Efficient Searches for r-Process-Enhanced, Metal-Poor Stars

Neutron-capture-enhanced, metal-poor stars are of central importance to developing an understanding of the operation of the r-process in the early Galaxy, thought to be responsible for the formation of roughly half of all elements beyond the iron peak. A handful of neutron-capture-rich, metal-poor stars with [Fe/H] < -2.0 have already been identified, including the well known r-process-enhanced stars CS 22892-052 and CS 31082-001. However, many questions of fundamental interest can only be addressed with the assemblage of a much larger sample of such stars, so that general properties can be distinguished. We describe a new effort, HERES: The Hamburg/ESO R-Process-Enhanced Star survey, nearing completion, which will identify on the order of 5-10 additional highly r-process-enhanced, metal-poor stars, and in all likelihood, a similar or greater number of mildly r-process-enhanced, metal-poor stars in the halo of the Galaxy. HERES is based on rapid "snapshot" spectra of over 350 candidate halo giants with [Fe/H] < -2.0, obtained at moderately high resolution, and with moderate signal-to-noise ratios, using the UVES spectrograph on the European VLT 8m telescope.Comment: Contributed paper to The Eighth Nuclei in the Cosmos conference, to appear (in refereed form) in Nuclear Physics

The central spheroids of Milky Way mass-sized galaxies

Indexación: Scopus.PBT, DM and AM acknowledge partial support from the Nucleo UNAB 2015 DI-677-15/N of Universidad Andres Bello. PBT acknowledges partial support from Fondecyt Regular 1150334 and the Southern Astrophysics Network (SAN) collaboration funded by Conicyt, and PICT 2011-0959 and PIP 2012-0396 (Mincyt, Argentina). DM and MZ are supported by the BASAL Center for Astrophysics and Associated Technologies (CATA) through grant PFB-06, and the Ministry for the Economy, Development, and Tourism, Programa Iniciativa Cientifica Milenio through grant IC120009, awarded to the Millennium Institute of Astrophysics (MAS), and by FONDECYT Regular grant No. 1130196. DC and TCB acknowledge partial support for this work from grant PHY 14-30152; Physics Frontier Center/JINA Center for the Evolution of the Elements (JINA-CEE), awarded by the US National Science Foundation. REGM acknowledges support from Ci?ncia sem Fronteiras (CNPq, Brazil).We study the properties of the central spheroids located within 10 kpc of the centre of mass of MilkyWay mass-sized galaxies simulated in a cosmological context. The simulated central regions are dominated by stars older than 10 Gyr, mostly formed in situ, with a contribution of ~30 per cent from accreted stars. These stars formed in well-defined starbursts, although accreted stars exhibit sharper and earlier ones. The fraction of accreted stars increases with galactocentric distance, so that at a radius of~8-10 kpc, a fraction of~40 per cent, on average, is detected. Accreted stars are slightly younger, lower metallicity, and more α-enhanced than in situ stars. A significant fraction of old stars in the central regions come from a few (2-3) massive satellites (~1010M⊙). The bulge components receive larger contributions of accreted stars formed in dwarfs smaller than ~109.5M⊙. The difference between the distributions of ages and metallicities of old stars is thus linked to the accretion histories - those central regions with a larger fraction of accreted stars are those with contributions from more massive satellites. The kinematical properties of in situ and accreted stars are consistent with the latter being supported by their velocity dispersions, while the former exhibit clear signatures of rotational support. Our simulations demonstrate a range of characteristics, with some systems exhibiting a co-existing bar and spheroid in their central regions, resembling in some respect the central region of the Milky Way. © 2016 The Authors.https://academic.oup.com/mnras/article/473/2/1656/422260

The Chemical Evolution of the Milky Way

The field of chemical evolution modeling of the Galaxy is experiencing in the last years a phase of high activity and important achievements. There are, however, several open questions which still need to be answered. In this review I summarize what have been the most important achievements and what are some of the most urgent questions to be answered.Comment: 10 pages including 3 figs, to appear in "The Chemical Evolution of the Milky Way. Stars vs Clusters", Proceedings of the Sept.1999 Vulcano Workshop, F.Giovannelli and F.Matteucci eds (Kluwer, Dordrecht) in pres

