- ID:
- ivo://CDS.VizieR/J/ApJ/898/150
- Title:
- High-res. MIKE obs. of metal-poor stars
- Short Name:
- J/ApJ/898/150
- Date:
- 21 Mar 2022 08:50:22
- Publisher:
- CDS
- Description:
- Extensive progress has recently been made in our understanding of heavy-element production via the r-process in the universe, specifically with the first observed neutron star binary merger (NSBM) event associated with the gravitational-wave signal detected by LIGO, GW170817. The chemical abundance patterns of metal-poor r-process-enhanced stars provide key evidence for the dominant site(s) of the r-process and whether NSBMs are sufficiently frequent or prolific r-process sources to be responsible for the majority of r-process material in the universe. We present atmospheric stellar parameters (using a nonlocal thermodynamic equilibrium analysis) and abundances from a detailed analysis of 141 metal-poor stars carried out as part of the R-Process Alliance (RPA) effort. We obtained high-resolution "snapshot" spectroscopy of the stars using the MIKE spectrograph on the 6.5m Magellan Clay telescope at Las Campanas Observatory in Chile. We find 10 new highly enhanced r-II (with [Eu/Fe]>+1.0), 62 new moderately enhanced r-I (+0.3<[Eu/Fe]<~+1.0), and 17 new limited-r ([Eu/Fe]<+0.3) stars. Among those, we find 17 new carbon-enhanced metal-poor (CEMP) stars, of which five are CEMP-no. We also identify one new s-process-enhanced ([Ba/Eu]>+0.5) and five new r/s (0.0<[Ba/Eu]<+0.5) stars. In the process, we discover a new ultra-metal-poor (UMP) star at [Fe/H]=-4.02. One of the r-II stars shows a deficit in {alpha} and Fe-peak elements, typical of dwarf galaxy stars. Our search for r-process-enhanced stars by RPA efforts has already roughly doubled the known r-process sample.
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Search Results
- ID:
- ivo://CDS.VizieR/J/ApJ/835/152
- Title:
- HST FGS-1r parallaxes for 8 metal-poor stars
- Short Name:
- J/ApJ/835/152
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Hubble Space Telescope (HST) fine guidance sensor observations were used to obtain parallaxes of eight metal-poor ([Fe/H]< -1.4) stars. The parallaxes of these stars determined by the new Hipparcos reduction average 17% accuracy, in contrast to our new HST parallaxes, which average 1% accuracy and have errors on the individual parallaxes ranging from 85 to 144{mu}as. These parallax data were combined with HST Advanced Camera for Surveys photometry in the F606W and F814W filters to obtain the absolute magnitudes of the stars with an accuracy of 0.02-0.03 mag. Six of these stars are on the main sequence (MS) (with -2.7<[Fe/H]< -1.8) and are suitable for testing metal-poor stellar evolution models and determining the distances to metal-poor globular clusters (GCs). Using the abundances obtained by O'Malley+ (2017ApJ...838...90O), we find that standard stellar models using the VandenBerg & Clem (2003AJ....126..778V) color transformation do a reasonable job of matching five of the MS stars, with HD 54639 ([Fe/H]=-2.5) being anomalous in its location in the color-magnitude diagram. Stellar models and isochrones were generated using a Monte Carlo analysis to take into account uncertainties in the models. Isochrones that fit the parallax stars were used to determine the distances and ages of nine GCs (with -2.4{<=}[Fe/H]{<=}-1.9). Averaging together the age of all nine clusters led to an absolute age of the oldest, most metal-poor GCs of 12.7+/-1.0Gyr, where the quoted uncertainty takes into account the known uncertainties in the stellar models and isochrones, along with the uncertainty in the distance and reddening of the clusters.
- ID:
- ivo://CDS.VizieR/IV/20
- Title:
- Identifications of Metal-Deficient F-M Stars
- Short Name:
- IV/20
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The catalog provides identifications in a variety of astronomical catalogs of 1555 metal-deficient stars from the MDSP catalog of metal-deficient F-M stars classified spectroscopically (Bartkevicius 1980) and from its first supplement, MDSPS1 (Bartkevicius 1984, Cat. <III/125>).
