Local Subdwarfs Research Articles

Constraints on the Distance Moduli, Helium and Metal Abundances, and Ages of Globular Clusters from their RR Lyrae and Non-variable Horizontal-branch Stars. III. M55 and NGC 6362

Abstract M55 (NGC 6809) and NGC 6362 are among the few globular clusters for which masses and radii have been derived to high precision for member binary stars. They also contain RR Lyrae variables, which, together with their non-variable horizontal-branch (HB) populations, provide tight constraints on the cluster reddenings and distance moduli through fits of stellar models to their pulsational and evolutionary properties. Reliable (m − M) V estimates yield M V and M bol values of comparable accuracy for binary stars, because the V-band bolometric corrections applicable to them have no more than a weak dependence on effective temperature ( ) and [Fe/H]. Chemical abundances derived from the binary mass–M V relations are independent of determinations based on their spectra. The temperatures of the binaries, which are calculated directly from their luminosities and the measured radii, completely rule out the low scale that has been determined for metal-deficient stars in some recent spectroscopic and interferometric studies. If [α/Fe] = 0.4 and [O/Fe] = 0.5 ± 0.1, we find that M55 has , [Fe/H] = −1.85 ± 0.1, and an age of 12.9 ± 0.8 Gyr, whereas NGC 6362 has , [Fe/H] = −0.90 ± 0.1, and an age of 12.4 ± 0.8 Gyr. The HB of NGC 6362 shows clear evidence for multiple stellar populations. Constraints from the RR Lyrae standard candle and from local subdwarfs (with Gaia DR2 parallaxes) are briefly discussed.

Metallicity and absolute magnitude calibrations for UBV photometry

Metallicity and absolute magnitude calibrations for UBV photometry

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.

Resolving the 47 Tucanae Distance Problem

Using a sample of 43 suitable local subdwarfs with newly acquired BVI photometry, we apply our main sequence fitting method to the metal rich Globular Cluster 47 Tucanae. Fitting in 2 colour planes, we find an apparent distance modulus of (m-M)v = 13.37 (+0.10/-0.11), leading to a dereddened distance modulus of (m-M)o = 13.25 (+0.06/-0.07). Consideration of the Red Clump in the cluster produces a distance modulus fully consistent with this result. The implied cluster age is 11 (+/-1.4) Gyr.

Models of Metal‐poor Stars with Gravitational Settling and Radiative Accelerations. II. The Age of the Oldest Stars

Isochrones for ages between 12 and 18 Gyr have been derived from the evolutionary tracks presented in Paper I (Richard et al.) for masses from 0.5 to 1.0 Mand initial chemical abundances corresponding to (1) Y ¼ 0:2352, Z ¼ 1:69 � 10 � 4 ð½Fe=H �¼� 2:31; ½�= Fe �¼ 0:3Þ and (2) Y ¼ 0:2370, Z ¼ 1:69 � 10 � 3 ð½Fe=H ¼� 1:31; ½�= Fe �¼ 0:3Þ. These are the first models for Population II stars in which both gravitational settling and radiative accelerations have been taken into account. Allowance for these diffusive processes leads to a 10%-12% reduction in age at a given turnoff luminosity. However, in order for the diffusive models to satisfy the constraints from Li and Fe abundance data (see Paper I) and to reproduce the observed mor- phologies of globular cluster (GC) color-magnitude diagrams (CMDs) in a straightforward way, extra mix- ing just below the boundary of the convective envelope seems to be necessary. Indeed, when additional turbulent mixing is invoked, the resultant models are able to satisfy all of these constraints, as well as those provided by the CMDs of local subdwarfs, rather well. Moreover, they imply an age near 13.5 Gyr for M92, which is one of the most metal-deficient (and presumably one of the oldest) of the Galaxy's GCs, if the field subgiant HD 140283 is used to derive the cluster distance. Comparisons of field subdwarfs and subgiants with a recently published fiducial for M5 suggests that the cluster has ½Fe=Hd � 1:4, in conflict with some esti- mates based on high-resolution spectroscopy, if the metallicities of the field stars are to be trusted. In addi- tion, an age of � 11.5 Gyr is found for M5, irrespective of whether diffusive or nondiffusive isochrones are employed in the analysis. The implications of our results for the extragalactic distance scale and for the Hub- ble constant are briefly discussed in the context of the presently favoredM � 0:35, � � � 0:65 cosmological model.

