Synthetic Horizontal Branch Models Research Articles

AstroSat Study of the Globular Cluster NGC 2298: Probable Evolutionary Scenarios of Hot Horizontal Branch Stars

Abstract We present the far-UV (FUV) photometry of images acquired with UVIT on AstroSat to probe the horizontal branch (HB) population of the Galactic globular cluster NGC 2298. UV-optical color–magnitude diagrams (CMDs) are constructed for member stars in combination with Hubble Space Telescope UV Globular Cluster Survey data for the central region and Gaia and ground-based photometric data for the outer region. A blue HB (BHB) sequence with a spread and four hot HB stars are detected in all FUV-optical CMDs and are compared with theoretical updated BaSTI isochrones and synthetic HB models with a range in helium abundance, suggesting that the hot HB stars are helium enhanced when compared to the BHB. The estimated effective temperature, radius, and luminosity of HB stars, using the best spectral energy distribution fits, were compared with various HB models. BHB stars span a temperature range from 7500 to 12,250 K. Three hot HB stars have 35,000–40,000 K, whereas one star has around ∼100,000 K. We suggest the following evolutionary scenarios: two stars are likely to be the progeny of extreme HB (EHB) stars formed through an early hot-flasher scenario, one is likely to be an EHB star with probable helium enrichment, and the hottest HB star, which is about to enter the white dwarf cooling phase, could have evolved from the BHB phase. Nevertheless, these are interesting spectroscopic targets to understand the late stages of evolution.

Evolutionary and pulsation properties of Type II Cepheids

We discuss the observed pulsation properties of Type II Cepheids (TIICs) in the Galaxy and in the Magellanic Clouds. We found that period (P) distributions, luminosity amplitudes, and population ratios of the three different sub-groups (BL Herculis [BLH, P < 5 days], W Virginis [WV, 5 ≤ P < 20 days], RV Tauri [RVT, P > 20 days]) are quite similar in different stellar systems, suggesting a common evolutionary channel and a mild dependence on both metallicity and environment. We present a homogeneous theoretical framework based on horizontal branch (HB) evolutionary models, showing that TIICs are mainly old (t ≥ 10 Gyr) low-mass stars. The BLH stars (BLHs) are predicted to be post-early asymptotic giant branch (PEAGB) stars (double shell burning) on the verge of reaching their AGB track (first crossing of the instability strip), while WV stars (WVs) are a mix of PEAGB and post-AGB stars (hydrogen shell burning) moving from the cool to the hot side (second crossing) of the Hertzsprung-Russell Diagram. This suggests that they are a single group of variable stars. The RVT stars (RVTs) are predicted to be a mix of post-AGB stars along their second crossing (short-period tail) and thermally pulsing AGB stars (long-period tail) evolving towards their white dwarf cooling sequence. We also present several sets of synthetic HB models by assuming a bi-modal mass distribution along the HB. Theory suggests, in agreement with observations, that TIIC pulsation properties marginally depend on metallicity. Predicted period distributions and population ratios for BLHs agree quite well with observations, while those for WVs and RVTs are almost a factor of two smaller and higher than observed, respectively. Moreover, the predicted period distributions for WVs peak at periods shorter than observed, while those for RVTs display a long-period tail not supported by observations. We investigate several avenues to explain these differences, but more detailed calculations are required to address these discrepancies.

