arXiv subjects

G. Piotto

Publications and source records attributed to G. Piotto.

At least 271 records · Page 15Linked to original sources

arXiv2011

Multiple stellar populations in 47 Tucanae

We use Hubble Space Telescope (HST) and ground-based imaging to study the multiple populations of 47 Tuc, combining high-precision photometry with calculations of synthetic spectra. Using filters covering a wide range of wavelengths, our HST photometry splits the main sequence (MS) into two branches, and we find that this duality is repeated in the subgiant and red-giant regions (SGB, RGB), and on the horizontal branch (HB). We calculate theoretical stellar atmospheres for MS stars, assuming different chemical composition mixtures, and we compare their predicted colors through the HST filters with our observed colors. We find that we can match the complex of observed colors with a pair of populations, one with primeval abundance and another with enhanced nitrogen and a small helium enhancement, but with depleted C and O. We confirm that models of RGB and red HB stars with that pair of compositions also give colors that fit our observations. We suggest that the different strengths of molecular bands of OH, CN, CH and NH, falling in different photometric bands, are responsible for the color splits of the two populations. Near the cluster center, in each portion of the color-magnitude diagram (CMD) the population with primeval abundances makes up only ~20% of the stars, a fraction that increases outwards, approachng equality in the outskirts of the cluster, with a fraction ~30% averaged over the whole cluster. Thus the second, He/N-enriched population is more concentrated and contributes the majority of the present-day stellar content of the cluster. We present evidence that the CMD of 47 Tuc consists of intertwined sequences of the two populations, whose separate identities can be followed continuously from the MS up to the RGB, and thence to the HB. A third population is visible only in the SGB, where it includes ~8% of the stars.

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arXiv2011

Luminosity and mass functions of the three main sequences of the globular cluster NGC 2808

High-precision HST photometry has recently revealed that the globular cluster (GC) NGC 2808 hosts a triple main sequence (MS) corresponding to three stellar populations with different helium abundances. We carried out photometry on ACS/WFC HST images of NGC 2808 with the main purpose of measuring the luminosity function (LF) of stars in the three different MSs, and the binary fraction in the cluster. We used isochrones to transform the observed LFs into mass functions (MFs). We estimated that the fraction of binary systems in NGC 2808 is f_bin ~0.05, and find that the three MSs have very similar LFs. The slopes of the corresponding MFs are alpha=-1.2+/-0.3 for the red MS, alpha =-0.9+/-0.3 for the middle MS, and alpha = -0.9+/-0.4 for the blue one, the same, to within the errors. There is marginal evidence of a MF flattening for masses M<~0.6 M_SUN for the the reddest (primordial) MS. These results represent the first direct measurement of the present day MF and LF in distinct stellar populations of a GC, and provide constraints on models of the formation and evolution of multiple generations of stars in these objects.

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arXiv2011

TASTE. II. A new observational study of transit time variations in HAT-P-13b

TASTE (The Asiago Search for Transit timing variations of Exoplanets) project is collecting high-precision, short-cadence light curves for a selected sample of transiting exoplanets. It has been claimed that the hot jupiter HAT-P-13b suddenly deviated from a linear ephemeris by $\sim 20$ min, implying that there is a perturber in the system. Using five new transits, we discuss the plausibility of this transit time variation (TTV), and show that a periodic signal should not be excluded. More follow-up observations are required to constrain the mass and the orbit of the hypothetical perturber.

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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

The two metallicity groups of the globular cluster M22: a chemical perspective

We present a detailed chemical composition analysis of 35 red giant stars in the globular cluster M22. High resolution spectra for this study were obtained at five observatories, and analyzed in a uniform manner. We have determined abundances of representative light proton-capture, alpha, Fe-peak and neutron-capture element groups. Our aim is to better understand the peculiar chemical enrichment history of this cluster, in which two stellar groups are characterized by a different content in iron, neutron capture elements Y, Zr and Ba, and alpha element Ca. The principal results of this study are: (i) substantial star-to-star metallicity scatter (-2.0<[Fe/H]<-1.6); (ii) enhancement of s-process/r-process neutron-capture abundance ratios in a fraction of giants, positively correlated with metallicity; (iii) sharp separation between the s-process rich and s-process poor groups by [La/Eu] ratio; (iv) possible increase of [Cu/Fe] ratios with increasing [Fe/H], suggesting that this element also has a significant s-process component; and (v) presence of Na-O and C-N anticorrelations in both the stellar groups.

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arXiv2011

TASTE: The Asiago Search for Transit timing variations of Exoplanets. I. Overview and improved parameters for HAT-P-3b and HAT-P-14b

A promising method for detecting earth-sized exoplanets is the timing analysis of a known transit. The technique allows a search for variations in either the transit duration or the center induced by the perturbation of a third body, e.g. a second planet or an exomoon. By applying this method, the TASTE (The Asiago Search for Transit Timing variations of Exoplanets) project will collect high-precision, short-cadence light curves for a selected sample of transits by using imaging differential photometry at the Asiago 1.82m telescope. The first light curves show that our project can achieve a competitive timing accuracy, as well as a significant improvement of the orbital parameters. We derived refined ephemerides for HAT-P-3b and HAT-P-14b with a timing accuracy of 11 and 25 s, respectively.

