CO and HCN isotopologue ratios in the outflows of AGB stars

Abstract

Context. 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 isotopologue ratio in the circumstellar envelope (CSE) may deviate considerably from the stellar atmospheric value. Aims. In this paper, we aim to examine how the 12CO/13CO and H12CN/H13CN abundance ratios vary radially due to chemical reactions in the outflows of asymptotic giant branch (AGB) stars and the effect of excitation and optical depth on the resulting line intensity ratios. We study both carbon-rich (C-type) and oxygen-rich (O-type) CSEs. Methods. We performed chemical modeling to derive radial abundance distributions of our selected species in the CSEs over a wide range of mass-loss rates (10−8 < Ṁ < 10−4 M⊙ yr−1). We used these as input in a non-local thermodynamic equilibrium radiative transfer code to derive the line intensities of several ground-state rotational transitions. We also test the influence of stellar parameters, physical conditions in the outflows, the intensity of the interstellar radiation field, and the importance of considering the chemical networks in our model results. Results. We quantified deviations from the atmospheric value for typical outflows. We find that the circumstellar value of 12CO/13CO can deviate from its atmospheric value by up to 25–94% and 6–60% for C- and O-type CSEs, respectively, in radial ranges that depend on the mass-loss rate. We show that variations of the intensity of the interstellar radiation field and the gas kinetic temperature can significantly influence the CO isotopologue abundance ratio in the outer CSEs of both C-type and O-type. On the contrary, the H12CN/H13CN abundance ratio is stable throughout the CSEs for all tested mass-loss rates. The radiative transfer modeling shows that the integrated line intensity ratio I12CO/I13CO of different rotational transitions varies significantly for stars with mass-loss rates in the range from 10−7 to 10−6 M⊙ yr−1 due to combined chemical and excitation effects. In contrast, the excitation conditions for the HCN isotopologues are the same for both isotopologues. Conclusions. We demonstrate the importance of using the isotopologue abundance profiles from detailed chemical models as inputs to radiative transfer models in the interpretation of isotopologue observations. Previous studies of circumstellar 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 molecule as a tracer of C isotope ratios is affected by fewer complicating problems, but we note that the corrections for high optical depths are very large in the case of high-mass-loss-rate C-type CSEs; and in O-type CSEs the H13CN lines may be too weak to detect.

CO line observations of OH/IR stars in the inner Galactic Bulge: Characteristics of stars at the tip of the AGB

“…Therefore, our detections of the rarer isotopologue, H 13 CN, were done at surprisingly high intensity levels, about 10-30% of that of the 12 CO(J = 3-2) lines. Thus, this result further strengthen the conclusion that our sources are rich in 13 C. We note here that, as opposed to the case of CO, HCN is photodissociated in the continuum and hence we do not expect any effects of circumstellar isotope-selective processes (Saberi et al 2020).…”

“…However, this can be counter-acted by the exchange reaction that favours the formation of 13 CO at the expense of 12 CO in low-temperature gas (Watson et al 1976). These CSE processes have been studied in detail by Saberi et al (2019Saberi et al ( , 2020, and the conclusion is that the circumstellar 12 CO/ 13 CO abundance ratio in the external part of the CSE of a high-mass-loss-rate object is only slightly above the initial 12 CO/ 13 CO value, which is determined by the stellar 12 C/ 13 C isotope ratio. This means that, if anything, we overestimate the latter.…”

“…This means that, if anything, we overestimate the latter. Saberi et al (2020) also tested the effect of a higher UV radiation field. It will lead to a further slight increase in the circumstellar 12 CO/ 13 CO abundance ratio in the external parts of the CSE, but the effect is marginal for the mass-loss rates of our objects.…”

Multiple components in the molecular outflow of the red supergiant NML Cyg

“…These ratios imply a low 12 C/ 13 C ratio (below 10), in line with those reported by and De Beck et al ( 2010 ) for evolved stars. We note the possibility for differences between derived 12 CO/ 13 CO and H 12 CN/H 13 CN considering the difference in photodissociation of CO (via lines) and HCN (via the continuum; Saberi et al 2020 ).…”

Multiple components in the molecular outflow of the red supergiant NML Cyg

“…These ratios imply a low 12 C/ 13 C ratio (below 10), in line with those reported by Milam et al (2009) and De Beck et al (2010) for evolved stars. We note the possibility for differences between derived 12 CO/ 13 CO and H 12 CN/H 13 CN considering the difference in photodissociation of CO (via lines) and HCN (via the continuum; Saberi et al 2020).…”