# Exploring circumstellar chemistry in X-ray emitting AGB stars

Source: [Exploring circumstellar chemistry in X-ray emitting AGB stars](<https://www.sota2.com/research/paper/68fa4ace1e209131ce9414b2>)

Collection index: [llms.txt](<https://www.sota2.com/research/llms.txt>)

Data updated: 2025-10-25T08:01:45.471Z

Authors: J. Alonso-Hern\\'andez, C. S\\'anchez Contreras, M. Ag\\'undez, R. Sahai, J. P. Fonfr\\'ia, L. Velilla-Prieto, G. Quintana-Lacaci, J. Cernicharo

Published: 2025-04-28T09:22:53.000Z

## Abstract

Aims. Our goal is to characterize the chemistry and physical conditions of the circumstellar envelopes (CSEs) of Asymptotic Giant Branch (AGB) binary candidate stars with UV-excess and X-ray emission, in particular, to identify the effects of the internal X-ray emission in the abundance of certain key molecules. Methods. We observed the 86.0-94.0 and 260.0-272.5 GHz spectral ranges searching for rotational transitions of the X-ray sensitive molecule $HCO^{+}$ in four AGB stars, two of them detected in both UV and X-ray emission and the other two detected only in UV. We derived the CSEs's physical parameters from previous CO observations and determined the molecular abundances of the detected species using radiative transfer models. We developed chemical kinetics models that account for the effects of internal X-ray emission (as well as UV radiation) and compared our predictions with observations. Results. We report the detection of $HCO^{+}$ in the X-ray emitting C-rich AGB T\\,Dra, while it remains undetected in the spectra of the other three sources. In T\\,Dra we also detect SiO, HCN, HNC, $HC\_{3}N$, $SiC\_{2}$, $C\_{2}H$ and SiS. For the other targets only HCN and SiO are detected. The high fractional abundance of $HCO^{+}$ derived for T\\,Dra ($\[1.5-3.0\]\\times 10^{-8}$) is in good agreement with the predictions from our chemical kinetics models including the effects of internal X-ray emission, and one order of magnitude higher than the values expected for C-rich AGB stars. Additionally, we identify abundance enhancements for HNC, and $HC\_{3}N$ alongside a depletion of CO in the innermost regions of T\\,Dra's envelope. Conclusions. An internal X-ray source can significantly alter molecular abundances in AGB CSEs, enhancing $HCO^{+}$, $N\_{2}H^{+}$, HNC, and $HC\_{3}N$ while depleting parent species like CO. UV radiation has a weaker effect unless the envelope is optically thin or porous.

## Recorded benchmark results

A research result applies to its stated dataset, split, metric and evaluation configuration. Compare like-for-like settings, preserve the metric's unit and direction, and distinguish paper-reported results from independent reproduction. The best result recorded here is not a claim of exhaustive, current global coverage.

## Original sources and implementation

[Original paper](<https://arxiv.org/abs/2504.19616>)

[PDF](<https://arxiv.org/pdf/2504.19616.pdf>)