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]
Characteristics
Spectral type C6,3e-C7,5e(N6e) [3]
B−V color index +5.43 [4]
Variable type SRa [3]
Astrometry
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  mas [1]
Distance approx. 2,000  ly
(approx. 700  pc)
Absolute magnitude  (MV)3.5 to +1.8 [6]
Orbit [7]
Period (P)8.5 yr
Eccentricity (e)≥0.6
Details
A
Mass 1.0 [8]   M
Radius 420 [9] –430 [7]   R
Luminosity 7,850 [8]   L
Surface gravity (log g)0.5 [8]   cgs
Temperature 2,650 [8]   K
Rotational velocity (v sin i)11 - 14 [8]  km/s
B
Mass <1 [7]   M
Other designations
V  Hya, BD 20°3283, HIP  53085, 2MASS  J10513724-2115002, IRAS  10491-2059, WDS  J10516-2115
Database references
SIMBAD data

V Hydrae (V Hya) is a carbon star in the constellation Hydra. To date perhaps uniquely in our galaxy it has plasma ejections/eruptions on a grand scale every 8.5 years caused by its near, unseen companion in an 8.5 year orbit, inferred by its ultraviolet excess and periastron passage likely through the outer parts of the star itself.

Contents

Variability

A visual band light curve for V Hydrae, plotted from AAVSO data VHyaLightCurve.png
A visual band light curve for V Hydrae, plotted from AAVSO data

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] [11]

Evolutionary stage

V Hydrae is a late 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. [12]

Companions

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 inferred by its ultraviolet excess. [13] [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. [13]

Bullets

Artist's illustration of plasma ejections from V Hydrae. Artist's illustration of scenario for plasma ejections from V Hydrae.jpg
Artist's illustration of plasma ejections from V Hydrae.

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 (AGB) 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] Microwave spectra of rotational transitions of carbon monoxide show that portions of the envelope, probably the jets, are moving away from the star at 200 km/sec. This is far faster than the ~15 km/sec stellar wind that is typically seen around AGB stars. [14]

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References

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