arXiv subjects

G. Piotto

Publications and source records attributed to G. Piotto.

At least 361 records · Page 20Linked to original sources

arXiv2001

The field of NGC 6397 as a test for Galactic models

Taking advantage of recent HST data for field stars in the region of the Galactic globular cluster NGC 6397, we tested the predictions of several Galactic models with star counts reaching a largely unexplored range of magnitudes, down to $V$$\sim$ 26.5. After updating the input stellar ($V-I$) colors, we found that the two-component Bahcall-Soneira (B&S) model can be put in satisfactory agreement with observations for suitable choices of disk/spheroid luminosity functions. However if one assumes the Gould, Bahcall and Flynn (1996, 1997) disk luminosity function (LF) together with the Gould, Flynn and Bahcall (1998) spheroid LF, there is no way to reconcile the predicted and observed $V $-magnitude distribution. We also analysed the agreement between observed and predicted magnitude and color distributions for two selected models with a thick disk component. Even in this case there are suitable combinations of model parameters and faint magnitude LFs which can give a reasonable agreement with observational star counts both in magnitude and in color, the above quoted combination of Gould et al. (1997, 1998) LFs giving again predictions in clear disagreement with observations.

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arXiv2001

Asymptotic Giant Branch predictions: theoretical uncertainties

In this paper we investigate the level of agreement between observations and "new" Asymptotic Giant Branch (AGB) models, as produced by updating the physical inputs adopted in previous stellar computations. One finds that the new physics increases the predicted luminosity of Horizontal Branch (HB) and AGB stellar structures by a similar amount, keeping unchanged the predictions about the difference in luminosity between these two evolutionary phases. The best fit of selected globular clusters appears rather satisfactory, disclosing the relevance of the assumption on the mass of the Red Giant Branch (RGB) progenitor in assessing the distance modulus of moderately metal rich clusters. The still existing uncertainties related either to the input physics or to the efficiency of some macroscopic mechanisms, like convection or microscopic diffusion, are critically discussed, ruling out the occurrence of the so called "breathing pulses" during the central He exhaustion, in agreement with earlier suggestions.

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arXiv2001

Luminosity and mass function of galactic open clusters: I. NGC 4815

We present deep V and I photometry for the open cluster NGC 4815 and four surrounding Galactic fields down to a limiting magnitude $V\sim25$. These data are used to study cluster spatial extension by means of star counts, and to derive the luminosity (LF) and mass function (MF). The radius turns out to be $3.6\pm0.3 arcmin$ at V=19.0 level, whereas the mass amounts at $880\pm230 m_{\odot}$ down to V=20.8. From the color-magnitude diagram, we obtain the LFs in the V and I bands, using both the standard histogram and an adaptive kernel. After correction for incompleteness and field star contamination, the LFs have been transformed into the present day mass functions (PDMF). The PDMFs from the V and I photometry can be represented as a power-law with a slope $α= 3.1\pm0.3$ and $α= 2.9\pm0.3$ (the \cite{salp55} MF in this notation has a slope $α= 2.35$) respectively, in the mass range $2.5 \leq \frac{m}{m_{\odot}} \leq 0.8$. Below this mass, the MF cannot be considered as representative of the cluster IMF, as it is the result of the combined effect of strong irregularities in the stellar background, probable internal dynamical evolution of the cluster and/or interaction of the cluster with the dense Galactic field. Unresolved binaries and mass segregation can only flatten the apparent derived IMF, so we expect that the real IMF must be steeper than the quoted slope by an unknown amount.

