Isochrone fitting of Galactic globular clusters – IV. NGC 6362 and NGC 6723

We present new isochrone fits to colour–magnitude diagrams of the Galactic globular clusters NGC 288, NGC 362, and NGC 6218 (M12). We utilize a lot of photometric bands from the ultraviolet to mid-infrared by use of data from the HST, Gaia, unWISE, Pan-STARRS, and other photometric sources. In our isochrone fitting, we use theoretical models and isochrones from the Dartmouth Stellar Evolution Program and Bag of Stellar Tracks and Isochrones for α-enhanced abundance [α/Fe] = +0.40, different helium abundances, and a metallicity of about [Fe/H] = −1.3 adopted from the literature. We derive the most probable distances 8.96 ± 0.05, 8.98 ± 0.06, and 5.04 ± 0.05 kpc, ages 13.5 ± 1.1, 11.0 ± 0.6, and 13.8 ± 1.1 Gyr, extinctions AV = 0.08 ± 0.03, 0.11 ± 0.04, and 0.63 ± 0.03 mag, and reddenings E(B − V) = 0.014 ± 0.010, 0.028 ± 0.011, and 0.189 ± 0.010 mag for NGC 288, NGC 362, and NGC 6218, respectively. The distance estimates from the different models are consistent, while those of age, extinction, and reddening are not. The uncertainties of age, extinction, and reddening are dominated by some intrinsic systematic differences between the models. However, the models agree in their relative age estimates: NGC 362 is 2.6 ± 0.5 Gyr younger than NGC 288 and 2.8 ± 0.5 Gyr younger than NGC 6218, confirming age as the second parameter for these clusters. We provide reliable lists of the cluster members and precise cluster proper motions from the Gaia Early Data Release 3.

We present a pilot study of Galactic globular cluster (GC) proper motion (PM) determinations using Gaia data. We search for GC stars in the Tycho-Gaia Astrometric Solution (TGAS) catalog from Gaia Data Release 1 (DR1), and identify five members of NGC 104 (47 Tucanae), one member of NGC 5272 (M3), five members of NGC 6121 (M4), seven members of NGC 6397, and two members of NGC 6656 (M22). By taking a weighted average of member stars, fully accounting for the correlations between parameters, we estimate the parallax (and, hence, distance) and PM of the GCs. This provides a homogeneous PM study of multiple GCs based on an astrometric catalog with small and well-controlled systematic errors and yields random PM errors similar to existing measurements. Detailed comparison to the available Hubble Space Telescope (HST) measurements generally shows excellent agreement, validating the astrometric quality of both TGAS and HST. By contrast, comparison to ground-based measurements shows that some of those must have systematic errors exceeding the random errors. Our parallax estimates have uncertainties an order of magnitude larger than previous studies, but nevertheless imply distances consistent with previous estimates. By combining our PM measurements with literature positions, distances, and radial velocities, we measure Galactocentric space motions for the clusters and find that these also agree well with previous analyses. Our analysis provides a framework for determining more accurate distances and PMs of Galactic GCs using future Gaia data releases. This will provide crucial constraints on the near end of the cosmic distance ladder and provide accurate GC orbital histories.

Dual supermassive black holes (SMBHs) at ∼kiloparsec scales are the progenitor population of SMBH mergers and play an important role in understanding the pairing and dynamical evolution of massive black holes in galaxy mergers. Because of the stringent resolution requirement and the apparent rareness of these small-separation pairs, there are scarce observational constraints on this population, with few confirmed dual SMBHs at <10 kpc separations at z > 1. Here we present results from a pilot search for kiloparsec-scale dual quasars selected with Gaia Data release 2 (DR2) astrometry and followed up with Hubble Space Telescope (HST) Wide Field Camera 3 dual-band (F475W and F814W) snapshot imaging. Our targets are quasars primarily selected with the varstrometry technique, i.e., light centroid jitter caused by asynchronous variability from both members in an unresolved quasar pair, supplemented by subarcsecond pairs already resolved by Gaia DR2. We find an overall high fraction of HST-resolved pairs among the varstrometry-selected quasars (unresolved in Gaia DR2), ∼30%–50%, increasing toward high redshift (∼60%–80% at z > 1.5). We discuss the nature of the 45 resolved subarcsecond pairs based on HST and supplementary data. A substantial fraction (∼40%) of these pairs are likely physical quasar pairs or gravitationally lensed quasars. We also discover a triple quasar candidate and a quadruply lensed quasar, which is among the smallest-separation quadruple lenses. These results provide important guidelines to improve varstrometry selection and follow-up confirmation of ~kiloparsec-scale dual SMBHs at high redshift.

