Long-term Variability of H₂CO Masers in Star-forming Regions

Abstract

We present results of a multi-epoch monitoring program on variability of 6 cm formaldehyde (H 2 CO) masers in the massive star forming region NGC 7538 IRS 1 from 2008 to 2015 conducted with the GBT, WSRT, and VLA. We found that the similar variability behaviors of the two formaldehyde maser velocity components in NGC 7538 IRS 1 (which was pointed out by Araya and collaborators in 2007) have continued. The possibility that the variability is caused by changes in the maser amplification path in regions with similar morphology and kinematics is discussed. We also observed 12.2 GHz methanol and 22.2 GHz water masers toward NGC 7538 IRS 1. The brightest maser components of CH 3 OH and H 2 O species show a decrease in flux density as a function of time. The brightest H 2 CO maser component also shows a decrease in flux density and has a similar LSR velocity to the brightest H 2 O and 12.2 GHz CH 3 OH masers. The line parameters of radio recombination lines and the 20.17 and 20.97 GHz CH 3 OH transitions in NGC 7538 IRS 1 are also reported. In addition, we observed five other 6 cm formaldehyde maser regions. We found no evidence of significant variability of the 6 cm masers in these regions with respect to previous observations, the only possible exception being the maser in G29.96−0.02. All six sources were also observed in the H 13 2 CO isotopologue transition of the 6 cm H 2 CO line; H

EVN observations of 6.7 GHz methanol maser polarization in massive star-forming regions

“…G25.83−0.18 is a very young SFR in an evolutionary stage prior to the UC H ii region phase at a kinematic distance of 5.0 ± 0.3 kpc (Araya et al 2008;Andreev et al 2017). Both 6.7 GHz and 12 GHz CH 3 OH maser emissions were detected (e.g., Walsh et al 1998;Błaszkiewicz & Kus 2004;Breen et al 2015Breen et al , 2016, which are ∼2 north of the 4.8 GHz H 2 CO maser detected at the center of an IR dark cloud (Araya et al 2008).…”

Thermal Formaldehyde Emission in NGC7538 IRS1

“…Using the 100 m Robert C. Byrd Green Bank Telescope (GBT), we detected 2 cm H 2 CO emission toward NGC 7538 IRS 1 (Andreev et al 2017). This emission was independently detected by Chen et al (2017a).…”

“…This emission was independently detected by Chen et al (2017a). Given the non-detection of a 2 cm maser in interferometric observations (Hoffman et al 2003), Chen et al (2017a) argued that the 2 cm H 2 CO emission in NGC 7538 IRS 1 is a variable maser, noting that variability has been detected in the 6 cm H 2 CO maser in the same source (e.g., Andreev et al 2017).…”

“…While thermal emission of high-frequency formaldehyde tran-transition (e.g., Chen et al 2017b, Ginsburg et al 2015, Araya et al 2007b, Hoffman et al 2003 2 . Currently, 10 Galactic regions are know to harbor 6 cm masers (Andreev et al 2017;Chen et al 2017b), one of them is NGC 7538 -the region where 6 cm H 2 CO masers were first confirmed (see review by Araya et al 2007b). NGC 7538 is an active site of high-mass star formation located at a distance of 2.65±0.12 kpc (Moscadelli et al 2009).…”

Studies of stellar evolution using masers

“…Also the 12.2 GHz methanol and 22.2 GHz water masers were observed, with a noticeable decrease in the flux. The variability may be caused by changes in the maser amplification path in regions with similar morphology and kinematics [1].…”