arXiv subjects

B. Barbuy

Publications and source records attributed to B. Barbuy.

At least 145 records · Page 8Linked to original sources

arXiv2004

The Metal Content of the Bulge Globular Cluster NGC 6528

High resolution spectra of five stars in the bulge globular cluster NGC 6528 were obtained at the 8m VLT UT2-Kueyen telescope with the UVES spectrograph. Out of the five stars, two of them showed evidence of binarity. The target stars belong to the horizontal and red giant branch stages, at 4000<T_eff<4800 K. Multiband V,I,J,H,Ks photometry was used to derive initial effective temperatures and gravities. The main purpose of this study is the determination of metallicity and elemental ratios for this template bulge cluster, as a basis for the fundamental calibration of metal-rich populations. The present analysis provides a metallicity [Fe/H] = -0.1\pm0.2 and the alpha-elements O, Mg and Si, show [alpha/Fe] ~ +0.1, whereas Ca and Ti are around the solar value or below, resulting in an overall metallicity Z ~ Zsun.

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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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arXiv2003

First stars IV.CS 29497-030: Evidence for operation of the s-process at very low metallicity

We present an abundance analysis of the very metal-poor, carbon-enhanced star CS 29497-030. Our results indicate that this unusually hot turnoff star (Teff = 6650K, 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 also exhibits a large enhancement in its neutron-capture elements; the pattern follows that expected to arise from the s-process. In particular, the Pb abundance is found to be very high with respect to iron ([Pb/Fe]= +3.5), and also with respect to the second peak s-process elements (e.g., Ba, La, Ce, Nd), which fits into the newly introduced classification of lead (Pb) stars. The known spectroscopic binary status of this star, along with the observed s-process abundance pattern, suggest that it has accreted matter from a companion, which formerly was an Asymptotic Giant-Branch(AGB) star. In a preliminary analysis, we have also identified broad absorption lines of metallic species that suggest a large axial rotational velocity for this star, which may be the result of spin-up associated with the accretion of material from its previous AGB companion. In addition, this star is clearly depleted in the light element Li. When considered along with its rather high inferred temperature, these observations are consistent with the expected properties of a very low metallicity halo blue straggler.

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arXiv2003

Gemini-Phoenix infrared high-resolution abundance analysis of five giants in the bulge globular cluster NGC 6553

A detailed abundance analysis of 5 giants of the metal-rich bulge globular cluster NGC 6553 was carried out using high resolution infrared spectra in the H band, obtained at the Gemini-South 8m telescope. JK photometry collected at ESO and VI photometry from the Hubble Space Telescope are used to derive effective temperatures. The present analysis provides a metallicity [Fe/H] = -0.20 +/- 0.10. An overabundance of oxygen of [O/Fe] = +0.20 is found from IR OH lines.

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arXiv2003

NTT infrared imaging of star cluster candidates towards the central parts of the Galaxy

We address the issue whether the central parts of the Galaxy harbour young clusters other than Arches, Quintuplet and the Nuclear Young Cluster. A large sample of centrally projected cluster candidates has been recently identified from the 2MASS J, H and Ks Atlas. We provide a catalogue of higher angular resolution and deeper images for 57 2MASS cluster candidates, obtained with the near-IR camera SOFI at the ESO NTT telescope. We classify 10 objects as star clusters, some of them deeply embedded in gas and/or dust clouds. Three other objects are probably star clusters, although the presence of dust in the field does not exclude the possibility of their being field stars seen through low-absorption regions. Eleven objects are concentrations of stars in areas of little or no gas, and are classified as dissolving cluster candidates. Finally, 31 objects turned out to be the blend of a few bright stars, not resolved as such in the low resolution 2MASS images. By combining the above results with other known objects we provide an updated sample of 42 embedded clusters and candidates projected within 7 degrees. As a first step we study Object 11 of Dutra & Bica (2000) projected at approximately 1 degree from the nucleus. We present H and Ks photometry and study the colour-magnitude diagram and luminosity function. Object 11 appears to be a less massive cluster than Arches or Quintuplet, and it is located at a distance from the Sun d=8 kpc, with a visual absorption Av=15.

