Anomalous HCN emission from warm giant molecular clouds

Abstract

Hydrogen cyanide (HCN) is considered a good tracer of the dense molecular gas that serves as fuel for star formation. However, recent large-scale surveys of giant molecular clouds (GMCs) have detected extended HCN rotational line emission far from star-forming cores. Such observations often spectroscopically resolve the HCN J = 1–0 (partially also the J = 2–1 and 3–2) hyperfine structure (HFS). A precise determination of the physical conditions of the gas requires treating the HFS line overlap effects. Here, we study the HCN HFS excitation and line emission using nonlocal radiative transfer models that include line overlaps and new HFS-resolved collisional rate coefficients for inelastic collisions of HCN with both para-H2 and ortho-H2 (computed via the scaled-infinite order sudden approximation up to Tk = 500 K). In addition, we account for the role of electron collisions in the HFS level excitation. We find that line overlap and opacity effects frequently produce anomalous HCN J = 1–0 HFS line intensity ratios (i.e., inconsistent with the common assumption of the same Tex for all HFS lines) as well as anomalous HFS line width ratios. Line overlap and electron collisions also enhance the excitation of the higher J rotational lines. Our models explain the anomalous HCN J = 1–0 HFS spectra observed in the Orion Bar and Horsehead photodissociation regions. As shown in previous studies, electron excitation becomes important for molecular gas with H2 densities below a few 105 cm−3 and electron abundances above ~10−5. We find that when electron collisions are dominant, the relative intensities of the HCN J = 1–0 HFS lines can be anomalous too. In particular, electron excitation can produce low-surface-brightness HCN emission from very extended but low-density gas in GMCs. The existence of such a widespread HCN emission component may affect the interpretation of the extragalactic relationship HCN luminosity versus star-formation rate. Alternatively, extended HCN emission may arise from dense star-forming cores and become resonantly scattered by large envelopes of lower density gas. There are two scenarios – namely, electron-assisted (weakly) collisionally excited versus scattering – that lead to different HCN J = 1–0 HFS intensity ratios, which can be tested on the basis of observations.

The Dense Gas Mass Fraction and the Relationship to Star Formation in M51

“…In addition, Goldsmith & Kauffmann (2017) demonstrate that in low-density environments with high ionization fractions, such as envelopes of molecular clouds or in the central regions of galaxies, electron collisions can also impact the HCN excitation, which can further decrease the effective critical density of the HCN J = 1 -0 line. Goicoechea et al (2022) find similar results when electron collisions dominate the excitation of the HCN molecules. For this study, we adopt the value of ¢ = n 10 4 cm −3 for the HCN J = 1 -0 transition that assumes a mixture of optically thin and thick emission and no electron excitation.…”

Hyperfine collisional excitation of ammonia by molecular hydrogen

“…At the other end, statistical rate coefficients can be immediately obtained provided the rate coefficients for pure rotational excitation are available. An intermediate approach is the IOS method, which has been used with some success for the hyperfine excitation of linear molecules by H 2 (Goicoechea et al 2022). Extending the IOS method and recoupling theory to asymmetric polyatomic molecules would be beneficial in order to generate sets of hyperfine-resolved rate coefficients for other molecules such as deuterated ammonia (NH 2 D, NHD 2 ) or H 2 CO.…”

BASECOL2023 scientific content

“…Using those highly accurate rate coefficients and an IOS recoupling method, Goicoechea et al (2022) calculated the hyperfine resolved rate coefficients for HCN in collision with o/p-H 2 , the two corresponding datasets (34 levels; T = 5-500 K; Goicoechea et al 2022) are in BASECOL2023. Those datasets supersede the hyperfine HCN-p-H 2 dataset by Ben Abdallah et al ( 2012) calculated with a simpler PES averaged over three orientations of H 2 .…”