arXiv subjects

F. Primas

Publications and source records attributed to F. Primas.

At least 73 records · Page 4Linked to original sources

arXiv2006

First stars VII. Lithium in extremely metal poor dwarfs

Aims. This study aims to determine the level and constancy of the Spite plateau as definitively as possible from homogeneous high-quality VLT-UVES spectra of 19 of the most metal-poor dwarf stars known. Methods. Our high-resolution (R ~ 43000), high S/N spectra are analysed with OSMARCS 1D LTE model atmospheres and turbospectrum synthetic spectra to determine effective temperatures, surface gravities, and metallicities, as well as Li abundances for our stars. Results. Eliminating a cool subgiant and a spectroscopic binary, we find 8 stars to have -3.5 < [Fe/H] < -3.0 and 9 stars with -3.0 < [Fe/H] < -2.5. Our best value for the mean level of the plateau is A(Li) =2.10 +- 0.09. The scatter around the mean is entirely explained by our estimate of the observational error and does not allow for any intrinsic scatter in the Li abundances. In addition, we conclude that a systematic error of the order of 200 K in any of the current temperature scales remains possible. The iron excitation equilibria in our stars support our adopted temperature scale, which is based on a fit to wings of the Halpha line, and disfavour hotter scales, which would lead to a higher Li abundance, but fail to achieve excitation equilibrium for iron. Conclusions. We confirm the previously noted discrepancy between the Li abundance measured in extremely metal-poor turnoff stars and the primordial Li abundance predicted by standard Big-Bang nucleosynthesis models adopting the baryonic density inferred from WMAP. We discuss recent work explaining the discrepancy in terms of diffusion and find that uncertain temperature scales remain a major question. (abridged)

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arXiv2006

The DART imaging and CaT survey of the Fornax Dwarf Spheroidal Galaxy

As part of the DART project we have used the ESO/2.2m Wide Field Imager in conjunction with the VLT/FLAMES GIRAFFE spectrograph to study the detailed properties of the resolved stellar population of the Fornax dwarf spheroidal galaxy out to and beyond its tidal radius. We re-derived the structural parameters of the Fornax dwarf spheroidal using our wide field imaging covering the galaxy out to its tidal radius, and analysed the spatial distribution of the Fornax stars of different ages as selected from Colour-Magnitude Diagram analysis. We have obtained accurate velocities and metallicities from spectra in the CaII triplet wavelength region for 562 Red Giant Branch stars which have velocities consistent with membership in Fornax dwarf spheroidal. We have found evidence for the presence of at least three distinct stellar components: a young population (few 100 Myr old) concentrated in the centre of the galaxy, visible as a Main Sequence in the Colour-Magnitude Diagram; an intermediate age population (2-8 Gyr old); and an ancient population (> 10Gyr),which are distinguishable from each other kinematically, from the metallicity distribution and in the spatial distribution of stars found in the Colour-Magnitude Diagram. From our spectroscopic analysis we find that the ``metal rich'' stars ([Fe/H]> -1.3) show a less extended and more concentrated spatial distribution, and display a colder kinematics than the ``metal poor'' stars ([Fe/H<-1.3). There is tentative evidence that the ancient stellar population in the centre of Fornax does not exhibit equilibrium kinematics. This could be a sign of a relatively recent accretion of external material.

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arXiv2006

First stars X. The nature of three unevolved Carbon-Enhanced Metal-Poor stars

From high-resolution VLT/UVES spectra (R~43,000), we determine abundances or upper limits for Li, C, N, O, and other important elements, as well as 12C/13C isotopic ratio for three Carbon enhanced metal poor stars. 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] ~ 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 12C/13C = 9, similar to most other CEMP stars without enhanced neutron-capture elements, while 12C/13C <= 40 in CS 31080-095. CS 31080-095 and CS 29528-041 have A(Li) ~ 1.7, below the Spite Plateau, while Li is not detected in CS 22958-042. 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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arXiv2006

CS 30322-023: an ultra metal-poor TP-AGB star?

