VLT/UVES Abundances in Four Nearby Dwarf Spheroidal Galaxies. I. Nucleosynthesis and Abundance Ratios

By M. Shetrone, K. Venn, E. Tolstoy, F. Primas, V. Hill, A. Kaufer. 2002

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We have used the Ultraviolet Echelle Spectrograph (UVES) on Kueyen (UT2) of the Very Large Telescope to take spectra of 15 individual red giants in the Sculptor, Fornax, Carina, and Leo I dwarf spheroidal galaxies (dSph's). We measure the abundances of α-, iron peak, first s-process, second s-process, and r-process elements. No dSph giants in our sample show the deep mixing abundance pattern (O and sometimes Mg depleted, while Na and Al are enhanced) seen in nearly all globular clusters. At a given metallicity the dSph giants exhibit lower [el/Fe] abundance ratios for the α-elements than stars in the Galactic halo. The low α abundances at low metallicities can be caused by a slow star formation rate and contribution from Type Ia SNe, and/or a small star formation event (low total mass) and mass-dependent Type II SN yields. In addition, Leo I and Sculptor exhibit a declining even-Z [el/Fe] pattern with increasing metallicity, while Fornax exhibits no significant slope. In contrast, Carina shows a large spread in the even-Z abundance pattern, even over small metallicity ranges, as might be expected from a bursting star formation history. The metal-poor stars in these dSph galaxies ([Fe/H] < -1) have halo-like s- and r-process abundances, but not every dSph exhibits the same evolution in the s- and r-process abundance pattern. Carina, Sculptor, and Fornax show a rise in the s-/r-process ratio with increasing metallicity, evolving from a pure r-process ratio to a solar-like s- and r-process ratio. On the other hand, Leo I, appears to show an r-process–dominated ratio over the range in metallicities sampled. At present, we attribute these differences in the star formation histories of these galaxies. Comparison of the dSph abundances with those of the halo reveals some consistencies with the Galactic halo. In particular, Nissen & Shuster found that their metal-rich, high Rmax high zmax halo stars exhibited low [α/Fe], [Na/Fe] and [Ni/Fe] abundance ratios. In the same abundance range our dSph exhibit the same abundance pattern, supporting their suggestions that disrupted dSph's may explain up to 50% of the metal-rich halo. Unfortunately, similar comparisons with the metal-poor Galactic halo have not revealed similar consistencies, suggesting that the majority of the metal-poor Galactic halo could not have been formed from objects similar to the dSph studied here. We use the dSph abundances to place new constraints on the nucleosynthetic origins of several elements. We attribute differences in the evolution of [Y/Fe] in the dSph stars versus the halo stars to a very weak AGB or SN Ia yield of Y (especially compared with Ba). That a lower and flatter Ba/Y ratio is seen in the halo is most likely a result of the pattern being erased by the large metallicity dispersion in the halo. Also, we find [Cu/Fe] and [Mn/Fe] are flat and halo-like over the metallicity city range -2 < [Fe/H] < -1.2, and that the [Cu/α] ratios are flat. Combining these abundances with knowledge of the age spread in these galaxies suggests that SNe Ia are not the main site for the production of Cu (and Mn) in very metal-poor stars. We suggest that metallicity-dependent SN yields may be more promising.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/cbca1ef36195d3254beadabc8ead7980662fd27b