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arXiv2003

On the calibration of the COBE/IRAS dust emission reddening maps

In this work we study the spectral properties (3600 - 6800 A) of the nuclear region of early-type galaxies at low (|b|<25, intermediate (including surroundings of the Magellanic Clouds) and high (South Polar Cap) Galactic latitudes. We determine the E(B-V) reddening values of the galaxies by matching their continuum distribution with respect to those of reddening-free spectral galaxy templates with similar stellar populations. We also compare the spectroscopic reddening value of each galaxy with that derived from 100 micron dust emission (E(B-V)_{FIR}) in its line of sight, and we find that there is agreement up to E(B-V)=0.25. Beyond this limit E(B-V)_{FIR} values are higher. Taking into account the data up to E(B-V) \approx 0.7, we derive a calibration factor of 0.016 between the spectroscopic E(B-V) values and Schlegel et al.'s (1998) opacities. By combining this result with an A_K extinction map built within ten degrees of the Galactic centre using Bulge giants as probes (Dutra et al. 2003), we extended the calibration of dust emission reddening maps to low Galactic latitudes down to |b|=4 and E(B-V)= 1.6 (A_V \approx 5). According to this new calibration, a multiplicative factor of approximately 0.75 must be applied to the COBE/IRAS dust emission reddening maps.

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arXiv2003

New infrared star clusters in the Northern and Equatorial Milky Way with 2MASS

We carried out a survey of infrared star clusters and stellar groups on the 2MASS J, H and K_s all-sky release Atlas in the Northern and Equatorial Milky Way (350 < l < 360, 0 < l < 230). The search in this zone complements that in the Southern Milky Way (Dutra et al. 2003a). The method concentrates efforts on the directions of known optical and radio nebulae. The present study provides 167 new infrared clusters, stellar groups and candidates. Combining the two studies for the whole Milky Way, 346 infrared clusters, stellar groups and candidates were discovered, whereas 315 objects were previously known. They constitute an important new sample for future detailed studies.

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arXiv2003

V, J, H and K Imaging of the Metal Rich Globular Cluster NGC 6528

New near-infrared observations of NGC6528 are presented. The JHK observations complement a previous HST/NICMOS data set by Ortolani et al. (2001), in that they sample a larger area, contain a more numerous sample of red giant stars, and include the K band. Also, archival HST data sets (separated by 6.093 years) were used to proper-motion decontaminate the near-infrared sample and extract a clean VJHK catalogue. Using the present wide colour baseline, we compared the cleaned colour-magnitude diagrams of NGC6528 with those of NGC 6553 and NGC104 and derived new estimates of reddening and distance, E(B-V)=0.55 and (m-M)o=14.44 (7.7 kpc). Moreover, the morphology and location of the cleaned red giant branch were used to derive a photometric estimate of the cluster metallicity. The average of 10 metallicity indicators yields a mean value of [M/H] ~ 0.0, and [Fe/H] ~-0.20 and +0.08 on the Zinn & West (1984) and Carretta & Gratton (1997) revised metallicity scale, respectively. The best isochrone fit to the cleaned K,V-K diagram is obtained for a 12.6 Gyr and Z=0.02 isochrone, i.e. the derived metallicity of NGC6528 turns out to be very close to the mean of stars in the Baade's Window. Five AGB variable star candidates, whose membership has to be confirmed spectroscopically, are bolometrically as bright as the known long period variable stars in NGC6553. As discussed in Guarnieri et al. (1997) for NGC6553, this may indicate that an `intermediate age' population is not needed to account for the brightest stars in external galaxies such as M32.

