The binary populations of eight globular clusters in the outer halo of the Milky Way

We analyse colour–magnitude diagrams of eight globular clusters (GCs) in the outer Galactic halo. Images were taken with the Wide Field Channel of the Advanced Camera for Survey and the Ultraviolet and Visual Channel of the Wide Field Camera 3 on board of the Hubble Space Telescope. We have determined the fraction of binary stars along the main sequence and combined results with those of a recent paper where some of us have performed a similar analysis on 59 Galactic GCs. In total, binaries have been now studied homogeneously in 67 GCs. We studied the radial and luminosity distributions of the binary systems, the distribution of their mass ratios and investigated univariate relations with several parameters of the host GCs. We confirm the anticorrelation between the binary fraction and the luminosity of the host cluster, and find that low-luminosity clusters can host a large population in excess of ∼40 per cent in the cluster core. However, our results do not support a significant correlation with the cluster age as suggested in the literature. In most GCs, binaries are more centrally concentrated than single stars. If the fraction of binaries is normalized to the core binary fraction the radial density profiles follow a common trend. It has a maximum in the centre and declines by a factor of 2 at a distance of about two core radii from the cluster centre. After dropping to its minimum at a radial distance of ∼5 core radii it stays approximately constant at larger radii. We also find that the mass ratio and the distribution of binaries as a function of the mass of the primary star is almost flat

The ACS Survey of Galactic Globular Clusters: M54 and Young Populations in the Sagittarius Dwarf Spheroidal Galaxy

We present new Hubble Space Telescope photometry of the massive globular cluster M54 (NGC 6715) and the superposed core of the tidally disrupted Sagittarius (Sgr) dSph galaxy as part of the ACS Survey of Galactic Globular Clusters. Our deep (F606W~26.5), high-precision photometry yields an unprecedentedly detailed color-magnitude diagram showing the extended blue horizontal branch and multiple main sequences of the M54+Sgr system. The distance and reddening to M54 are revised usingboth isochrone and main-sequence fitting to (m-M)_0=17.27 and E(B-V)=0.15. Preliminary assessment finds the M54+Sgr field to be dominated by the old metal-poor populations of Sgr and the globular cluster. Multiple turnoffs indicate the presence of at least two intermediate-aged star formation epochs with 4 and 6 Gyr ages and [Fe/H]=-0.4 to -0.6. We also clearly show, for the first time, a prominent, 2.3 Gyr old Sgr population of near-solar abundance. A trace population of even younger (0.1-0.8 Gyr old), more metal-rich ([Fe/H]\sim0.6) stars is also indicated. The Sgr age-metallicity relation is consistent with a closed-box model and multiple (4-5) star formation bursts over the entire life of the satellite, including the time since Sgr began disrupting.Comment: Accepted to ApJ Letter; 11 pages, 2 figures; figure 1 uploaded as jpg; paper in ApJ format with full-resolution figures available at: http://www.astro.ufl.edu/~ata/public_hstgc/paperIV/paperIV.p

Density Variations in the NW Star Stream of M31

The Pan Andromeda Archeological Survey (PAndAS) CFHT Megaprime survey of the M31-M33 system has found a star stream which extends about 120 kpc NW from the center of M31. The great length of the stream, and the likelihood that it does not significantly intersect the disk of M31, means that it is unusually well suited for a measurement of stream gaps and clumps along its length as a test for the predicted thousands of dark matter sub-halos. The main result of this paper is that the density of the stream varies between zero and about three times the mean along its length on scales of 2 to 20 kpc. The probability that the variations are random fluctuations in the star density is less than 10^-5. As a control sample we search for density variations at precisely the same location in stars with metallicity higher than the stream, [Fe/H]=[0, -0.5] and find no variations above the expected shot noise. The lumpiness of the stream is not compatible with a low mass star stream in a smooth galactic potential, nor is it readily compatible with the disturbance caused by the visible M31 satellite galaxies. The stream's density variations appear to be consistent with the effects of a large population of steep mass function dark matter sub-halos, such as found in LCDM simulations, acting on an approximately 10Gyr old star stream. The effects of a single set of halo substructure realizations are shown for illustration, reserving a statistical comparison for another study.Comment: ApJ revised version submitte

