Probing the mass-loss history of AGB and red supergiant stars from CO rotational line profiles

Abstract

Context. The evolution of intermediate and low-mass stars on the asymptotic giant branch is dominated by their strong dust-driven winds. More massive stars evolve into red supergiants with a similar envelope structure and strong wind. These stellar winds are a prime source for the chemical enrichment of the interstellar medium. Aims. We aim to (1) set up simple and general analytical expressions to estimate mass-loss rates of evolved stars, and (2) from those calculate estimates for the mass-loss rates of the asymptotic giant branch, red supergiant, and yellow hypergiant stars in our galactic sample. Methods. The rotationally excited lines of carbon monoxide (CO) are a classic and very robust diagnostic in the study of circumstellar envelopes. When sampling different layers of the circumstellar envelope, observations of these molecular lines lead to detailed profiles of kinetic temperature, expansion velocity, and density. A state-of-the-art, nonlocal thermal equilibrium, and co-moving frame radiative transfer code that predicts CO line intensities in the circumstellar envelopes of late-type stars is used in deriving relations between stellar and molecular-line parameters, on the one hand, and mass-loss rate, on the other. These expressions are applied to our extensive CO data set to estimate the mass-loss rates of 47 sample stars. Results. We present analytical expressions for estimating the mass-loss rates of evolved stellar objects for 8 rotational transitions of the CO molecule and thencompare our results to those of previous studies. Our expressions account for line saturation and resolving of the envelope, thereby allowing accurate determination of very high mass-loss rates. We argue that, for estimates based on a single rotational line, the CO(2-1) transition provides the most reliable mass-loss rate. The mass-loss rates calculated for the asympotic giant branch stars range from 4 × 10 −8 M yr −1 up to 8 × 10 −5 M yr −1 . For red supergiants they reach values between 2 × 10 −7 M yr −1 and 3 × 10 −4 M yr −1 . The estimates for the set of CO transitions allow time variability to be identified in the mass-loss rate. Possible mass-loss-rate variability is traced for 7 of the sample stars. We find a clear relation between the pulsation periods of the asympotic giant branch stars and their derived mass-loss rates, with a levelling off at ∼3 × 10 −5 M yr −1 for periods exceeding 850 days. Conclusions.

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

“…Khouri et al (2014a) derive a mass-loss rate for W Hya of 1.5 × 10 −7 M yr −1 , compared to 1.0 × 10 −7 M yr −1 in our previous modelling (Maercker et al 2008). Based on a grid of models using GASTRoNOoM, De Beck et al (2010) derive a formula to estimate the mass-loss rates from AGB stars based on the intensities of the CO lines. Using their formula results in mass-loss rates that are less than a factor two different from the values derived here.…”

“…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).…”

ALMA observations of the vibrationally excited rotational CO transition v = 1, J = 3 − 2 towards five AGB stars

“…TheṀ gas are those given by De Beck et al (2010, o Cet and π 1 Gru), Maercker et al (2016, R Scl), Danilovich et al (2014, W Aql), and Bujarrabal et al (2010, R Aqr). P and υ ∞ were retrieved from Bujarrabal et al (2010) for R Aqr and from De Beck et al (2010) for the other sources. …”

H2O vapor excitation in dusty AGB envelopes

“…The CSE has been modeled by several authors who report high gas-mass-loss rates ofṀ g ∼ 10 −5 −10 −4 M yr −1 (Netzer & Knapp 1987;Bowers & Johnston 1990;Justtanont & Tielens 1992;Loup et al 1993;Suh & Kim 2002;De Beck et al 2010). Owing to the high mass-loss rate, the density in the CSE is high enough for H 2 O ice to freeze out, shown by a strong absorption band around 3.1 μm Justtanont & Tielens 1992;Sylvester et al 1999 to 1342189961) taken in January 2010.…”