Phi² Hydrae

Phi² Hydrae, Latinized from φ² Hydrae, is a star in the constellation Hydra. It originally received the Flamsteed designation of 1 Crateris before being placed in the Hydra constellation.[9] Based upon an annual parallax shift of 4.31 mas as seen from Earth, it is located roughly 760 light years from the Sun. The star is faintly visible to the naked with an apparent visual magnitude of 6.09.[2] It forms a triangle with the fainter φ¹ Hydrae and the brighter φ³ Hydrae, between μ Hydrae and ν Hydrae.

Pulsations[5]

Period (days)	Amplitude (magnitude)
11.0	0.008
110.3	0.012
153.6	0.015

For other stars with this Bayer designation, see φ Hydrae

Phi² Hydrae

Observation data Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation	Hydra
Right ascension	10^h 36^m 16.65960^s[1]
Declination	−16° 20′ 39.5778″[1]
Apparent magnitude (V)	6.0943[2]
Characteristics
Spectral type	M1 III[3]
U−B color index	+1.95[4]
B−V color index	+1.64[4]
Variable type	SR[5]
Astrometry
Radial velocity (Rv)	+15.7±2.9[2] km/s
Proper motion (μ)	RA: −29.14[1] mas/yr Dec.: +2.31[1] mas/yr
Parallax (π)	4.31 ± 0.33[1] mas
Distance	760 ± 60 ly (230 ± 20 pc)
Absolute magnitude (MV)	−0.80[6]
Details
Luminosity	703[7] L☉
Temperature	3,791[7] K
Other designations
φ^2 Hya, BD−15°3087, HD 91880, HIP 51905, HR 4156, SAO 156093[8]
Database references
SIMBAD	data

This is an evolved red giant star with a stellar classification of M1 III.[10] It is currently on the asymptotic giant branch,[10] and is a semiregular variable that undergoes changes in luminosity according to three pulsation periods.[5] The star is radiating an estimated 703[7] times the Sun's luminosity from its photosphere at an effective temperature of 3,791 K.[7]

Phi² Hydrae has a faint visual companion: a magnitude 12.20 star at an angular separation of 3.50 arc seconds along a position angle of 280°, as of 1959.[11]

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.
2. de Bruijne, J. H. J.; Eilers, A.-C. (October 2012), "Radial velocities for the HIPPARCOS-Gaia Hundred-Thousand-Proper-Motion project", Astronomy & Astrophysics, 546: 14, arXiv:1208.3048, Bibcode:2012A&A...546A..61D, doi:10.1051/0004-6361/201219219, A61.
3. Houk, Nancy; Smith-Moore, M. (1978), Michigan catalogue of two-dimensional spectral types for the HD stars, 4, Ann Arbor: Dept. of Astronomy, University of Michigan, Bibcode:1988mcts.book.....H.
4. Mermilliod, J.-C. (1986), "Compilation of Eggen's UBV data, transformed to UBV (unpublished)", Catalogue of Eggen's UBV Data, SIMBAD, Bibcode:1986EgUBV........0M.
5. Tabur, V.; et al. (2009), "Long-term photometry and periods for 261 nearby pulsating M giants", Monthly Notices of the Royal Astronomical Society, 400 (4): 1945–61, arXiv:0908.3228, Bibcode:2009MNRAS.400.1945T, doi:10.1111/j.1365-2966.2009.15588.x.
6. 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.
7. McDonald, I.; et al. (2012), "Fundamental Parameters and Infrared Excesses of Hipparcos Stars", Monthly Notices of the Royal Astronomical Society, 427 (1): 343–57, arXiv:1208.2037, Bibcode:2012MNRAS.427..343M, doi:10.1111/j.1365-2966.2012.21873.x.
8. "* phi02 Hya". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2017-03-17.
9. Wagman, M. (August 1987), "Flamsteed's Missing Stars", Journal for the History of Astronomy, 18: 216, Bibcode:1987JHA....18..209W, doi:10.1177/002182868701800305.
10. Eggen, Olin J. (July 1992), "Asymptotic giant branch stars near the sun", Astronomical Journal, 104 (1): 275–313, Bibcode:1992AJ....104..275E, doi:10.1086/116239.
11. Mason, B. D.; et al. (2014), "The Washington Visual Double Star Catalog", The Astronomical Journal, 122: 3466–3471, Bibcode:2001AJ....122.3466M, doi:10.1086/323920.