Flavio Fusi Pecci

Distances, Ages, and Epoch of Formation of Globular Clusters*

We review the results on distances and absolute ages of Galactic globular clusters (GCs) obtained after the release of the Hipparcos catalog. Several methods aimed at the definition of the Population II local distance scale are discussed, and their results compared, exploiting new results for RR Lyraes in the Large Magellanic Cloud (LMC). We find that the so-called short distance and long distance scales may be reconciled whether or not a consistent reddening scale is adopted for Cepheids and RR Lyrae variables in the LMC. Emphasis is given in the paper to the discussion of distances and ages of GCs derived using Hipparcos parallaxes of local subdwarfs. We find that the selection criteria adopted to choose the local subdwarfs, as well as the size of the corrections applied to existing systematic biases, are the main culprit for the differences found among the various independent studies that first used Hipparcos parallaxes and the subdwarf fitting technique. We also caution that the absolute age of M92 (usually considered one of the oldest clusters) still remains uncertain due to the lack of subdwarfs of comparable metallicity with accurate parallaxes. Distances and ages for the nine clusters discussed in a previous paper by Gratton et al. are rederived using an enlarged sample of local subdwarfs, which includes about 90% of the metal-poor dwarfs with accurate parallaxes (Δπ/π ≤ 0.12) in the whole Hipparcos catalog. On average, our revised distance moduli are decreased by 0.04 mag with respect to Gratton et al. The corresponding age of the GCs is t = 11.5 ± 2.6 Gyr, where the error bars refer to the 95% confidence range. The relation between the zero-age horizontal branch (ZAHB) absolute magnitude and metallicity for the nine program clusters turns out to be MV(ZAHB) = (0.18 ± 0.09)([Fe/H] + 1.5) + (0.53 ± 0.12) Thanks to Hipparcos the major contribution to the total error budget associated with the subdwarf fitting technique has been moved from parallaxes to photometric calibrations, reddening, and metallicity scale. This total uncertainty still amounts to about ±0.12 mag. We then compare the corresponding (true) LMC distance modulus μLMC = 18.64 ± 0.12 mag with other existing determinations. We conclude that at present the best estimate for the distance of the LMC is μLMC = 18.54 ± 0.03 ± 0.06, suggesting that distances from the subdwarf fitting method are ~1 σ too long. Consequently, our best estimate for the age of the GCs is revised to Age = 12.9 ± 2.9 Gyr (95% confidence range). The best relation between ZAHB absolute magnitude and metallicity is MV(ZAHB) = (0.18 ± 0.09)( + 1.5) + (0.63 ± 0.07). Finally, we compare the ages of the GCs with the cosmic star formation rate recently determined by studies of the Hubble Deep Field (HDF), exploiting the determinations of ΩM = 0.3 and ΩΛ = 0.7 provided by Type Ia supernovae surveys. We find that the epoch of formation of the GCs (at z ~ 3) matches well the maximum of the star formation rate for elliptical galaxies in the HDF as determined by Franceschini et al.

