arXiv subjects

Pavel Kroupa

Publications and source records attributed to Pavel Kroupa.

At least 235 records · Page 13Linked to original sources

arXiv2012

Catch me if you can: is there a runaway-mass black hole in the Orion Nebula Cluster?

We investigate the dynamical evolution of the Orion Nebula Cluster (ONC) by means of direct N-body integrations. A large fraction of residual gas was probably expelled when the ONC formed, so we assume that the ONC was much more compact when it formed compared to its current size, in agreement with the embedded cluster radius-mass relation from Marks & Kroupa (2012). Hence, we assume that few-body relaxation played an important role during the initial phase of evolution of the ONC. In particular, three body interactions among OB stars likely led to their ejection from the cluster and, at the same time, to the formation of a massive object via runaway physical stellar collisions. The resulting depletion of the high mass end of the stellar mass function in the cluster is one of the important points where our models fit the observational data. We speculate that the runaway-mass star may have collapsed directly into a massive black hole (Mbh > 100Msun). Such a dark object could explain the large velocity dispersion of the four Trapezium stars observed in the ONC core. We further show that the putative massive black hole is likely to be a member of a binary system with appr. 70 per cent probability. In such a case, it could be detected either due to short periods of enhanced accretion of stellar winds from the secondary star during pericentre passages, or through a measurement of the motion of the secondary whose velocity would exceed 10 km/s along the whole orbit.

astro-ph.GA↗

arXiv2012

The emergence of super-canonical stars in R136-type star-burst clusters

[abridged] Among the most remarkable features of the stellar population of R136, the central, young, massive star cluster in the 30 Doradus complex of the Large Magellanic Cloud, are the single stars whose masses substantially exceed the canonical stellar upper mass limit of 150 M_sun. A recent study by us, viz., that of Banerjee, Kroupa & Oh (2012; Paper I), which involves realistic N-body computations of star clusters mimicking R136, indicates that such "super-canonical" (SC) stars can be formed out of a dense stellar population with a canonical initial mass function (IMF) through dynamically induced mergers of the most massive binaries. Here we study the formation of SC stars in the R136 models of Paper I in detail. To avoid forming extraneous SC stars from initially highly eccentric primordial binaries as in Paper I, we compute additional models with only initially circular primordial binaries. We also take into account the mass-evolution of the SC stars using detailed stellar evolutionary models that incorporate updated treatments of stellar winds. We find that SC stars begin to form via dynamical mergers of massive binaries from approx. 1 Myr cluster age. We obtain SC stars with initial masses up to approx. 250 M_sun from these computations. Multiple SC stars are found to remain bound to the cluster simultaneously within a SC-lifetime. These properties of the dynamically formed SC stars are consistent with those observed in R136. In fact, the stellar evolutionary models of SC stars imply that had they formed primordially along with the rest of the R136 cluster, i.e., violating the canonical upper limit, they would have evolved below the canonical 150 M_sun limit by approx. 3 Myr, the likely age of R136, and would not have been observable as SC stars at the present time in R136. This strongly supports the dynamical formation scenario of the observed SC stars in R136.

astro-ph.SR↗

arXiv2012

The dark matter crisis: falsification of the current standard model of cosmology

The current standard model of cosmology (SMoC) requires The Dual Dwarf Galaxy Theorem to be true according to which two types of dwarf galaxies must exist: primordial dark-matter (DM) dominated (type A) dwarf galaxies, and tidal-dwarf and ram-pressure-dwarf (type B) galaxies void of DM. Type A dwarfs surround the host approximately spherically, while type B dwarfs are typically correlated in phase-space. Type B dwarfs must exist in any cosmological theory in which galaxies interact. Only one type of dwarf galaxy is observed to exist on the baryonic Tully-Fisher plot and in the radius-mass plane. The Milky Way satellite system forms a vast phase-space-correlated structure that includes globular clusters and stellar and gaseous streams. Other galaxies also have phase-space correlated satellite systems. Therefore, The Dual Dwarf Galaxy Theorem is falsified by observation and dynamically relevant cold or warm DM cannot exist. It is shown that the SMoC is incompatible with a large set of other extragalactic observations. Other theoretical solutions to cosmological observations exist. In particular, alone the empirical mass-discrepancy--acceleration correlation constitutes convincing evidence that galactic-scale dynamics must be Milgromian. Major problems with inflationary big bang cosmologies remain unresolved.

