arXiv subjects

S. Portegies Zwart

Publications and source records attributed to S. Portegies Zwart.

61 records · Page 4Linked to original sources

arXiv2005

The Effect of Pre-Main Sequence Stars on Star Cluster Dynamics

We investigate the effects of the addition of pre-main sequence evolution to star cluster simulations. We allowed stars to follow pre-main sequence tracks that begin at the deuterium burning birthline and end at the zero age main sequence. We compared our simulations to ones in which the stars began their lives at the zero age main sequence, and also investigated the effects of particular choices for initial binary orbital parameters. We find that the inclusion of the pre-main sequence phase results in a slightly higher core concentration, lower binary fraction, and fewer hard binary systems. In general, the global properties of star clusters remain almost unchanged, but the properties of the binary star population in the cluster can be dramatically modified by the correct treatment of the pre-main sequence stage.

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arXiv2005

On the Origin of the most massive stars around R136

We discuss the signature of a peculiar constellation of very massive stars at a projected distance of 2-3 pc around R136a. We discuss various scenarios for its possible origin, such as independent clusters, triggered star formation, supernovae, and ejections via dynamical interactions. If the latter scenario were the correct one this would have significant implications on the way to probe the conditions in dense stellar cores, and on the evolution of massive clusters in general.

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arXiv2004

Recycled pulsars with black hole companions: the high mass analogues of PSR B2303+46

We investigate the possibility that mass transfer early in the evolution of a massive binary can effect a reversal of the end states of the two components, resulting in a neutron star which forms before a black hole. In this sense, such systems would comprise the high-mass analogues of white dwarf-neutron star systems such as PSR B2303+46. One consequence of this reversal is that a second episode of mass transfer from the black hole progenitor star can recycle the nascent neutron star, extending the life of the pulsar. An estimate of the formation rate through this channel is first performed via a simple analytical approach, and then refined using the results of the SeBa binary evolution package. The central role of kicks in determining the survival rate of these binaries is clearly demonstrated. The final result is expressed in terms of the number of field pulsars one can expect for every single neutron star-black hole (ns,bh) binary. We also calculate this figure for black hole-neutron star (bh,ns) systems formed through the usual channel. Assuming kicks drawn from the distribution of Cordes & Chernoff (1998), we find an expectation value of one (ns,bh) binary per 4 x 10^4 pulsars, and one (bh,ns) system for every 1500 pulsars. This helps to explain why neither system has been seen to date, though it suggests that detection of a (bh,ns) binary is imminent.

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arXiv2003

Dynamical Formation of Close Binary Systems in Globular Clusters

We know from observations that globular clusters are very efficient catalysts in forming unusual short-period binary systems or their offspring, such as low-mass X-ray binaries (LMXBs; neutron stars accreting matter from low-mass stellar companions), cataclysmic variables (CVs; white dwarfs accreting matter from stellar companions), and millisecond pulsars (MSPs; rotating neutron stars with spin periods of a few ms). Although there has been little direct evidence, the overabundance of these objects in globular clusters has been attributed by numerous authors to the high densities in the cores, which leads to an increase in the formation rate of exotic binary systems through close stellar encounters. Many such close binary systems emit X-radiation at low luminosities (L_x < 10^{34} erg/s) and are being found in large numbers through observations with the Chandra X-ray Observatory. Here we present conclusive observational evidence for a link between the number of close binaries observed in X-rays in a globular cluster and the stellar encounter rate of the cluster. We also make an estimate of the total number of LMXBs in globular clusters in our Galaxy.

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arXiv2003

Modest-2: A Summary

This is a summary paper of MODEST-2, a workshop held at the Astronomical Institute ``Anton Pannekoek'' in Amsterdam, 16-17 December 2002. MODEST is a loose collaboration of people interested in MOdelling DEnse STellar systems, particularly those interested in modelling these systems using all the available physics (stellar dynamics, stellar evolution, hydrodynamics and the interplay between the three) by defining interfaces between different codes. In this paper, we summarize 1) the main advances in this endeavour since MODEST-1; 2) the main science goals which can be and should be addressed by these types of simulations; and 3) the most pressing theoretical and modelling advances that we identified.

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arXiv2002

On the central structure of M15

We present a detailed comparison between the latest observational data on the kinematical structure of the core of M15, obtained with the Hubble STIS and WFPC2 instruments, and the results of dynamical simulations carried out using the special-purpose GRAPE-6 computer. The observations imply the presence of a significant amount of dark matter in the cluster core. In our dynamical simulations, neutron stars and/or massive white dwarfs concentrate to the center through mass segregation, resulting in a sharp increase in $M/L$ toward the center. While consistent with the presence of a central black hole, the Hubble data can also be explained by this central concentration of stellar-mass compact objects. The latter interpretation is more conservative, since such remnants result naturally from stellar evolution, although runaway merging leading to the formation of a black hole may also occur for some range of initial conditions. We conclude that no central massive object is required to explain the observational data, although we cannot conclusively exclude such an object at the level of $\sim500-1000$ solar masses. Our findings are unchanged when we reduce the assumed neutron-star retention fraction in our simulations from 100% to 0%.

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arXiv2002

Selection of the SIM Astrometric Grid

We investigate the choice of stellar population for use as the Astrometric Grid for the Space Interferometry Mission (SIM). SIM depends on the astrometric stability of about 2000 stars, the so called Grid, against which the science measures are referenced. Low metallicity, and thus relatively high luminosity K giants are shown to be the population of choice, when available. The alternative, nearby G dwarfs, are shown to be suseptable to unmodeled motions induced by gas-giant planetary companions, should there be a significant population of such companions. Radial velocity filtering is quite efficient in selecting Grid members from the K giants with yields exceeding 50% if filtering at 30m/s (1-sigma) is available. However if the binary fraction of the G dwarfs approaches 100% as some studies suggest, the yield of stable systems would be in the range of 15% at best (with 10m/s filtering). Use of the initial SIM measurement as a final filter is shown not to be critical in either case, although it could improve the yield of stable grid members. For a Grid composed of weak-lined K giants, the residual contamination by large unmodeled motions will amount to about 3% (and rises to about 6% if a 60m/s radial velocity criterion is used). The selective introduction of quadratic terms in the proper motion solutions during the post-mission phase of data reduction can reduce contamination to a remarkable 1% or better in either case. Analytic estimates based on circular orbits are developed which show how these results come about.

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