Carbon-Enhanced Metal-Poor Stars in the Early Galaxy

Very metal-deficient stars that exhibit enhancements of their carbon abundances are of crucial importance for understanding a number of issues -- the nature of stellar evolution among the first generations of stars, the shape of the Initial Mass Function, and the relationship between carbon enhancement and neutron-capture processes, in particular the astrophysical s-process. One recent discovery from objective-prism surveys dedicated to the discovery of metal-deficient stars is that the frequency of Carbon-Enhanced Metal-Poor (CEMP) stars increases with declining metallicity, reaching roughly 25% for [Fe/H] < -2.5. In order to explore this phenomenon in greater detail we have obtained medium-resolution (2 A) spectroscopy for about 350 of the 413 objects in the Christlieb et al. catalog of carbon-rich stars, selected from the Hamburg/ESO objective prism survey on the basis of their carbon-enhancement, rather than metal deficiency. Based on these spectra, and near-IR JHK photometry from the 2MASS Point Source Catalog, we obtain estimates of [Fe/H] and [C/Fe] for most of the stars in this sample, along with reasonably accurate determinations of their radial velocities. Of particular importance, we find that the upper envelope of carbon enhancement observed for these stars is nearly constant, at [C/H] ~ -1.0, over the metallicity range -4.0 < [Fe/H] < -2.0; this same level of [C/H] applies to the most iron-deficent star yet discovered, HE 0107-5240, at [Fe/H] = -5.3.Comment: Contributed paper to The Eigth Nuclei in the Cosmos conference, to appear (in refereed form) in Nuclear Physics

The Spite Lithium Plateau: Ultrathin but Postprimordial

We have studied 23 very metal-poor field turnoff stars, specifically chosen to enable a precise measurement of the dispersion in the lithium abundance of the Spite Li plateau

Jurassic: A chemically anomalous structure in the Galactic halo

Indexación ScopusDetailed elemental-abundance patterns of giant stars in the Galactic halo measured by the Apache Point Observatory Galactic Evolution Experiment (APOGEE-2) have revealed the existence of a unique and significant stellar subpopulation of silicon-enhanced ([Si/Fe] ≳ +0.5) metal-poor stars, spanning a wide range of metallicities (-1.5 ≲ [Fe/H] ≲-0.8). Stars with over-abundances in [Si/Fe] are of great interest because these have very strong silicon (28Si) spectral features for stars of their metallicity and evolutionary stage, offering clues about rare nucleosynthetic pathways in globular clusters (GCs). Si-rich field stars have been conjectured to have been evaporated from GCs, however, the origin of their abundances remains unclear, and several scenarios have been offered to explain the anomalous abundance ratios. These include the hypothesis that some of them were born from a cloud of gas previously polluted by a progenitor that underwent a specific and peculiar nucleosynthesis event or, alternatively, that they were due to mass transfer from a previous evolved companion. However, those scenarios do not simultaneously explain the wide gamut of chemical species that are found in Si-rich stars. Instead, we show that the present inventory of such unusual stars, as well as their relation to known halo substructures (including the in situ halo, Gaia-Enceladus, the Helmi Stream(s), and Sequoia, among others), is still incomplete. We report the chemical abundances of the iron-peak (Fe), the light-(C and N), the α-(O and Mg), the odd-Z (Na and Al), and the s-process (Ce and Nd) elements of 55 newly identified Si-rich field stars (among more than ∼600 000 APOGEE-2 targets), which exhibit over-abundances of [Si/Fe] as extreme as those observed in some Galactic GCs, and they are relatively well distinguished from other stars in the [Si/Fe]-[Fe/H] plane. This new census confirms the presence of a statistically significant and chemically-anomalous structure in the inner halo: Jurassic. The chemo-dynamical properties of the Jurassic structure is consistent with it being the tidally disrupted remains of GCs, which are easily distinguished by an over-abundance of [Si/Fe] among Milky Way populations or satellites. © J. G. Fernandez-Trincado et al. 2020.https://www.aanda.org/articles/aa/full_html/2020/12/aa39434-20/aa39434-20.htm