In an effort to bring more data to bear on the problem of the age and formation history of the inner regions of the Milky Way, I undertook a photometric and spectroscopic study of a small sample of globular clusters within 5 kpc of the Galactic center: NGC~6723, NGC~6352 and NGC~5927. The ages and chemical compositions of these clusters provide clues to the formation and chemical enrichment timescale of the Galactic (thick) disk and inner halo. Deep color-magnitude diagrams (CMDs) were constructed for the clusters from data obtained both from the ground and from the Hubble Space Telescope (HST). A total of 104 UBV frames of two fields in NGC~6723 were obtained during three separate runs at CTIO in 1989 and 1993. Twenty-eight images of NGC~5927 in filters F555W and F814W (similar to Johnson VI) were obtained during Cycle 4 with HST. All photometry of these images was done using DAOPHOT II and ALLSTAR (Stetson 1987, PASP, 99, 191 and updates) following standard reduction procedures. A self-consistent photometric reduction of all 110 NGC 6723 data frames was performed using ALLFRAME (Stetson 1994, PASP, 106, 260). NGC 6352 was observed during Cycle~2 with HST. Four exposures in each of two filters (F555W and F785LP) were obtained. Because these images suffered from the effects of spherical aberration in the telescope's primary mirror, a new flux-conserving deconvolution procedure was developed to allow accurate photometry for this data set. Ages were inferred from the magnitude differences between the clusters' horizontal branches (HBs) and main-sequence turnoffs (MSTOs) in the CMDs. A comparison between the age thus derived for NGC 6723 and that derived using an independent age estimator, the color difference between the MSTO and the base of the subgiant branch, indicated that the two methods produced similar relative ages for NGC~6723 with respect to other clusters. Three red giants in NGC~6723 were observed with the CTIO 4m echelle spectrograph in 1994. Usable wavelength coverage was from 5000-7800A. The spectra were extracted in the normal manner using standard IRAF routines. Using photometrically derived stellar parameters and preliminary metallicity estimates for the cluster, model atmospheres were computed using ATLAS9 and abundances were derived using WIDTH9 (Kurucz 1993, Model Atmospheres CD-ROM). Mean derived [Fe/H] and [alpha/Fe] ratios from all three stars were [Fe/H] = -1.26 ± 0.09~dex, [Si/Fe] = +0.68 ± 0.13~dex, [Ca/Fe] = +0.33 ± 0.13 dex, and [Ti/Fe] = +0.24 ± 0.15~dex. The inner halo cluster NGC 6723 was originally chosen for study because its estimated metallicity ([Fe/H] = - 1.09, Zinn & West 1984, ApJS, 55, 45) and richness in RR Lyraes made it a good "template" cluster for the Baade's Window (BW) RR Lyraes. However, this analysis showed that NGC 6723 is more metal-poor than previously thought, in keeping with its blue horizontal branch morphology. The lower metallicity weakens the association of NGC 6723 with the bulge RR Lyraes. The latter were used by Lee (1992, AJ, 104, 1780) to infer that bulge formation began 1-2 Gyr before formation of the outer halo. My age derivation demonstrated that NGC 6723 is comparable in age to other (old halo) globular clusters that have similar metallicities. The age differences between NGC~6723 and the oldest, most metal-poor globulars suggest that if NGC~6723 is representative, formation of the inner halo began between 1-3 Gyr later than the onset of halo formation, but occurred more or less simultaneously with the bulk of halo star formation. The ages determined for the metal-rich (thick) disk globular clusters NGC 5927 and NGC 6352 increase by half again the number of disk globulars for which accurate ages are known. NGC 6352 ([Fe/H] = -0.63 ± 0.04) was found to be comparable in age to other disk globulars with known ages. If these clusters are representative, then it appears that the thick disk began to form not long after the halo, in agreement with previous findings of Carney et al. (1990, AJ, 99, 572) and Marquez & Schuster (1994, A&AS, 108, 341). The derived age of NGC 5927 ([Fe/H] = -0.24 ± 0.06) depends critically on the adopted helium abundance, and the helium to hydrogen ratio in metal-rich clusters may be higher than in metal-poor clusters. Buzzoni et al. (1983, A&A, 128, 94) derived formulae that predict values of the helium fraction, Y, given R, the ratio of the number of horizontal branch (HB) stars to the number of red giant branch (RGB) stars. From the CMD, I derived R = 2.0 ± 0.5, which corresponds to Y = 0.29 ± 0.04 using equation 11 of Buzzoni et al. This helium abundance was used with appropriate stellar isochrones to derive an age 2--3 Gyr younger than the other disk globulars. Finally, the age of the bulge was derived using values from the literature for the Baade's Window MSTO magnitude, metallicity, helium abundance and distance. As far as possible, the age was derived in a manner fully consistent with those of the globular clusters. The derived "mean" bulge age was ~4 Gyr younger than the age of the typical disk globular. However, the large uncertainty in the magnitude of the bulge MSTO translated directly into a 5 Gyr uncertainty in the age difference, rendering its significance dubious.

