V Hydrae

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V Hydrae
Hydra constellation map.svg
Red circle.svg
Location of V Hydrae (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Hydra
Right ascension  10h 51m 37.25661s[1]
Declination −21° 15′ 00.3245″[1]
Apparent magnitude (V) 6.0 - 12.3[2]
Spectral type C6,3e-C7,5e(N6e)[3]
B−V color index +5.43[4]
Variable type SRa[3]
Radial velocity (Rv)−14.80[5] km/s
Proper motion (μ) RA: −11.02 ± 1.14[1] mas/yr
Dec.: 2.29 ± 1.16[1] mas/yr
Parallax (π)1.44 ± 1.41[1] mas
Distanceapprox. 2,000 ly
(approx. 700 pc)
Absolute magnitude (MV)−3.5 to +1.8[6]
Period (P)8.5 yr
Eccentricity (e)≥0.6
Mass1.0[8] M
Radius420[9]–430[7] R
Luminosity7,850[8] L
Surface gravity (log g)−0.5[8] cgs
Temperature2,650[8] K
Rotational velocity (v sin i)11 - 14[8] km/s
Mass<1[7] M
Other designations
V Hya, BD−20°3283, HIP 53085, 2MASS J10513724-2115002, IRAS 10491-2059, WDS J10516-2115
Database references

V Hydrae (V Hya) is a carbon star in the constellation Hydra.


Artist's illustration of plasma ejections from V Hydrae.

V Hydrae is a semiregular variable star of type SRa, sometimes considered to be a Mira variable, it pulsates with a period of 530 days and a brightness range of 1-2 magnitudes, but also shows deep fades at intervals of about 6,160 days when it may drop below magnitude 12.[2][10]

Evolutionary stage[edit]

V Hydrae is a carbon star, an asymptotic giant branch (AGB) star that has dredged up sufficient material from its interior to have more carbon in its atmosphere than oxygen; the rate of mass loss from V Hydrae indicates that it is almost at the end of the AGB stage and about to lose its atmosphere completely and form a planetary nebula. It is sometimes considered to be a post-AGB object.[11]


V Hydrae has a visible binary companion 46" distant, it is a magnitude 11.5 K0 giant.[6]

V Hydrae also has an unseen companion in an 8.5 year orbit, inferred by its ultraviolet excess and eruptions associated with the periastron passage.[12][7] It has been suggested that the steep drops in brightness every 17 years or so are caused by obscuration by a cloud associated with the companion passing in front of the giant star.[12]


V Hydrae has high-speed outflows of material collimated into jets, and also a disk of material around the star. Since the star itself is considered to be at the end of the Asymptotic Giant Branch phase of evolution and starting to generate a planetary nebula, the mechanism for the ejection of this material can give key insights to the formation of planetary nebulae; the ejections have been modelled as bullets of material fired out each time the compact companion passes close to the extended giant star during a highly eccentric orbit. The bullets are ejected in opposite directions in different orbits due to a flip-flop of the ejection mechanism.[7]


  1. ^ a b c d e Van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357.
  2. ^ a b Watson, Christopher (4 January 2010). "V Hydrae". The International Variable Star Index. American Association of Variable Star Observers. Retrieved 2016-10-12.
  3. ^ a b Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)". VizieR On-line Data Catalog: B/gcvs. Originally published in: 2009yCat....102025S. 1. Bibcode:2009yCat....102025S.
  4. ^ Ducati, J. R. (2002). "VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system". CDS/ADC Collection of Electronic Catalogues. 2237. Bibcode:2002yCat.2237....0D.
  5. ^ Gontcharov, G. A. (2006). "Pulkovo Compilation of Radial Velocities for 35 495 Hipparcos stars in a common system". Astronomy Letters. 32 (11): 759. arXiv:1606.08053. Bibcode:2006AstL...32..759G. doi:10.1134/S1063773706110065.
  6. ^ a b Gordon, Courtney P. (1968). "The Absolute Magnitudes of Carbon Stars". Publications of the Astronomical Society of the Pacific. 80: 597. Bibcode:1968PASP...80..597G. doi:10.1086/128694.
  7. ^ a b c d e Sahai, R.; Scibelli, S.; Morris, M. R. (2016). "High-speed Bullet Ejections during the AGB-to-Planetary Nebula Transition: HST Observations of the Carbon Star, V Hydrae". The Astrophysical Journal. 827 (2): 92. arXiv:1605.06728. Bibcode:2016ApJ...827...92S. doi:10.3847/0004-637X/827/2/92.
  8. ^ a b c d e Zhao-Geisler, R.; Quirrenbach, A.; Köhler, R.; Lopez, B. (2012). "Dust and molecular shells in asymptotic giant branch stars". Astronomy & Astrophysics. 545: A56. arXiv:1207.3767. Bibcode:2012A&A...545A..56Z. doi:10.1051/0004-6361/201118150.
  9. ^ Luttermoser, Donald G.; Brown, Alexander (1992). "A VLA 3.6 centimeter survey of N-type carbon stars". Astrophysical Journal. 384: 634. Bibcode:1992ApJ...384..634L. doi:10.1086/170905.
  10. ^ Olivier, Enrico A.; Whitelock, Patricia; Marang, Fred (2001). "Dust-enshrouded asymptotic giant branch stars in the solar neighbourhood". Monthly Notices of the Royal Astronomical Society. 326 (2): 490. arXiv:astro-ph/0103294. Bibcode:2001MNRAS.326..490O. doi:10.1046/j.1365-8711.2001.04511.x.
  11. ^ Knapp, G. R.; Crosas, M.; Young, K.; Ivezić, Željko (2000). "Atomic Carbon in the Envelopes of Carbon‐rich Post–Asymptotic Giant Branch Stars". The Astrophysical Journal. 534: 324. arXiv:astro-ph/9912496. Bibcode:2000ApJ...534..324K. doi:10.1086/308731.
  12. ^ a b Sahai, R.; Findeisen, K.; Gil De Paz, A.; Sánchez Contreras, C. (2008). "Binarity in Cool Asymptotic Giant Branch Stars: A GALEX Search for Ultraviolet Excesses" (PDF). The Astrophysical Journal. 689 (2): 1274–1278. arXiv:0807.1944. Bibcode:2008ApJ...689.1274S. doi:10.1086/592559.