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arXiv2003

New infrared star clusters in the southern Milky Way with 2MASS

We carried out a 2MASS J, H and K_s survey of infrared star clusters in the Milky Way sector 230$^{\circ}$ $< \ell <$ 350$^{\circ}$. This zone was the least studied in the literature, previously including only 12 infrared clusters or stellar groups with $|b|<$ 10$^{\circ}$, according to the recent catalogue by Bica et al. (2003). We concentrated efforts on embedded clusters, which are those expected in the areas of known radio and optical nebulae. The present study provides 179 new infrared clusters and stellar groups, which are interesting targets for detailed future infrared studies. The sample of catalogued infrared clusters and stellar groups in the Galaxy is now increased by 63%.

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arXiv2002

Elemental abundances of metal poor carbon rich lead star: CS29497-030

We present here the abundance analysis of a metal poor carbon rich lead star, CS29497-030. High resolution and high signal to noise spectra were obtained using the UVES spectrograph on the 8.2m VLT-Kueyen telescope. The observations were made as a part of the Large Programme 165.N-0276, P.I. R. Cayrel. Abundance analysis was done using the latest version of the MARCS model atmospheres (Plez et. al. 1992) and the turbospectrum spectrum synthesis code. We have derived Teff = 6650K from the FeI lines. Visible and infrared broad band colours using the Alonso et al. (1996) calibration, gives similar temperatures. A log g value of 3.5 was obtained from the ionisation equilibrium of FeI and FeII, we remark that this gravity also satisfies the MgI/MgII, TiI/TiII and MnI/MnII equilibria, within errors. The abundance analysis indicates a metallicity, [Fe/H] = --2.7. A large overabundance of carbon ([C/Fe]=2.7) was found. We have also found large enhancement in the s-process elements and in particular lead shows an extremely high abundance of [Pb/Fe]=3.5, which makes this the star with the highest Pb/Fe ratio, up to date. The Pb/Ba ratio is found to be high ([Pb/Ba]=1.2) and the same s true for other second-peak s-process elements(e.g La, Ce, Nd). The star is a known spectroscopic binary with a period of 346 days (Preston & Sneden 2000). The abundance pattern suggests that CS 29497-30 has accreted matter from its companion, when it was in the AGB phase.

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arXiv2002

Age and Metallicity Distribution of the Galactic Bulge from Extensive Optical and Near-IR Stellar Photometry

We present a new determination of the metallicity distribution, age, and luminosity function of the Galactic bulge stellar population. By combining near-IR data from the 2MASS survey, from the SOFI imager at ESO NTT and the NICMOS camera on board HST we were able to construct color-magnitude diagrams (CMD) and luminosity functions (LF) with large statistics and small photometric errors from the Asymptotic Giant Branch (AGB) and Red Giant Branch (RGB) tip down to $\sim 0.15 M_\odot$. This is the most extended and complete LF so far obtained for the galactic bulge. Similar near-IR data for a disk control field were used to decontaminate the bulge CMDs from foreground disk stars, and hence to set stronger constraint on the bulge age, which we found to be as large as that of Galactic globular clusters, or $\gsim 10$ Gyr. No trace is found for any younger stellar population. Synthetic CMDs have been constructed to simulate the effect of photometric errors, blending, differential reddening, metallicity dispersion and depth effect in the comparison with the observational data. By combining the near-IR data with optical ones, from the Wide Field Imager at the ESO/MPG 2.2m telescope, a disk-decontaminated $(M_K,V-K)$ CMD has been constructed and used to derive the bulge metallicity distribution, by comparison with empirical RGB templates. The bulge metallicity is found to peak at near solar value, with a sharp cutoff just above solar, and a tail towards lower metallicity that does not appreciably extend below [M/H]$\sim-1.5$.

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arXiv2002

A Catalogue of infrared star clusters and stellar groups

We compiled a catalogue of infrared star clusters in the Galaxy, which are most of them embedded. It condenses the growing literature information. We also include in the sample infrared stellar groups which are less dense than star clusters, such as those embedded in the dark clouds Taurus-Auriga and Chamaleon I. We provide galactic and equatorial coordinates, angular dimensions, different designations and related objects such as nebulae. A total of 189 infrared clusters and 87 embedded stellar groups are included. A fraction of 25 % of the embedded clusters are projected close to each other in pair or triplet systems, indicating that multiplicity plays an important role in star cluster formation.

