On the reliability of mass-loss-rate estimates for AGB stars

Abstract

Context. In the recent literature there has been some doubt as to the reliability of CO multi-transitional line observations as a massloss-rate estimator for AGB stars. Aims. Using new well-calibrated CO radio line observations, the main aim of the work presented here is to carefully evaluate the reliability of CO mass-loss-rate estimates for intermediate-to high-mass-loss-rate AGB stars with different photospheric chemistries. Methods. Mass-loss rates for 10 intermediate-to high-mass-loss-rate AGB stars are derived using a detailed non-LTE, non-local radiative transfer code based on the Monte-Carlo method to model the CO radio line intensities. The circumstellar envelopes are assumed to be spherically symmetric and formed by constant mass-loss rates. The energy balance is solved self-consistently and the effects of dust on the radiation field and thermal balance included. An independent estimate of the mass-loss rate is also obtained from the combination of dust radiative transfer modelling with a dynamical model of the gas and dust particles. Results. We find that the CO radio line intensities and shapes are successfully reproduced for the majority of our objects when assuming a constant mass-loss rate. Moreover, the CO line intensities are only weakly dependent on the adopted micro-turbulent velocity, in contrast to recent claims in the literature. The two methods used in the present work to derive mass-loss rates are consistent within a factor of ∼3 for intermediate-to high-mass-loss-rate objects, indicating that this is a lower limit to the uncertainty in present mass-loss-rate estimates. We find a tentative trend with chemistry. Mass-loss rates from the dust/dynamical model are systematically higher than those from the CO model for the carbon stars and vice versa for the M-type stars. This could be ascribed to a discrepancy in the adopted CO/H 2 -abundance ratio, but we caution that the sample is small and systematic errors cannot be excluded.

Carbon isotopic abundance ratios in S-type stars

“…Although the assumptions and approaches are very different from model to model, the results were very comparable (except from some surprising cases deeply discussed in that paper). The values ofṀ calculated by Ramstedt et al (2008) were similar to those given by other authors (Groenewegen et al 1998;Schöier & Olofsson 2001;Teyssier et al 2006), but of course not identical, with differences in general between 10% and 50%.…”

“…These general trends have been widely confirmed by observations and theoretical detailed models (see e.g. Ramstedt et al 2008;Teyssier et al 2006). …”

“…We can check the usually obtained differences in the discussion by Ramstedt et al (2008), who compared calculations and models by different authors to treat CO line observations. (They also discuss estimates of the mass-loss other than those based on CO lines, which is not relevant for us.)…”

A HIFI view on circumstellar H2O in M-type AGB stars: radiative transfer, velocity profiles, and H2O line cooling

“…Although detailed modelling of these parameters for individual sources may result in different values, the uncertainties in these individual models would complicate the comparative study that is the goal of this work. This basic modelling strategy has been thoroughly tested and used previously (e.g., Schöier & Olofsson 2001;Olofsson et al 2002;Ramstedt et al 2008;Maercker et al 2008Maercker et al , 2009Schöier et al 2011;Danilovich et al 2014).…”

“…Observations of molecular emission lines have been used to determine these properties. In particular, observations of CO emission lines have proven to be a useful tool to describe the mass loss (e.g., Schöier et al 2002;González Delgado et al 2003;Ramstedt et al 2008;De Beck et al 2010;Khouri et al 2014a). Observations of other molecules have led to important insights in the chemistry and structure of AGB CSEs (e.g., González Delgado et al 2003;Schöier et al 2007;Maercker et al 2008Maercker et al , 2009Decin et al 2010a;Cernicharo et al 2010Cernicharo et al , 2011De Beck et al 2012;Justtanont et al 2012;Khouri et al 2014b; Lombaert et al 2016).…”