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]
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 | |
Database references | |
SIMBAD | data |
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.[10] This combination of properties is exceptionally rare; only one other example has been found as of 2012.[11] 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.[10] In 1999, U Aqr was proposed to be a Thorne-Zytkow object, instead of being a simple R Coronae Borealis variable.[12]