This study is aimed at investigating the period-luminosity relation of SX Phoenicis (SX Phe) pulsators in Galactic globular clusters (GGCs) and Local Group dwarf galaxies. We verify isochrone-fitting ...distances of 46 GGCs by fitting their main sequences to a carefully chosen set of nearby, unevolved subdwarfs. We find that the difference between cluster distances obtained via isochrone fitting by Dotter et al. and those resulting from our subdwarf fits has a mean of 0.094 mag and a standard deviation of 0.098 mag. The cluster distances from Dotter et al. are used to calibrate an SX Phe period--luminosity relation based on radial double mode pulsators. The resulting empirical period-luminosity relation, which is insensitive to the inclusion of colour and/or metallicity terms, generally agrees well with previous empirical relations as well as theoretical predictions based on single-star pulsational and evolutionary models. However, there is a subset of 'subluminous' variables identified most notably in Fornax, Carina, NGC 2419 and Omega Centauri. We explore the possibility that, at least in GGCs, they represent blue stragglers which have enhanced helium content that was either inherited from second-generation progenitors or gained as a result of the blue straggler formation process.
ABSTRACT
RR Lyrae stars have long been popular standard candles, but significant advances in methodology and technology have been made in recent years to increase their precision as distance ...indicators. We present multiwavelength (optical UBVRcIc and Gaia G, BP, RP; near-infrared JHKs; mid-infrared 3.6, 4.5) period–luminosity–metallicity (PLZ), period–Wesenheit–metallicity (PWZ) relations, calibrated using photometry obtained from the Carnegie RR Lyrae Program and parallaxes from the Gaia second data release for 55 Galactic field RR Lyrae stars. The metallicity slope, which has long been predicted by theoretical relations, can now be measured in all passbands. The scatter in the PLZ relations is on the order of 0.2 mag, and is still dominated by uncertainties in the parallaxes. As a consistency check of our PLZ relations, we also measure the distance modulus to the globular cluster M4, the Large Magellanic Cloud and the Small Magellanic Cloud, and our results are in excellent agreement with estimates from previous studies.
ABSTRACT
We present the first internally consistent set of Fe/H values for 14 of the 15 old globular clusters in the Large Magellanic Cloud (LMC). We make use of periods and amplitudes of fundamental ...mode (ab-type) RR Lyrae stars in the vicinity of these LMC clusters, mostly taken from the Optical Gravitational Lensing Experiment (OGLE) data base, in order to calculate metallicities for the clusters. Our data set contains a total of 417 ab-type cluster RR Lyrae stars (289 from OGLE and the rest from the literature) and yields a set of homogeneous metallicities for 14 old LMC globular clusters all on the Zinn and West scale. No previously published work contains metallicities for as many of the old LMC globular clusters as presented herein. An analysis of a subset of our abundances compared with seven other metallicity studies in the peer-reviewed literature indicates very little overall agreement between these Fe/H values suggesting that significantly more effort needs to be devoted to measuring internally consistent Fe/Hs for the old LMC globular clusters.
ABSTRACT
We present a sample of 56 RR Lyrae variable stars (48 ab-type and 8 c-type) measured from a set of F475W (≈B filter) and F814W (≈I filter) archival images taken with the Hubble Space ...Telescope/Advanced Camera for Surveys of one field in the Local Group dwarf galaxy LGS-3 (also known as the Pisces Dwarf or Pisces I). The ab-type variables exhibit a mean period of =0.623 ± 0.007 d. Applying a relationship between Fe/H and period, we calculate a mean metallicity of 〈Fe/H〉 = −1.81 ± 0.04 on the Zinn & West scale. Finally, we conclude that the mean metal abundance of the RR Lyraes in LGS-3 is consistent with that of the LGS-3 field stellar population at an age of >10 Gyr.
ABSTRACT
We present an analysis of 50 F475W (≈B filter) and F814W (≈I filter) archival images taken with the Hubble Space Telescope (HST)/Advanced Camera for Surveys of one field in the isolated ...Local Group dwarf galaxy Wolf–Lundmark–Melotte (WLM). From point spread function fitting photometry of these images, we construct a colour–magnitude diagram and identify and characterize 90 RR Lyrae variables (76 ab-type and 14 c-type) determining their mean magnitudes, colours, periods, and amplitudes in the HST passbands. We have also uncovered 79 Cepheid-type variables, but because of the short time window of our observations (1.85 d), we are not confident in their period determinations. The mean period of the ab-type RR Lyraes is found to be 〈Pab〉 = 0.609 ± 0.058 (ran) corresponding to a mean metallicity of 〈Fe/H〉 = −1.74 ± 0.02 (ran) on the Carretta et al. scale. We also present a re-analysis of the Hodge et al. colour–magnitude diagram of the WLM globular cluster and conclude that it is much more metal-poor than Hodge et al. quote. Based on a comparison of the WLM RR Lyraes with those in the other seven ‘isolated’ dwarf galaxies of the Local Group (i.e. Leo A, LGS 3, Cetus, IC 1613, Aquarius, Tucana, and Phoenix), we conclude that there was one unique epoch of early star formation common to all of these isolated dwarfs.
