Abstract
HCl is supposed to be one of the main chlorine carriers in the interstellar medium (ISM). Then, accurate knowledge of chlorine chemistry requires accurate estimating the HCl abundance in molecularcloudswhichinturnrequiresthecalculationofcollisionalexcitationratecoefficients for the HCl molecule due to collisions with the most abundant collisional partner in the ISM. In this paper, we report theoretical calculations of the HCl–H2 rotationally inelastic rate coefficients. Using a recently developed potential energy surface, we have computed rate coefficients between the first 11 rotational levels of HCl for temperatures ranging from 5 to 300 K. These new HCl–H2 rate coefficients were compared to the available HCl–He rate coefficients currently used for astrophysical modelling. As one would expect, significant differences were found between new HCl–H2 and previous HCl–He rate coefficients. As a first application, we simulate the excitation of HCl in typical star-forming regions and in protostellar shocks. Electron-impact excitation is also included. It is found that the new H2 rate coefficients significantly increase the simulated line intensities. As a consequence, HCl abundance derived from the observations will be significantly reduced by the use of the present rate coefficients, confirming that HCl may not be the main chlorine carrier in the ISM.
Highlights
Chlorine has two stable isotopes, 35Cl and 37Cl, and a relatively low solar abundance of 3.2× 10−7 relative to atomic hydrogen (Asplund et al 2009)
Even if ortho-H2 is significantly less abundant than para-H2(j2 = 0) in the cold interstellar medium (ISM), the magnitude of the ortho-H2 rate coefficients make that this collisional partner has to be included as soon as the ortho-to-para ratio (OPR) of H2 is over 1/10
HCl–He rate coefficients are not recommended for modelling HCl emission from molecular clouds
Summary
Chlorine has two stable isotopes, 35Cl and 37Cl, and a relatively low solar abundance of 3.2× 10−7 relative to atomic hydrogen (Asplund et al 2009). HCl emission of the two isotopologues, H35Cl and H37Cl, was observed using HIFI-Herschel observations towards the carbon-rich star IRC+10216 (Cernicharo et al 2010b; Agundez et al 2011), where lines from j = 1−0 up to j = 7−6 were detected, towards the protostellar shock L1157-B1 (Codella et al 2012) and the star-forming region W3A (Cernicharo et al 2010a) These new observations have provided new insight into the chlorine chemistry in dense and warm (T > 150 K) regions. HCl infrared absorption lines have been detected in the warm circumstellar environment of the protostar CRL 2136, with a fractional abundance of 1−2 × 10−7 relative to H2 (Goto et al 2013) This value corresponds to approximately 20 per cent of the elemental chlorine abundance, as predicted by the models of dense molecular clouds.
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