Models for Old, Metal‐poor Stars with Enhanced α‐Element Abundances. III. Isochrones and Isochrone Population Functions

An isochrone population function (IPF) gives the relative distribution of stars along an isochrone. IPFs contain the information needed to calculate both luminosity functions and color functions, and they provide a straightforward way of generating synthetic stellar populations. An improved algorithm for interpolating isochrones and IPFs, based on the scheme introduced by Bergbusch & VandenBerg, is described. Software has been developed to permit such interpolations for any age encompassed by an input grid of stellar evolutionary tracks. Our first application of this software is to the models presented in this series of papers for 17 [Fe/H] values between -2.31 and -0.3, with three choices of [α/Fe] at each iron abundance (specifically, 0.0, 0.3, and 0.6). (These models do not treat gravitational settling or radiative acceleration processes, but otherwise they are based on up-to-date physics. Additional grids will be added to this database as they are completed.) The computer programs (written in FORTRAN 77) and the grids of evolutionary tracks that are presently available for processing by these codes into isochrones and IPFs are freely available to interested users. In addition, we add to the evidence presented in previous papers in this series in support of the Teff and color scales of our models. In particular, the temperatures derived by Gratton et al. for local Population II subdwarfs with accurate (Hipparcos) parallaxes are shown to be in excellent agreement with those predicted for them, when the Gratton et al. [Fe/H] scale is also assumed. Interestingly, the locus defined by local subdwarfs and subgiants on the (MV, log Teff)-plane and the morphologies of globular cluster (GC) color-magnitude diagrams are well matched by the present models, despite the neglect of diffusion, which suggests that some other process(es) must be at play to limit the expected effects of gravitational settling on predicted temperatures. The three field halo subgiants in our sample all appear to have ages 15 Gyr, which is favored for the Galaxy's most metal-poor GCs as well. (The settling of helium and heavy elements in the central regions of stars is expected to cause about a 10% reduction in these age estimates: this effect should persist even if some process, such as turbulence at the base of the convective envelope, counteracts diffusion in the surface layers.) Furthermore, our isochrones accurately reproduce the Da Costa & Armandroff red giant branch fiducials for M15, NGC 6752, NGC 1851, and 47 Tuc on the [MI, (V-I)0]-diagram. However, our models fail to predict the observed luminosities of the red giant bump by ≈0.25 mag: this could be an indication that there is some amount of inward overshooting of convective envelopes in red giants. For consistency reasons, the Zinn & West metallicities for intermediate metal-poor GCs (-1.8 [Fe/H] -1.1) seem to be preferred over recent spectroscopic results (based on the brightest cluster giants), suggesting that there is an inconsistency between current subdwarf and GC [Fe/H] scales.

On the existence of differences in luminosity between horizontal branch stars in globular clusters and in the field

The discrepancy between a LONG distance scale derived from Hipparcos based distances to globular clusters via main sequence fitting to local subdwarfs, and a SHORT distance scale derived from the absolute magnitude of field RR Lyraes via statistical parallaxes and the Baade-Wesselink method could be accounted for whether an intrinsic difference of about 0.1-0.2 mag was found to exist between horizontal branch (HB) stars populating the sparse general field and the dense globular clusters. In this paper we discuss the possible existence of such a systematic difference comparing the period-shifts observed for field and cluster RR Lyraes. Various approaches based on different parameters and data-sets for both cluster and field variables were used in order to establish the size of such a hypothetical difference, if any. We find that on the whole very small not significant differences exist between the period-metallicity distributions of field and cluster RR Lyraes, thus confirming with a more quantitative approach, the qualitative conclusions by Catelan (1998). This observational evidence translates into a very small difference between the horizontal branch luminosity of field and cluster stars, unless RR Lyraes in Globular Clusters are about 0.06 Mo more massive than field RR Lyrae at same metallicity, which is to be proven.

