Using Hubble Space Telescope Advanced Camera for Surveys (HST/ACS) and Wide Field Camera 3 (WFC3) observations from the Panchromatic Hubble Andromeda Treasury (PHAT), we present new period–luminosity ...(P-L) relations for Cepheid variables in M31. Cepheids from several ground-based studies are identified in the PHAT photometry to derive new P-L and Wesenheit P-L relations in the near infrared and visual filters. We derive a distance modulus to M31 of 24.51 ± 0.08 in the IR bands and 24.32 ± 0.09 in the visual bands, including the first P-L relations in the F475W and F814W filters for M31. Our derived visual and IR distance moduli disagree at slightly more than a 1σ level. Differences in the P-L relations between ground-based and HST observations are investigated for a subset of Cepheids. We find a significant discrepancy between ground-based and HST P-L relations with the same Cepheids, suggesting adverse effects from photometric contamination in ground-based Cepheid observations. Additionally, a statistically significant radial trend in the P-L relation is found which does not appear to be explained by metallicity.
In the present work we analyzed seven globular clusters (GCs) selected from their location in the Galactic bulge and with metallicity values in the range −1.30 Fe/H −0.50. The aim of this work is ...first to derive cluster ages assuming single stellar populations and second to identify the stars from first (1G) and second generations (2G) from the main sequence, subgiant, and red giant branches, and to derive their age differences. Based on a combination of UV and optical filters used in this project, we apply the Gaussian mixture models to distinguish the multiple stellar populations. Applying statistical isochrone fitting, we derive self-consistent ages, distances, metallicities, and reddening values for the sample clusters. An average age of 12.3 0.4 Gyr was obtained both using DSED and BaSTI (accounting atomic diffusion effects) isochrones, without a clear distinction between the moderately metal-poor and the more metal-rich bulge clusters, except for NGC 6717 and the inner halo NGC 6362 with ∼13.5 Gyr. We derived a weighted mean age difference between the multiple populations hosted by each GC of 41 170 Myr adopting canonical He abundances; whereas for higher He in 2G stars, this difference reduces to 17 170 Myr, but with individual uncertainties of 500 Myr.
The current generation of stellar isochrone models exhibits non-negligible discrepancies due to variations in the input physics. The success of each model is determined by how well it fits the ...observations, and this paper aims to disentangle contributions from the various physical inputs. New deep, wide-field optical and near-infrared photometry (UBVRIJHKS ) of the cluster M35 is presented, against which several isochrone systems are compared: Padova, PARSEC, Dartmouth, and Y super(2). Two different atmosphere models are applied to each isochrone: ATLAS9 and BT-Settl. For any isochrone set and atmosphere model, observed data are accurately reproduced for all stars more massive than 0.7 M sub(middot in circle). For stars less massive than 0.7 M sub(middot in circle), Padova and PARSEC isochrones consistently produce higher temperatures than observed. Dartmouth and Y super(2) isochrones with BT-Settl atmospheres reproduce optical data accurately; however, they appear too blue in IR colors. It is speculated that molecular contributions to stellar spectra in the near-infrared may not be fully explored, and that future study may reconcile these differences.
The UV-initiative Hubble Space Telescope Treasury survey of Galactic globular clusters provides a new window into the phenomena that shape the morphological features of the horizontal branch (HB). ...Using this large and homogeneous catalog of UV and blue photometry, we demonstrate that the HB exhibits discontinuities that are remarkably consistent in color (effective temperature). This consistency is apparent even among some of the most massive clusters hosting multiple distinct sub-populations (such as NGC 2808, omega Cen, and NGC 6715), demonstrating that these phenomena are primarily driven by atmospheric physics that is independent of the underlying population properties. However, inconsistencies arise in the metal-rich clusters NGC 6388 and NGC 6441, where the discontinuity within the blue HB (BHB) distribution shifts ~1000-2000 K hotter. We demonstrate that this shift is likely due to a large helium enhancement in the BHB stars of these clusters, which in turn affects the surface convection and evolution of such stars. Our survey also increases the number of Galactic globular clusters known to host blue-hook stars (also known as late hot flashers) from 6 to 23 clusters. These clusters are biased toward the bright end of the globular cluster luminosity function, confirming that blue-hook stars tend to form in the most massive clusters with significant self-enrichment.
Abstract
A number of scenarios for the formation of multiple populations in globular clusters (GCs) predict that second generation (2G) stars form in a compact and dense subsystem embedded in a more ...extended first-generation (1G) system. If these scenarios are accurate, a consequence of the denser 2G formation environment is that 2G binaries should be more significantly affected by stellar interactions and disrupted at a larger rate than 1G binaries. The fractions and properties of binary stars can thus provide a dynamical fingerprint of the formation epoch of multiple-population GCs and their subsequent dynamical evolution. We investigate the connection between binaries and multiple populations in five GCs, NGC 288, NGC 6121 (M 4), NGC 6352, NGC 6362, and NGC 6838 (M 71). To do this, we introduce a new method based on the comparison of Hubble Space Telescope observations of binaries in the F275W, F336W, F438W, F606W, and F814W filters with a large number of simulated binaries. In the inner regions probed by our data, we do not find large differences between the local 1G and the 2G binary incidences in four of the studied clusters, the only exception being M 4 where the 1G binary incidence is about three times larger than the 2G incidence. The results found are in general agreement with the results of simulations predicting significant differences in the global 1G and 2G incidences and in the local values in the clusters’ outer regions but similar incidences in the inner regions. The significant difference found in M 4 is consistent with simulations with a larger fraction of wider binaries. Our analysis also provides the first evidence of mixed (1G–2G) binaries, a population predicted by numerical simulations to form in a cluster’s inner regions as a result of stellar encounters during which one component of a binary is replaced by a star of a different population.
