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Post Info TOPIC: Draco Dwarf Spheroidal Galaxy


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Title: Carbon in the Draco Dwarf Spheroidal Galaxy
Authors: Matthew D. Shetrone, Graeme H. Smith, Laura M. Stanford, Michael H. Siegel, Howard E. Bond

Measurements of [C/Fe], [Ca/H], and [Fe/H] have been derived from Keck I LRISb spectra of 35 giants in the Draco dwarf spheroidal galaxy. The iron abundances are derived by a spectrum synthesis modelling of the wavelength region from 4850 to 5375 A, while calcium and carbon abundances are obtained by fitting the Ca II H and K lines and the CH G band respectively. A range in metallicity of -2.9 <= [Fe/H]} <= -1.6 is found within the giants sampled, with a good correlation between [Fe/H] and [Ca/H]. The great majority of stars in the sample would be classified as having weak absorption in the lambda 3883 CN band, with only a small scatter in band strengths at a given luminosity on the red giant branch. In this sense the behaviour of CN among the Draco giants is consistent with the predominantly weak CN bands found among red giants in globular clusters of metallicity [Fe/H] < -1.8. Over half of the giants in the Draco sample have [Fe/H] > -2.25, and among these there is a trend for the [C/Fe] abundance to decrease with increasing luminosity on the red giant branch. This is a phenomenon that is also seen among both field and globular cluster giants of the Galactic halo, where it has been interpreted as a consequence of deep mixing of material between the base of the convective envelope and the outer limits of the hydrogen-burning shell. However, among the six Draco giants observed that turn out to have metallicities -2.65 < [Fe/H] < -2.25 there is no such trend seen in the carbon abundance. This may be due to small sample statistics or primordial inhomogeneities in carbon abundance among the most metal-poor Draco stars. We identify a potential carbon-rich extremely metal-poor star in our sample. This candidate will require follow up observations for confirmation.

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Title: The Chemical Evolution of the Draco Dwarf Spheroidal Galaxy
Authors: Judith G. Cohen, Wenjin Huang

We present an abundance analysis based on high resolution spectra of 8 stars selected to span the full range in metallicity in the Draco dwarf spheroidal galaxy. We find [Fe/H] for the sample stars ranges from -1.5 to -3.0 dex. Combining our sample with previously published work for a total of 14 luminous Draco giants, we show that the abundance ratios [Na/Fe], [Mg/Fe] and [Si/Fe] for the Draco giants overlap those of Galactic halo giants at the lowest [Fe/H] probed, but are significantly lower for the higher Fe-metallicity Draco stars. For the explosive alpha-elements Ca and Ti, the abundance ratios for Draco giants with [Fe/H] > -2.4 dex are approximately constant and slightly sub-solar, well below values characteristic of Galactic halo stars. The s-process contribution to the production of heavy elements begins at significantly lower Fe-metallicity than in the Galactic halo.
Using a toy model we compare the behaviour of the abundance ratios within the sample of Draco giants with those from the literature of Galactic globular clusters, and the Carina and Sgr dSph galaxies. The differences appear to be related to the timescale for buildup of the heavy elements, with Draco having the slowest rate.
We note the presence of a Draco giant with [Fe/H] < -3.0 dex in our sample, and reaffirm that the inner Galactic halo could have been formed by early accretion of Galactic satellite galaxies and dissolution of young globular clusters, while the outer halo could have formed from those satellite galaxies accreted later.

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Title: The Variable Stars of the Draco Dwarf Spheroidal Galaxy - Revisited
Authors: K. Kinemuchi, H.C. Harris, Horace A. Smith, N.A. Silbermann, L.A. Snyder, A. P. LaCluyze, C.L. Clark

We present a CCD survey of variable stars in the Draco dwarf spheroidal galaxy. This survey, which has the largest area coverage since the original variable star survey by Baade & Swope, includes photometry for 270 RR Lyrae stars, 9 anomalous Cepheids, 2 eclipsing binaries, and 12 slow, irregular red variables, as well as 30 background QSOs. Twenty-six probable double-mode RR Lyrae stars were identified. Observed parameters, including mean V and I magnitudes, V amplitudes, and periods, have been derived. Photometric metallicities of the ab-type RR Lyrae stars were calculated according to the method of Jurcsik & Kovacs, yielding a mean metallicity of <[Fe/H]> = -2.19 0.03. The well known Oosterhoff intermediate nature of the RR Lyrae stars in Draco is reconfirmed, although the double-mode RR Lyrae stars with one exception have properties similar to those found in Oosterhoff type II globular clusters. The period-luminosity relation of the anomalous Cepheids is rediscussed with the addition of the new Draco anomalous Cepheids.

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