The Blue Straggler Population in the Globular Cluster M53 (NGC 5024): A Combined HST , LBT, and CFHT Study1 (original) (raw)

Mass segregation of different populations inside the cluster NGC 6101

Astronomy and Astrophysics, 2001

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, r h = 1.71 ′ ) 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 (V = 20.05 ± 0.05) 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 . 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, VZAHB = 16.59 ± 0.10, VTO = 20.05 ± 0.05, coupled with the values E(B-V) = 0.1, [Fe/H] = -1.80, Y = 0.23 yield a distance modulus (m − M )V = 16.23 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.

Peculiarities in velocity dispersion and surface density profiles of star clusters

Monthly Notices of the Royal Astronomical Society, 2010

Based on our recent work on tidal tails of star clusters we investigate star clusters of a few 10 4 M ⊙ by means of velocity dispersion profiles and surface density profiles. We use a comprehensive set of N -body computations of star clusters on various orbits within a realistic tidal field to study the evolution of these profiles with time, and ongoing cluster dissolution. From the velocity dispersion profiles we find that the population of potential escapers, i.e. energetically unbound stars inside the Jacobi radius, dominates clusters at radii above about 50% of the Jacobi radius. Beyond 70% of the Jacobi radius nearly all stars are energetically unbound. The velocity dispersion therefore significantly deviates from the predictions of simple equilibrium models in this regime. We furthermore argue that for this reason this part of a cluster cannot be used to detect a dark matter halo or deviations from Newtonian gravity. By fitting templates to the about 10 4 computed surface density profiles we estimate the accuracy which can be achieved in reconstructing the Jacobi radius of a cluster in this way. We find that the template of King (1962) works well for extended clusters on nearly circular orbits, but shows significant flaws in the case of eccentric cluster orbits. This we fix by extending this template with 3 more free parameters. Our template can reconstruct the tidal radius over all fitted ranges with an accuracy of about 10%, and is especially useful in the case of cluster data with a wide radial coverage and for clusters showing significant extra-tidal stellar populations. No other template that we have tried can yield comparable results over this range of cluster conditions. All templates fail to reconstruct tidal parameters of concentrated clusters, however. Moreover, we find that the bulk of a cluster adjusts to the mean tidal field which it experiences and not to the tidal field at perigalacticon as has often been assumed in other investigations, i.e. a fitted tidal radius is a cluster's time average mean tidal radius and not its perigalactic one. Furthermore, we study the tidal debris in the vicinity of the clusters and find it to be well represented by a power-law with a slope of -4 to -5. This steep slope we ascribe to the epicyclic motion of escaped stars in the tidal tails. Star clusters close to apogalacticon show a significantly shallower slope of up to -1, however. We suggest that clusters at apogalacticon can be identified by measuring this slope.

Blue Straggler Stars in the Unusual Globular Cluster NGC 6388

Astrophysical Journal, 2008

We have used multi-band high resolution HST WFPC2 and ACS observations combined with wide field ground-based observations to study the blue straggler star (BSS) population in the galactic globular cluster NGC 6388. As in several other clusters we have studied, the BSS distribution is found to be bimodal: highly peaked in the cluster center, rapidly decreasing at intermediate radii, and rising again at larger radii. In other clusters the sparsely populated intermediate-radius region (or ``zone of avoidance'') corresponds well to that part of the cluster where dynamical friction would have caused the more massive BSS or their binary progenitors to settle to the cluster center. Instead, in NGC 6388, BSS still populate a region that should have been cleaned out by dynamical friction effects, thus suggesting that dynamical friction is somehow less efficient than expected. As by-product of these observations, the peculiar morphology of the horizontal branch (HB) is also confirmed. In particular, within the (very extended) blue portion of the HB we are able to clearly characterize three sub-populations: ordinary blue HB stars, extreme HB stars, and blue hook stars. Each of these populations has a radial distribution which is indistinguishable from normal cluster stars.

