotational velocity (v sin i)≤10[12] km/s
Age52[11] Myr
"invisible"
Mass6.2[7] M☉
Radius2.2[12] R☉
Luminosity1,300[12] L☉
Surface gravity (log g)1.0[12] cgs
Temperature23,000[12] K
Rotational velocity (v sin i)250[12] km/s
Other designations
υ Sagittarii, υ Sgr, Upsilon Sgr, 46 Sagittarii, BD−16°5283, FK5 727, GC 26697, HD 181615, HD 181616, HIP 95176, HR 7342, PPM 235885, SAO 162518
Database references
SIMBADdata

Upsilon Sagittarii (Upsilon Sgr, υ Sagittarii, υ Sgr) is a spectroscopic binary star system in the constellation Sagittarius. Upsilon Sagittarii is the prototypical hydrogen-deficient binary (HdB), and one of only four such systems known. The unusual spectrum of hydrogen-deficient binaries has made stellar classification of Upsilon Sagittarii difficult.

System

υ Sgr is a binary system with an orbital period of 137.939 days and is approximately 1,800 light years from Earth. The primary star dominates the visible radiation and spectrum, but the secondary is hotter and more massive. Some sources consider the "invisible" component to be the primary on the basis of its mass.[8] There is also a disc of material being stripped from the primary and transferring material to the secondary, but no eclipses[9]

The system is classified as a single-lined spectroscopic binary, but high excitation lines from the secondary can be detected in the ultraviolet.[10] Radial velocity variations were discovered in 1899,[13] The first orbit was calculated in 1914, reasonably close to modern understanding of the system.[14] Analysis of optical interferometry finds the two stars to be separated by 1.3 mas and have a difference in brightness of about 3.5 magnitudes.[7]

Properties

Light curves for Upsilon Sagittarii, adapted from Malcolm and Bell (1986)[4]

The primary component appears as an A type supergiant, although published spectral types vary from F2p to B5II.[8] Contrasting components in the spectrum may originate from disc material, polar jets, or the star itself. The low mass and unusual chemical composition are also thought to produce misleading spectral calibrations, with the star not as massive or as luminous as the Ia luminosity class would suggest.[3]

The visible component is a helium star, almost entirely deficient of hydrogen.[15] It has also been described as a neon star, due to the very high relative levels of that element.[16] It has been stripped of its outer hydrogen layers after it expanded away from the main sequence.[15] It is thought to have originated as a main sequence star with around 8 M☉, expanded when it exhausted its core hydrogen, and now only 2.5 M☉ remains, highly inflated and giving the appearance of a supergiant star.[11] Other estimates give higher masses, as much as 5.45 M☉ and 8.56 M☉ at the known inclination of 50°.[8]

The supergiant component is also classified as an PV Telescopii variable, although it was originally catalogued as an eclipsing binary. It shows apparent magnitude fluctuations between +4.51 and +4.65 with a period of approximately 20 days.[5][17]

The companion is more massive than the supergiant primary, but so dim at visible frequencies as to be undetectable, although it can be seen in ultraviolet spectra. It is thought to be a B-type main sequence star accreting mass from the primary.[8]

Naming

υ Sagittarii has two entries in the Henry Draper Catalogue, HD 181615 and HD 181616.

In Chinese, 建 (Jiàn), meaning Establishment, refers to an asterism consisting of υ Sagittarii, ξ² Sagittarii, ο Sagittarii, π Sagittarii, 43 Sagittarii and ρ¹ Sagittarii. Consequently, the Chinese name for υ Sagittarii itself is 建六 (Jiàn liù, English: the Sixth Star of Establishment.)[18]