We present a Bayesian method to cross-match 5,827,988 high proper-motion Gaia sources (μ > 40 mas yr−1) to various photometric surveys: Two Micron All Sky Survey, AllWISE data release from the Wide-field Infrared Explorer (WISE) mission, Galaxy Evolution Explorer, Radial Velocity Experiment, Sloan Digital Sky Survey, and Panoramic Survey Telescope and Rapid Response System (Pan-STARRS). To efficiently associate these objects across catalogs, we develop a technique that compares the multidimensional distribution of all sources in the vicinity of each Gaia star to a reference distribution of random field stars obtained by extracting all sources in a region on the sky displaced 2′. This offset preserves the local field stellar density and magnitude distribution, allowing us to characterize the frequency of chance alignments. The resulting catalog with Bayesian probabilities >95% has a marginally higher match rate than current internal Gaia data release 2 (DR2) matches for most catalogs. However, a significant improvement is found with Pan-STARRS, where ∼99.8% of the sample within the Pan-STARRS footprint is recovered, as compared to a low ∼20.8% in Gaia DR2. Using these results, we train a Gaussian process regressor to calibrate two photometric metallicity relationships. For dwarfs of 3500 < T eff < 5280 K, we use metallicity values of 4378 stars from the Apache Point Observatory Galactic Evolution Experiment and Hejazi et al. to calibrate the relationship, producing results with a 1σ precision of 0.12 dex and few systematic errors. We then indirectly infer the metallicity of 4018 stars with 2850 < T eff < 3500 K, which are wide companions of primaries whose metallicities are estimated with our first regressor, to produce a relationship with a 1σ precision of 0.21 dex and significant systematic errors. Additional work is needed to better remove unresolved binaries from this second sample to reduce these systematic errors.

A deep understanding of the Milky Way galaxy, its formation and evolution requires observations of huge numbers of stars. Stellar photometry, therefore, provides an economical method to obtain intrinsic stellar parameters. With the addition of distance information – a prospect made real for more than a billion stars with the second Gaia data release – deriving reliable ages from photometry is a possibility. We have developed a Bayesian method that generates 2D probability maps of a star’s age and metallicity from photometry and parallax using isochrones. Our synthetic tests show that including a near-UV passband enables us to break the degeneracy between a star’s age and metallicity for certain evolutionary stages. It is possible to find well-constrained ages and metallicities for turn-off and sub-giant stars with colours including a U band and a parallax with uncertainty less than ∼20%. Metallicities alone are possible for the main sequence and giant branch. We find good agreement with the literature when we apply our method to the Gaia benchmark stars, particularly for turn-off and young stars. Further tests on the old open cluster NGC 188, however, reveal significant limitations in the stellar isochrones. The ages derived for the cluster stars vary with evolutionary stage, such that turn-off ages disagree with those on the sub-giant branch, and metallicities vary significantly throughout. Furthermore, the parameters vary appreciably depending on which colour combinations are used in the derivation. We identify the causes of these mismatches and show that improvements are needed in the modelling of giant branch stars and in the creation and calibration of synthetic near-UV photometry. Our results warn against applying isochrone fitting indiscriminately. In particular, the uncertainty on the stellar models should be quantitatively taken into account. Further efforts to improve the models will result in significant advancements in our ability to study the Galaxy.

