arXiv subjects

Pavel Kroupa

Publications and source records attributed to Pavel Kroupa.

341 records · Page 19Linked to original sources

arXiv1999

Constraints on Stellar-Dynamical Models of the Orion Nebula Cluster

The results obtained by Kroupa, Petr & McCaughrean (1999) for specific models of young compact binary-rich clusters are generalised using dynamical scaling relations, to infer the candidate set of possible birth models leading to the Orion Nebula Cluster (ONC), of which the Trapezium Cluster is the core. It is found that candidate sets of solutions exist which allow the ONC to be in virial equilibrium, expanding or contracting. The range of possible solutions is quite narrow. These results will serve as guidelines for future, CPU-intensive calculations of the stellar-dynamical and astrophysical evolution of the entire ONC. These, in turn, will be essential to quantify observables that will ultimately discriminate between models, thus allowing us to understand if the ONC is in the process of assembling a rich Galactic cluster, and, if this is the case, how it occurs.

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arXiv1999

Binary stars in young clusters: models versus observations of the Trapezium Cluster

The frequency of low-mass pre-main sequence binary systems is significantly lower in the Trapezium Cluster than in Taurus-Auriga. We investigate if this difference can be explained through stellar encounters in dense clusters. To this effect, a range of possible models of the well observed Trapezium Cluster are calculated using Aarseth's direct N-body code, which treats binaries accurately. The results are confronted with observational constraints. The range of models include clusters in virial equilibrium, expanding clusters as a result of instantaneous mass loss, as well as collapsing clusters. In all cases the primordial binary proportion is larger than 50 per cent, with initial period distributions as observed in Taurus-Auriga and the Galactic field. It is found that the expanding model, with an initial binary population as in the Galactic field, is most consistent with the observational constraints. This raises the possibility that the primordial group of OB stars may have expelled the cluster gas roughly 50 000 yr ago. The cluster's bulk expansion rate is thus a key observable that needs to be determined. The other models demonstrate that the rapidly decreasing binary proportion, its radial dependence and the form of the period distribution, together with structural and kinematical data, are very useful diagnostics on the present and past dynamical state of a young cluster. In particular, kinematical cooling from the disruption of wide binaries is seen for the first time.

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arXiv1998

An Expanding Trapezium Cluster?

Simulations with Aarseth's (1994) NBODY5 code are presented of an initially dense binary-rich cluster. It is assumed that the star formation efficiency is 50 per cent with instantaneous mass loss. The model central density and velocity dispersion agree with the observational constraints if expansion is only about 6x10^4 yr old. Additionally, the observed binary proportion constrains the primordial proportion to have been significantly less than in Taurus--Auriga. The claim that a variation of the birth binary proportion with gas cloud parameters has been detected, however, can only be verified if the cluster can be shown to be expanding rapidly.

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arXiv1998

The dynamical evolution of stellar super-clusters

Recent images taken with the Hubble Space Telescope (HST) of the interacting disk galaxies NGC 4038/4039 (the Antennae) reveal clusters of many dozens and possibly hundreds of young compact massive star clusters within projected regions spanning about 100 to 500 pc. It is shown here that a large fraction of the individual star clusters merge within a few tens to a hundred Myr. Bound stellar systems with radii of a few hundred pc, masses of less than about 10^9 Msun and relaxation times of 10^{11}-10^{12} yr may form from these. These spheroidal dwarf galaxies contain old stars from the pre-merger galaxy and much younger stars formed in the massive star clusters, and possibly from later gas-accretion events. The possibility that star formation in the outer regions of gas-rich tidal tails may also lead to super clusters is raised. The mass-to-light ratio of these objects is small, because they contain an insignificant amount of dark matter. After many hundred Myr such systems may resemble dwarf spheroidal satellite galaxies with large apparent mass-to-light ratia, if tidal shaping is important.

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arXiv1998

dSph Satellite Galaxies without Dark Matter: a Study of Parameter Space

A parameter study is underway in Heidelberg that aims to find and constrain possible solutions to the dSph satellite problem under the assumption that some of these systems may not be dark matter dominated. The present state of the parameter survey of tidally disrupting spheroidal dwarf galaxies is described, and examples of preliminary results are presented.

