The distribution of H13CN in the circumstellar envelope around IRC+10216

TL;DR

This study maps H13CN in IRC+10216's envelope, estimating its abundance and analyzing excitation conditions, revealing the dominant role of HCN in cooling and the importance of non-LTE effects in radiative transfer modeling.

Contribution

It provides detailed imaging and excitation analysis of H13CN in IRC+10216, offering new insights into molecular abundance and excitation mechanisms in circumstellar envelopes.

Findings

H13CN fractional abundance in the inner wind is about 4E-7

HCN abundance remains constant out to 4E16 cm before photodissociation

HCN line cooling can dominate over CO in the inner envelope

Abstract

H13CN J=8-7 sub-millimetre line emission produced in the circumstellar envelope around the extreme carbon star IRC+10216 has been imaged at sub-arcsecond angular resolution using the SMA. Supplemented by a detailed excitation analysis the average fractional abundance of H13CN in the inner wind (< 5E15 cm) is estimated to be about 4E-7, translating into a total HCN fractional abundance of 2E-5 using the isotopic ratio 12C/13C=50. Multi-transitional single-dish observations further requires the H13CN fractional abundance to remain more or less constant in the envelope out to a radius of about 4E16 cm, where the HCN molecules are effectively destroyed, most probably, by photodissociation. The large amount of HCN present in the inner wind provides effective line cooling that can dominate over that generated from CO line emission. It is also shown that great care needs to be taken in the radiative transfer modelling where non-local, and non-LTE, effects are important and where the radiation field from thermal dust grains plays a major role in exciting the HCN molecules. The amount of HCN present in the circumstellar envelope around IRC+10216 is consistent with predicted photospheric values based on equilibrium chemical models and indicates that any non-equilibrium chemistry occurring in the extended pulsating atmosphere has no drastic net effect on the fractional abundance of HCN molecules that enters the outer envelope. It further suggests that few HCN molecules are incorporated into dust grains.