- ID:
- ivo://CDS.VizieR/J/ApJ/703/2177
- Title:
- Metal-poor MS turnoff stars summary
- Short Name:
- J/ApJ/703/2177
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We identify 10 -- seven for the first time -- elements of cold halo substructure (ECHOS) in the volume within 17.5kpc of the Sun in the inner halo of the Milky Way. Our result is based on the observed spatial and radial velocity distribution of metal-poor main-sequence turnoff (MPMSTO) stars in 137 Sloan Extension for Galactic Understanding and Exploration lines of sight. We show that all of our detections are statistically significant and that we expect no false positives. These ECHOS represent the observable stellar debris of ancient merger events in the stellar accretion history of the Milky Way, and we use our detections and completeness estimates to infer a formal upper limit of 0.34^+0.02^_-0.02_ on the fraction of the MPMSTO population in the inner halo that belong to ECHOS.
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Search Results
- ID:
- ivo://CDS.VizieR/J/ApJ/874/148
- Title:
- Metal-poor star RAVE J093730.5-062655 abundances
- Short Name:
- J/ApJ/874/148
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- A new moderately r-process-enhanced metal-poor star, RAVEJ093730.5-062655, has been identified in the Milky Way halo as part of an ongoing survey by the R-Process Alliance. The temperature and surface gravity indicate that J0937-0626 is likely a horizontal branch star. At [Fe/H]=-1.86, J0937-0626 is found to have subsolar [X/Fe] ratios for nearly every light, {alpha}, and Fe-peak element. The low [{alpha}/Fe] ratios can be explained by an ~0.6dex excess of Fe; J0937-0626 is therefore similar to the subclass of "iron-enhanced" metal-poor stars. A comparison with Milky Way field stars at [Fe/H]=-2.5 suggests that J0937-0626 was enriched in material from an event, possibly a Type Ia supernova, that created a significant amount of Cr, Mn, Fe, and Ni and smaller amounts of Ca, Sc, Ti, and Zn. The r-process enhancement of J0937-0626 is likely due to a separate event, which suggests that its birth environment was highly enriched in r-process elements. The kinematics of J0937-0626, based on Gaia DR2 data, indicate a retrograde orbit in the Milky Way halo; J0937-0626 was therefore likely accreted from a dwarf galaxy that had significant r-process enrichment.
- ID:
- ivo://CDS.VizieR/J/ApJ/781/40
- Title:
- Metal-poor stars from HES survey. II. Spectroscopy
- Short Name:
- J/ApJ/781/40
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We report on the discovery of seven low-metallicity stars selected from the Hamburg/ESO Survey, six of which are extremely metal-poor (EMP, [Fe/H]{<=}-3.0), with four having [Fe/H]{<=}-3.5. Chemical abundances or upper limits are derived for these stars based on high-resolution (R~35000) Magellan/MIKE spectroscopy, and are in general agreement with those of other very and extremely metal-poor stars reported in the literature. Accurate metallicities and abundance patterns for stars in this metallicity range are of particular importance for studies of the shape of the metallicity distribution function of the Milky Way's halo system, in particular for probing the nature of its low-metallicity tail. In addition, taking into account suggested evolutionary mixing effects, we find that six of the program stars (with [Fe/H]{<=}-3.35) possess atmospheres that were likely originally enriched in carbon, relative to iron, during their main-sequence phases. These stars do not exhibit overabundances of their s-process elements, and hence may be, within the error bars, additional examples of the so-called CEMP-no class of objects.
- ID:
- ivo://CDS.VizieR/J/AJ/145/13
- Title:
- Metal-poor stars from SDSS/SEGUE. I. Abundances
- Short Name:
- J/AJ/145/13
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Chemical compositions are determined based on high-resolution spectroscopy for 137 candidate extremely metal-poor (EMP) stars selected from the Sloan Digital Sky Survey (SDSS) and its first stellar extension, the Sloan Extension for Galactic Understanding and Exploration (SEGUE). High-resolution spectra with moderate signal-to-noise (S/N) ratios were obtained with the High Dispersion Spectrograph of the Subaru Telescope. Most of the sample (approximately 80%) are main-sequence turnoff stars, including dwarfs and subgiants. Four cool main-sequence stars, the most metal-deficient such stars known, are included in the remaining sample. Good agreement is found between effective temperatures estimated by the SEGUE stellar parameter pipeline, based on the SDSS/SEGUE medium-resolution spectra, and those estimated from the broadband (V - K)_0_ and (g - r)_0_colors. Our abundance measurements reveal that 70 stars in our sample have [Fe/H] < -3, adding a significant number of EMP stars to the currently known sample. Our analyses determine the abundances of eight elements (C, Na, Mg, Ca, Ti, Cr, Sr, and Ba) in addition to Fe. The fraction of carbon-enhanced metal-poor stars ([C/Fe] > +0.7) among the 25 giants in our sample is as high as 36%, while only a lower limit on the fraction (9%) is estimated for turnoff stars. This paper is the first of a series of papers based on these observational results. The following papers in this series will discuss the higher-resolution and higher-S/N observations of a subset of this sample, the metallicity distribution function, binarity, and correlations between the chemical composition and kinematics of extremely metal-poor stars.