astro-ph.CO↗

arXiv2012

Inverse dynamical population synthesis: Constraining the initial conditions of young stellar clusters by studying their binary populations

Binary populations in young star clusters show multiplicity fractions both lower and up to twice as high as those observed in the Galactic field. We follow the evolution of a population of binary stars in dense and loose star clusters starting with an invariant initial binary population and a formal multiplicity fraction of unity, and demonstrate that these models can explain the observed binary properties in Taurus, Rho-Ophiuchus, Chamaeleon, Orion, IC 348, Upper Scorpius A, Praesepe, and the Pleiades. The model needs to consider solely different birth densities for these regions. The evolved theoretical orbital-parameter distributions are highly probable parent distributions for the observed ones. We constrain the birth conditions (stellar mass, M_ecl, and half-mass radius, r_h) for the derived progenitors of the star clusters and the overall present-day binary fractions allowed by the present model. The results compare very well with properties of molecular cloud clumps on the verge of star formation. Combining these with previously and independently obtained constraints on the birth densities of globular clusters, we identify a weak stellar mass -- half-mass radius correlation for cluster-forming cloud clumps, r_h / pc ~ (M_ecl / M_sun)^(0.13+-0.04). The ability of the model to reproduce the binary properties in all the investigated young objects, covering present-day densities from 1-10 stars pc^-3 (Taurus) to 2x10^4 stars pc^-3 (Orion), suggests that environment-dependent dynamical evolution plays an important role in shaping the present-day properties of binary populations in star clusters, and that the initial binary properties may not vary dramatically between different environments.

astro-ph.GA↗

arXiv2012

The influence of stellar-dynamical ejections and collisions on the relation between the maximum-star and star-cluster-mass

We perform the largest currently available set of direct N-body calculations of young star cluster models to study the dynamical influence, especially through the ejections of the most massive star in the cluster, on the current relation between the maximum-stellar-mass and the star-cluster-mass. We vary several initial parameters such as the initial half-mass radius of the cluster, the initial binary fraction, and the degree of initial mass segregation. Two different pairing methods are used to construct massive binaries for more realistic initial conditions of massive binaries. We find that lower mass clusters (<= 10^2.5 Msun) do not shoot out their heaviest star. In the case of massive clusters (>= 1000 Msun), no most-massive star escapes the cluster within 3 Myr regardless of the initial conditions if clusters have initial half-mass radii, r_0.5, >= 0.8 pc. However, a few of the initially smaller sized clusters (r_0.5 = 0.3 pc), which have a higher density, eject their most massive star within 3 Myr. If clusters form with a compact size and their massive stars are born in a binary system with a mass-ratio biased towards unity, the probability that the mass of the most massive star in the cluster changes due to the ejection of the initially most massive star can be as large as 20 per cent. Stellar collisions increase the maximum-stellar-mass in a large number of clusters when clusters are relatively dense (M_ecl >= 10^3 Msun and r_0.5 = 0.3 pc) and binary-rich. Overall, we conclude that dynamical effects hardly influence the observational maximum-stellar-mass -- cluster mass relation.

astro-ph.GA↗

arXiv2012

Evidence for top-heavy stellar initial mass functions with increasing density and decreasing metallicity

Residual-gas expulsion after cluster formation has recently been shown to leave an imprint in the low-mass present-day stellar mass function (PDMF) which allowed the estimation of birth conditions of some Galactic globular clusters (GCs) such as mass, radius and star formation efficiency. We show that in order to explain their characteristics (masses, radii, metallicity, PDMF) their stellar initial mass function (IMF) must have been top-heavy. It is found that the IMF is required to become more top-heavy the lower the cluster metallicity and the larger the pre-GC cloud-core density are. The deduced trends are in qualitative agreement with theoretical expectation. The results are consistent with estimates of the shape of the high-mass end of the IMF in the Arches cluster, Westerlund 1, R136 and NGC 3603, as well as with the IMF independently constrained for ultra-compact dwarf galaxies (UCDs). The latter suggests that GCs and UCDs might have formed along the same channel or that UCDs formed via mergers of GCs. A fundamental plane is found which describes the variation of the IMF with density and metallicity of the pre-GC cloud-cores simultaneously. The implications for the evolution of galaxies and chemical enrichment over cosmological times are expected to be major.

