The Blue Straggler Star Populations in Galactic Globular Clusters

We used a combination of high-resolution optical images acquired with the Hubble Space Telescope and near-IR wide-field data to investigate the stellar density profile and the population of blue straggler stars (BSSs) in the Galactic globular cluster NGC 6256, with the aim of probing its current stage of internal dynamical evolution. We found that the inner stellar density profile significantly deviates from a King model, while it is well reproduced by a steep cusp with a power-law slope α cusp = −0.89, thus implying that the cluster is currently in the post-core-collapse (PCC) phase. This is also confirmed by the very high segregation level of the BSS population measured through the parameter. We also found that the distribution of BSSs in the color–magnitude diagram is characterized by a collimated blue sequence and a red more sparse component, as already observed in three other PCC clusters. A comparison with appropriate collisional models demonstrates that the vast majority of the BSSs lying along the collimated blue sequence is consistent with a generation of coeval (1 Gyr old) stars with different masses originated by an event that highly enhanced the collisional rate of the system (i.e., the core collapse). This study confirms that the segregation level of BSSs is a powerful dynamical diagnostic also of star cluster in a very advanced stage of dynamical evolution. Moreover, it pushes forward the possibility of using the morphology of BSSs in a color–magnitude diagram as a tracer of the core-collapse and subsequent dynamical evolutionary phases.

We present a homogeneous study of blue straggler stars across ten outer halo globular clusters, three classical dwarf spheroidal and nine ultra-faint galaxies based on deep and wide-field photometric data taken with MegaCam on the Canada-France-Hawaii Telescope. We find blue straggler stars to be ubiquitous among these Milky Way satellites. Based on these data, we can test the importance of primordial binaries or multiple systems on blue straggler star formation in low density environments. For the outer halo globular clusters we find an anti-correlation between the specific frequency of blue straggler and absolute magnitude, similar to that previously observed for inner halo clusters. When plotted against density and encounter rate, the frequency of blue stragglers are well fitted by single trends with smooth transitions between dwarf galaxies and globular clusters, which points to a common origin for their blue stragglers. The fraction of blue stragglers stays constant and high in the low encounter rate regime spanned by our dwarf galaxies, and decreases with density and encounter rate in the range spanned by our globular clusters. We find that young stars can mimic blue stragglers in dwarf galaxies only if their ages are 2.5+/-0.5 Gyr and they represent ~1-7% of the total number of stars, which we deem highly unlikely. These results point to mass-transfer or mergers of primordial binaries or multiple systems as the dominant blue straggler formation mechanism in low density systems.

Forty-eight blue straggler stars have been discovered in NGC 5466, the only Galactic globular cluster known to contain an anomalous Cepheid of the sort found in dwarf galaxies. The stars were identified in color-magnitude diagrams constructed from photometry of deep photographic plates taken with the Canada-France-Hawaii 3.6 m telescope (calibrated with new UBV photoelectric photometry), and from point spread function photometry of CCD frames taken with the Palomar 5 m telescope. The stars typically have magnitudes ∼ 19 m .1 and colors ∼ 0 m .2. Forty-two of the 48 stars are situated inside of R=2.5 arcmin (see Fig.1), the projected radius containing half the cluster luminosity, and only six stars are found between 2.5 and 9 arcmin. A one-sided, two-sample Kolmogorov-Smirnov test (using the CCD data) establishes at the 98% significance level that the blue stragglers are more centrally concentrated than the subgiant stars of the same magnitude. By fitting multi-component King models to the projected radial distributions (Fig.2), the mean mass of the blue stragglers is shown to be ∼1.5 to two times larger than the masses of the stars that contributed the light from which the core and tidal radii were derived (i.e. M(Blue Str.)=1.3±0.3 M⊙). Because the central relaxation time for NGC 5466 is much less than the cluster age, the different radial distributions are attributed to mass segregation. A similar mass segregation is also observed in the globular cluster NGC 5053, where Nemec and Cohen (1986, in preparation) have recently identified ∼30 blue stragglers. The low stellar aensity and small escape velocity of NGC 5466 make a recent epoch of star formation (during which the blue stragglers might have formed as massive single stars) seem unlikely. Instead, the blue stragglers probably are either close binary systems that have transferred mass, or are coalesced stars. The very low frequency of stellar collisions expected in the center of NGC 5466 suggests that the blue stragglers are primordial binary systems. The simultaneous presence in NGC 5466 of the blue stragglers and the anomalous Cepheid V19, and their relative numbers, supports the hypothesis that there is an evolutionary connection between the two types of stars. By fitting theoretical isochrones to the photographic c-m diagram, NGC 5466 is estimated to have an age of 18±3 Gyr.

