Astronomical masers are useful probes of the physical conditions of the gas in which they are formed. Masers form under specific physical conditions and therefore, can be used to trace distinct environments, for example, star forming regions (SFRs), supernova remnants (SNRs), evolved stars, and outflows. In particular, collisionally excited 36 and 44 GHz methanol (CH3OH) and 1720 MHz hydroxl (OH) masers are found associated with gas shocked by the interaction between SNRs and neighboring molecular clouds (MCs). The overall goal of my thesis research is to combine modeling and observations to characterize the properties and formation of Class I CH3OH masers in these SNR/MC interaction regions. Developing a general model of the distribution of maser emission in these regions in all SNRs interacting with MCs will aid in the understanding of different processes that may be triggered through these interactions, namely induced star formation (SF) and cosmic ray (CR) acceleration.More accurate information on the density (and density gradients) in these turbulent regions could, for example, be used as inputs or constraints for models of galactic SNR CR acceleration and help explain if conditions are conducive for SF. In this thesis, I present results from calculations of the physical conditions necessary for the occurrence of collisionally pumped Class I 36, 44, 84, and 95 GHz CH3OH maser lines near SNRs, using an escape probability and level population code. The modeling shows that given a sufficient CH3OH abundance, CH3OH maser emission arises over a wide range of densities and temperatures, with optimal conditions at 10e4 < n < 10e6 per cubic cm and T > 60 K, overlapping with masing conditions for 1720 MHz OH masers. In addition, the 36 and 44 GHz transitions display more significant maser optical depths compared to the 84 and 95 GHz transitions over the majority of the physical conditions. The line intensity ratios between multiple transitions significantly change with altering physical conditions and can be used to constrain the physical parameters of the gas where masers are detected. I will show how I use the modeling results as a diagnostic tool to interpret the observational results of a sample of SNRs with previous and recent CH3OH maser detections (G1.4−0.1, W28, Sgr A East, G5.7−0.0, W44, and W51C). I also discuss how detections of CH3OH masers can be used along with other maser tracers, i.e. H2O masers, to pinpoint sights of SF near SNRs. Furthermore, I will discuss the close spatial and kinematic correlation of CH3OH masers in SNRs and bright ammonia (NH3 (3,3)) emission peaks and discuss how this relationship can be used to more accurately locate CH3OH masers near SNRs. Tighter constraints on the estimated physical conditions is achieved using modeling of the NH3 line in addition to the CH3OH modeling. By combining modeling and observations of different maser and molecular species, I aim to develop a comprehensive picture of the gas distribution in SNR/MC interaction regions.
Level of Degree
Physics & Astronomy
First Committee Member (Chair)
Second Committee Member
Third Committee Member
SNRs, Masers, Methanol, Ammonia, VLA, GBT
McEwen, Bridget. "Characterizing Supernova Remnant and Molecular Cloud Interaction Environments Using Class I Methanol Masers." (2016). http://digitalrepository.unm.edu/phyc_etds/41