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arXiv1998

On the Binary Properties and the Spatial and Kinematical Distribution of Young Stars

The effects which star cluster concentration and binarity have on observable parameters, that characterise the dynamical state of a population of stars after their birth aggregate dissolves, are investigated. To this end, the correlations between ejection velocity, binary proportion, mean system mass, binary orbital period and mass ratio are quantified for simulated aggregates. These consist of a few hundred low-mass binary and single stars, and have half-mass radii in the range 2.5 to 0.08 pc. The primordial binary-star population has a period distribution similar to that observed in Taurus-Auriga for pre-main sequence binaries. The findings presented here are useful for interpreting correlations between relative locations and proper motions, binary properties and masses of young stellar systems within and surrounding star forming regions, and of stellar systems escaping from Galactic clusters. (shortened)

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arXiv1998

Dynamical Interactions in Dense Stellar Clusters

This chapter reviews the dynamical processes in young stellar clusters. The accretion of gas by individual stars affects the dynamics of the cluster, and the masses of the stars. Dynamical mass segregation cannot explain the degree of mass segregation observed in clusters such as the Trapezium Cluster in Orion, implying that the location of the massive stars is an indication of where they formed. This can, however, be explained by the competitive accretion model. If most, if not all, stars form in binary systems, then the high proportion of pre-main sequence binary systems can evolve in clusters to the smaller proportion of Galactic field stars through binary-binary and binary-single interactions. These models make definite predictions for the distribution of binary properties. The early evolution of a cluster sensitively depends on the primordial binary star proportion. Close encounters between stars with circum-stellar discs have drastic effects on the discs and on the stellar orbits. The discs are truncated at radii comparable to the encounter peri-astron, limiting their lifetimes and affecting their potential for planet formation. If the gas represents a significant fraction of the total cluster mass, its removal on dynamical time-scales can unbind the cluster allowing the stars to escape and populate the field. (shortened)

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arXiv1998

Formation of dSph Satellites without Dark Matter

Recently it was shown that low-mass galaxies containing no dark matter can naturally evolve to dSph-like satellites in a tidal field. Such dSph-like satellites fake total domination by dark matter. If this is correct then it must be possible to assemble appropriate precursors of dSph satellites. The genesis of dwarf galaxies in tidal tails appears to be a likely possibility. This is documented by theoretical work and observational surveys. However, it is not yet clear if the Milky Way could have sustained one or two mergers or fly-bys that were mild enough not to have destroyed the Galactic disc. The tidal arms could have been pulled from the incoming galaxy. Also, this scenario has to account for the correlation between brightness and metal abundance displayed by the Galactic dSph satellites, and lead to at least some debris on bound eccentric orbits.

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arXiv1997

Dwarf Spheroidal Satellite Galaxies Without Dark Matter: Results From Two Different Numerical Techniques

Self-consistent simulations of the dynamical evolution of a low-mass satellite galaxy without dark matter are reported. The orbits have eccentricities 0.46 <= e <= 0.96 in a Galactic dark halo resembling our Milky Way. For the simulations, a particle-mesh code with nested sub-grids and a direct-summation N-body code running with the special purpose hardware device GRAPE are used. Initially, the satellite is spherical with an isotropic velocity distribution, and has a mass of 10^7 solar masses. Simulations with 1.3x10^5 up to 2x10^6 satellite particles are performed. The calculations proceed for many orbital periods until well after the satellite disrupts. In all cases the dynamical evolution converges to a remnant that contains roughly 1% of the initial satellite mass. The stable remnant results from severe tidal shaping of the initial satellite. To an observer from Earth these remnants look strikingly similar to the Galactic dwarf spheroidal satellite galaxies. Their apparent mass-to-light ratios are very large despite the fact that they contain no dark matter. These computations show that a remnant without dark matter displays larger line-of-sight velocity dispersions, sigma, for more eccentric orbits, which is a result of projection onto the observational plane. Assuming they are not dark matter dominated, it follows that the Galactic dSph satellites with sigma > 6km/s should have orbital eccentricities of e > 0.5. Some remnants have sub-structure along the line-of-sight that may be apparent in the morphology of the horizontal branch.

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arXiv1997

The Stellar Mass Function

Since the major review by Scalo (1986), significant progress has been achieved in constraining the mass function (MF) of low-mass stars. The break-throughs which today allow a much better understanding of the stellar luminosity function (LF) and the underlying MF are documented here and the resulting MF for Galactic field stars is confronted with microlensing data, the chemical enrichment history of the Galaxy, the Oort limit and well-studied open clusters.

