- ID:
- ivo://CDS.VizieR/J/ApJ/819/110
- Title:
- Extremely metal-poor galaxies in SDSS. II.
- Short Name:
- J/ApJ/819/110
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Extremely metal-poor (XMP) galaxies are defined to have a gas-phase metallicity smaller than a tenth of the solar value (12+log[O/H]<7.69). They are uncommon, chemically and possibly dynamically primitive, with physical conditions characteristic of earlier phases of the universe. We search for new XMPs in the Sloan Digital Sky Survey (SDSS) in a work that complements Paper I (Morales-Luis+, 2011, J/ApJ/743/77). This time, high electron temperature objects are selected; metals are a main coolant of the gas, so metal-poor objects contain high-temperature gas. Using the algorithm k-means, we classify 788677 spectra to select 1281 galaxies that have particularly intense [OIII]{lambda}4363 with respect to [OIII]{lambda}5007, which is a proxy for high electron temperature. The metallicity of these candidates was computed using a hybrid technique consistent with the direct method, rendering 196 XMPs. A less restrictive noise constraint provides a larger set with 332 candidates. Both lists are provided in electronic format. The selected XMP sample has a mean stellar mass around 10^8^M_{sun}_, with the dust mass ~10^3^M_{sun}_ for typical star-forming regions. In agreement with previous findings, XMPs show a tendency to be tadpole-like or cometary. Their underlying stellar continuum corresponds to a fairly young stellar population (<1Gyr), although young and aged stellar populations coexist at the low-metallicity starbursts. About 10% of the XMPs show large N/O. Based on their location in constrained cosmological numerical simulations, XMPs have a strong tendency to appear in voids and to avoid galaxy clusters. The puzzling 2%-solar low-metallicity threshold exhibited by XMPs remains.
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Search Results
- ID:
- ivo://CDS.VizieR/J/AJ/128/2402
- Title:
- Extremely metal-poor star candidates abundances
- Short Name:
- J/AJ/128/2402
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present chemical abundances for 110 stars identified in objective-prism surveys as candidates for being very metal-poor. The abundances are derived from high-S/N, intermediate-resolution spectra obtained with the Keck Observatory Echellette Spectrograph and Imager (ESI). An additional 25 stars with well-determined abundances ranging from [Fe/H]=-1.5 to -3.2 were observed and the results used to help calibrate our analysis and determine the accuracy of our abundance determinations. Abundances for the program stars were measured for Fe, Mg, Ca, Ti, Cr, and Ba with an accuracy of approximately 0.3dex. Fifty-three of the stars in our sample have [Fe/H]<=-2, 22 have [Fe/H]<=-2.5, and 13 have [Fe/H]<=-2.9. Surprisingly, approximately one-third of the sample is relatively metal-rich, with [Fe/H]>-1.5. In addition to identifying a number of extremely metal-poor stars, this study also shows that moderate-resolution spectra obtained with the Keck ESI yield relatively accurate abundances for stars as faint as V=14 with modest exposure time (~20 minutes). This capability will prove useful if the so-far elusive stars at [Fe/H]<-4 turn out to be mostly fainter than V=15.
- ID:
- ivo://CDS.VizieR/J/A+A/501/519
- Title:
- Extremely metal-poor turnoff stars abundances
- Short Name:
- J/A+A/501/519
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The detailed chemical abundances of extremely metal-poor (EMP) stars are key guides to understanding the early chemical evolution of the Galaxy. Most existing data, however, treat giant stars that may have experienced internal mixing later. We aim to compare the results for giants with new, accurate abundances for all observable elements in 18 EMP turnoff stars. VLT/UVES spectra at ~45000 and S/N ~130 per pixel (330-1000nm) are analysed with OSMARCS model atmospheres and the TURBOSPECTRUM code to derive abundances for C, Mg, Si, Ca, Sc, Ti, Cr, Mn, Co, Ni, Zn, Sr, and Ba. For Ca, Ni, Sr, and Ba, we find excellent consistency with our earlier sample of EMP giants, at all metallicities. However, our abundances of C, Sc, Ti, Cr, Mn and Co are ~0.2dex larger than in giants of similar metallicity. Mg and Si abundances are ~0.2dex lower (the giant [Mg/Fe] values are slightly revised), while Zn is again ~0.4dex higher than in giants of similar [Fe/H] (6 stars only). For C, the dwarf/giant discrepancy could possibly have an astrophysical cause, but for the other elements it must arise from shortcomings in the analysis. Approximate computations of granulation (3D) effects yield smaller corrections for giants than for dwarfs, but suggest that this is an unlikely explanation, except perhaps for C, Cr, and Mn. NLTE computations for Na and Al provide consistent abundances between dwarfs and giants, unlike the LTE results, and would be highly desirable for the other discrepant elements as well. Meanwhile, we recommend using the giant abundances as reference data for Galactic chemical evolution models.