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arXiv2000

The anomalous Galactic globular cluster NGC 2808

We present UBVI photometry for about 60,000 stars in a region of 44'x33' centered on the Galactic globular cluster (GGC) NGC 2808. The central region has been mapped with the HST WFPC2 camera. We cover a field which extends from the center to about 1.7 tidal radii. We confirm the anomalously elongated HB with gaps along it, and we show that the gaps are statistically significant. We find that: 1) the extended blue tail of HB (EBT) is present beyond 400''(=9r_h), and extends to V=21.2 also in these external regions; 2) also the gaps on the EBT are present at least out to 400'' from the cluster center, and possibly beyond that. The location of the gaps in the HB seems to be the same all over the cluster. 3) there are no significant radial gradients in the distribution of the stars in the HB. The observational facts presented in this paper seem to exclude the possibility that the EBTs originate from tidal stripping of the envelope of the RGB stars due to close encounters in high density environments or to mass transfer in close binaries. Also, it is not clear whether the presence of EBTs and gaps on them are the manifestation of the same physical phenomenon. We have also shown that the jump recently identified in the HB using Str"omgren-u photometry is clearly visible also in the U, U-B plane, at T_e~11,600K. Finally, we present the luminosity function (LF) of the RGB. The LF clearly shows the presence of the usual RGB bump, but also a second feature, 1.4 magnitudes brighter in V, that we have named RGB heap. The RGB heap, visible also in other GGCs, is a new feature that, because of its position on the RGB, we have tentatively associated with the recently discovered K giant variables.

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arXiv2000

Photometric catalog of nearby globular clusters (II)

In this paper we present the second and final part of a large and photometrically homogeneous CCD color-magnitude diagram (CMD) data base, comprising 52 nearby Galactic globular clusters (GGC) imaged in the V and I bands. The catalog has been collected using only two telescopes (one for each hemisphere). The observed clusters represent 75% of the known Galactic globulars with (m-M)_v <= 16.15 mag, cover most of the globular cluster metallicity range (-2.2 <= [Fe/H] <= -0.4), and span Galactocentric distances from ~1.2 to ~18.5 kpc. In particular, here we present the CMDs for the 13 GGCs observed in the Northern hemisphere. The remaining 39 Southern hemisphere clusters of the catalog have been presented in a companion paper (Rosenberg et al., 2000, A&AS 144, 5). We present the first CCD color magnitude diagram for NGC6779 (M56). All the CMDs extend from at least ~2 magnitudes below the turn-off (i.e. V_lim ~22) to the tip of the red giant branch. The calibration has been done using a large number of standard stars, and the absolute calibration is reliable to a ~0.02 mag level in both filters. This catalog, because of its homogeneity, is expected to represent a useful data base for the measurement of the main absolute and relative parameters characterizing the CMD of GGCs.

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arXiv2000

Comparison between Observed and Theoretical Red Giant Branch Luminosity Functions of Galactic Globular Clusters

V-band luminosity functions have been obtained for the upper main-sequence, sub-giant branch and red giant branch of 18 galactic globular clusters from HST data. A comparison with four sets of theoretical models has been performed. In contrast with what was found in several previous works, a Good general agreement has been found between the observed and theoretical LF at any metallicity [M/H]<-0.7. Possible discrepancies at higher metallicity, in the upper part of the RGB, need to be confirmed with further observational data and by extending all the models to the most metal rich regime. The SGB shape has been used to set an upper limit to the cluster age, and consequently a lower limit on the cluster distance. A discussion on the still open problem of the mismatch between the observed and theoretical RGB bump location is also presented.

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arXiv2000

New evidence for the complex struscture of the RGB of Omega Cen

We report on the complex structure of the red giant branch (RGB) of omega Cen, based on a new wide field and wide color baseline B and I photometry. Our color magnitude diagram (CMD) shows the presence of multiple populations along this branch, in particular we discovered an anomalous branch (RGB-a), which appears to be well separated from the bulk of the RGB stars. On the basis of our CMD and from the previous literature we conclude that (1) these stars, clearly identified as a separate population in our CMD, represent the extreme metal rich extension ([Ca/H]>-0.3) of the stellar content of omega Cen, and show anomalous abundances of s-process elements (as Ba and Zr) as well; (2) they are physical members of the omega Cen system; (3) they comprise ~ 5% of the stars of the whole system; (4) this component and the metal-intermediate one (-0.4>[Ca/H]>-1) have been found to share the same spatial distribution, both of them differing significantly from the most metal poor one ([Ca/H]<-1). This last evidence supports the hypothesis that metal rich components could belong to an independent (proto?) stellar system captured in the past by omega Cen.

