Sthα-55: a Carbon Mira, Not a Symbiotic Binary

We carried out a V RCIC photometric monitoring of StHα 55, and in addition we obtained low resolution absolute spectro-photometry and high resolution Echelle spectroscopy. Our data show that StHα 55 is a carbon Mira, pulsating with a 395 day period, with =13.1 mean brightness and ∆V=2.8 mag amplitude. It suffers from a low reddening (EB−V =0.15), lies at a distance of 5 kpc from the Sun and 1 kpc from the galactic plane, and its heliocentric systemic velocity is close to +22 km sec. The difference between the radial velocity of the optical absorption spectrum and that of the Hα emission is unusually small for a carbon Mira. The spectrum of StHα 55 can be classified as C-N5 C26 . Its C/C isotopic ratio is normal, and lines of BaII and other s-type elements, as well as LiI, have the same intensity as in field carbon stars of similar spectral type. The Balmer emission lines are very sharp and unlike those seen in symbiotic binaries. Their intensity changes in phase with the pulsation cycles in the same way as seen in field carbon Miras. We therefore conclude that StHα 55 is a bona fide, normal carbon Mira showing no feature supporting a symbiotic binary classification, as previously hypothesized.