- ID:
- ivo://CDS.VizieR/J/A+A/402/343
- Title:
- IR O I triplet, [O I] lines in F-K dwarfs/giants
- Short Name:
- J/A+A/402/343
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- In order to investigate the formation of O I 7771-5 and [O I] 6300/6363 lines, extensive non-LTE calculations for neutral atomic oxygen were carried out for wide ranges of model atmosphere parameters, which are applicable to early-K through late-F halo/disk stars of various evolutionary stages. The formation of the triplet O I lines was found to be well described by the classical two-level-atom scattering model, and the non-LTE correction is practically determined by the parameters of the line-transition itself without any significant relevance to the details of the oxygen atomic model. This simplifies the problem in the sense that the non-LTE abundance correction is essentially determined only by the line-strength (W_{lambda}_), if the atmospheric parameters of Teff, logg, and {xi} are given, without any explicit dependence of the metallicity; thus allowing a useful analytical formula with tabulated numerical coefficients. On the other hand, our calculations lead to the robust conclusion that LTE is totally valid for the forbidden [O I] lines.
- ID:
- ivo://CDS.VizieR/J/ApJ/890/119
- Title:
- Iron element abundances in 3 very metal-poor stars
- Short Name:
- J/ApJ/890/119
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have obtained new detailed abundances of the Fe-group elements Sc through Zn (Z=21-30) in three very metal-poor ([Fe/H]~-3) stars: BD+03 740, BD-13 3442, and CD-33 1173. High-resolution ultraviolet Hubble Space Telescope/Space Telescope Imaging Spectrograph spectra in the wavelength range 2300-3050{AA} were gathered, and complemented by an assortment of optical echelle spectra. The analysis featured recent laboratory atomic data for a number of neutral and ionized species for all Fe-group elements except Cu and Zn. A detailed examination of scandium, titanium, and vanadium abundances in large-sample spectroscopic surveys indicates that they are positively correlated in stars with [Fe/H]<=-2. The abundances of these elements in BD+03 740, BD-13 3442, CD-33 1173, and HD 84937 (studied in a previous paper of this series) are in accord with these trends and lie at the high end of the correlations. Six elements have detectable neutral and ionized features, and generally their abundances are in reasonable agreement. For Cr we find only minimal abundance disagreement between the neutral (mean of [CrI/Fe]=+0.01) and ionized species (mean of [CrII/Fe]=+0.08), unlike most studies in the past. The prominent exception is Co, for which the neutral species indicates a significant overabundance (mean of [CoI/H]=-2.53), while no such enhancement is seen for the ionized species (mean of [CoII/H]=-2.93). These new stellar abundances, especially the correlations among Sc, Ti, and V, suggest that models of element production in early high-mass metal-poor stars should be revisited.
- ID:
- ivo://CDS.VizieR/J/ApJ/566/880
- Title:
- ISO-SWS observations of H II regions
- Short Name:
- J/ApJ/566/880
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present mid-infrared Infrared Space Observatory Short-Wavelength Spectrometer (ISO-SWS) observations of the fine-structure emissions lines [Ne II] 12.8{mu}m, [Ne III] 15.6{mu}m, [Ne III] 36.0{mu}m, [Ar II] 6.99{mu}m, [Ar III] 8.99{mu}m, [S III] 18.7{mu}m, [S III] 33.5{mu}m, and [S IV] 10.5{mu}m and the recombination lines Br{alpha} and Br{beta} n a sample of 112 Galactic H II regions and 37 nearby extra-Galactic H II regions in the LMC, SMC, and M33.
- ID:
- ivo://CDS.VizieR/J/A+A/644/A83
- Title:
- Jurassic structure
- Short Name:
- J/A+A/644/A83
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Detailed 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 (^28^Si) 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 alpha- (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 ~600000 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 (MW) populations or satellites.
