Oxygen-rich disk in the V778 Cygni system resolved

Abstract

Aims. Various scenarios have been proposed to explain the presence of silicate features associated with carbon stars, such as V778 Cyg. We have attempted to constrain these theories by means of mapping water maser emission from V778 Cyg. Methods. The 22 GHz water maser emission from this star has been mapped using MERLIN with an astrometric accuracy of 25 mas.Results. The spatially-and kinematically-resolved maser complex is displaced by ∼190 mas from the position of the C-star as measured 10 years earlier using Tycho. Our simulations and analysis of available data show that this position difference is unlikely to be due to proper motion if V778 Cyg is at the assumed distance of 1.4 kpc. The maser components seem to form a distorted Sshaped structure extended over ∼18 mas with a clear velocity gradient. We propose a model which explains the observed water maser structure as an O-rich warped disk around a companion of the C-star in V 778 Cyg binary system, which is seen almost edge-on. Conclusions. Analysis of observational data, especially those obtained with MERLIN, suggests that V778 Cyg (and, by implication, other silicate carbon stars) are binary systems composed of a C-rich star and a companion which stores circumstellar O-rich material.

A self-consistent model of a 22 GHz water maser in a dusty environment near late-type stars

“…Water masers observed in some silicate carbon stars, e.g. V778 Cyg Engels & Leinert 1994), are not associated with the winds, but most probably arise in an oxygenrich disk around a companion star in a binary system (Szczerba et al 2005). …”

A New Catalog of Silicate Carbon Stars

“…And VLBA H 2 O maser observations toward EU And supported the circum-companion disk (Ohnaka & Boboltz 2008). For V778 Cyg, Szczerba et al (2006) obtained high resolution H 2 O maser maps at 22 GHz and they concluded that the object is composed of a C-rich star and a companion that stores an O-rich disk. Those observations are strong evidences for presence of an O-rich disk around a SCS which is actually a binary system.…”

A database of circumstellar OH masers

“…While the low number of M-supergiants is likely due to the low number of M-supergiants in general, the OH emitting planetary nebulae and silicate-carbon stars are peculiar within their parent population by having non-standard environments. In planetary nebulae amplification of the radio continuum emission might be held reponsible (Uscanga et al 2012), while the OH masers in silicate-carbon stars may arise like the H 2 O masers in oxygenrich disks associated with binary systems containing a carbon star (Szczerba et al 2006).…”