- ID:
- ivo://CDS.VizieR/J/AJ/154/52
- Title:
- Metal-poor stars from SDSS/SEGUE. I Unevolved stars
- Short Name:
- J/AJ/154/52
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present elemental abundances for eight unevolved extremely metal-poor (EMP) stars with T_eff_>5500K, among which seven have [Fe/H]{<}-3.5. The sample is selected from the Sloan Digital Sky Survey/Sloan Extension for Galactic Understanding and Exploration (SDSS/SEGUE) and our previous high-resolution spectroscopic follow-up with the Subaru Telescope. Several methods to derive stellar parameters are compared, and no significant offset in the derived parameters is found in most cases. From an abundance analysis relative to the standard EMP star G64-12, an average Li abundance for stars with [Fe/H]<-3.5 is A(Li)=1.90, with a standard deviation of {sigma}=0.10dex. This result confirms that lower Li abundances are found at lower metallicity, as suggested by previous studies, and demonstrates that the star-to-star scatter is small. The small observed scatter could be a strong constraint on Li-depletion mechanisms proposed for explaining the low Li abundance at lower metallicity. Our analysis for other elements obtained the following results: (i) a statistically significant scatter in [X/Fe] for Na, Mg, Cr, Ti, Sr, and Ba, and an apparent bimodality in [Na/Fe] with a separation of ~0.8dex, (ii) an absence of a sharp drop in the metallicity distribution, and (iii) the existence of a CEMP-s star at [Fe/H]{simeq}-3.6 and possibly at [Fe/H]{simeq}-4.0, which may provide a constraint on the mixing efficiency of unevolved stars during their main-sequence phase.
- ID:
- ivo://CDS.VizieR/J/AJ/142/188
- Title:
- Metal-poor stars from the HES using CH G-band
- Short Name:
- J/AJ/142/188
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We describe a new method to search for metal-poor candidates from the Hamburg/ESO objective-prism survey (HES) based on identifying stars with apparently strong CH G-band strengths for their colors. The hypothesis we exploit is that large overabundances of carbon are common among metal-poor stars, as has been found by numerous studies over the past two decades. The selection was made by considering two line indices in the 4300{AA} region, applied directly to the low-resolution prism spectra. This work also extends a previously published method by adding bright sources to the sample. The spectra of these stars suffer from saturation effects, compromising the index calculations and leading to an undersampling of the brighter candidates. A simple numerical procedure, based on available photometry, was developed to correct the line indices and overcome this limitation. Visual inspection and classification of the spectra from the HES plates yielded a list of 5288 new metal-poor (and by selection, carbon-rich) candidates, which are presently being used as targets for medium-resolution spectroscopic follow-up. Estimates of the stellar atmospheric parameters, as well as carbon abundances, are now available for 117 of the first candidates, based on follow-up medium-resolution spectra obtained with the SOAR 4.1m and Gemini 8m telescopes. There are eight newly discovered stars with [Fe/H] < -3.0 in our sample, including two with [Fe/H] < -3.5.
- ID:
- ivo://CDS.VizieR/J/A+A/640/A25
- Title:
- Metal-poor stars limb-darkening coefficients
- Short Name:
- J/A+A/640/A25
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Benchmark stars are crucial as validating standards for current as well as future large stellar surveys of the Milky Way. However, the number of suitable metal-poor benchmark stars is currently limited, owing to the difficulty in determining reliable effective temperatures (Teff) in this regime. We aim to construct a new set of metal-poor benchmark stars, based on reliable interferometric effective temperature determinations and a homogeneous analysis. The aim is to reach a precision of 1% in Teff, as is crucial for sufficiently accurate determinations of the full set of fundamental parameters and abundances for the survey sources. We observed ten late type metal-poor dwarf and giants: HD 2665, HD 6755, HD 6833, HD 103095, HD 122563, HD 127243, HD 140283, HD 175305, HD 221170 and HD 224930. Only three of the ten stars (HD 103095, HD 122563 and HD 140283) have previously been used as benchmark stars. For the observations, we used the high angular resolution optical interferometric instrument PAVO at the CHARA array. We modelled angular diameters using 3D limb darkening models and determined effective temperatures directly from the Stefan-Boltzmann relation, with an iterative procedure to interpolate over tables of bolometric corrections. Surface gravities (log(g)) were estimated from comparisons to Dartmouth stellar evolution model tracks. We collected spectroscopic observations from the ELODIE and FIES spectrographs and estimated metallicities ([Fe/H]) from a 1D non-LTE abundance analysis of unblended lines of neutral and singly ionized iron. We inferred Teff to better than 1% for five of the stars (HD 103095, HD 122563, HD 127243, HD 140283 and HD 224930). The effective temperatures of the other five stars are reliable to between 2-3%; the higher uncertainty on the Teff for those stars is mainly due to their having a larger uncertainty in the bolometric fluxes. We also determined log(g) and [Fe/H] with median uncertainties of 0.03dex and 0.09dex, respectively. This study presents reliable and homogeneous fundamental stellar parameters for ten metal-poor stars that can be adopted as a new set of benchmarks. The parameters are based on our consistent approach of combining interferometric observations, 3D limb darkening modelling and spectroscopic observations. The next paper in this series will extend this approach to dwarfs and giants in the metal-rich regime.
