Ground‐State SiO Maser Emission toward Evolved Stars

Abstract

We have made the first unambiguous detection of vibrational ground-statemaser emission from ^{28}SiO toward six evolved stars. Using theVery Large Array (VLA), we simultaneously observed the v=0,J=1-0, 43.4GHz ground-state transitions and the v=1,J=1-0, 43.1 GHz firstexcited-state transitions of ^{28}SiO toward the oxygen-richevolved stars IRC +10011, o Ceti, W Hya, RX Boo, NML Cyg, and R Cas andthe S-type star {χ} Cyg. We detected at least one v=0 SiO maserfeature from six of the seven stars observed, with peak maser brightnesstemperatures ranging from 10,000 to 108,800 K. In fact, four of theseven v=0 spectra show multiple maser peaks, a phenomenon that has notbeen previously observed. Ground-state thermal emission was detected forone of the stars, RX Boo, with a peak brightness temperature of 200 K.Comparing the v=0 and v=1 transitions, we find that the ground-statemasers are much weaker, with spectral characteristics different fromthose of the first excited-state masers. For four of the seven stars,the velocity dispersion is smaller for the v=0 emission than for the v=1emission; for one star, the dispersions are roughly equivalent; and fortwo stars (one of which is RX Boo), the velocity spread of the v=0emission is larger. In most cases, the peak flux density in the v=0emission spectrum does not coincide with the v=1 maser peak. Althoughthe angular resolution of these VLA observations was insufficient tocompletely resolve the spatial structure of the SiO emission, the SiOspot maps produced from the interferometric image cubes suggest that thev=0 masers are more extended than their v=1 counterparts.