Research Note

VLA survey of 22 GHz H₂O masers toward ten silicate carbon stars^⋆,⋆⋆

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: kohnaka@mpifr.de
² United States Naval Observatory, 3450 Massachusetts Avenue, NW, Washington, DC 20392-5420, USA
³ National Science Foundation, 4201 Wilson Boulevard, Arlington, VA, 22230, USA
e-mail: dboboltz@nsf.gov
⁴ Okayama Astrophysical Observatory, National Astronomical Observatory, Kamogata, Asakuchi, 719-0232 Okayama, Japan
⁵ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany

Received: 1 August 2013
Accepted: 8 October 2013

Abstract

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_⊙. The new mid-IR spectra of NC83, IRAS 06017+1011, and HD 189605 confirm the 10 μm silicate emission. The latter two stars show a bump at ~11.5 μm, 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.