Isochrones for hydrogen-burning globular cluster stars. I - The metallicity range (Fe/H) from -2 to -1 (original) (raw)
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Astronomy and Astrophysics
A new procedure is described to derive homogeneous relative ages from the Color-Magnitude Diagrams (CMDs) of Galactic globular clusters (GGCs). It is based on the use of a new observable, Delta V(0.05) , namely the difference in magnitude between an arbitrary point on the upper main sequence (V_{+0.05} -the V magnitude of the MS-ridge, 0.05 mag redder than the Main Sequence (MS) Turn-off, (TO)) and the horizontal branch (HB). The observational error associated to Delta V(0.05) is substantially smaller than that of previous age-indicators, keeping the property of being strictly independent of distance and reddening and of being based on theoretical luminosities rather than on still uncertain theoretical temperatures. As an additional bonus, the theoretical models show that Delta V(0.05) has a low dependence on metallicity. Moreover, the estimates of the relative age so obtained are also sufficiently invariant (to within ~ +/- 1 Gyr) with varying adopted models and transformations. Si...
The Hβ Index as an Age Indicator of Old Stellar Systems: The Effects of Horizontal-Branch Stars
The Astronomical Journal, 2000
The strength of the Hβ index has been computed for the integrated spectra of model globular clusters from the evolutionary population synthesis. These models take into account, for the first time, the detailed systematic variation of horizontal-branch (HB) morphology with age and metallicity. Our models show that the Hβ index is significantly affected by the presence of blue HB stars. Because of the contribution from blue HB stars, the Hβ does not monotonically decrease as metallicity increases at a given age. Instead, it reaches a maximum strength when the distribution of HB stars is centered around 9500 K, the temperature where the Hβ index becomes strongest. Our models indicate that the strength of the Hβ index increases as much as 0.75Å due to the presence of blue HB stars. The comparison of the recent Keck observations of the globular cluster system in the Milky Way Galaxy with those in giant elliptical galaxies, NGC 1399 and M87, shows a systematic shift in the Hβ against metallicity plane. Our models suggest that this systematic difference is understood if the globular cluster systems in giant elliptical galaxies are several billion years older, in the mean, than the Galactic counterpart. Further observations of globular cluster systems in the external galaxies from the large ground-based telescopes and space UV facilities will enable to clarify whether this difference is indeed due to the age difference or other explanations are also possible.
The H_beta Index and the Ages of Old Stellar Systems
Journal of Astronomy and Space Sciences, 1998
The H_beta and some metal line indices, such as Mg2, Fe52, Fe53 of single-age and single-metallicity populations are computed based on the method of evolutionary population synthesis, with careful consideration of the variation of the horizontal-branch morphology with metallicity and age. We find (a) that while metal lines are little affected, the H_beta index is severely enhanced (up to 30%) by the presence of the blue horizontal-branch stars, frustrating the current age-estimations from this index without careful consideration of these stars, and (b) that there is a systematic trend in the sense that the globular clusters in giant elliptical galaxies appear to be older than those in our Galaxy by several billion years. We also calculate these indices for the stellar populations with a metallicity spread, by adopting metallicity distribution functions predicted by chemical evolution models. The comparison of the models with the observed indices of the central regions of the early-t...