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arXiv2000

Galactic Globular Clusters as a test for Very Low-Mass stellar models

We make use of the Next Generation model atmospheres by Allard et al. (1997) and Hauschildt, Allard & Baron (1999) to compute theoretical models for low and very low-mass stars for selected metallicities in the range Z= 0.0002 to 0.002. On this basis, we present theoretical predictions covering the sequence of H-burning stars as observed in galactic globulars from the faint end of the Main Sequence up to, and beyond, the cluster Turn Off. The role played by the new model atmospheres is discussed, showing that present models appear in excellent agreement with models by Baraffe et al. (1997) as computed on quite similar physical basis. One finds that the theoretical mass-luminosity relations based on this updated set of models, are in good agreement with the empirical data provided by Henry & McCarthy (1993). Comparison with HST observation discloses that the location in the Color-Magnitude diagram of the lower Main Sequence in galactic globular clusters appears again in good agreement with the predicted sensitive dependence of these sequences on the cluster metallicity.

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arXiv2000

On the helium content of Galactic globular clusters via the R parameter

We estimate the empirical R parameter in 26 Galactic Globular Clusters covering a wide metallicity range, imaged by WFPC2 on board the HST. The improved spatial resolution permits a large fraction of the evolved stars to be measured and permits accurate assessment of radial populaton gradients and completeness corrections. In order to evaluate both the He abundance and the He to metal enrichment ratio, we construct a large set of evolutionary models by adopting similar metallicities and different He contents. We find an absolute He abundance which is lower than that estimated from spectroscopic measurements in HII regions and from primordial nucleosynthesis models. This discrepancy could be removed by adopting a C12O16 nuclear cross section about a factor of two smaller than the canonical value, although also different assumptions for mixing processes can introduce systematical effects. The trend in the R parameter toward solar metallicity is consistent with an upper limit to the He to metal enrichment ratio of the order of 2.5.

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arXiv2000

Photometric catalog of nearby globular clusters (I)

We present the first part of the first large and homogeneous CCD color-magnitude diagram (CMD) data base, comprising 52 nearby Galactic globular clusters (GGC) imaged in the V and I bands using only two telescopes (one for each hemisphere). The observed clusters represent 75% of the known Galactic globulars with (m-M)v<=16.15 mag, cover most of the globular cluster metallicity range (-2.2 <= [Fe/H] <= -0.4), and span Galactocentric distances from ~1.2 to ~18.5 kpc. In this paper, the CMDs for the 39 GGCs observed in the southern hemisphere are presented. The remaining 13 northern hemisphere clusters of the catalog are presented in a companion paper. For four clusters (NGC 4833, NGC 5986, NGC 6543, and NGC 6638) we present for the first time a CMD from CCD data. The typical CMD spans from the 22nd V magnitude to the tip of the red giant branch. Based on a large number of standard stars, the absolute photometric calibration is reliable to the ~0.02 mag level in both filters. This catalog, because of its homogeneity, is expected to represent a useful data base for the measurement of the main absolute and relative parameters characterizing the CMD of GGCs.

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arXiv1999

Galactic Globular Cluster Relative Ages

Based on a new large, homogeneous photometric database of 35 Galactic globular clusters (GGCs), a set of distance and reddening independent relative age indicators has been measured. The observed D(V-I)_2.5 and D(V)(HB-TO) vs. metallicity relations have been compared with the relations predicted by two recent updated libraries of isochrones. Using these models and two independent methods, we have found that self-consistent relative ages can be estimated for our GGC sample. Based on the relative age vs. metallicity distribution, we conclude that: (a) there is no evidence of an age spread for clusters with [Fe/H]<-1.2, all the clusters of our sample in this range being old and coeval; (b) for the intermediate metallicity group (-1.2<=[Fe/H]<-0.9) there is a clear evidence of age dispersion, with clusters up to ~25% younger than the older members; and (c) the clusters within the metal rich group ([Fe/H]>=-0.9) seem to be coeval within the uncertainties (except Pal12), but younger (~17%) than the bulk of the Galactic globulars. The latter result is totally model dependent. From the distribution of the GGC ages with the Galactocentric distance, we can present a possible scenario for the Milky Way formation: The GC formation process started at the same zero age throughout the halo, at least out to ~20 kpc from the Galactic center. According to the present stellar evolution models, the metal-rich globulars are formed at a later time (~ 17% lower age). And finally, significantly younger halo GGCs are found at any R(GC)>8 kpc. For these, a possible scenario associated with mergers of dwarf galaxies to the Milky Way is suggested.