Horizontal branch morphology: A new photometric parametrization

Context. Theory and observations indicate that the distribution of stars along the horizontal branch of Galactic globular clusters mainly depends on the metal content. However, the existence of globular clusters with similar metal content and absolute age but different horizontal branch morphologies, suggests the presence of another parameter affecting the star distribution along the branch. Aims. To investigate the variation of the horizontal branch morphology in Galactic globular clusters, we define a new photometric horizontal branch morphology index, overcoming some of the limitations and degeneracies affecting similar indices available in the literature. Methods. We took advantage of a sample of 64 Galactic globular clusters, with both space-based imaging data (Advanced Camera for Surveys survey of Galactic globular clusters) and homogeneous ground-based photometric catalogues in five different bands (U, B, V, R, I). The new index, τHB, is defined as the ratio between the areas subtended by the cumulative number distribution in magnitude (I) and in colour (V − I) of all stars along the horizontal branch. Results. This new index shows a linear trend over the entire range in metallicity (−2.35 ≤ [Fe/H] ≤ −0.12) covered by our Galactic globular cluster sample. We found a linear relation between τHB and absolute cluster ages. We also found a quadratic anti-correlation with [Fe/H], becoming linear when we eliminate the age effect on τHB values. Moreover, we identified a subsample of eight clusters that are peculiar according to their τHB values. These clusters have bluer horizontal branch morphology when compared to typical ones of similar metallicity. These findings allow us to define them as the ’second parameter’ clusters in the sample. A comparison with synthetic horizontal branch models suggests that they cannot be entirely explained with a spread in helium content.

Helium enrichment in intermediate-age Magellanic Clouds clusters: towards an ubiquity of multiple stellar populations?

Intermediate-age star clusters in the Magellanic Clouds harbour signatures of the multiple stellar populations long thought to be restricted to old globular clusters. We compare synthetic horizontal branch models with Hubble Space Telescope photometry of clusters in the Magellanic Clouds, with age between ~2 and ~10 Gyr, namely NGC 121, Lindsay 1, NGC 339, NGC 416, Lindsay 38, Lindsay 113, Hodge 6 and NGC 1978. We find a clear signature of initial helium abundance spreads (delta(Y)) in four out of these eight clusters (NGC 121, Lindsay 1, NGC 339, NGC 416) and we quantify the value of delta(Y). For two clusters (Lindsay 38, Lindsay 113) we can only determine an upper limit for delta(Y), whilst for the two youngest clusters in our sample (Hodge 6 and NGC 1978) no conclusion about the existence of an initial He spread can be reached. Our delta(Y) estimates are consistent with the correlation between maximum He abundance spread and mass of the host cluster found in Galactic globular clusters. This result strengthens the emerging view that the formation of multiple stellar populations is a standard process in massive star clusters, not limited to a high redshift environment.

Explaining the Multiple Populations in Globular Clusters by Multiple Episodes of Star Formation and Enrichment without Gas Expulsion from Massive Star Feedback

Abstract In order to investigate the origin of multiple stellar populations found in globular clusters (GCs) in the halo and bulge of the Milky Way, we have constructed chemical evolution models for their putative low-mass progenitors. In light of recent theoretical developments, we assume that supernova blast waves undergo blowout without expelling the pre-enriched ambient gas, while relatively slow winds of massive stars (WMSs), together with the winds and ejecta from low- to high-mass asymptotic giant branch stars, are all locally retained in these less massive systems. Interestingly, we find that the observed Na–O anti-correlations in metal-poor GCs can be reproduced when multiple episodes of starburst and enrichment are allowed to continue in these subsystems. A specific form of star formation history with decreasing time intervals between the successive stellar generations, however, is required to obtain this result, which is in good agreement with the parameters obtained from synthetic horizontal branch models. The “mass budget problem” is also much alleviated by our models without ad hoc assumptions on star formation efficiency, initial mass function, and the preferential loss of first-generation stars. We also apply these models to investigate the origin of super-He-rich red clump stars in the metal-rich bulge suggested by Lee et al. We find that chemical enrichment by the WMSs can naturally reproduce the required strong He enhancement in metal-rich subsystems. Our results further underscore that gas expulsion or retention is a key factor in understanding the multiple populations in GCs.

