Comparative chemistry of diffuse clouds V. Ammonia and formaldehyde

Using the VLA and NRAO 140′ telescopes we observed the λ1.3 cm (1, 1) and (2, 2) lines of NH3 in absorption and emission toward the compact extragalactic continuum sources NRAO150 (B0355+508) and 3C 111 (B0415+379). These sources are occulted by some seven local diffuse and translucent clouds showing molecular absorption in OH, CO, HCO and C2H: for the four features having NH3 absorption, we find rotational excitation temperatures 18–24 K, suggesting kinetic temperatures of at least 25–30 K. The abundance ratio N(NH3)/N(HCO) is comparable to values quoted for the cyanopolyne peak in TMC-1 (i.e., 2.5) in three of four cases where NH3 was seen. For clouds with higher column density N(HCO) >∼ 1012 cm−2, the NH3 column density N(NH3) is well correlated only with N(CS) (〈N(NH3)/N(CS)〉 ≈ 1.0) and N(H2CO) (〈N(NH3)/N(H2CO)〉 ≈ 0.4). N(H2CO) is well correlated with N(NH3) and N(CS) (〈N(H2CO)/N(CS)〉 ≈ 2.3) and the H2CO abundance relative to other species is like TMC-1, with (〈N(H2CO)/N(HCO+)〉 ≈ 2.3).