Abstract
Methyl cyanide (CH3CN) and propyne (CH3CCH) are two molecules commonly used as gas thermometers for interstellar gas. They are detected in several astrophysical environments and in particular towards protostars. Using data of the low-mass protostar IRAS 16293-2422 obtained with the IRAM 30m single-dish telescope, we constrained the origin of these two molecules in the envelope of the source. The line shape comparison and the results of a radiative transfer analysis both indicate that the emission of CH3CN arises from a warmer and inner region of the envelope than the CH3CCH emission. We compare the observational results with the predictions of a gas-grain chemical model. Our model predicts a peak abundance of CH3CCH in the gas-phase in the outer part of the envelope, at around 2000 au from the central star, which is relatively close to the emission size derived from the observations. The predicted CH3CN abundance only rises at the radius where the grain mantle ices evaporate, with an abundance similar to the one derived from the observations.
Highlights
Star-forming regions are ideal places for the development of the chemical complexity in the interstellar medium
We present a consistent analysis of CH3CCH and CH3CN
The computed excitation temperatures are different for these two molecules and much higher for CH3CN (75 K) as compared to CH3CCH (25 K)
Summary
Star-forming regions are ideal places for the development of the chemical complexity in the interstellar medium. All the species with 6 atoms or more and at least one atom of carbon, that are detected in the interstellar medium, are complex organic molecules (COMs, Herbst & van Dishoeck 2009) The formation of these species and their origin, either from gas-phase or dust surface reactions, is still highly debated. We focus on the study of these two complex molecules towards the solar-type protostar IRAS 16293– 2422 (hereafter IRAS16293), using data obtained with the IRAM-30m telescope This deeply embedded source, located in the ρ Ophiuchi cloud, is a binary composed of source A (South-East) and source B (North-West), which are separated by about 5 , i.e. about 750 au at a distance of about 141 pc (Ortiz-Leon et al 2017; Dzib et al 2018).
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