COMMON ORIGIN OF TWO RR LYRAE POPULATIONS AND THE DOUBLE RED CLUMP IN THE MILKY WAY BULGE

ABSTRACT A recent survey looking toward the Milky Way bulge has discovered two sequences of RR Lyrae stars on the period–amplitude diagram with a maximum period-shift of P ≈ 0.015 between the two populations. Here we show, from our synthetic horizontal-branch models, that this period-shift is most likely due to the small difference in helium abundance (ΔY = 0.012) between the first- and second-generation stars (G1 and G2), as is the case in our models for the inner halo globular clusters with similar metallicity ([Fe/H] ≈ −1.1). We further show that the observed double red clump (RC) in the bulge is naturally reproduced when these models are extended to solar metallicity following ΔY/ΔZ ≈ 6 for G2, as would be expected from the chemical evolution models. Therefore, the two populations of RR Lyrae stars and the double RC observed in the bulge appear to be different manifestations of the same multiple population phenomenon in the metal-poor and metal-rich regimes, respectively.

TheHubble Space TelescopeUV Legacy Survey of Galactic globular clusters – XI. The horizontal branch in NGC 6388 and NGC 6441

The Hubble Space Telescope UV Legacy survey of Galactic Globular Clusters (GC) is characterising many different aspects of their multiple stellar populations. The "Grundahl-jump" (G-jump) is a discontinuity in ultraviolet brightness of blue horizontal branch (HB) stars, signalling the onset of radiative metal levitation. The HB Legacy data confirmed that the G-jump is located at the same T$_{eff}$ ($\simeq$11,500 K) in nearly all clusters. The only exceptions are the metal-rich clusters NGC 6388 and NGC 6441, where the G-jump occurs at T$_{eff}\simeq$13-14,000K. We compute synthetic HB models based on new evolutionary tracks including the effect of helium diffusion, and approximately accounting for the effect of metal levitation in a stable atmosphere. Our models show that the G-jump location depends on the interplay between the timescale of diffusion and the timescale of the evolution in the T$_{eff}$ range 11,500 K$\lessapprox$T$_{eff}\lessapprox$14,000 K. The G-jump becomes hotter than 11,500 K only for stars that have, in this T$_{eff}$ range, a helium mass fraction Y>0.35. Similarly high Y values are also consistent with the modelling of the HB in NGC 6388 and NGC 6441. In these clusters we predict that a significant fraction of HB stars show helium in their spectra above 11,500 K, and full helium settling should only be found beyond the hotter G-jump.

The horizontal branch of the Sculptor dwarf galaxy

We have performed the first detailed simulation of the horizontal branch of the Sculptor dwarf spheroidal galaxy by means of synthetic modelling techniques,taking consistently into account the star formation history and metallicity evolution as determined from the main sequence and red giant branch spectroscopic observations. The only free parameter in the whole analysis is the integrated mass loss of red giant branch stars. This is the first time that synthetic horizontal branch models, consistent with the complex star formation history of a galaxy, are calculated and matched to the observations. We find that the metallicity range covered by the star formation history, as constrained by observations, plus a simple mass loss law, enable us to cover both the full magnitude and colour range of HB stars. In addition the number count distribution along the observed horizontal branch, can be also reproduced, provided that the red giant branch mass loss is mildly metallicity dependent, with a very small dispersion at fixed metallicity. The magnitude, metallicity and period distribution of the RR Lyrae stars are also well reproduced. There is no excess of bright objects that require enhanced-He models. The lack of signatures of enhanced-He stars along the horizontal branch is consistent with the lack of the O-Na anticorrelation observed in Sculptor and other dwarf galaxies, and confirms the intrinsic difference between Local Group dwarf galaxies and globular cluster populations. We also compare the brightness of the observed red giant branch bump with the synthetic counterpart, and find a discrepancy -- the theoretical bump being brighter -- similar to what is observed in Galactic globular clusters.

