44 GHz methanol masers and quasi-thermal emission in Sagittarius B2

The Very Large Array has been used to obtain approximate to 3'' resolution observations on the 44 GHz (7)0-->6(1)A(+) 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 x 4.3 pc (alpha x delta) region. Many of these are offset far from known molecular cores and ultracompact H II regions and may trace the interaction region of 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 x 10(-6) and 2.1 x 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 similar to 10(-6). It is argued that grain-surface chemistry is responsible for this high abundance.