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arXiv1997

The Theoretical Mass--Magnitude Relation of Low-Mass Stars and its Metallicity Dependence

We investigate the dependence of theoretically generated mass - (absolute magnitude) relations on stellar models. Using up to date physics we compute models in the mass range 0.1 < m < 1M_sun. We compare the solar-metallicity models with our older models, with recent models computed by others, and also with an empirical mass - (absolute magnitude) relation that best fits the observed data. At a given mass below 0.6M_sun the effective temperatures differ substantially from model to model. However taken individually each set of models is in good agreement with observations in the mass - luminosity plane. A minimum in the derivative dm/dM_V at M_V = 11.5, which is due to H_2 formation and establishment of a fully convective stellar interior, is present in all photometric bands, for all models. This minimum leads to a maximum in the stellar luminosity function for Galactic disk stars at M_V = 11.5, M_bol = 9.8. Stellar models should locate this maximum in the stellar luminosity function at the same magnitude as observations. Models which incorporate the most realistic theoretical atmospheres and the most recent equation of state and opacities can satisfy this constraint. These models are also in best agreement with the most recent luminosity - (effective temperature) and mass-luminosity data. Each set of our models of a given metallicity (with 0.2 > [Fe/H] > -2.3) shows a maximum in -dm/dM_bol, which moves to brighter bolometric magnitudes with decreasing metallicity. The change in location of the maximum, as a function of [Fe/H], follows the location of structure in luminosity functions for stellar populations with different metal abundances. This structure seen in all observed stellar populations can be accounted for by the mass--luminosity relation.

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arXiv1996

Dwarf Spheroidal Satellite Galaxies and the Galactic Tidal Field

The Milky Way is surrounded by nine or more dwarf-spheroidal (dSph) satellite galaxies that appear to consist primarily of dark matter. Here I summarise research that shows that initially spherical bound low-mass satellites without dark matter, that are on orbits within a massive Galactic dark corona, can evolve into remnants that are non-spherical, have a non-isotropic velocity dispersion tensor and are not in virial equilibrium, but are bright enough for sufficiently long times to be mistaken for dark-matter dominated dSph galaxies.

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arXiv1995

Star Cluster Evolution, Dynamical Age Estimation and the Kinematical Signature of Star Formation

We use N-body integration to follow the evolution of clusters of 200 binary systems with different initial half mass radii $R_{0.5}$. We also simulate single-star clusters. All clusters evolve according to the same $n(t)$ curve, where $n(t)$ is the number density of stars in the central 2~pc sphere at time $t$. $n(t)$ and the lifetime are independent of (i) the inital proportion of binaries and (ii) the initial $R_{0.5}$. Mass segregation measures the dynamical age of the cluster. The proportion of binaries in the central cluster region is a sensitive indicator of the initial cluster concentration. If most stars form in binaries in a typical embedded cluster which is located at the edge of a giant molecular cloud, then we estimate that at most about 10~per cent of all pre-main sequence stars achieve near escape velocities from the molecular cloud. The large ejection velocities resulting from close encounters between binary systems imply a `halo' distribution of young stars over large areas surrounding star forming sites which is expected to have a significantly reduced binary proportion and a significantly increased proportion of stars with depleted circumstellar disks. We compare the time dependent model single star and system luminosity function in the central cluster region with the observational Hyades and Pleiades luminosity functions and find no evidence for different dynamical properties of stellar systems at birth in the Hyades, Pleiades and Galactic field stellar samples. The observed proportion of binary stars in the very young Trapezium Cluster is consistent with the early dynamical evolution of a cluster with a very high initial stellar number density.

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arXiv1995

Inverse Dynamical Population Synthesis and Star Formation

We investigate the hypothesis that stars form in aggregates of binary systems and that the dynamical evolution of these aggregates leads to the observed properties of binary stars in the Galactic field. We assume that the initial distribution of periods is flat in logP, where P is the orbital period in days, and 3 7.5. After the cluster with R=0.8pc disintegrates we obtain a population which consists of about 60 per cent binary systems with a period distribution for logP>4 as is observed and in which the G-dwarf binaries have a mass ratio distribution which agrees with the observed distribution. This result indicates that the majority of Galactic field stars may originate from a clustered star formation mode. We invert the orbit depletion function and obtain an approximation to the initial binary star period distribution for star formation in the dominant mode cluster. Comparison with the measured distribution of orbits for pre-main sequence stars suggests that the initial distribution may not depend on the star formation environment. Inverse dynamical population synthesis suggests that the Galactic field stellar mass function may be related to the stellar density at birth in the most common, or dominant, mode of star formation.

