Other
Scientific paper
Mar 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt.......136p&link_type=abstract
PhD Thesis, University of Crete, Greece, 1995.
Other
2
Scientific paper
A systematic study of OH masers in star-forming regions has been undertaken with the aim to invert, at least partially, the maser problem and from the observations to infer the physical conditions in star-forming regions. The pumping mechanisms investigated, one at a time, are: Collisions, local line overlap, non-local line overlap and external infrared radiation field. The present work is an improvement over the work of previous researchers in two main aspects. First, a formalism has been developed to treat radiation transport of spectral lines that overlap locally or non-locally. By using the Sobolev or Large Velocity Gradient approximation, the radiative transfer equation has been solved and analytic expressions have been derived for the first time for the intensity and the mean intensity of the radiation field. Second, the collision rates between OH and H2 that are used distinguish for the first time between ortho-H2 and para-H2. All previous calculations considered para-H2 only. As our calculations have shown, it makes a big difference in which form H2 is in the interstellar medium. It is thus hoped that this work will help determine the abundance of ortho-H2. Our model calculations have shown that a combination of a FIR field, collisions and line overlap are necessary to reproduce the general features of HII/OH masers. The general conclusions from our study are: 1) If the 4660 MHz line is seen, it means that nH2 ~108 cm-3. If the 4751 MHz line is seen in the same spatial region, it confirms that nH2 ~108 cm-3. 2) In the presence of a FIR radiation field, it is more likely to see the 1612 MHz line in the same spatial region with the 1665 MHz line than with the 1667 MHz one. When the 1612 MHz line is seen in the same spatial region with the 1665 MHz one, it means that nH2 ~106 cm-3. 3) If both the 1665 MHz and the 1667 MHz lines are seen in the same spatial region, a FIR radiation field must be present and nH2 < mkern-14mu ~106 cm-3 there. The 1665 MHz line is typically the stronger of the two. 4) When the 1612 MHz line is observed, it means that nH2 > mkern-14mu ~ 106 cm-3. 5) Inversion of the 4766 MHz line means relatively small velocity gradients. We have found that this line is seen only for V < mkern-14mu ~1 km s-1 under all conditions we investigated. 6) The existence of the lines 1667 and 4766 MHz in the same spatial region, with the 1667 MHz one typically an order of magnitude brighter than the other, means TH2 > 150 K, fortho-H2 > mkern-14mu ~0.5 and relatively small velocities (typically V < 1 km s-1). 7) In the presence of moderate velocity gradients (V ~1 km s-1), if the 4766 MHz line is seen together with the 4751 MHz one in the same spatial region, then fortho-H2 < mkern-14mu ~0.5. The 4766 MHz line is much stronger than the 4751 MHz one. 8) In the presence of moderate velocity gradients (V ~1 km s-1), if the 4766 MHz line is observed together with the 1612 MHz one in the same spatial region, then fortho-H2 > mkern-14mu ~0.5. 9) In the presence of significant velocity gradients (V >= 2 km s-1), it is more likely to have the 1720 MHz line strongly inverted at nH2 ~few × 108 cm-3 than at nH2 < mkern-14μ ~few × 107 cm-3. 10) As the FIR radiation field increases, the 1665 MHz line increases in intensity and is inverted in a larger range of densities. 11) Contrary to common belief, the dust temperature need not be high and the optical depth of the dust need not be a large (p >= 2) power of frequency in order to have the 1665 MHz line stronger than the 1667 MHz one.
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