With [Fe/H] = -3.5, CS 30322-023 is the most metal-poor star to exhibit a clear s-process signature and the most metal-poor ``lead star'' known. CS 30322-023 is also remarkable in having the lowest surface gravity (log g <= -0.3) among the metal-poor stars studied to date. The available evidence indicates that this star is presently a thermally-pulsing asymptotic giant branch (TP-AGB) star, with no strong indication of binarity thus far (although a signal of period 192 d is clearly present in the radial-velocity data, this is likely due to pulsation of the stellar envelope). We show that low-mass TP-AGB stars are not expected to be exceedingly rare in a magnitude-limited sample such as the HK survey, because their high luminosities make it possible to sample them over a very large volume. The strong N overabundance and the low 12C/13C ratio (4) in this star is typical of the operation of the CN cycle. Coupled with a Na overabundance and the absence of a strong C overabundance, this pattern seems to imply that hot-bottom burning operated in this star, which should then have a mass of at least 2 Msun. However, the luminosity associated with this mass would put the star at a distance of about 50 kpc, in the outskirts of the galactic halo. We explore alternative scenarios in which the observed abundance pattern results from some mixing mechanism yet to be identified occurring in a single low-metallicity 0.8 Msun AGB star, or from pollution by matter from an intermediate-mass AGB companion which has undergone hot-bottom burning. We stress, however, that our abundances may be subject to uncertainties due to NLTE or 3D granulation effects which were not taken into consideration.

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arXiv2006

First stars IX -Mixing in extremely metal-poor giants. Variation of the 12C/13C, [Na/Mg] and [Al/Mg] ratios

Extremely metal-poor (EMP) stars preserve a fossil record of the composition of the ISM when the Galaxy formed. It is crucial, however, to verify whether internal mixing has modified their surface. We aim to understand the CNO abundance variations found in some, but not all EMP field giants analysed earlier. Mixing beyond the first dredge-up of standard models is required, and its origin needs clarification.The 12C/13C ratio is the most robust diagnostic of deep mixing, because it is insensitive to the adopted stellar parameters and should be uniformly high in near-primordial gas. We have measured 12C and 13C abundances in 35 EMP giants from high-quality VLT/UVES spectra. Correlations with other abundance data are used to study the depth of mixing.The 12C/13C ratio is found to correlate with [C/Fe] (and Li/H), and clearly anti-correlate with [N/Fe]. Evidence for such deep mixing is observed in giants above log L/Lsolar = 2.6, brighter than in less metal-poor stars, but matching the bump in the luminosity function in both cases. Three of the mixed stars are also Na-and Al-rich, another signature of deep mixing, but signatures of the ON cycle are not clearly seen in these stars. Extra mixing processes clearly occur in luminous RGB stars. The Na-and Al-rich giants could be AGB stars themselves, but an inhomogeneous early ISM or pollution from a binary companion remain possible alternatives (abridged).

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arXiv2006

A VLT-UVES spectrscopic analysis of C-rich Fe-poor stars

Large surveys of very metal-poor stars have revealed in recent years that a large fraction of these objects were carbon-rich, analogous to the more metal-rich CH-stars. The abundance peculiarities of CH-stars are commonly explained by mass-transfer from a more evolved companion. In an effort to better understand the origin and importance for Galactic evolution of Fe-poor, C-rich stars, we present abundances determined from high-resolution and high signal-to-noise spectra obtained with the UVES instrument attached to the ESO/VLT. Our analysis of carbon-enhanced objects includes both CH stars and more metal-poor objects, and we explore the link between the two classes. We also present preliminary results of our ongoing radial velocity monitoring.

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arXiv2006

Element abundances of unevolved stars in the open cluster M 67

We determined the metallicity ([Fe/H]), together with O, Na, Mg, Al, Si, Ca, Ti, Cr and Ni abundances for a sample of 10 unevolved or slightly evolved stars belonging to the open cluster M 67. We find an average metallicity [Fe/H]=0.03 +/- 0.01, in very good agreement with previous determinations. All the [X/Fe] abundance ratios are very close to solar. The star-to-star scatter in [Fe/H] and [X/Fe] ratios for all elements, including oxygen, is lower than 0.05 dex, implying that the large dispersion in lithium reported in previous studies is not due to differences in these element abundances. We also find that, when using a homogeneous scale, the abundance pattern of unevolved stars in our sample is very similar to that of evolved stars, suggesting that, at least in this cluster, RGB and clump stars have not undergone any chemical processing. Finally, our results show that M 67 has a chemical composition that is representative of the solar neighborhood.

