Dust mass-loss rates from AGB stars in the Fornax and Sagittarius dwarf Spheroidal galaxies

TL;DR

This study measures dust mass-loss rates from AGB stars in the Sagittarius and Fornax dwarf galaxies using mid-infrared photometry, confirming a correlation with metallicity and providing insights into stellar evolution.

Contribution

It presents new dust mass-loss rate measurements for AGB stars in low-metallicity dwarf galaxies using mid-infrared data and radiative transfer models, extending previous correlations across different galaxies.

Findings

Mass-loss rates range from 5×10^{-10} to 3×10^{-8} M_sun/yr in Sgr dSph.

Mass-loss rates are higher than nuclear burning rates, affecting AGB star evolution.

Some stars have lower mass-loss rates than theoretical predictions.

Abstract

To study the effect of metallicity on the mass-loss rate of asymptotic giant branch (AGB) stars, we have conducted mid-infrared photometric measurements of such stars in the Sagittarius (Sgr dSph) and Fornax dwarf spheroidal galaxies with the 10-$\mu$m camera VISIR at the VLT. We derive mass-loss rates for 29 AGB stars in Sgr dSph and 2 in Fornax. The dust mass-loss rates are estimated from the $K-[9]$ and $K-[11]$ colours. Radiative transfer models are used to check the consistency of the method. Published IRAS and Spitzer data confirm that the same tight correlation between $K-[12]$ colour and dust mass-loss rates is observed for AGB stars from galaxies with different metallicities, i.e. the Galaxy, the LMC and the SMC. The derived dust mass-loss rates are in the range 5$\times10^{-10}$ to 3$\times10^{-8}$ M$_{\odot}$yr$^{-1}$ for the observed AGB stars in Sgr dSph and around 5$\times10^{-9}$ M$_{\odot}$yr$^{-1}$ for those in Fornax; while values obtained with the two different methods are of the same order of magnitude. The mass-loss rates for these stars are higher than the nuclear burning rates, so they will terminate their AGB phase by the depletion of their stellar mantles before their core can grow significantly. Some observed stars have lower mass-loss rates than the minimum value predicted by theoretical models.