VLA Observations of Carbon 91$\alpha$ Recombination Line Emission in W49 North

Abstract

We have detected C91$\alpha$ (8.5891 GHz) emission toward 4 ultra-compact \HII regions (\UCHII s; W49G, J, L & C) in the W49 North massive star forming region with the Very Large Array (VLA) at 3\arcsec resolution. No carbon line emission was detected toward \UCHII s W49F, A, O, S and Q at this frequency to a 3$\sigma$ level of 2 mJy. We also observed the same region in the C75$\alpha$ line (15.3 GHz) with no detection at a 3$\sigma$ level of 6 mJy with a 1\arcsec.7 beam. Detection of line emission toward these sources add supporting data to the earlier result of \nocite{retal05a}Roshi et al (2005a) that many \UCHII s have an associated photo-dissociation region (PDR). Similarity of the LSR velocities of carbon recombination lines and H$_2$CO absorption toward \UCHII s in W49 North suggests that the PDRs reside in the dense interface zone surrounding these \HII regions. Combining the observed carbon line parameters at 8.6 GHz with the upper limits on line emission at 15.3 GHz, we obtain constraints on the physical properties of the PDRs associated with W49G and J. The upper limit on the number density of hydrogen molecule obtained from carbon line models is $\sim$ $5 \times 10^6$ \cmthree.