(SIO)-SI-29 V = 1 MASER EMISSION FROM EVOLVED STARS - PUMPING OF (SIO)-SI-29 BY (SIO)-SI-28 THROUGH INFRARED LINE OVERLAPS

We present the first detection of strong maser emission from high J rotational transitions of the first vibrational excited state of (SiO)-Si-29. Within the v = 1 level the only rational line showing strong maser emission is the J = 4-3. We have reached very low upper limits for the emission of the J = 2-1, J = 3-2 (except in VY CMa where this line has been detected), and J = 5-4 lines of the same vibrational level towards all the stars in our sample. This result could be understood as due to opacity effects if the maser intensity ratios R45 = T(B)(J = 4-3)/T(B)(J = 5-4) and R43 = T(B)(J = 4-3)/T(B)(J = 3-2) were only of almost-equal-to 2-3. However, the upper limits to the emission from the J = 5-4 line give R45 > 20-100 in all the observed stars. Similar results apply also to the ratios of the J = 4-3 to the J = 2-1 and J = 3-2 lines, except in the case of VY CMa, where the intensity ratio R43 is of the order of unity. We propose that the thermal overlap of the infrared lines of (SiO)-Si-28 with those of (SiO)-Si-29 is playing an important role in the pumping of the (SiO)-Si-29 masers in the v = 1 state. Infrared overlaps between these two isotopic species of silicon monoxide could also explain the (SiO)-Si-29 v = 0 masers.