High-resolution studies of the C-band and L-band radio continuum of the periodic methanol maser source G9.62+0.20E and surrounding sources

ABSTRACT High-resolution observations of the C-band and L-band radio continuum of the hypercompact H ii region $\rm G 9.62$+$\rm 0.20 E$ conducted with the e-MERLIN interferometric array are presented. A flux density of $\rm 2.25 \pm 0.20$ $\rm mJy$ and an angular size of $\rm 127 \pm 22$ $\rm mas$ were measured for the C-band continuum. At L band, an upper limit of $\rm 162$ $\rm \mu Jy$ on the integrated flux density was calculated at the continuum’s position. The surrounding sources C and D were detected at levels consistent with previous detections. The results for source E requires a steeper spectrum at L band than previous observations extrapolated from higher frequencies. This can be explained with a truncated inverse square law density distribution model. We obtain a C-band peak brightness temperature of about $\rm {4000\, \rm {K}}$, which assuming an electron temperature of $\rm {10^{4}}$$\rm {K}$ translates to an optical depth of $\rm {0.5}$, indicating the continuum is optically thin at C band going thick at L band. These results therefore place firm constraints on H ii region models in source E. The periodic variability of methanol masers, as seen in $\rm G 9.62$+$\rm 0.20 E$, has been explained with the pulsating star, accretion disc, and colliding wind binary models. Some models predict a hypercompact H ii region can provide pumping for methanol masers. In the colliding wind binary model context maser variability is attributed to seed photon modulation. Hence, future H ii models matching our observations could test predictions of these models in terms of the variability profiles of the $\rm {1.6 \, \rm {GHz}}$$\rm {OH}$ and $\rm {6.7 \, \rm {GHz} }$ methanol masers.