astro-ph.GA↗

arXiv2012

A natural formation scenario for misaligned and short-period eccentric extrasolar planets

Recent discoveries of strongly misaligned transiting exoplanets pose a challenge to the established planet formation theory which assumes planetary systems to form and evolve in isolation. However, the fact that the majority of stars actually do form in star clusters raises the question how isolated forming planetary systems really are. Besides radiative and tidal forces the presence of dense gas aggregates in star-forming regions are potential sources for perturbations to protoplanetary discs or systems. Here we show that subsequent capture of gas from large extended accretion envelopes onto a passing star with a typical circumstellar disc can tilt the disc plane to retrograde orientation, naturally explaining the formation of strongly inclined planetary systems. Furthermore, the inner disc regions may become denser, and thus more prone to speedy coagulation and planet formation. Pre-existing planetary systems are compressed by gas inflows leading to a natural occurrence of close-in misaligned hot Jupiters and short-period eccentric planets. The likelihood of such events mainly depends on the gas content of the cluster and is thus expected to be highest in the youngest star clusters.

astro-ph.EP↗

arXiv2012

Low-mass X-ray binaries indicate a top-heavy stellar initial mass function in ultra compact dwarf galaxies

It has been shown before that the high mass-to-light ratios of ultra compact dwarf galaxies (UCDs) can be explained if their stellar initial mass function (IMF) was top-heavy, i.e. that the IMF was skewed towards high mass stars. In this case, neutron stars and black holes would provide unseen mass in the UCDs. In order to test this scenario with an independent method, we use data on which fraction of UCDs has a bright X-ray source. These X-ray sources are interpreted as low-mass X-ray binaries (LMXBs), i.e. binaries where a neutron star accretes matter from an evolving low-mass star. We find that LMXBs are indeed up to 10 times more frequent in UCDs than expected if the IMF was invariant. The top-heavy IMF required to account for this overabundance is the same as needed to explain the unusually high mass-to-light ratios of UCDs and a top-heavy IMF appears to be the only simultaneous explanation for both findings. Furthermore, we show that the high rate of type II supernovae (SNII) in the star-burst galaxy Arp 220 suggests a top-heavy IMF in that system. This finding is consistent with the notion that star-burst galaxies are sites where UCDs are likely to be formed and that the IMF of UCDs is top-heavy. It is estimated that the IMF becomes top-heavy whenever the star formation rate per volume surpasses 0.1 Solar masses per year and cubic parsec in pc-scale regions.

astro-ph.CO↗

arXiv2012

Top-heavy IMFs in ultra compact dwarf galaxies?

Ultra compact dwarf galaxies (UCDs) are dense stellar systems at the border between massive star-clusters and small galaxies. Their on average high optical mass-to-light (M/L) ratio cannot be explained by stellar populations with the canonical stellar initial mass function (IMF), while it is doubtful that non-baryonic dark matter can accumulate enough on the scales of UCDs for influencing their dynamics significantly. UCDs in the Virgo galaxy cluster apparently also have an over-abundance of neutron stars, strongly suggesting a top-heavy IMF, which would explain both findings. This is because a top-heavy IMF can provide the unseen mass through an abundance of stellar remnants. The suggested variation of the IMF can be understood if UCDs represent a case of rapid star-formation in an extremely dense environment. While top-heavy IMFs imply a much heavier mass-loss shortly after the formation of a stellar system, this process does not necessarily dissolve the UCDs. Their formation with a top-heavy IMF would therefore not contradict their existence.

