Carbon Star Survey in the Local Group. VI. The Dwarf Spheroidal Galaxy NGC 205

We present a CFH12K survey of the carbon stars in NGC 205 and its surrounding field. We find that the number of C stars in NGC 205 is low (~500) for its luminosity and that very few C stars are seen outside of the 10' isophote, suggesting tidal stripping by M31. Their MI = -4.54, a magnitude nearly identical to what has been found in other galaxies with numerous C stars. Stars with accurate (R-I) photometry, to I ≈ 22.5, are used to determine the outer profile of NGC 205. A King profile with rt = 1977 ± 300 fits the data for distances larger than 7'. This rt implies that NGC 205 reached a minimum distance to M31 of ~42 kpc. The spatial distribution of C stars in the surrounding field reveals that some C stars belong to to the disk of M31, seen in projection, and that a surplus of C stars seen west of NGC 205 could be part of tidal debris left along its orbit. Finally, we report evidence of a transient period of enhanced star formation that occurred 1–2 Gyr ago in the northwest half of NGC 205

The extent of NGC 6822 revealed by its C stars population

Using the CFH12K camera, we apply the four band photometric technique to identify 904 carbon stars in an area 28' x 42' centered on NGC 6822. A few C stars, outside of this area were also discovered with the Las Campanas Swope Telescope. The NGC 6822 C star population has an average I of 19.26 mag leading to an average absolute I magnitude of -4.70 mag, a value essentially identical to the mean magnitude obtained for the C stars in IC 1613. Contrary to stars highlighting the optical image of NGC 6822, C stars are seen at large radial distances and trace a huge slightly elliptical halo which do not coincide with the huge HI cloud surrounding NGC6822. The previously unknown stellar component of NGC 6822 has a exponential scale length of 3.0' +/- 0.1' and can be traced to five scale lengths. The C/M ratio of NGC 6822 is evaluated to br 1.0 +/- 0.2.Comment: accepted, to be published in A

Nucleosynthesis And The Inhomogeneous Chemical Evolution Of The Carina Dwarf Galaxy

The detailed abundances of 23 chemical elements in nine bright red giant branch stars in the Carina dwarf spheroidal galaxy are presented based on high-resolution spectra gathered at the Very Large Telescope (VLT) and Magellan telescopes. A spherical model atmospheres analysis is applied using standard methods (local thermodynamic equilibrium and plane-parallel radiative transfer) to spectra ranging from 380 to 680 nm. Stellar parameters are found to be consistent between photometric and spectroscopic analyses, both at moderate and high resolution. The stars in this analysis range in metallicity from -2.9 < [Fe/H] < -1.3, and adopting the ages determined by Lemasle et al., we are able to examine the chemical evolution of Carina's old and intermediate-aged populations. One of the main results from this work is the evidence for inhomogeneous mixing in Carina and therefore for a poor statistical sampling of the supernova contributions when forming stars; a large dispersion in [Mg/Fe] indicates poor mixing in the old population, an offset in the [alpha/Fe] ratios between the old and intermediate-aged populations (when examined with previously published results) suggests that the second star formation event occurred in alpha-enriched gas, and one star, Car-612, seems to have formed in a pocket enhanced in SN Ia/II products. This latter star provides the first direct link between the formation of stars with enhanced SN Ia/II ratios in dwarf galaxies to those found in the outer Galactic halo (Ivans et al.). Another important result is the potential evidence for SNII driven winds. We show that the very metal-poor stars in Carina have not been enhanced in asymptotic giant branch or SN Ia products, and therefore their very low ratios of [Sr/Ba] suggests the loss of contributions from the early SNe II. Low ratios of [Na/Fe], [Mn/Fe], and [Cr/Fe] in two of these stars support this scenario, with additional evidence from the low [Zn/Fe] upper limit for one star. It is interesting that the chemistry of the metal-poor stars in Carina is not similar to those in the Galaxy, most of the other dwarf spheroidal galaxies, or the ultra faint dwarfs, and suggests that Carina may be at the critical mass where some chemical enrichments are lost through SN II driven winds.NSERCNSF AST 99-84073McDonald Observator

Chemical analysis of the Fornax Dwarf galaxy

This thesis is entitled “Chemical Analysis of the Fornax Dwarf Galaxy”, and it’s main goal is to determine what are the chemical elements present in the stars of this galaxy in order to try and understand it’s evolution. Galaxies are not “static” objects, they move, form stars and can interact with other galaxies. Studying the stars composing a galaxy can in principle, inform us about its past. Some stars can be as old as the galaxy itself, some can be much younger and we can use this information to study how the stellar spectra have varied with time over the entire history of star formation in this galaxy. Dwarf galaxies are in principle the most simple and straightforward type of galaxy and their study can be used to test numerous theories of the formation and evolution of stars and galaxies in a range of environments. Dwarf spheroidal galaxies are small, roughly spherical galaxies that are typically found in the vicinity of larger galaxies, such as the Milky Way. They typically do not have any ongoing star formation, nor do they appear to have any gas associated to them. The abundance ratios of different elements in individual stars with a range of ages provide a detailed insight into the various chemical enrichment processes (e.g., supernovae, stellar winds) which in turn improves our understanding of the global processes of formation and evolution of a galaxy as a whole. An important aspect of this thesis is the pipeline developed to analyse a large number of stellar spectra (~100) in a consistent and statistically robust manner, using tools that are typically used on spectra with twice the resolution and larger wavelength coverage. This required bringing together several complex tasks, including accurate stellar atmospheric models, atomic data for the absorption lines, codes of line formation, EW measurements and signal extraction methods, all of which need to be properly included and treated in order to obtain accurate results. The pipeline delivers stellar parameters and abundances in a controlled man- ner. This involved developing error analysis and diagnostics to carefully test the robustness of the results. The Fornax dwarf spheroidal galaxy contains five globular clusters (GCs) with a range of properties. Using VLT/UVES I have obtained the first detailed chemical abundances for nine individual stars in three of its GCs. From our results it is clear that they were formed promptly and early in the history of Fornax dSph, as were the Milky Way GCs. Thus, despite their very different mass, morphology and global star formation history, the abundance patterns of individual stars in the Fornax GCs are almost identical to those found in Milky Way globular clusters, including abundance patterns that are specific to GC stars (deep-mixing) and rare anomalies (europium-rich) also observed in other GCs. This suggests that stars in GCs are the same regardless of the type or size of the galaxy in which the GC is hosted. Thanks to the multi-fibre capability of VLT/FLAMES I have been able to make detailed abundance measurements of 81 RGB stars in the central part of Fornax, which is a significant, even dramatic, improvement on the previous UVES sample of three individual field stars. This sample of Fornax field stars exhibits unusually low [α/Fe] ratios, and its dependence with metallicity is different from the Milky Way, showing a different efficiency in gas enrichment. Fornax field stars are clearly predominantly enriched by s-process elements at high metallicity, showing the strong role of (metal poor) AGB stars. Our sample is dominated by a relatively young, relatively metal rich population. This means that we have obtained the most detailed picture of the chemical enrichment of Fornax during the last ~4 Gyrs. There is only one field star in our sample which appears old and metal poor, and it has abundance properties almost indistinguishable from the globular clusters in Fornax. These results confirm and deepen the difficulties found in earlier more limited surveys in understanding the role (if any) of this and of similar galaxies in the build up of our Milky Way. These results also challenge our understanding of basic nucleosynthetic processes, with for example, ratios of [Ni/Fe] that are well below what was typically thought possible.