ABSTRACT We develop a Bayesian model for globular clusters composed of multiple stellar populations, extending earlier statistical models for open clusters composed of simple (single) stellar ...populations. Specifically, we model globular clusters with two populations that differ in helium abundance. Our model assumes a hierarchical structuring of the parameters in which physical properties-age, metallicity, helium abundance, distance, absorption, and initial mass-are common to (i) the cluster as a whole or to (ii) individual populations within a cluster, or are unique to (iii) individual stars. An adaptive Markov chain Monte Carlo (MCMC) algorithm is devised for model fitting that greatly improves convergence relative to its precursor non-adaptive MCMC algorithm. Our model and computational tools are incorporated into an open-source software suite known as BASE-9. We use numerical studies to demonstrate that our method can recover parameters of two-population clusters, and also show how model misspecification can potentially be identified. As a proof of concept, we analyze the two stellar populations of globular cluster NGC 5272 using our model and methods. (BASE-9 is available from GitHub: https://github.com/argiopetech/base/releases).
NGC 6388 and NGC 6441 are two massive Galactic bulge globular clusters that share many properties, including the presence of an extended horizontal branch (HB), quite unexpected because of their high ...metal content. In this paper we use Hubble Space Telescope's WFPC2, ACS, and WFC3 images and present a broad multicolor study of their stellar content, covering all main evolutionary branches. The color-magnitude diagrams (CMDs) give compelling evidence that both clusters host at least two stellar populations, which manifest themselves in different ways. NGC 6388 has a broadened main sequence (MS), a split sub-giant branch (SGB), and a split red giant branch (RGB) that becomes evident above the HB in our data set; its red HB is also split into two branches. NGC 6441 has a split MS, but only an indication of two SGB populations, while the RGB clearly splits in two from the SGB level upward, and no red HB structure. The multicolor analysis of the CMDs confirms that the He difference between the two main stellar populations in the two clusters must be similar. This is observationally supported by the HB morphology, but also confirmed by the color distribution of the stars in the MS optical band CMDs. However, a MS split becomes evident in NGC 6441 using UV colors, but not in NGC 6388, indicating that the chemical patterns of the different populations are different in the two clusters, with C, N, and O abundance differences likely playing a major role. We also analyze the radial distribution of the two populations.
Hubble Space Telescope (HST) fine guidance sensor observations were used to obtain parallaxes of eight metal-poor (Fe/H < −1.4) stars. The parallaxes of these stars determined by the new Hipparcos ...reduction average 17% accuracy, in contrast to our new HST parallaxes, which average 1% accuracy and have errors on the individual parallaxes ranging from 85 to 144 as. These parallax data were combined with HST Advanced Camera for Surveys photometry in the F606W and F814W filters to obtain the absolute magnitudes of the stars with an accuracy of 0.02-0.03 mag. Six of these stars are on the main sequence (MS) (with −2.7 < Fe/H < −1.8) and are suitable for testing metal-poor stellar evolution models and determining the distances to metal-poor globular clusters (GCs). Using the abundances obtained by O'Malley et al., we find that standard stellar models using the VandenBerg & Clem color transformation do a reasonable job of matching five of the MS stars, with HD 54639 (Fe/H = −2.5) being anomalous in its location in the color-magnitude diagram. Stellar models and isochrones were generated using a Monte Carlo analysis to take into account uncertainties in the models. Isochrones that fit the parallax stars were used to determine the distances and ages of nine GCs (with −2.4 ≤ Fe/H ≤ −1.9). Averaging together the age of all nine clusters led to an absolute age of the oldest, most metal-poor GCs of 12.7 1.0 Gyr, where the quoted uncertainty takes into account the known uncertainties in the stellar models and isochrones, along with the uncertainty in the distance and reddening of the clusters.
We use Cycle 21 Hubble Space Telescope (HST) observations and HST archival ACS Treasury observations of 30 Galactic globular clusters to characterize two distinct stellar populations. A sophisticated ...Bayesian technique is employed to simultaneously sample the joint posterior distribution of age, distance, and extinction for each cluster, as well as unique helium values for two populations within each cluster and the relative proportion of those populations. We find the helium differences among the two populations in the clusters fall in the range of ~0.04 to 0.11. Because adequate models varying in carbon, nitrogen, and oxygen are not presently available, we view these spreads as upper limits and present them with statistical rather than observational uncertainties. Evidence supports previous studies suggesting an increase in helium content concurrent with increasing mass of the cluster and we also find that the proportion of the first population of stars increases with mass as well. Our results are examined in the context of proposed globular cluster formation scenarios. Additionally, we leverage our Bayesian technique to shed light on the inconsistencies between the theoretical models and the observed data.
We used deep observations collected with Advanced Camera for Surveys (ACS) at Hubble Space Telescope (HST) to derive the fraction of binary systems in a sample of 13 low-density Galactic globular ...clusters. By analysing the colour distribution of main-sequence stars we derived the minimum fraction of binary systems required to reproduce the observed colour–magnitude diagram morphologies. We found that all the analysed globular clusters contain a minimum binary fraction larger than 6 per cent within the core radius. The estimated global fractions of binary systems range from 10 to 50 per cent depending on the cluster. A dependence of the relative fraction of binary systems on the cluster age has been detected, suggesting that the binary disruption process within the cluster core is active and can significantly reduce the binary content in time.
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