Effects of Metallicity on the Chemical Composition of Carbon Stars

TL;DR

This study analyzes how metallicity influences dust and molecular features in carbon stars' circumstellar envelopes, revealing effects on spectral features and proposing a method to trace dust condensation history across different metallicity environments.

Contribution

It introduces a new method to study dust condensation sequences in carbon stars, accounting for metallicity effects, using spectral feature measurements.

Findings

C2H2 spectral features are affected by dust dilution, less so in the Magellanic Clouds.

No strong metallicity dependence observed for the 13.7 micron C2H2 Q branch.

Shifts in SiC emission are mainly due to molecular absorption, not metallicity.

Abstract

We present \emph{Spitzer} IRS data on 19 asymptotic giant branch (AGB) stars in the Large Magellanic Cloud, complementing existing published data sets of carbon stars in both Magellanic Clouds and the Milky Way, to investigate the effects of metallicity on dust and molecular spectral features arising from the circumstellar envelope. We find that the C$_2$H$_2$ P and R branches at 7.5 micron are affected by dust dilution at higher mass-loss rates -- albeit to a lesser extent for sources in the Magellanic Clouds, compared to the Milky Way -- while the narrow 13.7 micron C$_2$H$_2$ Q branch only shows the effect of dust dilution at low mass-loss rates. A strong metallicity dependence is not observed for the Q branch. Independent of metallicity, we also provide an explanation for the observed shifts in the central wavelength of the SiC emission feature, as we show that these are largely caused by molecular band absorption on either side of the dust emission feature, dominating over shifts in the central wavelength caused by self-absorption. We have devised a method to study the dust condensation history in carbon-rich AGB stars in different metallicity environments, by measuring the strength of the 11.3 \um SiC and 30 \um MgS features with respect to the continuum, as a function of mass-loss rate. With this method, it is possible to distinguish in what order SiC and graphite condense, which is believed to be sensitive to the metallicity, prior to the eventual deposit of the MgS layer.