Abstract
Isocyanic acid (HNCO), the most stable of the simplest molecules containing the four main elements essential for organic chemistry, has been observed in several astrophysical environments such as molecular clouds, star-forming regions, external galaxies and comets. In this work, we model HNCO spectral line profiles toward the low-mass solar type protostar IRAS 16293$-2$422 observed with the ALMA interferometer, the IRAM, JCMT and APEX single-dish radio telescopes, and the HIFI instrument on board the Herschel Space Observatory. In star-forming environments, the HNCO emission is not always in Local Thermodynamical Equilibrium (LTE). A non-LTE radiative transfer approach is necessary to properly interpret the line profiles, and accurate collisional rate coefficients are needed. Here, we used the RADEX package with a completely new set of collisional quenching rates between HNCO and both ortho-H$_2$ and para-H$_2$ obtained from quantum chemical calculations yielding a novel potential energy surface in the rigid rotor approximation. We find that the lines profiles toward IRAS 16293$-$2422 are very well reproduced if we assume that the HNCO emission arises from a compact, dense and hot physical component associated with the hot corino, a warm component associated with the internal part of the protostellar envelope, and a cold and more extended component associated with the outer envelope. The derived HNCO abundances from our model agree well with those computed with the Nautilus chemical code.
Highlights
Towards molecular clouds in the direction of the Galactic centre (e.g. Turner 1991; Martın et al 2008), and revealed its presence in vari
To identify and model the relevant HNCO spectral lines, we used CASSIS4 (Caux et al 2011), a software developed at IRAPUPS/CNRS which makes use of the Cologne Data base for Molecular Spectroscopy3 (CDMS) data base (Muller et al 2001, 2005). 35 transitions are present in the IRAM observations, 16 in the Atacama Pathfinder Experiment (APEX) ones, and 6 in the James Clerk Maxwell Telescope (JCMT) ones
3.3 Model fitting In CASSIS, it is possible to model with RADEX an observed spectrum with a set of physical components, each of them defined with six physical parameters that will serve as input for RADEX: the size of the component, its density n(H2), its kinetic temperature Tkin, the Full Width at Half Maximum (FWHM) and the velocity relative to the Local Standard of Rest VLSR of the lines, and the column density N of the studied species
Summary
Towards molecular clouds in the direction of the Galactic centre (e.g. Turner 1991; Martın et al 2008), and revealed its presence in vari-. Fedoseev et al 2015; Noble et al 2015) Regardless, both gas phase and grain surface chemistry need to be taken into account to correctly model the observed abundances in astrophysical sources of HNCO and other related molecules of potential exobiological interest. HNCO has been detected towards the compact sources A and B (e.g. Bisschop et al 2008) as well as in the surrounding large-scale envelope (e.g. van Dishoeck et al 1995) This makes I16293 an ideal target to study the chemistry of HNCO at multiple scales in a low-mass star-forming environment (e.g. Bisschop et al 2008; Marcelino et al 2010).
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