TIDAL DISRUPTION, GLOBAL MASS FUNCTION, AND STRUCTURAL PARAMETER EVOLUTION IN STAR CLUSTERS

The Astrophysical Journal, 2010

We present a unified picture for the evolution of star clusters on the twobody relaxation timescale. We use direct N-body simulations of star clusters in a galactic tidal field starting from different multi-mass King models, up to 10% of primordial binaries and up to N tot = 65536 particles. An additional run also includes a central Intermediate Mass Black Hole. We find that for the broad range of initial conditions we have studied the stellar mass function of these systems presents a universal evolution which depends only on the fractional mass loss. The structure of the system, as measured by the core to half mass radius ratio, also evolves toward a universal state, which is set by the efficiency of heating on the visible population of stars induced by dynamical interactions in the core of the system. Interactions with dark remnants (white dwarfs, neutron stars and stellar mass black holes) are dominant over the heating induced by a moderate population of primordial binaries (3-5%), especially under the assumption that most of the neutron stars and black holes are retained in the system. All our models without primordial binaries undergo a deep gravothermal collapse in the radial mass profile. However their projected light distribution can be well fitted by medium concentration King models (with parameter W 0 ∼ 8), even though there tends to be an excess over the best fit for the innermost points of the surface brightness. This excess is consistent with a shallow cusp in the surface brightness (µ ∼ R −ν with ν ∼ 0.4−0.7), like it has been observed for many globular clusters from high-resolution HST imaging. Generally fitting a King profile to derive the structural parameters yields to larger fluctuations in the core size than defining the core as the radius where the surface brightness is one half of its central value. Classification of core-collapsed globular clusters based on their surface brightness profile may thus fail in systems that appear to have already bounced back to lower concentrations, particularly if the angular resolution of the observations is limited and the core is not well resolved.

Quantitative analysis of clumps in the tidal tails of star clusters

Monthly Notices of the Royal Astronomical Society, 2009

Tidal tails of star clusters are not homogeneous but show well defined clumps in observations as well as in numerical simulations. Recently an epicyclic theory for the formation of these clumps was presented. A quantitative analysis was still missing. We present a quantitative derivation of the angular momentum and energy distribution of escaping stars from a star cluster in the tidal field of the Milky Way and derive the connection to the position and width of the clumps. For the numerical realization we use star-by-star N -body simulations. We find a very good agreement of theory and models. We show that the radial offset of the tidal arms scales with the tidal radius, which is a function of cluster mass and the rotation curve at the cluster orbit. The mean radial offset is 2.77 times the tidal radius in the outer disc. Near the Galactic centre the circumstances are more complicated, but to lowest order the theory still applies. We have also measured the Jacobi energy distribution of bound stars and showed that there is a large fraction of stars (about 35%) above the critical Jacobi energy at all times, which can potentially leave the cluster. This is a hint that the mass loss is dominated by a self-regulating process of increasing Jacobi energy due to the weakening of the potential well of the star cluster, which is induced by the mass loss itself.

THE DYNAMICAL STATE OF THE GLOBULAR CLUSTER M10 (NGC 6254)

The Astrophysical Journal, 2010

Studying the radial variation of the stellar mass function in globular clusters (GCs) has proved a valuable tool to explore the collisional dynamics leading to mass segregation and core collapse. Recently, Pasquato et al. (2009) used the mass segregation profile to investigate the presence of an intermediate-mass black hole (IMBH) in NGC 2298. As a relaxed cluster with a large core, M 10 (NGC 6254) is suitable for a similar investigation. In order to study the radial dependence of the luminosity and mass function of M 10, we used deep high resolution archival images obtained with the Advanced Camera for Survey (ACS) on board the Hubble Space Telescope (HST), reaching out to approximately the cluster's half-mass radius (r hm ), combined with deep Wide Field and Planetary Camera 2 (WFPC2) images that extend our radial coverage to more than 2 r hm . From our photometry, we derived a radial mass segregation profile and a global mass function that we compared with those of simulated clusters containing different energy sources (namely hard binaries and/or an IMBH) able to halt core collapse and to quench mass segregation. A set of direct N-body simulations of GCs, with and without an IMBH of mass 1% of the total cluster mass, comprising different initial mass functions (IMFs) and primordial binary fractions, was used to predict the observed mass segregation profile and mass function.

