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Stellar Chemical Signatures and Hierarchical Galaxy Formation
To compare the chemistries of stars in the Milky Way dwarf spheroidal(dSph) satellite galaxies with stars in the Galaxy, we have compiled alarge sample of Galactic stellar abundances from the literature. Whenkinematic information is available, we have assigned the stars tostandard Galactic components through Bayesian classification based onGaussian velocity ellipsoids. As found in previous studies, the[α/Fe] ratios of most stars in the dSph galaxies are generallylower than similar metallicity Galactic stars in this extended sample.Our kinematically selected stars confirm this for the Galactic halo,thin-disk, and thick-disk components. There is marginal overlap in thelow [α/Fe] ratios between dSph stars and Galactic halo stars onextreme retrograde orbits (V<-420 km s-1), but this is notsupported by other element ratios. Other element ratios compared in thispaper include r- and s-process abundances, where we find a significantoffset in the [Y/Fe] ratios, which results in a large overabundance in[Ba/Y] in most dSph stars compared with Galactic stars. Thus, thechemical signatures of most of the dSph stars are distinct from thestars in each of the kinematic components of the Galaxy. This resultrules out continuous merging of low-mass galaxies similar to these dSphsatellites during the formation of the Galaxy. However, we do not ruleout very early merging of low-mass dwarf galaxies, since up to one-halfof the most metal-poor stars ([Fe/H]<=-1.8) have chemistries that arein fair agreement with Galactic halo stars. We also do not rule outmerging with higher mass galaxies, although we note that the LMC and theremnants of the Sgr dwarf galaxy are also chemically distinct from themajority of the Galactic halo stars. Formation of the Galaxy's thickdisk by heating of an old thin disk during a merger is also not ruledout; however, the Galaxy's thick disk itself cannot be comprised of theremnants from a low-mass (dSph) dwarf galaxy, nor of a high-mass dwarfgalaxy like the LMC or Sgr, because of differences in chemistry.The new and independent environments offered by the dSph galaxies alsoallow us to examine fundamental assumptions related to thenucleosynthesis of the elements. The metal-poor stars ([Fe/H]<=-1.8)in the dSph galaxies appear to have lower [Ca/Fe] and [Ti/Fe] than[Mg/Fe] ratios, unlike similar metallicity stars in the Galaxy.Predictions from the α-process (α-rich freeze-out) would beconsistent with this result if there have been a lack of hypernovae indSph galaxies. The α-process could also be responsible for thevery low Y abundances in the metal-poor stars in dSph's; since [La/Eu](and possibly [Ba/Eu]) are consistent with pure r-process results, thelow [Y/Eu] suggests a separate r-process site for this light(first-peak) r-process element. We also discuss SNe II rates and yieldsas other alternatives, however. In stars with higher metallicities([Fe/H]>=-1.8), contributions from the s-process are expected; [(Y,La, and Ba)/Eu] all rise as expected, and yet [Ba/Y] is still muchhigher in the dSph stars than similar metallicity Galactic stars. Thisresult is consistent with s-process contributions from lower metallicityAGB stars in dSph galaxies, and is in good agreement with the slowerchemical evolution expected in the low-mass dSph galaxies relative tothe Galaxy, such that the build-up of metals occurs over much longertimescales. Future investigations of nucleosynthetic constraints (aswell as galaxy formation and evolution) will require an examination ofmany stars within individual dwarf galaxies.Finally, the Na-Ni trend reported in 1997 by Nissen & Schuster isconfirmed in Galactic halo stars, but we discuss this in terms of thegeneral nucleosynthesis of neutron-rich elements. We do not confirm thatthe Na-Ni trend is related to the accretion of dSph galaxies in theGalactic halo.
| Thorium and Uranium Chronometers Applied to CS 31082-001
We use the classical r-process model to explore the implications of therecently reported first observation of U in the extremely metal-poor,r-process element-enriched halo star CS 31082-001 for U and Thcosmochronometry. Using updated nuclear physics input and performing anew, conservative, analysis of the remaining uncertainties in theclassical r-process model, we confirm that U (together with Th)abundance observations in metal-poor stars are a promising tool fordating r-process events in the early Galaxy, independent of assumptionson Galactic chemical evolution. We show that nuclear physicsuncertainties limit the present accuracy of estimated U/Th ages to about2 Gyr. Critical nuclear data that are required to lower this uncertaintyinclude β-delayed fission branchings and reliable predictions ofthe onset of deformation in the vicinity of the N=184 shell closurearound 244Tl, as both directly affect predicted U/Th ratiosin r-process models. In this paper we apply, for the first time, the newHFBCS-1 mass model within the framework of the classical r-processmodel. We find that the predicted U and Th abundances are incompatiblewith the solar U and Th abundances and trace this back to a differentprediction of the onset of deformation around 244Tl. In thecase of CS 31082-001, we find it likely that the zero-age U and Thabundances were enhanced by about a factor of 2.5 compared to both (1) atheoretical extrapolation from the observed stable elements using theclassical r-process model and (2) the zero-age abundances of Th and U inother r-process-enhanced, metal-poor halo stars. Although presently adhoc, this ``actinide boost'' assumption solves the apparent problem ofthe relative age difference compared with other metal-poor halo starsand, at the same time, the problem of the inconsistency of ages based onU/(stable nucleus), Th/(stable nucleus) and U/Th ratios. There clearlyexist differences, among some r-process-enhanced, metal-poor stars, inthe level of the elemental abundances of actinides beyond the thirdr-process peak. Whether CS 31082-001 is a relatively rare case orcommonplace awaits the identification of larger numbers ofr-process-enhanced, metal-poor stars in which both U and Th can bemeasured. Using the U/Th ratio, we obtain a best age estimate for ther-process elements in CS 31082-001 of 15.5+/-3.2 Gyr. Futureobservations of Pb and Bi and a better determination of the r-processcontribution to solar Pb are needed to put the age estimates for thisand other stars on a more solid basis. For our most likely scenario, weprovide predictions of the expected upper and lower limits on theabundances of the elements Pb and Bi in CS 31082-001.
| The r-Process in the Early Galaxy
We report Sr, Pd, and Ag abundances for a sample of metal-poor fieldgiants and analyze a larger sample of Y, Zr, and Ba abundances. The[Y/Zr] and [Pd/Ag] abundance ratios are similar to those measured forthe r-process-rich stars CS 22892-052 and CS 31082-001. The [Pd/Ag]ratio is larger than predicted from the solar system r-processabundances. The constant [Y/Zr] and [Sr/Y] values in the field starsplace strong limits on the contributions of the weak s-process and themain s-process to the light neutron-capture elements. Stars in theglobular cluster M15 possess lower [Y/Zr] values than the field stars.There is a large dispersion in [Y/Ba]. Because the r-process isresponsible for the production of the heavy elements in the earlyGalaxy, these dispersions require varying light-to-heavy ratios inr-process yields.
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Observation and Astrometry data
| Constellation: | Virgo |
| Right ascension: | 11h52m24.37s |
| Declination: | +07°32'41.4" |
| Apparent magnitude: | 9.725 |
| Proper motion RA: | 4.5 |
| Proper motion Dec: | -11.9 |
| B-T magnitude: | 11.624 |
| V-T magnitude: | 9.882 |
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