- ID:
- ivo://CDS.VizieR/J/ApJ/743/77
- Title:
- Extremely metal-poor (XMP) galaxies in SDSS
- Short Name:
- J/ApJ/743/77
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We carry out a systematic search for extremely metal-poor (XMP) galaxies in the spectroscopic sample of Sloan Digital Sky Survey (SDSS) data release 7 (DR7). The XMP candidates are found by classifying all the galaxies according to the form of their spectra in a region 80{AA} wide around H{alpha}. Due to the data size, the method requires an automatic classification algorithm. We use k-means. Our systematic search renders 32 galaxies having negligible [NII] lines, as expected in XMP galaxy spectra. Twenty-one of them have been previously identified as XMP galaxies in the literature - the remaining 11 are new. This was established after a thorough bibliographic search that yielded only some 130 galaxies known to have an oxygen metallicity 10 times smaller than the Sun (explicitly, with 12+log(O/H)<=7.65). XMP galaxies are rare; they represent 0.01% of the galaxies with emission lines in SDSS/DR7. Although the final metallicity estimate of all candidates remains pending, strong-line empirical calibrations indicate a metallicity about one-tenth solar, with the oxygen metallicity of the 21 known targets being 12+log(O/H)~7.61+/-0.19. Since the SDSS catalog is limited in apparent magnitude, we have been able to estimate the volume number density of XMP galaxies in the local universe, which turns out to be (1.32+/-0.23)x10^-4^Mpc^-3^. The XMP galaxies constitute 0.1% of the galaxies in the local volume, or ~0.2% considering only emission-line galaxies. All but four of our candidates are blue compact dwarf galaxies, and 24 of them have either cometary shape or are formed by chained knots.
- ID:
- ivo://CDS.VizieR/J/MNRAS/358/13
- Title:
- Faint dwarfs in NGC 2547
- Short Name:
- J/MNRAS/358/13
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Intermediate resolution spectroscopy from the European Southern Observatory Very Large Telescope is analysed for 63 photometrically selected low-mass (0.08-0.30M_{sun}_) candidates of the open cluster NGC 2547. We have confirmed membership for most of these stars using radial velocities, and found that lithium remains undepleted for cluster stars with I>17.54+/-0.14 and Ks>14.86+/-0.12. From these results, several pre-main-sequence evolutionary models give almost model independent ages of 34-36Myr, with a precision of 10%. These ages are only slightly larger than the ages of 25-35(+/-5)Myr obtained using the same models to fit isochrones to higher mass stars descending towards the zero-age main-sequence, both in empirically calibrated and theoretical colour-magnitude diagrams. This agreement between age determinations in different mass ranges is an excellent test of the current generation of low-mass pre-main-sequence stellar models and lends confidence to ages determined with either method between 30 and 120Myr.
- ID:
- ivo://CDS.VizieR/J/ApJ/417/209
- Title:
- Faint planetary nebulae in Magellanic Clouds
- Short Name:
- J/ApJ/417/209
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The accuracy of spectrophotometry for Magellanic Cloud planetary nebulae is limited by a number of physical effects and operational difficulties: atmospheric dispersion, wavelength-dependent seeing, pointing and guiding errors, and a background of numerous stars and diffuse emission. We describe procedures to minimize the impact of some of these. We then compare our results for both bright and faint objects with published values. There are a few exceptional cases (e.g., LMC 89), but generally we find that bright planetary nebulae in the Clouds have been observed with high accuracy. On the other hand, observations of the faint sample from Jacoby (1980ApJS...42....1J) suffer seriously from many of these effects. We also compare published {lambda}5007 photometry with new CCD photometry for nine faint planetary nebulae. We find the photographic photometry presented by Jacoby (1980ApJS...42....1J) to be accurate to 0.26mag. Fluxes for the fainter objects that are derived from spectrophotometry (Boroson & Liebert, 1989ApJ...339..844B) are found to be less reliable. In comparison, spectrophotometry using the observational techniques presented in this paper provides reliable absolute fluxes. With these improved observations, we review the correlations presented by Kaler & Jacoby (1990ApJ...362..491K, 1991ApJ...382..134K) between abundance ratios and central star mass. The new results fit our earlier correlation for N/O, strengthen that for He/H, and change little about those for C/O and O/H.
- ID:
- ivo://CDS.VizieR/J/A+A/384/491
- Title:
- Fast-rotating nearby solar-type stars. I.
- Short Name:
- J/A+A/384/491
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the results of high-resolution spectroscopic and high-precision photometric observations on a sample of 129 late-F and G-type nearby stars selected on the basis of their large rotational velocity. Using also data from the Hipparcos satellite, CORAVEL and from the ROSAT satellite database, we infer spectral types, compute radial velocities, vsini, Li abundances and X-ray luminosities and investigate the single or binary nature of the sample stars. Such a careful analysis of our sample shows a large fraction of binaries (~62%) and of young single disk stars. In particular, at least 9 stars can be considered bona-fide PMS or ZAMS objects, and 30 stars are identified as SBs for the first time. Information on the presence of Ca II K emission and on optical variability is given for some of the stars of the sample. The photometry has been obtained at the European Southern Observatory (ESO-La silla, Chile) during several observing runs (19/11-03/12 1993, 24/11-04/12 1994, 15-28/01/1995 and 1-12/10/1995), by using the 50cm ESO telescope. The figures were published in paper II.
- ID:
- ivo://CDS.VizieR/J/A+A/527/A134
- Title:
- Fe abundances for 28 XMM galaxy clusters
- Short Name:
- J/A+A/527/A134
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The abundances of Fe in the intracluster medium of nearby (z<0.08) clusters were measured up to 0.3~0.5r_180_. We analyzed 28 clusters of galaxies observed with XMM-Newton. We derived Fe abundances from the flux ratios of Fe lines to the continuum within an energy range of 3.5-6keV to minimize and evaluate systematic uncertainties.
- ID:
- ivo://CDS.VizieR/J/A+A/522/A26
- Title:
- Fe Abundances in metal-poor stars
- Short Name:
- J/A+A/522/A26
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The determination of Lithium abundance A(Li) is extremely sensitive to the adopted temperature scale, so that a Teff bias might mimic a trend in A(Li). We present here VLT-UVES Li abundances for 28 halo dwarf stars between [Fe/H]=-2.5 and -3.5, 10 of which have [Fe/H]<-3. Four different Teff scales have been used: Direct Infrared Flux Method (IRFM) has been used on the basis of 2MASS infrared photometry; H{alpha} wings have been fitted against two synthetic grids computed by means of 1D LTE atmosphere models, and a grid of H{alpha} profiles computed by means of 3D hydrodynamical atmosphere models. As a result, we confirm previous claims that A(Li) does not exhibit a plateau below [Fe/H]=-3. A strong positive correlation with [Fe/H] appears, not influenced by the choice of the Teff estimator. From a linear fit, we obtain a strong slope of about 0.30 dex in A(Li) per dex in [Fe/H], significant to 2-3, and consistent within 1 among all the four Teff estimators. A significant slope is also detected in the A(Li)-Teff plane, driven mainly by the coolest stars in the sample (Teff<6250), which appear Li-poor. However, removing such stars does not alter significantly the behavior in the A(Li)-[Fe/H] plane. When the full sample is considered, the scatter in A(Li) increases by a factor of 2 towards lower metallicities, while the plateau appears very thin above [Fe/H]=-2.8. At this metallicity, the plateau lies at A(Li)3D,NLTE=2.199+/-0.086.
- ID:
- ivo://CDS.VizieR/J/PASP/121/117
- Title:
- Fe and Ti abundances of 12 low-metallicity M stars
- Short Name:
- J/PASP/121/117
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have calculated Fe and Ti abundances in 12 low-metallicity main sequence M stars using high-resolution spectra. These subdwarf and extreme-subdwarf stars allow us to extend our calibration of a method to determine cool-dwarf-star metallicities using molecular band strength indices from low-resolution spectra. Our calibration can now be used to determine metallicity to within +/-0.3dex for stars with [Fe/H] between -1.5 and +0.05 and temperatures between 3500 and 4000K. We also report a method to estimate temperatures for M dwarfs using equivalent width measurements of the infrared CaII triplet and the KI line at 7699{AA}. Our metallicity measurements show that the recently proposed classification system for low-mass stars (dwarfs, subdwarfs, extreme subdwarfs, and ultrasubdwarfs) does represent a metallicity sequence, with the ultrasubdwarfs the most metal-poor stars.
