VLA observations of carbon 91α recombination line emission in W49 north

Abstract

We have detected C91α (8.5891 GHz) emission toward four ultracompact H II regions (UCHs; W49G, J, L, and C) in the W49 North massive star-forming region with the Very Large Array (VLA) at 3" resolution. No carbon line emission was detected toward UCHs W49F, A, O, S, and Q at this frequency to a 3 σ level of 2 mJy. We also observed the same region in the C75α line (15.3 GHz) with no detection at a 3 σ level of 6 mJy with a 1".7 beam. Detection of line emission toward these sources add supporting data to the 2005 result of Roshi et al. that many UCHs have an associated photodissociation region (PDR). The similarity of the LSR velocities of carbon recombination lines and H₂CO absorption toward UCHs in W49 North suggests that the PDRs reside in the dense interface zone surrounding these H II 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 ~5 × 10⁶ cm^-3.