- ID:
- ivo://CDS.VizieR/J/ApJS/96/175
- Title:
- Kinematics of Metal-Poor Stars. I.
- Short Name:
- J/ApJS/96/175
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We discuss the kinematic properties of a sample of 1936 Galactic stars, selected without kinematic bias, and with abundances [Fe/H] <= -0.6. The stars selected for this study all have measured radial velocities, and the majority have abundances determined from spectroscopic and narrow-/intermediate-band photometric techniques. In contrast to previous examinations of the kinematics of the metal-poor stars in the Galaxy, our sample contains large numbers of stars that are located at distances in excess of 1 kpc from the Galactic plane. Thus, a much clearer picture of the nature of the metal-deficient populations in the Galaxy can now be drawn. Our present data can be well described in terms of a two-component kinematic model consisting of a thick disk, rotating at roughly 200 km/s (independent of metal abundance), and an essentially nonrotating halo. The kinematics of these two components suggest a very broad overlap in metallicity; the thick disk is shown to possess an extremely metal-weak tail, extending to abundances even lower than previously reported, down to at least [Fe/H] ~ -2.0. A "minimal-assumptions" maximum-likelihood model is used to show that below [Fe/H] = -1.5 roughly 30% of stars in the solar neighbourhood can be kinematically associated with the thick disk. Over the metallicity interval -1.6 <= [Fe/H] <= -1.0, the thick-disk proportion rises to 60%. This fraction is only slightly smaller than contribution of thick-disk stars derived by Morrison, Flynn, and Freeman in the same metallicity interval (80%). Our confirmation that significant numbers of stars with thick-disk-like kinematics exist in the solar neighbourhood at arbitrarily low metal abundance suggests that previous disagreements about the correlation of population rotation velocities and metal abundance (e.g. Sandage & Fouts vs. Norris) may be due primarily to the selection criteria employed, and the resulting different contribution of metal-weak thick-disk stars to the respective data sets. The non-Gaussian nature of the velocity distribution of extremely metal-poor stars ([Fe/H] <= -1.5) in the directions of the Galactic poles reported by previous workers can also be understood as a consequence of the overlap between a cold metal-weak thick-disk population and a hot halo population. A maximum-likelihood technique has been developed in order to estimate the velocity ellipsoids of the thick-disk and halo components of the Galaxy. From the 349 stars in our sample with -1.0 <= [Fe/H] <= -0.6 and |z| <= 1 kpc, the velocity ellipsoid of the thick disk is (sigma_U, sigma_V, sigma_W) = (63 +/- 7, 42 +/- 4, 38 +/- 4) km/s. These values are in remarkably good accord with the predicted thick-disk velocity ellipsoid obtained by Quinn, Hernquist, and Fullagar from simulations of a satellite-merger formation scenario. Based on this velocity ellipsoid, a radial scale length for thick-disk stars of h_R = 4.7 +/- 0.5 kpc is obtained, larger than reported by Morrison, and similar to the value obtained for the old-disk population. However, the apparent equality of sigma_V and sigma_W is evidence that the thick disk is kinematically distinct from the old-disk population, where sigma_V:sigma_W ~ 2^{1/2}:1. We find a substantially smaller asymmetric-drift velocity gradient for presumed thick-disk stars (delta Vrot/delta |z| = -13 +/- 6 km/s/kpc) than reported by Majewski (delta Vrot/delta |z| = -21 +/- 1 km/s/kpc). From 887 stars in our sample with [Fe/H] <= -1.5 the local velocity ellipsoid of the halo is (sigma_r, sigma_phi, sigma_theta) = (153 +/- 10, 93 +/- 18, 107 +/- 7) km/s, that is, strongly radially peaked, as indicated by previous studies. We find little difference in the velocity ellipsoids of this sample when it is split into two roughly equal pieces with -2.2 <= [Fe/H] <= -1.5 and [Fe/H] <= -2.2, which indicates a lack of radial metallicity gradient in the halo, as found from studies of the Galactic globular cluster system. The velocity ellipsoid obtained from the small number of stars in our sample with Galactocentric distances r > 10 kpc (N = 61) is (sigma_r, sigma_phi, sigma_theta) = (115 +/- 18, 138 +/- 78, 110 +/- 24) km/s, much less radially elongated than found for the local sample.
