Mass segregation

About: Mass segregation is a research topic. Over the lifetime, 1024 publications have been published within this topic receiving 57729 citations.

The evolution of the global stellar mass function of star clusters: an analytic description

Abstract: The evolution of the global stellar mass function of star clusters is studied based on a large set of N-body simulations of clusters with a range of initial masses, initial concentrations, in circular or elliptical orbits in different tidal environments. Models with and without initial mass segregation are included. The depletion of low-mass stars in initially Roche-volume (tidal) filling clusters starts typically on a time-scale of the order of the core collapse time. In clusters that are initially underfilling their Roche-volume it takes longer because the clusters have to expand to their tidal radii before dynamical mass-loss becomes important. We introduce the concept of the differential mass function (DMF), which describes the changes with respect to the initial mass function (IMF). We show that the evolution of the DMF can be described by a set of very simple analytic expressions that are valid for a wide range of initial cluster parameters and for different IMFs. The agreement between this description and the models is very good, except for initially Roche-volume underfilling clusters that are severely mass segregated.

Automated star counts in the southern globular-cluster ngc-6809(m55)

Abstract: THE ASTRONOMICAL JOURNAL VOLUME 89, NUMBER 1 AUTOMATED STAR COUNTS IN THE SOUTHERN GLOBULAR CLUSTER NGC 6809 (M55) MICHAEL J. IRWIN Institute of Astronomy, Madingley Road, Cambridge CB3 OHA, England VIRGINIA TRIMBLE Department of Physics, University of California, Irvine, California 92717 and Astronomy Program, University of Maryland, College Park, Maryland 20742 Received 6 June 1983; revised 23 September I 983 ABSTRACT We report star counts, as a function of position and apparent magnitude, in the rich, relatively open southern globular cluster NGC 6809 (M55). Three AAO 150q plates were scanned by the Automatic Plate Measuring System (APM) at the Institute of Astronomy, Cambridge, and 20 825 images were counted by its associated software. Previously known features of rich globular clusters which appear in the raw counts include a flattening of the luminosity function below M 3 ~ + 5.2, increased central concentration of bright stars relative to faint ones (normally interpreted as mass segregation), and mild deviations in radial profile from King models. Crowding of the field, which causes the counting proce- dure to miss faint stars preferentially near the cluster center, contributes to all of these, and may be responsible for all of the apparent mass segregation, but not for all of the other two eflqects. The deviations from a smooth radial profile are real in the sense of showing up in other counts of this and other clusters and are of marginal statistical significance (1-20 for a Poisson distribution). The luminosi- ty function is the most detailed so far published for a globular cluster, and its precise shape near main- sequence turnofi‘ may be usable as an age indicator. Automated subtraction and comparison of plate pairs rediscovered five of the six known RR Lyrae variables and found one additional candidate variable of about the same brightness, as well as eight possible variables near the faint limit of the scans. Three of these are confirmed as variable by Liller, using conventional techniques. These are of appropriate luminosity, periods, and number to be either W UMa (contact binary) or BY Dra (spotted, rotating) stars and so are deserving of further investigation. We conclude that the APM in its normal image analysis mode did at least as well in its first try at measuring stars in a crowded globular cluster field as a novice human observer would. An experienced human observer can, however, extract more reliable information in the centers of such clusters, and the APM software can probably be modified to imitate JANUARY 1984 some of the fruits of such experience. I. INTRODUCING THE APM Counting stars as a function of apparent brightness and position in the sky has been part of astronomy at least since the time of Newton (1962; Hoskin 1982). Star counts, prop- erly done, can be remarkably informative, yielding informa- tion on problems ranging from the overall structure of our galaxy (Herschel 1785; Bahcall and Soneira 1981) to the de- tailed distribution of obscuring matter (Bok 1977; Tomita et al. 1979; Trimble 1977). They can also be remarkably tedious to perform (Weistrop 1968), requiring great patience and great consistency on the part of the counter. This is particu- larly so for the crowded fields of globular clusters, where star counts remain a major source of information on cluster structure and dynamics (King et al. 1968; Harris and Racine 1979) Interesting points not yet fully clarified include the Initial Mass Function of the main-sequence stars, the extent of mass segregation within clusters, deviations of the radial profiles from classic King (1966) models, and the differences in these things from one cluster to another. Da Costa (1982) and others have begun to address these questions with star counts done by traditional methods. We here ask the addi- tional question: can we be replaced by a machine? The an- swer, perhaps predictably, proves to be a qualified yes. Her- zog and Illingworth (1977) reached a similar conclusion using a PDS scanner and a CDC 6400 computer, but their counts do not seem to have been published. The machine in our case is the Automatic Plate Measur- ing System (APM) at the Institute of Astronomy in Cam- 83 Astron. J. 89 (1), January 1984 0004-6256/84/010083-122500.90 bridge (Kibblewhite et al. 1983). It is the product of nearly a decade of SERC-sponsored development (under the leader- ship of Edward Kibblewhite) and came into regular use in 1980. Its distinctive features include a laser spot scanner, permitting very rapid scanning and low measurement noise, and on-line processing capability that includes provision for density wedge calibration, sky background estimation, noise removal, and automatic image detection and computation of image parameters (integrated isophotal intensities, posi- tions, second order moments, peak intensities, and areal pro- files). The high scanning speed of the APM enables us to take extra care in determining the sky background. All measure- ments are done in two passes. On the first pass, the sky back- ground is estimated. The plate is partitioned into roughly fix §-mm pixels, and, for each of these background pixels, an array of 64X 64 intensity measurements is made. The inter- polated mode of this array is used as an initial estimate of the sky background. When the full 2D array of initial back- ground intensity measurements has been made, it is further processed through a nonlinear filter to give the final back- ground values. The purpose of the filter is to detect and cor- rect background values contaminated by the presence of re- solved astronomical images and to smooth the final values. By making use of all the background information in this way, it is possible to estimate local background levels over extended objects such as globular clusters or nearby large galaxies. On the second pass over the plate, a threshold is defined as a fixed additive isophote above the sky background, and © 1984 Am. Astron. Soc. 83 © American Astronomical Society 0 Provided by the NASA Astrophysics Data System

