Abstract
Abstract We report the detection of the strongest 6 cm H2CO emission in our Galaxy. The detection toward the massive star-forming region G339.88-1.26 was made using the Shanghai Tianma radio telescope (TMRT). The G339.88-1.26 star formation region hosts one of the strongest 6.7 GHz methanol masers and has an accompanying collimated, ionized jet seen in radio continuum free–free emission. The peak flux density of the detected H2CO emission is ∼19 Jy, one order of magnitude stronger than the nine previously known H2CO maser sources. The corresponding luminosity is also brighter than has been observed in previous H2CO maser sources, even those detected in the Central Molecular Zone of our Galaxy. A TMRT on-the-fly map of the region shows a point-like source structure and this, combined with the spectral characteristics of the H2CO emission (multiple, narrow components), leads us to conclude that the detected H2CO emission is masing (with a brightness temperature in excess of 104 K). The detection of a very strong H2CO maser in G339.88-1.26 suggests that ionized jet/outflow environments might provide efficient maser pumping for this transition, with the ionized jet/outflow-driven shock causing collisional excitation, and are also able to provide additional seed photons for maser amplification of the radio continuum emission.
Highlights
A number of surveys for H2CO masers in the 101–111 6 cm transition have been made toward massive young stellar objects (MYSOs; e.g., Araya et al 2004, 2007b, 2008) and evolved stars (Araya et al 2015)
We found that the H2CO emission spectrum was the same when quotient spectra were produced using either of the two different off-positions, demonstrating that there is no significant contribution to the spectrum from any background H2CO absorptions at the off-position locations
Compared with the H2CO masers in the Central Molecular Zone (CMZ) of the Milky Way, associated with Sgr B2 (Hoffman et al 2007) and Cloud C (Ginsburg et al 2015), we find that the luminosity of the H2CO emission in G339.88-1.26 is similar to the brightest emission in the CMZ
Summary
A number of surveys for H2CO masers in the 101–111 6 cm transition have been made toward massive young stellar objects (MYSOs; e.g., Araya et al 2004, 2007b, 2008) and evolved stars (Araya et al 2015). A model with radiation caused by radio continuum from a background compact H II region with emission measure greater than 108 pc cm−6 can explain the 6 cm H2CO maser detected toward NGC 7538 (Boland & de Jong 1981). The detection is from the massive star-forming region G339.88-1.26, which is one of the strongest (1520 Jy; Caswell et al 2011) 6.7 GHz methanol masers This source has been well studied in a range of maser and thermal molecular transitions (e.g., Norris et al 1993; de Buizer et al 2002; Ellingsen et al 2004, 2011) and in the continuum emission at radio and infrared bands (e.g., Ellingsen et al 1996, 2005; Purser et al 2016).
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