H₂CO and CS in diffuse clouds: Excitation and abundance

¹ LERMA, Observatoire de Paris, PSL University, CNRS, Sorbonne Université, 75014 Paris, France
e-mail: maryvonne.gerin@obspm.fr
² National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
³ IRAM, 300 rue de la Piscine, 38406 Saint Martin d’Hères, France
⁴ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France

Received: 3 January 2024
Accepted: 6 March 2024

Abstract

Context. Diffuse interstellar clouds present an active chemistry despite their relatively low density and the ubiquitous presence of far-UV radiation.

Aims. To provide constraints on the chemical processes responsible for the observed columns of organic species, we used the NOEMA interferometer to observe the sight line toward NRAO150 (B0355+508) in the 2 mm spectral window.

Methods. We targeted the low excitation lines of ortho H₂ CO (2_1,1–1_1,0) and para H₂ CO (2_0,2–1_0,1) as well as the nearby transitions of CS (3–2) and c-C₃H₂ (3_1,2–2_2,1), (4_1,4–3_0,3), and (2_2,0–1_1,1). We combined these data with previous observations of the same sight line to determine the excitation conditions, column densities, and abundances relative to H₂ in the different velocity components. We performed non-LTE radiative transfer calculations including collision cross sections with ortho and para H₂ and with electrons. New collision cross sections with electrons were computed for ortho and para formaldehyde.

Results. All targeted lines were detected. The c-C₃H₂ line profiles are very similar to those of HCO⁺ and CCH, while the CS absorption features are narrower and mostly concentrated in two main velocity components at V_LSR = −17.2 and −10.4 km s⁻¹. H₂CO absorption lines present an intermediate pattern with absorption in all velocity components but larger opacities in the two main velocity components. The ortho-to-para ratios of H₂CO and c-C₃H₂ are consistent with the statistical value of three. While the excitation temperature of all c-C₃H₂ velocity components is consistent with the Cosmic Microwave Background (CMB), the two strong components detected in CS show a clear excess over the CMB indicating that CS resides at higher densities than other species along this particular sightline, n(H₂) ~ 2500 cm⁻³ while n(H₂) < 500 cm⁻³ for the other velocity components. We detected faint absorption from o-H₂¹³CO and C³⁴S allowing us to derive isotopic ratios: o-H₂CO/o-H₂¹³CO = 61 ± 12 and C³²S/C³⁴S = 24 ± 6. The excitation of the 1_1,0−1_1,1 line of formaldehyde at 4.8 GHz is sensitive to the electron fraction and its excitation temperature is predicted to be lower than the CMB at low and moderate electron fractions (x(e) < 6 × 10⁻⁵), and to rise above the CMB at high electron fractions (x(e) > 10⁻⁴).