Long-term Variability of H${}_2$CO Masers in Star-forming Regions

Abstract

We present results of a multi-epoch monitoring program on variability of 6cm formaldehyde (H${}_2$CO) masers in the massive star forming region NGC7538IRS1 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 NGC7538IRS1 (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 12.2GHz methanol and 22.2GHz water masers toward NGC7538IRS1. 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 12.2GHz CH${}_3$OH masers. The line parameters of radio recombination lines and the 20.17 and 20.97GHz CH${}_3$OH transitions in NGC7538IRS1 are also reported. In addition, we observed five other 6cm formaldehyde maser regions. We found no evidence of significant variability of the 6cm masers in these regions with respect to previous observations, the only possible exception being the maser in G29.96$-$0.02. All six sources were also observed in the H${}_2^{13}$CO isotopologue transition of the 6cm H${}_2$CO line; H${}_2^{13}$CO absorption was detected in five of the sources. Estimated column density ratios [H${}_2^{12}$CO]/[H${}_2^{13}$CO] are reported.

29 pages, 9 figures