Evidence for a Companion to BM Gem, a Silicate Carbon Star

Abstract

Balmer and Paschen continuum emission, as well as Balmer series lines of P Cygni-type profile from Hγ through H23, are revealed in the violet spectra of BM Gem, a carbon star associated with an oxygen-rich circumstellar shell (silicate carbon star). The blueshifted absorption in the Balmer lines indicates the presence of an outflow, the line-of-sight velocity of which is at least 400 km s−1. The Balmer lines show a significant change in profile over a period of 75 days. We argue that the observed unusual features in BM Gem are strong evidence for the presence of a companion, which should form an accretion disk that gives rise to both an ionized gas region and a high-velocity variable outflow. The estimated luminosity of ~0.2 (0.03-0.6) L☉ for the ionized gas can be maintained by a mass accretion rate for a dwarf companion of ~10−8 M☉ yr−1, while ~10−10 M☉ yr−1 is sufficient for accretion to a white dwarf companion. These accretion rates are feasible for some detached binary configurations on the basis of the Bondi-Hoyle-type accretion process. Therefore, we conclude that the carbon star BM Gem is in a detached binary system with a companion of low mass and low luminosity. However, we are unable to determine whether this companion object is a dwarf or a white dwarf, although the gas outflow velocity of 400 km s−1, as well as the nondetection in the X-ray survey, favor its identity as a dwarf star. The upper limits for binary separation are 210 and 930 AU for a dwarf and a white dwarf, respectively, in the case of circular orbit. We also note that the observed features of BM Gem mimic those of Mira (o Cet), which may suggest actual similarities in their binary configurations and circumstellar structures.