arXiv subjects

M. Zoccali

Publications and source records attributed to M. Zoccali.

At least 163 records · Page 9Linked to original sources

arXiv2007

Distances of the bulge globular clusters Terzan 5, Liller 1, UKS 1 and Terzan 4 based on HST NICMOS photometry

A large number of pulsars and X-rays sources are detected in globular clusters. To understand the structure and content of these clusters, accurate distances are required. We derive the distances of Terzan 5, Liller 1 and UKS 1 using as a reference a recent distance determination of NGC 6528, based on HST/NICMOS and NTT/SOFI infrared photometry. The distance of the metal-poor cluster Terzan 4 was derived from a comparison with M92 in NICMOS bands. Distances of the metal-rich clusters are obtained by comparison of the Horizontal Branch (HB) level of the clusters, relative to the reddening line passing through the HB of NGC 6528. We use methods based on NICMOS bands and transformations to J and H magnitudes with different assumptions. Liller~1 and Terzan~4 are found to be at the central bulge distance, UKS~1 is beyond the Galactic center, while Terzan 5 is closer to the Sun than the other four clusters. The distance of Terzan 5 is of paramount importance, given the impact of its population of 21 pulsars, which is related to the high cluster density. The distance of Terzan 5 is found to be d$_{\odot}$=5.5$\pm$0.9 kpc from the Sun, thus closer to us than values given in studies of pulsars in Terzan 5. As a consequence, the higher cluster density is even more favourable for formation of the millisecond pulsars recently detected in this cluster.

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arXiv2007

The isotopic 6Li/7Li ratio in Cen X-4 and the origin of Li in X-ray binaries

Context: Cool stars, companions to compact objects, are known to show Li abundances which are high compared to field stars of the same spectral type, which are heavily Li depleted. This may be due either to Li production or Li preservation in these systems. Aims: To measure the lithium isotopic ratio in the companion star of the neutron star X-ray binary Cen X-4. Method: We use UVES spectra obtained in years 2000 and 2004 around the orbital quadratures. The spectra are analysed with spectrum synthesis techniques and the errors estimated with Monte Carlo simulations. Results: We measure A(Li)=2.87+-0.20 and 6Li/7Li = 0.12+0.08-0.05 at 68% confidence level. We also present updated system parameters with a refined determination of the orbital period and component masses i.e. 1.14+-0.45 Msun and 0.23+-0.10 Msun for the neutron star and companion, respectively. Conclusions: In our view the low level of 6Li favours Li preservation scenarios, although Li production mechanisms cannot be ruled out. In the case of preservation, no Li is freshly created in the binary, but the tidally-locked companion has preserved its original Li by some mechanism, possibly inhibited destruction due to its fast rotation.

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arXiv2006

Oxygen, Sodium, Magnesium and Aluminium as tracers of the Galactic Bulge Formation

This paper investigates the peculiar behaviour of the light even (alpha-elements) and odd atomic number elements in red giants in the galactic bulge, both in terms of the chemical evolution of the bulge, and in terms of possible deep-mixing mechanisms in these evolved stars. Using UVES on VLT, we measure the abundances of the four light elements O, Na, Mg and Al in 13 red clump and 40 red giant branch stars in four fields spanning the bulge from b=-3 to -12°. We show that the resulting abundance patterns point towards a chemical enrichment dominated by massive stars at all metallicities. O, Mg and Al ratios with respect to iron are overabundant with respect to both galactic disks (thin and thick) for [Fe/H]$>-0.5$. A formation timescale for the galactic bulge shorter than for both the thin and thick disks is therefore inferred. Using Mg as a proxy for metallicity (instead of Fe) we further show that: (i) the bulge stars [O/Mg] ratio follows and extend to higher metallicities the decreasing trend of [O/Mg] found in the galactic disks. (ii) the [Na/Mg] ratio trend with increasing [Mg/H] is found to increase in three distinct sequences in the thin disk, the thick disk and the bulge. The bulge trend is well represented by the predicted metallicity-dependent yields of massive stars, whereas the galactic disks have too high Na/Mg ratios at low metallicities, pointing to an additional source of Na from AGB stars. (iii) Contrary to Na, there appears to be no systematic difference in the [Al/Mg] ratio between bulge and disk stars, and the theoretical yields by massive stars agree with the observed ratios, leaving no space for AGB contribution to Al.