2010
Isochrones for ages > ∼ 4 Gyr and metallicities in the range −2.5 < ∼ [Fe/H] < ∼ +0.3 that take the diffusion of helium and recent advances in stellar physics into account are compared with observations in the Johnson-Cousins BV (RI) C photometric system for several open and globular star clusters. The adopted color-T eff relations include those which we have derived from the latest MARCS model atmospheres and the empirical transformations for dwarf and subgiant stars given by Casagrande et al. (2010, A&A, 512, 54; hereafter CRMBA). Those reported by VandenBerg & Clem (2003, AJ, 126, 778) have also been considered, mainly to resolve some outstanding questions concerning them. Indeed, for the latter, V − I C colors should be corrected by ≈ −0.02 mag, for all metal abundances, in order to obtain consistent interpretations of the observed (B − V, V)-, (V − R C , V)-, and (V − I C , V)-diagrams for M 67 and the Hyades, as well as for local subdwarfs. Remarkably, when the subdwarfs in the CRMBA data set that have σ π /π ≤ 0.15 are superimposed on a set of 12 Gyr isochrones spanning a
Ages of globular clusters: a new approach
Monthly Notices of the Royal Astronomical Society, 1996
We have applied a new method to analyze the horizontal branch (HB) morphology in relation to the distribution of stars near the red giant branch (RGB) tip for the globular clusters M22, M5, M68, M107, M72, M92, M3 and 47 Tuc. This new method permits determination of the cluster ages to greater accuracy than conventional isochrone fitting. Using the method in conjunction with our new high-quality photometric data for RGB and HB stars in the first five of these clusters, we discuss the origins of the spread in color on the HB and its relation to the 'second parameter' problem. The oldest clusters in our sample are found to have relatively low ages (13.5 ± 2 Gyr). A 1σ uncertainty in each of the parameters of mass and helium content combined with the effects of helium diffusion gives a lower limit for the age of the oldest clusters of 9.7 Gyr.
Stellar mass and age determinations
Astronomy & Astrophysics, 2012
Aims. We present dense grids of stellar models suitable for comparison with observable quantities measured with great precision, such as those derived from binary systems or planet-hosting stars. Methods. We computed new Geneva models without rotation at metallicities Z = 0.006, 0.01, 0.014, 0.02, 0.03 and 0.04 (i.e. [Fe/H] from −0.33 to +0.54) and with mass in small steps from 0.5 to 3.5 M . Great care was taken in the procedure for interpolating between tracks in order to compute isochrones. Results. Several properties of our grids are presented as a function of stellar mass and metallicity. Those include surface properties in the Hertzsprung-Russell diagram, internal properties including mean stellar density, sizes of the convective cores, and global asteroseismic properties. Conclusions. We checked our interpolation procedure and compared interpolated tracks with computed tracks. The deviations are less than 1% in radius and effective temperatures for most of the cases considered. We also checked that the present isochrones provide nice fits to four couples of observed detached binaries and to the observed sequences of the open clusters NGC 3532 and M67. Including atomic diffusion in our models with M < 1.1 M leads to variations in the surface abundances that should be taken into account when comparing with observational data of stars with measured metallicities. For that purpose, iso-Z surf lines are computed. These can be requested for download from a dedicated web page together with tracks at masses and metallicities within the limits covered by the grids. The validity of the relations linking Z and [Fe/H] is also re-assessed in light of the surface abundance variations in low-mass stars.
Astronomy & Astrophysics, 2016
Context. The Mount Wilson Ca ii index log(R HK) is the accepted standard metric of calibration for the chromospheric activity versus age relation for FGK stars. Recent results claim its inability to discern activity levels, and thus ages, for stars older than ∼2 Gyr, which would severely hamper its application to date disk stars older than the Sun. Aims. We present a new activity-age calibration of the Mt. Wilson index that explicitly takes mass and [Fe/H] biases into account; these biases are implicit in samples of stars selected to have precise ages, which have so far not been appreciated. Methods. We show that these selection biases tend to blur the activity-age relation for large age ranges. We calibrate the Mt. Wilson index for a sample of field FGK stars with precise ages, covering a wide range of mass and [Fe/H], augmented with data from the Pleiades, Hyades, M 67 clusters, and the Ursa Major moving group. Results. We further test the calibration with extensive new Gemini/GMOS log(R HK) data of the old, solar [Fe/H] clusters, M 67 and NGC 188. The observed NGC 188 activity level is clearly lower than M 67. We correctly recover the isochronal age of both clusters and establish the viability of deriving usable chromospheric ages for solar-type stars up to at least ∼6 Gyr, where average errors are ∼0.14 dex provided that we explicitly account for the mass and [Fe/H] dimensions. We test our calibration against asteroseismological ages, finding excellent correlation (ρ = +0.89). We show that our calibration improves the chromospheric age determination for a wide range of ages, masses, and metallicities in comparison to previous age-activity relations.
