Collurania Teramo Observatory

Context. We present the second Gaia data release, Gaia DR2, consisting of astrometry, photometry, radial velocities, and information on astrophysical parameters and variability, for sources brighter than magnitude 21. In addition epoch astrometry and photometry are provided for a modest sample of minor planets in the solar system. Aims. A summary of the contents of Gaia DR2 is presented, accompanied by a discussion on the differences with respect to Gaia DR1 and an overview of the main limitations which are still present in the survey. Recommendations are made on the responsible use of Gaia DR2 results. Methods. The raw data collected with the Gaia instruments during the first 22 months of the mission have been processed by the Gaia Data Processing and Analysis Consortium (DPAC) and turned into this second data release, which represents a major advance with respect to Gaia DR1 in terms of completeness, performance, and richness of the data products. Results. Gaia DR2 contains celestial positions and the apparent brightness in G for approximately 1.7 billion sources. For 1.3 billion of those sources, parallaxes and proper motions are in addition available. The sample of sources for which variability information is provided is expanded to 0.5 million stars. This data release contains four new elements: broad-band colour information in the form of the apparent brightness in the G BP (330–680 nm) and G RP (630–1050 nm) bands is available for 1.4 billion sources; median radial velocities for some 7 million sources are presented; for between 77 and 161 million sources estimates are provided of the stellar effective temperature, extinction, reddening, and radius and luminosity; and for a pre-selected list of 14 000 minor planets in the solar system epoch astrometry and photometry are presented. Finally, Gaia DR2 also represents a new materialisation of the celestial reference frame in the optical, the Gaia -CRF2, which is the first optical reference frame based solely on extragalactic sources. There are notable changes in the photometric system and the catalogue source list with respect to Gaia DR1, and we stress the need to consider the two data releases as independent. Conclusions. Gaia DR2 represents a major achievement for the Gaia mission, delivering on the long standing promise to provide parallaxes and proper motions for over 1 billion stars, and representing a first step in the availability of complementary radial velocity and source astrophysical information for a sample of stars in the Gaia survey which covers a very substantial fraction of the volume of our galaxy.

Context. We present the early installment of the third Gaia data release, Gaia EDR3, consisting of astrometry and photometry for 1.8 billion sources brighter than magnitude 21, complemented with the list of radial velocities from Gaia DR2. Aims. A summary of the contents of Gaia EDR3 is presented, accompanied by a discussion on the differences with respect to Gaia DR2 and an overview of the main limitations which are present in the survey. Recommendations are made on the responsible use of Gaia EDR3 results. Methods. The raw data collected with the Gaia instruments during the first 34 months of the mission have been processed by the Gaia Data Processing and Analysis Consortium and turned into this early third data release, which represents a major advance with respect to Gaia DR2 in terms of astrometric and photometric precision, accuracy, and homogeneity. Results. Gaia EDR3 contains celestial positions and the apparent brightness in G for approximately 1.8 billion sources. For 1.5 billion of those sources, parallaxes, proper motions, and the ( G BP − G RP ) colour are also available. The passbands for G , G BP , and G RP are provided as part of the release. For ease of use, the 7 million radial velocities from Gaia DR2 are included in this release, after the removal of a small number of spurious values. New radial velocities will appear as part of Gaia DR3. Finally, Gaia EDR3 represents an updated materialisation of the celestial reference frame (CRF) in the optical, the Gaia -CRF3, which is based solely on extragalactic sources. The creation of the source list for Gaia EDR3 includes enhancements that make it more robust with respect to high proper motion stars, and the disturbing effects of spurious and partially resolved sources. The source list is largely the same as that for Gaia DR2, but it does feature new sources and there are some notable changes. The source list will not change for Gaia DR3. Conclusions. Gaia EDR3 represents a significant advance over Gaia DR2, with parallax precisions increased by 30 per cent, proper motion precisions increased by a factor of 2, and the systematic errors in the astrometry suppressed by 30–40% for the parallaxes and by a factor ~2.5 for the proper motions. The photometry also features increased precision, but above all much better homogeneity across colour, magnitude, and celestial position. A single passband for G , G BP , and G RP is valid over the entire magnitude and colour range, with no systematics above the 1% level

