arXiv subjects

P. Bonifacio

Publications and source records attributed to P. Bonifacio.

263 records · Page 15Linked to original sources

arXiv2000

The INT Search for Metal-Poor Stars. Spectroscopic Observations and Classification via Artificial Neural Networks

With the dual aims of enlarging the list of extremely metal-poor stars identified in the Galaxy, and boosting the numbers of moderately metal-deficient stars in directions that sample the rotational properties of the thick disk, we have used the 2.5m Isaac Newton Telescope and the Intermediate Dispersion Spectrograph to carry out a survey of brighter (primarily northern hemisphere) metal-poor candidates selected from the HK objective-prism/interference-filter survey of Beers and collaborators. Over the course of only three observing runs (15 nights) we have obtained medium-resolution (resolving power ~ 2000) spectra for 1203 objects (V ~ 11-15). Spectral absorption-line indices and radial velocities have been measured for all of the candidates. Metallicities, quantified by [Fe/H], and intrinsic (B-V)o colors have been estimated for 731 stars with effective temperatures cooler than roughly 6500 K, making use of artificial neural networks (ANNs), trained with spectral indices. We show that this method performs as well as a previously explored Ca II K calibration technique, yet it presents some practical advantages. Among the candidates in our sample, we identify 195 stars with [Fe/H] <= -1.0, 67 stars with [Fe/H] <= -2.0, and 12 new stars with [Fe/H] <= -3.0. Although the EFECTIVE YIELD of metal-poor stars in our sample is not as large as previous HK survey follow-up programs, the rate of discovery per unit of telescope time is quite high.

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arXiv1999

The Galactic Lithium Evolution Revisited

The evolution of the 7Li abundance in the Galaxy has been computed by taking into account several 7Li sources: novae, massive AGB stars, C-stars, Type II SNe and GCRs. The theoretical predictions for the evolution of the 7Li abundance in the solar neighborhood have been compared to a new compilation of data. A critical analysis of extant observations revealed a possible extension of the Li plateau towards higher metallicities (up to [Fe/H] = -0.5 or even -0.3) with a steep rise afterwards. We conclude that 1) the 7Li contribution from novae is required in order to reproduce the shape of the growth of A(Li) versus [Fe/H], 2) the contribution from Type II SNe should be lowered by at least a factor of two, and 3) the 7Li production from GCRs is probably more important than previously estimated, in particular at the highest metallicities.

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arXiv1999

The 1999 outburst of the eclipsing and recurrent nova U Scorpii

The spectroscopic and photometric evolution of the 1999 outburst of the eclipsing and recurrent nova U Sco is presented. The photometric evolution closely matches that of the previous events. The FWZI=10,000 km/sec for emission lines at maximum has decreased to 4000 km/sec by day +23, with continuous and dramatic changes in the line profiles. No nebular line has become visible and the ionization degree has increased during the brightness decline. A not previously reported and quite puzzling splitting of the emission lines into three components after the first two weeks is outstanding in our spectra. The radiated luminosity is found to be a tiny fraction of that of classical novae for any reasonable distance to U Sco.

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arXiv1999

Abundances of metal-weak thick-disc candidates

High resolution spectra of 5 candidate metal-weak thick-disc stars suggested by Beers & Sommer-Larsen (1995) are analyzed to determine their chemical abundances. The low abundance of all the objects has been confirmed with metallicity reaching [Fe/H]=-2.9. However, for three objects, the astrometric data from the Hipparcos catalogue suggests they are true halo members. The remaining two, for which proper-motion data are not available, may have disc-like kinematics. It is therefore clear that it is useful to address properties of putative metal-weak thick-disc stars only if they possess full kinematic data. For CS 22894-19 the abundance pattern similar to those of typical halo stars is found, suggesting that chemical composition is not a useful discriminant between thick-disc and halo stars. CS 29529-12 is found to be C enhanced with [C/Fe]=+1.0; other chemical peculiarities involve the s process elements: [Sr/Fe]=-0.65 and [Ba/Fe]=+0.62, leading to a high [Ba/Sr] considerably larger than what is found in more metal-rich carbon-rich stars, but similar to LP 706-7 and LP 625-44 discussed by Norris et al (1997a). Hipparcos data have been used to calculate the space velocities of 25 candidate metal-weak thick-disc stars, thus allowing us to identify 3 bona fide members, which support the existence of a metal-poor tail of the thick-disc, at variance with a claim to the contrary by Ryan & Lambert (1995).