We fit various color–magnitude diagrams (CMDs) of the high-latitude Galactic globular clusters NGC 5024 (M53), NGC 5053, NGC 5272 (M3), NGC 5466, and NGC 7099 (M30) by isochrones from the Dartmouth Stellar Evolution Database and Bag of Stellar Tracks and Isochrones for α–enrichment [α/Fe] = +0.4. For the CMDs, we use data sets from Hubble Space Telescope, Gaia, and other sources utilizing, at least, 25 photometric filters for each cluster. We obtain the following characteristics with their statistical uncertainties for NGC 5024, NGC 5053, NGC 5272, NGC 5466, and NGC 7099, respectively: metallicities [Fe/H] = −1.93 ± 0.02, −2.08 ± 0.03, −1.60 ± 0.02, −1.95 ± 0.02, and −2.07 ± 0.04 dex with their systematic uncertainty 0.1 dex; ages 13.00 ± 0.11, 12.70 ± 0.11, 11.63 ± 0.07, 12.15 ± 0.11, and 12.80 ± 0.17 Gyr with their systematic uncertainty 0.8 Gyr; distances (systematic uncertainty added) 18.22 ± 0.06 ± 0.60, 16.99 ± 0.06 ± 0.56, 10.08 ± 0.04 ± 0.33, 15.59 ±0.03 ± 0.51, and 8.29 ± 0.03 ± 0.27 kpc; reddenings E(B − V) = 0.023 ± 0.004, 0.017 ± 0.004, 0.023 ± 0.004, 0.023 ± 0.003, and 0.045 ± 0.002 mag with their systematic uncertainty 0.01 mag; extinctions A V = 0.08 ± 0.01, 0.06 ± 0.01, 0.08 ± 0.01, 0.08 ± 0.01, and 0.16 ± 0.01 mag with their systematic uncertainty 0.03 mag, which suggest the total Galactic extinction A V = 0.08 across the whole Galactic dust to extragalactic objects at the North Galactic Pole. The horizontal branch morphology difference of these clusters is explained by their different metallicity, age, mass-loss efficiency, and loss of low-mass members in the evolution of the core-collapse cluster NGC 7099 and loose clusters NGC 5053 and NGC 5466.

Following an approach by Paczy\'nski & Stanek we compare red clump stars with parallaxes known to better than 10% in the Hipparcos catalog with the red clump stars observed in three fields in M31 using the HST. There are $\sim 600$ and $\sim 6,300$ such stars in the two data sets, respectively. The local red clump luminosity function is well represented by a Gaussian with the peak at $M_{I,m}=-0.23$, and the dispersion $\sigma_{RC}\approx0.2 $mag. This allows a single step determination of the distance modulus to M31 $\mu_{0,M31} = 24.471\pm 0.035 \pm 0.045 $mag (statistical plus systematic error) and the corresponding distance $R_{M31}= 784\pm 13\pm 17 kpc$. The number of red clump stars is large enough that the formal statistical error in the distance is only $\lesssim 2$%. We also correct the treatment of the local interstellar extinction by Paczy\'nski & Stanek and we obtain the Galactocentric distance modulus $\mu_{0,GC}=14.57 \pm0.04 \pm 0.04 $mag (statistical plus systematic error), and the corresponding Galactocentric distance $R_0=8.2 \pm0.15 \pm 0.15 kpc$.

