- ID:
- ivo://CDS.VizieR/J/ApJ/807/171
- Title:
- SkyMapper Survey metal-poor star spectroscopy
- Short Name:
- J/ApJ/807/171
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The SkyMapper Southern Sky Survey is carrying out a search for the most metal-poor stars in the Galaxy. It identifies candidates by way of its unique filter set which allows for estimation of stellar atmospheric parameters. The set includes a narrow filter centered on the CaII K 3933{AA} line, enabling a robust estimate of stellar metallicity. Promising candidates are then confirmed with spectroscopy. We present the analysis of Magellan Inamori Kyocera Echelle high-resolution spectroscopy of 122 metal-poor stars found by SkyMapper in the first two years of commissioning observations. Forty-one stars have [Fe/H]<=-3.0. Nine have [Fe/H]<=-3.5, with three at [Fe/H]~-4. A 1D LTE abundance analysis of the elements Li, C, Na, Mg, Al, Si, Ca, Sc, Ti, Cr, Mn, Co, Ni, Zn, Sr, Ba, and Eu shows these stars have [X/Fe] ratios typical of other halo stars. One star with low [X/Fe] values appears to be "Fe-enhanced", while another star has an extremely large [Sr/Ba] ratio: >2 Only one other star is known to have a comparable value. Seven stars are "CEMP-no" stars ([C/Fe]>0.7, [Ba/Fe]<0). 21 stars exhibit mild r-process element enhancements (0.3<=[Eu/Fe]<1.0), while four stars have [Eu/Fe]>=1.0. These results demonstrate the ability to identify extremely metal-poor stars from SkyMapper photometry, pointing to increased sample sizes and a better characterization of the metal-poor tail of the halo metallicity distribution function in the future.
Number of results to display per page
Search Results
- ID:
- ivo://CDS.VizieR/J/A+A/436/687
- Title:
- SMC NGC 346-11 and AV 304 spectra
- Short Name:
- J/A+A/436/687
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- An analysis of high-resolution VLT/UVES spectra of two B-type main sequence stars, NGC 346-11 and AV 304, in the Small Magellanic Cloud (SMC), has been undertaken, using the non-LTE TLUSTY model atmospheres to derive the stellar parameters and chemical compositions of each star. The chemical compositions of the two stars are in reasonable agreement. Moreover, our stellar analysis agrees well with earlier analyses of HII regions. The results derived here should be representative of the current base-line chemical composition of the SMC interstellar medium as derived from B-type stars.
- ID:
- ivo://CDS.VizieR/J/A+A/423/683
- Title:
- Sodium abundances in nearby disk stars
- Short Name:
- J/A+A/423/683
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present sodium abundances for a sample of nearby stars. All results have been derived from NLTE statistical equilibrium calculations. The influence of collisional interactions with electrons and hydrogen atoms is evaluated by comparison of the solar spectrum with very precise fits to the Na I line cores.
- ID:
- ivo://CDS.VizieR/J/AZh/80/458
- Title:
- Sodium abundances in stellar atmospheres
- Short Name:
- J/AZh/80/458
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The non-LTE sodium abundances of 100 dwarf, giant and subgiant stars with metallicities -3<[Fe/H]<0.3 are determined using high-dispersion spectra with high signal-to-noise ratios.
- ID:
- ivo://CDS.VizieR/J/A+A/554/A96
- Title:
- Spectra of LiI(670.8nm) line in metal-poor stars
- Short Name:
- J/A+A/554/A96
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Un-evolved, very metal-poor stars are the most important tracers of the cosmic abundance of lithium in the early universe. Combining the standard Big Bang nucleosynthesis model with Galactic production through cosmic ray spallation, these stars at [Fe/H]<-2 are expected to show an undetectably small ^6^Li/^7^Li isotopic signature. Evidence to the contrary may necessitate an additional pre-galactic production source or a revision of the standard model of Big Bang nucleosynthesis. It would also cast doubts on Li depletion from stellar atmospheres as an explanation for the factor 3-5 discrepancy between the predicted primordial ^7^Li from the Big Bang and the observed value in metal-poor dwarf/turn-off stars. We revisit the isotopic analysis of four halo stars, two with claimed ^6^Li-detections in the literature, to investigate the influence of improved model atmospheres and line formation treatment. For the first time, a combined 3D, non-local thermodynamic equilibrium (NLTE) modelling technique for Li, Na, and Ca lines is utilised to constrain the intrinsic line-broadening and to determine the Li isotopic ratio. We discuss the influence of 3D NLTE effects on line profile shapes and assess the realism of our modelling using the Ca excitation and ionisation balance. By accounting for NLTE line formation in realistic 3D hydrodynamical model atmospheres, we can model the Li resonance line and other neutral lines with a consistency that is superior to LTE, with no need for additional line asymmetry caused by the presence of ^6^Li. Contrary to the results from 1D and 3D LTE modelling, no star in our sample has a significant (2 sigma) detection of the lighter isotope in NLTE. Over a large parameter space, NLTE modelling systematically reduces the best-fit Li isotopic ratios by up to five percentage points. As a bi-product, we also present the first ever 3D NLTE Ca and Na abundances of halo stars, which reveal significant departures from LTE. The observational support for a significant and non-standard 6Li production source in the early universe is substantially weakened by our findings.
