arXiv subjects

S. Villanova

Publications and source records attributed to S. Villanova.

At least 91 records · Page 5Linked to original sources

arXiv2012

Spectroscopy of horizontal branch stars in Omega Centauri

We analyze the reddening, surface helium abundance and mass of 115 horizontal branch (HB) and blue hook (BH) stars in OmegaCentauri, spanning the HB from the blue edge of the instability strip to Teff~50000K. The mean cluster reddening is E(B-V)=0.115+-0.004, in good agreement with previous estimates, but we evidence a pattern of differential reddening in the cluster area. The stars in the western half are more reddened than in the southwest quadrant by 0.03-0.04 magnitudes. We find that the helium abundances measured on low-resolution spectra are systematically lower by ~0.25 dex than the measurements based on higher resolution. No difference in helium abundance is detected between OmegaCentauri and three comparison clusters, and the stars in the range 11500-20000K follow a trend with temperature, which probably reflects a variable efficiency of the diffusion processes. There is mild evidence that two families of extreme HB (EHB) stars (Teff>20000K) could exist, as observed in the field, with ~15% of the objects being helium depleted by a factor of ten with respect to the main population. The distribution of helium abundance above 30000K is bimodal, but we detect a fraction of He-poor objects lower than previous investigations. The observations are consistent with these being stars evolving off the HB. Their spatial distribution is not uniform, but this asymmetric distribution is only marginally significative. We also find that EHB stars with anomalously high spectroscopic mass could be present in OmegaCentauri, as previously found in other clusters. The derived temperature-color relation reveals that stars hotter than 11000K are fainter than the expectations of the canonical models in the U band, while no anomaly is detected in B and V. This behavior, not observed in NGC6752, is a new peculiarity of OmegaCentauri HB stars.

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arXiv2012

Lithium and sodium in the globular cluster M4. Detection of a Li-rich dwarf star: preservation or pollution?

Context. The abundance inhomogeneities of light elements observed in Globular Clusters (GCs), and notably the ubiquitous Na-O anti-correlation, are generally interpreted as evidence that GCs comprise several generations of stars. There is an on-going debate as to the nature of the stars which produce the inhomogeneous elements, and investigating the behavior of several elements is a way to shed new light on this problem. Aims. We aim at investigating the Li and Na content of the GC M 4, that is known to have a well defined Na-O anti-correlation. Methods. We obtained moderate resolution (R=17 000-18 700) spectra for 91 main sequence (MS)/sub-giant branch stars of M 4 with the Giraffe spectrograph at the FLAMES/VLT ESO facility. Using model atmospheres analysis we measured lithium and sodium abundances. Results. We detect a weak Li-Na anti-correlation among un-evolved MS stars. One star in the sample, # 37934, shows the remarkably high lithium abundance A(Li)=2.87, compatible with current estimates of the primordial lithium abundance. Conclusions. The shallow slope found for the Li-Na anti-correlation suggests that lithium is produced in parallel to sodium. This evidence, coupled with its sodium-rich nature, suggests that the high lithium abundance of star # 37934 may originate by pollution from a previous generations of stars. The recent detection of a Li-rich dwarf of pollution origin in the globular cluster NGC 6397 may also point in this direction. Still, no clear cut evidence is available against a possible preservation of the primordial lithium abundance for star # 37934.

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arXiv2012

The Unique Na:O Abundance Distribution in NGC 6791: The First Open(?) Cluster with Multiple Populations

Almost all globular clusters investigated exhibit a spread in their light element abundances, the most studied being a Na:O anticorrelation. In contrast, open clusters show a homogeneous composition and are still regarded as Simple Stellar Populations. The most probable reason for this difference is that globulars had an initial mass high enough to retain primordial gas and ejecta from the first stellar generation and thus formed a second generation with a distinct composition, an initial mass exceeding that of open clusters. NGC 6791 is a massive open cluster, and warrants a detailed search for chemical inhomogeneities. We collected high resolution, high S/N spectra of 21 members covering a wide range of evolutionary status and measured their Na, O and Fe content. We found [Fe/H]=+0.42$\pm 0.01$, in good agreement with previous values, and no evidence for a spread. However, the Na:O distribution is completely unprecedented. It becomes the first open cluster to show intrinsic abundance variations that cannot be explained by mixing, and thus the first discovered to host multiple populations. It is also the first star cluster to exhibit two subpopulations in the Na:O diagram with one being chemically homogeneous while the second has an intrinsic spread that follows the anticorrelation so far displayed only by globular clusters. NGC 6791 is unique in many aspects, displaying certain characteristics typical of open clusters, others more reminiscent of globulars, and yet others, in particular its Na:O behavior investigated here, that are totally unprecedented. It clearly had a complex and fascinating history.