- ID:
- ivo://CDS.VizieR/J/ApJS/238/16
- Title:
- LAMOST-DR3 very metal-poor star catalog
- Short Name:
- J/ApJS/238/16
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the result of a search for very metal-poor (VMP, [Fe/H]{<}-2.0) stars in the Milky Way based on low-resolution spectra from Large sky Area Multi-Object fiber Spectroscopic Telescope (LAMOST) DR3, significantly enlarging the current candidate sample of these low-metallicity objects. The selection procedure results in a sample of 10008 VMP stars covering a large area of sky in the Northern Hemisphere, and includes over 6800 targets brighter than V~16. This LAMOST DR3 VMP sample provides the largest number of VMP candidates to date that are sufficiently bright for follow-up high-resolution observation with 4-10m telescopes, greatly expanding the VMP stars discovered in the northern sky, and can be used to balance the spatial distribution of VMP stars with high-resolution spectroscopic analyses. Comparison with stars having existing high-resolution analyses and Tycho Gaia Astrometric Solution parallaxes indicates that the derived stellar parameters and distance estimates are reliable. The sample reaches beyond 40kpc in the halo, and contains over 670 candidates of extremely metal-poor ([Fe/H]{<}-3.0) and ultra-metal-poor ([Fe/H]{<}-4.0) stars. The distribution of V{phi} indicates that the sample consists of two halo components, with the retrograde component likely to be associated with the outer-halo population. A new criterion is proposed to select carbon-enhanced metal-poor (CEMP) star candidates, using line indices G1 and EGP over the range 4000K<Teff<7000K, resulting in 636 CEMP candidates from the LAMOST DR3 VMP sample.
- ID:
- ivo://CDS.VizieR/J/ApJ/891/39
- Title:
- LAMOST DR3 very metal-poor stars of the Galactic halo
- Short Name:
- J/ApJ/891/39
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We search for dynamical substructures in the LAMOST DR3 very metal-poor (VMP) star catalog. After cross-matching with Gaia DR2, there are ~3300 VMP stars with available high-quality astrometric information that have halo-like kinematics. We apply a method based on the self-organizing map StarGO to find groups clustered in the 4D space of orbital energy and angular momentum. We identify 57 dynamically tagged groups (DTGs), which we label DTG-1 to DTG-57. Most of them belong to existing massive substructures in the nearby halo, such as the Gaia Sausage or Sequoia. The stream identified by Helmi+ (1999Natur.402...53H) is recovered, but the two disjointed portions of the substructure appear to have distinct dynamical properties. The very retrograde substructure Rg5 found previously by Myeong+ (2018MNRAS.478.5449M) is also retrieved. We report six new DTGs with highly retrograde orbits, two with very prograde orbits, and 12 with polar orbits. By mapping other data sets (APOGEE halo stars, and catalogs of r-process-enhanced and carbon-enhanced metal-poor [CEMP] stars) onto the trained neuron map, we can associate stars with detailed chemical abundances with the DTGs and look for associations with chemically peculiar stars. The highly eccentric Gaia Sausage groups contain representatives of both debris from the satellite itself (which is {alpha}-poor) and the Splashed Disk, sent up into eccentric halo orbits from the encounter (and which is {alpha}-rich). The new prograde substructures also appear to be associated with the Splashed Disk. The DTGs belonging to the Gaia Sausage host two relatively metal-rich r-II stars and six CEMP stars in different subclasses, consistent with the idea that the Gaia Sausage progenitor is a massive dwarf galaxy. Rg5 is dynamically associated with two highly r-process-enhanced stars with [Fe/H]~-3. This finding indicates that its progenitor might be an ultrafaint dwarf galaxy that has experienced r-process enrichment from neutron star mergers.