Death of a cluster: the destruction of M67 as seen by the SDSS

Abstract: We probe the spatial and dynamical structure of the old open cluster M67 using photometric data from the Sloan Digital Sky Survey's sixth data release. Making use of an optimal contrast, or matched filter, algorithm, we map the distribution of high probability members of M67. We find an extended and elongated halo of likely members to a radius of nearly 60'. Our measured core radius of Rcore = 8.'24+/-0.'60 is somewhat larger than that of previous estimates. We attribute the larger core radius measurement to the SDSS probing lower mass main sequence stars than has been done before for similar studies of M67, and the exclusion of post main sequence M67 members in the SDSS sample. We estimate the number of M67 members in our SDSS sample to be 1385+/-67 stars. A lower limit on the binary fraction in M67 is measured to be 45%. A higher fraction of binary stars is measured in the core as compared to the halo, and the luminosity function of the core is found to be more depleted of low-mass stars. Thus the halo is consistent with mass segregation within the cluster. The galactic orbit of M67 is calculated from recent proper motion and radial velocity determinations. The elongated halo is roughly aligned to the proper motion of the cluster. This appears to be a result of mass segregation due to the galactic tidal field. Our algorithm is run on 2MASS photometry to directly compare to previous studies of M67. Decreasing core radii are found for stars with greater masses. We test the accuracy of our algorithm using 1000 artificial cluster Monte Carlo simulations. It is found that the matched filter technique is suitable for recovering low-density spatial structures, as well as measuring the binary fraction of the cluster.

Structure and stellar content analysis of the open cluster M11 with 2MASS photometry

Abstract: An overall analysis of the structure and stellar content of M11 is presented, thanks to the wide-angle 2MASS spatial coverage. We derive photometric and structural parameters and discuss the spatial dependance of the luminosity and mass functions. Photometric parameters basically agree with previous ones mostly based on the optical. We obtained a core radius of 1.23pc and a tidal radius of 29pc. In particular, the cluster is populous enough so that the tidal radius could be obtained by fitting the three-parameter King profile to the radial distribution of stars. We analyzed the spatial distribution of mass functions, finding that the the slope changes from -0.73 in the core to +2.88 in the outer halo. The spatial distribution of mass function slopes derived from 2MASS agrees with that derived from optical CCD data, which further confirms the reliability of 2MASS data for future analyses of this kind at comparable observational limits. We detect mass segregation up to distances from the center of ~20arcmin. We emphasize that the mass function slope in the core is flatter than anywhere else as a consequence of mass segregation. The derived total cluster mass is ~11000 solar masses.

Photometric and structural properties of ngc 6544: a combined vvv-hubble space telescope study

Abstract: We combine archival Hubble Space Telescope imaging with wide-field near-infrared photometry to study the neglected metal-poor Galactic globular cluster NGC 6544. A high spatial resolution map of differential reddening over the inner portion of the cluster is constructed, revealing variations of up to half of the total reddening, and the resulting corrected color-magnitude diagrams reveal a sparse blue horizontal branch and centrally concentrated blue straggler population, verified via relative proper motions. Using the corrected photometry to investigate the cluster distance, reddening, and age via direct comparison to well-calibrated photometry of clusters with similar metallicities, we estimate (m – M)0 = 11.96, E(B – V) = 0.79, and an age coeval with M13 to within the relevant uncertainties. Although our data are insufficient to place tight constraints on the reddening law toward NGC 6544, we find no strong evidence that it is non-standard at optical or near-infrared wavelengths. We also provide near-infrared fiducial sequences extending nearly 2 mag below the cluster main sequence turnoff, generated from a statistically decontaminated sample of cluster stars. Lastly, we redetermine the cluster center and construct a radial number density profile which is well fit by an atypically flat power law with a slope of about 1.7. We discuss this result, together with a flattened main sequence luminosity function and inverted mass function, in the context of mass segregation and tidal stripping via interactions with Milky Way potential.