Global and nonglobal parameters of horizontal-branch morphology of globular clusters

The horizontal-branch (HB) morphology of globular clusters (GCs) is mainly determined by metallicity. However, the fact that GCs with almost the same metallicity exhibit different HB morphologies demonstrates that at least one more parameter is needed t

The ACS survey of galactic globular clusters. XI. The three-dimensional orientation of the Sagittarius dwarf spheroidal galaxy and its globular clusters

We use observations from the Hubble Space Telescope Advanced Camera for Surveys (HST/ACS) study of Galactic globular clusters to investigate the spatial distribution of the inner regions of the disrupting Sagittarius dwarf spheroidal galaxy (Sgr). We combine previously published analyses of four Sgr member clusters located near or in the Sgr core (M54, Arp 2, Terzan 7, and Terzan 8) with a new analysis of diffuse Sgr material identified in the background of five low-latitude Galactic bulge clusters (NGC 6624, 6637, 6652, 6681, and 6809) observed as part of the ACS survey. By comparing the bulge cluster color-magnitude diagrams to our previous analysis of the M54/Sgr core, we estimate distances to these background features. The combined data from four Sgr member clusters and five Sgr background features provide nine independent measures of the Sgr distance and, as a group, provide uniformly measured and calibrated probes of different parts of the inner regions of Sgr spanning 20° over the face of the disrupting dwarf. This allows us, for the first time, to constrain the three-dimensional orientation of Sgr's disrupting core and globular cluster system and compare that orientation to the predictions of an N-body model of tidal disruption. The density and distance of Sgr debris are consistent with models that favor a relatively high Sgr core mass and a slightly greater distance (28-30kpc, with a mean of 29.4kpc). Our analysis also suggests that M54 is in the foreground of Sgr by ∼ 2 kpc, projected on the center of the Sgr dSph. While this would imply a remarkable alignment of the cluster and the Sgr nucleus along the line of sight, we cannot identify any systematic effect in our analysis that would falsely create the measured 2kpc separation. Finally, we find that the cluster Terzan 7 has the most discrepant distance (25kpc) among the four Sgr core clusters, which may suggest a different dynamical history than the other Sgr core clusters

Multiple Stellar Populations in Metal-Poor Globular Clusters with JWST: a NIRCam view of M 92

Recent work on metal-intermediate globular clusters (GCs) with [Fe/H]=$-1.5$ and $-0.75$ has illustrated the theoretical behavior of multiple populations in photometric diagrams obtained with the James Webb Space Telescope (JWST). These results are confirmed by observations of multiple populations among M-dwarfs of 47 Tucanae. Here, we explore the multiple populations in metal-poor GCs with [Fe/H]=$-$2.3. We take advantage of synthetic spectra and isochrones that account for the chemical composition of multiple populations to identify photometric diagrams that separate the distinct stellar populations of GCs. We derived high-precision photometry and proper motion for main-sequence stars in the metal-poor GC M 92 from JWST and Hubble Space Telescope (HST) images. We identified a first generation (1G) and two main groups of second-generation stars (2G$_{\rm A}$ and 2G$_{\rm B}$) and investigated their kinematics and chemical composition. We find isotropic motions with no differences among the distinct populations. The comparison between the observed colors of M 92 stars and the colors derived by synthetic spectra reveals that helium abundance of 2G$_{\rm A}$ and 2G$_{\rm B}$ stars are higher than that of the 1G by $\Delta Y \sim 0.01$ and $0.04$, respectively. The $m_{\rm F090W}$ vs. $m_{\rm F090W}-m_{\rm F277W}$ color-magnitude diagram shows that below the knee, MS stars exhibit a wide color broadening due to multiple populations. We constrain the amount of oxygen variation needed to reproduce the observed MS width, which is consistent with results on red-giant branch stars. We conclude that multiple populations with masses of $\sim$0.1-0.8$M_{\odot}$ share similar chemical compositions.Comment: 15 pages, 9 figures. Submitted to ApJ on April 6th, 202

Active Learning for Computationally Efficient Distribution of Binary Evolution Simulations

Binary stars undergo a variety of interactions and evolutionary phases, critical for predicting and explaining observed properties. Binary population synthesis with full stellar-structure and evolution simulations are computationally expensive requiring a large number of mass-transfer sequences. The recently developed binary population synthesis code POSYDON incorporates grids of MESA binary star simulations which are then interpolated to model large-scale populations of massive binaries. The traditional method of computing a high-density rectilinear grid of simulations is not scalable for higher-dimension grids, accounting for a range of metallicities, rotation, and eccentricity. We present a new active learning algorithm, psy-cris, which uses machine learning in the data-gathering process to adaptively and iteratively select targeted simulations to run, resulting in a custom, high-performance training set. We test psy-cris on a toy problem and find the resulting training sets require fewer simulations for accurate classification and regression than either regular or randomly sampled grids. We further apply psy-cris to the target problem of building a dynamic grid of MESA simulations, and we demonstrate that, even without fine tuning, a simulation set of only $\sim 1/4$ the size of a rectilinear grid is sufficient to achieve the same classification accuracy. We anticipate further gains when algorithmic parameters are optimized for the targeted application. We find that optimizing for classification only may lead to performance losses in regression, and vice versa. Lowering the computational cost of producing grids will enable future versions of POSYDON to cover more input parameters while preserving interpolation accuracies.Comment: 20 pages (16 main text), 10 figures, submitted to Ap

The halo+cluster system of the Galactic globular cluster NGC 1851

NGC 1851 is surrounded by a stellar component that extends more than 10 times beyond the tidal radius. Although the nature of this stellar structure is not known, it has been suggested to be a sparse halo of stars or associated with a stellar stream. We analyse the nature of this intriguing stellar component surrounding NGC 1851 by investigating its radial velocities and chemical composition, in particular in comparison with those of the central cluster analysed in a homogeneous manner. In total we observed 23 stars in the halo with radial velocities consistent with NGC 1851, and for 15 of them we infer [Fe/H] abundances. Our results show that (i) stars dynamically linked to NGC 1851 are present at least up to ̃2.5 tidal radii, supporting the presence of a halo of stars surrounding the cluster; (ii) apart from the NGC 1851 radial velocity-like stars, our observed velocity distribution agrees with that expected from Galactic models, suggesting that no other substructure (such as a stream) at different radial velocities is present in our field; (iii) the chemical abundances for the s-process elements Sr and Ba are consistent with the s-normal stars observed in NGC 1851; (iv) all halo stars have metallicities, and abundances for the other studied elements Ca, Mg and Cr, consistent with those exhibited by the cluster. The complexity of the whole NGC 1851 cluster+halo system may agree with the scenario of a tidally disrupted dwarf galaxy in which NGC 1851 was originally embedded

The remnants of galaxy formation from a panoramic survey of the region around M31

In hierarchical cosmological models, galaxies grow in mass through the continual accretion of smaller ones. The tidal disruption of these systems is expected to result in loosely bound stars surrounding the galaxy, at distances that reach $10 - 100$ times the radius of the central disk. The number, luminosity and morphology of the relics of this process provide significant clues to galaxy formation history, but obtaining a comprehensive survey of these components is difficult because of their intrinsic faintness and vast extent. Here we report a panoramic survey of the Andromeda galaxy (M31). We detect stars and coherent structures that are almost certainly remnants of dwarf galaxies destroyed by the tidal field of M31. An improved census of their surviving counterparts implies that three-quarters of M31's satellites brighter than $M_V < -6$ await discovery. The brightest companion, Triangulum (M33), is surrounded by a stellar structure that provides persuasive evidence for a recent encounter with M31. This panorama of galaxy structure directly confirms the basic tenets of the hierarchical galaxy formation model and reveals the shared history of M31 and M33 in the unceasing build-up of galaxies.Comment: Published in Nature. Supplementary movie available at https://www.astrosci.ca/users/alan/PANDAS/Latest%20news%3A%20movie%20of%20orbit.htm