Variable stars have historically served as tools and laboratories in our understanding of stellar formation and evolution, the formation of star clusters, the calibration of distance determination methods, and a variety of other areas. In particular, the study of eclipsing binaries (EBs) in globular clusters (GCs) can provide direct distance estimates to clusters as well as constraints for the turnoff masses of GC stars. Knowing precise distances to GCs would constitute an independent check of widely used distance determination methods in astronomy. Obtaining masses of GC turnoff stars provides a fundamental test of low-metallicity stellar models that calculate ages for these stars—and thus the GCs themselves—and thereby allow a lower limit estimate of the age of the universe. My dissertation research with M. Mateo at the University of Michigan consists of a monitoring survey of 10 Galactic GCs with the aim of identifying photometrically variable EBs around or below the main-sequence turnoff (MSTO). My thesis work comprises the results of our research on the GCs NGC 3201, M10, and M12. Our observing strategy is aimed at detecting variables with periods between 0.2 and around 5 days and , thereby optimizing our chance to find the 16.5 ! V ! 20 valuable detached EBs in the target clusters. In addition, we correct for differential reddening variations across the cluster fields of up to several tenths of magnitudes in V by internally creating a differential extinction map with an arcminute resolution from our cluster photometry data and calculate the additional reddening zero point using isochrone fitting. The resulting improvement in the appearance of the color-magnitude diagrams (CMDs) of the clusters is considerable (see K. von Braun & M. Mateo 2001, AJ, 121, 1522). We obtained approximately 200 VI epochs with about 20,000 stars per image for each of the three clusters. Analysis of these data revealed the existence of 14 variable stars (11 EBs) in the field of NGC 3201, three variables (one EB) in the field of M10, and two EBs in the field of M12. Spectroscopic followup work showed that only one variable (a blue straggler W Ursa Majoris contact EB) in the field of NGC 3201 is associated with the cluster. Another W UMa EB is most likely a member of M12, based on its location in M12’s CMD and its empirically calculated absolute magnitude. The rest of the variable stars we detected are members of the Galactic disk (see K. von Braun & M. Mateo 2002, AJ, 123, 279; and K. von Braun et al. 2002, AJ, 124, 2067 for details). We thus calculate a ratio of observable, short-period, main-sequence binaries to main-sequence stars of around 1/500 for the Galactic disk (consistent with literature estimates) and 1/9000 for GC members. The latter value is considerably smaller than the commonly quoted value of 1/1000. While the discrepancy may be due to small number statistics, we attribute it to the fact that we lose about 20% of our fields to crowding toward the cluster center where the binary fraction should be highest after a few cluster relaxation times. In my dissertation, I show the cluster fields and CMDs (before and after dereddening) with the locations of the variable stars and our differential extinction maps, as well as the phased light curves and spectra (wherever applicable) of the variable stars. For additional information or to obtain a copy of the dissertation, please contact me or visit http://www.ciw.edu/vonbraun.

We report the discovery of a hot white dwarf (WD) companion to a blue straggler star (BSS) in the globular cluster (GC) NGC 5466, based on observations from the Ultra-Violet Imaging Telescope (UVIT) on board AstroSat. The Spectral Energy Distribution (SED) of the Far-UV detected BSS NH 84 was constructed by combining the flux measurements from four filters of UVIT, with GALEX, GAIA, and other ground-based observations. The SED of NH 84 reveals the presence of a hot companion to the BSS. The temperature and radius of the BSS (T K, R/R ) derived from Gemini spectra and SED fitting using Kurucz atmospheric models are consistent with each other. The temperature and radius of the hotter companion of NH 84 (T K, R/R ) derived by fitting Koester WD models to the SED suggest that it is likely to be a hot WD. The radial velocity derived from the spectra along with the proper motion from GAIA DR2 confirms NH 84 to be a kinematic member of the cluster. This is the second detection of a BSS-WD candidate in a GC, and the first in the outskirts of a low-density GC. The location of this BSS in NGC 5466 along with its dynamical age supports the mass-transfer pathway for BSS formation in low-density environments.

