Mass segregation

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

The Modification of Planetary Orbits in Dense Open Clusters

Abstract: We show that Jovian planets will frequently experience orbital disruption due to interactions with binary stars in their birth clusters. We attack the problem using a Monte Carlo approach and estimate the effective cross section for severe orbital disruption: σ=(230 AU)2. Combining the numerically determined cross section with typical cluster densities n and velocity dispersions v, we find a typical interaction rate of Γ=nσv ≈ 0.13 disruptive encounters per star per 100 million years. This scattering mechanism naturally accounts for extrasolar systems such as 14 Her or 16 Cyg B, in which a Jovian planet is found in an eccentric and reasonably close orbit. This mechanism can also produce systems with extremely small orbits, such as 51 Peg. However, the efficiency is too low to account for the observed frequency of such close systems, and hence some other mechanism for orbital migration is almost certainly at work. Because of mass segregation within the birth cluster, we predict that higher mass stars will have a larger fraction of planets with eccentric orbits than the low-mass stars that comprise the bulk of the stellar population.

A 500 Parsec Halo Surrounding the Galactic Globular NGC 1851

Abstract: Using imaging that shows 4 mag of main-sequence stars, we have discovered that the Galactic globular cluster NGC 1851 is surrounded by a halo that is visible from the tidal radius of 700 arcsec (41 pc) to more than 4500 arcsec (>250 pc). This halo is symmetric and falls in density as a power law of r {sup -1.24}. It contains approximately 0.1% of the dynamical mass of NGC 1851. There is no evidence for tidal tails. Current models of globular cluster evolution do not explain this feature, although simulations of tidal influences on dwarf spheroidal galaxies qualitatively mimic these results. Given the state of published models, it is not possible to decide between creation of this halo from either isolated cluster evaporation or from tidal or disk shocking, or from destruction of a dwarf galaxy in which this object may have once been embedded.

A stellar census in globular clusters with MUSE : Binaries in NGC 3201

Abstract: We utilise multi-epoch MUSE spectroscopy to study binary stars in the core of the Galactic globular cluster NGC 3201. Our sample consists of 3553 stars with 54 883 spectra in total comprising 3200 main-sequence stars up to 4 magnitudes below the turn-off. Each star in our sample has between 3 and 63 (with a median of 14) reliable radial velocity measurements within five years of observations. We introduce a statistical method to determine the probability of a star showing radial velocity variations based on the whole inhomogeneous radial velocity sample. Using HST photometry and an advanced dynamical MOCCA simulation of this specific cluster we overcome observational biases that previous spectroscopic studies had to deal with. This allows us to infer a binary frequency in the MUSE field of view and enables us to deduce the underlying true binary frequency of (6.75 ± 0.72)% in NGC 3201. The comparison of the MUSE observations with the MOCCA simulation suggests a large portion of primordial binaries. We can also confirm a radial increase in the binary fraction towards the cluster centre due to mass segregation. We discovered that in the core of NGC 3201 at least (57.5 ± 7.9)% of blue straggler stars are in a binary system. For the first time in a study of globular clusters, we were able to fit Keplerian orbits to a significant sample of 95 binaries. We present the binary system properties of eleven blue straggler stars and the connection to SX Phoenicis-type stars. We show evidence that two blue straggler formation scenarios, the mass transfer in binary (or triple) star systems and the coalescence due to binary-binary interactions, are present in our data. We also describe the binary and spectroscopic properties of four sub-subgiant (or red straggler) stars. Furthermore, we discovered two new black hole candidates with minimum masses (M sin i ) of (7.68 ± 0.50) M ⊙ , (4.4 ± 2.8) M ⊙ , and refine the minimum mass estimate on the already published black hole to (4.53 ± 0.21) M ⊙ . These black holes are consistent with an extensive black hole subsystem hosted by NGC 3201.

Statistics of N-body simulations — III. Unequal masses

Abstract: We describe results from large numbers of $N$-body simulations containing from $250$ to $1000$ stars each. The distribution of stellar masses is a power law, and the systems are isolated. While the collapse of the core exhibits the expected segregation of different masses, we find that the post-collapse evolution is, at a first approximation, homologous. This is quite surprising because there is no reason for supposing that mass segregation should not continue to have a substantial effect on the evolution of the cluster. In fact the spatial distribution of the mean stellar mass is nearly static throughout the post-collapse regime, except for the overall expansion of the systems, and this helps to explain why the post-collapse evolution is nearly self-similar. Self-similarity is also exhibited by the distribution of anisotropy and the profile of departures from equipartition, which show little change during the post-collapse phase. The departures from energy equipartition and isotropy are small in the core and increase with radius. During post-collapse evolution massive stars (mainly) are removed from the system by binary activity. This effect dominates the preferential escape of low-mass stars due to standard two-body relaxation processes.

The CFHT Open Star Cluster Survey. IV. Two Rich, Young Open Star Clusters: NGC 2168 (M35) and NGC 2323 (M50)

Abstract: We continue our study of rich Galactic clusters by presenting deep CCD observations of both NGC 2168 (M35) and NGC 2323 (M50). Both clusters are found to be rich (NGC 2168 contains at least 1000 stars brighter than V = 22, and NGC 2323 contains ~2100 stars brighter than our photometric limit of V ~ 23) and young (NGC 2168 age = 180 Myr and for NGC 2323 age = 130 Myr). The color-magnitude diagrams for the clusters exhibit clear main sequences stretching over 14 mag in the (V, B-V)-plane. Comparing these long main sequences with those of earlier clusters in the survey, as well as with the Hyades, has allowed for accurate distances to be established for each cluster (for NGC 2168 d = 912 pc and for NGC 2323 d = 1000 pc). Analysis of the luminosity and mass functions suggests that, despite their young ages, both clusters are somewhat dynamically relaxed, exhibiting signs of mass segregation. This is especially interesting in the case of NGC 2323, which has an age of only 1.3 times the dynamical relaxation time. The present photometry is also deep enough to detect all of the white dwarfs in both clusters. We discuss some interesting candidates that may be the remnants of quite massive (M ≥ 5 M⊙) progenitor stars. The white dwarf cooling age of NGC 2168 is found to be in good agreement with the main-sequence turnoff age. These objects are potentially very important for setting constraints on the white dwarf initial-final mass relationship and the upper mass limit for white dwarf production.