Pumping of the 4.8 GHz H₂CO masers and its implications for the periodic masers in G37.55+0.20

Abstract

Context. Periodic or regular flaring of class II CH 3 OH masers in nine high mass star forming regions is now a well established phenomenon. Amongst the nine star forming regions, G37.55+0.20 is the only case at present where apart from the presence of a periodic class II CH 3 OH maser, correlated flaring of another masing species, H 2 CO in this case, has been detected. Aims. We perform numerical calculations to investigate under which conditions the 1 10 −1 11 transition (4.8 GHz) of ortho-H 2 CO is inverted in order to address the question of the correlated flaring of the 6.7 GHz CH 3 OH and 4.8 GHz H 2 CO masers in G37.55+0.20. Methods. We developed a numerical code to study the population inversion of o-H 2 CO. Equilibrium solutions for the level populations are found by integrating the rate equations using Heun's method. Results. It is found that collisional excitation with H 2 as well as radiative excitation by the free-free radio continuum radiation from a nearby ultra-or hyper-compact H  region can invert the 1 10 −1 11 transition. It is also found that radiative excitation by the dust infrared radiation field does not lead to an inversion of the 1 10 −1 11 transition. The 2 11 −2 12 (14.5 GHz) and 3 12 −3 13 (28.9 GHz) transitions are inverted only in the presence of the free-free continuum radiation field of a very compact H  region. Conclusions. Due to the different pumping mechanisms of the H 2 CO and CH 3 OH masers it is unlikely that the near simultaneous flaring of the CH 3 OH and H 2 CO masers in G37.55+0.20 is due to changes in the pumping of the masers.

On the pumping of the CS(υ = 0) masers in W51 e2e

“…For the Orion hot core Chandler & Wood (1997) derived a CS abundance of 1.2× 10 −8 , which is only slightly higher than the maximum CS abundance for L134N. Similar abundances were also found for W43MM1, IRAS18264-1152, IRAS05358+3543, IRAS18162-2048 (Herpin et al 2009) and AFGL2591 (van der Tak et al 2003). Bergin et al (1997) derived CS abundances relative to CO of between 2 × 10 −5 and 1 × 10 −4 for M17 and Cepheus respectively, which gives CS abundances relative to H 2 of the order 10 −8 − 10 −9 using [CO]/[H 2 ] = 10 −4 (Kruegel 2003).…”

Formaldehyde Masers: Exclusive Tracers of High-Mass Star Formation

“…However, with the exception of some 6 cm H 2 CO maser regions in Sgr B2 (e.g., Mehringer et al 1994), most 6 cm H 2 CO masers do not seem to be directly associated with UC H II regions, but instead, they are associated with younger phases of high-mass star formation characterized by hot molecular core environments, hyper-compact H II regions and/or weak (or undetectable) radio continuum sources (e.g., Araya et al 2007aAraya et al , 2007cAraya et al , 2008. Even though a fully satisfactory theoretical explanation for H 2 CO masers has not been achieved yet, the current H 2 CO maser pumping models require special physical conditions in terms of density, abundance, coherent path length, and/or high emission measure from a background H II region (Boland & de Jong 1981;van der Walt 2014), thus the exclusive association of H 2 CO masers with high-mass star-forming regions is also expected from a theoretical perspective.…”

A Census of Early-phase High-mass Star Formation in the Central Molecular Zone

“…Third, pulsars, X-ray binaries, stellar winds from massive stars, and background active galactic nuclei (AGNs) can contribute to radio continuum emission. These targets are usually seen as point sources in X-ray emission, while UC H II regions may present diffuse X-ray emission ( 0.2 pc, 5 at the distance of the CMZ; e.g., Tsujimoto et al 2006) therefore can be distinguished. We cross match with Chandra Xray point source catalogs of Muno et al (2006Muno et al ( , 2009 and Zhu et al (2018) with a search radius of 1 , and find the following matches: C1, C5, C25, C41, C48, C51, C55, C60, C63, and C86.…”