We present a large and updated stellar evolution database for low-, intermediate- and high-mass stars in a wide metallicity range, suitable for studying Galactic and extragalactic simple and composite stellar populations using population synthesis techniques. The stellar mass range is between \\sim0.5Mo and 10Mo with a fine mass spacing. The metallicity [Fe/H] comprises 10 values ranging from -2.27 to 0.40, with a scaled solar metal distribution. The initial He mass fraction ranges from Y=0.245, for the more metal-poor composition, up to 0.303 for the more metal-rich one, with Delta Y/Delta Z\\sim 1.4. For each adopted chemical composition, the evolutionary models have been computed without and with overshooting from the Schwarzschild boundary of the convective cores during the central H-burning phase. The whole set of evolutionary models can be used to compute isochrones in a wide age range, from \\sim30 Myr to \\sim15Gyr. Both evolutionary tracks and isochrones are available in several observational planes, employing updated set of bolometric corrections and color-Te relations computed for this project. The number of points along the models and the resulting isochrones is selected in such a way that interpolation for intermediate metallicities not contained in the grid is straightforward. We compare our isochrones with results from different stellar evolution databases and perform some empirical tests for the reliability of our models. Since this work is devoted to scaled solar compositions, we focus our attention on the Galactic disk populations, employing multicolor photometry of unevolved field MS stars with precise Hipparcos parallaxes, well-studied open clusters and one eclipsing binary system with precise measurements of masses, radii and [Fe/H] of both components.

The recently improved information on the stellar (n,gamma) cross sections of neutron-magic nuclei at N = 82, and in particular of 142Nd, turned out to represent a sensitive test for models of s-process nucleosynthesis. While these data were found to be incompatible with the classical approach based on an exponential distribution of neutron exposures, they provide significantly better agreement between the solar abundance distribution of s nuclei and the predictions of models for low mass AGB stars. Particular attention is paid to a consistent description of s-process branchings in the region of the rare earth elements. It is shown that - in certain cases - the nuclear data are sufficiently accurate that the resulting abundance uncertainties can be completely attributed to stellar modelling. Thus, the s process becomes important for testing the role of different stellar masses and metallicities as well as for constraining the assumptions for describing the low neutron density provided by the 13C source.

Context. Gaia Data Release 2 provides high-precision astrometry and three-band photometry for about 1.3 billion sources over the full sky. The precision, accuracy, and homogeneity of both astrometry and photometry are unprecedented. Aims. We highlight the power of the Gaia DR2 in studying many fine structures of the Hertzsprung-Russell diagram (HRD). Gaia allows us to present many different HRDs, depending in particular on stellar population selections. We do not aim here for completeness in terms of types of stars or stellar evolutionary aspects. Instead, we have chosen several illustrative examples. Methods. We describe some of the selections that can be made in Gaia DR2 to highlight the main structures of the Gaia HRDs. We select both field and cluster (open and globular) stars, compare the observations with previous classifications and with stellar evolutionary tracks, and we present variations of the Gaia HRD with age, metallicity, and kinematics. Late stages of stellar evolution such as hot subdwarfs, post-AGB stars, planetary nebulae, and white dwarfs are also analysed, as well as low-mass brown dwarf objects. Results. The Gaia HRDs are unprecedented in both precision and coverage of the various Milky Way stellar populations and stellar evolutionary phases. Many fine structures of the HRDs are presented. The clear split of the white dwarf sequence into hydrogen and helium white dwarfs is presented for the first time in an HRD. The relation between kinematics and the HRD is nicely illustrated. Two different populations in a classical kinematic selection of the halo are unambiguously identified in the HRD. Membership and mean parameters for a selected list of open clusters are provided. They allow drawing very detailed cluster sequences, highlighting fine structures, and providing extremely precise empirical isochrones that will lead to more insight in stellar physics. Conclusions. Gaia DR2 demonstrates the potential of combining precise astrometry and photometry for large samples for studies in stellar evolution and stellar population and opens an entire new area for HRD-based studies.