Cumulative physical uncertainty in modern stellar models
Astronomy & Astrophysics, 2013
Context. Theoretical stellar evolutionary models are still affected by not negligible uncertainties due to the errors in the adopted physical inputs. Aims. In this paper, using our updated stellar evolutionary code, we quantitatively evaluate the effects of the uncertainties in the main physical inputs on the evolutionary characteristics of low mass stars, and thus of old stellar clusters, from the main sequence to the zero age horizontal branch (ZAHB). To this aim we calculated more than 3000 stellar tracks and isochrones, with updated solar mixture, by changing the following physical inputs within their current range of uncertainty: 1 H(p, νe +) 2 H, 14 N(p, γ) 15 O, and triple-α reaction rates, radiative and conductive opacities, neutrino energy losses, and microscopic diffusion velocities. Methods. The analysis was conducted performing a systematic variation on a fixed grid, in a way to obtain a full crossing of the perturbed input values. The effect of the variations of the chosen physical inputs on relevant stellar evolutionary features, such as the turn-off luminosity, the central hydrogen exhaustion time, the red-giant branch tip luminosity, the helium core mass, and the ZAHB luminosity in the RR Lyrae region are analyzed in a statistical way. Results. We find that, for a 0.9 M model, the cumulative uncertainty on the turn-off, the red-giant branch tip, and the ZAHB luminosities accounts for ±0.02 dex, ±0.03 dex, and ±0.045 dex respectively, while the central hydrogen exhaustion time varies of about ±0.7 Gyr. For all examined features the most relevant effect is due to the radiative opacities uncertainty; for the later evolutionary stages the second most important effect is due to the triple-α reaction rate uncertainty. For an isochrone of 12 Gyr, we find that the isochrone turn-off log luminosity varies of ±0.013 dex, the mass at the isochrone turn-off varies of ±0.015 M , and the difference between ZAHB and turn-off log-luminosity varies of ±0.05 dex. The effect of the physical uncertainty affecting the age inferred from turn-off luminosity and from the vertical method are of ±0.375 Gyr and ±1.25 Gyr, respectively.
Revised ages for stars in the solar neighbourhood
Astronomy & Astrophysics - ASTRON ASTROPHYS, 1998
New ages are computed for the stars from the Edvardsson et al. (1993) data set. The revised values are systematically larger toward older ages (t>4 Gyr), while they are slightly lower for t<4 Gyr. A similar, but considerably smaller trend is present when the ages are computed with the distances based on Hipparcos parallaxes. The resulting age-metallicity relation has a small, but distinct slope of ~ em0.07 dex/Gyr. Tables 3\to8 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or WWW at URL http://cdsweb.u-strasbg.fr/Abstract.html
The Astrophysical Journal, 2014
We present an analysis of photometric and spectroscopic data of the Carina dSph galaxy, testing a new approach similar to that used to disentangle multiple populations in Galactic globular clusters (GCs). We show that a proper color combination is able to separate a significant fraction of the red giant branch (RGB) of the two main Carina populations (the old one, ∼12 Gyr, and the intermediate-age one, 4-8 Gyr). In particular, the c U,B,I = (U − B) − (B − I) pseudo-color allows us to follow the RGB of both populations along a relevant portion of the RGB. We find that the oldest stars have a more negative c U,B,I pseudo-color than intermediateage ones. We correlate the pseudo-color of RGB stars with their chemical properties, finding a significant trend between the iron content and the c U,B,I. Stars belonging to the old population are systematically more metal-poor ([Fe/H] = −2.32 ± 0.08 dex) than the intermediate-age ones ([Fe/H] = −1.82 ± 0.03 dex). This gives solid evidence of the chemical evolution history of this galaxy, and we have a new diagnostic that can allow us to break the age-metallicity degeneracy of H-burning advanced evolutionary phases. We compared the distribution of stars in the c U,B,I plane with theoretical isochrones, finding that no satisfactory agreement can be reached with models developed in a theoretical framework based on standard heavy element distributions. Finally, we discuss possible systematic differences when compared with multiple populations in GCs.