Theta Hydrae
Binary star system in the constellation Hydra
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Theta Hydrae, Latinized from θ Hydrae, is a binary star[8] system in the constellation Hydra. It is visible to the naked eye with an apparent visual magnitude of 3.9.[2] The star system has a high proper motion[8] with an annual parallax shift of 28.4 mas,[1] indicating a distance of about 115 light years. Theta Hydrae forms a double with a magnitude 9.9 star located at an angular separation of 29 arcseconds.[10]
 | |
| Observation data Epoch J2000.0 Equinox J2000.0 (ICRS) | |
|---|---|
| Constellation | Hydra |
| Right ascension | 09h 14m 21.866s[1] |
| Declination | +02° 18′ 51.64″[1] |
| Apparent magnitude (V) | 3.888[2] |
| Characteristics | |
| Spectral type | B9.5 V + DA 1.6[3] |
| U−B color index | −0.118[2] |
| B−V color index | −0.065[2] |
| Astrometry | |
| Radial velocity (Rv) | −10.7±0.3[4] km/s |
| Proper motion (μ) | RA: +128.152[1] mas/yr Dec.: −327.709[1] mas/yr |
| Parallax (π) | 28.4019±0.3682 mas[1] |
| Distance | 115 ± 1 ly (35.2 ± 0.5 pc) |
| Absolute magnitude (MV) | +0.92[5] |
| Details | |
| θ Hya A | |
| Mass | 2.52[3] M☉ |
| Luminosity | 52[5] L☉ |
| Surface gravity (log g) | 3.80±0.08[6] cgs |
| Temperature | 10,099±145[6] K |
| Metallicity [Fe/H] | −0.42±0.09[6] dex |
| Rotational velocity (v sin i) | 95[7] km/s |
| θ Hya B | |
| Mass | 0.68[8] or 1.21[3] M☉ |
| Temperature | 30,700[3] K |
| Other designations | |
| θ Hya, 22 Hydrae, BD+02°2167, FK5 347, HD 79469, HIP 45336, HR 3665, SAO 117527[9] | |
| Database references | |
| SIMBAD | data |
The primary component of this system is a B-type main sequence star with a stellar classification of B9.5 V.[3] It is a candidate Lambda Boötis star, indicating it displays an underabundance of iron peak elements.[11] However, it is also underabundant in oxygen, a characteristic not shared by other Lambda Boötis stars. Instead, it may be a peculiar B star.[12]
An orbiting white dwarf companion was discovered in 1998 from its X-ray emission. This degenerate star must have evolved from a progenitor that was once more massive than the current primary.[8] Burleigh and Barstow (1999) gave a mass estimate of 0.68[8] times the mass of the Sun, whereas Holberg et al. (2013) put it as high as 1.21[3] times the Sun's mass. The latter would put it beyond the theoretical upper limit for white dwarf remnants of typical single stars that did not undergo a merger or mass loss.[13]