Stellar Evolutionary Tracks Research Articles

The Victoria‐Regina Stellar Models: Evolutionary Tracks and Isochrones for a Wide Range in Mass and Metallicity that Allow for Empirically Constrained Amounts of Convective Core Overshooting

Seventy-two grids of stellar evolutionary tracks, along with the means to generate isochrones and luminosity/color functions from them, are presented in this investigation. Sixty of them extend (and encompass) the sets of models reported by VandenBerg et al. for 17 [Fe/H] values from -2.31 to -0.30 and ?-element abundances corresponding to [?/Fe] = 0.0, 0.3, and 0.6 (at each iron abundance) to the solar metallicity and to sufficiently high masses (up to ~2.2 M?) that isochrones may be computed for ages as low as 1 Gyr. The remaining grids contain tracks for masses from 0.4 to 4.0 M? and 12 [Fe/H] values between -0.60 and +0.49 (assuming solar metal-to-hydrogen number abundance ratios): in this case, isochrones may be calculated down to ~0.2 Gyr. The extent of convective core overshooting has been modeled using a parameterized version of the Roxburgh criterion, in which the value of the free parameter at a given mass and its dependence on mass have been determined from analyses of binary star data and the observed color-magnitude diagrams for several open clusters. Because the calculations reported herein satisfy many empirical constraints, they should provide useful probes into the properties of both simple and complex stellar populations.

A Spectroscopic Analysis of Blue Stragglers, Horizontal Branch Stars, and Turnoff Stars in Four Globular Clusters

We present a spectroscopic analysis of HST STIS and FOS low- and intermediate-resolution spectroscopy of 55 stars in four globular clusters (47 Tucanae, M3, NGC 6752, and NGC 6397). Stars hotter than Teff = 5750 K and with a signal-to-noise ratio larger than 15 were analyzed with non-local thermodynamic equilibrium model atmospheres, and values for their effective temperatures and gravities were obtained. Using photometric fluxes, we also obtained radii, luminosities, and spectroscopic masses. Twenty-four stars in our sample are blue stragglers (BSs). Their photometric colors and magnitudes place these BSs above and redward of the clusters' zero-age main sequence: this is consistent with the gravities we find for these stars, which are lower than zero-age main-sequence gravities. A comparison with stellar evolutionary tracks shows that almost all of our BSs are in the Hertzsprung gap. This is contrary to theory, because of the short timescale expected for stars in this evolutionary phase. The mean BS mass is 1.04 M☉ for 14 nonvariable stars, or 1.07 M☉ counting all 24 BSs in our sample. For the nonvariable stars the mean BS masses for individual clusters are 1.73, 1.01, 0.95, and 0.72 M☉ for NGC 6397, NGC 6752, 47 Tuc, and M3, respectively. Adding the variable stars (which improves the statistics but increases the uncertainty), the mean masses become 1.27, 1.05, 0.99, and 0.99 M☉, respectively. Although there is considerable scatter, the BS spectroscopic masses correlate with both effective temperature and brightness of the stars, as expected. The mean nonvariable turnoff star mass (0.58 M☉) is significantly below the values determined for the BSs and below the main-sequence turnoff mass. The mean nonvariable horizontal-branch (HB) star mass is higher than expected (0.79 M☉). In particular, several HB stars have masses well above the main-sequence turnoff mass. Some of these HB stars are suspected of actually being BSs, since most of them reside at ambiguous locations on the CMD, making them prone to misclassification. Values and limits to the stellar rotation rates (v sin i) are imposed by fitting weak metal lines, the Ca II K line wings, or the helium lines for the hotter stars. Five BSs with reasonably constrained rotations show average and median v sin i values of 109 and 100 km s-1, respectively, suggesting v ~ 160 km s-1. At least some GC BSs are very rapid rotators, but this information cannot yet constrain their origin as stellar collision or binary mergers because of the lack of clear theoretical predictions. Six extreme HB stars have rotation rates v sin i between 50 and 200 km s-1, which are high for these stars and might indicate a binary origin. De Marco et al. found that four BSs and two HB stars in our sample have Balmer jumps that are too large for the effective temperatures implied by the slopes of their Paschen continua. Two additional HB stars are now identified in the current study as having the same feature. For these stars, the presence of a disk of partly ionized material is suspected, although high stellar rotation rates could also partly explain the data.

