StHa 55: a carbon Mira, not a symbiotic binary

TL;DR

This study confirms that StHa 55 is a typical carbon Mira variable star with specific pulsation and spectral characteristics, refuting earlier claims of it being a symbiotic binary.

Contribution

The paper provides detailed photometric and spectroscopic analysis establishing StHa 55 as a normal carbon Mira, challenging previous symbiotic binary classification assumptions.

Findings

StHa 55 pulsates with a 395-day period.

Spectral features match those of typical carbon Miras.

No evidence of symbiotic binary features found.

Abstract

We carried out a VRI photometric monitoring of StHa 55, and in addition we obtained low resolution absolute spectro-photometry and high resolution Echelle spectroscopy. Our data show that StHa 55 is a carbon Mira, pulsating with a 395 day period, with <V>=13.1 mean brightness and 2.8 mag amplitude in V band. It suffers from a low reddening (E(B-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 Ha emission is unusually small for a carbon Mira. The spectrum of StHa 55 can be classified as C-N5 C2 6-. Its 13C/12C 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 StH 55 is a bona fide, normal carbon Mira showing no feature supporting a symbiotic binary classification, as previously hypothesized.