On the nature of the SiO maser in the Kleinmann-Low nebula
Recently several emissionlines from rotational transitions in vibrationally excited levels of SiO have been detected. The first source was found in the Kleinmann-Low (KL) nebula in Orion. Most likely the radiation source is an astrophysical maser, perhaps associated with the Becklin Neugebauer (BN) object. Following the detection of the SiO maser in Orion, the same radiation has been found from a number of late type stars associated with OH/H2O line emission. However, no SiO masers have been found at the positions of OH masers in H II regions. This is somewhat puzzling since the KL nebula and the BN object are generally considered to be very young objects.
It is known from the work of Larsonthat during the last stages of the formation of massive stars the central, embryonal star is already on the main sequence, while still accreting matter at a high rate. It can be shown that the high gas density in the envelope pushes the Strömgren sphere of the star onto the stellar surface. We suggest that in the neutral gas envelope close to the star the conditions are favourable for SiO maser formation. More specifically we propose that the BN object is the source of maser emission observed in the direction of the KL nebula.
We have constructed a model of the BN object to explain its near-infrared spectrum using the methods of Larson (1969, Monthly Not. R.A.S. 145, 297) but adding foreground extinction. The parameters of the best fit are a luminosity of the central star of L = 5000 L⊙, a mass inflow rate in the cocoon of M = 2 x 10−5 M⊙ and an extinction in the KL nebula of Av = 30m. The physical conditions at the point in the cocoon where we expect maser emission are as follows: n(H2) ≈ 108 cm−3, the infall velocity u ≈ 20 km sec−1 the kinetic temperature of the gas is ≈ 500 K, the temperature of the radiation field ≈2000 K diluted by a factor of about 10–2 to 10−3. These conditions are very similar to those in the expanding envelopes of late-type stars, for which detailed SiO maser models have recently been published by Kwan and Scoville (1974, Ap. J. Lett. 194, L97). Thus the SiO maser lines in the KL nebula could very well be produced in the cocoon around the BN star.
In general we expect that infrared sources associated with H II regions with spectra peaking at near-infrared wavelengths are good candidates for SiO maser emission.
(A more extensive account of this work has been submitted to Astronomy and Astrophysics). *** DIRECT SUPPORT *** A3418034 00006