Evolutionary population synthesis: models, analysis of the ingredients and application to high-z galaxies

Evolutionary population synthesis models for a wide range of metallicities, ages, star formation histories, initial mass functions and horizontal branch morphologies, including blue morphologies at high metallicity, are computed. The model output comprises spectral energy distributions, colours, stellar M/L ratios, bolometric corrections and near-infrared (IR) spectral line indices. The energetics of the post main sequence evolutionary phases are evaluated with the fuel consumption theorem. The impact on the models of the stellar evolutionary tracks (in particular with and without overshooting) is assessed. We find modest differences in synthetic broad-band colours as induced by the use of different tracks in our code [e.g. �( V − K ) ∼ 0.08 mag, �( B − V ) ∼ 0.03 mag]. Noticeably, these differences are substantially smaller than the scatter among other models in the literature, even when the latter adopt the same evolutionary tracks. The models are calibrated with globular cluster data from the Milky Way for old ages, and the Magellanic clouds plus the merger remnant galaxy NGC 7252, both for young ages of ∼0.1‐2 Gyr, in a large wavelength range from the U band to the K band. Particular emphasis is put on the contribution from the thermally pulsing asymptotic giant branch (TP-AGB) phase. We show that this evolutionary phase is crucial for the modelling of young stellar populations by direct comparison with observed spectral energy distributions of Magellanic cloud clusters, which are characterized by relatively high fluxes, both blueward and redward of the V band. We find that the combination of the near-IR spectral indices C2 and H2O can be used to determine the metallicity of ∼1 Gyr stellar populations. As an illustrative application, we re-analyse the spectral energy distributions of some of the high-z galaxies (2.4 z 2.9) observed with the Spitzer Space Telescope by Yan et al. Their high rest-frame near-IR fluxes is reproduced very well with the models including TP-AGB stars for ages in the range ∼0.6‐1.5 Gyr, suggesting formation redshifts for these objects around z ∼ 3‐6. Ke yw ords: stars: AGB and post-AGB ‐ stars: evolution ‐ galaxies: evolution ‐ galaxies: stellar content ‐ cosmology: early Universe.

Luminosity and Mass Limits for the Progenitor of the Type Ic Supernova 2004gt in NGC 4038

We report our attempts to locate the progenitor of the type Ic SN 2004gt in NGC 4038. We use high resolution HST ACS images of SN 2004gt and have compared them with deep pre-explosion HST WFPC2 F336W, F439W, F555W and F814W images. We identify the SN location on the pre-explosion frames with an accuracy of 5mas. We show that the progenitor is below the detection thresholds of all the pre-explosion images. These detection limits are used to place luminosity and mass limits on the progenitor, by comparing them with stellar evolution tracks on the Hertzsprung-Russell diagram. The progenitor of SN 2004gt seems to be restricted to a low-luminosity high-temperature star, either a single WC star with initial mass $>40M_{\odot}$ or a low mass star in a binary. The pre-explosion data cannot distinguish between the two scenarios.

The Effective Temperature Scale of Galactic Red Supergiants: Cool, but Not as Cool as We Thought

We use moderate-resolution optical spectrophotometry and the new MARCS stellar atmosphere models to determine the effective temperatures of 74 Galactic red supergiants. From these we find a new effective temperature scale that is significantly warmer than those in the literature. We show that this temperature scale, along with the newly derived bolometric corrections, gives much better agreement between our red supergiants and stellar evolutionary tracks. This agreement provides an independent verification of our new temperature scale. The combination of effective temperature and bolometric luminosities allows us to calculate stellar radii; the coolest and most luminous stars have radii of roughly 1500 solar radii (7 AU), in excellent accordance with the largest stellar radii predicted from current evolutionary theory. We find that similar results are obtained for the effective temperatures and bolometric luminosities using only the de-reddened V-K colors, providing a powerful demonstration of the self-consistency of the MARCS models.