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arXiv2012

Spectroscopic analysis of the two subgiant branches of the globular cluster NGC1851

NGC1851 possibly shows a spread in [Fe/H], but the relation between this spread and the division in the SGB is unknown. We obtained blue (3950-4600 A) intermediate resolution (R~8,000) spectra for 47 stars on the bright and 30 on the faint SGB of NGC 1851 (b-SGB and f-SGB, respectively). The determination of the atmospheric parameters to extremely high internal accuracy leads to small errors when comparing different stars in the cluster. We found that the b-SGB is slightly more metal-poor than the f-SGB, with [Fe/H]=-1.227+/-0.009 and [Fe/H]=-1.162+/- 0.012, respectively. This implies that the f-SGB is only slightly older by ~0.6 Gyr than the b-SGB if the total CNO abundance is constant. There are more C-normal stars in the b-SGB than in the f-SGB. This is consistent with what is found for HB stars, if b-SGB are the progenitors of red HB stars, and f-SGB those of blue HB ones. The abundances of the n-capture elements Sr and Ba have a bimodal distribution, reflecting the separation between f-SGB (Sr and Ba-rich) and b-SGB stars (Sr and Ba-poor). In both groups, there is a clear correlation between [Sr/Fe] and [Ba/Fe], suggesting that there is a real spread in the abundances of n-capture elements. There is some correlation between C and Ba abundances, while the same correlation for Sr is much more dubious. We identified six C-rich stars, which have a moderate overabundance of Sr and Ba and rather low N abundances. This group of stars might be the progenitors of these on the anomalous RGB in the (v, v-y) diagram. These results are discussed within different scenarios for the formation of NGC1851. It is possible that the two populations originated in different regions of an inhomogeneous parent object. However, the striking similarity with M22 calls for a similar evolution for these two clusters. Deriving reliable CNO abundances for the two sequences would be crucial.

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arXiv2012

Detailed abundances in stars belonging to ultra-faint dwarf spheroidal galaxies

We report preliminary results concerning the detailed chemical composition of metal poor stars belonging to close ultra-faint dwarf galaxies (hereafter UfDSphs). The abundances have been determined thanks to spectra obtained with X-Shooter, a high efficiency spectrograph installed on one of the ESO VLT units. The sample of ultra-faint dwarf spheroidal stars have abundance ratios slightly lower to what is measured in field halo star of the same metallicity.We did not find extreme abundances in our Hercules stars as the one found by Koch for his 2 Hercules stars. The synthesis of the neutron capture elements Ba and Sr seems to originate from the same nucleosynthetic process in operation during the early stages of the galactic evolution.

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arXiv2012

The helium content of globular clusters: NGC6121 (M4)

He has been proposed as a key element to interpret the observed multiple MS, SGB, and RGB, as well as the complex horizontal branch (HB) morphology. Stars belonging to the bluer part of the HB, are thought to be more He rich (ΔY=0.03 or more) and more Na-rich/O-poor than those located in the redder part. This hypothesis was only partially confirmed in NGC 6752, where stars of the redder zero-age HB showed a He content of Y=0.25+-0.01, fully compatible with the primordial He content of the Universe, and were all Na-poor/O-rich. Here we study hot blue HB (BHB) stars in the GC NGC 6121 (M4) to measure their He plus O/Na content. We observed 6 BHB stars using the UVES@VLT2 spectroscopic facility. In addition to He, O, Na, and Fe abundances were estimated. Stars turned out to be all Na-rich and O-poor and to have a homogeneous enhanced He content with a mean value of Y=0.29+-0.01(random)+-0.01(systematic). The high He content of blue HB stars in M4 is also confirmed by the fact that they are brighter than red HB stars (RHB). Theoretical models suggest the BHB stars are He-enhanced by ΔY=0.02-0.03 with respect to the RHB stars. The whole sample of stars has a metallicity of [Fe/H]=-1.06+-0.02 (internal error). This is a rare direct measurement of the (primordial) He abundance for stars belonging to the Na-rich/O-poor population of GC stars in a temperature regime where the He content is not altered by sedimentation or extreme mixing as suggested for the hottest, late helium flash HB stars. Our results support theoretical predictions that the Na-rich/O-poor population is also more He-rich than the Na-poor/O-rich generation and that a leading contender for the 2^{nd} parameter is the He abundance.

