G. Clementini

The O-Na and Mg-Al anticorrelations in turn-off and early subgiants in globular clusters

High dispersion spectra (R > 40 000) for a quite large number of stars at the main sequence turn-o and at the base of the giant branch in NGC 6397 and NGC 6752 were obtained with the UVES on Kueyen (VLT UT2). The (Fe/H) values we found are 2:03 0:02 0:04 and 1:42 0:02 0:04 for NGC 6397 and NGC 6752 respectively, where the rst error bars refer to internal and the second ones to systematic errors (within the abundance scale dened by our analysis of 25 subdwarfs with good Hipparcos parallaxes). In both clusters the (Fe/H)s obtained for TO-stars agree perfectly (within a few percent) with that obtained for stars at the base of the RGB. The (O=Fe) = 0:21 0:05 value we obtain for NGC 6397 is quite low, but it agrees with previous results obtained for giants in this cluster. Moreover, the star-to-star scatter in both O and Fe is very small, indicating that this small mass cluster is chemically very homogenous. On the other hand, our results show clearly and for the rst time that the O-Na anticorrelation (up to now seen only for stars on the red giant branches of globular clusters) is present among unevolved stars in the globular cluster NGC 6752, a more massive cluster than NGC 6397. A similar anticorrelation is present also for Mg and Al, and C and N. It is very dicult to explain the observed Na-O, and Mg-Al anticorrelation in NGC 6752 stars by a deep mixing scenario; we think it requires some non internal mechanism.

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.

Distances and ages of NGC 6397, NGC 6752 and 47 Tuc

New improved distances and absolute ages for the Galactic globular clusters NGC 6397, NGC 6752, and 47 Tuc are obtained using the Main Sequence Fitting Method. We derived accurate estimates of reddening and metal abundance for these three clusters using a strictly dierential procedure, where the Johnson B V and Stromgren b y colours and UVES high resolution spectra of turn-o stars and early subgiants belonging to the clusters were compared to similar data for field subdwarfs with accurate parallaxes measured by Hipparcos. The use of a reddening free temperature indicator (the profile of H) allowed us to reduce the error bars in reddening determinations to about 0.005 mag, and in metal abundances to 0.04 dex, in the scales defined by the local subdwarfs. Error bars in distances are then reduced to about 0.07 mag for each cluster, yielding ages with typical random errors of about 1 Gyr. We find that NGC 6397 and NGC 6752 have ages of 13:9 1:1 and 13:8 1: 1G yr respectively, when standard isochrones without microscopic diusion are used, while 47 Tuc is probably about 2.6 Gyr younger, in agreement with results obtained by other techniques sensitive to relative ages. If we use models that include the eects of sedimentation due to microscopic diusion in agreement with our observations of NGC 6397, and take into account various sources of possible systematic errors with a statistical approach, we conclude that the age of the oldest globular clusters in the Galaxy is 13:4 0:8 0:6 Gyr, where the first error bar accounts for random eects, and the second one for systematic errors. This age estimate is fully compatible with the very recent results from WMAP, and indicates that the oldest Galactic globular clusters formed within the first 1.7 Gyr after the Big Bang, corresponding to a redshift of z 2:5, in a standardCDM model. The epoch of formation of the (inner halo) globular clusters lasted about 2.6 Gyr, ending at a time corresponding to a redshift of z 1:3. On the other hand, our new age estimate once combined with values of H0 given by WMAP and by the HST Key Project, provides a robust upper limit at 95% level of confidence of M < 0:57, independently of type Ia SNe, and strongly supports the need for a dark energy. The new cluster distances lead to new estimates of the horizontal branch luminosity, that may be used to derive the zero point of the relation between the horizontal branch absolute magnitude and metallicity: we obtain MV (HB)= (0:22 0:05)((Fe=H)+ 1:5)+ (0:56 0:07). This zero point is 0.03 mag shorter than obtained by Carretta et al. (2000) and within the error bar it agrees with, but it is more precise than most of the previous individual determinations of the RR Lyrae absolute magnitude. When combined with the apparent average luminosity of the RR Lyrae stars in the LMC by Clementini et al. (2003), this zero point provides a new estimate of the distance modulus to the LMC: (m M)0= 18:50 0:09.

Age Determination of Six Intermediate-Age Small Magellanic Cloud Star Clusters with Hst/acs

We present a photometric analysis of the star clusters Lindsay 1, Kron 3, NGC 339, NGC 416, Lindsay 38, and NGC 419 in the Small Magellanic Cloud (SMC), observed with the Hubble Space Telescope Advanced Camera for Surveys (ACS) in the F555W and F814W filters. Our color-magnitude diagrams (CMDs) extend ~3.5 mag deeper than the main-sequence turnoff points, deeper than any previous data. Cluster ages were derived using three different isochrone models: Padova, Teramo, and Dartmouth, which are all available in the ACS photometric system. Fitting observed ridgelines for each cluster, we provide a homogeneous and unique set of low-metallicity, single-age fiducial isochrones. The cluster CMDs are best approximated by the Dartmouth isochrones for all clusters, except for NGC 419 where the Padova isochrones provided the best fit. Using Dartmouth isochrones we derive ages of 7.5 ± 0.5 Gyr (Lindsay 1), 6.5 ± 0.5 Gyr (Kron 3), 6 ± 0.5 Gyr (NGC 339), 6 ± 0.5 Gyr (NGC 416), and 6.5 ± 0.5 Gyr (Lindsay 38). The CMD of NGC 419 shows several main-sequence turnoffs, which belong to the cluster and to the SMC field. We thus derive an age range of 1.2-1.6 Gyr for NGC 419. We confirm that the SMC contains several intermediate-age populous star clusters with ages unlike those of the Large Magellanic Cloud and the Milky Way. Interestingly, our intermediate-age star clusters have a metallicity spread of ~0.6 dex, which demonstrates that the SMC does not have a smooth, monotonic age-metallicity relation. We find an indication for centrally-concentrated blue straggler star candidates in NGC 416, while these are not present for the other clusters. Using the red clump magnitudes, we find that the closest cluster, NGC 419 (~50 kpc), and the farthest cluster, Lindsay 38 (~67 kpc), have a relative distance of ~17 kpc, which confirms the large depth of the SMC. The three oldest SMC clusters (NGC 121, Lindsay 1, and Kron 3) lie in the northwestern part of the SMC, while the youngest (NGC 419) is located near the SMC main body.

Distance to the Large Magellanic Cloud: The RR Lyrae Stars

New photometry and spectroscopy for more than a hundred RR Lyrae stars in two fields located close to the bar of the Large Magellanic Cloud (LMC) are used to derive new accurate estimates of the average magnitude, the local reddening, the luminosity-metallicity relation, and the distance to the LMC. The average apparent luminosity of the RR Lyraes with complete V and B light curves is V(RR) = 19.412 ± 0.019 (σ = 0.153), B(RR) = 19.807 ± 0.022 (σ = 0.172) in our field A (62 stars) and V(RR) = 19.320 ± 0.023 (σ = 0.159), B(RR) = 19.680 ± 0.024 (σ = 0.163) in our field B (46 stars). The average V apparent luminosity of the clump stars in the same areas is 0.108 and 0.029 mag brighter than the RR Lyrae level (Vclump = 19.304 ± 0.002 and 19.291 ± 0.003, in field A: 6728 stars, and B: 3851 stars, respectively). Metallicities from low-resolution spectra obtained with the Very Large Telescope have been derived for 101 RR Lyrae stars, finding an average value of [Fe/H] = -1.48 ± 0.03 (σ = 0.29, on the Harris metallicity scale). An estimate of the reddening within the two fields was obtained (1) from the Sturch method applied to the fundamental-mode pulsators (RRabs) with known metal abundance and (2) from the colors of the edges of the instability strip defined by the full sample of RR Lyrae variable stars. We obtained E(B-V) = 0.116 ± 0.017 and 0.086 ± 0.017 mag in fields A and B, respectively, with a clear-cut indication of a 0.03 mag differential reddening between the two fields. We find that reddening in field A is 0.028 mag smaller than derived by OGLE-II in the same area. On average, the new reddenings are also 0.035 mag larger than derived from Cepheids with projected distances within 2° from the centers of our fields. The new metallicities were combined with the apparent average V0 luminosities to determine the slope of the luminosity-metallicity relation for the RR Lyrae stars. We derived ΔMV(RR)/Δ [Fe/H] = 0.214 ± 0.047, with no clear evidence for the change in slope at [Fe/H] = -1.5, as recently suggested by evolutionary/pulsation and horizontal-branch models. The dereddened apparent average luminosity of the RR Lyraes defined by the present photometry is V(RR)0 = 19.064 ± 0.064 at [Fe/H] = -1.5. When coupled with the absolute magnitude derived from the Baade-Wesselink and the statistical parallaxes methods (MV(RR) = 0.68 ± 0.15 and 0.76 ± 0.13 mag at [Fe/H] = -1.5), both methods known to favor the short distance scale, this value leads to distance moduli for the LMC of μLMC = 18.38 ± 0.16 and μLMC = 18.30 ± 0.14, respectively. If we use instead the absolute magnitude from the new main-sequence fitting of Galactic globular clusters from Gratton et al. [MV(RR) = 0.61 ± 0.07 mag at [Fe/H] = -1.5], we derive μLMC = 18.45 ± 0.09. The average I apparent luminosity of the clump stars derived by the present photometry is Iclump = 18.319 ± 0.002 and 18.307 ± 0.003, in field A (σ = 0.190, 6728 stars) and B (σ = 0.184, 3851 stars), respectively. These values, once corrected for our new reddening estimates, lead to I0 = 18.12 ± 0.06 mag and move the clump distance modulus to the LMC to 18.42 ± 0.07 and 18.45 ± 0.07 when Udalski or Popowski metallicity-I luminosity relations for the clump stars are adopted. All these values are only 1 σ shorter than provided by the Population I distance indicators and make it possible to reconcile the short- and long-distance scale on a common value for the distance modulus of the LMC of μLMC = 18.515 ± 0.085 mag.

The lithium content of the globular cluster NGC 6397

We make use of high resolution, high signal-to-noise ratio spectra of 12 turn-o stars in the metal-poor globular cluster NGC 6397 to measure its lithium content. We conclude that they all have the same lithium abundance A(Li)= 2:34 with a standard deviation of 0.056 dex. We use this result, together with Monte Carlo simulations, to estimate that the maximum allowed intrinsic scatter is of the order of 0.035 dex. This is a new stringent constraint to be fulfilled by stellar models which predict Li depletion. We argue that although a mild depletion of 0.1-0.2 dex, as predicted by recent models, cannot be ruled out, there is no compelling reason for it. This fact, together with the good agreement with the Li abundance observed in field stars, supports the primordial origin of lithium in metal-poor stars. Taking the above value as the primordial lithium abundance implies a cosmic baryonic density which is eitherbh 2 = 0:016 0:004 orbh 2 = 0:005 +0:0026 0:0006 , from the predictions of standard big bang nucleosynthesis. The high baryonic density solution is in agreement with recent results on the primordial abundance of deuterium and 3 He and on the estimates derived from the fluctuations of the cosmic microwave background.

I Zw 18 Revisited with HST ACS and Cepheids: New Distance and Age*

We present new V- and I-band HST ACS photometry of I Zw 18, the most metal-poor blue compact dwarf (BCD) galaxy in the nearby universe. It has been argued in the past that I Zw 18 is a very young system that started forming stars only 500 Myr ago, but other work has hinted that older (1 Gyr) red giant branch (RGB) stars may also exist. Our new data, once combined with archival HST ACS data, provide a deep and uncontaminated optical color-magnitude diagram (CMD) that now strongly indicates an RGB. The RGB tip (TRGB) magnitude yields a distance modulus (m - M)0 = 31.30 ± 0.17, i.e., D = 18.2 ± 1.5 Mpc. The time-series nature of our observations allows us to also detect and characterize for the first time three classical Cepheids in I Zw 18. The time-averaged Cepheid V and I magnitudes are compared to the VI reddening-free Wesenheit relation predicted from new nonlinear pulsation models specifically calculated at the metallicity of I Zw 18. For the one bona fide classical Cepheid with a period of 8.63 days this implies a distance modulus (m - M)0 = 31.42 ± 0.26. The other two Cepheids have unusually long periods (125.0 and 129.8 days) but are consistent with this distance. The coherent picture that emerges is that I Zw 18 is farther away than previously assumed and older than suggested by some previous works. The presence of an RGB population rules out the possibility that I Zw 18 is a truly primordial galaxy formed recently (z 0.1) in the local universe.

Metal abundances of RR Lyrae stars in the bar of the Large Magellanic Cloud

Metallicities ((Fe/H)) from low resolution spectroscopy obtained with the Very Large Telescope (VLT) are presented for 98 RR Lyrae and 3 short period Cepheids in the bar of the Large Magellanic Cloud. Our metal abundances have typical errors of ±0.17 dex. The average metallicity of the RR Lyrae stars is (Fe/H) = −1.48 ± 0.03 ± 0.06 on the scale of Harris (1996). The star-to-star scatter (0.29 dex) is larger than the observational errors, indicating a real spread in metal abundances. The derived metallicities cover the range −2.12 −1. For the ab-type variables we compared our spectroscopic abundances with those obtained from the Fourier decomposition of the light curves. We find good agreement between the two techniques, once the systematic offset of 0.2 dex between the metallicity scales used in the two methods is taken into account. The spectroscopic metallicities were combined with the dereddened apparent magnitudes of the variables to derive the slope of the luminosity-metallicity relation for the LMC RR Lyrae stars: the resulting value is 0.214 ± 0.047 mag/dex. Finally, the 3 short period Cepheids have (Fe/H) values in the range −2.0 < (Fe/H) < −1.5. They are more metal-poor than typical LMC RR Lyrae stars, thus they are more likely to be Anomalous Cepheids rather than the short period Classical Cepheids that are often found in a number of dwarf Irregular galaxies.

Homogeneous photometry and metal abundances for a large sample of Hipparcos metal-poor stars *

Homogeneous photometric data (Johnson V, B-V, V-K, Cousins V-I and Stromgren b-y), radial velocities, and abundances of Fe, O, Mg, Si, Ca, Ti, Cr and Ni are presented for 99 stars with high-precision parallaxes measured by the Hipparcos satellite. These data have been previously used to assist the derivation of accurate distances and ages of Galactic globular clusters. Magnitudes and colours for the programme stars were obtained by combining and standardizing carefully selected literature data available in the Simbad data base, and V and B-V values measured by the Hipparcos/Tycho mission. Comparison of colours for our targets suggests: (i) ground-based and Tycho B-V values agree well for colours bluer than 0.75 mag, but have a lot of scatter for redder colours; (ii) the Hipparcos V-I colours have a very large scatter and a zero-point offset of +0.02 mag compared to the literature values. The programme stars have metal abundances in the range −2.5 200) spectra obtained at the Asiago and McDonald Observatories for 66 stars. The analysis is carried out following the same precepts as those used in previous papers of this series, and includes corrections for departures from LTE in the formation of the O lines. The main results are: (i) the equilibrium of ionization of Fe is well satisfied in late F--early K dwarfs; (ii) oxygen and α-elements are overabundant by ∼0.3 dex. This large homogeneous abundance data set has been used to recalibrate the abundance scales of Schuster & Nissen, Ryan & Norris and Carney et al. (1994).