Berkeley 22, an old and distant open cluster towards the Galactic anticentre

We present deep CCD BVI photometry of the distant, old open cluster Berkeley 22, covering from the red giants branch (RGB) to about 6 magnitudes below the main sequence (MS) turn-off. Using the synthetic Colour - Magnitude Diagram method with three different types of stellar evolutionary tracks, we estimate values and theoretical uncertainty of distance modulus mod0, reddening E(B-V), age tau and approximate metallicity. The best fit to the data is obtained for 13.8 <= mod0 <= 14.1, 0.64 <= E(B-V) <= 0.65, 2 <= tau <= 2.5 Gyr (depending on the amount of overshooting from convective regions adopted in the stellar models) and solar metallicity.

Optimization of Starburst99 for Intermediate‐Age and Old Stellar Populations

We have incorporated the latest release of the Padova models into the evolutionary synthesis code Starburst99. The Padova tracks were extended to include the full asymptotic giant branch (AGB) evolution until the final thermal pulse over the mass range 0.9 to 5 solar mass. With this addition, Starburst99 accounts for all stellar phases that contribute to the integrated light of a stellar population with arbitrary age from the extreme ultraviolet to the near-infrared. AGB stars are important for ages between 0.1 and 2 Gyr, with their contribution increasing at longer wavelengths. We investigate similarities and differences between the model predictions by the Geneva and the Padova tracks. The differences are particularly pronounced at ages > 1 Gyr, when incompleteness sets in for the Geneva models. We also perform detailed comparisons with the predictions of other major synthesis codes and found excellent agreement. Our synthesized optical colors are compared to observations of old, intermediate-age, and young populations. Excellent agreement is found for the old globular cluster system of NGC 5128 and for old and intermediate-age clusters in NGC 4038/39. In contrast, the models fail for red supergiant dominated populations with sub-solar abundances. This failure can be traced back to incorrect red supergiant parameters in the stellar evolutionary tracks. Our models and the synthesis code are publicly available as version 5.0 of Starburst99 at this http URL

Evolutionary stellar population synthesis at high spectral resolution: optical wavelengths

We present the single stellar population (SSP) synthesis results of our new synthetic stellar atmosphere models library with a spectral sampling of 0.3 A, covering the wavelength range from 3000 to 7000 A for a wide range of metallicities (twice solar, solar, half solar and 1/10 solar). The stellar library is composed of 1650 spectra computed with the latest improvements in stellar atmospheres. In particular, it incorporates non-local thermodynamic equilibrium (LTE) line-blanketed models for hot (T eff ≥ 27 500 K), and LTE line-blanketed models (Phoenix) for cool (3000 ≤ T eff ≤ 4500 K) stars. Because of the high spectral resolution of this library, evolutionary synthesis models can be used to predict the strength of numerous weak absorption lines and the evolution of the profiles of the strongest lines over a wide range of ages. The SSP results have been calculated for ages from 1 Myr to 17 Gyr using the stellar evolutionary tracks provided by the Geneva and Padova groups. For young stellar populations, our results have a very detailed coverage of high-temperature stars with similar results for the Padova and Geneva isochrones. For intermediate and old stellar populations, our results, once degraded to a lower resolution, are similar to the ones obtained by other groups (limitations imposed by the stellar evolutionary physics notwidthstanding). The limitations and advantages of our models for the analysis of integrated populations are described. The full set of the stellar library and the evolutionary models are available for retrieval at the websites http://www.iaa.csic.es/∼rosa and http://www.iaa.csic.es/∼mcs/sed@, or on request from the first two authors.

The M / L * Ratio of Young Star Clusters in Galactic Mergers

We point out a strong time evolution of the mass-to-light conversion factor η commonly used to estimate masses of dense star clusters from observed cluster radii and stellar velocity dispersions. We use a gasdynamical model coupled with the Cambridge stellar evolution tracks to compute line-of-sight velocity dispersions and half-light radii weighted by the luminosity. Stars at birth are assumed to follow the Salpeter initial mass function in the range of 0.15-17 M☉. We find that η, and hence the estimated cluster mass, increases by factors as large as 3 over timescales of 20 million years. Increasing the upper mass limit to 50 M☉ leads to a sharp rise of similar amplitude but in as little as 10 million years. Fitting truncated isothermal (Michie-King) models to the projected light profile leads to overestimates of the concentration parameter c of δc ≈ 0.3 compared to the same functional fit applied to the projected mass density.

