Context. Despite their carbon-rich photospheres, silicate carbon stars show evidence of oxygen-rich circumstellar material, which is thought to exist in disks. Silicate carbon stars represent interesting cases that allow us to study the possible effects of binarity on stellar evolution and mass loss accompanied by the formation of disks.
Aims: We present a small survey of 22 GHz H2O masers toward ten silicate carbon stars with much better sensitivity than is the case for previous observations.
Methods: We observed our sample with the Karl G. Jansky Very Large Array (VLA) using the most expanded configuration (A-configuration) with a maximum baseline of 36 km. For some of our program stars with noisy Infrared Astronomical Satellite (IRAS) Low Resolution Spectra (LRS), we present new mid-IR spectra obtained with the Very Large Telescope Interferometer and the Spitzer Space Telescope.
Results: We detected H2O masers toward five out of ten silicate carbon stars (EU And, V778 Cyg, IRAS 06017+1011, V1415 Cyg, and NC83 = V1945 Cyg), with NC83 being a new detection. No H2O masers were detected toward BM Gem, IRAS 07221-0431, IRAS 08002-3803, IRAS 18006-3213, and HD 189605. The velocity separation between the most blue- and red-shifted maser features is 10-14 km s-1. If we assume that the masers originate in circum-companion disks, the measured velocity separations translate into a lower limit of the rotational velocity of 5-7 km s-1, and the upper limit of the radius of the maser emitting region is estimated to be 10-68 AU for a companion mass of 0.5-1.7 M&sun. The new mid-IR spectra of NC83, IRAS 06017+1011, and HD 189605 confirm the 10 mum silicate emission. The latter two stars show a bump at ~11.5 mum, which is presumably due to SiC originating in the ongoing mass loss from the carbon-rich primary star, not due to crystalline silicate. We also report on the detection of the UV flux at 2271 Å toward HD 189605. Based on MIDI observations made with the Very Large Telescope Interferometer of the European Southern Observatory. Program ID: 076.D-0250.This work is based (in part) on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. |