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arXiv2011

The C+N+O abundance of Omega Centauri giant stars: implications on the chemical enrichment scenario and the relative ages of different stellar populations

We present a chemical-composition analysis of 77 red-giant stars in Omega Centauri. We have measured abundances for carbon and nitrogen, and combined our results with abundances of O, Na, La, and Fe that we determined in our previous work. Our aim is to better understand the peculiar chemical-enrichment history of this cluster, by studying how the total C+N+O content varies among the different-metallicity stellar groups, and among stars at different places along the Na-O anticorrelation. We find the (anti)correlations among the light elements that would be expected on theoretical ground for matter that has been nuclearly processed via high-temperature proton captures. The overall [(C+N+O)/Fe] increases by 0.5 dex from [Fe/H] -2.0 to [Fe/H] -0.9. Our results provide insight into the chemical-enrichment history of the cluster, and the measured CNO variations provide important corrections for estimating the relative ages of the different stellar populations.

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arXiv2011

Multiple stellar populations in the Globular Clusters NGC1851 and NGC6656 (M22)

In the last years, photometric and spectroscopic evidence has demonstrated that many, maybe all the Globular Clusters (GC) host multiple stellar populations. High-resolution spectroscopy has established that, while most GCs are mono-metallic with no significant abundance spread in $s$-elements, in all the globulars studied to date the presence of different stellar generation is inferred by the Na-O and the C-N anticorrelations. In this context, NGC 1851 and NGC 6656 are among the most intriguing clusters. Contrary to the majority of GCs, they host two groups of stars with different s-elements abundance that are clearly associated to the two distinct sub-giant and red-giant branches detected in their color-magnitude diagrams (CMD). In the case of NGC 6656 s-rich stars are also enriched in iron and calcium. Each $s$-element group exhibits its own Na-O and C-N anticorrelations thus indicating the presence of sub-populations and suggesting that the parent clusters have experienced a very complex star-formation history. In this paper we summarize the properties of multiple populations in NGC 1851 and NGC 6656.

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arXiv2011

Lithium-rich giants in the Galactic thick disk

Context: Lithium is a fragile element, which is easily destroyed in the stellar interior. The existence of lithium-rich giants still represents a challenge for stellar evolution models. Aims: We have collected a large database of high-resolution stellar spectra of 824 candidate thick-disk giants having 2\,MASS photometry and proper motions measured by the Southern Proper-Motion Program (SPM). In order to investigate the nature of Li-rich giants, we searched this database for giants presenting a strong Li\,I resonance line. Methods: We performed a chemical abundance analysis on the selected stars with the MOOG code along with proper ATLAS-9 model atmospheres. The iron content and atmospheric parameters were fixed by using the equivalent width of a sample of Fe lines. We also derive abundances for C, N, and O and measure or derive lower limits on the $^{12}$C/$^{13}$C isotopic ratios, which is a sensible diagnostic of the stars evolutionary status. Results: We detected five stars with a lithium abundance higher than 1.5, i.e. Li-rich according to the current definition. One of them (SPM-313132) has A(Li)$>$3.3 and, because of this, belongs to the group of the rare super Li-rich giants. Its kinematics makes it a likely thin-disk member and its atmospheric parameters are compatible with it being a 4\,M$_\odot$ star either on the red giant branch (RGB) or the early asymptotic giant branch. This object is the first super Li-rich giant detected at this phase. The other four are likely low-mass thick-disk stars evolved past the RGB luminosity bump, as determined from their metallicities and atmospheric parameters. The most evolved of them lies close to the RGB-tip. It has A(Li)$>$2.7 and a low $^{12}$C/$^{13}$C isotopic ratio, close to the cool bottom processing predictions.