Stellar Evolution with Enriched Surface Convection Zones. I. General Effects of Planet Consumption

Abundance analyses of stars with planets have revealed that their metallicities are enhanced relative to field stars. Such a trend was originally suggested to be due to accretion of iron-rich planetary material. Based on this assumption, we have developed a stellar evolution code to model stars with non-uniform metallicity distributions. We have calculated ``polluted'' stellar evolution tracks for stars with M=0.9-1.2 M_sun. Our models encompass a range of initial metal content from Z=0.01 to 0.03, and include metallicity enhancements within the stellar convection zone corresponding to Delta-Z=0.005-0.03. We find that the primary effects of metal enhancement on stellar structure and evolution are expansion of the convection zone and downward shift of effective temperature. In addition, we have computed the surface metallicities expected for stars of different mass for fixed quantities of pollution; there appears to be no correlation with present observational data on the metallicities of stars known to harbor planets.

Evolution and Yields of Extremely Metal‐poor Intermediate‐Mass Stars

Intermediate mass stellar evolution tracks from the main sequence to the tip of the AGB for five initial masses (2 to 6Msun) and metallicity Z=0.0001 have been computed. The detailed 1D structure and evolution models include exponential overshooting, mass loss and a detailed nucleosynthesis network with updated nuclear reaction rates. The network includes a two-particle heavy neutron sink for approximating neutron density in the He-shell flash. It is shown how the neutron-capture nucleosynthesis is important in models of very low metallicity for the formation of light neutron-heavy species, like sodium or the heavy neon and magnesium isotopes. The models have high resolution, as required for modeling the third dredge-up. All sequences have been followed from the pre-main sequence to the end of the AGB when all envelope mass is lost. Detailed structural and chemical model properties as well as yields are presented. This set of stellar models is based on standard assumptions and updated input physics. It can be confronted with observations of extremely-metal poor stars and may be used to assess the role of AGB stars in the origin of abundance anomalies of some Globular Cluster members of correspondingly low metallicity.

Color Transformations and Bolometric Corrections for Galactic Halo Stars: α‐Enhanced versus Scaled‐Solar Results

We have performed the first extensive analysis of the impact of an [?/Fe] > 0 metal distribution on broadband colors in the parameter space (surface gravity, effective temperature, and metal content) covered by Galactic globular cluster stars. A comparison of updated and homogeneous ATLAS 9 UBVRIJHKL synthetic photometry, for both ?-enhanced and scaled-solar metal distributions, has shown that it is impossible to reproduce ?-enhanced (B - V) and (U - B) color transformations with simple rescalings of the scaled-solar ones. At [Fe/H] ~ -2.0, ?-enhanced transformations are well reproduced by scaled-solar ones with the same [Fe/H], but this good agreement breaks down at [Fe/H] larger than about -1.6. As a general rule, (B - V) and (U - B) ?-enhanced colors are bluer than scaled-solar ones at either the same [Fe/H] or the same [M/H], and the differences increase with increasing metallicity and decreasing Teff. A preliminary analysis of the contribution of the various ?-elements to the stellar colors shows that the magnesium abundance (and to a lesser extent oxygen and silicon) is mainly responsible for these differences. On the contrary, the bolometric correction to the V-band and other infrared colors predicted by ?-enhanced transformations are well reproduced by scaled-solar results because of their weak dependence on the metal content. Key parameters, such as the turnoff and zero-age horizontal branch V magnitudes, as well as the red giant branch tip I magnitude obtained from theoretical isochrones, are in general unaffected when using the appropriate ?-enhanced transformations in place of scaled-solar ones. We have also studied, for the first time, the effect of boundary conditions obtained from appropriate ?-enhanced model atmospheres on the stellar evolutionary tracks in the log L/L?-Teff plane. We find that, for both scaled-solar and ?-enhanced metal mixtures, the integration of a solar T(?) relationship provides?at least for masses larger than 0.5-0.6 M??tracks very similar to those computed using boundary conditions from the appropriate model atmospheres.

