By Olivia Jones, Postdoctoral Fellow at STScI
The evolution of dust in galaxies is intrinsically linked with that of the stars, with the formation of new grain material coinciding with stellar deaths. Ultimately this material is returned (via gentle stellar outflows or explosively in the case of supernovae) to the interstellar medium, where it resides for approximately 10^8 years before it is gradually consumed by subsequent generations of star and planet formation. This perpetual recycling and associated enrichment of the gas and dust gradually alters the chemical composition of a galaxy.
Asymptotic Giant Branch (AGB) stars are important contributors to enrichment of the interstellar medium. AGB stars are cool, luminous giants that lose mass at high rates via pulsation-enhanced, dust-driven winds. During this phase of evolution the effective stellar temperature is low enough for dust grains and molecules, notably CO, to condense in their winds, forming substantial circumstellar envelopes detectable in the infrared and millimeter domains.
AGB stars are classified into two major spectral types: carbon-rich stars and oxygen-rich stars. The classification depends on the C/O ratio, which is decisive in the future chemical evolution. In oxygen-rich environments (where the C/O ratio < 1), the dust tends to be composed of amorphous or crystalline forms of silicates and metal-oxides such as amorphous aluminum oxide. Conversely, carbonaceous dust species such as amorphous carbon, SiC and MgS dominate the dust production in carbon-stars (C/O >1). Observational estimates suggest that two thirds of the dust we detect in the Milky Way has been produced by oxygen-rich AGB stars (Gehrz 1989). However, at sub-solar metallicities their total dust contribution relative to supernovae, the efficiency of dust condensation and the chemical composition of the dust remains uncertain.
Figure 1 (click on plots to enlarge): Example Spitzer-IRS spectra of oxygen-rich and carbon-rich evolved stars with a dust excess in the Small Magellanic Cloud (Ruffel et al. submitted). The key spectral features due to silicates and carbonaceous dust are marked in the top panel.
Figure 2 (click on plots to enlarge): Variation in the chemical composition of the crystalline silicate dust alters the peak positions and shapes of the narrow spectral features produced by resonances in the crystalline silicate lattice. Comparing the relative strengths of spectral features (at 23- and 28- microns) which are attributed to different crystalline silicate species in the Large Magellanic Cloud, Small Magellanic Cloud and Milky Way, show a change in the crystalline silicate dust mineralogy with metallicity (Jones et al. 2012). The solid symbols indicate that both features are present in the spectra.
With Spitzer we have observed evolved stars across a range of metal content; from present day solar abundances to metallicities that resemble star-forming galaxies at high redshift (e.g Meixner et al. 2006). These observations show that metallicity has a significant influence on the production and chemical composition of the oxygen-rich dust. In oxygen-rich stars silicates become oxygen poor at lower metallicity, with forsterite becoming less common than enstatite in the Magellanic Clouds, compared to the Galaxy (Jones et al. 2012). This is also seen in low-mass globular cluster stars, where conventional silicate features are seen to disappear in stars below [Fe/H] = -1; instead a featureless mid-IR excess is seen which is possibly caused by metallic iron dust (McDonald et al. 2010). Amorphous alumina oxide is also reduced compared to the Milky Way (Jones et al. 2014). This is because the molecules available for dust production in oxygen-rich stars are limited by the abundances of heavy elements in the photosphere, which reflect the initial abundances of the molecular cloud from which the star formed; this is not the case for carbon stars which produce their own carbon, so the resulting dust condensation sequence in carbon-rich AGB stars shows only minor changes with metallicity (Sloan et al. 2014). Furthermore, at lower metallicities there is less initial oxygen and the numerical ratio of carbon to oxygen atom abundances increases. Consequently, the timescales before dredge-up causes the star to become carbon rich are shorter. Observations of the Magellanic Clouds suggest that the mass return from AGB stars is dominated by carbon stars (Riebel et. al 2012), and cumulatively AGB stars may account for over 50% of the dust production in these galaxies (Boyer et al. 2012, Matsuura et al. 2013).
Recent observational results indicate that large amounts of dust can form in very metal-poor AGB stars (Fe/H] < -2 ; Boyer et al. 2015), and that they contribute significantly to the total dust budgets of metal-poor, high-redshift galaxies. However, it unclear how AGB stars in metal-poor environments are able to produce substantial quantities of dust. Understanding how the conditions in the early universe affect the production and chemical composition of the dust grains will require the superior sensitivity and mid-infrared spectroscopy available with the JWST.
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