- ID:
- ivo://CDS.VizieR/J/AJ/103/267
- Title:
- Spectrocopy of hot stars in galactic halo
- Short Name:
- J/AJ/103/267
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The paper presents spectroscopy for 769 hot stars from the HK objective-prism/interference-filter survey of Beers et al. (1985), 193 of which have also available broadband UBV colors. When available, photometric information is used to obtain estimates of the surface temperature for degenerates and for O- and B-type subdwarfs, based on previously derived calibrations. Several extremely hot (T(eff) greater than 50,000 K) He-rich sdO stars are identified. Out of 769 stars in this sample, 551 are clearly identified on the basis of their Balmer line profiles as field horizontal-branch (FHB) or mid-to late-type A stars. The A stars exhibit rotation and line-of-sight dispersion consistent with membership in the galactic thick disk. The FHB stars exhibit kinematic properties which suggest a transition from thick disk to halo.
- ID:
- ivo://CDS.VizieR/J/A+A/649/A49
- Title:
- Spectroscopic study of CEMP-(s and r/s) stars
- Short Name:
- J/A+A/649/A49
- Date:
- 22 Feb 2022
- Publisher:
- CDS
- Description:
- The origin of the enhanced abundances of both s- and r-process elements observed in a subclass of Carbon-Enhanced Metal-Poor (CEMP) stars, i.e., CEMP-r/s stars, still remains poorly understood. The i-process nucleosynthesis has been suggested as one of the most promising mechanisms for the origin of these stars. To better understand the chemical signatures and formation mechanism(s) of five previously claimed potential CH star candidates HE 0017+0055, HE 2144-1832, HE 2339-0837, HD 145777 and CD-27 14351 through a detailed systematic follow-up spectroscopic study based on high-resolution spectra. The stellar atmospheric parameters, the effective temperature T_eff_, the microturbulent velocity {zeta}, the surface gravity logg, and the metallicity [Fe/H] are derived from local thermodynamic equilibrium analyses using model atmospheres. Elemental abundances of C, N, {alpha}-elements, iron-peak elements and several neutron-capture elements are estimated using the equivalent width measurement technique as well as spectrum synthesis calculations in some cases. In the context of the double enhancement observed in four of the programme stars, we have critically examined if the literature i-process model yields ([X/Fe]) of heavy elements can explain the observed abundance distribution. The estimated metallicity [Fe/H] of the programme stars ranges from -1.63 to -2.74. All the five stars show enhanced abundance for Ba and four of them exhibit enhanced abundance for Eu. Based on our analysis, HE 0017+0055, HE 2144-1832 and HE 2339-0837 are found to be CEMP-r/s stars, whereas HD 145777 and CD-27 14351 show characteristic properties of CEMP-s stars. From a detailed analysis of different classifiers of CEMP stars, we have identified the one which best describes the CEMP-s and CEMP-r/s stars. We found that for both CEMP-s and CEMP-r/s stars, [Ba/Eu] and [La/Eu] exhibit positive values and [Ba/Fe]>=1.0. However, CEMP-r/s stars satisfy [Eu/Fe]>=1.0, 0.0<=[Ba/Eu]<=1.0 and/or 0.0<=[La/Eu]<=0.7. CEMP-s stars normally show [Eu/Fe]<1.0 with [Ba/Eu]>0.0 and/or [La/Eu]>0.5. If [Eu/Fe]~>=1.0, then the condition on [Ba/Eu] and/or [La/Eu] for a star to be a CEMP-s star is [Ba/Eu]>1.0 and/or [La/Eu]>0.7. Using a large sample of similar stars from the literature we have examined if [hs/ls] alone can be used as a classifier, and if there are any limiting values for [hs/ls] ratio that can be used to distinguish CEMP-s and CEMP-r/s stars. In spite of peaking at different values of [hs/ls], CEMP-s and CEMP-r/s stars show an overlap in the range 0.0<[hs/ls]<1.5 ,and hence, this ratio cannot be used to distinguish CEMP-s and CEMP-r/s stars. We have noticed a similar overlap in the case of [Sr/Ba] ratio as well, in the range -1.6<[Sr/Ba]<-0.5 and hence, this ratio also cannot be used to separate the two subclasses, CEMP-s and CEMP-r/s.
