Abstract. High dispersion spectra (R ∼ > 40 000) for a quite large number of stars at the main sequence turn-off 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 first error bars refer to internal and the second ones to systematic errors (within the abundance scale defined 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 first 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 difficult 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.

Abstract. 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 differential procedure, where the Johnson B − V and Strömgren b − y colours and UVES high resolution spectra of turn-off 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 Hα) 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 ± 1.1 Gyr respectively, when standard isochrones without microscopic diffusion 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 effects of sedimentation due to microscopic diffusion 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 effects, 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 standard ΛCDM 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 H 0 given by WMAP and by the HST Key Project, provides a robust upper limit at 95% level of confidence of Ω 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 M V (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.

Mass‐loss of red giant branch (RGB) stars is still poorly determined, despite its crucial role in the chemical enrichment of galaxies. Thanks to the recent detection of solar‐like oscillations in G–K giants in open clusters with Kepler, we can now directly determine stellar masses for a statistically significant sample of stars in the old open clusters NGC 6791 and 6819. The aim of this work is to constrain the integrated RGB mass‐loss by comparing the average mass of stars in the red clump (RC) with that of stars in the low‐luminosity portion of the RGB [i.e. stars with L≲L(RC)]. Stellar masses were determined by combining the available seismic parameters νmax and Δν with additional photometric constraints and with independent distance estimates. We measured the masses of 40 stars on the RGB and 19 in the RC of the old metal‐rich cluster NGC 6791. We find that the difference between the average mass of RGB and RC stars is small, but significant [ (random) ±0.04 (systematic) M⊙]. Interestingly, such a small does not support scenarios of an extreme mass‐loss for this metal‐rich cluster. If we describe the mass‐loss rate with Reimers prescription, a first comparison with isochrones suggests that the observed is compatible with a mass‐loss efficiency parameter in the range 0.1 ≲η≲ 0.3. Less stringent constraints on the RGB mass‐loss rate are set by the analysis of the ∼2 Gyr old NGC 6819, largely due to the lower mass‐loss expected for this cluster, and to the lack of an independent and accurate distance determination. In the near future, additional constraints from frequencies of individual pulsation modes and spectroscopic effective temperatures will allow further stringent tests of the Δν and νmax scaling relations, which provide a novel, and potentially very accurate, means of determining stellar radii and masses.

15131514 KANN ET AL.Vol. 720 ABSTRACT We have gathered optical photometry data from the literature on a large sample of Swift-era gamma-ray burst (GRB) afterglows including GRBs up to 2009 September, for a total of 76 GRBs, and present an additional three pre-Swift GRBs not included in an earlier sample. Furthermore, we publish 840 additional new photometry data points on a total of 42 GRB afterglows, including large data sets for GRBs 050319, 050408, 050802, 050820A, 050922C, 060418, 080413A, and 080810. We analyzed the light curves of all GRBs in the sample and derived spectral energy distributions for the sample with the best data quality, allowing us to estimate the host-galaxy extinction. We transformed the afterglow light curves into an extinction-corrected z = 1 system and compared their luminosities with a sample of pre-Swift afterglows. The results of a former study, which showed that GRB afterglows clustered and exhibited a bimodal distribution in luminosity space, are weakened by the larger sample. We found that the luminosity distribution of the two afterglow samples (Swift-era and pre-Swift) is very similar, and that a subsample for which we were not able to estimate the extinction, which is fainter than the main sample, can be explained by assuming a moderate amount of line-of-sight host extinction. We derived bolometric isotropic energies for all GRBs in our sample, and found only a tentative correlation between the prompt energy release and the optical afterglow luminosity at 1 day after the GRB in the z = 1 system. A comparative study of the optical luminosities of GRB afterglows with echelle spectra (which show a high number of foreground absorbing systems) and those without, reveals no indication that the former are statistically significantly more luminous. Furthermore, we propose the existence of an upper ceiling on afterglow luminosities and study the luminosity distribution at early times, which was not accessible before the advent of the Swift satellite. Most GRBs feature afterglows that are dominated by the forward shock from early times on. Finally, we present the first indications of a class of long GRBs, which form a bridge between the typical highluminosity, high-redshift events and nearby low-luminosity events (which are also associated with spectroscopic supernovae) in terms of energetics and observed redshift distribution, indicating a continuous distribution overall.

The measurement of the cosmic microwave background has strongly constrained the cosmological parameters of the Universe 1 . When the measured density of baryons (ordinary matter) is combined with standard Big Bang nucleosynthesis calculations 2,3 , the amounts of hydrogen, helium and lithium produced shortly after the Big Bang can be predicted with unprecedented precision 1,4 . The predicted primordial lithium abundance is a factor of two to three higher than the value measured in the atmospheres of old stars 5,6 . With estimated errors of 10 to 25 %, this cosmological lithium discrepancy seriously challenges our understanding of stellar physics, Big Bang nucleosynthesis or both. Certain modifications to nucleosynthesis have been proposed 7 , but found experimentally not to be viable 8 . Diffusion theory, however, predicts atmospheric abundances of stars to vary with time 9 , which offers a possible explanation of the discrepancy. Here we report spectroscopic observations of stars in the metal-poor globular cluster NGC 6397 that reveal trends of atmospheric abundance with evolutionary stage for various elements. These element-specific trends are reproduced by stellar-evolution models with diffusion and turbulent mixing 10 . We thus conclude that diffusion is predominantly responsible for the low apparent stellar lithium abundance in the atmospheres of old stars by transporting the lithium deep into the star. Diffusive processes altering the elemental composition in stars have been studied for decades 9,11 . Evidence for their importance comes from helioseismology 12 and the study of hot stars with peculiar abundance patterns 13 . Among solar-type stars, the effects of diffusion are expected to be more pronounced in old, very metal-poor stars. Given their greater age, diffusion has had more time to produce sizeable effects than in younger stars like the Sun. Detailed element-by-element predictions from models including effects of atomic diffusion and radiative accelerations became available a few years ago 14 , but these early models produced strong abundance trends that are incompatible with measurements of, in particular, the abundance of lithium common among stars of the Galactic halo over a wide range of metallicities (the so-called Spite plateau of lithium). However, the recent inclusion of turbulent mixing 10 brings model predictions into better agreement with observations. According to the predictions from such model calculations, stars leaving the main-sequence (turn-off stars) are expected to show the largest variations relative to the composition of the gas from which the stars originated. Giant stars, however, have deep surface convection zones which erase most effects of diffusion and restore the original composition. One notable exception is lithium which disintegrates in layers with T ≥ 2.1 million K. The destruction of lithium inside the star leads to a successive dilution of the surface lithium because the convective envelope expands when the star becomes a red giant. We performed spectroscopic ob...