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arXiv2011

A lack of close binaries among hot horizontal branch stars in globular clusters. II. NGC\,2808

Models based on their binary origin have been very successful in reproducing the properties of field subdwarf-B stars, but the observations of their analogues in globular clusters has posed new problems, while the discovery of multiple populations offered an appealing alternative scenario for the formation of these stars. We search for binaries of period P<200 days among a sample of blue horizontal branch stars (Teff=12000-22000 K) in NGC2808, a cluster known to host three distinct stellar populations and a multimodal horizontal branch. The final sample consists of 64 targets. The radial velocity of the targets was measured in fourteen epochs, spanning a temporal interval of about 75 days. We detect no RV variable object among stars cooler than the photometric G1 gap at 17000 K, while two close (P<10 days) and two intermediate-period (P=10-50 days) systems are found among hotter targets. The close and intermediate-period binary fraction for stars cooler than the gap are fc<5% and fip<10%, respectively, with 95% confidence. The most probable values among hotter stars are fc~20% and fip~30%, but the 90%- confidence level intervals are large (6-42% and 11-72%, respectively). The G1 gap appears as a discontinuity in the binary faction, with a higher incidence of binaries among hotter stars, but a constant increase in f with temperature rather than a discontinuity cannot be excluded from our observations. We find that intermediate-period binaries, never investigated before among cluster HB stars, could play an important role, being more than ~15-20% of the hottest stars of our sample. Our results indicate that fc among hot HB stars is most probably higher for younger clusters, confirming the recently proposed age-fc relation. However, the large observed difference in binary fraction between clusters (e.g. NGC2808 and NGC6752) is still not reproduced by binary population synthesis models.

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arXiv2010

Sodium-Oxygen anticorrelation among Horizontal Branch Stars in the Globular Cluster M4

The horizontal branch (HB) morphology of globular clusters (GC) is mainly governed by metallicity. The second parameter problem, well known since the 60's, states that metallicity alone is not enough to describe the observed HB morphology of many GCs. Despite many efforts to resolve this issue, the second parameter phenomenon still remains without a satisfactory explanation. We have analyzed blue, red-HB, and RR-Lyrae stars in the GC M4 and studied their Fe, Na, and O abundances. Our goal is to investigate possible connections between the bimodal HB of M4 and the chemical signatures of the two stellar populations recently discovered among red giants of this cluster. We obtained FLAMES-UVES/GIRAFFE spectra of a sample of 22 stars covering the HB from the red to the blue region. While iron has the same abundance in both the red and blue-HB segment, the red-HB is composed of stars with scaled-solar sodium abundances, while the blue-HB stars are all sodium enhanced and oxygen depleted. The RR-Lyrae are Na-poor, as the red-HB stars, and O-rich. This is what we expect if the blue-HB consists of a second generation of stars formed from the ejecta produced by an earlier stellar population through high-temperature hydrogen-burning processes that include the CNO, NeNa, and MgAl cycles and are therefore expected to be He-rich. According to this scenario, the sodium and oxygen pattern detected in the blue and red-HB segments suggests helium as the second parameter that rules the HB morphology in M4.