The horizontal branch (HB) morphology of globular clusters (GCs) is most strongly influenced by metallicity. The second parameter phenomenon, first described in the 1960s, acknowledges that ...metallicity alone is not enough to describe the HB morphology of all GCs. In particular, astronomers noticed that the outer Galactic halo contains GCs with redder HBs at a given metallicity than are found inside the solar circle. Thus, at least a second parameter was required to characterize HB morphology. While the term 'second parameter' has since come to be used in a broader context, its identity with respect to the original problem has not been conclusively determined. Here we analyze the median color difference between the HB and the red giant branch, hereafter denoted as Delta *D(V - I), measured from Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) photometry of 60 GCs within ~20 kpc of the Galactic center. Analysis of this homogeneous data set reveals that, after the influence of metallicity has been removed from the data, the correlation between Delta *D(V - I) and age is stronger than that of any other parameter considered. Expanding the sample to include HST ACS and Wide Field Planetary Camera 2 photometry of the six most distant Galactic GCs lends additional support to the correlation between Delta *D(V - I) and age. This result is robust with respect to the adopted metallicity scale and the method of age determination, but must bear the caveat that high-quality, detailed abundance information is not available for a significant fraction of the sample. Furthermore, when a subset of GCs with similar metallicities and ages is considered, a correlation between Delta *D(V - I) and central luminosity density is exposed. With respect to the existence of GCs with anomalously red HBs at a given metallicity, we conclude that age is the second parameter and central density is most likely the third. Important problems related to HB morphology in GCs, notably multi-modal distributions and faint blue tails, remain to be explained.
Abstract
A detailed analysis of ground-based CCD
UBV
photometry and space-based Gaia Data Release 3 (DR3) data for the open clusters King 6 and NGC 1605 was performed. Using the
pyUPMASK
algorithm on ...Gaia astrometric data to estimate cluster membership probabilities, we have identified 112 stars in King 6 and 160 stars in NGC 1605 as the statistically most likely members of each cluster. We calculated reddening and metallicity separately using
UBV
two-color diagrams to estimate parameter values via independent methods. The color excess
E
(
B
−
V
) and photometric metallicity Fe/H for King 6 are 0.515 ± 0.030 mag and 0.02 ± 0.20 dex, respectively. For NGC 1605, they are 0.840 ± 0.054 mag and 0.01 ± 0.20 dex, respectively. With reddening and metallicity kept constant, we have estimated the distances and cluster ages by fitting PARSEC isochrones to color–magnitude diagrams based on the Gaia and
UBV
data. The photometric distances are 723 ± 34 pc for King 6 and 3054 ± 243 pc for NGC 1605. The cluster ages are 200 ± 20 Myr and 400 ± 50 Myr for King 6 and NGC 1605, respectively. The mass function slopes were found to be 1.29 ± 0.18 and 1.63 ± 0.36 for King 6 and NGC 1605, respectively. These values are in good agreement with the value of Salpeter. The relaxation times were estimated as 5.8 Myr for King 6 and 60 Myr for NGC 1605. These indicate that both clusters are dynamically relaxed since these times are less than the estimated cluster ages. A Galactic orbit analysis shows that both clusters formed outside the solar circle and are members of the young thin-disk population.
ABSTRACT
We present a purely differential line-of-sight distance between M31 and M33 using ab-type RR Lyrae variables observed in each galaxy by the Hubble Space Telescope Advanced Camera for Surveys ...in the F606W filter. Using 1481 RR Lyraes in 13 M31 fields and 181 RR Lyraes in 6 M33 fields, and placing all of these stars on a uniform photometric scale with internally consistent corrections for metal abundance and extinction, we find a relative absolute distance modulus of Δ(m − M)o = –0.298 ± 0.016 in the sense of (m − M)o, M31 – (m − M)o, M33. Adopting an absolute distance modulus of (m− M)o = 24.46 ± 0.10 for M31 places M33 115 kpc beyond M31 in line-of-sight distance.
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; a/Fe = -0.2, ,0.,0.,0.6, and 0.8; and three initial He abundances for masses from 0.1 to 1.8 M 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 semiempirical, 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 at the Dartmouth Stellar Evolution Database and the Multimission Archive at the Space Telescope Science Institute.
Abstract
Our understanding of the kinematic properties of multiple stellar populations (mPOPs) in Galactic globular clusters (GCs) is still limited compared to what we know about their chemical and ...photometric characteristics. Such limitation arises from the lack of a comprehensive observational investigation of this topic. Here we present the first homogeneous kinematic analysis of mPOPs in 56 GCs based on high-precision proper motions computed with Hubble Space Telescope data. We focused on red-giant-branch stars, for which the mPOP tagging is clearer, and measured the velocity dispersion of stars belonging to first (1G) and second generations (2G). We find that 1G stars are generally kinematically isotropic even at the half-light radius, whereas 2G stars are isotropic at the center and become radially anisotropic before the half-light radius. The radial anisotropy is induced by a lower tangential velocity dispersion of 2G stars with respect to the 1G population, while the radial component of the motion is comparable. We also show possible evidence that the kinematic properties of mPOPs are affected by the Galactic tidal field, corroborating previous observational and theoretical results suggesting a relation between the strength of the external tidal field and some properties of mPOPs. Although limited to the GCs’ central regions, our analysis leads to new insights into the mPOP phenomenon, and provides the motivation for future observational studies of the internal kinematics of mPOPs.
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