Simultaneous 22 GHz Water and 44 GHz Methanol Maser Survey of Ultracompact H ii Regions

Abstract

We have carried out a multi-epoch simultaneous survey of 22 GHz H$_2$O and 44 GHz Class I CH$_3$OH masers toward 103 ultracompact HII regions (UCHIIs) between 2010 and 2017. We detected H$_2$O and CH$_3$OH maser emission in 74 (72%) and 55 (53%) UCHIIs, respectively. Among them, 3 H$_2$O and 27 CH$_3$OH maser sources are new detections. These high detection rates suggest that the occurrence periods of both maser species are significantly overlapped with the UCHII phase of massive star formation. The CH$_3$OH maser lines always have small (< 10 km s$^{-1}$) relative velocities with respect to the natal molecular cores, while H$_2$O maser lines often show larger relative velocities. Twenty four H$_2$O maser-detected sources have maser lines at relative velocities > 30 km s$^{-1}$, and thirteen of them show extremely high-velocity (> 50 km s$^{-1}$) components. The appearance and disappearance of H$_2$O maser lines are quite frequent over six-month or one-year time intervals. In contrast, CH$_3$OH maser lines usually do not exhibit significant variations in the line intensity, velocity, or shape for the same periods. The isotropic luminosities of both masers appear to correlate with the bolometric luminosities of the central stars. This correlation becomes much stronger in the case that data points in the low- and intermediate-mass regimes are added. The maser luminosities also tend to increase with the radio continuum luminosities of UCHIIs and the submillimeter continuum luminosities of the associated dense cores.