Theta2 Sagittarii

Theta2 Sagittarii, Latinized from θ2 Sagittarii, is a solitary[10] star in the zodiac constellation of Sagittarius. It is faintly visible to the naked eye with an apparent visual magnitude of +5.30.[2] The star is progressing in the general direction of the Sun with a radial velocity of −17.60 km/s.[4] Based upon an annual parallax shift of 20.62 mas as seen from Earth,[1] it is located around 158 light years from the Sun. The star is drifting closer with a radial velocity of −17.6 km/s.[4]

For other stars with this Bayer designation, see θ Sagittarii.
Theta2 Sagittarii
Location of θ2 Sagittarii (circled)
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Sagittarius
Right ascension 19h 59m 51.35684s[1]
Declination −34° 41′ 52.0797″[1]
Apparent magnitude (V) +5.30[2]
Characteristics
Spectral type A4/A5 IV[3]
U−B color index +0.06[2]
B−V color index +0.17[2]
Astrometry
Radial velocity (Rv)−17.60[4] km/s
Proper motion (μ) RA: +108.23[1] mas/yr
Dec.: −69.51[1] mas/yr
Parallax (π)20.62 ± 0.28 mas[1]
Distance158 ± 2 ly
(48.5 ± 0.7 pc)
Absolute magnitude (MV)+1.87[5]
Details
Mass1.93[6] M☉
Luminosity14[7] L☉
Surface gravity (log g)4.07±0.14[6] cgs
Temperature8,113±276[6] K
Rotational velocity (v sin i)45.2±0.5[8] km/s
Age809[6] Myr
Other designations
θ2 Sgr, CPD−35° 8701, HD 189118, HIP 98421, HR 7624, SAO 211717, WDS J19599-3442[9]
Database references
SIMBADdata

The spectrum of Theta2 Sagittarii matches a stellar classification of A4/A5 IV,[3] indicating that, at the estimated age of 809 million years,[6] this is an evolving A-type subgiant star. It is a suspected Am star and may display photometric variability, at least during a 1992 observation period.[11] The star has an estimated 1.93[6] times the mass of the Sun and is radiating 14[7] times the Sun's luminosity from its photosphere at an effective temperature of 8,113 K.[6] It is spinning with a projected rotational velocity of 45.2 km/s.[8]

Theta2 Sagittarii has a pair of visual companions. Component B is a magnitude 11.3 star at an angular separation of 32.8 arc seconds along a position angle of 165°, as of 2000. Component C lies at an angular separation of 1.5 arc seconds along a position angle of 104° from component B, as of 1965.[12] Neither is physically associated with Theta2 Sagittarii itself.[10]

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. Mendoza, E. E.; et al. (June 1978), "UBVRI photometry of 225 AM stars", Astronomical Journal, 83: 606–614, Bibcode:1978AJ.....83..606M, doi:10.1086/112242.
  3. Houk, Nancy (1979), Michigan catalogue of two-dimensional spectral types for the HD stars, vol. 3, Ann Arbor, Michigan: Dept. of Astronomy, University of Michigan, Bibcode:1982mcts.book.....H.
  4. Gontcharov, G. A. (November 2006), "Pulkovo Compilation of Radial Velocities for 35495 Hipparcos stars in a common system", Astronomy Letters, 32 (11): 759–771, arXiv:1606.08053, Bibcode:2006AstL...32..759G, doi:10.1134/S1063773706110065, S2CID 119231169.
  5. Anderson, E.; Francis, Ch. (2012), "XHIP: An extended hipparcos compilation", Astronomy Letters, 38 (5): 331, arXiv:1108.4971, Bibcode:2012AstL...38..331A, doi:10.1134/S1063773712050015, S2CID 119257644.
  6. David, Trevor J.; Hillenbrand, Lynne A. (2015), "The Ages of Early-Type Stars: Strömgren Photometric Methods Calibrated, Validated, Tested, and Applied to Hosts and Prospective Hosts of Directly Imaged Exoplanets", The Astrophysical Journal, 804 (2): 146, arXiv:1501.03154, Bibcode:2015ApJ...804..146D, doi:10.1088/0004-637X/804/2/146, S2CID 33401607.
  7. McDonald, I.; et al. (2012), "Fundamental Parameters and Infrared Excesses of Hipparcos Stars", Monthly Notices of the Royal Astronomical Society, 427 (1): 343–57, arXiv:1208.2037, Bibcode:2012MNRAS.427..343M, doi:10.1111/j.1365-2966.2012.21873.x, S2CID 118665352.
  8. Díaz, C. G.; et al. (July 2011), "Accurate stellar rotational velocities using the Fourier transform of the cross correlation maximum", Astronomy & Astrophysics, 531: A143, arXiv:1012.4858, Bibcode:2011A&A...531A.143D, doi:10.1051/0004-6361/201016386, S2CID 119286673.
  9. "tet02 Sgr". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2017-07-07.{{cite web}}: CS1 maint: postscript (link)
  10. Eggleton, P. P.; Tokovinin, A. A. (September 2008), "A catalogue of multiplicity among bright stellar systems", Monthly Notices of the Royal Astronomical Society, 389 (2): 869–879, arXiv:0806.2878, Bibcode:2008MNRAS.389..869E, doi:10.1111/j.1365-2966.2008.13596.x, S2CID 14878976.
  11. Anders, G. J. (September 1992), "Theta2 Sgr: Serendipitous Discovery of Variability in an Am Star?", Information Bulletin on Variable Stars, 3768: 1, Bibcode:1992IBVS.3768....1A.
  12. Mason, B. D.; et al. (2014), "The Washington Visual Double Star Catalog", The Astronomical Journal, 122 (6): 3466, Bibcode:2001AJ....122.3466M, doi:10.1086/323920, retrieved 2015-07-22.
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