WSRT and VLA observations of the 6 centimeter and 2 centimeter lines of H2CO in the direction of W58C1 (on 3) and W58C2

Absorption in the J(K-,K+) = 2(11)-2(12) transition of formaldehyde at 2 cm toward the ultracompact H II regions C1 and C2 of W58 has been observed wi th the Very Large Array with an angular resolution of similar to0."2 and a velocity resolution of similar to1 km s(-1). The high-resolution continuum image of C1 (also known as ON) shows a partial shell that opens to the nort heast. Strong H2CO absorption is observed against W58C1. The highest optica l depth (tau > 2) occurs in the southwest portion of C1 near the edge of th e shell, close to the continuum peak. The absorption is weaker toward the n earby, more diffuse compact H II region C2, tau less than or equal to 0.3. The H2CO velocity (-21.2 km s(-1)) toward C1 is constant and agrees with th e velocity of CO emission, main-line OH masers, and the H76 alpha recombina tion line, but differs from the velocity of the 1720 MHz OH maser emission (similar to -13 km s(-1)). Observations of the absorption in the J(K-,K+) = 1(10)-1(11) transition of formaldehyde at 6 cm toward W58C1 and C2 carried out earlier with the Westerbork Aperture Synthesis Telescope at lower reso lution (similar to4 " x 7 ") show comparable optical depths and velocities to those observed at 2 cm. Based on the mean optical depth profiles at 6 an d 2 cm, the volume density of molecular hydrogen n(H-2) and the formaldehyd e column density N(H2CO) were determined. The n(H-2) is similar to6 x 10(4) cm(-3) toward C1. N(H2CO) for C1 is similar to8 x 10(14) cm(-2), while tha t toward C2 is similar to8 x 10(13) cm(-2).