Ammonia in circumstellar environment of V Cyg

TL;DR

This study models ammonia in the circumstellar envelope of V Cyg, revealing insights into its abundance and formation mechanisms through non-LTE radiative transfer analysis including vibrational pumping effects.

Contribution

It presents the first non-LTE radiative transfer modeling of ammonia in V Cyg, incorporating vibrational pumping, to constrain ammonia abundance and formation processes.

Findings

Ammonia abundance exceeds theoretical predictions for C-rich stars.

Radiative pumping significantly affects ammonia excitation.

Photodissociation radius estimated from chemical modeling.

Abstract

The HIFI instrument on board of the Herschel Space Observatory (HSO) has been very successful in detecting molecular lines from circumstellar envelopes around evolved stars, like massive red supergiants, Asymptotic Giant Branch (AGB) and post-AGB stars, as well as planetary nebulae. Among others, ammonia has been found in circumstellar envelopes of C-rich AGB stars in amounts that significantly exceeded theoretical predictions for C-rich stars. Few scenarios have been proposed to resolve this problem: formation of ammonia behind the shock front, photochemical processes in the inner part of the envelope partly transparent to UV background radiation due to the clumpy structure of the gas, and formation of ammonia on dust grains. Careful analysis of observations may help to put constraints on one or another mechanism of ammonia formation. Here, we present results of the non-LTE radiative transfer modeling of ammonia transitions including a crucial process of radiative pumping via v$_2$ = 1 vibrational band (at $\sim$10 $\mu$m) for V Cyg. Only ground-based ammonia transition NH$_{3}$ J = 1$_{0}$ - 0$_{0}$ at 572.5 GHz has been observed by HIFI. Therefore, to determine abundance of ammonia we estimate a photodissociation radius of NH$_{3}$ using chemical model of the envelope consistent with dust grain properties concluded from the spectral energy distribution.