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arXiv2005

Oxygen abundances in metal-poor subgiants as determined from [O I], O I and OH lines

The debate on the oxygen abundances of metal-poor stars has its origin in contradictory results obtained using different abundance indicators. To achieve a better understanding of the problem we have acquired high quality spectra with the Ultraviolet and Visual Echelle Spectrograph at VLT, with a signal-to-noise of the order of 100 in the near ultraviolet and 500 in the optical and near infrared wavelength range. Three different oxygen abundance indicators, OH ultraviolet lines around 310.0 nm, the [OI] line at 630.03 nm and the OI lines at 777.1-5 nm were observed in the spectra of 13 metal-poor subgiants with -3.0<=[Fe/H]<=-1.5. Oxygen abundances were obtained from the analysis of these indicators which was carried out assuming local thermodynamic equilibrium and plane-parallel model atmospheres. Abundances derived from OI were corrected for departures from local thermodynamic equilibrium. Stellar parameters were computed using Teff-vs-color calibrations based on the infrared flux method and Balmer line profiles, Hipparcos parallaxes and FeII lines. [O/Fe] values derived from the forbidden line at 630.03 nm are consistent with an oxygen/iron ratio that varies linearly with [Fe/H] as [O/Fe]}=-0.09(+/-0.08)[Fe/H]+0.36(+/-0.15). Values based on the OI triplet are on average 0.19+/-0.22 dex(s.d.) higher than the values based on the forbidden line while the agreement between OH ultraviolet lines and the forbidden line is much better with a mean difference of the order of -0.09+/-0.25 dex(s.d.). In general, our results follow the same trend as previously published results with the exception of the ones based on OH ultraviolet lines. In that case our results lie below the values which gave rise to the oxygen abundance debate for metal-poor stars.

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arXiv2005

Li and Be depletion in metal-poor subgiants

A sample of metal-poor subgiants has been observed with the UVES spectrograph at the Very Large Telescope and abundances of Li and Be have been determined. Typical signal-to-noise per spectral bin values for the co-added spectra are of the order of 500 for the Li I line (670.78 nm) and 100 for the Be II doublet lines (313.04 nm). The spectral analysis of the observations was carried out using the Uppsala suite of codes and MARCS (1D-LTE) model atmospheres with stellar parameters from photometry, parallaxes, isochrones and Fe II lines. Abundance estimates of the light elements were corrected for departures from local thermodynamic equilibrium in the line formation. Effective temperatures and Li abundances seem to be correlated and Be abundances correlate with [O/H]. Standard models predict Li and Be abundances approximately one order of magnitude lower than main-sequence values which is in general agreement with the observations. On average, our observed depletions seem to be 0.1 dex smaller and between 0.2 and 0.4 dex larger (depending on which reference is taken) than those predicted for Li and Be, respectively. This is not surprising since the initial Li abundance, as derived from main-sequence stars on the Spite plateau, may be systematically in error by 0.1 dex or more, and uncertainties in the spectrum normalisation and continuum drawing may affect our Be abundances systematically.

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arXiv2005

The Chemical Composition of Cepheids in the Milky Way and the Magellanic Clouds

We have measured the elemental abundances of 68 Galactic and Magellanic Cepheids from FEROS and UVES high-resolution and high signal-to-noise spectra in order to establish the influence of the chemical composition on the properties of these stars (see Romaniello et al. 2005). Here we describe the robust analytical procedure we have developed to accurately determine them. The resulting iron abundances span a range between ~ -0.80 dex for stars in the Small Magellanic Cloud and ~ +0.20 dex for the most metal-rich ones in the Galaxy. While the average values for each galaxy are in good agreement with non-pulsating stars of similar age, Cepheids display a significant spread. Thus it is fundamental to measure the metallicity of individual stars.

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arXiv2005

The Metallicity Dependence of the Cepheid Period-Luminosity Relation

We have assessed the influence of the stellar iron content on the Cepheid Period-Luminosity (PL) relation by relating the V band residuals from the Freedman et al (2001) PL relation to [Fe/H] for 68 Galactic and Magellanic Cloud Cepheids. The iron abundances were measured from FEROS and UVES high-resolution and high signal-to-noise optical spectra. Our data indicate that the stars become fainter as metallicity increases, until a plateau or turnover point is reached at about solar metallicity. This behavior appears at odds both with the PL relation being independent from iron abundance and with Cepheids becoming monotonically brighter as metallicity increases (e.g. Kennicutt et al 1998, Sakai et al 2004).

