Ground‐State SiO Maser Emission toward Evolved Stars

We have made the first unambiguous detection of vibrational ground-statemaser emission from SiO toward six evolved stars. Using the Very Large Array,we simultaneously observed the v=0, J=1-0, 43.4-GHz, ground-state and the v=1,J=1-0, 43.1-GHz, first excited-state transitions of SiO toward the oxygen-richevolved stars IRC+10011, o Ceti, W Hya, RX Boo, NML Cyg, and R Cas and theS-type star chi Cyg. We detected at least one v=0 SiO maser feature from six ofthe seven stars observed, with peak maser brightness temperatures ranging from10,000 K to 108,800 K. In fact, four of the seven v=0 spectra show multiplemaser peaks, a phenomenon which has not been previously observed. Ground-statethermal emission was detected for one of the stars, RX Boo, with a peakbrightness temperature of 200 K. Comparing the v=0 and the v=1 transitions, wefind that the ground-state masers are much weaker with spectral characteristicsdifferent from those of the first excited-state masers. For four of the sevenstars the velocity dispersion is smaller for the v=0 emission than for the v=1emission, for one star the dispersions are roughly equivalent, and for twostars (one of which is RX Boo) the velocity spread of the v=0 emission islarger. In most cases, the peak flux density in the v=0 emission spectrum doesnot coincide with the v=1 maser peak. Although the angular resolution of theseVLA observations were insufficient to completely resolve the spatial structureof the SiO emission, the SiO spot maps produced from the interferometric imagecubes suggest that the v=0 masers are more extended than their v=1counterparts