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arXiv2002

Extinction within 10 degrees of the Galactic Centre using 2MASS

We extract J and K_s magnitudes from the 2MASS Point Source Catalog for approximately 6 million stars with 8 < K_s < 13 in order to build an A_K extinction map within 10 degrees of the Galactic centre. The extinction was determined by fitting the upper giant branch of (K_s, J-K_s) colour-magnitude diagrams to a dereddened upper giant branch mean locus built from previously studied Bulge fields. The extinction values vary from A_K=0.05 in the edges of the map up to A_K=3.2 close to the Galactic centre. The 2MASS extinction map was compared to that recently derived from DENIS data. Both maps agree very well up to A_K=1.0. The 2MASS extinction values were also compared to those obtained from dust emission in the far infrared using DIRBE/IRAS. Several systematic effects likely to bias this comparison were addressed, including the presence of dust on the background of the bulk of 2MASS stars used in the extinction determination. For the region with $3^{\circ}<|b|<5^{\circ}$, where the dust contribution on the far side of the Galaxy is $\approx$ 5%, the two extinction determinations correlate well, but the dust emission A_K values are systematically higher than those from 2MASS. A calibration correction factor of 76% for the DIRBE/IRAS dust emission extinction is needed to eliminate this systematic effect. Similar comparisons were also carried out for the $1^{\circ}<|b|<3^{\circ}$ and $|b| < 0.5^{\circ}$ strips. An asymmetry relative to the Galactic plane is observed in the dust maps, roughly in the sense that A_K values are 60% smaller in the south than in the north for $1^{\circ}<|b|<5^{\circ}$. This asymmetry is due to the presence of foreground dust clouds mostly in the northern region of the Bulge.

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arXiv2002

Chemical Abundances in Twelve Red Giants of the Large Magellanic Cloud from High-Resolution Infrared Spectroscopy

High-resolution infrared spectra (R=50,000) have been obtained for twelve red-giant members of the LMC with the Gemini South 8.3-meter telescope plus Phoenix spectrometer. Quantitative chemical abundances of carbon-12, carbon-13, nitrogen-14, and oxygen-16 were derived from molecular lines of CO, CN, and OH, while sodium, scandium, titanium, and iron abundances were derived from neutral atomic lines. The LMC giants have masses from about 1 to 4 solar masses and span a metallicity range from [Fe/H]= -1.1 to -0.3. The program red giants all show evidence of first dredge-up mixing, with low 12C/13C ratios, and low 12C correlated with high 14N abundances. Comparisons of the oxygen-to-iron ratios in the LMC and the Galaxy indicate that the trend of [O/Fe] versus [Fe/H] in the LMC falls about 0.2 dex below the Galactic trend. Such an offset can be modeled as due to an overall lower rate of supernovae per unit mass in the LMC relative to the Galaxy, as well as a slightly lower ratio of supernovae of type II to supernovae of type Ia.

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arXiv2002

A photometric study of the young open cluster NGC 1220

We present UBV CCD observations obtained in the field of the northern open cluster NGC 1220, for which little information is available. We provide also BV CCD photometry of a field 5$^{\prime}$ northward of NGC 1220 to take into account field star contamination. We argue that NGC 1220 is a young compact open cluster, for which we estimate a core radius in the range $1.5-2.0$ arcmin. We identify 26 likely candidate members with spectral type earlier than $A5$, down to $V_o$=15.00 mag on the basis of the position in the two-colour Diagram and in the Colour Magnitude Diagrams (CMDs). By analyzing the distribution of these stars in the colour-colour and CMDs, we find that NGC 1220 has a reddening E$(B-V)=0.70\pm0.15$ mag, is placed $1800\pm200$ pc distant from the Sun, and has an age of about 60 Myrs. The cluster turns out to be located about 120 pc above the Galactic plane, relatively high with respect to its age.