Models for Old, Metal‐Poor Stars with Enhanced α‐Element Abundances. II. Their Implications for the Ages of the Galaxy’s Globular Clusters and Field Halo Stars

The ages of globular clusters (GCs) and post-turnoff field stars in the Galactic halo are derived using new stellar evolutionary models that explicitly take into account the observed abundances of the α-elements. Whereas the distances of the field subgiants considered in this study are based on Hipparcos parallax measurements, theoretical zero-age horizontal-branch (ZAHB) loci have been used to set the GC distance scale. (As reported in Paper I, the latter imply RR Lyrae luminosities that are within 0.1-0.15 mag, on the high side, of those found for field variables from Baade-Wesselink, trignometic parallax, and statistical parallax studies.) Both the field and cluster observations indicate that the most metal-deficient stars are 14 Gyr old while those with [Fe/H] -1.3 are 2-3 Gyr younger. Unless the O/Fe (or, more generally, the α/Fe) number abundance ratio rises quite steeply with decreasing [Fe/H], it seems unlikely that a significant age-metallicity relation can be avoided. Using what is effectively the ΔV method of determining relative cluster ages, we find that the dispersion in age at any [Fe/H] less than ≈-1.0 is small (10%-15%). Even Arp 2 and IC 4499, which had been previously categorized as young GCs, probably have near-normal ages for their metallicities—though a more definitive conclusion must await improved photometry for these systems. Ruprecht 106 appears no more than 1-1.5 Gyr younger than M3, as opposed to the ~4 Gyr age difference that others have found. However, Palomar 12 and Terzan 7 are undoubtedly young, and they provide strong evidence that, at [Fe/H] -1.0, the ages of GCs differ by as much as 4 Gyr (or ~25%). (A much larger sample of metal-rich clusters must be studied to ascertain whether the distribution of ages at higher metallicities is broad or narrow: Pal 12 and Ter 7 could well be atypical and represent the tail of a distribution that is strongly peaked near 12 Gyr.) Importantly, for the best-observed systems, there is no obvious conflict between the relative age estimates based on the ΔV method, on the one hand, and the Δ(B-V)TO,RGB technique, on the other. However, there are some exceptions, like M68, M53, M13, and NGC 288 for which the two approaches give slightly different results, indicating that something in addition to, or besides, age must be playing a role. From isochrone/ZAHB fits to the C-M diagrams (CMDs) of the outer-halo GCs Pal 3, Pal 4, and Eridanus, we conclude that there is no more than a small dependence of age on Galactocentric distance. Pal 4 seems to have very close to the same age as NGC 362, NGC 1261, and NGC 1851, while Eridanus and Pal 3 appear to be 1 Gyr younger than most inner-halo systems of the same metallicity. Unless our understanding of the HB phase of evolution is seriously in error, cluster-to-cluster age differences (at a given metallicity) are much too small for age to be the most important of the possible second parameters in determining the morphology of the horizontal branch (the first parameter being [Fe/H]). Finally, we suggest that the GC distance scale as inferred from studies of nearby RR Lyraes is not necessarily in conflict with that based on local subdwarfs. A reconciliation of the two approaches is possible simply by adopting a particular metallicity scale for the globular clusters—one that is much closer to the Zinn-West scale than to that recently proposed by Carretta & Gratton. Indeed, such a simple way of achieving consistency between subdwarf-based distances and those inferred from RR Lyraes seems compelling. If this suggestion is correct, then the long distance scale and low estimates of GC ages are not tenable. In fact, the adoption of the short distance scale, implying ages 15 Gyr for the most metal-poor GCs (and 18.30 for the true distance modulus of the Large Magellanic Cloud), leads to much improved agreement between synthetic and observed CMDs in the vicinity of the turnoff. It does cause a mismatch between predicted and observed HB luminosities, in the sense that the models are too bright by ~0.1-0.15 mag, but this may simply be an indication that current conductive opacities or the assumed chemical abundance parameters (Y, [O/Fe], and/or [α/Fe]) are not quite right.

