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تسجيل مستخدم جديدAGB Stars in the Fornax Dwarf Spheroidal Galaxy
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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.
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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 reddes
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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-$mu$
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resent in the outer regions, and measure the luminosity of the AGB bump. We measure a corrected distance modulus (m-M)o = 20.70+/-0.12, based on the RGB tip luminosity, which is in good agreement with that obtained from the mean magnitude of old HB stars [(m-M)o=20.76+/-0.04] and with previous results. The (B-I) distribution of the RG stars can be approximately described as the superposition of two populations. The dominant component, comprising ~70% of the red giant stars, consists of relatively metal-enriched intermediate-age stars, whose mean metallicity is [Fe/H]=-1.39+/-0.15 (the age-corrected metallicity would be [Fe/H] ~ -1.0+/-0.15). The dominant intermediate-age component has an intrinsic color dispersion sigma(B-I)=0.06+/-0.01mag, corresponding to a relatively low abundance dispersion, sigma[Fe/H]=0.12+/-0.02dex. In addition, there is a small population of giants on the blue side of the RGB, whose spatial distribution is consistent with that of old horizontal branch stars, and completely different from that of the younger population. This unambiguously qualifies them as old and metal-poor. The exceptional color width of the Fornax RGB is therefore due to the presence of two main populations, yielding a large abundance range (-2.0<[Fe/H]<-0.7). This evidence suggests a scenario in which this galaxy started forming a stellar halo and its surrounding clusters together about 10-13 Gyr ago, then a major SF episode occurred after several Gyr.
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