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Register a new userDust mass-loss rates from AGB stars in the Fornax and Sagittarius dwarf Spheroidal galaxies
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Eric Lagadec
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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.
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We report on a multi-epoch study of the Fornax dwarf spheroidal galaxy, made with the Infrared Survey Facility, over an area of about 42x42. The colour-magnitude diagram shows a broad well-populated giant branch with a tip that slopes down-wards from red to blue, as might be expected given Fornaxs known range of age and metallicity. The extensive AGB includes seven Mira variables and ten periodic semi-regular variables. Five of the seven Miras are known to be carbon rich. Their pulsation periods range from 215 to 470 days, indicating a range of initial masses. Three of the Fornax Miras are redder than typical LMC Miras of similar period, probably indicating particularly heavy mass-loss rates. Many, but not all, of the characteristics of the AGB are reproduced by isochrones from Marigo et al. for a 2 Gyr population with a metallicity of Z=0.0025. An application of the Mira period-luminosity relation to these stars yields a distance modulus for Fornax of 20.69+/-0.04 (internal), +/-0.08 (total) (on a scale that puts the LMC at 18.39 mag) in good agreement with other determinations. Various estimates of the distance to Fornax are reviewed.
The existence of blue straggler stars (BSSs) in dwarf spheroidal galaxies (dSphs) is still an open question. In fact, many BSS candidates have been observed in the Local Group dSphs, but it is unclear whether they are real BSSs or young stars. Shedding light on the nature of these BSS candidates is crucial, in order to understand the star formation history of dSphs. In this paper, we consider BSS candidates in Sculptor and Fornax. In Fornax there are strong hints that the BSS population is contaminated by young stars, whereas in Sculptor there is no clear evidence of recent star formation. We derive the radial and luminosity distribution of BSS candidates from wide field imaging data extending beyond the nominal tidal radius of these galaxies. The observations are compared with the radial distribution of BSSs expected from dynamical simulations. In Sculptor the radial distribution of BSS candidates is consistent with that of red horizontal branch (RHB) stars and is in agreement with theoretical expectations for BSSs generated via mass transfer in binaries. On the contrary, in Fornax the radial distribution of BSS candidates is more concentrated than that of all the considered stellar populations. This result supports the hypothesis that most of BSS candidates in Fornax are young stars and is consistent with previous studies.
We have observed five carbon-rich AGB stars in the Fornax dwarf spheroidal (dSph) galaxy, using the Infrared Spectrometer on board the Spitzer Space Telescope. The stars were selected from a near-infrared survey of Fornax and include the three reddest stars, with presumably the highest mass-loss rates, in that galaxy. Such carbon stars probably belong to the intermediate-age population (2-8 Gyr old and metallicity of [Fe/H] -1) of Fornax. The primary aim of this paper is to investigate mass-loss rate, as a function of luminosity and metallicity, by comparing AGB stars in several galaxies with different metallicities. The spectra of three stars are fitted with a radiative transfer model. We find that mass-loss rates of these three stars are 4-7x10^-6 Msun yr-1. The other two stars have mass-loss rates below 1.3x10^-6 Msun yr-1. We find no evidence that these rates depend on metallicity, although we do suggest that the gas-to-dust ratio could be higher than at solar metallicity, in the range 240 to 800. The C2H2 bands are stronger at lower metallicity because of the higher C/O ratio. In contrast, the SiC fraction is reduced at low metallicity, due to low silicon abundance. The total mass-loss rate from all known carbon-rich AGB stars into the interstellar medium of this galaxy is of the order of 2x10^-5 Msun yr-1. This is much lower than that of the dwarf irregular galaxy WLM, which has a similar visual luminosity and metallicity. The difference is attributed to the younger stellar population of WLM. The suppressed gas-return rate to the ISM accentuates the difference between the relatively gas-rich dwarf irregular and the gas-poor dwarf spheroidal galaxies. Our study will be useful to constrain gas and dust recycling processes in low metallicity galaxies.
A radiative transfer code is used to model the spectral energy distributions of 57 mass-losing Asymptotic Giant Branch (AGB) stars and red supergiants (RSGs) in the Large Magellanic Cloud (LMC) for which ISO spectroscopic and photometric data are available. As a result we derive mass-loss rates and bolometric luminosities. A gap in the luminosity distribution around M_bol = -7.5 mag separates AGB stars from RSGs. The luminosity distributions of optically bright carbon stars, dust-enshrouded carbon stars and dust-enshrouded M-type stars have only little overlap, suggesting that the dust-enshrouded AGB stars are at the very tip of the AGB and will not evolve significantly in luminosity before mass loss ends their AGB evolution. Derived mass-loss rates span a range from Mdot about 10^-7 to 10^-3 M_sun/yr. More luminous and cooler stars are found to reach higher mass-loss rates. The highest mass-loss rates exceed the classical limit set by the momentum of the stellar radiation field, L/c, by a factor of a few due to multiple scattering of photons in the circumstellar dust envelope. Mass-loss rates are lower than the mass consumption rate by nuclear burning, Mdot_nuc, for most of the RSGs. Two RSGs have Mdot >> Mdot_nuc, however, suggesting that RSGs shed most of their stellar mantles in short phases of intense mass loss. Stars on the thermal pulsing AGB may also experience episodes of intensified mass loss, but their quiescent mass-loss rates are usually already higher than Mdot_nuc.
We present radial surface brightness profiles for all five globular clusters in the Fornax dwarf spheroidal galaxy, and for the four present members of the Sagittarius dwarf spheroidal galaxy. These profiles are derived from archival Hubble Space Telescope observations, and have been calculated using the same techniques with which we measured profiles in our previous studies of LMC and SMC clusters (astro-ph/0209031 and astro-ph/0209046 respectively), apart from some small modifications. From the surface brightness profiles, we have determined structural parameters for each cluster, including core radii and luminosity and mass estimates. We also provide a brief summary of literature measurements of other parameters for these clusters, including their ages, metallicities and distances. Our core radius measurements are mostly in good agreement with those from previous lower resolution studies, although for several clusters our new values are significantly different. The profile for Fornax cluster 5 does not appear to be well fit by a King-type model and we suggest that it is a post core-collapse candidate. We examine the distribution of cluster core radii in each of the two dwarf galaxy systems, and compare these with the distribution of core radii for old LMC clusters. The three distributions match within the limits of measurement errors and the small sample sizes. We discuss the implications of this in the context of the radius-age trend we have previously highlighted for the Magellanic Cloud clusters.
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