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arXiv2011

NGC 2419: a large and extreme second generation in a currently undisturbed cluster

We analyse complementary HST and SUBARU data for the globular cluster NGC 2419. We make a detailed analysis of the horizontal branch (HB), that appears composed by two main groups of stars: the luminous blue HB stars ---that extend by evolution into the RR Lyrae and red HB region--- and a fainter, extremely blue population. We examine the possible models for this latter group and conclude that a plausible explanation is that they correspond to a significant (~30 %) extreme second generation with a strong helium enhancement (Y~0.4). We also show that the color dispersion of the red giant branch is consistent with this hypothesis, while the main sequence data are compatible with it, although the large observational error blurs the possible underlying splitting. While it is common to find an even larger (50 -- 80) percentage of second generation in a globular cluster, the presence of a substantial and extreme fraction of these stars in NGC 2419 might be surprising, as the cluster is at present well inside the radius beyond which the galactic tidal field would be dominant. If a similar situation had been present in the first stages of the cluster life, the cluster would have retained its initial mass, and the percentage of second generation stars should have been quite small (up to ~10 %). Such a large fraction of extreme second generation stars implies that the system must have been initially much more massive and in different dynamical conditions than today. We discuss this issue in the light of existing models of the formation of multiple populations in globular clusters.

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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

TASTE: The Asiago Survey for Timing transit variations of Exoplanets

A promising method to detect earth-sized exoplanets is the timing analysis of a known transit. The technique allows a search for variations in transit duration or center induced by the perturbation of a third body, e.g. a second planet or an exomoon. To this aim, TASTE (The Asiago Survey for Timing transit variations of Exoplanets) project will collect high-precision, short-cadence light curves for a selected sample of transits by using imaging differential photometry at the Asiago 1.82m telescope. The first light curves show that our project can already provide a competitive timing accuracy, as well as a significant improvement over the orbital parameters. We derived refined ephemerides for HAT-P-3b and HAT-P-14b with only one transit each, thanks to a timing accuracy of 11 and 25 s, respectively.

astro-ph.EP↗

arXiv2010

X-shooter observations of main sequence stars in the globular cluster NGC 2808: first chemical tagging of a He-normal and a He-rich dwarf

We present the first chemical composition study of two unevolved stars in the globular cluster NGC 2808, obtained with the spectrograph X-shooter@VLT. NGC 2808 shows three discrete, well separated main sequences. The most accepted explanation for this phenomenon is that their stars have different helium contents. We observed one star on the bluest main sequence, (bMS, claimed to have high helium content, Y~0.4), and one on the reddest main sequence (rMS, consistent with a canonical helium content, Y=0.245). We analyzed features of NH, CH, Na, Mg, Al, and Fe. While Fe, Ca, and other elements have the same abundances in the two stars, the bMS star shows a huge enhancement of N, a depletion of C, an enhancement of Na and Al, and small depletion of Mg with respect to the rMS star. This is exactly what is expected if stars on the bMS formed from the ejecta produced by an earlier stellar generation in the complete CNO and MgAl cycles whose main product is helium. The elemental abundance pattern differences in these two stars are consistent with the differences in helium content suggested by the color-magnitude diagram positions of the stars.

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arXiv2010

Fast rotating Blue Stragglers in the globular cluster M4

We have used high resolution spectra obtained with the spectrograph FLAMES at the ESO Very Large Telescope to determine the kinematical properties and the abundance patterns of 20 blue straggler stars (BSSs) in the globular cluster M4. We found that ~ 40% of the measured BSSs are fast rotators (with rotational velocities > 50 km/s). This is the largest frequency of rapidly rotating BSSs ever detected in a globular cluster. In addition, at odds with what has been found in 47 Tucanae, no evidence of carbon and/or oxygen depletion has been revealed in the sample of 11 BSSs for which we were able to measure the abundances. This could be due either to low statistics, or to a different BSS formation process acting in M4.

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arXiv2010

Detailed abundances of a large sample of giant stars in M 54 and in the Sagittarius nucleus

