Infrared properties of SiO maser sources in late-type stars

The results of an SiO maser survey for late-type stars selected by the IRTS (Infrared Telescope in Space) are presented. We have detected SiO J = 1-0, v = 1 and/or v = 2 lines in 27 stars out of 59 stars. The maser intensity increases with the depth of the H2O absorption in the infrared spectra and redness of the 2.2 and 12 mum color. The column densities of the water vapo r in the target stars were estimated from the depth of the water absorption in the IRTS spectra. We found that the SiO maser was detected mostly in th e stars with a column density of water vapor higher than 3 x 10(19)-3 x 10( 20) cm(-2). We further estimated the density of hydrogen molecules in the o uter atmosphere corresponding to these column densities, obtaining 10(9)-10 (10) cm(-3) as a lower limit. These values are roughly in agreement with th e critical hydrogen density predicted by models for the excitation of SiO m asers. It is possible that the SiO masers are excited in clumps with column density even higher than this value. The present results provide useful in formation for the understanding of the physical conditions of the outer atm ospheres in late-type stars.