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arXiv2010

Testing Newtonian gravity with distant globular clusters: NGC1851 and NGC1904

Globular clusters are useful to test the validity of Newtonian dynamics in the low acceleration regime typical of galaxies, without the complications of non-baryonic dark matter. Specifically, in absence of disturbing effects, e.g. tidal heating, their velocity dispersion is expected to vanish at large radii. If such behaviour is not observed, and in particular if, as observed in elliptical galaxies, the dispersion is found constant at large radii below a certain threshold acceleration, this might indicate a break down of Newtonian dynamics. To minimise the effects of tidal heating in this paper we study the velocity dispersion profile of two distant globular clusters, NGC 1851 and NGC 1904. The velocity dispersion profile is derived from accurate radial velocities measurements, obtained at the ESO 8m VLT telescope. Reliable data for 184 and 146 bona fide cluster star members, respectively for NGC 1851 and NGC 1904, were obtained. These data allow to trace the velocity dispersion profile up to ~2r0, where r0 is the radius at which the cluster internal acceleration of gravity is a0 = 10e-8 cm/s/s. It is found that in both clusters the velocity dispersion becomes constant beyond ~r0. These new results are fully in agreement with those found for other five globular clusters previously investigated as part of this project. Taken all together, these 7 clusters support the claim that the velocity dispersion is constant beyond r0, irrespectively of the specific physical properties of the clusters: mass, size, dynamical history, and distance from the Milky Way. The strong similarly with the constant velocity dispersion observed in elliptical galaxies beyond r0 is suggestive of a common origin for this phenomenon in the two class of objects, and might indicate a breakdown of Newtonian dynamics below a0.

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arXiv2010

The lithium content of omega Centauri. New clues to the cosmological Li problem from old stars in external galaxies

A discrepancy has emerged between the cosmic lithium abundance inferred by the WMAP satellite measurement coupled with the prediction of the standard big-bang nucleosynthesis theory, and the constant Li abundance measured in metal-poor halo dwarf stars (the so-called Spite plateau). Several models are being proposed to explain this discrepancy, involving either new physics, in situ depletion, or the efficient depletion of Li in the pristine Galaxy by a generation of massive first stars. The realm of possibilities may be narrowed considerably by observing stellar populations in different galaxies, which have experienced different evolutionary histories. The WCen stellar system is commonly considered as the remnant of a dwarf galaxy accreted by the Milky Way (MW). We investigate the Li content of a conspicuous sample of unevolved stars in this object. We obtained moderate resolution (R=17000) spectra for 91 main-sequence/early sub-giant branch (MS/SGB) WCen stars using the FLAMES-GIRAFFE/VLT spectrograph. Li abundances were derived by matching the equivalent width of the LiI resonance doublet at 6708A, to the prediction of synthetic spectra computed with different Li abundances. Synthetic spectra were computed using the SYNTHE code along with ATLAS9 model atmospheres. The stars effective temperatures are derived by fitting the wings of the Ha line with synthetic profiles. We obtain a mean content of A(Li)=2.19+-0.14~dex for WCen MS/SGB stars. This is comparable to what is observed in Galactic halo field stars of similar metallicities and temperatures. The Spite plateau seems to be an ubiquitous feature of old, warm metal-poor stars. It exists also in external galaxies, if we accept the current view about the origin of WCen. This implies that the mechanism(s) that causes the "cosmological lithium problem" may be the same in the MW and other galaxies.

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arXiv2010

VLT multi-epoch radial velocity survey toward NGC 6253. Analysis of three transiting planetary candidates

We measured the radial velocity of 139 stars in the region of NGC 6253, discussing cluster's membership and binarity in this sample, complementing our analysis with photometric, proper motion, and radial velocity data available from previous studies of this cluster, and analyzing three planetary transiting candidates we found in the field of NGC 6253. Spectra were obtained with the UVES and GIRAFFE spectrographs at the VLT, during three epochs in August 2008. The mean radial velocity of the cluster is -29.11+/-0.85 km/s. Using both radial velocities and proper motions we found 35 cluster's members, among which 12 are likely cluster's close binary systems. One star may have a sub-stellar companion, requiring a more intensive follow-up. Our results are in good agreement with past radial velocity and photometric measurements. Furthermore, using our photometry, astrometry and spectroscopy we identified a new sub-giant branch eclipsing binary system, member of the cluster. The cluster's close binary frequency at 29% +/- 9% (34% +/-10% once including long period binaries), appears higher than the field binary frequency equal to (22% +/- 5%, though these estimates are still consistent within the uncertainties. Among the three transiting planetary candidates the brightest one (V=15.26) is worth to be more intensively investigated with higher percision spectroscopy. We discussed the possibility to detect sub-stellar companions (brown dwarfs and planets) with the radial velocity technique (both with UVES/GIRAFFE and HARPS) around turn-off stars of old open clusters [abridged].