- ID:
- ivo://CDS.VizieR/J/AJ/155/256
- Title:
- Spectroscopic validation of RAVE metal-poor stars
- Short Name:
- J/AJ/155/256
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present results from a medium-resolution (R~2000) spectroscopic follow-up campaign of 1694 bright (V<13.5), very metal-poor star candidates from the RAdial Velocity Experiment (RAVE). Initial selection of the low-metallicity targets was based on the stellar parameters published in RAVE Data Releases 4 (Cat. III/272) and 5 (Cat. III/279). Follow up was accomplished with the Gemini-N and Gemini-S, the ESO/NTT, the KPNO/Mayall, and the SOAR telescopes. The wavelength coverage for most of the observed spectra allows for the determination of carbon and {alpha}-element abundances, which are crucial for considering the nature and frequency of the carbon-enhanced metal-poor (CEMP) stars in this sample. We find that 88% of the observed stars have [Fe/H]=< -1.0, 61% have [Fe/H]=< -2.0, and 3% have [Fe/H]=< -3.0 (with four stars at [Fe/H]=< -3.5). There are 306 CEMP star candidates in this sample, and we identify 169 CEMP Group I, 131 CEMP Group II, and 6 CEMP Group III stars from the A(C) versus [Fe/H] diagram. Inspection of the [{alpha}/C] abundance ratios reveals that five of the CEMP Group II stars can be classified as "mono-enriched second-generation" stars. Gaia DR1 matches were found for 734 stars, and we show that transverse velocities can be used as a confirmatory selection criteria for low-metallicity candidates. Selected stars from our validated list are being followed-up with high-resolution spectroscopy to reveal their full chemical-abundance patterns for further studies.
- ID:
- ivo://CDS.VizieR/J/ApJ/862/174
- Title:
- Spectroscopy obs. of RAVE J094921.8-161722
- Short Name:
- J/ApJ/862/174
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a high-resolution (R~35000), high signal-to-noise ratio (S/N>200) Magellan/MIKE spectrum of the star RAVE J094921.8-161722, a bright (V=11.3) metal-poor red giant star with [Fe/H]=-2.2, identified as a carbon-enhanced metal-poor (CEMP) star from the RAVE survey. We report its detailed chemical abundance signature of light fusion elements and heavy neutron-capture elements. We find J0949-1617 to be a CEMP star with s-process enhancement that must have formed from gas enriched by a prior r-process event. Light neutron-capture elements follow a low-metallicity s-process pattern, while the heavier neutron-capture elements above Eu follow an r-process pattern. The Pb abundance is high, in line with an s-process origin. Thorium is also detected, as expected from an r-process origin, as Th is not produced in the s-process. We employ nucleosynthesis model predictions that take an initial r-process enhancement into account, and then determine the mass transfer of carbon and s-process material from a putative more massive companion onto the observed star. The resulting abundances agree well with the observed pattern. We conclude that J0949-1617 is the first bonafide CEMP-r+s star identified. This class of objects has previously been suggested to explain stars with neutron-capture element patterns that originate from neither the r- nor the s-process alone. We speculate that J0949-1617 formed in an environment similar to those of ultra-faint dwarf galaxies like TucanaIII and ReticulumII, which were enriched in r-process elements by one or multiple neutron star mergers at the earliest times.
- ID:
- ivo://CDS.VizieR/J/ApJ/870/122
- Title:
- Spectroscopy of low-metallicity star candidates
- Short Name:
- J/ApJ/870/122
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present results from an observing campaign to identify low-metallicity stars in the Best & Brightest Survey. From medium-resolution (R~1200-2000) spectroscopy of 857 candidates, we estimate the stellar atmospheric parameters (T_eff_, logg, and [Fe/H]), as well as carbon and {alpha}-element abundances. We find that 69% of the observed stars have [Fe/H]<=-1.0, 39% have [Fe/H]<=-2.0, and 2% have [Fe/H]<=-3.0. There are also 133 carbon-enhanced metal-poor (CEMP) stars in this sample, with 97 CEMP Group I and 36 CEMP Group II stars identified in the A(C) versus [Fe/H] diagram. A subset of the confirmed low-metallicity stars were followed-up with high-resolution spectroscopy, as part of the R-process Alliance, with the goal of identifying new highly and moderately r-process-enhanced stars. Comparison between the stellar atmospheric parameters estimated in this work and from high-resolution spectroscopy exhibit good agreement, confirming our expectation that medium-resolution observing campaigns are an effective way of selecting interesting stars for further, more targeted, efforts.