IAC-STAR: A Code for Synthetic Color-Magnitude Diagram Computation

The code IAC-star is presented. It generates synthetic HR and color-magnitude diagrams (CMDs) and is mainly aimed to star formation history studies in nearby galaxies. Composite stellar populations are calculated on a star by star basis, by computing the luminosity, effective temperature and gravity of each star by direct bi-logarithmic interpolation in the metallicity and age grid of a library of stellar evolution tracks. Visual (broad band and HST) and infrared magnitudes are also provided for each star after applying bolometric corrections. The Padua (Bertelli et al. 1994, Girardi et al. 2000) and Teramo (Pietrinferni et al. 2004) stellar evolution libraries and various bolometric corrections libraries are used in the current version. A variety of star formation rate functions, initial mass functions and chemical enrichment laws are allowed and binary stars can be computed. Although the main motivation of the code is the computation of synthetic CMDs, it also provides integrated masses, luminosities and magnitudes as well as surface brightness fluctuation luminosities and magnitudes for the total synthetic stellar population, and therefore it can also be used for population synthesis research. The code is offered for free use and can be executed at the site {\tt http://iac-star.iac.es}, with the only requirement of referencing this paper and crediting as indicated in the site.

Berkeley 29, the most distant old open cluster

We present CCD BVI photometry of the old open cluster (OC) Berkeley 29, located in the anticentre direction. Using the synthetic colour-magnitude diagrams (CMDs) technique we estimate at the same time its age, reddening, distance and approximate metallicity using three types of stellar evolutionary tracks. The best solutions give: age = 3.4 or 3.7 Gyr, (m - M) 0 = 15.6 or 15.8 with E(B - V) = 0.13 or 0.10, and metallicity lower than solar (Z = 0.006 or 0.004), depending on the adopted stellar models. Using these derived values, Be 29 turns out to be the most distant OC known, with Galactocentric distance R GC = 21.4 to 22.6 kpc. Hence, Be 29 qualitatively follows both the age-metallicity relation and the metal abundance gradient typical of Galactic disc objects. The cluster position and radial velocity, however, appear to link Be 29 to the family of the Canis Maior debris.

Integrated spectral energy distributions and absorption-feature indices of single stellar populations

Using evolutionary population synthesis, we present integrated spectral energy distributions and absorption-line indices defined by the Lick Observatory image dissector scanner (referred to as Lick/IDS) system, for an extensive set of instantaneous burst single stellar populations (SSPs). The ages of the SSPs are in the range 1-19 Gyr and the metallicities [Fe/H] are in the range -2.3 - 0.2. Our models use the rapid single stellar evolution algorithm of Hurley, Pols and Tout for the stellar evolutionary tracks, the empirical and semi-empirical calibrated BaSeL-2.0 model of Lejeune, Cuisinier and Buser for the library of stellar spectra and the empirical fitting functions of Worthey, Faber, Gonzalez and Burstein for the Lick/IDS spectral absorption-feature indices. Applying our synthetic Lick/IDS absorption-line indices to the merit function, we obtain the age and the metallicity of the central region of M32, which can be well explained by an instantaneous SSP with an age of 6.5 Gyr and a metallicity similar to solar. Applying the derived age and the metallicity from the merit function to a number of index-index diagrams, we find that the plots of Hbeta-Fe5015 and Hbeta-Fe5782 are the best index-index diagrams from which we can directly obtain reasonable age and metallicity.

