V Hydrae (V Hya) is a

carbon star A carbon star (C-type star) is typically an asymptotic giant branch star, a luminous red giant, whose atmosphere contains more carbon than oxygen. The two elements combine in the upper layers of the star, forming carbon monoxide, which consumes mos ...

in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The origins of the e ...

Hydra Hydra generally refers to: * Lernaean Hydra, a many-headed serpent in Greek mythology * ''Hydra'' (genus), a genus of simple freshwater animals belonging to the phylum Cnidaria Hydra or The Hydra may also refer to: Astronomy * Hydra (constel ...

. 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.

Variability

V Hydrae is a

semiregular variable star In astronomy, a semiregular variable star, a type of variable star, is a giant or supergiant of intermediate and late (cooler) spectral type showing considerable periodicity in its light changes, accompanied or sometimes interrupted by various irre ...

of type SRa, sometimes considered to be a

Mira variable Mira variables (named for the prototype star Mira) are a class of pulsating stars characterized by very red colours, pulsation periods longer than 100 days, and amplitudes greater than one magnitude in infrared and 2.5 magnitude at visual wavelen ...

. 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.

Evolutionary stage

V Hydrae is a late

, an

asymptotic giant branch The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (about 0.5 to 8 solar masses) lat ...

(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.

Companions

V Hydrae has a visible binary companion 46" distant. It is a magnitude 11.5 K0 giant. 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. 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.

Bullets

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 The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (about 0.5 to 8 solar masses) lat ...

(AGB) phase of evolution and starting to generate a

planetary nebula A planetary nebula (PN, plural PNe) is a type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives. The term "planetary nebula" is a misnomer because they are unrelated to ...

, the mechanism for the ejection of this material can give key insights to the formation of

e. 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. Microwave spectra of rotational transitions of

carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...

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.

References

{{DEFAULTSORT:V Hydrae Hydra (constellation) Carbon stars Hydrae, V Asymptotic-giant-branch stars Semiregular variable stars 053085 Durchmusterung objects