Epsilon Hydri

Epsilon Hydri, Latinized from ε Hydri, is a single,^[8] blue-white hued star in the southern constellation of Hydrus. It is a faint star with an apparent visual magnitude of 4.12,^[2] but it can be seen with the naked eye. Measurements made by the Hipparcos spacecraft showed an annual parallax shift of 21.48 mas,^[1] which provides a distance estimate of 152 light years. The star is moving away from the Sun with a radial velocity of +13.6 km/s.^[4] It is a member of the Tucana-Horologium moving group, an association of stars that share a common motion through space.^[9]

ε Hydri

Location of ε Hydri (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Hydrus
Right ascension	02h 39m 35.36121s[1]
Declination	−68° 16′ 01.0103″[1]
Apparent magnitude (V)	4.12[2]
Characteristics
Spectral type	B9 Va[2]
U−B color index	−0.14[3]
B−V color index	−0.06[3]
Astrometry
Radial velocity (Rv)	+13.6±0.9[4] km/s
Proper motion (μ)	RA: +87.30[1] mas/yr Dec.: +0.09[1] mas/yr
Parallax (π)	21.48 ± 0.09 mas[1]
Distance	151.8 ± 0.6 ly (46.6 ± 0.2 pc)
Absolute magnitude (MV)	+0.78[2]
Details
Mass	2.64[5] M☉
Radius	2.2[6] R☉
Luminosity	60[5] L☉
Surface gravity (log g)	4.33[7] cgs
Temperature	10,970±373[7] K
Rotational velocity (v sin i)	96[5] km/s
Age	133[7] Myr
Other designations
ε Hyi, CPD−68° 161, FK5 95, GC 3240, HD 16978, HIP 12394, HR 806, SAO 248621
Database references
SIMBAD	data

The stellar classification for this star is B9 Va,^[2] indicating that is it a B-type main-sequence star that is generating energy through hydrogen fusion at its core. It is a young star, just 133^[7] million years in age, and has a high rate of spin with a projected rotational velocity of 96 km/s.^[5] This is giving the star a mild oblate shape with an equatorial bulge that is 5% greater than the polar radius.^[10] Epsilon Hydri has an estimated 2.64 times the mass of the Sun and 2.2 times the Sun's radius.^[5] It is radiating 60^[5] times the Sun's luminosity from its photosphere at an effective temperature of around 10,970 K.^[7]

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. Vizier catalog entry
2. 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. Vizier catalog entry
3. Mallama, A. (2014). "Sloan Magnitudes for the Brightest Stars". The Journal of the American Association of Variable Star Observers. 42 (2): 443. Bibcode:2014JAVSO..42..443M.Vizier catalog entry
4. Gontcharov, G. A. (2006). "Pulkovo Compilation of Radial Velocities for 35 495 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. Zorec, J.; Royer, F. (2012). "Rotational velocities of A-type stars". Astronomy & Astrophysics. 537: A120. arXiv:1201.2052. Bibcode:2012A&A...537A.120Z. doi:10.1051/0004-6361/201117691. S2CID 55586789. Vizier catalog entry
6. Allende Prieto, C.; Lambert, D. L. (1999). "Fundamental parameters of nearby stars from the comparison with evolutionary calculations: Masses, radii and effective temperatures". Astronomy and Astrophysics. 352: 555–562. arXiv:astro-ph/9911002. Bibcode:1999A&A...352..555A. Vizier catalog entry
7. 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. Vizier catalog entry
8. 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.
9. Bell, Cameron P. M.; et al. (November 2015), "A self-consistent, absolute isochronal age scale for young moving groups in the solar neighbourhood", Monthly Notices of the Royal Astronomical Society, 454 (1): 593–614, arXiv:1508.05955, Bibcode:2015MNRAS.454..593B, doi:10.1093/mnras/stv1981
10. Belle, G. T. (2012). "Interferometric observations of rapidly rotating stars". The Astronomy and Astrophysics Review. 20 (1): 51. arXiv:1204.2572. Bibcode:2012A&ARv..20...51V. doi:10.1007/s00159-012-0051-2. S2CID 119273474.