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arXiv2005

The Lithium Content of the Galactic Halo Stars

Thanks to the accurate determination of the baryon density of the universe by the recent cosmic microwave background experiments, updated predictions of the standard model of Big Bang nucleosynthesis now yield the initial abundance of the primordial light elements with an unprecedented precision. In the case of $^7$Li, the CMB+SBBN value is significantly higher than the generally reported abundances for Pop II stars along the so-called Spite plateau. In view of the crucial importance of this disagreement which has cosmological, galactic and stellar implications, we decided to tackle the most critical issues of the problem by revisiting a large sample of literature Li data in halo stars that we assembled following some strict selection criteria on the quality of the original analyses. [Abridged]

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arXiv2005

First stars VI - Abundances of C, N, O, Li, and mixing in extremely metal-poor giants. Galactic evolution of the light elements

We have investigated the poorly-understood origin of nitrogen in the early Galaxy by determining N abundances in 35 extremely metal-poor halo giants (22 stars have [Fe/H]<-3.0) using the C and O abundances determined in Paper V. Because any dredge-up of CNO processed material to the surface may complicate the interpretation of CNO abundances in giants, we have also measured the surface abundance of lithium. Our sample shows a clear dichotomy between two groups of stars. The first group shows evidence of C to N conversion through CN cycling and strong Li dilution, a signature of mixing. The second group shows no evidence for C to N conversion, and Li is only moderately diluted, and we conclude that their C and N abundances are very close to those of the gas from which they formed in the early Galaxy. These "unmixed" stars reflect the abundances in the early Galaxy: the [C/Fe] ratio is constant (about +0.2 dex) and the [C/Mg] ratio is close to solar at low metallicity, favouring a high C production by massive zero-metal supernovae. The [N/Fe] and [N/Mg] ratios scatter widely. The larger values of these ratios define a flat upper plateau ([N/Mg]= 0.0, [N/Fe]= +0.1), which could reflect higher values within a wide range of yields of zero-metal Sne II. Alternatively, by analogy with the DLA's, the lower abundances ([N/Mg]= -1.1, [N/Fe]= -0.7) could reflect generally low yields from the first Sne II, the other stars being N enhanced by winds of massive Asymptotic Giant Branch (AGB) stars. At present it cannot be decided whether primary N is produced primarily in SNe II or in massive AGB stars, or in both. The stellar N abundances and [N/O] ratios are compatible with those found in Damped Lyman-alpha (DLA) systems.

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arXiv2004

The influence of chemical composition on the properties of Cepheid stars I - Period-Luminosity relation vs iron abundance

We have assessed the influence of the stellar iron content on the Cepheid Period-Luminosity (PL) relation by relating the V band residuals from the Freedman et al (2001) PL relation to [Fe/H] for 37 Galactic and Magellanic Clouds Cepheids. The iron abundances were measured from FEROS and UVES high-resolution and high-signal to noise optical spectra. Our data indicate that the stars become fainter as metallicity increases, until a plateau or turnover point is reached at about solar metallicity. Our data are incompatible with both no dependence of the PL relation on iron abundance, and with the linearly decreasing behavior often found in the literature (e.g. Kennicutt et al 1998, Sakai et al 2004). On the other hand, non-linear theoretical models of Fiorentino et al (2002) provide a fairly good description of the data.

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arXiv2004

Two distinct ancient components in the Sculptor Dwarf Spheroidal Galaxy: First Results from DART

We have found evidence for the presence of two distinct ancient stellar components (both geq 10 Gyr old) in the Sculptor dwarf spheroidal galaxy. We used the ESO Wide Field Imager (WFI) in conjunction with the VLT/FLAMES spectrograph to study the properties of the resolved stellar population of Sculptor out to and beyond the tidal radius. We find that two components are discernible in the spatial distribution of Horizontal Branch stars in our imaging, and in the [Fe/H] and v_hel distributions for our large sample of spectroscopic measurements. They can be generally described as a ``metal-poor'' component ([Fe/H]< -1.7) and a ``metal-rich'' component ([Fe/H]> -1.7). The metal-poor stars are more spatially extended than the metal-rich stars, and they also appear to be kinematically distinct. These results provide an important insight into the formation processes of small systems in the early universe and the conditions found there. Even this simplest of galaxies appears to have had a surprisingly complex early evolution.

