The Populations of Carina. I. Decoding the Color-Magnitude Diagram

© 2017. The American Astronomical Society. All rights reserved.. We investigate the color-magnitude diagram (CMD) of the Carina dwarf spheroidal galaxy using data of Stetson et al. and synthetic CMDs based on isochrones of Dotter et al., in terms of the parameters [Fe/H], age, and [α/Fe] , for the cases when (i) [α/Fe] is held constant and (ii) [α/Fe] is varied. The data are well described by four basic epochs of star formation, having [Fe/H] = -1.85, -1.5, -1.2, and ∼-1.15 and ages ∼13, 7, ∼3.5, and ∼1.5 Gyr, respectively (for [α/Fe] = 0.1, constant [α/Fe], and [α/Fe] = 0.2, 0.1, -0.2, -0.2, variable [α/Fe]), with small spreads in [Fe/H] and age of order 0.1 dex and 1-3 Gyr. Within an elliptical radius 13.′1, the mass fractions of the populations, at their times of formation, were (in decreasing age order) 0.34, 0.39, 0.23, and 0.04. This formalism reproduces five observed CMD features (two distinct subgiant branches of old and intermediate-age populations, two younger, main-sequence components, and the small color dispersion on the red giant branch (RGB). The parameters of the youngest population are less certain than those of the others, and given it is less centrally concentrated, it may not be directly related to them. High-resolution spectroscopically analyzed RGB samples appear statistically incomplete compared with those selected using radial velocity, which contain bluer stars comprising ∼5%-10% of the samples. We conjecture these objects may, at least in part, be members of the youngest population. We use the CMD simulations to obtain insight into the population structure of Carina's upper RGB

Carbon-enhanced metal-poor stars in the SDSS-APOGEE data base

We identify six new carbon-enhanced metal-poor (CEMP) stars ([C/Fe]>+ 0.7 and [Fe/H] < -1.8) and another seven likely candidates within the APOGEE data base following Data Release 12. These stars have chemical compositions typical of metal-poor halo stars, e.g. mean [a/Fe]=+0.24 +/- 0.24, based on the APOGEE Stellar Parameters and Chemical Abundances Pipeline results. A lack of heavy-element spectral lines impedes further sub-classification of these CEMP stars, however, based on radial velocity (RV) scatter, we predict most are not CEMP-s stars which are typically found in binary systems. Only one object, 2M15312547+4220551, may be in a binary since it exhibits a scatter in its RV of 1.7 +/- 0.6 km s(-1) based on three visits over a 25.98 d baseline. Optical observations are now necessary to confirm the stellar parameters and low metallicities of these stars, to determine the heavy-element abundance ratios and improve the precision in the derived abundances, and to examine their CEMP sub-classifications

The Populations of Carina. II. Chemical Enrichment

Chemical abundances are presented for 19 elements in a sample of 63 red giants in the Carina dwarf spheroidal galaxy (dSph), based on homogeneous 1D/LTE model atmosphere analyses of our own observations (32 stars) and data available in the literature (a further 31 independent stars). The (Fe) metallicity and [$\alpha$/Fe] distribution functions have mean values and dispersions of -1.59 and 0.33 dex ([Fe/H] range: -2.68 to -0.64) and 0.07 and 0.13 dex ([$\alpha$/Fe] range: -0.27 to 0.25), respectively. We confirm the finding of Venn et al. that a small percentage (some 10% in the present investigation) of the sample shows clear evidence for significant enrichment by Type Ia supernova (SN Ia) ejecta. Calcium, with the most accurately determined abundance of the $\alpha$-elements, shows an asymmetric distribution toward smaller values of [Ca/Fe] at all [Fe/H], most significantly over -2.0 < [Fe/H] < -1.0, suggestive of incomplete mixing of the ejecta of SNe Ia with the ambient medium of each of Carina's generations. Approximate color-magnitude diagram age estimates are presented for the sample, and together with our chemical abundances, compared with the results of our previous synthetic color-magnitude diagram analysis, which reported the details of Carina's four well-defined populations. We searched for the Na-O anticorrelation universally reported in the Galaxy's globular clusters and confirm that this phenomenon does not exist in Carina. We also found that one of the 32 stars in our sample has an extremely enhanced lithium abundance - A(Li)$_\text{NLTE}$ = +3.36, consistent with membership of the ~1% group of Li-rich stars in dSph described by Kirby et al.Studies at RSAA, ANU, of the Galaxy’s most metal-poor stars and its dwarf galaxy satellite systems by J.E.N. and D.Y. are supported by Australian Research Council grants DP0663562, DP0984924, DP120100475, DP150100862, and FT140100554. K.A.V. acknowledges support from the Canadian NSERC Discovery Grants program. This work was partly supported by the European Union FP7 program through ERC grant no. 320360