ON THE PERIOD DISTRIBUTION OF CLUSTER RR LYRAE STARS TO CONSTRAIN THEIR HELIUM CONTENT: THE CASE OF ω CENTAURI

We present new sets of nonlinear, time-dependent convective hydrodynamical models of RR Lyrae stars assuming two metal (Z = 0.0005, Z = 0.001) and three helium abundances (Y = 0.24, 0.30, 0.38). For each chemical composition, we constructed a grid of fundamental and first overtone models covering a broad range of stellar masses and luminosities. To constrain the impact of the helium content on RR Lyrae properties, we adopted two observables—period distribution and luminosity amplitudes—that are independent of distance and reddening. The current predictions confirm that the helium content has a marginal effect on the pulsation properties. The key parameter causing the difference between canonical and He-enhanced observables is the luminosity. We compared current predictions with the sample of 189 RR Lyrae stars in ω Cen and found that the period range of He-enhanced models is systematically longer than observed. These findings apply to metal-poor and metal-intermediate He-enhanced models. To further constrain the impact of He-enhanced structures on the period distribution, we also computed a series of synthetic horizontal branch (HB) models and found that the predicted period distribution, based on a Gaussian sampling in mass, agrees quite well with observations. This applies not only to the minimum fundamentalized period of RR Lyrae stars (0.39 versus 0.34 days) but also to the fraction of Type II Cepheids (2% versus 3%). We also computed a series of synthetic HB models assuming a mixed HB population in which the 80% is made of canonical HB structures, while the 20% is made of He-enhanced (Y = 0.30) HB structures. We found that the fraction of Type II Cepheids predicted by these models is almost a factor of two larger than observed (5% versus 3%). This indicates that the fraction of He-enhanced structures in ω Cen cannot be larger than 20%.

Horizontal-Branch Morphology and Mass Loss in Globular Clusters

The connection between mass loss on the red giant branch (RGB) and horizontal-branch (HB) morphology in globular clusters (GCs) has long been acknowledged but the mechanisms governing mass loss remain poorly understood from a theoretical perspective. The present study uses synthetic HB models to demonstrate for the first time that α-enhancement and a simple relation between mass loss and metallicity can explain the entire range of HB morphology (characterized by the HB type index) observed in old, coeval GCs. The mass loss-metallicity relation accounts naturally for the fact that the most metal-poor GCs ([Fe/H] < –2) have redder HBs than is typical of GCs with –2 < [Fe/H] < –1.5 without invoking younger ages. These results may prove useful in studying the contribution of HB stars to integrated light via stellar population synthesis.

The ACS Survey of Galactic Globular Clusters. II. Stellar Evolution Tracks, Isochrones, Luminosity Functions, and Synthetic Horizontal-Branch Models

The ACS Survey of Galactic Globular Clusters, an HST Treasury Project, will deliver high quality, homogeneous photometry of 65 globular clusters. This paper introduces a new collection of stellar evolution tracks and isochrones suitable for analyzing the ACS Survey data. Stellar evolution models were computed at [Fe/H]= -2.5, -2.0, -1.5, -1.0, -0.5, and 0; [alpha/Fe]= -0.2, 0, 0.2, 0.4, 0.6, and 0.8; and three initial He abundances for masses from 0.1 to 1.8 Msun and ages from 2 to 15 Gyr. Each isochrone spans a wide range in luminosity from Mv~14 up to the tip of the red giant branch. These are complemented by a set of He-burning tracks that extend from the zero age horizontal branch to the onset of thermal pulsations on the asymptotic giant branch. In addition, a set of computer programs are provided that make it possible to interpolate the isochrones in [Fe/H], generate luminosity functions from the isochrones, and create synthetic horizontal branch models. The tracks and isochrones have been converted to the observational plane with two different color-Teff transformations, one synthetic and one semi-empirical, in ground-based B, V, and I, and F606W and F814W for both ACS-WFC and WFPC2 systems. All models and programs presented in this paper are available from http://stellar.dartmouth.edu/~models/

A Pulsational Distance to ω Centauri Based on Near‐Infrared Period‐Luminosity Relations of RR Lyrae Stars

We present new Near-Infrared (J,K) magnitudes for 114 RR Lyrae stars in the globular cluster Omega Cen (NGC 5139) which we combine with data from the literature to construct a sample of 180 RR Lyrae stars with J and K mean magnitudes on a common photometric system. This is presently the largest such sample in any stellar system. We also present updated predictions for J,K-band Period-Luminosity relations for both fundamental and first-overtone RR Lyrae stars, based on synthetic horizontal branch models with metal abundance ranging from Z=0.0001 to Z=0.004. By adopting for the Omega Cen variables with measured metal abundances an alpha-element enhancement of a factor of 3 (about 0.5 dex) with respect to iron we find a true distance modulus of 13.70 (with a random error of 0.06 and a systematic error of 0.06), corresponding to a distance d=5.5 Kpc (with both random and systematic errors equal to 0.03 Kpc). Our estimate is in excellent agreement with the distance inferred for the eclipsing binary OGLEGC-17, but differ significantly from the recent distance estimates based on cluster dynamics and on high amplitude Delta Scuti stars.

