2004ApJ...608..480B

We have made the first unambiguous detection of vibrational ground-state maser emission from ²⁸SiO toward six evolved stars. Using the Very Large Array (VLA), we simultaneously observed the v=0,J=1-0, 43.4 GHz ground-state transitions and the v=1,J=1-0, 43.1 GHz first excited-state transitions of ²⁸SiO toward the oxygen-rich evolved stars IRC +10011, o Ceti, W Hya, RX Boo, NML Cyg, and R Cas and the S-type star χ Cyg. We detected at least one v=0 SiO maser feature from six of the seven stars observed, with peak maser brightness temperatures ranging from 10,000 to 108,800 K. In fact, four of the seven v=0 spectra show multiple maser peaks, a phenomenon that has not been previously observed. Ground-state thermal emission was detected for one 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-state masers are much weaker, with spectral characteristics different from those 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=1 emission; for one star, the dispersions are roughly equivalent; and for two stars (one of which is RX Boo), the velocity spread of the v=0 emission is larger. In most cases, the peak flux density in the v=0 emission spectrum does not coincide with the v=1 maser peak. Although the angular resolution of these VLA observations was insufficient to completely resolve the spatial structure of the SiO emission, the SiO spot maps produced from the interferometric image cubes suggest that the v=0 masers are more extended than their v=1 counterparts.