Dynamical Mass Constraints on Low‐Mass Pre–Main‐Sequence Stellar Evolutionary Tracks: An Eclipsing Binary in Orion with a 1.0 M ⊙ Primary and a 0.7 M ⊙ Secondary

Abstract : We report the discovery of a double-lined, spectroscopic, eclipsing binary in the Orion star-forming region. We analyze the system spectroscopically and photometrically to empirically determine precise, distance-independent masses, radii, effective temperatures, and luminosities for both components. The measured masses for the primary and secondary, accurate to approx. 1%, are 1.01 and 0.73 M(solar), respectively; thus, the primary is a definitive pre-main-sequence solar analog, and the secondary is the lowest-mass star yet discovered among pre-main-sequence eclispsing binary systems. We use these fundamental measurements to test the predictions of per-main-sequence stellar evolutionary tracks. None of the models we examined correctly predict the masses of the two components simultaneously, and we implicate differences between the theoretical and empirical effective temperature scales for this failing. All of the models predict the observed slope of the mass-radius relationship reasonably well, though the observations tend to favor models with low convection efficiencies. Indeed considering our newly determined mass measurements together with other dynamical mass measurements of pre-main-sequence stars in the literature, as ell as measurements of Li abundances in these stars, we show that the data strongly favor evolutionary models with inefficient convection in the stellar interior, even though such models cannot reproduce the properties of the present-day Sun.

Synthetic stellar populations: single stellar populations, stellar interior models and primordial protogalaxies

ABSTRACT We present a new set of stellar interior and synthesis models for predicting the inte-grated emission from stellar populations in star clusters and galaxies of arbitrary ageand metallicity. This work differs from existing spectral synthesis codes in a number ofimportant ways, namely (1) the incorporation of new stellar evolutionary tracks, withsufficient resolution in mass to sample rapid stages of stellar evolution; (2) a phys-ically consistent treatment of evolution in the HR diagram, including the approachto the main sequence and the effects of mass loss on the giant and horizontal-branchphases. Unlike several existing models, ours yield consistent ages when used to datea coeval stellar population from a wide range of spectral features and colour indexes.We use Hipparcos data to support the validity of our new evolutionary tracks. Werigorously discuss degeneracies in the age-metallicity plane and show that inclusion ofspectral features blueward of 4500 ˚A suffices to break any remaining degeneracy andthat with moderate S/N spectra (10 per 20˚A resolution element) age and metallicityare not degenerate. We also study sources of systematic errors in deriving the ageof a single stellar population and conclude that they are not larger than 10 − 15%.We illustrate the use of single stellar populations by predicting the colors of primor-dial proto-galaxies and show that one can first find them and then deduce the formof the IMF for the early generation of stars in the universe. Finally, we provide ac-curate analytic fitting formulas for ultra fast computation of colors of single stellarpopulations.Key words: galaxies: stellar populations; stars: stellar evolution

Hubble Space TelescopeUltraviolet Spectroscopy of Central Stars of the LMC Planetary Nebulae

In the quest to understand the origin of Planetary Nebula (PN) morphology, correlations have been sought between the nebular shapes and the evolutionary status of the central stars. Several of the mechanisms proposed to explain asymmetric shapes have a direct link with the central star's evolutionary status. Among the possible mechanisms invoked to produce asymmetric PNs, stellar rotation is certainly an effective one, as several hydrodynamics models have shown. In this work we present Space Telescope Imaging Spectrograph (STIS) UV spectra of a sample of bright LMC PNs and their central stars. LMC PNs distances are known, thus the observed characteristics of the stars translate into absolute physical quantities readily, and provide the safest dataset for theory-observation comparisons. We are planing to fit stellar models to the observed spectra using black-body SEDs and synthetic photometry to estimate stellar temperature and reddening. We are going to use the stellar evolutionary tracks to compare the post-AGB age with the dynamical age from the nebula. P-Cygni profiles will be analyzed in order to determine an approximation of the mass loss rate. Stellar characteristics will be related to the morphology of the nebulae observed by our group with HST STIS optical slitless spectroscopy.

