U Aquarii

Not to be confused with Upsilon 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.^[3] 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.^[5] It appears to lie several kiloparsecs below the galactic plane, and thus may belong to an old stellar population.^[4]

U Aquarii

The visual band light curve of U Aquarii, from AAVSO data[1]
Observation data Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation	Aquarius
Right ascension	22h 03m 19.69905s[2]
Declination	−16° 37′ 35.2811″[2]
Apparent magnitude (V)	10.6 to 15.9[3]
Characteristics
Spectral type	C-Hd[4]
U−B color index	0.07[5]
B−V color index	0.66[5]
Variable type	R CrB[3]
Astrometry
Radial velocity (Rv)	+89.49±1.54[6] km/s
Proper motion (μ)	RA: 2.780[2] mas/yr Dec.: −1.907[2] mas/yr
Parallax (π)	0.0859 ± 0.0222 mas[2]
Distance	approx. 38,000 ly (approx. 12,000 pc)
Details
Mass	4.5[7] M☉
Surface gravity (log g)	2.28[7] cgs
Temperature	5,500[5] K
Metallicity [Fe/H]	−0.99[7] dex
Rotational velocity (v sin i)	8.4[8] km/s
Other designations
U Aqr, BD−17°6424, HIP 108876, IRAS 22006-1652[9]
Database references
SIMBAD	data

Christian Heinrich Friedrich Peters discovered that U Aquarii is a variable star based on observations made from 1875 to 1878.^[10][11] Its variable star designation was published in Annie Jump Cannon's Second catalogue of variable stars in 1907, at which time the class of variable star it belonged to was still unknown.^[10]

The stellar classification of this star is C-Hd,^[4] and it is classified as a R Coronae Borealis variable.^[3] It is a carbon star with a hydrogen-deficient spectra that also shows evidence of s-process elements,^[4] including overabundances of strontium and yttrium, but no barium.^[12] This combination of properties is exceptionally rare; only one other example has been found as of 2012.^[13] 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.^[12] In 1999, U Aqr was proposed to be a Thorne-Zytkow object, instead of being a simple R Coronae Borealis variable.^[14]

References

1. "Download Data". aavso.org. AAVSO. Retrieved 1 October 2021.
2. Brown, A. G. A.; et al. (Gaia collaboration) (2021). "Gaia Early Data Release 3: Summary of the contents and survey properties". Astronomy & Astrophysics. 649: A1. arXiv:2012.01533. Bibcode:2021A&A...649A...1G. doi:10.1051/0004-6361/202039657. S2CID 227254300. (Erratum: doi:10.1051/0004-6361/202039657e). Gaia EDR3 record for this source at VizieR.
3. 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.
4. 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.
5. 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.
6. 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.
7. 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. S2CID 131780028.
8. 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.
9. "U Aqr". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2021-01-01.
10. Cannon, Annie J. (1907). "Second catalogue of variable stars". Annals of Harvard College Observatory. 55: 1–94. Bibcode:1907AnHar..55....1C. Retrieved 20 November 2024.
11. "U Aqr". The International Variable Star Index. AAVSO. Retrieved 20 November 2024.
12. 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.
13. Goswami, Aruna (2012). Prugniel, Philippe; Singh, Harinder P. (eds.). Spectroscopic characterization 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. Vol. 6. p. 189. arXiv:1204.1806. Bibcode:2012ASInC...6..189G. ISBN 978-81-922926-4-9.
14. 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.