Evidence for Rapid Rotation of the Carbon Star V Hydrae
Abstract
The carbon star V Hya is known to have a bipolar circumstellar outflow and an unusual photospheric spectrum. After measuring the changes in the photospheric velocity and carrying out a spectral broadening analysis over 10 photometric phases, we conclude that rotation due to spin-up by a companion in a common envelope configuration is the most compelling explanation for all the data taken together. After considering the spectral and velocity behavior of 74 other carbon stars, we find that V Hya is alone in our sample in having such obvious phase-dependent broadening.
In this paper we detail the broadening results and explore other possible causes of spectral broadening in late-type stars. In particular, we find that: (1) the spectral broadening in V Hya varies about a mean value of 13.5 km s ^{1 }(with total variation of 9 km s 1) in concert with the photometric phase and with a phase relationship consistent with that expected for a rapidly rotating star that conserves angular momentum as it pulsates, (2) V Hya's unusual, very long secondary period of 6500 days might be attributed to a coupling of radial pulsation (529 day period) with rotation, (3) the upper limit on the ultraviolet continuum flux favors a common envelope versus detached binary system, (4) the variability type, Mira versus semiregular variable, is uncertain since V Hya demonstrates spectral characteristics of both, and (5) the evolutionary placement on the asymptotic giant branch is in question since V Hya appears to lack 99Tc, a signature element of third dredge-up whose absence suggests that a mass transfer event in the past might be responsible for V Hya's carbon and s-process element enhancement.- Publication:
-
The Astrophysical Journal
- Pub Date:
- September 1995
- DOI:
- 10.1086/176190
- Bibcode:
- 1995ApJ...450..862B
- Keywords:
-
- STARS: AGB AND POST-AGB;
- STARS: CARBON;
- STARS: INDIVIDUAL CONSTELLATION NAME: V HYDRAE;
- STARS: ROTATION