We have surveyed the blue straggler star population of the Galactic globular cluster M5 using high-resolution images of the core along with wide-field ground-based images reaching to more than 19 core radii. To gauge M5's relative efficiency of producing stragglers, we compared our sample to five studies of other globular clusters (mainly Ferraro et al. 1997b; Ferraro et al 2003; and Piotto et al. 2004). Using a "bright" sample selected in the same way as Ferraro et al. 1997b, we found a bimodal radial distribution similar to those found in three other luminous clusters. When the radial distributions for different clusters are scaled using the core radius, there is good cluster-to-cluster agreement in the size of the core straggler sample and the center of the "zone of avoidance". However, M5 has the smallest fraction of stragglers in the zone of avoidance of any of the clusters measured to date, and its zone of avoidance appears to be wider (in r / r_c) than that of M3, which has a very similar surface brightness profile. Both of these facts indicate that M5's straggler population has dynamically evolved to a larger extent than M3. Using an ultraviolet sample from Hubble Space Telescope selected in the same way as Ferraro et al. 2003 and Ferraro et al. 2004, we find that the frequency of blue stragglers in M5 is lower than all but two of the clusters examined. We also identified seven bright blue stragglers that were previously misidentified as HB stars by Sandquist et al. (2004). These bright stragglers are most likely the result of stellar collisions involving binary stars.

UBV CCD photometry for 7217 stars in the loose Galactic globular cluster IC 4499 is presented. The new colour-magnitude diagram (CMD) reaches 2mag below the main-sequence turn-off (TO), yielding the first view of the subgiant branch and the upper main sequence. Beyond confirming the already known overall morphology of the horizontal and giant branches, these new data lead to the following results. (i) The cluster interstellar reddening, independently obtained from the two-colour diagram, is |$E(B-V)=0.25\pm0.05.$| (ii) The mean metallicity derived via different photometric observables measured in the CMD turns out to be [Fe/H]=-1.75±-0.20, about 0.2-0.3 dex lower than that obtained via spectroscopy. (iii) 64 candidate blue stragglers (BSS) have been identified. They are more concentrated than the subgiant stars of similar brightness (at >3σ). (iv) Coupling the apparent luminosity of the horizontal branch, |$V_\text {HB}=17.65\pm0.04$|⁠, with |$V_\text {TO}=20.90\pm0.10$| [and |$(B-V)_\text {TO}=0.657\pm 0.003$|], one gets |$\Delta V^\text {TO}_\text {HB}=3.25\pm0.12$|⁠, a value much lower than 3.55±0.09 found by Buonanno, Corsi & Fusi Pecci for 18 well-studied clusters. With the adopted metallicity, this suggests that IC 4499 is younger (by 3-4 Gyr) than most globular clusters of similar metallicity. This conclusion is further supported by differential comparisons of the main branches in the CMDs of a wide set of reference clusters. (v) Together with Pal 12, Ruprecht 106, Arp 2 and Terzan 7, this is the fifth Galactic globular cluster showing significant evidence for large age differences in the medium-far Galactic halo.

In this talk, we will simply show the consequences of binary population synthesis for blue stragglers, such as the integrated spectral energy distribution (ISED), the color-magnitude diagram, the specific frequency, and the influences on colors. Blue stragglers have been found in all stellar populations and they are an important population component in both stellar evolution and star clusters. Much evidence shows that blue stragglers are relevant to primordial binaries. The binary population synthesis study shows that primordial binary evolution may produce blue stragglers at any given times. The specific frequency in this way decreases with time first, then increases when the age is larger than 10 Gyr, while that from angular momentum loss induced by magnetic braking in low-mass binaries increases with time and exceeds that of primordial binary evolution in a population older than 3 Gyr. Meanwhile, blue stragglers resulting from primordial binary evolution are dominant contributors to the ISEDs in ultraviolet and blue bands in a population between 0.3 and 2.0 Gyr. The mass fraction of the lost matter from the primary accreted by the secondary, beta, significantly affects on the final results, e.g. the specific frequency of blue stragglers decreasing with beta, blue stragglers produced from a high value of beta being more massive, then contributing more to the ISEDs of the host clusters. For old open clusters, it is appropriate to adopt a higher value of beta when the primary is in HG at the onset of mass transfer. Our study also shows that, for most Galactic open clusters, the specific frequency of blue stragglers obtained from our simulations is much lower than that of observations, which has been discussed in this talk.

