Mass segregation

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

The Abundance Spread Among Giants and Subgiants in the Globular Cluster Omega Centauri

Abstract: We present spectroscopic abundances and radial velocities for giant stars in the Galactic globular cluster omega Centauri based on the CaII infrared triplet. Two samples of stars were observed: 234 stars at M_V = 1.25 on the lower giant branch at radial distances between 8 and 23arcmin, and 145 stars at M_V = -1.3 at radial distances between 3 and 22arcmin. Previous metallicity studies found a non-gaussian metallicity distribution containing a tail of metal-rich stars. We confirm these results except our unbiased cluster metallicity distributions are significantly narrower. They contain the following key features: (1) No very metal-poor stars, (2) a sudden rise in the metal-poor distribution to a modal [Fe/H] value of --1.70 consistent with an homogeneous metallicity unresolved at the 0.07 dex level, (3) a tail to higher metallicities with more stars than predicted by simple chemical evolution models, and (4) a weak correlation between metallicity and radius such that the most metal-rich stars are concentrated to the cluster core. The unresolved metal-weak tail implies that the gas out of which omega Cen formed was well-mixed up to the modal metallicity of the cluster. Therefore, omega Cen like other Galactic globular clusters, seems to have formed in a pre-enriched and homogenized (up to the modal metallicity) environment. The existence of a weak metallicity gradient supports the idea that omega Cen self-enriched, with the enriched gas sinking to the cluster center due to gas dissipation processes. We also note, however, that the metal-rich stars are more massive than the bulk of the stars in the cluster, and may have sunk to the center by dynamical mass segregation over the lifetime of the cluster.

Deep Hubble Space Telescope WFPC2 Photometry of NGC 288. I. Binary Systems and Blue Stragglers

Abstract: We present the first results of a deep WFPC2 photometric survey of the loose galactic globular cluster NGC 288. The fraction of binary systems is estimated from the color distribution of objects near the main sequence (MS) with a method analogous to that introduced by Rubenstein & Bailyn. We have unequivocally detected a significant population of binary systems with a radial distribution that has been significantly influenced by mass segregation. In the inner region of the cluster (r < 1rh 1.6rc) the binary fraction (fb) lies in the range 0.08–0.38 regardless of the assumed distribution of mass ratios, F(q). The most probable fb lies between 0.10 and 0.20 depending on the adopted F(q). On the other hand, in the outer region (r ≥ 1rh), fb must be less than 0.10, and the most likely value is 0.0, independently of the adopted F(q). The detected population of binaries is dominated by primordial systems. The specific frequency of blue stragglers (BSs) is exceptionally high, suggesting that the BS production mechanism via binary evolution can be very efficient. A large population of BSs is possible even in low-density environments if a sufficient reservoir of primordial binaries is available. The observed distribution of BSs in the color-magnitude diagram is not compatible with a rate of BS production that has been constant in time, if it is assumed that all the BSs are formed by the merging of two stars.

The Star Formation History and Mass Function of the Double Cluster h and Chi Persei

Abstract: The h and Chi Per "double cluster" is examined using wide-field (0.98 deg x 0.98 deg) CCD UBV imaging supplemented by optical spectra of several hundred of the brightest stars. Restricting our analysis to near the cluster nuclei, we find identical reddenings (E(B-V)=0.56+/-0.01), distance moduli (11.85+/-0.05), and ages (12.8+/-1.0 Myr) for the two clusters. In addition, we find an IMF slope for each of the cluster nuclei that is quite normal for high-mass stars, Gamma=-1.3+/-0.2, indistinguishable from a Salpeter value. We derive masses of 3700 M_Sun (h) and 2800 M_Sun (Chi) integrating the PDMF from 1 to 120 M_Sun. There is evidence of mild mass segregation within the cluster cores. Our data are consistent with the stars having formed at a single epoch; claims to the contrary are very likely due to the inclusion of the substantial population of early-type stars located at similar distances in the Perseus spiral arm, in addition to contamination by G and K giants at various distances. We discuss the uniqueness of the double cluster, citing other examples of such structures in the literature, but concluding that the nearly identical nature of the two cluster cores is unusual. We fail to settle the long-standing controversy regarding whether or not the double cluster is the core of the Per OB1 association, and argue that this may be unanswerable with current techniques. We also emphasize the need for further work on the pre-main sequence population of this nearby and highly interesting region.

Evidence for primordial mass segregation in globular clusters

Abstract: We have studied the dissolution of initially mass-segregated and unsegregated star clusters due to two-body relaxation in external tidal fields, using Aarseth's collisional N-body code NBODY4 on GRAPE6 special-purpose computers. When extrapolating results of initially non-mass-segregated models to globular clusters, we obtain a correlation between the time until destruction and the slope of the mass function, in the sense that globular clusters that are closer to dissolution are more strongly depleted in low-mass stars. This correlation fits observed mass functions of most globular clusters. The mass functions of several globular clusters are, however, more strongly depleted in low-mass stars than is suggested by these models. Such strongly depleted mass functions can be explained if globular clusters started initially mass segregated. Primordial mass segregation also explains the correlation between the slope of the stellar mass function and the cluster concentration that was recently discovered by De Marchi and coworkers. In this case, it is possible that all globular clusters started with a mass function similar to that seen in young open clusters in the present-day universe, at least for stars below m = 0.8 M☉. This argues for a near universality of the mass function for different star formation environments and metallicities in the range -2 < [ Fe/H ] < 0. We finally describe a novel algorithm that can initialize stationary mass-segregated clusters with an arbitrary density profile and amount of mass segregation.

Using the minimum spanning tree to trace mass segregation

Abstract: We present a new method to detect and quantify mass segregation in star clusters. It compares the minimum spanning tree (MST) of massive stars with that of random stars. If mass segregation is present, the MST length of the most massive stars will be shorter than that of random stars. This difference can be quantified (with an associated significance) to measure the degree of mass segregation. We test the method on simulated clusters in both 2D and 3D and show that the method works as expected. We apply the method to the Orion Nebula Cluster (ONC) and show that the method is able to detect the mass segregation in the Trapezium with a `mass segregation ratio' \Lambda_{MSR}=8.0 \pm 3.5 (where \Lambda_{MSR}=1 is no mass segregation) down to 16 \Msun, and also that the ONC is mass segregated at a lower level (~2.0 \pm 0.5) down to 5 \Msun. Below 5 \Msun we find no evidence for any further mass segregation in the ONC.