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arXiv2004

Lead abundance in the uranium star CS 31082-001

In a previous paper we were able to measure the abundance of uranium and thorium in the very-metal poor halo giant BPS CS 31082-001, but only obtained an upper limit for the abundance of lead (Pb). We have got from ESO 17 hours of additional exposure on this star in order to secure a detection of the minimum amount of lead expected to be present in CS 31082-001, the amount arising from the decay of the original content of Th and U in the star. We report here this successful detection. We find an LTE abundance log(Pb/H)+12=-0.55 \pm 0.15 dex, one dex below the upper limits given by other authors for the similar stars CS 22892-052 and BD +17d3248, also enhanced in r-process elements. From the observed present abundances of Th and U in the star, the expected amount of Pb produced by the decay of 232Th, and 238U alone, over 12-15 Gyr is -0.73\pm 0.17 dex. The decay of 235U is more difficult to estimate, but is probably slightly below the contribution of 238U, making the contribution of the 3 actinides only slightly below, or even equal to, the measured abundance. The contribution from the decay of 234U has was not included, for lack of published data. In this sense our determination is a lower limit to the contribution of actinides to lead production. We comment this result, and we note that if a NLTE analysis, not yet possible, doubles our observed abundance, the decay of the 3 actinides will still represent 50 per cent of the total lead, a proportion higher than the values considered so far in the literature.

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arXiv2004

The metallicity dependence of the Cepheid PL-relation

A sample of 37 Galactic, 10 LMC and 6 SMC cepheids is compiled for which individual metallicity estimates exist and BVIK photometry in almost all cases. The Galactic cepheids all have an individual distance estimate available. For the MC objects different sources of photometry are combined to obtain improved periods and mean magnitudes. A multi-parameter Period-Luminosity relation is fitted to the data which also solves for the distance to the LMC and SMC. When all three galaxies are considered, without metallicity effect, a significant quadratic term in log P is found, as previously observed and also predicted in some theoretical calculations. For the present sample it is empirically determined that for log P < 1.65 linear PL-relations may be adopted, but this restricts the sample to only 4 LMC and 1 SMC cepheid. Considering the Galactic sample a metallicity effect is found in the zero point in the VIWK PL-relation (-0.6 \pm 0.4 or -0.8 \pm 0.3 mag/dex depending on the in- or exclusion of one object), in the sense that metal-rich cepheids are brighter. The small significance is mostly due to the fact that the Galactic sample spans a narrow metallicity range. The error is to a significant part due to the error in the metallicity determinations and not to the error in the fit. Including the 5 MC cepheids broadens the observed metallicity range and a metallity effect of about -0.27 \pm 0.08 mag/dex in the zero point is found in VIWK, in agreement with some previous empirical estimates, but now derived using direct metallicity determinations for the cepheids themselves.

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arXiv2003

First stars V - Abundance patterns from C to Zn and supernova yields in the early Galaxy

In the framework of an ESO Large Programme high quality spectra of 70 very metal-poor dwarfs and giants were obtained with the VLT-UVES combination. This paper reports the detailed analyses of the 35 giants of the sample. At the exception of a few template stars, all the targets are from the H&K Beers et al. survey. No less than 22 stars have [Fe/H] below -3.0. The imprint of yields of the first galactic or protogalactic supernovae is sampled in the photospheres of these stars. If [C/Fe], [Na/Fe] and [Al/Fe] show a spread significantly above the observational scatter, several abundance ratios present an amazing low scatter when plotted against [Fe/H], as [Cr/Fe] but also odd elements, as [K/Fe] or [Sc/Fe]. The abundance of oxygen has been derived from the [O I] line for 16 of these giants, in spite of the extreme weakness of the line. No significant slope with [Fe/H] is found. Similar plots have been made using Mg instead of Fe as reference element. With Mg, no clear trend with metallicity is obvious below [Mg/H]=-3., except perhaps for [Zn/Mg]. It is contemplated that the mild slope found in the very low scatter diagram [Cr/Fe] versus [Fe/H] might be spurious due for example to the LTE approximation plus use of 1-D models. This would relax the absence of scatter in the dilution of the SN ejectas within the ambient medium, as needed if [Fe/H] is a well defined function of [Cr/Fe].

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