Three-micron spectra of AGB stars and supergiants in nearby galaxies

The dependence of stellar molecular bands on the metallicity is studied using infrared L-band spectra of AGB stars (both carbon-rich and oxygen-rich) and M-type supergiants in the Large and Small Magellanic Clouds (LMC and SMC) and in the Sagittarius Dwarf Spheroidal Galaxy. The spectra cover SiO bands for oxygen-rich stars, and acetylene (C2H2), CH and HCN bands for carbon-rich AGB stars. The equivalent width of acetylene is found to be high even at low metallicity. The high C2H2 abundance can be explained with a high carbon-to-oxygen (C/O) ratio for lower metallicity carbon stars. In contrast, the HCN equivalent width is low: fewer than half of the extra-galactic carbon stars show the 3.5micron HCN band, and only a few LMC stars show high HCN equivalent width. HCN abundances are limited by both nitrogen and carbon elemental abundances. The amount of synthesized nitrogen depends on the initial mass, and stars with high luminosity (i.e. high initial mass) could have a high HCN abundance. CH bands are found in both the extra-galactic and Galactic carbon stars. None of the oxygen-rich LMC stars show SiO bands, except one possible detection in a low quality spectrum. The limits on the equivalent widths of the SiO bands are below the expectation of up to 30angstrom for LMC metallicity. Several possible explanations are discussed. The observations imply that LMC and SMC carbon stars could reach mass-loss rates as high as their Galactic counterparts, because there are more carbon atoms available and more carbonaceous dust can be formed. On the other hand, the lack of SiO suggests less dust and lower mass-loss rates in low-metallicity oxygen-rich stars. The effect on the ISM dust enrichment is discussed.Comment: accepted for A&

Booms and Busts: the Burstiness of Star Formation in Nearby Dwarf Galaxies

In this review I summarise recent advances in our understanding of the importance of starburst events to the evolutionary histories of nearby galaxies. Ongoing bursts are easily diagnosed in emission-line surveys, but assessing the timing and intensity of fossil bursts requires more effort, usually demanding color-magnitude diagrams or spectroscopy of individual stars. For ages older than ~1 Gyr, this type of observation is currently limited to the Local Group and its immediate surroundings. However, if the Local Volume is representative of the Universe as a whole, then studies of the age and metallicity distributions of star clusters and resolved stellar populations should give statistical clues as to the frequency and importance of bursts to the histories of galaxies in general. Based on starburst statistics in the literature and synthetic colour-magnitude diagram studies of Local Group galaxies, I attempt to distinguish between systemic starbursts that strongly impact galaxy evolution and stochastic bursts that can appear impressive but are ultimately of little significance on gigayear timescales. As a specific case, it appears as though IC 10, the only starburst galaxy in the Local Group, falls into the latter category and is not fundamentally different from other nearby dwarf irregular galaxies.Comment: Accepted by the Publications of the Astronomical Society of Australia (PASA). Summary of a review talk given at the Southern Cross Astrophysics Conference on "Galaxy Metabolism" held in Sydney, 22-26 June 2009. 9 pages, 2 figure

Discovery of long-period variable stars in the very-metal-poor globular cluster M15

We present a search for long-period variable (LPV) stars among giant branch stars in M15 which, at [Fe/H] ~ -2.3, is one of the most metal-poor Galactic globular clusters. We use multi-colour optical photometry from the 0.6-m Keele Thornton and 2-m Liverpool Telescopes. Variability of delta-V ~ 0.15 mag is detected in K757 and K825 over unusually-long timescales of nearly a year, making them the most metal-poor LPVs found in a Galactic globular cluster. K825 is placed on the long secondary period sequence, identified for metal-rich LPVs, though no primary period is detectable. We discuss this variability in the context of dust production and stellar evolution at low metallicity, using additional spectra from the 6.5-m Magellan (Las Campanas) telescope. A lack of dust production, despite the presence of gaseous mass loss raises questions about the production of dust and the intra-cluster medium of this cluster.Comment: 13 pages, 9 figures, accepted by MNRA

Very Large Telescope three micron spectra of dust-enshrouded red giants in the Large Magellanic Cloud

We present ESO/VLT spectra in the 2.9--4.1 micron range for a large sample of infrared stars in the Large Magellanic Cloud (LMC), selected on the basis of MSX and 2MASS colours to be extremely dust-enshrouded AGB star candidates. Out of 30 targets, 28 are positively identified as carbon stars, significantly adding to the known population of optically invisible carbon stars in the LMC. We also present spectra for six IR-bright stars in or near three clusters in the LMC, identifying four of them as carbon stars and two as oxygen-rich supergiants. We analyse the molecular bands of C2H2 at 3.1 and 3.8 micron, HCN at 3.57 micron, and sharp absorption features in the 3.70--3.78 micron region that we attribute to C2H2. There is evidence for a generally high abundance of C2H2 in LMC carbon stars, suggestive of high carbon-to-oxygen abundance ratios at the low metallicity in the LMC. The low initial metallicity is also likely to have resulted in less abundant HCN and CS. The sample of IR carbon stars exhibits a range in C2H2:HCN abundance ratio. We do not find strong correlations between the properties of the molecular atmosphere and circumstellar dust envelope, but the observed differences in the strengths and shapes of the absorption bands can be explained by differences in excitation temperature. High mass-loss rates and strong pulsation would then be seen to be associated with a large scale height of the molecular atmosphere.Comment: Accepted for publication in Astronomy and Astrophysics. 20 pages. Figure 11 is degraded for posting on astro-p

Obscured Asymptotic Giant Branch Variables in the Large Magellanic Cloud and the Period-Luminosity Relation

The characteristics of oxygen-rich and carbon-rich, large amplitude (dK>0.4 mag), asymptotic giant branch variables in the Large Magellanic Clouds are discussed, with an emphasis on those obscured by dust. Near-infrared photometry, obtained over about 8 years, is combined with published mid-infrared observations from IRAS and ISO to determine bolometric magnitudes for 42 stars. Pulsation periods of the O-rich stars are in the range 116<P<1393 days, while those for C-rich stars have 298<P<939 days. In addition to the regular pulsations, one O-rich star and four C-rich stars show large amplitude, dK> 0.6 mag, secular or very long period variations which may be associated with changes in their mass-loss rates. We discuss and compare various methods of determining the bolometric magnitudes and show, perhaps surprisingly, that most of the very long period stars seem to follow an extrapolation of the period-luminosity relation determined for stars with shorter periods - although the details do depend on how the bolometric magnitudes are calculated. Three stars with thin shells, which are clearly more luminous than the obscured AGB stars, are undergoing hot bottom burning, while other stars with similar luminosities have yet to be investigated in sufficient detail to determine their status in this regard. We suggest that an apparent change in slope of the period luminosity relation around 400-420 days is caused by variables with luminosities brighter than the predictions of the core-mass luminosity relation, due to excess flux from hot bottom burning.Comment: 20 pages, 20 figures, accepted for MNRA

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

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.Comment: 12 pages, 9 figures, accepted for publication in MNRA