CLASS I METHANOL (CH₃OH) MASER CONDITIONS NEAR SUPERNOVA REMNANTS

Abstract

We present results from calculations of the physical conditions necessary for the occurrence of 36.169 (4 −1 − 3 0 E), 44.070 (7 0 − 6 1 A + ), 84.521 (5 −1 − 4 0 E), and 95.169 (8 0 − 7 1 A + ) GHz methanol (CH 3 OH) maser emission lines near supernova remnants (SNRs), using the MOLPOP-CEP program. The calculations show that given a sufficient methanol abundance, methanol maser emission arises over a wide range of densities and temperatures, with optimal conditions at n ∼ 10 4 − 10 6 cm −3 and T > 60 K. The 36 GHz and 44 GHz transitions display more significant maser optical depths compared to the 84 GHz and 95 GHz transitions over the majority of physical conditions. It is also shown that line ratios are an important and applicable probe of the gas conditions. The line ratio changes are largely a result of the E-type transitions becoming quenched faster at increasing densities. The modeling results are discussed using recent observations of CH 3 OH and hydroxyl (OH) masers near the supernova remnants G1.4−0.1, W28, and Sgr A East.

A New 95 GHz Methanol Maser Catalog. I. Data

“…Class I methanol masers, on the other hand, are often found offset by ∼1′ (∼0.1-1 pc) from other maser species, infrared sources, etc. The collisional pumping of class I methanol masers is not only suggested by their association with astrophysical shocks but also supported by theoretical predictions (Cragg et al 1992;Menten 1996;McEwen et al 2014;Leurini et al 2016).…”

44 GHZ CLASS I METHANOL (CH3OH) MASER SURVEY IN THE GALACTIC CENTER

“…This means they are most likely formed in different regions in the shocked gas Pihlström et al 2014;McEwen et al 2014). The majority of the 44 GHz masers in the NE region have slightly lower velocity spread compared to the OH masers, which have an average of ∼ 57 km s −1 , and are located just to the SW of the group of 44 GHz CH 3 OH masers (in the rectangle region).…”

“…The lowest flux density ratios between these two maser species in this region is ∼5. Based on modeling results from McEwen et al (2014), this implies a high density region (n > 10 6 cm −3 ). In addition, the high concentration of masers in this region is spatially coincident with strong SiO (2 − 1) emission (between 20 and 50 km s −1 ), as can be seen in Yusef-Zadeh et al (2013), which is also indicative of high density shocked gas.…”

“…are the optimal masing conditions for the 36 GHz transition (McEwen et al 2014;Nesterenok 2016). Because of the large overlap in conditions, these transitions can be found co-spatially, but brighter 36 GHz CH 3 OH masers are expected to trace higher density regions (e.g., near the actual shock front in SNR/MC interaction regions).…”

A 95 GHz methanol emission survey toward eight small supernova remnants

“…Therefore, the presence of γ-ray emission such as at Gev or Tev energies marks a possibility of enhanced abundance of CH 3 OH (such as at 36, 44 and 95 GHz), and hence may raise the possibility of detection. Although the excitation conditions for these three masers are different, all three maser emission outputs are presented over a wide range of environments (see e.g., McEwen et al, 2014;Leurini et al, 2016). In order to expect expect a high detection rate and save time, the selection criteria were that the SNRs were small ( 10 ′ ) and associated with class I methanol 36/44 GHz maser or GeV/TeV emission.…”