- ID:
- ivo://CDS.VizieR/J/A+A/300/751
- Title:
- Metal-poor stars spectroscopy. II
- Short Name:
- J/A+A/300/751
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Models of post-main sequence stellar evolution of VandenBerg & Bell have been applied to determine spectroscopic masses and distances for metal-poor stars. Careful consideration of the most important error sources published in more recent papers such as VandenBerg for the first time allow us to draw firm statistical conclusions. It is shown that the evolutionary calculations qualitatively fit to the observed stellar parameters whereas quantitatively they predict too high ages for metal-poor stars. As an important result we confirm that evolutionary sequences need to be calibrated with respect to their metal abundance in order to use their absolute predictions of temperature and luminosity. In our spectroscopic analyses the strong dependence between surface gravity and abundances determined from Fe I lines restricts the accuracy of Fe abundances in subgiants to 0.1 dex at best. The most remarkable result of our evolutionary and kinematic investigations of halo stars refers to the large fraction of slightly evolved subgiants among the so-called subdwarfs. Since conventional photometric approaches often assume that the great majority of metal-poor stars are dwarfs this results in distances that are systematically too low for their samples.
- ID:
- ivo://CDS.VizieR/J/A+A/587/A124
- Title:
- Metal-poor stars towards the Galactic bulge
- Short Name:
- J/A+A/587/A124
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a comprehensive chemical abundance analysis of five red giants and two horizontal branch (HB) stars towards the south- ern edge of the Galactic bulge, at (l, b)~(0{deg}, -11{deg}). Based on high-resolution spectroscopy obtained with the Magellan/MIKE spectrograph, we derived up to 23 chemical element abundances and identify a mixed bag of stars, representing various populations in the central regions of the Galaxy. Although cosmological simulations predict that the inner Galaxy was host to the first stars in the Universe, we see no chemical evidence of the ensuing massive supernova explosions: all of our targets exhibit halo-like, solar [Sc/Fe] ratios, which is in contrast to the low values predicted from Population III nucleosynthesis. One of the targets is a CEMP-s star at [Fe/H]=-2.52dex, and another target is a moderately metal-poor ([Fe/H]=-1.53dex) CH star with strong enrichment in s-process elements (e.g., [Ba/Fe]=1.35). These individuals provide the first contenders of these classes of stars towards the bulge. Four of the carbon-normal stars exhibit abundance patterns reminiscent of halo star across a metallicity range spanning -2.0 to -2.6dex, i.e., enhanced {alpha}-elements and solar Fe-peak and neutron-capture elements, and the remaining one is a regular metal-rich bulge giant. The position, distance, and radial velocity of one of the metal-poor HB stars coincides with simulations of the old trailing arm of the disrupted Sagittarius dwarf galaxy. While their highly uncertain proper motions prohibit a clear kinematic separation, the stars' chemical abundances and distances suggest that these metal-poor candidates, albeit located towards the bulge, are not of the bulge, but rather inner halo stars on orbits that make them pass through the central regions. Thus, we caution similar claims of detections of metal-poor stars as true habitants of the bulge.
- ID:
- ivo://CDS.VizieR/J/A+A/445/939
- Title:
- Metal-poor stars uvby-beta photometry. XI.
- Short Name:
- J/A+A/445/939
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- New uvby-beta data are provided for 442 high-velocity and metal-poor stars; 90 of these stars have been observed previously by us, and 352 are new. When combined with our previous two photometric catalogues, the data base is now made up of 1533 high-velocity and metal-poor stars, all with photometry and complete kinematic data. Hipparcos, plus a new photometric calibration for M_v_ also based on the Hipparcos parallaxes, provide distances for nearly all of these stars; our previous photometric calibrations give values for E(b-y) and [Fe/H]. The [Fe/H],V(rot) diagram allows us to separate these stars into different Galactic stellar population groups, such as old-thin-disk, thick-disk, and halo. The X histogram, where X is our stellar-population discriminator combining V(rot) and [Fe/H], and contour plots for the [Fe/H],V(rot) diagram both indicate two probable components to the thick disk.