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arXiv2010

Chemical composition of stellar populations in Omega Centauri

We derive abundances of Fe, Na, O, and s-elements from GIRAFFE@VLT spectra for more than 200 red giant stars in the Milky Way satellite Omega Centauri. Our preliminary results are that: (i) we confirm that Omega Centauri exibiths large star-to-star metallicity variations ($\sim$ 1.4 dex); (ii) the metallicity distribution reveals the presence of at least five stellar populations with different [Fe/H]; (iii) a clear Na-O anticorrelation is clearly observed for the metal-poor and metal-intermediate populations while apparently the anticorrelation disappears for the most metal-rich populations. Interestingly the Na level grows with iron.

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arXiv2009

Multiple stellar populations in the Galactic globular cluster NGC 6752

We have carried out high-precision photometry on a large number of archival HST images of the Galactic globular cluster NGC 6752, to search for signs of multiple stellar populations. We find a broadened main sequence, and demonstrate that this broadening cannot be attributed either to binaries or to photometric errors. There is also some indication of a main-sequence split. No significant spread could be found along the subgiant branch, however. Ground-based photometry reveals that in the U vs. (U-B) color-magnitude diagram the red-giant branch exhibits a clear color spread, which we have been able to correlate with variations in Na and O abundances. In particular the Na-rich, O-poor stars identified by Carretta et al. (2007) define a sequence on the red side of the red-giant branch, while Na-poor, O-rich stars populate a bluer, more dispersed portion of the red-giant branch.

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arXiv2009

A double stellar generation in the Globular Cluster NGC6656 (M 22). Two stellar groups with different iron and s-process element abundance

AIMS. In this paper we present the chemical abundance analysis from high resolution UVES spectra of seventeen bright giant stars of the Globular Cluster M~22. RESULTS. We obtained an average iron abundance of [Fe/H]=-1.76\pm0.02 (internal errors only) and an αenhancement of 0.36\pm0.04 (internal errors only). Na and O, and Al and O follow the well known anti-correlation found in many other GCs. We identified two groups of stars with significantly different abundances of the s-process elements Y, Zr and Ba. The relative numbers of the two group members are very similar to the ratio of the stars in the two SGBs of M22 recently found by Piotto (2009). Y and Ba abundances do not correlate with Na, O and Al. The s-element rich stars are also richer in iron and have higher Ca abundances. The results from high resolution spectra have been further confirmed by lower resolution GIRAFFE spectra of fourteen additional M22 stars. GIRAFFE spectra show also that the Eu -- a pure r-process element -- abundance is not related to the iron content. We discuss the chemical abundance pattern of M22 stars in the context of the multiple stellar populations in GC scenario.

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arXiv2009

The Helium content of Globular Clusters: light element abundance correlations and HB morphology. I. NGC6752

Helium has been proposed as the key element to interpret the observed multiple main sequences (MS), subgiant branches (SGB) and red giant branches (RGB), as well as the complex horizontal branch (HB) morphology in Globular Clusters (GC). However, up to now, He was never directly measured in suitable GC stars (8500<Teff<11500 K) with the purpose of verify this hypothesis. We studied 7 hot blue horizontal branch (BHB) stars (Teff<11500 K) in the GC NGC 6752 with the purpose to measure their Helium content. In addition Fe,Cr,Si,Ti,O,Na, and Ba abundances were measured. We could measure He abundance only for stars warmer than Teff=8500 K. All our targets with measurable He are zero age HB (ZAHB) objects and turned out to have a homogeneous He content with a mean value of Y=0.245+-0.012, compatible with the most recent measurements of the primordial He content of the Universe (Y~0.25). The whole sample of stars have a metallicity of [Fe/H]=-1.56+-0.03 and [alpha/Fe]=+0.21+-0.03. Our HB targets show the same Na-O anticorrelation identified among the TO-SGB-RGB stars. This is the first direct measurement of the He abundance for a significative sample of GC stars in a temperature regime where the He content is not altered by sedimentation processing or extreme mixing as suggested for the hottest, late helium flasher HB stars.

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