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arXiv1995

The Dynamical Properties of Stellar Systems in the Galactic Disc

We postulate that most stars are born in aggregates of binary systems which are dynamically equivalent to the `dominant mode cluster'. The initial binary orbits are consitent with pre-main sequence data. Stellar masses are paired at random from the KTG(1.3) mass function. We develop a simple model which establishes the observed correlations between eccentricity, mass ratio and period. Stimulated evolution of orbital parameters within the cluster places a few per cent of all orbits into the eigenevolution region of the eccentricity- period diagram. Observations should reveal the odd binary with eccentric orbits at periods smaller than the circularisation cutoff period. We show that the birth eccentricity distribution must be approximately dynamically relaxed because stimulated evolution in the dominant mode cluster cannot sufficiently thermalise a significantly different distribution. After disintegration of the dominant mode cluster we have a proportion of binaries which compares favourably with the observed proportion. The shape of the period distribution agrees well with the observed distribution. The mass-ratio distribution of G dwarf binaries is depleted at small mass ratios and has the shape of the main sequence distribution. We predict and tabulate the mass ratio distribution for main sequence binaries with a primary star less massive than $1.1\,M_\odot$. Our model Galactic field stellar population has a binary proportion among G-, K- and M-dwarfs in good agreement with the observational data. Too few triple and quadruple systems form by capture to account for the number of observed systems. We compare the specific angular momentum distribution of our initial binary star population with the observed distribution of specific angular momenta of molecular cloud cores.

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arXiv1995

UNIFICATION OF THE NEARBY AND PHOTOMETRIC STELLAR LUMINOSITY FUNCTIONS

We introduce a model Galactic field low-mass stellar population that has a proportion of binary systems as observed, with a mass ratio distribution consistent with observational constraints. The model single star and system luminosity function agrees with the nearby and the Malmquist corrected photometric luminosity function, respectively. We tabulate the model luminosity functions in the photometric V-, I- and K-bands, and in bolometric magnitudes. Unresolved binary systems are thus a natural explanation for the difference between the nearby and photometric luminosity functions. A local overdensity of faint stars needs not be postulated to account for the difference, and is very unlikely. We stress that the nearby luminosity function can only be used to weakly constrain the stellar mass function below $0.5\,M_\odot$, because of the small sample size. The photometric luminosity function can only be used to put lower limits on the stellar mass function because most binary systems are not resolved. However, taken together the nearby and photometric stellar luminosity function data do not imply a mass function with a peak at a mass of $0.2-0.3\,M_\odot$. Instead, the data are consistent with a power-law mass function between 0.08$\,M_\odot$ and $0.5\,M_\odot$. We urge researchers to only use star count data that are properly corrected for {\it all} contributions to cosmic scatter, photometric uncertainties, and unresolved binaries, and to be aware of the severe limitations of theoretical mass--luminosity relations for low mass stars, when drawing conclusions about structure in the stellar mass function.

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arXiv1995

ARE THE NEARBY AND PHOTOMETRIC STELLAR LUMINOSITY FUNCTIONS DIFFERENT?

The stellar luminosity function derived from the sample of stars within 5.2--20~pc is the nearby luminosity function. The luminosity function obtained from deep low spatial resolution surveys to distances of typically 100--200~pc is the photometric luminosity function. We obtain and tabulate a best estimate of the parent distribution from which Malmquist corrected photometric luminosity functions are sampled. This is presently the most {\it accurate} and {\it precise} available estimate of the true distribution of stars with absolute magnitude for $M_{\rm V}\le16.5$, as derived from low-resolution surveys. Using rigorous statistical analysis we find that the hypothesis that the nearby and photometric luminosity functions estimate the same parent luminosity function can be discarded with 99~per~cent confidence or better for $M_{\rm V}>13,M_{\rm bol}>10$.

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