Linking dwarf galaxies to halo building blocks with the most metal-poor star in Sculptor

Current cosmological models indicate that the Milky Way's stellar halo was assembled from many smaller systems. Based on the apparent absence of the most metal-poor stars in present-day dwarf galaxies, recent studies claimed that the true Galactic building blocks must have been vastly different from the surviving dwarfs. The discovery of an extremely iron-poor star (S1020549) in the Sculptor dwarf galaxy based on a medium-resolution spectrum cast some doubt on this conclusion. However, verification of the iron-deficiency and measurements of additional elements, such as the alpha-element Mg, are mandatory for demonstrating that the same type of stars produced the metals found in dwarf galaxies and the Galactic halo. Only then can dwarf galaxy stars be conclusively linked to early stellar halo assembly. Here we report high-resolution spectroscopic abundances for 11 elements in S1020549, confirming the iron abundance of less than 1/4000th that of the Sun, and showing that the overall abundance pattern mirrors that seen in low-metallicity halo stars, including the alpha-elements. Such chemical similarity indicates that the systems destroyed to form the halo billions of years ago were not fundamentally different from the progenitors of present-day dwarfs, and suggests that the early chemical enrichment of all galaxies may be nearly identical.Comment: 16 pages, including 2 figures. Accepted for publication in Nature. It is embargoed for discussion in the press until formal publication in Natur

The evolution of rotating stars

First, we review the main physical effects to be considered in the building of evolutionary models of rotating stars on the Upper Main-Sequence (MS). The internal rotation law evolves as a result of contraction and expansion, meridional circulation, diffusion processes and mass loss. In turn, differential rotation and mixing exert a feedback on circulation and diffusion, so that a consistent treatment is necessary. We review recent results on the evolution of internal rotation and the surface rotational velocities for stars on the Upper MS, for red giants, supergiants and W-R stars. A fast rotation is enhancing the mass loss by stellar winds and reciprocally high mass loss is removing a lot of angular momentum. The problem of the ``break-up'' or $\Omega$-limit is critically examined in connection with the origin of Be and LBV stars. The effects of rotation on the tracks in the HR diagram, the lifetimes, the isochrones, the blue to red supergiant ratios, the formation of W-R stars, the chemical abundances in massive stars as well as in red giants and AGB stars, are reviewed in relation to recent observations for stars in the Galaxy and Magellanic Clouds. The effects of rotation on the final stages and on the chemical yields are examined, as well as the constraints placed by the periods of pulsars. On the whole, this review points out that stellar evolution is not only a function of mass M and metallicity Z, but of angular velocity $\Omega$ as well.Comment: 78 pages, 7 figures, review for Annual Review of Astronomy and Astrophysics, vol. 38 (2000

The stellar halo of the Galaxy

Stellar halos may hold some of the best preserved fossils of the formation history of galaxies. They are a natural product of the merging processes that probably take place during the assembly of a galaxy, and hence may well be the most ubiquitous component of galaxies, independently of their Hubble type. This review focuses on our current understanding of the spatial structure, the kinematics and chemistry of halo stars in the Milky Way. In recent years, we have experienced a change in paradigm thanks to the discovery of large amounts of substructure, especially in the outer halo. I discuss the implications of the currently available observational constraints and fold them into several possible formation scenarios. Unraveling the formation of the Galactic halo will be possible in the near future through a combination of large wide field photometric and spectroscopic surveys, and especially in the era of Gaia.Comment: 46 pages, 16 figures. References updated and some minor changes. Full-resolution version available at http://www.astro.rug.nl/~ahelmi/stellar-halo-review.pd