44 GHz Methanol Masers and Quasi-Thermal Emission in Sagittarius B2

The Very Large Array has been used to obtain ~3'' resolution observations on the 44 GHz 7₀ → 6₁A⁺ transition of CH₃OH (methanol) in the Sgr B2 massive star-forming complex. A total of 18 compact regions showing maser emission are found, which are spread over 2.1 pc × 4.3 pc (α × δ) region. Many of these are offset far from known molecular cores and ultracompact H II regions and may trace the interaction region of a cloud-cloud collision. There is no spatial coincidence between 44 GHz and 6.7 GHz CH₃OH masers in this region, as expected, because the pumping mechanisms for these two transitions are different. Isotropic maser luminosities range between 1 × 10^-6 and 2.1 × 10^-5 L_☉. In addition, 17 regions with broad line width quasi-thermal 44 GHz CH₃OH emission are identified, many of which are close to known molecular hot cores, in particular those associated with the Sgr B2(N) and Sgr B2(M) continuum sources. In Sgr B2(N), quasi-thermal emission appears to be associated with two 10'' diameter ionized shells. These ionized shells may have swept up and shocked molecular material as they expanded. Also, a quasi-thermal core is observed to be coincident with a source of continuum emission from dust and emission from more complex species. In Sgr B2(M), CH₃OH quasi-thermal emission arises predominantly from the western portion of this region. The CH₃OH fractional abundance in most of the quasi-thermal cores appears to be quite high at ~10^-6. It is argued that grain-surface chemistry is responsible for this high abundance.