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The s process: Nuclear physics, stellar models, and observations (2010, background): “Measurements of heavy elements in stars of dwarf spheroidal galaxies around the Milky Way have been made in the past decade (e.g. Sadakane et al., 2004; Shetrone et al., 2001, 2003).”
The effect of photoionization on the cooling rates of enriched, astrophysical plasmas (2008, background): “For instance, since [O/Fe] is observed to vary with environment by factors of two or so at a fixed metallicity (e.g., Shetrone et al. 2003), we can expect similar variations in the cooling rates.”
A REVISED PARALLEL-SEQUENCE MORPHOLOGICAL CLASSIFICATION OF GALAXIES: STRUCTURE AND FORMATION OF S0 AND SPHEROIDAL GALAXIES (2011, background): “The α element abundances in dSph galaxies are not much enhanced with respect to solar, also consistent with prolonged star formation histories (Shetrone et al. 1998, 2001, 2003, 2009; Tolstoy et al. 2003; Venn et al. 2004; Geisler et al. 2005).”
Two Distinct Ancient Components in the Sculptor Dwarf Spheroidal Galaxy: First Results from the Dwarf Abundances and Radial Velocities Team (2004, background): “There have also been studies of the abundances of small samples of individual red giant branch (RGB) stars at low and high resolution (Shetrone et al. 2003; Tolstoy et al. 2001, 2003).”
Complexity on Small Scales: The Metallicity Distribution of the Carina Dwarf Spheroidal Galaxy (2005, background): “The high-resolution measurements from Shetrone et al. (2003) quoted above in fact also provide an external check of our metallicity estimates.”
The Spitzer Survey of the Small Magellanic Cloud: S3MC Imaging and Photometry in the Mid- and Far-Infrared Wave Bands (2006, background): “Recentwork on dwarf spheroidal galaxies, for example, has been based on detailed spectral analyses of RGB stars (e.g., Shetrone et al. 2003; Tolstoy et al. 2003; Sadakane et al. 2004).”
THE CHEMICAL EVOLUTION OF THE DRACO DWARF SPHEROIDAL GALAXY (2009, methodology): “Shetrone et al (2003) and his earlier work demonstrated that dSph galaxies clearly have a chemical inventory which reveals signatures of a lower star formation rate and these low luminosity satellites appear to have a smaller fractional contribution of SNII in the total chemical inventory than in…”
Elemental abundance survey of the Galactic thick disc (2005, background): “It may be noted too that abundance analyses of stars in dwarf spheroidal galaxies and the Magellanic Clouds (Shetrone et al. 2003; Hill 2004) show a variety of [X/Fe] versus [Fe/H] relations, each unlike those of the thick disc.”
Λ Cold Dark Matter, Stellar Feedback, and the Galactic Halo Abundance Pattern (2005, background): “Additionally, stars in dwarf spheroidal (dSph) galaxies around the Milky Way have intermediateα-element abundances at low-metallicity, [α/Fe]∼ +0.15 (Smecker-Hane & McWilliam 1999; Shetrone et al. 2001; Smecker-Hane & McWilliam 2003; Shetrone et al. 2003; Venn et al. 2004; Geisler et al. 2005).”
The Emergence of the Thick Disk in a Cold Dark Matter Universe (2004, background): “In addition, recent high resolution spectroscopic observations of individual stars in dwarf spheroidals (dSphs), find solar α element to iron abundance ratios (e.g. Shetrone et al. 2001, 2003), which differs significantly from observed [α/Fe] and [Fe/H] in the thick disk.”
Detailed abundances of a large sample of giant stars in M 54 and in the Sagittarius nucleus (2010, background): “Whenever data are indeed available for both GCs and field stars in the same galaxy (Fornax – Letarte et al. 2006; Shetrone et al. 2003, Sgr – this study), only stars in clusters do actually contribute to the Na-O anticorrelation.”
Abundances of Baade’s Window Giants from Keck HIRES Spectra. II. The Alpha and Light Odd Elements (2006, background): “The idea that Type Ia produce Si, Ca and Ti is supported by the composition of dwarf spheroidal galaxies from Smecker-Hane & McWilliam (2002), McWilliam & Smecker-Hane (2005) and the work of Shetrone et al. (2003) and Geisler et al. (2004), as reviewed by Venn et al. (2004).”
HIGH-RESOLUTION SPECTROSCOPY OF EXTREMELY METAL-POOR STARS IN THE LEAST EVOLVED GALAXIES: URSA MAJOR II and COMA BERENICES (2009, background): “The past decade has provided a wealth of new information about chemical abundances in dSphs in general (e.g., Shetrone et al. 1998, 2001, 2003; Tolstoy et al. 2003; Venn et al. 2004; Geisler et al. 2005), but the ultra-faint dwarf galaxies were discovered too recently to have been included in these…”
Multiple populations in globular clusters (2012, background): “For instance, the overabundance of α−elements with respect . to Fe has been used to conclude that the majority of field halo stars cannot have formed in objects such as present-day dSph’s (see Shetrone et al. 2003; Tolstoy et al. 2009).”
Formation of isolated dwarf galaxies with feedback (2009, result): “…the minimum lifetime is increased above several hundred Myrs, the [α/Fe] ratios are too high, and there is no visible turnover point on the [α/Fe]/[Fe/H] diagram, contrary to the observed distribution of red giants in Local Group dwarf spheroidals (e.g. Shetrone et al. 2003; Tolstoy et al. 2003).”