References

  1. 1 2 3 4 5 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. 1 2 3 Ducati, J. R. (2002). "VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system". CDS/ADC Collection of Electronic Catalogues. 2237. Bibcode:2002yCat.2237....0D.
  3. 1 2 3 4 Kipper, Tõnu; Klochkova, Valentina G. (2012). "High-Resolution Spectroscopy of the Hydrogen-Deficient Binary Upsilon Sgr". Baltic Astronomy. 21 (3): 219. Bibcode:2012BaltA..21..219K. doi:10.1515/astro-2017-0382.
  4. 1 2 Malcolm, G. J.; Bell, S. A. (1986). "Evidence for pulsation in the hydrogen-deficient binary Upsilon Sagittarii". Monthly Notices of the Royal Astronomical Society. 222 (3): 543. Bibcode:1986MNRAS.222..543M. doi:10.1093/mnras/222.3.543.
  5. 1 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.
  6. ↑ Wilson, Ralph Elmer (1953). "General catalogue of stellar radial velocities". Washington. Bibcode:1953GCRV..C......0W.
  7. 1 2 3 Hutter, D. J.; Tycner, C.; Zavala, R. T.; Benson, J. A.; Hummel, C. A.; Zirm, H. (2021). "Surveying the Bright Stars by Optical Interferometry. III. A Magnitude-limited Multiplicity Survey of Classical be Stars". The Astrophysical Journal Supplement Series. 257 (2): 69. arXiv:2109.06839. Bibcode:2021ApJS..257...69H. doi:10.3847/1538-4365/ac23cb. S2CID 237503492.
  8. 1 2 3 4 5 Koubský, P.; Harmanec, P.; Yang, S.; Netolický, M.; Škoda, P.; Šlechta, M.; Korčáková, D. (2006). "Properties and nature of Be stars. 25. A new orbital solution and the nature of a peculiar emission-line binary υ Sagittarii". Astronomy and Astrophysics. 459 (3): 849. Bibcode:2006A&A...459..849K. doi:10.1051/0004-6361:20065274.
  9. 1 2 Netolický, M.; Bonneau, D.; Chesneau, O.; Harmanec, P.; Koubský, P.; Mourard, D.; Stee, P. (2009). "The circumbinary dusty disk around the hydrogen-deficient binary star υ Sagittarii". Astronomy and Astrophysics. 499 (3): 827. Bibcode:2009A&A...499..827N. doi:10.1051/0004-6361/200811192.
  10. 1 2 3 Dudley, R. E.; Jeffery, C. S. (1990). "The Mass Ratio of Upsilon-Sagittarii from Ultraviolet Radial Velocities". Monthly Notices of the Royal Astronomical Society. 247: 400. Bibcode:1990MNRAS.247..400D.
  11. 1 2 3 Leushin, V. V. (2001). "Atmospheric Iron Abundance in the Primary Component of upsilon Sgr". Astronomy Letters. 27 (10): 634–644. Bibcode:2001AstL...27..634L. doi:10.1134/1.1404457. S2CID 122291723.
  12. 1 2 3 4 5 6 7 8 9 Gilkis, Avishai; Shenar, Tomer (2023). "Ups!... I did it again: Unveiling the hidden companion in Upsilon Sagittarii, a unique binary system at a second mass transfer stage". Monthly Notices of the Royal Astronomical Society. 518 (3): 3541–3555. arXiv:2209.14315. Bibcode:2023MNRAS.518.3541G. doi:10.1093/mnras/stac3375.
  13. ↑ Campbell, W. W. (1895). "Stars whose spectra contain both bright and dark hydrogen lines". Astrophysical Journal. 2: 177. Bibcode:1895ApJ.....2..177C. doi:10.1086/140127.
  14. ↑ Wilson, R. E. (1915). "The orbit of the spectroscopic binary upsilon Sagittarii". Lick Observatory Bulletin. 8: 132–133. Bibcode:1915LicOB...8..132W. doi:10.5479/ADS/bib/1915LicOB.8.132W.
  15. 1 2 Leushin, V. V.; Snezhko, L. I.; Chuvenkov, V. V. (1998). "History of the chemical evolution of the primary component of the binary system υ Sagittarius". Astronomy Letters. 24 (1): 39. Bibcode:1998AstL...24...39L.
  16. ↑ Leushin, V. V. (2000). "The primary component of υ Sagittarius is a neon star". Bulletin of the Special Astrophysical Observatory. Russian Academy of Sciences. 50: 60. Bibcode:2000BSAO...50...60L.
  17. ↑ Jeffery, C. Simon (2008). "Variable Star Designations for Extreme Helium Stars". Information Bulletin on Variable Stars. 5817: 1. Bibcode:2008IBVS.5817....1J.
  18. ↑ (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 5 月 11 日

Further reading

  • Dudley, R. E., Jeffery, C. S., 1990. Mon. Not. R. astr. Soc. 247, 400
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