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 catalogue. Several methods for the Population II local distance scale are discussed, exploiting NEW RESULTS for RR Lyraes in the Large Magellanic Cloud (LMC). We find that the so-called Short and Long Distance Scales may be reconciled whether a consistent reddening scale is adopted for Cepheids and RR Lyrae variables in the LMC. Distances and ages for the 9 clusters discussed in Paper I are re-derived using an enlarged sample of local subdwarfs, which includes about 90% of the metal-poor dwarfs with accurate parallaxes (Delta p/p < 0.12) in the whole Hipparcos catalogue. On average, our revised distance moduli are decreased by 0.04 mag with respect to Paper I. The corresponding age of the GCs is t=11.5+-2.6 Gyr (95% confidence range). The relation between Mv(ZAHB) and metallicity for the nine programme 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. Comparing the corresponding (true) LMC distance modulus 18.64+-0.12 mag with other existing determinations, we conclude that at present the best estimate for the distance of the LMC is: 18.54+-0.03+-0.06, suggesting that distances from the subdwarf fitting method are 1 sigma 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 Mv(ZAHB) and [Fe/H] is: Mv(ZAHB) =(0.18+-0.09)([Fe/H]+1.5)+(0.63+-0.07).

Models for Old, Metal‐poor Stars with Enhanced α‐Element Abundances. I. Evolutionary Tracks and ZAHB Loci; Observational Constraints

Stellar evolutionary tracks have been computed for 17 [Fe/H] values from -2.31 to -0.30 assuming, in each case, [α/Fe] = 0.0, 0.3, and 0.6. The helium abundance was assumed to vary from Y = 0.2352 at [Fe/H] = -2.31 to Y = 0.2550 at [Fe/H] = -0.30 and held constant for the different choices of [α/Fe] at a fixed iron content. Masses in the range 0.5 ≤ ☉ ≤ 1.0, in 0.1 ☉ steps, were generally considered, though sequences for higher mass values were computed, as necessary, to ensure that isochrones as "young" as 8 Gyr could be generated for each grid. All of the stellar models are based on an equation of state that treats nonideal effects, the latest nuclear reaction and neutrino cooling rates, and opacities that were computed specifically for the adopted chemical mixtures. The tracks were extended to the tip of the giant branch or to an age of 30 Gyr, whichever came first, and zero-age horizontal-branch (ZAHB) loci were constructed using the helium core masses and chemical profiles from appropriate red giant precursors. Selected models have been compared with those computed by A. V. Sweigart, for the same masses and chemical compositions, to demonstrate that the results obtained from two entirely independent stellar evolution codes agree well with one another when very similar input physics is assumed. In the case of extremely metal-deficient stars, an enhancement in the abundance of the α-elements causes a single, fairly significant bump in the opacity at a temperature just above 106 K, which is caused by absorption processes involving the K shell of oxygen. This peak becomes steadily more pronounced as the overall metallicity increases and a second bump, arising from the L edges of Ne, Mg, and Si, eventually appears near log T = 5.6. As far as the tracks and isochrones are concerned, we find that, as already reported by others, it is possible to mimic the computations for [α/Fe] > 0 remarkably well by those for scaled-solar mixes simply by requiring the total mass-fraction abundance of the heavy elements, Z, to be the same. However, this result holds only for metallicities significantly less than solar. Above [Fe/H] ≳ -0.8, tracks and isochrones for enhanced α-element mixtures begin to have systematically hotter/bluer turnoffs and red giant branches than those for scaled-solar mixtures of the heavy elements. Also addressed is the extent to which our models satisfy the constraints posed by the local subdwarfs, the distances of which are based on Hipparcos parallax measurements. Our analysis suggests that the predicted metallicity dependence of the location of the lower main sequence on the C-M diagram is in good agreement with the observed dependence. In fact, we do not find any compelling evidence from the local Population II calibrators that the colors of our models require significant adjustments. In further support of our calculations, we find that, both in zero point and slope, the computed giant branches on the (Mbol, log Teff)-plane agree well with those inferred for globular clusters from observations in the infrared. Moreover, our ZAHB models have luminosities that are just outside the 1 σ error bars of the mean MV's inferred for RR Lyrae stars from Baade-Wesselink, statistical parallax, and trigonometric parallax studies. Lower helium contents or higher α-element abundances or an increase in the conductive opacities are among the possible ways of reducing the differences that remain. To facilitate comparisons with observations, the tracks/ZAHBs are provided with predicted BV(RI)C photometry.