The extended structure of the remote cluster B514 in M 31

Astronomy and Astrophysics, 2007

Aims. We present a study of the density profile of the remote M31 globular cluster B514, obtained from HST/ACS observations. Methods. Coupling the analysis of the distribution of the integrated light with star counts we are able to reliably follow the profile of the cluster out to r ∼ 35 ′′ , corresponding to ≃ 130 pc. The profile is well fitted, out to ∼ 15 core radii, by a King Model having C=1.65. With an estimated core radius r c = 0.38 ′′ , this corresponds to a tidal radius of r t ∼ 17 ′′ (∼ 65 pc). The analysis of the light profile allows also the estimate of the ellipticity and position angle of the isophotes within r ≤ 20 ′′ . Results. We find that both the light and the star counts profiles show a departure from the best fit King model for r > ∼ 8 ′′ -as a surface brightness excess at large radii, and the star counts profile shows a clear break in correspondence of the estimated tidal radius. Both features are interpreted as the signature of the presence of extra tidal stars around the cluster. It is also shown that B514 has a half-light radius significantly larger than ordinary globular clusters of the same luminosity. In the M V vs. logr h plane, B514 lies in a region inhabited by peculiar clusters, like ω Cen, G1, NGC2419 and others, as well as by the nuclei of dwarf elliptical galaxies.

The Dynamical State and Blue Straggler Population of the Globular Cluster NGC 6266 (M62

Astronomical Journal, 2006

The millisecond pulsar population (whose members are all in binary systems) and the X-ray emitting population (more than 50 sources within the cluster half mass radius) suggest that NGC 6266 is in a dynamical phase particularly active in generating binaries through dynamical encounters. UV observations of the central region have been used to probe the population of blue straggler stars, whose origin might be also affected by dynamical interactions. The comparison with other globular clusters observed with a similar strategy shows that the blue straggler content in NGC 6266 is relatively low, suggesting that the formation channel that produces binary systems hosting neutron stars or white dwarfs is not effective in significantly increasing the blue straggler population. Moreover, an anticorrelation between millisecond pulsar content and blue straggler specific frequency in globular cluster seems emerging with increasing evidence.

Another Nonsegregated Blue Straggler Population in a Globular Cluster: the Case of NGC 2419

The Astrophysical Journal, 2008

We have used a combination of ACS-HST high-resolution and wide-field SUB-ARU data in order to study the Blue Straggler Star (BSS) population over the entire extension of the remote Galactic globular cluster NGC 2419. The BSS population presented here is among the largest ever observed in any stellar system, with more than 230 BSS in the brightest portion of the sequence. The radial distribution of the selected BSS is essentially the same as that of the other cluster stars. In this sense the BSS radial distribution is similar to that of ω Centauri and unlike that of all Galactic globular clusters studied to date, which have highly centrally segregated distributions and, in most cases, a pronounced upturn in the external regions. As in the case of ω Centauri, this evidence indicates that NGC 2419 is not yet relaxed even in the central regions. This observational fact is in agreement with estimated half-mass relaxation time, which is of the order of the cluster age.

Dynamics of the Globular Cluster System Associated with M87 (NGC 4486). II. Analysis

The Astrophysical Journal, 2001

1 2 DYNAMICS OF THE M87 GLOBULAR CLUSTER SYSTEM ABSTRACT We present a dynamical analysis of the globular cluster system associated with M87 (= NGC 4486), the cD galaxy near the dynamical center of the Virgo cluster. The analysis utilizes a new spectroscopic and photometric database which is described in a companion paper ). Using a sample of 278 globular clusters with measured radial velocities and metallicities, and new surface density profiles based on wide-field Washington photometry, we study the dynamics of the M87 globular cluster system both globally -for the entire cluster sample -and separately -for the metal-rich and metal-poor globular cluster samples. This constitutes the largest sample of radial velocities for pure Population II tracers yet assembled for any galaxy. Our principal findings are summarized as follows: