Dissertation

Theoretical modelling of astrophysical masers: applications to and results for oh and sio

Doel, Robert Colin

1991

Overview

In the case of OH we model the lowest 48 hypcrfinc states using the Sobolev, or 'Large Velocity Gradient' approximation and include the effects of collisions with para-H2 in J = 0, a continuum radiation field (with components from the dust within the masing region, dust external to the masing region and, optionally, from a nearby star) and a velocity field. Far-infrared line overlap is included in the model. We use a new theory which describes the effects of both the thermal width of the line profiles ('thermal overlap') and the Doppler shifting due to the velocity field ('velocity overlap') in all cases where a significant amount of overlap between the line profiles (taken to be Gaussian) occurs. The observed behaviour of Hll/OH regions (strong inversions in all four ground state transitions, and in the excited rotational states 2n3/2 J - 5/2 anti 2rii/2 J = l/j over a narrow range of conditions, the association of various masing frequencies in different rotational states, the prevalence of the main line masers over those of the satellite lines) is reproduced well under the following conditions: H2 number density 106-10s cm\"\"3, kinetic and local dust temperature 100-200 K, external dust temperature 150-250 K, diluted spatially by a factor of 0.25, OH abundance 10~6-2xlO-6, velocity gradient 500-1500 km s\"1 pc~*. It is found that a consequence of FIR line overlap in accelerative velocity fields is to increase the gain coefficient for 1665 MHz above that of 1667 MHz. This is a commonly observed feature that is not otherwise predicted. Masers in star-forming areas without H1I regions and in OH/IR stars arc modelled briefly and a new, as yet unobserved, phenomenon of FIR lasers at 115 ftm and 135 fim is predicted to occur near volumes of hot (> 500 K) dust which may be found in star-forming regions.In the case of SiO in circumstcllar envelopes we represent the lowest 40 rotational states (J = 0-39) in each of the first 5 vibrational states (V = 0-4). The less abundant isotopic forms 20SilcO and 30SilcO may optionally be included also. We model collisions with para-H2 in J = 0, the background continuum radiation field (with contributions from the evolved star, dust external to the masing region and/or dust within it) and the effects of a velocity field using the Sobolev approximation. The SiO masers characteristic of M-Miras and supergiants are reproduced with moderate success, and those of OH/IR stars rather better using the following conditions: Hj number density 109-101U cm~3, kinetic temperature 1100-1500 K, dust (either local to the masing region, or external and surrounding it) temperature 1100-1500 K, SiO abundance lO\"5-10~4, velocity gradient 1000-10000 km s\"1 pc-1. These conditions also give rise to an inversion in the transition of the less abundant isotopes that has been observed to mase, i.e. V = 0 J = 1-0. It is suggested thai mascr action in high lying rotational states occurs in the absence of optically thick hot dust (conditions as above excepting TXD < 400 K) possibly reflecting a low heavy element (particularly Aluminium) abundance in some evolved star envelopes. From thermodynamic arguments it is shown that accurate collision rate coefficients for SiO and both atomic hydrogen and rotation-ally excited molecular hydrogen arc highly desirable if the lull range of potential conditions for SiO masers arc to be explored accurately.The problem of competitive gain in SiO is addressed briefly. It is shown that where only the one or two lowest lying transitions within a vibralional state are inverted, then a saturating mascr in J = 1-0 can cause an inversion in the J = 1-0 transition of the next higher vibrational slate. This reflects the observed association of J = 1-0 in V = 1 and 2. Conversely, if a greater number of transitions within a vibrational state arc all inverted then simultaneous saturation in each of those results in a stronger gain in ihc next higher lying transition within the same vibra

• Publisher: ProQuest Dissertations & Theses
• Subject: Physical chemistry