We present a re-analysis of the Geneva-Copenhagen survey, which benefits from the infrared flux method to improve the accuracy of the derived stellar effective temperatures and uses the latter to build a consistent and improved metallicity scale. Metallicities are calibrated on high-resolution spectroscopy and checked against four open clusters and a moving group, showing excellent consistency. The new temperature and metallicity scales provide a better match to theoretical isochrones, which are used for a Bayesian analysis of stellar ages. With respect to previous analyses, our stars are on average 100 K hotter and 0.1 dex more metal rich, which shift the peak of the metallicity distribution function around the solar value. From Strömgren photometry we are able to derive for the first time a proxy for [α/Fe] abundances, which enables us to perform a tentative dissection of the chemical thin and thick disc. We find evidence for the latter being composed of an old, mildly but systematically alpha-enhanced population that extends to super solar metallicities, in agreement with spectroscopic studies. Our revision offers the largest existing kinematically unbiased sample of the solar neighbourhood that contains full information on kinematics, metallicities, and ages and thus provides better constraints on the physical processes relevant in the build-up of the Milky Way disc, enabling a better understanding of the Sun in a Galactic context.

We present a new analysis of neutron capture occurring in low-mass asymptotic giant branch (AGB) stars suffering recurrent thermal pulses. We use dedicated evolutionary models for stars of initial mass in the range 1 to 3 M☉ and metallicity from solar to half solar. Mass loss is taken into account with the Reimers parameterization. The third dredge-up mechanism is self-consistently found to occur after a limited number of pulses, mixing with the envelope freshly synthesized 12C and s-processed material from the He intershell. During thermal pulses, the temperature at the base of the convective region barely reaches T8 ~ 3 (T8 being the temperature in units of 108 K), leading to a marginal activation of the 22Ne(α, n)25Mg neutron source. The alternative and much faster reaction 13C(α, n)16O must then play the major role. However, the 13C abundance left behind by the H shell is far too low to drive the synthesis of the s-elements. We assume instead that at any third dredge-up episode, hydrogen downflows from the envelope penetrate into a tiny region placed at the top of the 12C-rich intershell, of the order of a few 10-4 M☉. At H reignition, a13C-rich (and 14N-rich) zone is formed. Neutrons by the major 13C source are then released in radiative conditions at T8 ~ 0.9 during the interpulse period, giving rise to an efficient s-processing that depends on the 13C profile in the pocket. A second small neutron burst from the 22Ne source operates during convective pulses over previously s-processed material diluted with fresh Fe seeds and H-burning ashes. The main features of the final s-process abundance distribution in the material cumulatively mixed with the envelope through the various third dredge-up episodes are discussed. Contrary to current expectations, the distribution cannot be approximated by a simple exponential law of neutron irradiations. The s-process nucleosynthesis mostly occurs inside the 13C pocket; the form of the distribution is built through the interplay of the s-processing occurring in the intershell zones and the geometrical overlap of different pulses.

Context. Gaia Early Data Release 3 ( Gaia EDR3) contains astrometry and photometry results for about 1.8 billion sources based on observations collected by the European Space Agency Gaia satellite during the first 34 months of its operational phase. Aims. In this paper, we focus on the photometric content, describing the input data, the algorithms, the processing, and the validation of the results. Particular attention is given to the quality of the data and to a number of features that users may need to take into account to make the best use of the Gaia EDR3 catalogue. Methods. The processing broadly followed the same procedure as for Gaia DR2, but with significant improvements in several aspects of the blue and red photometer (BP and RP) preprocessing and in the photometric calibration process. In particular, the treatment of the BP and RP background has been updated to include a better estimation of the local background, and the detection of crowding effects has been used to exclude affected data from the calibrations. The photometric calibration models have also been updated to account for flux loss over the whole magnitude range. Significant improvements in the modelling and calibration of the Gaia point and line spread functions have also helped to reduce a number of instrumental effects that were still present in DR2. Results. Gaia EDR3 contains 1.806 billion sources with G -band photometry and 1.540 billion sources with G BP and G RP photometry. The median uncertainty in the G -band photometry, as measured from the standard deviation of the internally calibrated mean photometry for a given source, is 0.2 mmag at magnitude G = 10–14, 0.8 mmag at G ≈ 17, and 2.6 mmag at G ≈ 19. The significant magnitude term found in the Gaia DR2 photometry is no longer visible, and overall there are no trends larger than 1 mmag mag −1 . Using one passband over the whole colour and magnitude range leaves no systematics above the 1% level in magnitude in any of the bands, and a larger systematic is present for a very small sample of bright and blue sources. A detailed description of the residual systematic effects is provided. Overall the quality of the calibrated mean photometry in Gaia EDR3 is superior with respect to DR2 for all bands.

Accurate photometry with HST/ACS shows that the main sequence of the globular cluster NGC 2808 splits into three separate branches. The three MS branches may be associated with complexities of the cluster's horizontal branch and of its abundance distribution. We attribute the MS branches to successive rounds of star formation, with different helium abundances; we discuss possible sources of helium enrichment. Some other massive globulars also appear to have complex populations; we compare them with NGC 2808.

[Abridged] We present a large, new set of stellar evolution models and isochrones for an alpha-enhanced metal distribution typical of Galactic halo and bulge stars; it represents a homogeneous extension of our stellar model library for a distribution already presented in Pietrinferni et al.(2004). The effect of the alpha-element enhancement has been properly taken into account in the nuclear network, opacity, equation of state and, for the first time, the bolometric corrections, and color transformations. This allows us to avoid the inconsistent use - common to all alpha-enhanced model libraries currently available - of scaled-solar bolometric corrections and color transformations for alpha-enhanced models and isochrones. We show how bolometric corrections to magnitudes obtained for the U,B portion of stellar spectra for T_{eff}<=6500K, are significantly affected by the metal mixture, especially at the higher metallicities. We also provide complete sets of evolutionary models for low-mass, He-burning stellar structures covering the whole metallicity range, to enable synthetic horizontal branch simulations. We compare our database with several widely used stellar model libraries from different authors, as well as with various observed color magnitude and color-color diagrams (Johnson-Cousins BVI and near infrared magnitudes, Stromgren colors) of Galactic field stars and globular clusters. We also test our isochrones comparing integrated optical colors and Surface Brightness Fluctuation magnitudes with selected globular cluster data. We find a general satisfactory agreement with the empirical constraints.

Context. Aims. The goal of this paper is to demonstrate the outstanding quality of the second data release of the Gaia mission and its power for constraining many different aspects of the dynamics of the satellites of the Milky Way. We focus here on determining the proper motions of 75 Galactic globular clusters, nine dwarf spheroidal galaxies, one ultra-faint system, and the Large and Small Magellanic Clouds. Methods. Using data extracted from the Gaia archive, we derived the proper motions and parallaxes for these systems, as well as their uncertainties. We demonstrate that the errors, statistical and systematic, are relatively well understood. We integrated the orbits of these objects in three different Galactic potentials, and characterised their properties. We present the derived proper motions, space velocities, and characteristic orbital parameters in various tables to facilitate their use by the astronomical community. Results. Our limited and straightforward analyses have allowed us for example to ( i ) determine absolute and very precise proper motions for globular clusters; ( ii ) detect clear rotation signatures in the proper motions of at least five globular clusters; ( iii ) show that the satellites of the Milky Way are all on high-inclination orbits, but that they do not share a single plane of motion; ( iv ) derive a lower limit for the mass of the Milky Way of 9.1 -2.6 +6.2 × 10 11 M ⊙ based on the assumption that the Leo I dwarf spheroidal is bound; ( v ) derive a rotation curve for the Large Magellanic Cloud based solely on proper motions that is competitive with line-of-sight velocity curves, now using many orders of magnitude more sources; and ( vi ) unveil the dynamical effect of the bar on the motions of stars in the Large Magellanic Cloud. Conclusions. All these results highlight the incredible power of the Gaia astrometric mission, and in particular of its second data release.

We present HST observations that show a bifurcation of colors in the middle main sequence of the globular cluster Omega Centauri. We see this in three different fields, observed with different cameras and filters. We also present high precision photometry of a central ACS field which shows a number of main-sequence turnoffs and subgiant branches. The double main sequence, the multiple turnoffs and subgiant branches, and other population sequences discovered in the past along the red giant branch of this cluster add up to a fascinating but frustrating puzzle. We suggest various explanations, none of them very conclusive.

High dispersion spectra () 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 and 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 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.

We revise the scenario of the formation of Galactic globular clusters (GCs) by adding the observed detailed chemical composition of their different stellar generations to the set of their global parameters. We exploit the unprecedented set of homogeneous abundances of more than 1200 red giants in 19 clusters, as well as additional data from literature, to give a new definition of bona fide GCs, as the stellar aggregates showing the Na-O anticorrelation. We propose a classification of GCs according to their kinematics and location in the Galaxy in three populations: disk/bulge, inner halo, and outer halo. We find that the luminosity function of GCs is fairly independent of their population, suggesting that it is imprinted by the formation mechanism only marginally affected by the ensuing evolution. We show that a large fraction of the primordial population should have been lost by the proto-GCs. The extremely low Al abundances found for the primordial population of massive GCs indicate a very fast enrichment process before the formation of the primordial population. We suggest a scenario for the formation of GCs that includes at least three main phases: i) the formation of a precursor population (likely due to the interaction of cosmological structures similar to those that led to the formation of dwarf spheroidals, but residing at smaller Galactocentric distances, with the early Galaxy or with other structures); ii) the triggering of a long episode of star formation (the primordial population) from the precursor population; and iii) the formation of the current GC, mainly within a cooling flow formed by the slow winds of a fraction of the primordial population. The precursor population is very effective in raising the metal content in massive and/or metal-poor (mainly halo) clusters, while its rle is minor in small and/or metal-rich (mainly disk) ones. Finally, we use principal component analysis and multivariate relations to study the phase of metal enrichment from first to second generation. We conclude that most of the chemical signatures of GCs may be ascribed to a few parameters, the most important being metallicity, mass, and cluster age. Location within the Galaxy (as described by the kinematics) also plays some rle, while additional parameters are required to describe their dynamical status.

In this paper we describe a new UV-initiative Hubble Space Telescope project (GO-13297) that will complement the existing F606W and F814W database of the Advanced Camera for Surveys Globular Cluster (GC) Treasury by imaging most of its clusters through UV/blue WFC3/UVIS filters F275W, F336W, and F438W. This "magic trio" of filters has shown an uncanny ability to disentangle and characterize multiple population (MP) patterns in GCs in a way that is exquisitely sensitive to C, N, and O abundance variations. Combination of these passbands with those in the optical also gives the best leverage for measuring helium enrichment. The dozen clusters that had previously been observed in these bands exhibit a bewildering variety of MP patterns, and the new survey will map the full variance of the phenomenon. The ubiquity of multiple stellar generations in GCs has made the formation of these cornerstone objects more intriguing than ever; GC formation and the origin of their MPs have now become one and the same problem. In this paper we will describe the database and our data reduction strategy, as well as the uses we intend to make of the final photometry, astrometry, and PMs. We will also present preliminary color-magnitude diagrams from the data so far collected. These diagrams also draw on data from GO-12605 and GO-12311, which served as a pilot project for the present GO-13297.

We review the theory of electron-conduction opacity, a fundamental ingredient in the computation of low-mass stellar models; shortcomings and limitations of the existing calculations used in stellar evolution are discussed. We then present new determinations of the electron-conduction opacity in stellar conditions for an arbitrary chemical composition, that improve over previous works and, most importantly, cover the whole parameter space relevant to stellar evolution models (i.e., both the regime of partial and high electron degeneracy). A detailed comparison with the currently used tabulations is also performed. The impact of our new opacities on the evolution of low-mass stars is assessed by computing stellar models along both the H- and He-burning evolutionary phases, as well as Main Sequence models of very low-mass stars and white dwarf cooling tracks.

We present a re-analysis of the Geneva-Copenhagen survey, which benefits from the infrared flux method to improve the accuracy of the derived stellar effective temperatures and uses the latter to build a consistent and improved metallicity scale. Metallicities are calibrated on high-resolution spectroscopy and checked against four open clusters and a moving group, showing excellent consistency. The new temperature and metallicity scales provide a better match to theoretical isochrones, which are used for a Bayesian analysis of stellar ages. With respect to previous analyses, our stars are on average 100 K hotter and 0.1 dex more metal rich, which shift the peak of the metallicity distribution function around the solar value. From Stromgren photometry we are able to derive for the first time a proxy for alpha elements, which enables us to perform a tentative dissection of the chemical thin and thick disc. We find evidence for the latter being composed of an old, mildly but systematically alpha-enhanced population that extends to super solar metallicities, in agreement with spectroscopic studies. Our revision offers the largest existing kinematically unbiased sample of the solar neighbourhood that contains full information on kinematics, metallicities, and ages and thus provides better constraints on the physical processes relevant in the build-up of the Milky Way disc, enabling a better understanding of the Sun in a Galactic context.

We use high-precision photometry of red-giant-branch (RGB) stars in 57 Galactic globular clusters (GCs), mostly from the 'Hubble Space Telescope (HST ) UV Legacy Survey of Galactic GCs', to identify and characterize their multiple stellar populations. For each cluster the pseudo-two-colour diagram (or 'chromosome map') is presented, built with a suitable combination of stellar magnitudes in the F275W, F336W, F438W, and F814W filters that maximizes the separation between multiple populations. In the chromosome map of most GCs (type-I clusters), stars separate in two distinct groups that we identify with the first (1G) and the second generation (2G). This identification is further supported by noticing that 1G stars have primordial (oxygen-rich, sodium-poor) chemical composition, whereas 2G stars are enhanced in sodium and depleted in oxygen. This 1G-2G separation is not possible for a few GCs where the two sequences have apparently merged into an extended, continuous sequence. In some GCs (type-II clusters) the 1G and/or the 2G sequences appear to be split, hence displaying more complex chromosome maps. These clusters exhibit multiple subgiant branches (SGBs) also in purely optical colour-magnitude diagrams, with the fainter SGB joining into a red RGB which is populated by stars with enhanced heavy-element abundance. We measure the RGB width by using appropriate colours and pseudo-colours. When the metallicity dependence is removed, the RGB width correlates with the cluster mass. The fraction of 1G stars ranges from ~8 per cent to ~67 per cent and anticorrelates with the cluster mass, indicating that incidence and complexity of the multiple population phenomenon both increase with cluster mass.

We derive a strong bound on the axion-photon coupling ${g}_{a\ensuremath{\gamma}}$ from the analysis of a sample of 39 Galactic Globular Clusters. As recognized long ago, the $R$ parameter, i.e., the number ratio of stars in horizontal over red giant branch of old stellar clusters, would be reduced by the axion production from photon conversions occurring in stellar cores. In this regard, we have compared the measured $R$ with state-of-the-art stellar models obtained under different assumptions for ${g}_{a\ensuremath{\gamma}}$. We show that the estimated value of ${g}_{a\ensuremath{\gamma}}$ substantially depends on the adopted He mass fraction $Y$, an effect often neglected in previous investigations. Taking as a benchmark for our study the most recent determinations of the He abundance in H ii regions with $\mathrm{O}/\mathrm{H}$ in the same range of the Galactic Globular Clusters, we obtain an upper bound ${g}_{a\ensuremath{\gamma}}&lt;0.66\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}\text{ }\text{ }{\mathrm{GeV}}^{\ensuremath{-}1}$ at 95% confidence level. This result significantly improves the constraints from previous analyses and is currently the strongest limit on the axion-photon coupling in a wide mass range.

Having shown in a recent paper that the main sequence of ω Centauri is split into two distinct branches, we now present spectroscopic results showing that the bluer sequence is less metal-poor. We have carefully combined GIRAFFE@VLT spectra of 17 stars on each side of the split into a single spectrum for each branch, with adequate S/N to show clearly that the stars of the blue main sequence are less metal poor by 0.3 dex than those of the dominant red one. From an analysis of the individual spectra, we could not detect any abundance spread among the blue main sequence stars, while the red main sequence stars show a 0.2 dex spread in metallicity. We use stellar-structure models to show that only greatly enhanced helium can explain the color difference between the two main sequences, and we discuss ways in which this enhancement could have arisen. Subject headings: globular clusters: individual (NGC 5139) — Hertzsprung-