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arXiv1999

HST Observations of Galactic Globular Cluster Cores. II. NGC 6273 and the Problem of Horizontal-Branch Gaps

We present observations of the center of the Galactic globular cluster NGC 6273, obtained with the HST/WFPC2 camera as part of the snapshot program GO-7470. A B, V color-magnitude diagram (CMD) for ~28,000 stars is presented and discussed. The most prominent feature of the CMD, identified for the first time in this paper, is the extended horizontal branch blue tail (EBT) with a clear double-peaked distribution and a significant gap. The EBT of NGC 6273 is compared with the EBTs of seven other globular clusters for which we have a CMD in the same photometric system. From this comparison we conclude that all the globular clusters in our sample with an EBT show at least one gap along the HB, which could have similar origins. A comparison with theoretical models suggests that at least some of these gaps may be occuring at a particular value of the stellar mass, common to a number of different clusters. From the CMD of NGC 6273 we obtain a distance modulus (m-M)_V=16.27+-0.20. We also estimate an average reddening E(B-V)=0.47+-0.03, though the CMD is strongly affected by differential reddening, with the relative reddening spanning a 0.2 magnitude interval in E(B-V) in the WFPC2 field. A luminosity function for the evolved stars in NGC 6273 is also presented and compared with the most recent evolutionary models.

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arXiv1999

Comparison between predicted and empirical DV(Bump -HB) in Galactic Globular Clusters

We present observational estimates of DV(Bump-HB) in a sample of 28 Galactic Globular Clusters (GGCs) observed by HST. The photometric accuracy and the sizable number of stars measured in each cluster allowed us to single out the RGB Bump both in metal-poor and in metal-rich GGCs. Empirical values are compared with homogeneous theoretical predictions which account for both H and He burning phases over a wide range of metal abundances (0.0001 < Z < 0.02). We found that, within current observational uncertainties on both iron and $α$-element abundances, theory and observations are in very good agreement, provided that the metallicity scale by Carretta & Gratton (1997) as extended by Cohen et al. (1999) is adopted. Interesting enough, we also found that both theoretical and observed values show a change in the slope of the DV(Bump-HB)-[M/H] relation toward higher metal contents.

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arXiv1999

HST Luminosity Functions of the Globular Clusters M10, M22, and M55. A comparison with other clusters

From a combination of deep Hubble Space Telescope V and I images with groundbased images in the same bands, we have obtained color-magnitude diagrams of M10, M22, and M55, extending from just above the hydrogen burning limit to the tip of the red giant branch, down to the white dwarf cooling sequence. We have used the color-magnitude arrays to extract main sequence luminosity functions (LFs) from the turnoff to about 0.13 solar masses. The LFs of M10 is significantly steeper than that for the other two clusters. The difference cannot be due to a difference in metallicity. A comparison with the LFs from Piotto, Cool, and King (1997), shows a large spread in the LF slopes. This spread is also present in the local mass functions (MFs) obtained from the observed LFs using different theoretical mass--luminosity relations. The dispersion in the MF slopes remains also after removing the mass segregation effects by using multimass King-Michie models. The globular cluster MF slopes are also flatter than the MF slope of the field stars and of the Galactic clusters in the same mass interval. We interpret the MF slope dispersion and the MF flatness as an evidence of dynamical evolution which makes the present day globular cluster stellar MFs different from the intial MFs. The slopes of the present day MFs exclude that the low mass star can be dynamically relevant for the Galactic globular clusters.

