| 1. |
- Sivarani, T., et al.
(författare)
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First stars X. The nature of three unevolved carbon-enhanced metal-poor stars
- 2006
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 459:1, s. 125-135
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Tidskriftsartikel (refereegranskat)abstract
- Context. On the order of 20% of the very metal-poor stars in the Galaxy exhibit large carbon enhancements. It is important to establish which astrophysical sites and processes are responsible for the elemental abundance patterns of this early Galactic population. Aims. We seek to understand the nature of the progenitors of three main-sequence turnoff Carbon-Enhanced Metal-Poor (CEMP) stars, CS 31080-095, CS 22958-042, and CS 29528-041, based on a detailed abundance analysis. Methods. From high-resolution VLT/UVES spectra (R similar to 43 000), we determine abundances or upper limits for Li, C, N, O, and other important elements, as well as C-12/C-13 isotopic ratios. Results. All three stars have -3.30 <= [Fe/H]<= -2.85 and moderate to high CNO abundances. CS 22958-042 is one of the most carbon-rich CEMP stars known ([C/Fe] = +3.2), while CS 29528-041 (one of the few N-enhanced metal-poor stars known) is one of the most nitrogen rich ([N/Fe] = +3.0). Oxygen is very high in CS 31080-095 ([O/Fe] = +2.35) and in CS 22958-042 ([O/Fe] = +1.35). All three stars exhibit [Sr/Fe] < 0; Ba is not detected in CS 22958-042 ([Ba/Fe] < -0.53),but it is moderately enhanced ([Ba/Fe] similar to 1) in the other two stars. CS 22958-042 displays one of the largest sodium overabundances yet found in CEMP stars ([Na/Fe] = +2.8). CS 22958-042 has C-12/C-13 = 9, similar to most other CEMP stars without enhanced neutron-capture elements, while C-12/C-13 = 40 in CS 31080-095. CS 31080-095 and CS 29528-041 have A(Li) similar to 1.7, below the Spite Plateau, while Li is not detected in CS 22958-042. Conclusions. CS 22958-042 is a CEMP-no star, but the other two stars are in no known class of CEMP star and thus either constitute a new class or are a link between the CEMP-no and CEMP-s classes, adding complexity to the abundance patterns for CEMP stars. We interpret the abundance patterns in our stars to imply that current models for the presumed AGB binary progenitors lack an extra-mixing process, similar to those apparently operating in RGB stars.
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| 2. |
- Bonifacio, P., et al.
(författare)
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First stars XII. Abundances in extremely metal-poor turnoff stars, and comparison with the giants
- 2009
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 501:2, s. 519-530
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Tidskriftsartikel (refereegranskat)abstract
- Context. 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. Aims. We aim to compare the results for giants with new, accurate abundances for all observable elements in 18 EMP turno. stars. Methods. VLT/UVES spectra at R similar to 45 000 and S/N similar to 130 per pixel (lambda lambda 330-1000 nm) 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. Results. 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 similar to 0.2 dex larger than in giants of similar metallicity. Mg and Si abundances are similar to 0.2 dex lower (the giant [Mg/Fe] values are slightly revised), while Zn is again similar to 0.4 dex higher than in giants of similar [Fe/H] (6 stars only). Conclusions. 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.
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| 3. |
- Cayrel, R., et al.
(författare)
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Determination of [O/Fe] in BD +23 3130 from ESO VLT-UVES observations
- 2001
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Ingår i: New Astronomy Reviews. - 1872-9630. ; 45:8, s. 533-535
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Tidskriftsartikel (refereegranskat)abstract
- We report a new determination of [O/Fe, the relative logarithmicabundance of O/Fe with respect to the Sun, for the very metal-poor starBD+23 3130 ([Fe/H=-2.6). The value was derived from the forbidden line[O I at 630 nm and from six weak Fe II lines, with a S/N ratiosubstantially larger than those obtained before, thanks to theefficiency of the VLT-UVES instrument at Paranal. We obtain[O/Fe=0.71+/-0.25, a value 0.36 dex higher than the value obtained fromthe same lines by Fulbright and Kraft [AJ 118 (1999) 527, but 0.46lower than the one derived by Israelian et al. [ApJ 507 (1998) 805 fromthe UV OH bands.
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| 4. |
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| 5. |
- Cayrel, R., et al.