CCD Photometry of the Globular Cluster ω Centauri. I. Metallicity of RR Lyrae Stars from [CLC]Ca[ITAL]by[/ITAL][/CLC] Photometry

We present new measurements of the metallicity of 131 RR Lyrae stars in the globular cluster ω Centauri, using the hk index of the Caby photometric system. The hk method has distinct advantages over ΔS and other techniques in determining the metallicity of RR Lyrae stars and has allowed us to obtain the most complete and homogeneous metallicity data to date for the RR Lyrae stars in this cluster. For RR Lyrae stars in common with the ΔS observations of Butler, Dickens, &Epps and Gratton, Tornambe, &Ortolani, we have found that our metallicities, [Fe/H]hk, deviate systematically from their ΔS metallicity, while our [Fe/H]hk for well-observed, field RRab stars are consistent with previous spectroscopic measurements. We conclude that this is because of the larger errors associated with the previous ΔS observations for this cluster. The MV(RR)-[Fe/H] and period shift–[Fe/H] relations obtained from our new data are consistent with the evolutionary models predicted by Y.-W. Lee, confirming that the luminosity of RR Lyrae stars depends on evolutionary status as well as metallicity. Using the period-amplitude diagram, we have also identified highly evolved RRab stars in the range - 1.9 ≤ [Fe/H] < -1.5, as predicted from the synthetic horizontal-branch models.

The Metallicity Dependence of RR Lyrae Absolute Magnitudes from Synthetic Horizontal-Branch Models

A grid of synthetic horizontal-branch (SHB) models based on horizontal-branch (HB) evolutionary tracks with improved physics has been constructed to reconsider the theoretical calibration of the dependence of Mv(RR) on metallicity in globular clusters and the slope of the mean MV(RR)-[Fe/H] relation. The SHB models confirm Lee's earlier finding that the slope of the MV(RR)-[Fe/H] relation is itself a function of the metallicity range considered and that, in addition, for a given [Fe/H] RR Lyrae luminosities depend on HB morphology. This is due to the fact that HB stars pass through the RR Lyrae instability strip at different evolutionary stages, depending on their original position on the HB. At [Fe/H] = -1.9, and for HB type 0, the models yield MV(RR) = 0.47 ± 0.10. The mean slope for the zero-age HB models is 0.204. Since there is no simple universal relation between Mv(RR) and metallicity that is applicable to all globular clusters, the HB morphology of each individual cluster must be taken into account, in addition to [Fe/H], in deriving the appropriate Mv(RR). Taking HB morphology into account, we find that the slope of the mean MV(RR)-[Fe/H] relation varies between 0.36 for the clusters with Galactocentric distances Rgc less than 6 kpc and 0.22 for clusters with 6 kpc < Rgc ≤ 20 kpc. Implications for interpreting observations of field RR Lyrae variables and for absolute globular cluster ages and Galactic chronology are briefly discussed.