A Preliminary Seismic Analysis of 51 Pegasi: Large and Small Spacings from Standard Models

We present a preliminary theoretical seismic study of the astronomically famous star 51 Peg. This is done by first performing a detailed analysis within the Hertzsprung-Russell diagram (HRD). Using the Yale stellar evolution code (YREC), a grid of stellar evolutionary tracks has been constructed for the masses 1.00, 1.05, and 1.10 M☉, in the metallicity range Z = 0.024-0.044, and for values of the Galactic helium enrichment ratio (ΔY/ΔZ) in the range 0-2.5. Along these evolutionary tracks, we select 75 stellar model candidates that fall within the 51 Peg observational error box in the HRD (all turn out to have masses of 1.05 and 1.10 M☉). The corresponding allowable age range for these models, which depends sensitively on the parameters of the model, is relatively large, ~2.5-5.5 Gyr. For each of the 75 models, a nonradial pulsation analysis is carried out and the large- and small-frequency spacings are calculated. The results show that just measuring the large- and small-frequency spacings will greatly reduce the present uncertainties in the derived physical parameters and in the age of 51 Peg. Finally, we briefly discuss refinements in the physics of the models and in the method of analysis, which will have to be included in future models to make the best of the precise frequency determinations expected from space observations.

The Age of the Oldest Stars in the Local Galactic Disk from Hipparcos Parallaxes of G and K Subgiants11Part review of the history of the discovery of subgiants and part new results from Hipparcos.

ABSTRACTWe review the history of the discovery of field subgiant stars, the role that they played in the development of the early understanding of stellar evolution, and their importance in the age dating of the Galactic disk. We use the cataloged data from the Hipparcos satellite in this latter capacity.Based on Hipparcos parallaxes with relative accuracies of σπ/π≤0.10, the absolute magnitude of the lower envelope of the nearly horizontal subgiant sequence for field stars in the H‐R diagram for B−V colors between 0.85 and 1.05 is measured to be MV = 4.03 ± 0.06. New stellar evolutionary tracks calculated for metal abundances in the range -0.29≤[Fe/H ]≤ +0.37 are fitted to the main‐sequence, subgiant, and giant‐star distributions in the Hipparcos H‐R diagram. Isochrones for [Fe/H ] = +0.37 provide the best fit to the reddest giants between +3≳MV≳0, as well as to the envelope of the reddest main‐sequence stars at MV≳5. The red edge of the densest part of the distribution of field giants is, however, most readily matched by isochrones having [Fe/H ] ≈ +0.23. Such high metal abundances are evidently confirmed by the spectroscopically observed high metallicity (between [Fe/H ] = +0.2 and +0.4) of the old thick‐disk Galactic cluster NGC 6791, whose color‐magnitude diagram can be made to fit either of these red boundaries by adopting suitable values for the reddening and distance modulus (from within the observed ranges of uncertainties of these quantities).The age of the field stars in the solar neighborhood is found to be 7.9 ± 0.7 Gyr (or 7.4 ± 0.7 Gyr if the stellar models allow for the effects of diffusive processes) by fitting the theoretical isochrones for [Fe/H ] = +0.37 to the lower envelope of the Hipparcos subgiants. However, this age is a function of metallicity. The models show a dependence of δt = -3.99([Fe/H] - 0.37) at MV∼ +4 for metallicities between 0.00 and +0.37. The same grid of isochrones yields ages, in turn, of 4.0 ± 0.2, 6.2 ± 0.5, and 7.5 to 10 Gyr (depending on the assumed reddening) for the old Galactic clusters M67, NGC 188, and NGC 6791 using metal abundances, distance moduli, and reddenings adopted in the text. Although the distance (and hence age) of NGC 6791 is somewhat uncertain, the ages of both the Galactic disk in the solar neighborhood and of NGC 6791 are, nevertheless, likely between 3 and 5 Gyr younger than the oldest halo globular clusters, which have ages of ∼13.5 Gyr. The conclusion is the same as that reached earlier by Liu & Chaboyer, who used Hipparcos parallax stars that were mainly near the main‐sequence turnoff rather than at the lower bound of the subgiant luminosity that we determine here. The most significant results are (1) the supermetallicity of the oldest local disk stars and (2) the large age difference between the most metal poor component of the halo and the thick and thin disk in the solar neighborhood, confirming Liu & Chaboyer. These facts are undoubtedly related and pose again the problem of the proper scenario for the timing of events in the formation of the halo and the Galactic disk in the solar neighborhood.