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arXiv2002

First Stars II. Elemental abundances in the extremely metal-poor star CS 22949--037: A diagnostic of early massive supernovae

CS 22949--037 is one of the most metal-poor giants known ([Fe/H]$ \approx-4.0$), and it exhibits large overabundances of carbon and nitrogen (Norris et al.). Using VLT-UVES spectra of unprecedented quality, regarding resolution and S/N ratio, covering a wide wavelength range (from $λ= 350$ to 900 nm), we have determined abundances for 21 elements in this star over a wide range of atomic mass. The major new discovery is an exceptionally large oxygen enhancement, [O/Fe] $= 1.97\pm0.1$, as measured from the [OI] line at 630.0 nm. We find an enhancement of [N/Fe] of $2.56\pm 0.2$, and a milder one of [C/Fe] $= 1.17\pm$0.1, similar to those already reported in the literature. This implies $Z_{\star}=0.01 Z_{\odot}$. We also find carbon isotopic ratios $^{12}$C/$^{13}$C$ =4\pm2.0$ and $^{13}$C/$^{14}$N$ =0.03 ^{+0.035}_{-0.015}$, close to the equilibrium value of the CN cycle. Lithium is not detected. Na is strongly enhanced ([Na/Fe] $= +2.1 \pm 0.2$), while S and K are not detected. The silicon-burning elements Cr and Mn are underabundant, while Co and Zn are overabundant ([Zn/Fe]$ = +0.7$). Zn is measured for the first time in such an extremely metal-poor star. The abundances of the neutron-capture elements Sr, Y, and Ba are strongly decreasing with the atomic number of the element: [Sr/Fe] $\approx +0.3$, [Y/Fe] $\approx -0.1$, and [Ba/Fe] $\approx -0.6$. Among possible progenitors of CS 22949--037, we discuss the pair-instability supernovae. Such very massive objects indeed produce large amounts of oxygen, and have been found to be possible sources of primary nitrogen. Other scenarios are also discussed. A 30-40$M_{\odot}$ supernova, with fallback, seems the most likely progenitor for CS 22949--037.

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arXiv2002

The extreme r-element rich, iron-poor halo giant CS31082-001: Implications for the r-process site(s) and radioactive cosmochronology

We present a high-resolution spectroscopic analysis of the bright (V=11.7), extreme halo giant CS31082-001 ([Fe/H] = -2.9), obtained in an ESO-VLT Large Programme dedicated to very metal-poor stars. We find CS31082-001 to be extremely rich in r-process elements, comparable in this respect only to the similarly metal-poor, but carbon-enriched, giant CS22892-052. 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 are reported in CS31082-001, almost half of the entire periodic table. All elements with 56 \leq Z \leq 72 follow the Solar r-element pattern, reduced by about 1.25 dex ([r/Fe]=+1.7 dex, a factor 50). 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. Thus, while a universal production ratio for the r-process elements seems to hold in the interval 56 \leq Z \leq 72, it breaks down in the actinide region. When available, the U/Th is thus preferable to Th/Eu for radioactive dating: (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 CS31082-001 is 14.0+/-2.4 Gyr.

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arXiv2001

Using Cepheids to determine the galactic abundance gradient I. The solar neighbourhood

A number of studies of abundance gradients in the galactic disk have been performed in recent years. The results obtained are rather disparate: from no detectable gradient to a rather significant slope of about -0.1 dex kpc -1. The present study concerns the abundance gradient based on the spectroscopic analysis of a sample of classical Cepheids. These stars enable one to obtain reliable abundances of a variety of chemical elements. Additionally, they have well determined distances which allow an accurate determination of abundance distributions in the galactic disc. Using 236 high resolution spectra of 77 galactic Cepheids, the radial elemental distribution in the galactic disc between galactocentric distances in the range 6-11 kpc has been investigated. Gradients for 25 chemical elements (from carbon to gadolinium) are derived...

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