RR Lyrae Luminosity Differences between Oosterhoff Group I and II Cluster Systems and the Origin of the Oosterhoff Dichotomy

We present a comparative study of the Oosterhoff II cluster M2 and the Oosterhoff I cluster M3. Both have similar metallicities, [Fe/H] = -1.62 for M2 and -1.66 for M3, but very different horizontal-branch (HB) morphologies (B-R)/(B + V + R) = 0.92 for M2 and 0.08 for M3. A period shift analysis and main-sequence fitting show that RRab variables in M2 are about 0.2 mag brighter than those in M3. Comparisons of the M2 period shift with Oosterhoff I clusters NGC 3201 and NGC 7006 also yield similar results, while a comparison between M2 and the Oosterhoff II cluster NGC 5986 reveals that the RR Lyrae luminosities are very similar. The luminosity difference is thought to be due to the evolutionary effect described in 1990 by Lee, Demarque, & Zinn: the M2 RRab variables have evolved away from the zero-age horizontal branch (ZAHB), while most M3 RRab variables lie near the ZAHB. A comparison of the mean period change rates of two clusters supports this hypothesis. Our relative age estimation using the difference in color between the base of giant branch and turn-off point shows that M2 is about 2 Gyr older than M3. Our result strongly suggests that the Oosterhoff dichotomy is due to age differences between Oosterhoff group I and II. This is consistent with the idea that the global second parameter is age. We discuss the kinematic differences between Oosterhoff group I and II clusters. Our result shows that the Oosterhoff group I clusters have zero or retrograde rotation with vrot = -68 ± 56 km s-1 and σlos = 131 ± 28 km s-1, while the Oosterhoff group II clusters have prograde rotation with vrot = +94 ± 47 km s-1 and σlos = 115 ± 29 km s-1, confirming a similar conclusion of van den Bergh. The difference in kinematics and ages between Oosterhoff group I and II clusters suggests that they may have different origins: The Oosterhoff II clusters were formed very early in the proto-Galaxy while the Oosterhoff I clusters were formed at different locations and at a later time, and were probably merger events. The period distributions of an unbiased sample of field RRab variables with | Z | ≤ 3 kpc and | Z | ≥ 5 kpc indicate that they may belong to different populations, with peak periods of 0.65 and 0.55 days, respectively. If the hypothesis that the Oosterhoff dichotomy is due to evolution is correct, then this period shift among the field RR Lyrae variables suggests that the RRab population with | Z | ≤ 3 kpc is somewhat older than the RRab population with | Z | ≥ 5 kpc. This also suggests different formation histories. In an appendix, we discuss that the frequently used Gaussian HB mass-dispersion rate (i.e., the mass-loss rate at the red giant branch [RGB] tip) in synthetic HB model calculations cannot fully explain the extended blue HB population and the pulsational properties of RR Lyrae variables in M2. Comparisons with synthetic HB models strongly suggests that an enhanced mass loss is required that extends the HB toward lower HB masses. We also discuss statistical effects on the metallicity estimate using P0,min for field RRab variables reported by Castellani, Maceroni, & Tosi in 1983. Our calculations suggest that the statistical effect is sufficient to explain the apparent gradient in P0,min without introducing a metallicity gradient.

METAL-RICH GLOBULAR CLUSTERS OF THE GALAXY: MORPHOLOGY OF THE HELIUM-BURNING "CLUMP," AND THE DETERMINATION OF RELATIVE AGES THROUGH THE "DELTA-V" METHOD

ABSTRACT Morphological aspects of the V, B-V diagram of metal-rich globular clusters are analyzed, on the basis of stellar evolution models for horizontal-branch (HB) and red giant branch stars. These have been incorporated into detailed synthetic HB models, the influence of differential reddening upon which is also discussed. The synthetic HB models are found to be very clumpy in the color-magnitude diagram, though more extended structures occasionally result due to the effect of evolution away from the zero-age HB. In particular, "sloped" clumps may be naturally expected for sufficiently high helium abundances. Differential reddening is found to have a smaller influence than evolution itself upon the morphology of the clump. The synthetic HB models are also used to study the age difference between metal-rich clusters on the basis of the "Delta-V" method. We emphasize the importance of knowing in advance the relative abundances of helium and metals in order to estimate this age difference quantitatively. We present chemical evolution models that serve to illustrate the effect, and show that, for two clusters of identical [Fe/H] and Delta-V, the one enriched predominantly by ejecta from supernovae type II explosions will appear younger than the other which also suffered enrichment from supernovae type Ia ejecta. The implied locations of the red giant branch feature known as the "bump" are also discussed on the basis of recent observational and theoretical results. Our corresponding predictions should be checked against accurate observational data obtained with the Hubble Space Telescope.