Ages of Globular Clusters from Hipparcos Parallaxes of Local Subdwarfs

We report here initial but strongly conclusive results for absolute ages of Galactic globular clusters (GGCs). This study is based on high-precision trigonometric parallaxes from the HIPPARCOS satellite coupled with accurate metal abundances ([Fe/H], [O/Fe], and [α/Fe]) from high-resolution spectroscopy for a sample of about thirty subdwarfs. Systematic effects due to star selection (Lutz-Kelker corrections to parallaxes) and the possible presence of undetected binaries in the sample of bona fide single stars are examined, and appropriate corrections are estimated. They are found to be small for our sample. The new data allow us to reliably define the absolute location of the main sequence (MS) as a function of metallicity. These results are then used to derive distances and ages for a carefully selected sample of nine globular clusters having metallicities determined from high-dispersion spectra of individual giants according to a procedure totally consistent with that used for the field subdwarfs. Very precise and homogeneous reddening values have also been independently determined for these clusters. Random errors for our distance moduli are ±0.08 mag, and systematic errors are likely of the same order of magnitude. These very accurate distances allow us to derive ages with internal errors of ~12% (±1.5 Gyr). The main results are: 1. HIPPARCOS parallaxes are smaller than corresponding ground-based measurements, leading, in turn, to longer distance moduli (~0.2 mag) and younger ages (~2.8 Gyr). 2. The distance to NGC 6752 derived from our MS fitting is consistent with that determined using the white dwarf cooling sequence. 3. The relation between the zero-age HB (ZAHB) absolute magnitude and metallicity for the nine program clusters is This relation is fairly consistent with some of the most recent theoretical models. Within quoted errors, the slope is in agreement with that given by the Baade-Wesselink (BW) analysis of RR Lyrae stars by Fernley and Clementini et al., while it is somewhat shallower than the relation given by Sandage. The zero-point is 0.2 to 0.3 mag brighter than that obtained with BW, while it agrees fairly well with that given by Sandage. A comparison with alternative relationships is briefly discussed. 4. The corresponding LMC distance modulus is (m - M)0 = 18.60 ± 0.07, in good agreement with the recent values of 18.70 ± 0.10 and 18.54 ± 0.2 derived by Feast & Catchpole and van Leeuwen et al., respectively, from HIPPARCOS parallaxes of Galactic Cepheid and Mira variables. 5. The age of the bona fide old globular clusters (Oosterhoff II and BHB), based on the absolute magnitude of the turnoff (a theoretically robust indicator) is where the error bar is the 95% confidence range. The rms scatter of individual ages around the mean value is ~10%, in agreement with expectations from observational errors alone (that is, we do not find it necessary to introduce a real age scatter among these clusters). A reliable study of the relative ages requires the use of age indicators better suited to this purpose and data for a larger sample of GGCs. 6. Allowing for a minimum delay of 0.5 Gyr from the birth of the universe until the formation of globular clusters, our age estimate is compatible with an Einstein-de Sitter model if H0 ≤ 64 km s-1 Mpc-1, or H0 ≤ 83 km s-1 Mpc-1 in a flat universe with Ωm = 0.2. Since these upper limits are well within the confidence range of most determinations of H0, we conclude that the present age of globular clusters does not conflict with standard inflationary models of the universe.

A New Infrared Array Photometric Survey of Galactic Globular Clusters: A Detailed Study of the Red Giant Branch Sequence as a Step toward the Global Testing of Stellar Models

We present high-quality near-infrared color-magnitude diagrams of 10 Galactic globular clusters (GCs) spanning a wide metallicity range (-2.15 < [Fe/H] < -0.2). This homogeneous database has been used to perform a detailed analysis of the red giant branch (RGB), adopting a variety of observables to describe its physical and chemical properties. First, a set of metallicity indicators have been measured, namely, (1) the RGB (J-K) and (V-K) colors at different magnitude levels, (2) the RGB K magnitude at different colors, and (3) the RGB slope. For these parameters we present new calibrations in terms of both spectroscopic iron abundance and global metallicity, including the ?-element enhancement. These relations can be used to derive a photometric estimate of the GC metal content from the RGB morphology and location. Second, the location in luminosity of the main RGB features (namely, the bump and the tip) and their dependence on metallicity have been studied, yielding quantitative observational relationships. Finally, adopting new transformations between the observational and theoretical quantities, the mean ridge lines for the clusters of our sample have been reported in the plane. This allows us to study the RGB location in terms of effective temperature, bolometric luminosity of the main RGB features, and their calibrations with varying metallicity. Direct comparisons between updated theoretical models and observations show an excellent overall agreement.

