Comparative chemistry of diffuse clouds

V. Ammonia and formaldehyde

¹ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA, 22903-2475, USA e-mail: hliszt@nrao.edu
² Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 Saint Martin d'Hères, France
³ Obs. de Paris, 61 Av. de l'Observatoire, 75014 Paris, France

Accepted: 18 August 2005

Abstract

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 cyanopolyyne 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⁺) , the NH₃ column density N(NH₃) is well correlated only with N(CS) ((NH₃)/N(CS)) and N(H₂CO) ((NH₃)/N(H₂CO)). N(H₂CO) is well correlated with N(NH₃) and N(CS) (N(H₂CO)/N(CS)) and the H₂CO abundance relative to other species is like TMC-1, with (N(H₂CO)/N(HCO⁺)).