The Distance to the Galactic Center

ABSTRACTThe Optical Gravitational Lensing Experiment (OGLE) data on high‐amplitude δ Scuti stars (HADS) and RR Lyrae stars have been analyzed to determine the distance to the Galactic bulge. Individual color excesses and V absorption for each individual variable star have been computed, which lead to accurate 〈V0〉 magnitudes. The period‐luminosity relation of the HADS is utilized to calculate distance moduli. We find that the metal‐strong HADS are concentrated strongly to the bulge. The best distance modulus to the bulge from the HADS data is 14.49 ± 0.06 mag. The dependence of Mv on period for the bulge RR Lyrae stars can be found from the reddening‐free magnitude VV - I. The zero point of the Mv–log P relation is set with the horizontal branches of Oosterhoff type I globular clusters. Mv≅0.44 mag (long‐distance scale) in the log P interval −0.29 to −0.22 as inferred by main‐sequence fitting of the clusters to local subdwarfs and utilizing short‐period HADS in the clusters. The best distance modulus to the bulge from the RR Lyrae (ab) stars is 14.45 mag, which compares favorably with the distance modulus of 14.49 mag found from the HADS. We adopt the mean value 14.47 ± 0.08 mag for the bulge distance modulus. A small additive correction leads to 7.9 ± 0.3 kpc as the best distance to the Galactic center.

On the Existence of Luminosity Differences between HB Field and Cluster Stars

AbstractThe discrepancy between the long distance scale as derived, e.g., from Hipparcos-based distances to globular clusters via main sequence fitting to local subdwarfs, and the short distance scale as derived, e.g., from the absolute magnitude of field RR Lyrae stars via statistical parallaxes and the Baade–Wesselink method, could be accounted for if an intrinsic difference in luminosity of about 0.1−0.2 mag were found to exist between horizontal branch (HB) stars populating the sparse general field and the dense globular clusters.

Globular Cluster Ages from Hipparcos Subdwarfs

The new Hipparcos parallax data for local subdwarfs allow a much more reliable determination of the distance to globular clusters (by direct sequence fitting) than was previously possible. Earlier studies (Reid 1997, Gratton et al. 1997) have reported larger distances than expected, especially for the most metal-poor clusters, implying much younger ages. In our study of Hipparcos subdwarfs applied to M92 — representative of the oldest, most metal-poor clusters — we find however a distance only slightly in excess of previous expectations. We show, using Monte Carlo simulations, that most of the difference may be explained by our more detailed treatment of the Lutz-Kelker and selection biases. With up-to-date stellar evolution models, we derive a minimum age of 13 Gyr for the Universe. This value, although lower than previous estimates, still imposes a rather strict upper limit to Ho in the context of standard cosmological models.

The absolute magnitude of field metal-poor horizontal branch stars

HIPPARCOS satellite parallaxes for 22 metal-poor field horizontal branch stars with Vo<9 are used to derive their absolute magnitude. The weighted mean value is Mv=+0.69+/-0.10 for an average metallicity of [Fe/H]=-1.41; a somewhat brighter average magnitude of Mv=+0.60+/-0.12 for an average metallicity of [Fe/H]=-1.51 is obtained eliminating HD17072, that might be on the first ascent of the giant branch rather than on the horizontal branch. The present values agree with determinations based on proper motions and application of the Baade-Wesselink method to field RR Lyraes; they are from 0.1 to 0.2 mag fainter than those based on calibration of cluster distances obtained by using local subdwarfs, and on alternative distance calibrators for the LMC. The possibility that there is a real difference between the luminosity of the horizontal branch for clusters and the field is briefly commented.

