Nanni, A. ; Groenewegen, M.A.T., ; Aringer, B ; Rubele, S. ; Bressan, A. ; van Loon, J.Th. ; Goldman, S.R.

Poster presented at EWASS, Lyon (FR) on 2019-06-24

Abstract: We present a recent investigation aimed to estimate the mass-loss and dust production rates of the carbon stars in the Magellanic Clouds (MCs). To achieve this goal we perform the spectral energy distribution (SED) fitting of the entire populations of carbon stars in the MCs over pre-calculated grids of spectra. The grids of spectra are computed as a function of the stellar parameters by consistently following the dust growth of different dust species, i.e. SiC and amorphous carbon, and the wind dynamics. Differently from the usual approach in the literature, our method allows to compute the dust production rates, gas-to-dust ratio, dust chemistry and expansion velocity of the outflow, and to estimate the mass-loss rates of carbon stars. The optical constants for amorphous carbon are selected on the basis of their ability to reproduce different observations in the infrared and optical bands of Gaia Data Release 2. An excess of extreme mass-losing carbon stars in the Large Magellanic Cloud (LMC) with low gas-to-dust ratios over the carbon stars in the Small Magellanic Cloud (SMC) is found. Typical gas-to-dust ratios is around 700 for the extreme stars in both galaxies, but the value can be down to approximately 160-200 and 100 for a few sources in the SMC and in the LMC, respectively. Such low values of the gas-to-dust ratios indicate that the process of dust condensation is particularly efficient around the reddest carbon stars. The extreme carbon stars in the LMC observed with the Atacama Large Millimeter Array (ALMA) and their wind speed are studied in detail. Our method is able to reproduce the SED as well as the expansion velocities of these sources. For the most dust-obscured star in this sample we estimate a large value of the mass-loss rate of about 6.3e-5 Msun/yr. The method introduced will be useful to plan and interpret upcoming observations of evolved stars performed with ALMA and other facilities. For this purpose, the grids of spectra and dusty models employed for this analysis will be publicly available.