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arXiv1998

Young Galactic globular clusters II. The case of Palomar 12

We present broadband V,I CCD photometry for ca. 1700 stars towards the Galactic globular cluster Palomar 12, covering a field of 10.7' x 10.7'. From these data, a color-magnitude diagram from the red giant branch tip to ca. 2 mag below the cluster's turn-off is obtained. From a comparison with the color magnitude diagrams of 47 Tuc and M5, and using different theoretical models, we confirm that Pal 12 is younger, finding an age 68% +/- 10% that of both template clusters. Revised structural parameters are also obtained: r(c) = 37.8 +/- 0.6 and c = 1.08 +/- 0.02.

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arXiv1998

The Galactic globular cluster NGC 1851: its dynamical and evolutionary properties

We have completely mapped the globular cluster NGC 1851 with VI CCD photometry, obtaining a CMD for ca. 20500 stars. From the apparent luminosity of the HB we derive a distance modulus (m-M)o=15.44+/-0.20. The presence of 7 "supra-HB" stars is confirmed. Six of them are identified as evolved descendants from HB progenitors. Synthetic HR diagrams demonstrate that the problem of the HB morphology could be solved by assuming a bimodal efficiency of the mass loss along the RGB. We find evidence that the radial distribution of the blue HB stars is different from that of the red HB and SGB stars. The first measurement of the mean absolute I magnitude for 22 known RR Lyr variables ( =0.12+/-0.20 at [Fe/H]=-1.28), is given. We found 7 new RR Lyr candidates (6 ab type and 1 c type), lowering the ratio N(c)/N(ab) to 0.38. From a sample of 25 globular clusters (including NGC 1851) a new calibration for DV(bump,HB) as a function of metallicity is derived, and we find some evidence for an age-metallicity relation among GCs. We identify 13 blue straggler stars, which do not show any sign of variability. The blue stragglers are less concentrated than the SGB stars with similar magnitudes for r>80". Finally, a radial dependence of the LF, a sign of mass segregation, is found. Transforming the LF into a mass function and correcting for mass segregation by means of multi-mass King-Michie models, we find a global MF exponent x(0)=0.2+/-0.3.

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arXiv1997

The Luminosity and Mass Function of the Globular Cluster NGC1261

I-band CCD images of two large regions of the Galactic globular cluster NGC 1261 have been used to construct stellar luminosity functions (LF) for 14000 stars in three annuli from 1.4' from the cluster center to the tidal radius. The LFs extend to M_I~8 and tend to steepen from the inner to the outer annulus, in agreement with the predictions of the multimass King-Michie model that we have calculated for this cluster. The LFs have been transformed into mass functions. Once corrected for mass segregation the global mass function of NGC 1261 has a slope x_0=0.8+/-0.5

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arXiv1997

Pal 1: Another Young Galactic Halo Globular Cluster?

Deep V and I CCD images of the loosely populated Galactic globular cluster Pal 1 and the surrounding field have been obtained with the INT. A color magnitude diagram down to V(lim) ~ 24, and a luminosity function complete down to V(lim) ~ 23.25 have been constructed. Adopting a reddening E(V-I)=0.20+/-0.04 and comparing the CMD of Pal 1 with 47 Tucanae, we obtain a distance modulus (m-M)o=15.25+/-0.25, indicating that Pal 1 is located 3.7+/-0.4 kpc above the Galactic disc and 17.3+/-1.6 kpc from the Galactic center. Comparison with 47 Tuc and M71 shows that Pal 1 must be significantly younger. The best fitting isochrones (Bertelli et al, 1994) give an age between 6.3 and 8 Gyrs, which would make Pal 1 the youngest Galactic globular cluster so far identified, casting some doubt on the real nature of this object. The luminosity function shows some evidence for mass segregation, consistent with the very short relaxation time. The global mass function can be fitted with a power law of slope x=-1.4+/-0.7. This mass function is anomalously flat, suggesting either a strong dynamical evolution or an initial mass function significantly different from most of the other halo globular clusters. A discussion on the possible nature of Pal 1 is presented.

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