How do methanol masers manage to appear in the youngest star vicinities and isolated molecular clumps?

Abstract

Abstract. General characteristics of methanol (CH3 OH) maser emission are summarized. It is shown that methanol maser sources are concentrated in the spiral arms. Most of the methanol maser sources from the Perseus arm are associated with embedded stellar clusters and a considerable portion is situated close to compact Hii regions. Almost 1/3 of the Perseus Arm sources lie at the edges of optically identified Hii regions which means that massive star formation in the Perseus Arm is to a great extent triggered by local phenomena. A multiline analysis of the methanol masers allows us to determine the physical parameters in the regions of maser formation. Maser modelling shows that class II methanol masers can be pumped by the radiation of the warm dust as well as by free-free emission of a hypercompact region (hcHii) with a turnover frequency exceeding 100 GHz. Methanol masers of both classes can reside in the vicinity of hcHiis. Modelling shows that periodic changes of maser fluxes can be reproduced by variations of the dust temperature by a few percent which may be caused by variations in the brightness of the central young stellar object reflecting the character of the accretion process. Sensitive observations have shown that the masers with low flux densities can still have considerable amplification factors. The analysis of class I maser surveys allows us to identify four distinct regimes that differ by the series of their brightest lines.

Gas kinematics in high-mass star-forming regions from the Perseus spiral arm

“…Maser sources at 6.7 GHz belong to the Class II type. Class I methanol masers are collisionally excited and are associated with places where molecular outflows interact with the surrounding medium, Sobolev et al (2007). Radiatively pumped Class II masers are found in less violent regions close to young stellar objects (see observational evidences in recent papers by Hu et al (2016); Chibueze et al (2017) and theoretical investigations by Cragg et al (2005); Sobolev et al (2005)).…”

“…It is natural to expect that radial velocities of Class II masers should be close to the system velocity of molecular gas in regions of star formation. Hopefully it is possible to use the radial velocities of masers for estimation of kinematic distances in statistical studies, Pestalozzi et al (2005); Xu et al (2009) ;Sobolev et al (2007). For example, Sobolev et al (2005) show that there are remarkable peaks on the dependence "number of maser sources -distance from the Galactic center".…”

“…van der Walt et al (2007). They compared velocities of CS (2-1) and methanol maser emission at 6.7 GHz for 337 maser features in 63 sources.…”

Class I Methanol Masers in the Galactic Center

“…The number density and temperature required for 36 GHz methanol masers are near those modeled for 1720 MHz OH masers, with n ∼ 10 4 − 10 5 cm −3 and T < 100 K (Morimoto, Kanzawa & Ohishi 1985, Cragg et al 1992, Liechti & Wilson 1996. At least in SFRs, higher densities and temperatures, n ∼ 10 5 − 10 6 cm −3 and T = 80 − 200 K, the Class I 44 GHz line will have optimized maser output, while the 36 GHz maser eventually becomes quenched (Pratap et al 2008, Sobolev et al 2005, Sobolev et al 2007). These methanol masers may therefore constrain the density in the shocked SNR regions.…”

Procura de Estrelas de Alta Massa em Formação

“…Emissões maser de metanol de Classe II (detectadas em 6.7 e 12 GHz) são coincidentes com a posição de núcleos protoestelares, regiões UCH ii e fontes que também apresentam emissão maser de OH. Esse tipo de emissão maser ocorre via bombeamento radiativo (Sobolev et al, 2007;Fontani et al, 2010 …”