Multimodal Distributions along the Horizontal Branch

We report on the Hubble Space Telescope (HST) Wide Field Planetary Camera 2 (WFPC2) U, V, and far-ultraviolet observations of three galactic globular clusters (GGCs), NGC 5272 = M3, NGC 6205 = M13, and NGC 6093 = M80. Two of these clusters (namely, M13 and M80) have horizontal-branch (HB) tails that extend to the helium-burning main sequence, with the hottest stars reaching theoretical effective temperatures above 35,000 K. In both clusters, groups of stars are found to be separated by narrow gaps along the blue HB sequence. These gaps appear at similar locations in the color-magnitude diagrams of the two clusters. While stochastic effects may give rise to variations in the color distribution along the HB, the coincidence of gaps in different clusters effectively rules this out as the primary cause. The comparison among the clusters strongly suggests that there are separate physical processes operating during the earlier red giant phase of evolution to produce mass loss.

The First Empirical Mass-Loss Law for Population II Giants*

Using the Spitzer IRAC camera we have obtained mid-IR photometry of the red giant branch stars in the Galactic globular cluster 47 Tuc. About 100 stars show an excess of mid-IR light above that expected from their photospheric emission. This is plausibly due to dust formation in mass flowing from these stars. This mass loss extends down to the level of the horizontal branch and increases with luminosity. The mass loss is episodic, occurring in only a fraction of stars at a given luminosity. Using a simple model and our observations we derive mass-loss rates for these stars. Finally, we obtain the first empirical mass-loss formula calibrated with observations of Population II stars. The dependence on luminosity of our mass-loss rate is considerably shallower than the widely used Reimers law. The results presented here are the first from our Spitzer survey of a carefully chosen sample of 17 Galactic globular clusters, spanning the entire metallicity range from about one hundredth up to almost solar.

ISOCAM Observations of Galactic Globular Clusters: Mass Loss along the Red Giant Branch

Deep images in the 10 ?m spectral region have been obtained for five massive Galactic globular clusters, NGC 104 (=47 Tuc), NGC 362, NGC 5139 (=? Cen), NGC 6388, NGC 7078 (=M15), and NGC 6715 (=M54) in the Sagittarius Dwarf Spheroidal using the ISOCAM on board the Infrared Space Observatory in 1997. A significant sample of bright giants have an ISOCAM counterpart, but only fewer than 20% of these have a strong mid-IR excess indicative of dusty circumstellar envelopes. From a combined physical and statistical analysis we derive mass-loss rates and frequency. We find that (1) significant mass loss occurs only at the very end of the Red Giant Branch evolutionary stage and is episodic, (2) the modulation timescales must be greater than a few decades and less than a million years, and (3) mass-loss occurrence does not show a crucial dependence on the cluster metallicity.

Deep Hubble Space Telescope WFPC2 Photometry of NGC 288. I. Binary Systems and Blue Stragglers

We present the first results of a deep WFPC2 photometric survey of the loose galactic globular cluster NGC 288. The fraction of binary systems is estimated from the color distribution of objects near the main sequence (MS) with a method analogous to that introduced by Rubenstein & Bailyn. We have unequivocally detected a significant population of binary systems with a radial distribution that has been significantly influenced by mass segregation. In the inner region of the cluster (r < 1rh 1.6rc) the binary fraction (fb) lies in the range 0.08–0.38 regardless of the assumed distribution of mass ratios, F(q). The most probable fb lies between 0.10 and 0.20 depending on the adopted F(q). On the other hand, in the outer region (r ≥ 1rh), fb must be less than 0.10, and the most likely value is 0.0, independently of the adopted F(q). The detected population of binaries is dominated by primordial systems. The specific frequency of blue stragglers (BSs) is exceptionally high, suggesting that the BS production mechanism via binary evolution can be very efficient. A large population of BSs is possible even in low-density environments if a sufficient reservoir of primordial binaries is available. The observed distribution of BSs in the color-magnitude diagram is not compatible with a rate of BS production that has been constant in time, if it is assumed that all the BSs are formed by the merging of two stars.