A new analysis of the red giant branch 'tip' distance scale and the value of the Hubble constant

The theoretical evaluations of the red giant branch tip (TRGB) luminosity presented previously by Salaris &38; Cassisi are extended to higher metallicities, and compared with analogous independent results recently published. The present sets of stellar models agree quite well in the determination of the TRGB brightness. Relations between TRGB bolometric and l (Cousins) magnitude and zero-age horizontal branch (ZAHB) V magnitude with respect to the metallicity are provided by adopting empirical, semi-empirical and theoretical evaluations of bolometric corrections, after a careful calibration of the zero-point of the bolometric correction scales. The comparison between our ZAHB and TRGB distance scales for galactic globular clusters presented in Paper I is now supplemented with a comparison with the Hipparcos distance scale set by local subdwarfs with accurate parallax determinations. The overall agreement between ZAHB and Hipparcos distances is quite good. The TRGB distances for globular clusters are compatible with the ZAHB distances in the limit of the small sample of red giants observed. The ZAHB and TRGB distances to resolved galaxies are in good agreement, whereas the comparison between TRGB and Cepheid distances, computed by using the calibration suggested earlier by Madore &38; Freedman, reveals a systematic discrepancy of the order of 0.12 mag. The TRGB distances are systematically longer in comparison with the Cepheid ones. This result supports the case for a revision of the zero-point of the Cepheid distance scale, as already suggested by other authors on the basis of Hipparcos parallaxes. We do not find any clear correlation of the difference between TRGB and Cepheid distances with metal content. The application of our TRGB distance scale to NGC 3379 provides a distance to the Leo I group that is about 8 per cent higher than the one obtained recently by Sakai et al. adopting the TRGB-metallicity calibration by Lee, Freedman &38; Madore. Our distance to the Leo I group, coupled with recent independent determinations of the Coma cluster-Leo I distance, obtained differentially by means of secondary distance indicators, provides a determination of H0 at the Coma cluster in the range H0 = 60 ± 11 km s−1 Mpc−1. For choices of Ω in agreement with the observations (0.3 ≤ Ω ≤ 1) and cosmological constant equal to zero, our derived H0 value is compatible with the most recent determinations of galactic globular cluster ages, thus removing the long-standing conflict between the Hubble age and the age of the oldest stars in the Galaxy.

Confirmation of previous ground-based Cepheid P-L zero-points using Hipparcos trigonometric parallaxes

Comparisons show agreement at the 0.1-mag level between the calibration of the Cepheid period–luminosity (P–L) relation by Feast & Catchpole (FC) using the early release of Hipparcos data and four previous ground-based calibrations, three of which are either largely or totally independent of the distance to the Large Magellanic Cloud (LMC). Each of the comparisons has the sense that the FC calibration is brighter, but only at the level of ≲0.1 mag. In contrast, FC argue that their Hipparcos recalibration leads to a 0.2-mag revision in the distance to the LMC, and thereby to a 10 per cent decrease in the Hubble constant. We argue differently. The comparison of the Hipparcos recalibration with others should be made using only local Galactic Cepheids, not based on Cepheids in the LMC that require a set of precepts that are not germane to the direct Hipparcos recalibration. The comparison made here, using only Galactic Cepheids, gives a correction of ∼4 per cent or less to our value of H0 based on Type Ia supernovae, keeping all other factors and precepts the same. A second success of the Hipparcos mission is the calibration of the position of the main sequence in the Hertzsprung–Russell diagram as a function of metallicity using local subdwarfs. These data have been used by Reid and by Gratton et al. to obtain, similarly to FC, a brighter absolute magnitude of RR Lyrae stars by ∼0.3 mag from that often currently adopted. These new calibrations confirm the earlier brighter calibrations by Walker, by Sandage, and by Mazzitelli, D'Antona & Caloi, thereby reducing the ages of globular clusters by ∼30 per cent. This removes most of the cosmological time-scale problem if H0∼55 km s−1 Mpc−1. A similar conclusion, based on pulsation theory and MACHO data, has been reached by Alcock et al.

Ages of Globular Clusters fromHIPPARCOSParallaxes 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.

