The ALMA detection of CO rotational line emission in AGB stars in the Large Magellanic Cloud

Abstract

Context. Low-and intermediate-mass stars lose most of their stellar mass at the end of their lives on the asymptotic giant branch (AGB). Determining gas and dust mass-loss rates (MLRs) is important in quantifying the contribution of evolved stars to the enrichment of the interstellar medium. Aims. Attempt to, for the first time, spectrally resolve CO thermal line emission in a small sample of AGB stars in the Large Magellanic Cloud. Methods. The Atacama Large Millimeter Array was used to observe 2 OH/IR stars and 4 carbon stars in the LMC in the CO J= 2-1 line. Results. We present the first measurement of expansion velocities in extragalactic carbon stars. All four C-stars are detected and wind expansion velocities and stellar velocities are directly measured. Mass-loss rates are derived from modelling the spectral energy distribution and Spitzer/IRS spectrum with the DUSTY code. Gas-to-dust ratios are derived that make the predicted velocities agree with the observed ones. The expansion velocities and MLRs are compared to a Galactic sample of well-studied relatively low MLRs stars supplemented with "extreme" C-stars that have properties more similar to the LMC targets. Gas MLRs derived from a simple formula are significantly smaller than derived from the dust modelling, indicating an order of magnitude underestimate of the estimated CO abundance, time-variable mass loss, or that the CO intensities in LMC stars are lower than predicted by the formula derived for Galactic objects. This could be related to a stronger interstellar radiation field in the LMC. Conclusions. Although the LMC sample is small and the comparison to Galactic stars is non-trivial because of uncertainties in their distances (hence luminosities) it appears that for C stars the wind expansion velocities in the LMC are lower than in the solar neighbourhood, while the MLRs appear similar. This is in agreement with dynamical dust-driven wind models.

Mass loss of stars on the asymptotic giant branch

“…For their LMC sample with OH 1612 MHz line emission, Goldman et al (2017) concluded that at similar luminosity the LMC sources have lower υ ∞ (they center around 10 km s −1 ) than those of their Galactic comparison samples. The CO line detections of four carbon stars in the LMC by Groenewegen et al (2016) are consistent with this conclusion.…”

Fundamental parameters and infrared excesses of Tycho–Gaia stars

“…In (metal-poor) globular clusters and the Magellanic Clouds, the onset appears between 700 and 1500 L⊙ McDonald et al 2011a,c;Boyer et al 2015). While the total mass-loss rate (at least in older stars) does not appear to be strongly linked to metallicity (van Loon et al 2008;, the onset luminosity is likely to have some metallicity dependence (e.g McDonald et al 2010b), as the dust column density should scale approximately with metallicity (van Loon 2006; Groenewegen et al 2016). However, the onset is hard to trace in solar metallicity populations due to distance or contamination.…”

The mass-loss, expansion velocities, and dust production rates of carbon stars in the Magellanic Clouds

“…For the SMC the metallicity value is taken from Rubele et al (2018), while for the LMC carbon stars we select the typical value derived from trilegal simulations (Girardi et al 2005) based on the star formation history derived by Harris & Zaritsky (2009) (Marigo, private communication). The same reference value for the LMC has been assumed in other works (Groenewegen et al 2016a;Lebzelter et al 2018). For the Z = 0.004 the spectra have been calculated by employing a denser sampling of dust density profile employed in the radiative transfer calculations with respect to Nanni et al (2018).…”

“…For each of the sources the initial and current stellar masses are constrained by employing the TP-AGB tracks from Marigo et al (2013) with initial masses of 1.8 and 3.0 M ⊙ , and metallicity Z = 0.006. The same procedure as in Groenewegen et al (2016b) is adopted: the current mass of the star is the one of the model in the track with the period and luminosity closer to the observed ones. The luminosity value adopted is the one derived from this work.…”

“…Furthermore, in order to estimate the dust production and mass-loss rates the outflow expansion velocity and gas-to-dust ratio usually need to be assumed. In only few works the measured wind speeds are used to estimate the gas-to-dust ratios (Marshall et al 2004;Groenewegen et al 2016a;Goldman et al 2017). All these assumptions affect the final estimates of the dust production and mass-loss rates.…”