We present a statistical approach to investigating the dynamical evolution of the old open cluster Trumpler 19. We identified 810 cluster members using an ensemble-based unsupervised machine learning method applied to Gaia Data Release 3 astrometric data. From the color–magnitude diagram, we identified 18 blue straggler stars (BSSs) in Trumpler 19. The mass function of the cluster shows a flatter slope, indicating strong mass segregation and advanced dynamical evolution. We fitted the radial surface density profile and found that the concentration parameter c > 1, suggesting that the cluster has formed a clear core–halo structure as a result of dynamical evolution. We characterized the mass segregation among the cluster members as well as BSSs using the Minimum Spanning Trees method, indicating a significant central concentration. Additionally, the sedimentation level of the BSSs is measured as A r h + = 0.28 ± 0.05 , further supporting radial segregation. To probe the BSS formation mechanisms, we estimated their fractional mass excess (M e ), supporting binary mass transfer and mergers as the dominant channels. This is further supported by the presence of six variable BSSs. The dynamical evolution of the cluster is further assessed through its tidal interaction with the Galaxy. Trumpler 19 appears to be within the tidal regime, where strong Galactic tidal forces have a significant influence on the dynamical evolution. This indicates that the cluster may have undergone significant mass-loss processes, potentially leading to its eventual disruption, which is further supported by the orbit analysis of the cluster. We found that Trumpler 19 may have lost more than 95% of its initial mass due to dynamical evolution.

This chapter explores how we might use the observed statistics of blue stragglers in globular clusters to shed light on their formation. This means we will touch on topics also discussed elsewhere in this book, such as the discovery and implications of bimodal radial distributions and the “double sequences” of blue stragglers that have recently been found in some clusters. However, we will focus particularly on the search for a “smoking gun” correlation between the number of blue stragglers in a given globular cluster and a physical cluster parameter that would point towards a particular formation channel. As we shall see, there is little evidence for an intrinsic correlation between blue straggler numbers and stellar collision rates, even in dense cluster cores. On the other hand, there is a clear correlation between blue straggler numbers and the total (core) mass of the cluster. This would seem to point towards a formation channel involving binaries, rather than dynamical encounters. However, the correlation between blue straggler numbers and actual binary numbers—which relies on recently determined empirical binary fractions—is actually weaker than that with core mass. We explain how this surprising result may be reconciled with a binary formation channel if binary fractions depend almost uniquely on core mass. If this is actually the case, it would have significant implications for globular cluster dynamics more generally.KeywordsBlue StragglersGlobular ClustersDynamic EncounterDense Cluster CoresStellar CollisionsThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

We have used ESO telescopes at La Silla and the Hubble Space Telescope (HST) in order to obtain accurate B, V, I CCD photometry for the stars located within 200″ (2 half-mass radii, ) from the center of the cluster NGC 6101. Color-Magnitude Diagrams (CMDs) extending from the red-giant tip to about 5 magnitudes below the main-sequence turnoff MSTO () have been constructed. The following results have been obtained from the analysis of the CMDs: a) The overall morphology of the main branches confirms previous results from the literature, in particular the existence of a sizeable population of 73 "blue stragglers" (BSS), which had been already partly detected (27). They are considerably more concentrated than either the subgiant branch (SGB) or the main sequence (MS) stars, and have the same spatial distribution as the horizontal branch (HB) stars (84% probability from K-S test). An hypothesis on the possible BSS progeny is also presented. b) The HB is narrow and the bulk of stars is blue, as expected for a typical metal-poor globular cluster. c) The derived magnitudes for the HB and the MSTO, , , coupled with the values , , yield a distance modulus and an age similar to other "old" metal-poor globular clusters. In particular, from the comparison with theoretical isochrones, we derive for this cluster an age of 13 Gyrs. d) By using the large statistical sample of Red Giant Branch (RGB) stars, we detected with high accuracy the position of the bump in the RGB luminosity function. This observational feature has been compared with theoretical prescriptions, yielding a good agreement within the current theoretical and observational uncertainties.

THE nearby globular cluster NGC6397, 2.2 kpc from us, is an evolved cluster which has passed through core collapse1–3; its core region is expanding after an earlier dynamical phase of contraction. Its proximity means that observations of it with CCD (charge-coupled device) detectors in arcsecond-seeing conditions simulate Hubble Space Telescope observations of more-distant objects, such as the archetypal post-core-collapse globular cluster M15, and are capable of testing the predictions of some recent models4,5 of post-collapse cluster dynamics. We have discovered in the central region of NGC6397 a population of 'blue stragglers'6, stars that are too hot for their luminosity in relation to subgiant stars. Five of these blue stragglers are within 0.07 pc of the centre of the cluster, and are brighter than the rest by one magnitude. We suggest that the blue stragglers are either coalesced stars resulting from stellar collisions expected to occur especially during core-collapse7,8, or belong to a population of primordial close binaries that could provide the energy for the expansion of the core4.