The combination of visual and spectroscopic orbits in binary systems enables precise distance measurements without additional assumptions, making them ideal for examining the parallax zero-point offset (PZPO) at bright magnitudes (G < 13) in Gaia. We compiled 249 orbital parallaxes from 246 binary systems and used Markov chain Monte Carlo simulations to exclude binaries where orbital motion significantly impacts parallaxes. After removing systems with substantial parallax errors, large discrepancies between orbital and Gaia parallaxes, and selecting systems with orbital periods under 100 days, a final sample of 44 binaries was retained.The weighted mean PZPO for this sample is −38.9 ± 10.3 μas, compared to −58.0 ± 10.1 μas for the remaining systems, suggesting that orbital motion significantly affects parallax measurements. These formal uncertainties of the PZPO appear to be underestimated by a factor of approximately 2.0. For bright stars with independent trigonometric parallaxes from Very Long Baseline Interferometry and Hubble Space Telescope, the weighted mean PZPOs are −14.8 ± 10.6 and −31.9 ± 14.1 μas, respectively. Stars with G ≤ 8 exhibit a more pronounced parallax bias, with some targets showing unusually large deviations, likely due to systematic calibration errors in Gaia for bright stars. The orbital parallaxes dataset compiled in this work serves as a vital resource for validating parallaxes in future Gaia data releases.

Using the WMAP determination of the baryon density, the standard model of big bang nucleosynthesis yields relatively precise predictions of the primordial light-element abundances. Currently there are two significantly different observational determinations of the primordial helium abundance, and if only statistical errors in 4He abundance determinations are considered, the discrepancies between the observational determinations and the value favored by the WMAP results are significant. Here we examine in detail some likely sources of systematic uncertainties that may resolve the differences between the two determinations. We conclude that the observational determination of the primordial helium abundance is completely limited by systematic errors and that these systematic errors have not been fully accounted for in any published observational determination of the primordial helium abundance. In principle, the observed metal-poor H II region spectra should be analyzed in a nonparametric way, such that the H II region physical conditions and the helium abundance are derived solely from the relative flux ratios of the helium and hydrogen emission lines. In practice, there are very few H II region spectra with the quality that allows this, so that most analyses depend on assumed ranges or relationships between physical parameters, resulting in parametric solutions with underestimated error bars. A representative result of our analysis yields Yp = 0.249 ± 0.009. We stress that the main result of the present work is the increase in the size of the uncertainty rather than the shift in the primordial value. Furthermore, given that most of the spectra analyzed to date do not significantly constrain the primordial helium abundance, we argue in favor of a range of allowed values of 0.232 ≤ Yp ≤ 0.258. This easily allows for concordance between measurements of the baryon-to-photon ratio (η) from WMAP, deuterium abundances, and helium abundance (although the discrepancy with lithium remains).

We analysed 30 RR Lyrae stars (RRLs) located in the Large Magellanic Cloud (LMC) globular cluster Reticulum that were observed in the 3.6 and 4.5 $\mu$m passbands with the Infrared Array Camera (IRAC) on board of the Spitzer Space Telescope. We derived new mid-infrared (MIR) period-luminosity PL relations. The zero points of the PL relations were estimated using the trigonometric parallaxes of five bright Milky Way (MW) RRLs measured with the Hubble Space Telescope (HST) and, as an alternative, we used the trigonometric parallaxes published in the first Gaia data release (DR1) which were obtained as part of the Tycho-Gaia Astrometric Solution (TGAS) and the parallaxes of the same stars released with the second Gaia data release (DR2). We determined the distance to Reticulum using our new MIR PL relations and found that distances calibrated on the TGAS and DR2 parallaxes are in a good agreement and, generally, smaller than distances based on the HST parallaxes, although they are still consistent within the respective errors. We conclude that Reticulum is located ~3 kpc closer to us than the barycentre of the LMC.

We have used HST-WFPC2 multiband observations of a field around SN 1987A in the Large Magellanic Cloud to measure its distance from the Sun. The observations allowed us to carefully determine the interstellar extinction along the line of sight to a large number of stars and to measure the LMC distance by using two stellar distance indicators: the Red Clump and the Tip of the Red Giant Branch. From an application of the Red Clump method we obtain a distance modulus (m-M)o,rc(LMC)=18.59+-0.04+-0.08 mag (statistical plus systematic error), in good agreement with the distance derived by using the Tip of the Red Giant Branch stars, namely (m-M)o,trgb(LMC)=18.69+-0.25+-0.06 mag (statistical plus systematic error). Both values agree well with the distance to the SN 1987A as determined from a study of its inner ring fluorescent echo ((m-M)(SN87A)=18.55+-0.05 mag, Panagia 1998), thus excluding distance moduli lower than 18.43 to a 99.7% significance level. Differences with respect to previous results obtained using the same distance indicators are discussed.