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arXiv2006

VLT-UVES abundance analysis of four giants in NGC 6553

Metal-rich globular clusters trace the formation of bulges. Abundance ratios in the metal-rich globular clusters such as NGC 6553 can constrain the formation timescale of the Galactic bulge. The purpose of this study is determine the metallicity and elemental ratios in individual stars of the metal-rich bulge globular cluster NGC 6553. A detailed abundance analysis of four giants in NGC 6553 is carried out, based on optical high-resolution echelle spectra obtained with UVES at the ESO VLT-UT2 Kueyen telescope. A metallicity [Fe/H]= -0.20 dex is derived, together with $α$-element enhancement of Mg and Si ([Mg/Fe]=+0.28, [Si/Fe]=+0.21), solar Ca and Ti ([Ca/Fe]=+0.05, [Ti/Fe]=-0.01), and a mild enhancement of the r-process element Eu with [Eu/Fe] = +0.10. A mean heliocentric radial velocity of -1.86 km s$^{-1}$ is measured. We compare our results with previous investigations of the cluster.

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arXiv2006

Oxygen abundances in the Galactic Bulge: evidence for fast chemical enrichment

AIMS: We spectroscopically characterize the Galactic Bulge to infer its star formation timescale, compared to the other Galactic components, through the chemical signature on its individual stars. METHODS: We derived iron and oxygen abundances for 50 K giants in four fields towards the Galactic bulge. High resolution (R=45,000) spectra for the target stars were collected with FLAMES-UVES at the VLT. RESULTS: Oxygen, as measured from the forbidden line at 6300 Å, shows a well-defined trend with [Fe/H], with [O/Fe] higher in bulge stars than in thick disk ones, which were known to be more oxygen enhanced than thin disk stars. CONCLUSIONS: These results support a scenario in which the bulge formed before and more rapidly than the disk, and therefore the MW bulge can be regarded as a prototypical old spheroid, with a formation history similar to that of early-type (elliptical) galaxies.

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arXiv2005

Dwarf elliptical galaxies in Centaurus A group: stellar populations in AM 1339-445 and AM 1343-452

We study the red giant populations of two dE galaxies, AM 1339-445 and AM 1343-452, with the aim of investigating the number and luminosity of any upper asymptotic giant branch (AGB) stars present. The galaxies are members of the Centaurus A group (D~3.8 Mpc) and are classified as outlying (R~350 kpc) satellites of Cen A. The analysis is based on near-IR photometry for individual red giant stars, derived from images obtained with ISAAC on the VLT. The photometry, along with optical data derived from WFPC2 images retrieved from the HST science archive, enable us to investigate the stellar populations of the dEs in the vicinity of the red giant branch (RGB) tip. In both systems we find stars above the RGB tip, which we interpret as intermediate-age upper-AGB stars. The presence of such stars is indicative of extended star formation in these dEs similar to that seen in many, but not all, dEs in the Local Group. For AM 1339-445, the brightest of the upper-AGB stars have Mbol~-4.5 while those in AM 1343-452 have Mbol~-4.8 mag. These luminosities suggest ages of approximately 6.5+/-1 and 4+/-1 Gyr as estimates for the epoch of the last episode of significant star formation in these systems. In both cases the number of upper-AGB stars suggests that ~15% of the total stellar population is in the form of intermediate-age stars, considerably less than is the case for outlying dE satellites of the Milky Way such as Fornax and LeoI.

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arXiv2005

Dynamical Mass Estimates for Two Luminous Star Clusters in Galactic Merger Remnants

We present high-dispersion spectra of two extremely massive star clusters in galactic merger remnants. One cluster, W30, is located in the ~500 Myr old merger remnant NGC 7252 and has a velocity dispersion and effective radius of sigma=27.5+-2.5 km/s and R_eff=9.3+-1.7 pc, respectively. The other cluster, G114, located in the ~3 Gyr old merger remnant NGC 1316, is much more compact, R_eff=4.08+-0.55 pc, and has a velocity dispersion of sigma=42.1+-2.8 km/s. These measurements allow an estimate of the virial mass of the two clusters, yielding M_dyn(W30)=1.59(+-0.26) * 10^7 Msun and M_dyn(G114)=1.64(+-0.13) *10^7 Msun. Both clusters are extremely massive, being more than three times heavier than the most massive globular clusters in the Galaxy. For both clusters we measure light-to-mass ratios, which when compared to simple stellar population (SSP) models of the appropriate age, are consistent with a Kroupa-type stellar mass function. Using measurements from the literature we find a strong age dependence on how well SSP models (with underlying Kroupa or Salpeter-type stellar mass functions) fit the light-to-mass ratio of clusters. Based on this result we suggest that the large scatter in the light-to-mass ratio of the youngest clusters is not due to variations in the underlying stellar mass function, but instead to the rapidly changing internal dynamics of young clusters. Finally, based on the positions of old globular clusters, young massive clusters (YMCs), ultra-compact dwarf galaxies (UCDs) and dwarf-globular transition objects (DGTOs) in kappa-space we conclude that 1) UCDs and DGTOs are consistent with the high mass end of star clusters and 2) YMCs occupy a much larger parameter space than old globular clusters, consistent with the idea of preferential disruption of star clusters. (abridged)

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arXiv2005

Completing the Census of (Bright) Variable Stars in Galactic Globular Clusters

We present a long-term project aimed at completing the census of (bright) variable stars in Galactic globular clusters. While our main aim is to obtain a reliable assessment of the populations of RR Lyrae and type II Cepheid stars in the Galactic globular cluster system, due attention is also being paid to other types of variables, including SX Phoenicis stars, long-period variables, and eclipsing binaries.