Hubble Space Telescope Ultraviolet Observations of the Cores of M3 and M13

We present preliminary results from Hubble Space Telescope (HST)/WFPC2 observations of the central regions of the of the Galactic globular clusters M13 and M3. The clusters are almost identical in most respects, including chemical composition, but there are dramatic differences in both the horizontal-branch (HB) and blue straggler populations. The M13 HB has a long blue tail extending 4.5 mag in V, reaching well below the level of the main-sequence turnoff. M3 has no such feature. M3 and M13 are thus an extreme case of the second-parameter problem in HB morphology. Also present in the M13 HB are two gaps similar to those seen in the clusters NGC 6752 and NGC 2808. M3 has a specific frequency of blue stragglers 3 times larger than that of M13. Our results imply that neither age nor cluster density, two popular second-parameter candidates, are likely to be responsible for the observed differences.

Globular cluster mass functions

Deep imaging data in the I bandpass for three globular clusters were used to construct their luminosity and mass functions. These new data for ω Cen, M5, and NGC 6752, together with existing data for M13, NGC 6397, and M71 were then employed to investigate any symptomatics of the cluster mass function slopes and their evolution. A related question also considered was the relation between the currently observed and the IMF slopes.

The Large Magellanic Cloud Globular Cluster NGC 1866: New Data, New Models, New Analysis*

We present a new deep (down to V ~ 24) photometry of a wide region (~6 × 6) around the Large Magellanic Cloud globular cluster NGC 1866. Our sample is much larger (by more than a factor of 3) than any previous photometry and with a main sequence (MS) that may be considered complete, down to at least 3 mag below the brightest MS star; such an occurrence allows a meaningful and robust comparison with various theoretical scenarios produced by means of models computed with the evolutionary code FRANEC. Both age and present-day mass function slope, α, are derived by a fit to the available MS and by the use of the parameter Δσ, which is simply the difference, in σ, between the observed and the predicted integrated MS luminosity functions. Our main conclusions are as follows: (1) The adoption of standard models (i.e., computed by adopting the Schwarzschild criterion to fix the border of the convective core) allows a fair fit to the MS for an age of the order of 100–140 Myr and a present-day mass function having a slope α between 2.3 and 1.9, the exact values depending on the adopted distance modulus. It is moreover possible to reproduce the average He clump luminosity while the total number of stars predicted in the He clump is twice the observed value; this means that we reobtain and confirm the first finding of Becker & Mathews, according to whom the simple adoption of a classical scenario leads to a neat discrepancy concerning the prediction of the number of stars in the He clump. (2) The adoption of models computed by increasing the size of the convective core by a certain amount—i.e., 0.25Hp—leads to a fair fit to the MS only for a visual distance modulus (m - M)V 18.6, an age t 200 Myr, and a mass function slope α 2.2. In this case, the total number of He clump stars is well reproduced, although the luminosity function of the He clump itself is predicted to be systematically less luminous than observed. The previous conclusions are based on the assumption that there is no appreciable population of binaries in NGC 1866. Though there are not yet sufficient data on the frequency of binary systems in these clusters, we analyze how the previous scenarios would change if a consistent (30%) population of binary systems were present in the cluster. This choice is based on the fact that a fraction of binaries of the order of 30% has already been found in NGC 1818, a cluster similar to NGC 1866. The inclusion of a 30% binary population leads to the following additional conclusions: (3) The adoption of the standard models now leads to a good fit to the entire luminosity function—i.e., MS, turnoff, and He clump stars—for a visual distance modulus (m - M)V = 18.8, an age t 100 Myr, and a mass function slope α 2.4, thus largely removing the classical discrepancy between observed and predicted number of stars in the He-burning clump. The quoted visual distance modulus constrains the unreddened distance modulus (m - M)0 within 18.50 and 18.62, depending on the reddening (whose most common values available in the literature range from 0.05 to 0.10). (4) At variance with the last point, the fit obtained by using models computed with an enlarged convective core gets worse when a binary component is taken into account. This is because of the fact that the presence of binary systems increases the existing discrepancy between the observed and predicted clump luminosity, since the He clump is predicted to be even less luminous than in absence of binaries. As a consequence of this analysis, we think that the next step toward a proper understanding of NGC 1866 and similar clusters, must include the accurate determination of the frequency of binary systems, which we hope will be performed with the incoming Cycle 8 Hubble Space Telescope observations of NGC 1866.