CO and HCN isotopologue ratios in the outflows of AGB stars

Abstract

Isotopologue line intensity ratios of circumstellar molecules have been widely used to trace the photospheric elemental isotopic ratios of evolved stars. However, depending on the molecular species and the physical conditions of the environment, the circumstellar isotopologue ratio may deviate considerably from the stellar atmospheric value. In this paper, we aim to examine how the CO and HCN abundance ratios vary radially due to chemical reactions in the outflows of AGB stars and the effect of excitation and optical depth on the resulting line intensity ratios. We find that the circumstellar 12CO/13CO can deviate from its atmospheric value by up to 25-94% and 6-60% for C- and O-type CSEs, respectively. We show that variations of the intensity of the ISRF and the gas kinetic temperature can significantly influence the CO isotopologue ratio in the outer CSEs. On the contrary, the H12CN/H13CN ratio is stable for all tested mass-loss rates. The RT modeling shows that the integrated line intensity ratio of CO of different rotational transitions varies significantly for stars with intermediate mass-loss rates due to combined chemical and excitation effects. In contrast, the excitation conditions for the both HCN isotopologues are the same. We demonstrate the importance of using the isotopologue abundance profiles from chemical models as inputs to RT models in the interpretation of isotopologue observations. Previous studies of CO isotopologue ratios are based on multi-transition data for individual sources and it is difficult to estimate the errors in the reported values due to assumptions that are not entirely correct according to this study. If anything, previous studies may have overestimated the circumstellar 12CO/13CO abundance ratio. The use of the HCN as a tracer of C isotope ratios is affected by fewer complicating problems, provided one accounts corrections for high optical depths.

arXiv comment14 pages, 11 figures

References in corpus (17)

• The UMIST database for astrochemistry 2012
• The Dawes Review 2: Nucleosynthesis and stellar yields of low and intermediate-mass single stars
• Rotational quenching of CO due to H${}_2$ collisions
• Probing the mass-loss history of AGB and red supergiant stars from CO rotational line profiles - II. CO line survey of evolved stars: derivation of mass-loss rate formulae
• Models of circumstellar molecular radio line emission: Mass loss rates for a sample of bright carbon stars
• The abundance of HCN in circumstellar envelopes of AGB stars of different chemical types
• Circumstellar water vapour in M-type AGB stars: Radiative transfer models, abundances and predictions for HIFI
• New observations and models of circumstellar CO line emission of AGB stars in the Herschel SUCCESS programme
• Carbon Fractionation in PDRs
• The wind of W Hya as seen by Herschel - I. The CO envelope
• Detailed modelling of the circumstellar molecular line emission of the S-type AGB star W Aquilae
• Nucleosynthesis in AGB stars traced by oxygen isotopic ratios. I - Determining the stellar initial mass by means of the ${}^{17}$O/${}^{18}$O ratio
• Ejection of globular cluster interstellar media through ionization by white dwarfs
• Photodissociation of CO in the outflow of evolved stars
• A detailed view of the gas shell around R Sculptoris with ALMA
• ALMA observations of the variable 12CO/13CO ratio around the asymptotic giant branch star R Sculptoris
• H12CN and H13CN excitation analysis in the circumstellar outflow of R Scl

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• Do All Low-Mass Stars Undergo Extra Mixing Processes?
• The Nearby Evolved Stars Survey III: First data release of JCMT CO-line observations
• Exploring circumstellar chemistry in X-ray emitting AGB stars