Younger and Brighter - New Distances to Globular Clusters Based on HIPPARCOS Parallax Measurements of Local Subdwarfs

view Abstract Citations (239) References (67) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Younger and Brighter - New Distances to Globular Clusters Based on HIPPARCOS Parallax Measurements of Local Subdwarfs Reid, I. Neill Abstract We have used new parallax measurements, obtained by the Hipparcos satellite, of fifteen nearby, metal-poor stars to re-define the subdwarf main-sequence. All of these stars have parallaxes determined to an accuracy of at least 12%. Comparing these measurements against previous ground-based data for nine stars reveals a systematic offset of 5%, in the sense that the Hipparcos parallaxes are smaller (i.e., the inferred distances are larger). The availability of the Hipparcos observations expands the local subdwarf sample to the extent that we can separate the stars by abundance into intermediate ([Fe/H]∼-1.4) and extreme ([Fe/H] ∼-2) subsets. Main-sequence fitting techniques are then used to match stars of the appropriate abundance range to the color-magnitude diagrams of the seven globular clusters: M5, NGC 6752, M13, M13, M92, M30, and M68. We derive respective distance moduli of 14.45, 13.17, 14.48, 15.38, 14.93, 14.95, and 15.29 magnitudes, with formal uncertainties of ±0.1 mag. The metal-poor systems M68, M15, and M30 have moderate foreground reddening, and varying by ±0.02 mag can change the derived distances by up to ±7%. With the exception of NGC 6752, however, our derived distances exceed previous estimates, particularly in the case of the four [Fe/H]∼-2.1 globulars, where our distance moduli are 0.3 mag higher than the current standard values. We discuss briefly how these findings affect the RR Lyrae distance scale, isochrone-based estimates of the age of globular clusters and our picture of the early stages of star formation in the Galaxy. We note that our results go some way towards reconciling the apparent contradiction between the cluster ages and recent determinations of the Hubble constant. Publication: The Astronomical Journal Pub Date: July 1997 DOI: 10.1086/118462 arXiv: arXiv:astro-ph/9704078 Bibcode: 1997AJ....114..161R Keywords: GALAXIES: DISTANCES AND REDSHIFTS; GLOBULAR CLUSTERS: GENERAL; Astrophysics E-Print: to appear in AJ full text sources arXiv | ADS | data products SIMBAD (54) NED (1)

Hipparcos subdwarfs and globular cluster ages: towards reliable absolute ages

The maximum age of galactic globular clusters provides the best observational constraint on the minimum age of the Universe. One of the main “missing link” in the globular cluster age determination has been the lack of a precise calibration, with local subdwarfs, of the position of the subdwarf sequence at different [Fe/H].Hipparcos data may change this situation. As many precise parallaxes become available for local subdwarfs, the distance to globular clusters can be estimated directly from ZAMS fitting to the subdwarf locus. The ages can then be inferred from the turnoff luminosity (a robust prediction of stellar evolution models), rather than using secondary indicators such as Horizontal-Branch position, or indicators depending on the uncertain colour scale such as turnoff colour.Combining Hipparcos parallaxes with [Fe/H] values determined with the CORAVEL spectrometer, we are studying the position of the subdwarfs in the Colour-Magnitude Diagram from a sample of more than 900 subdwarf candidates. Preliminary results are presented here. It is shown that the distances of many subdwarfs had been underestimated in previous studies, mainly because a large fraction of them is in fact evolved off the main sequence into the turnoff or the subgiant branch.

NEW INSIGHTS INTO GLOBULAR CLUSTERS FROM MAIN-SEQUENCE PHOTOMETRY

CCD photometry of main-sequence stars is presented for four globular clusters, NGC 288, NGC 362, NGC 6752, and NGC 7099. Analysis of (B -V) color distributions of stars on the main sequences of the clusters indicates a high degree of chemical homogeneity among the stars in a cluster. The most stringent limit is for the cluster NGC 362, for which an upper limit on the standard deviation of the [M/H] distribution of a = 0.08 dex is set. The incidence of photometrically-detectable binary stars in NGC 362 and NGC 7099 is very low (< 0.05%). In NGC 288 there appears to be a second sequence to the redward of the main sequence composed of 14 objects. This sequence is 0.75 mag above the main sequence and may be comprised of equal-mass binary systems. Distances have been determined to NGC 288, NGC 362, and NGC 7099 by fitting the lower main sequences of these clusters to the local subdwarfs using the recently-revised parallax values and Lutz-Kelker corrections. The distance moduli are measured to be (m M ) = 14.65 ±0.2 for NGC 288 and NGC 362, and (m -M) = 14.50 ±0.2 for NGC 7099. The implied values for Mv