We use stellar proper motions (PM) from Gaia Data Release 2 for studying the internal kinematics of Milky Way globular clusters. In addition to statistical measurement errors, there are significant spatially correlated systematic errors, which cannot be ignored when studying the internal kinematics. We develop a mathematically consistent procedure for incorporating the spatial correlations in any model-fitting approach, and use it to determine rotation and velocity dispersion profiles of a few dozen clusters. We confirm detection of rotation in the sky plane for ∼10 clusters reported in previous studies, and discover a few more clusters with rotation amplitudes exceeding ∼0.05 mas yr−1. However, in more than half of these cases the significance of this rotation signature is rather low when taking into account the systematic errors. We find that the PM dispersion is not sensitive to systematic errors in PM, however, it is quite sensitive to the selection criteria on the input sample, most importantly, in crowded central regions. When using the cleanest possible samples, PM dispersion can be reliably measured down to 0.1 mas yr−1 for ∼60 clusters.

Aims. Recently, the study of globular cluster (GC) color-magnitude diagrams (CMDs) has shown that some of them harbor multiple populations with different chemical compositions and/or ages. In the first case, the most common candidate is a spread in the initial helium abundance, but this quantity is difficult to determine spectroscopically due to the fact that helium absorption lines are not present in cooler stars, whereas for hotter GC stars gravitational settling of helium becomes important. As a consequence, indirect methods to determine the initial helium abundance among populations are necessary. For that reason, in this series of papers, we investigate the effects of a helium enrichment in populations covering the range of GC metallicities. Methods. In this first paper, we present the theoretical evolutionary tracks, isochrones, and zero-age horizontal branch (ZAHB) loci calculated with the Princeton-Goddard-PUC (PGPUC) stellar evolutionary code, which has been updated with the most recent input physics and compared with other theoretical databases. The chemical composition grid covers 9 metallicities ranging from Z = 1.60 10 -4 to 1.57 10 -2 (-2.25 < [Fe/H] < -0.25), 7 helium abundances from Y = 0.230 to 0.370, and an alpha-element enhancement of [/Fe] = 0.3. Results. The effects of different helium abundances that can be observed in isochrones are: splits in the main sequence (MS), differences in the luminosity (L) and effective temperature (T eff ) of the turn off point, splits in the sub giant branch being more prominent for lower ages or higher metallicities, splits in the lower red giant branch (RGB) being more prominent for higher ages or higher metallicities, differences in L of the RGB bump (with small changes in T eff ), and differences in L at the RGB tip. At the ZAHB, when Y is increased there is an increase of L for low T eff , which is affected in different degrees depending on the age of the GC being studied. Finally, the ZAHB morphology distribution depending on the age explains how for higher GC metallicities a population with higher helium abundance could be hidden at the red ZAHB locus.

We use the Gaia data release 1 (DR1) to study the proper motion (PM) fields of the Large and Small Magellanic Clouds (LMC, SMC). This uses the Tycho-Gaia Astrometric Solution (TGAS) PMs for 29 Hipparcos stars in the LMC and 8 in the SMC. The LMC PM in the West and North directions is inferred to be , and the SMC PM . These results have similar accuracy and agree to within the uncertainties with existing Hubble Space Telescope (HST) PM measurements. Since TGAS uses different methods with different systematics, this provides an external validation of both data sets and their underlying approaches. Residual DR1 systematics may affect the TGAS results, but the HST agreement implies this must be below the random errors. Also in agreement with prior HST studies, the TGAS LMC PM field clearly shows the clockwise rotation of the disk, even though it takes the LMC disk in excess of 108 years to complete one revolution. The implied rotation curve amplitude for young LMC stars is consistent with that inferred from line of sight (LOS) velocity measurements. Comparison of the PM and LOS rotation curves implies a kinematic LMC distance modulus , consistent but not yet competitive with photometric methods. These first results from Gaia on the topic of Local Group dynamics provide an indication of how its future data releases will revolutionize this field.