Evolutionary Aspects of the CMD of NGC 6553

We have investigated the V, B – V “clump” morphology of the globular cluster NGC 6553 (Ortolani et al. 1990, OBB90) through synthetic horizontal branch (SHB) models. Catelan's (1993) computations were extended to more metal-rich compositions, following Sweigart (1987) and Castellani et al. (1991), and transposed to the observational plane on the basis of VandenBerg's (1992) colour transformations and bolometric corrections. Observational scatter has also been added. In general, the SHB models are very clumpy, unlike the observed feature, which seems extended and peculiarly tilted. However, for particular combinations of helium abundance, metallicity, and mean mass on the HB, tilted models result, being however significantly less sloped and wider than observed. The NGC 6553 field is differentially reddened by ΔE(B – V) ≈ 0.06 (OBB90), which has been modelled, but which implies a CMD scatter which is smaller than the one originating from evolution away from the zero-age HB alone. We have also investigated the age of the cluster (ΔV method) and location of the red giant branch “bump,” in comparison with 47 Tuc. Since the helium and α-elements abundances are not known for NGC 6553, three chemical evolution scenarios have been considered, following the method of de Freitas Pacheco (1993). Details can be found elsewhere (Catelan et al. 1994).

The horizontal-branch stars in globular clusters. 2: The second parameter phenomenon

view Abstract Citations (441) References (151) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Horizontal-Branch Stars in Globular Clusters. II. The Second Parameter Phenomenon Lee, Young-Wook ; Demarque, Pierre ; Zinn, Robert Abstract Using synthetic horizontal-branch models, we have investigated the origin of the systematic variation in horizontal-branch (HB) morphology with galactocentric distance (RG) among globular clusters. The variations in He abundance, CNO abundance, and core mass required separately to explain this effect are inconsistent with either the observed properties of the RR Lyrae variables or the observed main-sequence turnoffs in the clusters. There is also no clear evidence that the trend with RG is related to the central concentrations, central densities, or absolute magnitudes of the clusters. The variations in cluster age required to explain this effect are not in conflict with any observations. A detailed comparison of our synthetic HB calculations with pairs of clusters of very different HB morphology but similar (Fe/H) reveals reasonably good agreement between the age differences inferred from HB morphology and the main-sequence turnoff. The major source of uncertainty is the need for ad hoc hypotheses in the modeling of the HB morphologies of a few peculiar clusters (e.g., NGC 6752). Nonetheless, there is firm evidence for age variations of several gigayears (as much as approximately 5 Gyr) among the halo globular clusters. Our results support the hypothesis of Searle & Zinn that the inner halo is more uniform in age and is older in the mean than the outer halo, and we estimate this difference to be approximately 2 Gyr. Publication: The Astrophysical Journal Pub Date: March 1994 DOI: 10.1086/173803 Bibcode: 1994ApJ...423..248L Keywords: Globular Clusters; Horizontal Branch Stars; Star Formation; Stellar Models; Abundance; Stellar Composition; Stellar Luminosity; Astrophysics; GALAXY: FORMATION; GALAXY: GLOBULAR CLUSTERS: GENERAL; STARS: EVOLUTION; STARS: HORIZONTAL-BRANCH full text sources ADS | data products SIMBAD (83) Related Materials (1) Part 1: 1990ApJ...350..155L