(författare)
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First stars V - Abundance patterns from C to Zn and supernova yields in the early Galaxy
- 2004
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 416:3, s. 1117-1138
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Tidskriftsartikel (refereegranskat)abstract
- In the framework of the ESO Large Programme ``First Stars'', veryhigh-quality spectra of some 70 very metal-poor dwarfs and giants wereobtained with the ESO VLT and UVES spectrograph. These stars are likelyto have descended from the first generation(s) of stars formed after theBig Bang, and their detailed composition provides constraints on issuessuch as the nature of the first supernovae, the efficiency of mixingprocesses in the early Galaxy, the formation and evolution of the haloof the Galaxy, and the possible sources of reionization of the Universe.This paper presents the abundance analysis of an homogeneous sample of35 giants selected from the HK survey of Beers et al. (cite{BPS92},cite{Be99}), emphasizing stars of extremely low metallicity: 30 of our35 stars are in the range -4.1 <[Fe/H]< -2.7, and 22 stars have[Fe/H] < -3.0. Our new VLT/UVES spectra, at a resolving power ofR∼45 000 and with signal-to-noise ratios of 100-200 per pixel overthe wavelength range 330-1000 nm, are greatly superior to those of theclassic studies of McWilliam et al. (cite{MPS95}) and Ryan et al.(cite{RNB96}).The immediate objective of the work is to determine precise,comprehensive, and homogeneous element abundances for this large sampleof the most metal-poor giants presently known. In the analysis wecombine the spectral line modeling code ``Turbospectrum'' with OSMARCSmodel atmospheres, which treat continuum scattering correctly and thusallow proper interpretation of the blue regions of the spectra, wherescattering becomes important relative to continuous absorption (λ< 400 nm). We obtain detailed information on the trends of elementalabundance ratios and the star-to-star scatter around those trends,enabling us to separate the relative contributions of cosmic scatter andobservational/analysis errors.Abundances of 17 elements from C to Zn have been measured in all stars,including K and Zn, which have not previously been detected in starswith [Fe/H] < -3.0. Among the key results, we discuss the oxygenabundance (from the forbidden [OI] line), the different and sometimescomplex trends of the abundance ratios with metallicity, the very tightrelationship between the abundances of certain elements (e.g., Fe andCr), and the high [Zn/Fe] ratio in the most metal-poor stars. Within theerror bars, the trends of the abundance ratios with metallicity areconsistent with those found in earlier literature, but in many cases thescatter around the average trends is much smaller than found in earlierstudies, which were limited to lower-quality spectra. We find that thecosmic scatter in several element ratios may be as low as 0.05 dex.The evolution of the abundance trends and scatter with decliningmetallicity provides strong constraints on the yields of the firstsupernovae and their mixing into the early ISM. The abundance ratiosfound in our sample do not match the predicted yields frompair-instability hypernovae, but are consistent with element productionby supernovae with progenitor masses up to 100 M⊙.Moreover, the composition of the ejecta that have enriched the matterBased on observations obtained in the frame of the ESO programme ID165.N-0276(A).Full Tables 3 and 8 are available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/416/1117 This work hasmade use of the SIMBAD database.
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| 6. |
- Cayrel, R., et al.
(författare)
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Measurement of stellar age from uranium decay
- 2001
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 409:6821, s. 691-692
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Tidskriftsartikel (refereegranskat)abstract
- The ages of the oldest stars in the Galaxy indicate when star formationbegan, and provide a minimum age for the Universe. Radioactive dating ofmeteoritic material and stars relies on comparing the present abundanceratios of radioactive and stable nuclear species to the theoreticallypredicted ratios of their production. The radioisotope 232Th(half-life 14Gyr) has been used to date Galactic stars, but it decays byonly a factor of two over the lifetime of the Universe. 238U(half-life 4.5Gyr) is in principle a more precise age indicator, buteven its strongest spectral line, from singly ionized uranium at awavelength of 385.957nm, has previously not been detected in stars. Herewe report a measurement of this line in the very metal-poor starCS31082-0018, a star which is strongly overabundant in itsheavy elements. The derived uranium abundance, log(U/H) = -13.7 +/- 0.14+/- 0.12 yields an age of 12.5 +/- 3Gyr, though this is still modeldependent. The observation of this cosmochronometer gives the mostdirect age determination of the Galaxy. Also, with improved theoreticaland laboratory data, it will provide a highly precise lower limit to theage of the Universe.
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| 7. |
- Depagne, E., et al.