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arXiv1998

Spectroscopy of red giants of the Sagittarius dwarf galaxy

The photometric study of the Sagittarius dwarf galaxy by Marconi et al (1998) has suggested the presence of a spread in metallicity (-0.7< [Fe/H] < -1.6), which may result from different bursts of star formation. We present the results from a Multi Object Slit spectroscopy program carried out at the NTT with the aim to determine spectroscopic abundances of Sagittarius giants selected from the CCD photometry. From our intermediate resolution (R about 1500) spectra radial velocities are determined to confirm the membership and metallicities are derived by using spectral synthesis codes for stars down to V about18, i.e. below the RGB clump. Out of 57 observed stars 23 have a radial velocity consistent with Sagittarius membership, here we present results for 8 of these. No star with [Fe/H]<-1.0 is found, three stars are found to be metal-rich.

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arXiv1998

Spectroscopy of Giants of the Sagittarius Dwarf Galaxy

In this paper we present the first results of the analysis of intermediate resolution ($Δλ\sim 3.5 Å$) spectra of giants of the Sagittarius dwarf galaxy acquired using the ESO NTT telescope. From the deep CCD photometry of Marconi et al (1998a) we have selected a sample of giants representative of the metallicity spread suggested by the comparison of the colour-magnitude diagram of Sagittarius with those of galactic globular clusters. The spectra have been used to measure radial velocities, to confirm the membership to Sagittarius, and to provide a metallicity estimate by using spectral synthesis techniques. The analyzed stars show a spread in metallicities in the range -1.0<[Fe/H]<+0.7, some 0.5 dex more metal-rich than the photometric estimates.

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arXiv1998

Nitrogen Abundances in Damped Ly alpha Galaxies

Nitrogen abundances have been derived in Damped Ly alpha (DLA) galaxies at Zabs = 2.309, 2.827 and 3.025 toward the QSOs 0100+1300, 1425+6039 and 0347-3819 respectively. The behaviour of nitrogen relative to iron-peak and alpha-elements has been investigated by considering all the extant NI determinations for a total of 9 DLA galaxies. We have estimated the fraction of iron locked into dust grains to convert the observed [N/Fe] ratios into overall (dust plus gas) relative abundances, [N/Fe]corr. The ratios [N/alpha] have been mostly determined by using sulphur as a tracer of alpha-elements which is unaffected by dust. The [N/Fe] and [N/alpha] ratios show high dispersions, of one order of magnitude or more, which have no equivalent in other element-to-element ratios in DLAs. The lowest values of the [N/Fe]corr and [N/alpha] ratios are at variance with the values measured in Galactic halo stars of similar metallicity suggesting that part of the DLA galaxies do not follow the chemical evolution of the Milky Way. The DLA nitrogen abundances and their dispersion show some similarities with those observed in dwarf galaxies. The behaviour of nitrogen abundance ratios can be ascribed, in general to the delayed release of nitrogen in the course of evolution. However it is difficult to conciliate this interpretation with the lowest [N/alpha] values measured, since an expected enhancement of alpha-elements respect to the iron-peak elements is not observed simultaneously in these DLA galaxies. In two cases, relatively high [N/alpha] values are observed which require also a more complex chemical evolution to be explained.

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arXiv1997

CS 22957-027: A carbon-rich extremely-metal-poor star

We present a high-resolution spectroscopic analysis of the extremely-metal-poor star CS 22957-027, which Beers et al (1992) noted to exhibit a rather strong G-band. For Teff = 4839 K, derived from broadband photometry, our analysis obtains log g = 2.25, and a very low metallicity of [Fe/H]=-3.43 +/- 0.12. The carbon-to-iron ratio is found to be enhanced by about 2 dex relative to the solar value, similar to the handful of other carbon-rich metal-deficient stars discussed by Norris et al (1997a) and Barbuy et al (1997). From the 12C13C (1,0) Swan bandhead and 13CH lines the isotope ratio 12C/13C is about 10, indicating a 13C enrichment. Nitrogen is also found to be enhanced by about 1 dex. The s-process elements Sr and Ba are, surprisingly, found to be under-abundant relative to solar, with [Sr/Fe]=-0.91 and [Ba/Fe]=-0.93. The star's low luminosity requires that the chemical enrichment arises from mass transfer from an evolved companion, rather than self-polluting dredge-up processes. However, the s-process elements Sr and Ba are unusually low, with a solar ratio to one another, at variance with what found in the ``classical'' CH stars. Finally we note that a feature due to 13CH is present in coincidence with the Th II 401.9129 nm resonance line. The blend is of relevance for the nucleo-chronology use of the Th II line in these extremely metal poor stars, when significant 13C is present.

