On the effects of subvirial initial conditions and the birth temperature of R136

Caputo, Daniel P.; Vries, Nathan de; Zwart, Simon Portegies

doi:10.1093/mnras/stu1769

Cited by 9 publications

(7 citation statements)

References 43 publications

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“…It is important to see whether this remains true for statistics of higher-N systems or systems with a dominant mass. An example of a higher-N system where precision might play a role is a young star cluster (without gas) going through the process of cold collapse [37]. At the moment of deepest collapse, a fraction of stars will obtain large accelerations, so that a small error in the acceleration can cause large errors in the position and velocity.…”

Section: Resultsmentioning

confidence: 99%

On the reliability of N-body simulations

Boekholt

Zwart

2015

Comput. Astrophys.

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The general consensus in the N-body community is that statistical results of an ensemble of collisional N-body simulations are accurate, even though individual simulations are not. A way to test this hypothesis is to make a direct comparison of an ensemble of solutions obtained by conventional methods with an ensemble of true solutions. In order to make this possible, we wrote an N-body code called Brutus, that uses arbitrary-precision arithmetic. In combination with the Bulirsch-Stoer method, Brutus is able to obtain converged solutions, which are true up to a specified number of digits.We perform simulations of democratic 3-body systems, where after a sequence of resonances and ejections, a final configuration is reached consisting of a permanent binary and an escaping star. We do this with conventional double-precision methods, and with Brutus; both have the same set of initial conditions and initial realisations. The ensemble of solutions from the conventional simulations is compared directly to that of the converged simulations, both as an ensemble and on an individual basis to determine the distribution of the errors.We find that on average at least half of the conventional simulations diverge from the converged solution, such that the two solutions are microscopically incomparable. For the solutions which have not diverged significantly, we observe that if the integrator has a bias in energy and angular momentum, this propagates to a bias in the statistical properties of the binaries. In the case when the conventional solution has diverged onto an entirely different trajectory in phase-space, we find that the errors are centred around zero and symmetric; the error due to divergence is unbiased, as long as the time-step parameter, η ≤ 2 -5 and when simulations which violate energy conservation by more than 10% are excluded. For resonant 3-body interactions, we conclude that the statistical results of an ensemble of conventional solutions are indeed accurate.

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Section: Resultsmentioning

confidence: 99%

On the reliability of N-body simulations

Boekholt

Zwart

2015

Comput. Astrophys.

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“…Finally, in regions undergoing violent relaxation, the most massive stars attain larger velocity dispersions than the average stars in the region. This is not due to their being in close binary systems, which would inflate the velocity dispersion (Gieles et Parker et al 2014b;Caputo et al 2014) has shown that violent relaxation leads to strong dynamical mass segregation. If this mass segregation were a signature of energy equipartition we would expect the most massive stars to be moving more slowly than the average star, which is the opposite to what we see in the simulations.…”

Section: Discussionmentioning

confidence: 99%

Dynamical evolution of star-forming regions – II. Basic kinematics

Parker

Wright

2016

Mon. Not. R. Astron. Soc.

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We follow the dynamical evolution of young star-forming regions with a wide range of initial conditions and examine how the radial velocity dispersion, σ, evolves over time. We compare this velocity dispersion to the theoretically expected value for the velocity dispersion if a region were in virial equilibrium, σ vir and thus assess the virial state (σ/σ vir ) of these systems. We find that in regions that are initially subvirial, or in global virial equilibrium but subvirial on local scales, the system relaxes to virial equilibrium within several million years, or roughly 25 -50 crossing times, according to the measured virial ratio. However, the measured velocity dispersion, σ, appears to be a bad diagnostic of the current virial state of these systems as it suggests that they become supervirial when compared to the velocity dispersion estimated from the virial mass, σ vir . We suggest that this discrepancy is caused by the fact that the regions are never fully relaxed, and that the early non-equilibrium evolution is imprinted in the one-dimensional velocity dispersion at these early epochs. If measured early enough (<2 Myr in our simulations, or ∼20 crossing times), the velocity dispersion can be used to determine whether a region was highly supervirial at birth without the risk of degeneracy. We show that combining σ, or the ratio of σ to the interquartile range (IQR) dispersion, with measures of spatial structure, places stronger constraints on the dynamical history of a region than using the velocity dispersion in isolation.

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“…Another point of discussion is the virial state of a cluster before gas expulsion (Caputo et al 2014). Apart from the cluster being in equilibrium, sub-and super-Keplerian situations could in principle be possible.…”

Section: Methodsmentioning

confidence: 99%

Which young clusters and associations are we missing today?

Pfalzner

Vincke

Xiang

2015

A&A

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Context. Currently clusters and association of stars are mainly detected as surface density enhancements relative to the background field. While clusters form, their surface density increases. It likely decreases again at the end of the star formation process when the system expands as a consequence of gas expulsion. Therefore the surface density of a single cluster can change considerably in young clusters during the first 20 Myr of their development. Aims. We investigate the effect of the gas expulsion on the detectability of clusters and associations typical of the solar neighbourhood, where the star formation efficiency is <35%. The main focus is on the dependence on the initial cluster mass. Methods. N-body methods are used to determine the cluster and association dynamics after gas expulsion. Results. We find that, even for low background densities, only clusters and associations with initial central surface densities exceeding a few 1000 M /pc 2 will be detected as clusters at ages 5 Myr. Even the Orion nebula cluster, one of the most massive nearby clusters, would only be categorised as a small co-moving group with current methods after 5 Myr of development. This means that cluster expansion leads to a selection effect -at ages of <1-2 Myr the full range of clusters and associations is observed whereas at ages >4 Myr only the most massive clusters are identified, while systems with initially M c < 3000 M are missing. Conclusions. The temporal development of stellar properties is usually determined by observing clusters of different ages. The potentially strong inhomogeneity of the cluster sample makes this method highly questionable. However, Gaia could provide the means to rectify this situation because it will be able to detect lower mass clusters.

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On the effects of subvirial initial conditions and the birth temperature of R136

Cited by 9 publications

References 43 publications

On the reliability of N-body simulations

On the reliability of N-body simulations

Dynamical evolution of star-forming regions – II. Basic kinematics

Which young clusters and associations are we missing today?

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