Age differences between old stellar populations from the HB morphology-metallicity diagram

view Abstract Citations (40) References (48) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Age Difference Between Old Stellar Populations From the HB Morphology- Metallicity Diagram Catelan, M. ; de Freitas Pacheco, J. A. Abstract Isochrones in the horizontal-branch (HB) morphology-metallicity diagram have been constructed on the basis of (1) detailed synthetic HB models, as recently presented by Catelan (1993), and herein extended to competely red and blue HB morphologies; and (2) the red giant branch (RGB) models of Sweigart & Gross (1978). The effect of variations in the several relevant input parameters that characterize the present (standard) isochrones -- namely, main-sequence helium content (YMS), abundances of the alpha-capture elements ((alpha/Fe)), and mean overall mass loss on the RGB (Delta script M) -- has been extensively studied. We find that the age difference which may be estimated between any two (old) stellar populations from their distributions in the HB morphology-(Fe/H) plane, holding YMS, (alpha/Fe), and Delta script M fixed, depends strongly on the absolute values which are assumed for the latter quantities. In other words, unless the absolute values for such parameters can be accurately determined for the objects subject to examination, the HB morphology-(Fe/H) diagram does not constrain age differences in a reliable way. As examples, we have analyzed several following cases. Our isochrones are for fixed YMS, (alpha/Fe), and Delta script M. The identification of the true second parameter that controls HB morphology (besides (Fe/H)) is thus seen to depend on the accurate determination of cluster-to-cluster differences in (at least) the three parameters listed above, whereas the magnitude of the problem itself depends on the absolute values of those parameters which may have been excluded as second-parameter candidates. We also find that similar age differences between any two populations, as obtained for different (YMS, (alpha/Fe), Delta script M) combinations, imply similar absolute ages for these populations. On the basis of this result, we show that the classical second-parameter pair NGC 288-NGC 362 can only be explained in terms of an age difference as small as approximately equal to 3 Gyr, if both clusters are younger than approximately 10 Gyr. This follows irrespective of the (YMS, (alpha/Fe), Delta script M) combination chosen which is capable of reproducing the reported age difference of 3 Gyr between these two clusters. The far-reaching implications of this result must clearly be further explored. Publication: The Astronomical Journal Pub Date: November 1993 DOI: 10.1086/116771 Bibcode: 1993AJ....106.1858C Keywords: Abundance; Age Factor; Astronomical Catalogs; Chronology; Globular Clusters; Horizontal Branch Stars; Metallicity; Morphology; Stellar Models; Galactic Bulge; Iron; Oxygen; Ratios; Red Giant Stars; Astronomy; GALAXY: EVOLUTION; STARS: POPULATION II; GALAXY: FORMATION full text sources ADS | data products SIMBAD (3)

Analysis of evolutionary period-shift predictions for RR Lyrae stars in Galactic globular clusters

view Abstract Citations (2) References (11) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Analysis of Evolutionary Period-Shift Predictions for RR Lyrae Stars in Galactic Globular Clusters Catelan, M. ; de Freitas Pacheco, J. A. Abstract On the basis of detailed synthetic horizontal-branch (SHB) models, we analyze how different choices of HB evolutionary tracks can affect the predicted period shifts for the RR Lyrae stars in Galactic globular clusters. Previous suggestions that choosing Sweigart's (1987) models leads to a decrease in the predicted period shifts between Oosterhoff type I and II clusters with respect to those obtained through use of the Lee and Demarque (1990) tracks are confirmed and quantified. For OoII clusters with not-too-blue HB morphologies, we find that the increase in predicted period-shift values which results from the inclusion of the final 5-10 percent of HB evolution in the evolutionary tracks is clearly not sufficient to overcome the decrease which results from the adoption of the former tracks instead of the latter. The choice of Sweigart's models leads to a smaller slope for the evolutionary Delta log P(T/eff/)-(Fe/H) relation by about 0.010 if the mean luminosity level of the RR Lyrae variables in the SHB models are employed as a reference for reading the period values, and by about 0.017 if the lower envelopes of the corresponding distributions are used instead - thus further complicating the explanation of the Sandage period-shift effect. Publication: The Astronomical Journal Pub Date: July 1993 DOI: 10.1086/116627 Bibcode: 1993AJ....106..175C Keywords: Galactic Clusters; Globular Clusters; Horizontal Branch Stars; Lyra Constellation; Stellar Evolution; Milky Way Galaxy; Morphology; Stellar Models; Astrophysics; GLOBULAR CLUSTERS: GENERAL; STARS: VARIABLES: RR LYRAE full text sources ADS | data products SIMBAD (8)