Homogeneous abundances of light elements, alpha and Fe-group elements from high-resolution FLAMES spectra are presented for 76 red giant stars in M54, a massive globular cluster (GC) lying in the nucleus of the Sagittarius dwarf galaxy. We also derived detailed abundances for 27 red giants belonging to the Sgr nucleus. Our abundances assess the intrinsic metallicity dispersion (~0.19 dex, rms scatter) of M54, with the bulk of stars peaking at [Fe/H]~-1.6 and a long tail extending to higher metallicities, similar to omega Cen. The spread in these probable nuclear star clusters exceeds those of most GCs: these massive clusters are located in a region intermediate between normal GCs and dwarf galaxies. M54 shows the Na-O anticorrelation, typical signature of GCs, which is instead absent in the Sgr nucleus. The light elements (Mg, Al, Si) participating to the high temperature Mg-Al cycle show that the pattern of (anti)correlations produced by proton-capture reactions in H-burning is clearly different between the most metal-rich and most metal-poor components in the two most massive GCs in the Galaxy, confirming early result based on the Na-O anticorrelation. As in omega Cen, stars affected by most extreme processing, i.e. showing the signature of more massive polluters, are those of the metal-rich component. This can be understood if the burst of star formation giving birth to the metal-rich component was delayed by as much as 10-30 Myr with respect to the metal-poor one. The evolution of these massive GCs can be reconciled in the general scenario for the formation of GCs sketched in Carretta et al.(2010a) taking into account that omega Cen could have already incorporated the surrounding nucleus of its progenitor and lost the rest of the hosting galaxy while the two are still observable as distinct components in M54 and the surrounding field.

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arXiv2010

The helium spread in the Globular cluster 47 Tuc

Spectroscopy has shown the presence of the CN band dicothomy and the Na-O anticorrelations for 50--70% of the investigated samples in the cluster 47 Tuc, otherwise considered a "normal" prototype of high metallicity clusters from the photometric analysis. Very recently, the re-analysis of a large number of archival HST data of the cluster core has been able to put into evidence the presence of structures in the Sub Giant Branch: it has a brighter component with a spread in magnitude by $\sim$0.06 mag and a second one, made of about 10% of stars, a little fainter (by $\sim$0.05 mag). These data also show that the Main Sequence of the cluster has an intrinsic spread in color which, if interpreted as due to a small spread in helium abundance, suggests $Δ$Y$\sim$0.027. In this work we examine in detail whether the Horizontal Branch morphology and the Sub Giant structure provide further independent indications that a real --although very small-helium spread is present in the cluster. We re--analyze the HST archival data for the Horizontal Branch of 47 Tuc, obtaining a sample of $\sim$500 stars with very small photometric errors, and build population synthesis based on new models to show that its particular morphology can be better explained by taking into account a spread in helium abundance of 2% in mass. The same variation in helium is able to explain the spread in luminosity of the Sub Giant Branch, while a small part of the second generation is characterized by a small C+N+O increase and provides an explanation for the fainter Sub Giant Branch. We conclude that three photometric features concur to form the paradigm that a small but real helium spread is present in a cluster that has no spectacular evidence for multiple populations like those shown by other massive clusters.

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arXiv2010

Absolute proper motion of the Galactic open cluster M67

We derived the absolute proper motion (PM) of the old, solar-metallicity Galactic open cluster M67 using observations collected with CFHT (1997) and with LBT (2007). About 50 galaxies with relatively sharp nuclei allow us to determine the absolute PM of the cluster. We find (mu_alpha cos(delta),mu_delta)_J2000.0 = (-9.6+/-1.1,-3.7+/-0.8) mas/yr. By adopting a line-of-sight velocity of 33.8+/-0.2 km/s, and assuming a distance of 815+/-50 pc, we explore the influence of the Galactic potential, with and without the bar and/or spiral arms, on the galactic orbit of the cluster.

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arXiv2010

M 54 + Sagittarius = omega Centauri

We derive homogeneous abundances of Fe, O, Na and alpha-elements from high resolution FLAMES spectra for 76 red giant stars in NGC 6715 (M 54) and for 25 red giants in the surrounding nucleus of the Sagittarius (Sgr) dwarf galaxy. Our main findings are that: (i) we confirm that M 54 shows intrinsic metallicity dispersion, ~0.19 dex r.m.s.; (ii) when the stars of the Sgr nucleus are included, the metallicity distribution strongly resembles that in omega Cen; the relative contribution of the most metal-rich stars is however different in these two objects; (iii) in both GCs there is a very extended Na-O anticorrelation, signature of different stellar generations born within the cluster, and (iv) the metal-poor and metal-rich components in M 54 (and omega Cen) show clearly distinct extension of the Na-O anticorrelation, the most heavily polluted stars being those of the metal-rich component. We propose a tentative scenario for cluster formation that could explain these features. Finally, similarities and differences found in the two most massive GCs in our Galaxy can be easily explained if they are similar objects (nuclear clusters in dwarf galaxies) observed at different stages of their dynamical evolution.

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arXiv2010

The end of the white dwarf cooling sequence in M 67

In this paper, we present for the first time a proper-motion-selected white dwarf (WD) sample of the old Galactic open cluster M 67, down to the bottom of the WD cooling sequence (CS). The color-magnitude diagram is based on data collected with the LBC-Blue camera at the prime focus of LBT. As first epoch data, we used CFHT-archive images collected 10 years before LBC data. We measured proper motions of all the identified sources. Proper motions are then used to separate foreground and background objects from the cluster stars, including WDs. Finally, the field-object cleaned WD CS in the V vs. B-I color-magnitude diagram is compared with the models. We confirm that the age derived from the location of the bottom of the WD CS is consistent with the turn off age.

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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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