Discovery of weak 6.7 GHz CH₃OH masers in a sample of high-mass Hi-GAL sources

Abstract

Context. Maser lines from different molecular species, including water, hydroxyl, and methanol, are common observational phenomena associated with massive star-forming regions. In particular, since its discovery, the 6.7 GHz methanol maser has been recognized as one of the clearest signposts in the formation of young high-mass stars. Aims. The methanol maser thus appears as an ideal tool for studying the early phases of massive star formation. However, it is difficult to establish the exact start of the methanol maser phase, and it would then be interesting to detect and study low flux-density methanol masers (i.e., 0.1 Jy or even 0.1 Jy), in order to determine if they can be used effectively to mark a specific evolutionary phase in high-mass star formation. Methods. Past surveys have been unable to systematically detect many low flux-density methanol masers, and thus we do not yet know how many such masers exist in the Galaxy and what their physical nature is. A large sample of massive cores can now be found in the Herschel infrared GALactic Plane Survey (Hi-GAL), which we have used to search for methanol and excited OH masers toward a sample of starless and protostellar high-mass clumps using the Arecibo telescope. Results. Out of a sample of 107 observed Hi-GAL sources, we detected a total of 37 methanol masers, with 22 sources being new and weak (median peak flux density 0.07 Jy) detections, in the Galactic longitude range [32.• 0, 59.• 8]. We also detected 12 6.035 GHz OH maser regions, with 9 new detections. Our survey covers a similar range of source distances to the "Arecibo Methanol Maser Galactic Plane Survey" (or AMGPS, ), but the methanol masers detected by us are clearly shifted toward lower integrated flux densities. Conclusions. The newly detected methanol masers mostly have low luminosity (assuming isotropic emission), and except for some sources, their weakness is not due to distance effects or positional offsets. No specific correlation is found with the physical parameters of the Hi-GAL clumps, except for sources with both CH 3 OH and OH masers, which tend to have higher mass and luminosity. The intensity of the methanol masers correlates well with the velocity range of the maser emission, which suggests that the low brightness of these masers is related to the number of maser spots in the emitting region and to their evolution with time.

Formaldehyde Masers: Exclusive Tracers of High-Mass Star Formation

“…Given that 6 cm H 2 CO masers are typically found toward regions where other Note. Positions and peak flux densities of 6.7 GHz CH 3 OH and 6.035 GHz OH masers are from Olmi et al (2014). The typical rms of the 6.035 GHz OH nondetection from Olmi et al (2014) was between ∼5 and 10 mJy.…”

“…Positions and peak flux densities of 6.7 GHz CH 3 OH and 6.035 GHz OH masers are from Olmi et al (2014). The typical rms of the 6.035 GHz OH nondetection from Olmi et al (2014) was between ∼5 and 10 mJy. molecular masers have previously been found (e.g., Araya et al 2008), our selection criteria consisted of Hi-GAL sources in the Arecibo sky with 6.7 GHz CH 3 OH maser detections from Olmi et al (2014).…”

Herschel-HIFI observations of H2O, NH3, and N2H+toward high-mass starless and protostellar clumps identified by the Hi-GAL survey

“…The methanol masers associated with sources l59-441/ l59-445 and l59-444 were found by comparing our observations with those carried out by Olmi et al (2014b) toward the = 30 • and = 59 • regions in search of 6.7 GHz CH 3 OH and 6.0 GHz OH masers. The masers associated with l59-441/ l59-445 and l59-444 are G59.63−0.19 and G59.78+0.63, respectively, and they have peak flux densities of 0.58 and 0.03 Jy.…”

“…The masers associated with l59-441/ l59-445 and l59-444 are G59.63−0.19 and G59.78+0.63, respectively, and they have peak flux densities of 0.58 and 0.03 Jy. As discussed by Olmi et al (2014b), these two methanol masers belong to a new class of low-brightness masers that could represent an earlier stage of evolution.…”

The shortest periodic and flaring flux variability of a methanol maser emission at 6.7 GHz in G 014.23−00.50

“…Long-term and frequent flux monitoring was conducted toward source samples only less than 20%, which was estimated from ∼200 sources (e.g., Goedhart et al 2004;Szymczak et al 2015;Maswanganye et al 2016) in more than 1000 methanol masers samples (e.g., Pestalozzi et al 2005;Xu et al 2009;Caswell et al 2010Caswell et al , 2011Green et al 2010Green et al , 2012bOlmi et al 2014;Sun et al 2014;Breen et al 2015, and references therein). Furthermore, most of the monitored sources have been selected simply by their peak flux densities > 5 Jy in Szymczak, Wolak, and Bartkiewicz (2015).…”