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Register a new userEvidence for primordial mass segregation in globular clusters
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We have studied the dissolution of initially mass segregated and unsegregated star clusters due to two-body relaxation in external tidal fields, using Aarseths collisional N-body code NBODY4 on GRAPE6 special-purpose computers. When extrapolating results of initially not 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 which 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 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 which was recently discovered by De Marchi et al. (2007). 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 Msun. 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 which can initialise stationary mass segregated clusters with arbitrary density profile and amount of mass segregation.
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We present a model to explain the mass segregation and shallow mass functions observed in the central parts of dense and young starburst stellar clusters. The model assumes that the initial pre-stellar cores mass function resulting from the turbulent fragmentation of the proto-cluster cloud is significantly altered by the cores coalescence before they collapse to form stars. With appropriate, yet realistic parameters, this model based on the competition between cores coalescence and collapse reproduces the mass spectra of the well studied Arches cluster. Namely, the slopes at the intermediate and high mass ends are reproduced, as well as the peculiar bump observed at 6 M_sol. This coalescence-collapse process occurs on short timescale of the order of one fourth the free fall time of the proto-cluster cloud (i.e., a few 10^{4} years), suggesting that mass segregation in Arches and similar clusters is primordial. The best fitting model implies the total mass of the Arches cluster is 1.45 10^{5} M_sol, which is slightly higher than the often quoted, but completeness affected, observational value of a few 10^{4} M_sol. The derived star formation efficiency is ~30 percent which implies that the Arches cluster is likely to be gravitationally bound.
Observations of young star-forming regions suggest that star clusters are born completely mass segregated. These initial conditions are, however, gradually lost as the star cluster evolves dynamically. For star clusters with single stars only and a canonical initial mass function, it has been suggested that traces of these initial conditions vanish at a time $tau_mathrm{v}$ between 3 and $3.5,t_mathrm{rh}$ (initial half-mass relaxation times). Since a significant fraction of stars are observed in binary systems and it is widely accepted that most stars are born in binary systems, we aim to investigate what role a primordial binary population (even up to $100,%$ binaries) plays in the loss of primordial mass segregation of young star clusters. We used numerical $N$-body models similar in size to the Orion Nebula Cluster (ONC) -- a representative of young open clusters -- integrated over several relaxation times to draw conclusions on the evolution of its mass segregation. We also compared our models to the observed ONC. We found that $tau_mathrm{v}$ depends on the binary star fraction and the distribution of initial binary parameters that include a semi-major axis, eccentricity, and mass ratio. For instance, in the models with $50,%$ binaries, we find $tau_mathrm{v} = (2.7 pm 0.8),t_mathrm{rh}$, while for $100,%$ binary fraction, we find a lower value $tau_mathrm{v} = (2.1 pm 0.6),t_mathrm{rh}$. We also conclude that the initially completely mass segregated clusters, even with binaries, are more compatible with the present-day ONC than the non-segregated ones.
Observational results of young star-forming regions suggest that star clusters are completely mass segregated at birth. As a star cluster evolves dynamically, these initial conditions are gradually lost. For star clusters with single stars only and a canonical IMF, it has been suggested that traces of these initial conditions vanish at $tau_{rm v}$ between 3 and 3.5 half-mass relaxation times. By the means of numerical models, here we investigate the role of the primordial binary population on the loss of primordial mass segregation. We found that $tau_{rm v}$ does not seem to depend on the binary star distribution, yielding $3 < tau_{rm v} / t_{rm rh} < 3.5$. We also conclude that the completely mass segregated clusters, even with binaries, are more compatible with the present-day ONC than the non-segregated ones.
We used a combination of Hubble Space Telescope and ground based data to probe the dynamical state of the low mass Galactic globular cluster NGC 6101. We have re-derived the structural parameters of the cluster by using star counts and we find that it is about three times more extended than thought before. By using three different indicators, namely the radial distribution of Blue Straggler Stars, that of Main Sequence binaries and the luminosity (mass) function, we demonstrated that NGC 6101 shows no evidence of mass segregation, even in the innermost regions. Indeed, both the BSS and the binary radial distributions fully resemble that of any other cluster population. In addition the slope of the luminosity (mass) functions does not change with the distance, as expected for non relaxed stellar systems. NGC 6101 is one of the few globulars where the absence of mass segregation has been observed so far. This result provides additional support to the use of the dynamical clock calibrated on the radial distribution of the Blue Stragglers as a powerful indicator of the cluster dynamical age.
Using X-ray sources as sensitive probes of stellar dynamical interactions in globular clusters (GCs), we study the mass segregation and binary burning processes in $omega$ Cen. We show that the mass segregation of X-ray sources is quenched in $omega$ Cen, while the X-ray source abundance of $omega$ Cen is much smaller than other GCs, and the binary hardness ratio (defined as $L_{rm X}/(L_{rm K}f_{b})$, with $f_{b}$ the binary fraction, $L_{rm X}$ and $L_{rm K}$ the cumulative X-ray and K band luminosity of GCs, respectively) of $omega$ Cen is located far below the $L_{rm X}/(L_{rm K}f_{b})-sigma_{c}$ correlation line of the dynamically normal GCs. These evidences suggest that the binary burning processes are highly suppressed in $omega$ Cen, and other heating mechanisms, very likely a black hole subsystem (BHS), are essential in the dynamical evolution of $omega$ Cen. Through the black hole burning processes (i.e., dynamical hardening of the BH binaries), the BHS is dominating the energy production of $omega$ Cen, which also makes $omega$ Cen a promising factory of gravitational-wave sources in the Galaxy.
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