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arXiv2005

On the Horizontal Branch of the Galactic Globular NGC2808

We present new UV (F218W) data for stars in the central region of the Galactic globular cluster NGC2808, collected with the WFPC2 camera on board of the Hubble Space Telescope. These data together with F439W and F555W-band data and previous ground based observations provide a multifrequency coverage of the cluster stellar population extending up to a distance of 1.7 times the cluster core radius. We discuss this complete sample of stars, which includes 764 Red Giant Branch (RGB) stars brighter than the Horizontal Branch (HB) luminosity level, 1239 HB stars, 119 Asymptotic Giant Branch (AGB), and 22 AGB-manque' stellar structures. As already known, we find that blue HB stars separate into three distinct groups. However, our multiband photometry indicates that several stars in the two hotter HB groups show a flat spectrum, thus suggesting the binarity of these objects. Artificial star experiments suggest that at most 50% of them might be photometric blends. Moreover, at variance with previous claims one finds that canonical Zero Age Horizontal Branch (ZAHB) models do reach effective temperatures typical of observed hot HB stars. We also show that the ratio between HB and RGB stars brighter than the HB luminosity level steadly increases when moving from the cluster center to the periphery, passing from R=1.37 +/- 0.14 in the cluster core to R=1.95 +/- 0.26 in the outer cluster regions. We discuss the possible origin of such a radial gradient in the context of the Blue Tails phenomenon, advancing some suggestions concerning the clumpy stellar distribution along the HB.

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arXiv2004

Variable stars in the bar of the Large Magellanic Cloud: the photometric catalogue

The catalogue of the Johnson-Cousins B,V and I light curves obtained for 162 variable stars (135 RR Lyrae, 4 candidate Anomalous Cepheids, 11 Classical Cepheids, 11 eclipsing binaries and 1 delta Scuti star) in two areas close to the bar of the Large Magellanic Cloud is presented along with coordinates, finding charts, periods, epochs, amplitudes, and mean quantities (intensity- and magnitude-averaged luminosities) of the variables with full coverage of the light variations. A star by star comparison is made with MACHO and OGLE II photometries based on both variable and constant stars in common, and the transformation relationships to our photometry are provided. The pulsation properties of the RR Lyrae stars in the sample are discussed in detail. Parameters of the Fourier decomposition of the light curves are derived for the fundamental mode RR Lyrae stars with complete and regular curves (29 stars). They are used to estimate metallicities, absolute magnitudes, intrinsic (B-V)o colours, and temperatures of the variable stars, according to Jurcsik and Kovacs (1996), and Kovacs and Walker (2001) method. Quantities derived from the Fourier parameters are compared with the corresponding observed quantities. In particular, the "photometric" metallicities are compared with the spectroscopic metal abundances derived by Gratton et al. (2004) from low resolution spectra obtained with FORS at the Very Large Telescope.

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arXiv2004

A Homogenous Set of Globular Cluster Relative Distances and Reddenings

We present distance modulus and reddening determinations for 72 Galactic globular clusters from the homogeneous photometric database of Piotto et al. (2002), calibrated to the HST flight F439W and F555W bands. The distances have been determined by comparison with theoretical absolute magnitudes of the ZAHB. For low and intermediate metallicity clusters, we have estimated the apparent Zero Age Horizontal Branch (ZAHB) magnitude from the RR Lyrae level. For metal rich clusters, the ZAHB magnitude was obtained from the fainter envelope of the red HB. Reddenings have been estimated by comparison of the HST colour-magnitude diagrams (CMD) with ground CMDs of low reddening template clusters. The homogeneity of both the photometric data and the adopted methodological approach allowed us to obtain highly accurate relative cluster distances and reddenings. Our results are also compared with recent compilations in the literature.

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arXiv2004

Detailed chemical composition of the open cluster IC 4651: The iron peak, alpha elements, and Li

We present a detailed chemical analysis of 22 stars along the colour-magnitude sequence of the intermediate-age (1.7 Gyr) open cluster IC 4651. We find for the cluster a well-defined Fe abundance of [Fe/H]= 0.10 +/- 0.03 (internal errors), with a reddening E(b-y)=0.091. We also derive abundances for the alpha elements Mg, Si, Ti, and Ca and find a moderate enhancement of the three former elements. Among the Fe group elements, Cr and Ni are slightly overabundant ([X/Fe] ~ 0.05). The Na abundance among the giants is more than 0.2 dex higher than in the dwarfs. We interpret this as due to dredge-up of 23Na in the giants. Li abundances show a well-defined pattern: the Solar-type stars have an almost constant Li abundance, just below that of the Hyades, and the Li-dip is pronounced and well determined. Turnoff stars just above the dip have a `cosmic' Li abundance, but within a very small range of magnitudes (0.25 mag) higher on the turnoff, Li drops by more than a factor 10 implying that some extra mixing is required. Among the giants, two probable clump stars show detectable Li, while all the other (likely) RGB stars do not. We show that rotating stellar models including the most recent developments for meridional circulation and turbulence by shear instabilities explain very well the observed Li pattern. The possibility remains open that the giants exhibiting the highest Li abundances are actual RGB bump stars which have just been through the so-called "lithium flash".

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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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arXiv2004

Spectral Types and Masses of White Dwarfs in Globular Clusters

White dwarfs in globular clusters offer additional possibilities to determine distances and ages of globular clusters, provided their spectral types and masses are known. We therefore started a project to obtain spectra of white dwarfs in the globular clusters NGC 6397 and NGC 6752. All observed white dwarfs show hydrogen-rich spectra and are therefore classified as DA. Analysing the multi-colour photometry of the white dwarfs in NGC 6752 yields an average gravity of log g = 7.84 and 0.53 Msolar as the most probable average mass for globular cluster white dwarfs. Using this average gravity we try to determine independent temperatures by fitting the white dwarf spectra. While the stellar parameters determined from spectroscopy and photometry usually agree within the mutual error bars, the low resolution and S/N of the spectra prevent us from setting constraints stronger than the ones derived from the photometry alone. For the same reasons the white dwarf spectra obtained for NGC 6397 unfortunately do not provide an independent distance estimate of sufficient accuracy to distinguish between the long and short distance scale for globular clusters.

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arXiv2004

Simulating the recent star formation history in the halo of NGC 5128

Simulated color-magnitude diagrams (CMDs) are used to investigate the recent star formation history in NGC 5128. The comparison of the simulations with the observed UV CMD for a field in the NE shell, where recent star formation is present, constrains the initial mass function (IMF) and the duration of the star formation episode. The star formation burst is still on-going or at most has stopped some 2 Myr ago. The look-back time on the main sequence is set by the incompleteness of the U-band observations at V~26, and is of the order of 50 Myr. The post main sequence phases have a look-back time up to 100 Myr, setting the maximum observable time for the star formation in this field. The comparison of the observed and simulated luminosity functions and the number of post main sequence vs. total number of stars favours models with active star formation in the last 100 Myr. The data are best fitted by an IMF with Salpeter slope (α=2.35), and are also consistent with slopes in the range of 2<~α<~2.6. They exclude steeper IMF slopes. The mean star formation rate for a Salpeter IMF in the range of masses 0.6<~M<~100 M_sun within the last 100 Myr is 1.6x10^-4 M_sun yr^-1 kpc^-2.

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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

The Red Giant Branch Luminosity Function Bump

We present observational estimates of the magnitude difference between the luminosity function RGB bump and the HB (Delta(bump-zahb)), and of star counts in the bump region (R_bump), for a sample of 54 Galactic globular clusters (GCs) observed by the HST. The large sample of stars resolved in each cluster, and the high photometric accuracy of the data allowed us to detect the bump also in a number of metal poor clusters. To reduce the photometric uncertainties, empirical values are compared with theoretical predictions obtained from a set of updated canonical stellar evolution models which have been transformed directly into the HST flight system. We found an overall qualitative agreement between theory and observations. Quantitative estimates of the confidence level are hampered by current uncertainties on the GCs metallicity scale, and by the strong dependence of Delta(bump-zahb) on the cluster metallicity. In case of the R_bump parameter, which is only weakly affected by the metallicity, we find a very good quantitative agreement between theoretical canonical models and observations. For our full cluster sample the average difference between predicted and observed R_bump values is practically negligible, and ranges from -0.002 to -0.028, depending on the employed metallicity scale. The observed dispersion around these values is entirely consistent with the observational errors on R_bump. As a comparison, the value of R_bump predicted by theory in case of spurious bump detections would be \~0.10 smaller than the observed ones. We have also tested the influence on the predicted Delta(bump-zahb) and R_bump values of an He-enriched component in the cluster stellar population. We find that the predicted Delta(bump-zahb) and R_bump values are only marginally affected.

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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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