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 embedded star clusters

Abstract: We study the evolution of embedded clusters. The equations of motion of the stars in the cluster are solved by direct N-body integration while taking the effects of stellar evolution and the hydrodynamics of the natal gas content into account. The gravity of the stars and the surrounding gas are coupled self-consistently to allow the realistic dynamical evolution of the cluster. While the equations of motion are solved, a stellar evolution code keeps track of the changes in stellar mass, luminosity and radius. The gas liberated by the stellar winds and supernovae deposits mass and energy into the gas reservoir in which the cluster is embedded. We examine cluster models with 1000 stars, but we varied the star formation efficiency (between 0.05 and 0.5), cluster radius (0.1–1.0 pc), the degree of virial support of the initial population of stars (0–100 per cent) and the strength of the feedback. We find that an initial star fraction M★/Mtot > 0.05 is necessary for cluster survival. Survival is more likely if gas is not blown out violently by a supernova and if the cluster has time to approach virial equilibrium during outgassing. While the cluster is embedded, dynamical friction drives early and efficient mass segregation in the cluster. Stars of m≳ 2 M⊙ are preferentially retained, at the cost of the loss of less massive stars. We conclude that the degree of mass segregation in open clusters such as the Pleiades is not the result of secular evolution but a remnant of its embedded stage.

On the evolution of a star cluster and its multiple stellar systems following gas dispersal

Abstract: We investigate the evolution, following gas dispersal, of a star cluster produced from a hydrodynamical calculation. We find that when the gas, initially comprising 60% of the mass, is removed, the system settles into a bound cluster containing 30-40% of the stellar mass surrounding by an expanding halo of ejected stars. The bound cluster expands from an initial radius of <0.05 pc to 1-2 pc over 4-10 Myr, depending on how quickly the gas is removed, implying that stellar clusters may begin with far higher stellar densities than usually assumed. With rapid gas dispersal the most massive stars are found to be mass segregated for the first ~1 Myr of evolution, but classical mass segregation only develops for cases with long gas removal timescales. Eventually, many of the most massive stars are expelled from the bound cluster. Despite the high initial stellar density and the extensive dynamical evolution of the system, we find that the stellar multiplicity is almost constant during the 10 Myr of evolution. This is because the primordial multiple systems are formed in a clustered environment and, thus, by their nature are already resistant to further evolution. The majority of multiple system evolution is confined to the decay of high-order systems and the formation of a significant population of very wide (10^4-10^5 AU) multiple systems in the expanding halo. This formation mechanism for wide binaries potentially solves the problem of how most stars apparently form in clusters and yet a substantial population of wide binaries exist in the field. Many of these wide binaries and the binaries produced by the decay of high-order multiple systems have unequal mass components, potentially solving the problem that hydrodynamical simulations of star formation are found to under-produce unequal-mass solar-type binaries.

On the distribution of stellar remnants around massive black holes: slow mass segregation, star cluster inspirals, and correlated orbits

Abstract: We use N-body simulations as well as analytical techniques to study the long-term dynamical evolution of stellar black holes (BHs) at the Galactic center (GC) and to put constraints on their number and mass distribution. Starting from models that have not yet achieved a state of collisional equilibrium, we find that timescales associated with cusp regrowth can be longer than the Hubble time. Our results cast doubts on standard models that postulate high densities of BHs near the GC and motivate studies that start from initial conditions that correspond to well-defined physical models. For the first time, we consider the distribution of BHs in a dissipationless model for the formation of the Milky Way nuclear cluster (NC), in which massive stellar clusters merge to form a compact nucleus. We simulate the consecutive merger of ~10 clusters containing an inner dense sub-cluster of BHs. After the formed NC is evolved for ~5 Gyr, the BHs do form a steep central cusp, while the stellar distribution maintains properties that resemble those of the GC NC. Finally, we investigate the effect of BH perturbations on the motion of the GC S-stars as a means of constraining the number of the perturbers. We find that reproducing the quasi-thermal character of the S-star orbital eccentricities requires gsim 1000 BHs within 0.1 pc of Sgr A*. A dissipationless formation scenario for the GC NC is consistent with this lower limit and therefore could reconcile the need for high central densities of BHs (to explain the S-stars orbits) with the "missing-cusp" problem of the GC giant star population.

On strong mass segregation around a massive black hole: implications for lower-frequency gravitational-wave astrophysics

Abstract: We present, for the first time, a clear N-body (NB) realization of the strong mass segregation solution for the stellar distribution around a massive black hole (MBH). We compare our NB results with those obtained by solving the orbit-averaged Fokker-Planck (FP) equation in energy space. The NB segregation is slightly stronger than in the FP solution, but both confirm the robustness of the regime of strong segregation when the number fraction of heavy stars is a (realistically) small fraction of the total population. In view of recent observations revealing a dearth of giant stars in the sub-parsec region of the Milky Way, we show that the timescales associated with cusp re-growth are not longer than (0.1 – 0.25) × Trlx (rh ). These timescales are shorter than a Hubble time for black holes masses M • 4 × 106 M ☉ and we conclude that quasi-steady, mass-segregated, stellar cusps may be common around MBHs in this mass range. Since extreme mass ratio inspirals detection rates by Laser Interferometer Space Antenna are expected to peak for M • ~ 4 × 105-106 M ☉, a good fraction of these events should originate from strongly segregated stellar cusps.

The Mass Function of the Arches Cluster from Gemini Adaptive Optics Data

Abstract: We have analysed high resolution adaptive optics (AO) science demonstration data of the young, massive stellar cluster Arches near the Galactic Center, obtained with the Gemini North telescope in combination with the University of Hawai'i AO system Hokupa'a. The AO H and K 0 photometry is calibrated using HST/NICMOS observations in the equivalent filters F160W and F205W obtained by Figer et al. (1999). The calibration procedure allows a detailed comparison of the ground-based adaptive optics observations against diraction limited space-based photometry. The spatial resolution as well as the overall signal-to-noise ratio of the Gemini/Hokupa'a data is comparable to the HST/NICMOS data. The low Strehl ratio of only a few percent is the dominant limiting factor in the Gemini AO science demonstration data as opposed to space-based observations. After a thorough technical comparison, the Gemini and HST data are used in combination to study the spatial distribution of stellar masses in the Arches cluster. Arches is one of the densest young clusters known in the Milky Way, with a central density of3 10 5 M pc 3 and a total mass of about 10 4 M. A strong colour gradient is observed over the cluster field. The visual extinction increases byAV 10 mag over a distance of 15 00 from the cluster core. Extinction maps reveal a low-extinction cavity in the densest parts of Arches (R 5 00 ), indicating the depletion of dust due to stellar winds or photo-evaporation. We correct for the change in extinction over the field and show that the slope of the mass function is strongly influenced by the eects of dierential extinction. We obtain present-day mass function slopes of 0:8 0: 2i n the mass range 6 10 00 , in accordance with a Salpeter slope ( = 1:35). The bias in the mass function towards high-mass stars in the Arches center is a strong indication for mass segregation. The dynamical and relaxation timescales for Arches are estimated, and possible mass segregation eects are discussed with respect to cluster formation models.