Context. In globular clusters (GCs), blue straggler stars (BSS) are heavier than the average star, so dynamical friction strongly affects them. The radial distribution of BSS, normalized to a reference population, appears bimodal in a fraction of Galactic GCs, with a density peak in the core, a prominent zone of avoidance at intermediate radii, and again higher density in the outskirts. The zone of avoidance appears to be located at larger radii the more relaxed the host cluster, acting as a sort of dynamical clock. Aims. We use a new method to compute the evolution of the BSS radial distribution under dynamical friction and diffusion. Methods. We evolve our BSS in the mean cluster potential under dynamical friction plus a random fluctuating force, solving the Langevin equation with the Mannella quasi symplectic scheme. This is a new simulation method that is much faster and simpler than direct N-body codes, but retains their main feature: diffusion powered by strong, if infrequent, kicks. Results. We compute the radial distribution of initially unsegregated BSS normalized to a reference population as a function of time. We trace the evolution of its minimum, corresponding to the zone of avoidance. We compare the evolution under kicks extracted from a Gaussian distribution to that obtained using a Holtsmark distribution. The latter is a fat-tailed distribution which correctly models the effects of close gravitational encounters. We find that the zone of avoidance moves outwards over time, as expected based on observations, only when using the Holtsmark distribution. Thus, the correct representation of near encounters is crucial to reproduce the dynamics of the system. Conclusions. We confirm and extend earlier results that showed how the dynamical clock indicator depends on dynamical friction and on effective diffusion powered by dynamical encounters. We demonstrated the high sensitivity of the clock to the details of the mechanism underlying diffusion, which may explain the difficulties in reproducing the motion of the zone of avoidance across different simulation methods.

The sedimentation level of blue straggler stars (BSS) has been shown to be a great tool to investigate the dynamical states of globular clusters (GCs). The area enclosed between the cumulative radial distributions of BSS and a reference population up to the half-mass radius of the clusters, $A^+_{\mathrm{rh}}$, is known to be a measure of the sedimentation of BSS in GCs. In this work, we calculate $A^+_{\mathrm{rh}}$ for 11 open clusters (OCs) using a combined list of main-sequence turn-off stars, sub-giant branch stars, and red giant branch stars as reference population. The BSS, the reference populations, and the cluster members are identified using the proper motions and parallaxes from the Gaia DR2 data. In a subset of clusters, the BSS are confirmed cluster members on the basis of radial velocity information available in the literature. Using the Pearson and Spearman rank correlation coefficients, we find weak correlations between the estimated values of $A^+_{\mathrm{rh}}$ and other markers of dynamical ages of the clusters, i.e. the number of central relaxations a cluster has experienced since its formation, and the structural parameters of the clusters. Based on statistical tests, we find that these correlations are similar to the corresponding correlations among the less evolved GCs, albeit within large errors.

The present paper describes the behavior of the rotational velocity in\nmetal--poor stars ([Fe/H]<-0.5 dex) in different evolutionary stages, based on\nVsini values from the literature. Our sample is comprised of stars in the field\nand some Galactic globular clusters, including stars on the main sequence, the\nred giant branch (RGB), and the horizontal branch (HB). The metal--poor stars\nare, mainly, slow rotators, and their Vsini distribution along the HR diagram\nis quite homogeneous. Nevertheless, a few moderate to high values of Vsini are\nfound in stars located on the main sequence and on the HB. We show that the\noverall distribution of Vsini values is basically independent of metallicity\nfor the stars in our sample. In particular, the fast-rotating main sequence\nstars in our sample present similar rotation rates as their metal-rich\ncounterparts, suggesting that some of them may actually be fairly young, in\nspite of their low metallicity, or else that at least some of them would be\nbetter classified as blue straggler stars. We do not find significant evidence\nof evolution in Vsini values as a function of position on the RGB; in\nparticular, we do not confirm previous suggestions that stars close to the RGB\ntip rotate faster than their less evolved counterparts. While the presence of\nfast rotators among moderately cool blue HB stars has been suggested to be due\nto angular momentum transport from a stellar core that has retained significant\nangular momentum during its prior evolution, we find that any such transport\nmechanisms must likely operate very fast as the star arrives on the zero-age HB\n(ZAHB), since we do not find a link between evolution off the ZAHB and Vsini\nvalues.\n We present an extensive tabulation of all quantities discussed in this paper,\nincluding rotation velocities, temperatures, gravitie\n