(författare)
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First Stars. II. Elemental abundances in the extremely metal-poor star CS 22949--037. A diagnostic of early massive supernovae
- 2002
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 390:1, s. 187-198
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Tidskriftsartikel (refereegranskat)abstract
- CS 22949-037 is one of the most metal-poor giants known ([Fe/H]~-4.0),and it exhibits large overabundances of carbon and nitrogen (Norris etal.). Using VLT-UVES spectra of unprecedented quality, regardingresolution and S/N ratio, covering a wide wavelength range (from lambda= 350 to 900 nm), we have determined abundances for 21 elements in thisstar over a wide range of atomic mass. The major new discovery is anexceptionally large oxygen enhancement, [O/Fe] = 1.97+/-0.1, as measuredfrom the [O I] line at 630.0 nm. We find an enhancement of [N/Fe] of2.56+/- 0.2, and a milder one of [C/Fe] = 1.17+/-0.1, similar to thosealready reported in the literature. This implies Zstar =0.01Zsun. We also find carbon isotopic ratios12C/13C =4+/-2.0 and 13C/14N=0.03 +0.035-0.015, close to the equilibrium valueof the CN cycle. Lithium is not detected. Na is strongly enhanced([Na/Fe] = +2.1 +/- 0.2), while S and K are not detected. Thesilicon-burning elements Cr and Mn are underabundant, while Co and Znare overabundant ([Zn/Fe]=+0.7). Zn is measured for the first time insuch an extremely metal-poor star. The abundances of the neutron-captureelements Sr, Y, and Ba are strongly decreasing with the atomic number ofthe element: [Sr/Fe] ~ +0.3, [Y/Fe] ~ -0.1, and [Ba/Fe] ~ -0.6. Amongpossible progenitors of CS 22949-037, we discuss the pair-instabilitysupernovae. Such very massive objects indeed produce large amounts ofoxygen, and have been found to be possible sources of primary nitrogen.However, the predicted odd/even effect is too large, and the predictedZn abundance much too low. Other scenarios are also discussed. Inparticular, the yields of a recent model (Z35Z) from Heger and Woosleyare shown to be in fair agreement with the observations. The onlydiscrepant prediction is the very low abundance of nitrogen, possiblycurable by taking into account other effects such as rotationallyinduced mixing. Alternatively, the absence of lithium in our star, andthe values of the isotopic ratios 12C/13C and13C/14N close to the equilibrium value of the CNcycle, suggest that the CNO abundances now observed might have beenaltered by nuclear processing in the star itself. A 30-40Msun supernova, with fallback, seems the most likelyprogenitor for CS 22949-037. Based on observations made with the ESOVery Large Telescope at Paranal Observatory, Chile (programme ID165.N-0276(A)).
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| 8. |
- François, P., et al.
(författare)
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First Stars. III. A detailed elemental abundance study of four extremely metal-poor giant stars
- 2003
-
Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 403:3, s. 1105-1114
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Tidskriftsartikel (refereegranskat)abstract
- This paper reports detailed abundance analyses for four extremelymetal-poor (XMP) giant stars with [Fe/H]<-3.8, based onhigh-resolution, high-S/N spectra from the ESO VLT (Kueyen/UVES) and LTEmodel atmosphere calculations. The derived [alpha /Fe] ratios in oursample exhibit a small dispersion, confirming previous findings in theliterature, i.e. a constant overabundance of the alpha -elements with avery small (if any) dependence on [Fe/H]. In particular, the very smallscatter we determine for [Si/Fe] suggests that this element shows aconstant overabundance at very low metallicity, a conclusion which couldnot have been derived from the widely scattered [Si/Fe] values reportedin the literature for less metal-poor stars. For the iron-peak elements,our precise abundances for the four XMP stars in our sample confirm thedecreasing trend of Cr and Mn with decreasing [Fe/H], as well as theincreasing trend for Co and the absence of any trend for Sc and Ni. Incontrast to the significant spread of the ratios [Sr/Fe] and [Ba/Fe], wefind [Sr/Ba] in our sample to be roughly solar, with a much lowerdispersion than previously found for stars in the range -3.5 < [Fe/H]< -2.5.Based on observations made with the ESO Very Large Telescope at ParanalObservatory, Chile (Large Programme ID 165.N-0276(A)).The complete version of Table 5 is only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5) or viahttp:/ /cdsweb.u-strasbg.fr/cgi-bin/qcat?J /A+A/403/1105
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| 9. |
- Hill, V., et al.
(författare)
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First stars. I. The extreme r-element rich, iron-poor halo giant CS31082-001. Implications for the r--process site(s) and radioactive cosmochronology
- 2002
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 387:2, s. 560-579
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Forskningsöversikt (refereegranskat)abstract
- We present a high-resolution ( R= 75 000, S/ N ) spectroscopic analysis of the bright ( V= 11.7), extreme halo giant CS 31082-001([Fe/H] = -2.9), obtained in an ESO-VLT Large Programme dedicated to very metal-poor stars. We find CS 31082-001 to be extremely rich in r-process elements, comparable in this respect only to the similarly metal-poor, but carbon-enriched, giant CS 22892-052. As a result of the extreme overabundance of the heaviest r-process elements, and negligible blending from CH and CN molecular lines, a reliable measurement is obtained of the U II line at 386 nm, for the first time in a halo star, along with numerous lines of Th II, as well as lines of 25 other r-process elements. Abundance estimates for a total of 43 elements (44 counting Hydrogen) are reported in CS 31082-001, almost half of the entire periodic table. The main atmospheric parameters of CS 31082-001 are as follows: K, (cgs), [Fe/H] = -2.9 (in LTE), and microturbulence 1.8 0.2 km s -1. Carbon and nitrogen are not significantly enhanced relative to iron. As usual in giant stars, Li is depleted by dilution ( (Li/H) = 0.85). The -elements show the usual enhancements with respect to iron, with [O/Fe] (from [O I] 6300 Å), [Mg/Fe] , [Si/Fe] , and [Ca/Fe] , while [Al/Fe] is near -0.5. The r-process elements show unusual patterns: among the lightest elements ( 40), Sr and Zr follow the Solar r-element distribution, but Ag is down by 0.8 dex. All elements with 56 Z 72 follow the Solar r-element pattern, reduced by about 1.25 dex. Accordingly, the [ r/Fe] enhancement is about +1.7 dex (a factor of 50), very similar to that of CS 22892-052. Pb, in contrast, seems to be below the shifted Solar r-process distribution, possibly indicating an error in the latter, while thorium is more enhanced than the lighter nuclides. In CS 31082-001, log(Th/Eu) is , higher than in the Solar System (-0.46) or in CS 22892-052 (-0.66). If CS 31082-001 and CS 22892-052 have similar ages, as expected for two extreme halo stars, this implies that the production ratios were different by about 0.4 dex for the two objects. Conversely, if the Th/Eu production ratio were universal, an age of 15 Gyr for CS 22892-052 would imply a negative age for CS 31082-001. Thus, while a universal production ratio for the r-process elements seems to hold in the interval 56 Z 72, it breaks down in the actinide region. When available, the U/Th is thus preferable to Th/Eu for radioactive dating, for two reasons: (i) because of its faster decay rate and smaller sensitivity to observational errors, and (ii) because the inital production ratio of the neighboring nuclides 238U and 232Th is more robustly predicted than the 151Eu/ 232Th ratio. Our current best estimate for the age of CS 31082-001 is Gyr. However, the computed actinide production ratios should be verified by observations of daughter elements such as Pb and Bi in the same star, which are independent of the subsequent history of star formation and nucelosynthesis in the Galaxy.
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| 10. |
- Sivarani, T., et al.
(författare)
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First stars IV. CS 29497-030: Evidence for operation of the s-process at very low metallicity
- 2004
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 413:3, s. 65-1073
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Forskningsöversikt (refereegranskat)abstract
- We present an abundance analysis of the very metal-poor, carbon-enhancedstar CS 29497-030. Our results indicate that this unusually hot turnoffstar (Teff = 6650 K, log g = 3.5) has a metallicity [Fe/H] =-2.8, and exhibits large overabundances of carbon ([C/Fe] = +2.38),nitrogen ([N/Fe] = +1.88), and oxygen ([O/Fe] = +1.67). This star alsoexhibits a large enhancement in its neutron-capture elements; thepattern follows that expected to arise from the s-process. Inparticular, the Pb abundance is found to be very high with respect toiron ([Pb/Fe] = +3.5), and also with respect to the second peaks-process elements (e.g., Ba, La, Ce, Nd), which fits into the newlyintroduced classification of lead (Pb) stars. The known spectroscopicbinary status of this star, along with the observed s-process abundancepattern, suggest that it has accreted matter from a companion, whichformerly was an Asymptotic Giant-Branch (AGB) star. In a preliminaryanalysis, we have also identified broad absorption lines of metallicspecies that suggest a large axial rotational velocity for this star,which may be the result of spin-up associated with the accretion ofmaterial from its previous AGB companion. In addition, this star isclearly depleted in the light element Li. When considered along with itsrather high inferred temperature, these observations are consistent withthe expected properties of a very low metallicity halo blue straggler.Based on observations made with the ESO Very Large Telescope at ParanalObservatory, Chile (program ID 165.N-0276(A)).Table ef{tab6} is only available in electronic form athttp://www.edpsciences.org
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