U Aquarii

U Aquarii, abbreviated U Aqr, is a variable star in the equatorial constellation of Aquarius. It is invisible to the naked eye, having an apparent visual magnitude that ranges from 10.6 down to as low as 15.9.[2] Based on parallax measurements, the distance to this star is approximately 38 kly (12 kpc). In 1990, W. A. Lawson and associates provided a distance estimate of 43 kly (13.2 kpc) based on the assumption of a bolometric magnitude of −5.[4] It appears to lie several kiloparsecs below the galactic plane, and thus may belong to an old stellar population.[3]

U Aquarii
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Aquarius
Right ascension 22h 03m 19.69905s[1]
Declination −16° 37′ 35.2811″[1]
Apparent magnitude (V) 10.6 to 15.9[2]
Characteristics
Spectral type C-Hd[3]
U−B color index 0.07[4]
B−V color index 0.66[4]
Variable type R CrB[2]
Astrometry
Radial velocity (Rv)+89.49±1.54[5] km/s
Proper motion (μ) RA: 2.780[1] mas/yr
Dec.: −1.907[1] mas/yr
Parallax (π)0.0859 ± 0.0222[1] mas
Distanceapprox. 38,000 ly
(approx. 12,000 pc)
Details
Mass4.5[6] M☉
Surface gravity (log g)2.28[6] cgs
Temperature5,500[4] K
Metallicity [Fe/H]−0.99[6] dex
Rotational velocity (v sin i)8.4[7] km/s
Other designations
U Aqr, BD−17°6424, HIP 108876, IRAS 22006-1652[8]
Database references
SIMBADdata

The stellar classification of this star is C-Hd,[3] and it is classified as a R Coronae Borealis variable.[2] It is a carbon star with a hydrogen-deficient spectra that also shows evidence of s-process elements,[3] including overabundances of strontium and yttrium, but no barium.[9] This combination of properties is exceptionally rare; only one other example has been found as of 2012.[10] The elemental abundances are explained as the result of a single neutron exposure event, which is difficult to reconcile with a conjecture that this may be a post-AGB-type star.[9] In 1999, U Aqr was proposed to be a Thorne-Zytkow object, instead of being a simple R Coronae Borealis variable.[11]

References
  1. Brown, A. G. A.; et al. (Gaia collaboration). "Gaia Early Data Release 3: Summary of the contents and survey properties". Astronomy & Astrophysics (in press). arXiv:2012.01533. doi:10.1051/0004-6361/202039657. S2CID 227254300. Gaia EDR3 record for this source at VizieR.
  2. Samus', N. N; Kazarovets, E. V; Durlevich, O. V; Kireeva, N. N; Pastukhova, E. N (2017). "General catalogue of variable stars: Version GCVS 5.1". Astronomy Reports. 61 (1): 80. Bibcode:2017ARep...61...80S. doi:10.1134/S1063772917010085. S2CID 125853869.
  3. Bond, H. E.; et al. (October 1979). "The extraordinary composition of U Aquarii". Astrophysical Journal. 233: 205–210. Bibcode:1979ApJ...233..205B. doi:10.1086/157382.
  4. Lawson, W. A.; et al. (November 1990). "The photometric characteristics of cool hydrogen-deficient carbon stars". Monthly Notices of the Royal Astronomical Society. 247: 91. Bibcode:1990MNRAS.247...91L.
  5. Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.
  6. Anders, F.; Khalatyan, A.; Chiappini, C.; Queiroz, A. B.; Santiago, B. X.; Jordi, C.; Girardi, L.; Brown, A. G. A.; Matijevič, G.; Monari, G.; Cantat-Gaudin, T.; Weiler, M.; Khan, S.; Miglio, A.; Carrillo, I.; Romero-Gómez, M.; Minchev, I.; De Jong, R. S.; Antoja, T.; Ramos, P.; Steinmetz, M.; Enke, H. (2019). "Photo-astrometric distances, extinctions, and astrophysical parameters for Gaia DR2 stars brighter than G = 18". Astronomy and Astrophysics. 628: A94. arXiv:1904.11302. Bibcode:2019A&A...628A..94A. doi:10.1051/0004-6361/201935765.
  7. Majewski, Steven R.; Schiavon, Ricardo P.; Frinchaboy, Peter M.; Allende Prieto, Carlos; Barkhouser, Robert; Bizyaev, Dmitry; Blank, Basil; Brunner, Sophia; Burton, Adam; Carrera, Ricardo; Chojnowski, S. Drew; Cunha, Kátia; Epstein, Courtney; Fitzgerald, Greg; García Pérez, Ana E.; Hearty, Fred R.; Henderson, Chuck; Holtzman, Jon A.; Johnson, Jennifer A.; Lam, Charles R.; Lawler, James E.; Maseman, Paul; Mészáros, Szabolcs; Nelson, Matthew; Nguyen, Duy Coung; Nidever, David L.; Pinsonneault, Marc; Shetrone, Matthew; Smee, Stephen; et al. (2017). "The Apache Point Observatory Galactic Evolution Experiment (APOGEE)". The Astronomical Journal. 154 (3): 94. arXiv:1509.05420. Bibcode:2017AJ....154...94M. doi:10.3847/1538-3881/aa784d. S2CID 119293367.
  8. "U Aqr". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2021-01-01.
  9. Malaney, R. A. (October 1985). "On the nature of the neutron exposure event of U Aquarii". Monthly Notices of the Royal Astronomical Society. 216 (3): 743–752. Bibcode:1985MNRAS.216..743M. doi:10.1093/mnras/216.3.743.
  10. Goswami, Aruna (2012). Prugniel, Philippe; Singh, Harinder P. (eds.). Spectroscopic charaterization of FHLC stars from the Hamburg/ESO survey and a newly found HdC star. International Workshop on Stellar Libraries, Proceedings of a conference held 5-9 December, 2011 at University of Delhi, India. Astronomical Society of India Conference Series. 6. p. 189. arXiv:1204.1806. Bibcode:2012ASInC...6..189G. ISBN 978-81-922926-4-9.
  11. Vanture, Andrew D.; et al. (April 1, 1999). "Is U Aquarii a Thorne-Żytkow Object?". The Astrophysical Journal. 514 (2): 932–938. Bibcode:1999ApJ...514..932V. doi:10.1086/306956.
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