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arXiv1997

Lithium in very metal poor thick disk stars

A search for lithium is performed on seven metal poor dwarfs with metallicities ranging from [Fe/H]=-1.5 down to [Fe/H]=-3.0 but showing disk-like kinematics. These stars belong to the metal poor tail of the Galactic thick disk and they may be also the result of an accretion event (Beers and Sommer-Larsen 1995). The Li 6707.8 A line is present in all the seven dwarfs. The weighted average of the Li abundance for the stars is A(Li)=2.20 (+/-0.06) and is consistent within the errors with the plateau Li abundance of A(Li)=2.24(+/- 0.012) found in genuine halo stars in the same range of metallicities (Bonifacio and Molaro 1997). One of the stars, CS 22182-24, shows somewhat lower Li abundance (A(Li)=1.6(+/-0.40)) and is a candidate to being a Li-poor star. Whether this group of stars belongs to the oldest stars in the disk or to the old population of an external galaxy accreted by the Milky Way, the present observations provide support to the universality of a pre-Galactic Li abundance as is observed in the Galactic halo stars.

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arXiv1996

New beryllium observations in low-metallicity stars

We present observations of the Be II 313.0 nm resonance doublet in 14 halo and old disk stars with metallicities ranging from [Fe/H]=-0.4 to about -3.0 obtained with the CASPEC spectrograph of the ESO 3.6m telescope at a FWHM about 8.6 km/s resolution. Abundances are derived by means of the synthetic spectra technique employing Kurucz (1993) atmospheric models, with enhanced alpha-elements and no overshooting. The derived abundances together with those available in literature show that for -2.7 < [Fe/H] < -0.8 Be correlates linearly with iron [Be] proportional to 1.07(+/- 0.08)[Fe/H], giving strength to previous results. However, a steeper correlation is still possible at metallicities lower than [Fe/H]<-1.4 with [Be] proportional to 1.6(+/- 0.44)[Fe/H]. When iron is replaced with oxygen, Be is found tracking closely oxygen up to solar values, without signs of breaking in correspondence of the onset of the Galactic disk. No evidence of intrinsic dispersion is found, ought to the large errors involved in the Be abundance determinations, but for three stars (HD 106516, HD 3795, HD 211998) a significant upper limit in the Be abundance can be placed at about 1 dex below the mean trend of the Be-Fe relation. For such stars non conventional mixing is required to explain Be depletion. Be observations can be used to discriminate strongly Li-depleted stars. These are the stars which show less Li than that expected by high energy cosmic rays production as deduced from Be observations. The available Be observations imply that some of the stars which contribute to the scatter in the Li-Fe diagramme are Li-depleted stars.

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arXiv1995

Lithium Abundance of Halo Dwarfs Revised

Lithium abundances in a sample of halo dwarfs have been redetermined by using the new T$_{eff}$ derived by Fuhrmann et al (1994) from modelling of the Balmer lines. These T$_{eff}$ are reddening independent, homogeneous and of higher quality than those based on broad band photometry. Abundances have been derived by generating new atmospheric models by using the ATLAS-9 code by Kurucz (1993) with enhanced $α$-elements and without the overshooting option. The revised abundances show a remarkably flat {\it plateau} in the Li-T$_{eff}$ plane for T$_{eff}$$>$ 5700 K with no evidence of trend with T$_{eff}$ or falloff at the hottest edge. Li abundances are not correlated with metallicity for [Fe/H]$<$ -1.4 in contrast with Thorburn (1994). All the determinations are consistent with the same pristine lithium abundance and the errors estimated for individual stars fully account for the observed dispersion. The weighted average Li value for the 24 stars of the plateau with T$_{eff}$$> $ 5700 K and [Fe/H]$\le$ -1.4, is [Li] = 2.210 $\pm$ 0.013, or 2.224 when non-LTE corrections by Carlsson et al (1994) are considered.

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