2001AJ....121..391D

Absorption in the J(K_–,K₊)=2₁₁-2₁₂ transition of formaldehyde at 2 cm toward the ultracompact H II regions C1 and C2 of W58 has been observed with the Very Large Array with an angular resolution of ∼0".2 and a velocity resolution of ∼1 km.s^–1. The high-resolution continuum image of C1 (also known as ON) shows a partial shell that opens to the northeast. Strong H₂CO absorption is observed against W58C1. The highest optical depth (τ>2) occurs in the southwest portion of C1 near the edge of the shell, close to the continuum peak. The absorption is weaker toward the nearby, more diffuse compact H II region C2, τ≤0.3. The H₂CO velocity (-21.2 km.s^–1) toward C1 is constant and agrees with the velocity of CO emission, main-line OH masers, and the H76α recombination line, but differs from the velocity of the 1720 MHz OH maser emission (~-13 km.s^–1). Observations of the absorption in the J(K_–,K₊)=1₁₀-1₁₁ transition of formaldehyde at 6 cm toward W58C1 and C2 carried out earlier with the Westerbork Aperture Synthesis Telescope at lower resolution (∼4''x7'') show comparable optical depths and velocities to those observed at 2 cm. Based on the mean optical depth profiles at 6 and 2 cm, the volume density of molecular hydrogen n(H₂) and the formaldehyde column density N(H₂CO) were determined. The n(H₂) is ∼6x10⁴ cm^–3 toward C1. N(H₂CO) for C1 is ∼8x10¹⁴ cm^–2, while that toward C2 is ∼8x10¹³ cm^–2.