Upsilon Sagittarii

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Not to be confused with the Cepheid variable U Sagittarii.
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])
Distance
approx. 1,800 ly
(approx. 550 pc)
Absolute magnitude (MV)−4.73 / −1.14[7]
Semi-amplitude (K1)
(primary)
49.6[10] km/s
Semi-amplitude (K2)
(secondary)		29.7[10] km/s
Details
"visible"
Rotational velocity (v sin i)
250[12] km/s
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

spectrum of hydrogen-deficient binaries has made stellar classification
of Upsilon Sagittarii difficult.

System

υ Sgr is a

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

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 name for υ Sagittarii itself is 建六 (Jiàn liù, English: the Sixth Star of Establishment.)[18]

References

  1. ^
    S2CID 18759600
    .
  2. ^
    Bibcode:2002yCat.2237....0D
    .
  3. ^
    doi:10.1515/astro-2017-0382
    .
  4. ^
    doi:10.1093/mnras/222.3.543
    .
  5. ^
    Bibcode:2009yCat....102025S
    .
  6. Bibcode:1953GCRV..C......0W
    .
  7. ^
    S2CID 237503492
    .
  8. ^
    doi:10.1051/0004-6361:20065274
    .
  9. ^
    doi:10.1051/0004-6361/200811192
    .
  10. ^
    Bibcode:1990MNRAS.247..400D
    .
  11. ^
    S2CID 122291723
    .
  12. ^
    doi:10.1093/mnras/stac3375
    .
  13. doi:10.1086/140127
    .
  14. doi:10.5479/ADS/bib/1915LicOB.8.132W
    .
  15. ^
    Bibcode:1998AstL...24...39L
    .
  16. Bibcode:2000BSAO...50...60L
    .
  17. 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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