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Register a new userThe blue straggler population of the old open cluster Berkeley 17
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Added by
Souradeep Bhattacharya
Publication date
2019
fields
Physics
and research's language is
English
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Blue Straggler Stars (BSSs) are observed in Galactic globular clusters and old open clusters. The radial distribution of BSSs has been used to diagnose the dynamical evolution of globular clusters. For the first time, with a reliable sample of BSSs identified with Gaia DR2, we conduct such an analysis for an open cluster. We identify members, including BSSs, of the oldest known Galactic open cluster Berkeley 17 with the Gaia DR2 proper motions and parallaxes. We study the radial distribution of the BSS population to understand the dynamical evolution of the cluster. We select cluster members to populate the colour magnitude diagram in the Gaia filters. Cluster parameters are derived using the brightest members. The BSSs and giant branch stars are identified, and their radial distributions are compared. The segregation of BSSs is also evaluated with respect to the giant branch stars using the Minimum Spanning Tree analysis. We determine Berkeley 17 to be at $3138.6^{+285.5}_{-352.9}$ pc. We find 23 BSS cluster members, only two of which were previously identified. We find a bimodal radial distribution of BSSs supported by findings from the MST method. The bimodal radial distribution of BSSs in Berkeley 17 indicates that they have just started to sink towards the cluster center, placing Berkeley 17 with globular clusters of intermediate dynamical age. This is the first such determination for an open cluster.
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Blue Stragglers are stars located in an unexpected region of the color-magnitude diagram of a stellar population, as they appear bluer and more luminous than the stars in the turnoff region. They are ubiquitous, since they have been found among Milky Way field stars, in open and globular clusters, and also in other galaxies of the Local Group. Here we present a study on the blue straggler population of the old and metal-rich open cluster Collinder 261, based on Gaia DR2 data and on a multi-epoch radial velocity survey conducted with FLAMES@VLT. We also analyze the radial distribution of the blue straggler population to probe the dynamical status of the cluster. Blue straggler candidates were identified first with Gaia DR2, according to their position on the CMD, proper motions, and parallaxes. Their radial distribution was compared with those of the main sequence, red giant, and red clump stars, to evaluate mass segregation. Additionally, their radial velocities (and the associated uncertainties) were compared with the mean radial velocity and the velocity dispersion of the cluster. When possible, close binaries and long-period binaries were also identified, based on the radial velocity variations for the different epochs. We also looked for yellow stragglers, i.e., possible evolved blue stragglers. We found 53 blue stragglers members of Collinder 261, six of them already identified in previous catalogs. Among the blue straggler candidates with radial velocity measurements, we found one long-period binary, five close-binary systems, three non-variable stars; we also identified one yellow straggler.
We present the analysis of the morphological shape of Berkeley 17, the oldest known open cluster (~10 Gyr), using a probabilistic star counting of Pan-STARRS point sources, and confirm its core-tail shape, plus an antitail, previously detected with the 2MASS data. The stellar population, as diagnosed by the color-magnitude diagram and theoretical isochrones, shows many massive members in the cluster core, whereas there is a paucity of such members in both tails. This manifests mass segregation in this aged star cluster with the low-mass members being stripped away from the system. It has been claimed that Berkeley 17 is associated with an excessive number of blue straggler candidates. Comparison of nearby reference fields indicates that about half of these may be field contamination.
By combining high-resolution HST and wide-field ground based observations, in ultraviolet and optical bands, we study the Blue Stragglers Star (BSS) population of the galactic globular cluster M5 (NGC 5904) from its very central regions up to its periphery. The BSS distribution is highly peaked in the cluster center, decreases at intermediate radii and rises again outward. Such a bimodal distribution is similar to those previously observed in other globular clusters (M3, 47Tucanae, NGC6752). As for these clusters, dynamical simulations suggest that, while the majority of BSS in M5 could be originated by stellar collisions, a significant fraction (20-40%) of BSS generated by mass transfer processes in primordial binaries is required to reproduce the observed radial distribution. A candidate BSS has been detected beyond the cluster tidal radius. If confirmed, this could represent an interesting case of an evaporating BSS.
The most massive star clusters include several generations of stars with a different chemical composition (mainly revealed by an Na-O anti-correlation) while low-mass star clusters appear to be chemically homogeneous. We are investigating the chemical composition of several clusters with masses of a few 10^4 Msun to establish the lower mass limit for the multiple stellar population phenomenon. Using FLAMES@VLT spectra we determine abundances of Fe, O, Na, and several other elements (alpha, Fe-peak, and neutron-capture elements) in the old open cluster Berkeley 39. This is a massive open cluster: M~10^4 Msun, approximately at the border between small globular clusters and large open clusters. Our sample size of about 30 stars is one of the largest studied for abundances in any open cluster to date, and will be useful to determine improved cluster parameters, such as age, distance, and reddening when coupled with precise, well-calibrated photometry. We find that Berkeley 39 is slightly metal-poor, <[Fe/H]>=-0.20, in agreement with previous studies of this cluster. More importantly, we do not detect any star-to-star variation in the abundances of Fe, O, and Na within quite stringent upper limits. The r.m.s. scatter is 0.04, 0.10, and 0.05 dex for Fe, O, and Na, respectively. This small spread can be entirely explained by the noise in the spectra and by uncertainties in the atmospheric parameters. We conclude that Berkeley 39 is a single-population cluster.
To study the crucial range of Galactocentric distances between 12 and 16 kpc, where little information is available, we have obtained VI CCD imaging of Berkeley 20 and BVI CCD imaging of Berkeley 66 and Tombaugh 2, three distant, old open clusters. Using the synthetic colour magnitude diagram (CMD) technique with three types of evolutionary tracks of different metallicities, we have determined age, distance, reddening and indicative metallicity of these systems. The CMD of Be 20 is best reproduced by stellar models with a metallicity about half of solar (Z=0.008 or 0.01), in perfect agreement with high resolution spectroscopic estimates. Its age is between 5 and 6 Gyr from stellar models with overshooting and between 4.3 and 4.5 Gyr from models without it. The distance modulus from the best fitting models is always (m-M)0=14.7 (corresponding to a Galactocentric radius of about 16 kpc), and the reddening E(B-V) ranges between 0.13 and 0.16. A slightly lower metallicity (Z ~ 0.006) appears to be more appropriate for Be 66. This cluster is younger, (age of 3 Gyr), and closer, (m-M)0=13.3 (i.e., at 12 kpc from the Galactic centre), than Be 20, and suffers from high extinction, 1.2 < E(B-V) < 1.3, variable at the 2-3 per cent level. Finally, the results for To 2 indicate that it is an intermediate age cluster, with an age of about 1.4 Gyr or 1.6-1.8 Gyr for models without and with overshooting, respectively. The metallicity is about half of solar (Z=0.006 to 0.01), in agreement with spectroscopic determinations. The distance modulus is (m-M)0=14.5, implying a distance of about 14 kpc from the Galactic centre; the reddening E(B-V) is 0.31-0.4, depending on the model and metallicity, with a preferred value around 0.34.
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