astro-ph.CO↗

arXiv2011

Star cluster formation and some implications for GAIA

Stars form in spatially and temporarily correlated star formation events (CSFEs) and the dynamical processes within these "embedded clusters" leave imprints in the stellar populations in galactic fields. Such imprints are correlations in phase space (e.g. gravitationally bound star clusters, tidal streams), in the binary properties of stars and in the present-day stellar mass functions in the surviving clusters. The dynamical processes include expulsion of massive stars from cluster cores, disruption of CSFEs due to residual gas expulsion and energy-equipartition driven evaporation of stars from clusters leading to dark star clusters and cold kinematical streams with epicyclic overdensities. The properties of such phase-space structures in the Milky Way (MW) field depend on the effective gravitational potential of the MW. GAIA data will significantly constrain all of these aspects, and will in particular impact on gravitational dynamics via the properties of cold streams and on star-formation via the constraint on the gas expulsion process through the expanding unbound populations that must be associated with every CSFE.

astro-ph.GA↗

arXiv2011

Do all O stars form in star clusters?

The question whether or not massive stars can form in isolation or only in star clusters is of great importance for the theory of (massive) star-formation as well as for the stellar initial mass function of whole galaxies (IGIMF-theory). While a seemingly easy question it is rather difficult to answer. Several physical processes (e.g. star-loss due to stellar dynamics or gas expulsion) and observational limitations (e.g. dust obscuration of young clusters, resolution) pose severe challenges to answer this question. In this contribution we will present the current arguments in favour and against the idea that all O stars form in clusters.

astro-ph.GA↗

arXiv2011

Runaway massive stars from R136: VFTS 682 is very likely a "slow runaway"

We conduct a theoretical study on the ejection of runaway massive stars from R136 --- the central massive, star-burst cluster in the 30 Doradus complex of the Large Magellanic Cloud. Specifically, we investigate the possibility of the very massive star (VMS) VFTS 682 being a runaway member of R136. Recent observations of the above VMS, by virtue of its isolated location and its moderate peculiar motion, have raised the fundamental question whether isolated massive star formation is indeed possible. We perform the first realistic N-body computations of fully mass-segregated R136-type star clusters in which all the massive stars are in primordial binary systems. These calculations confirm that the dynamical ejection of a VMS from a R136-like cluster, with kinematic properties similar to those of VFTS 682, is common. Hence the conjecture of isolated massive star formation is unnecessary to account for this VMS. Our results are also quite consistent with the ejection of 30 Dor 016, another suspected runaway VMS from R136. We further note that during the clusters' evolution, mergers of massive binaries produce a few single stars per cluster with masses significantly exceeding the canonical upper-limit of 150 solar mass. The observations of such single super-canonical stars in R136, therefore, do not imply an IMF with an upper limit greatly exceeding the accepted canonical 150 solar mass limit, as has been suggested recently, and they are consistent with the canonical upper limit.

astro-ph.GA↗

arXiv2011

The dynamical fingerprint of gas-expulsion: Insights into the assembly of the Milky Ways' old GC system

Since the oldest globular clusters (GCs) are early residuals from the formation of the Milky Way (MW), GCs were exposed to the likely evolving potential of our Galaxy. The expulsion of the residual-gas from the GC's embedded progenitors is sensitive to the conditions in the pre-MW gas cloud. By means of N-body computations it is shown that gas throw-out from initially mass-segregated GCs affect the shape of the low-mass stellar mass function (MF) and that its imprint might still be visible in the present-day MF (PDMF). The strength of the tidal-field at birth influences the degree of gas-expulsion driven low-mass-star depletion and therefore the PDMF probes the MW potential at the time of GC formation. It is argued that among the old GC population in the MW, younger GCs show stronger low-mass-star loss than older GCs. This is shown to be consistent with a contracting and self-gravitating cloud in which fluctuations in the pre-MW potential grow with time. An initially relatively smooth tidal field evolved into a grainy potential within a dynamical time-scale of the collapsing cloud (based on Marks & Kroupa 2010, MNRAS, 406, 2000).

astro-ph.GA↗

arXiv2011

A new type of compact stellar population: dark star clusters

Among the most explored directions in the study of dense stellar systems is the investigation of the effects of the retention of supernova remnants, especially that of the massive stellar remnant black holes (BHs), in star clusters. By virtue of their eventual high central concentration, these stellar mass BHs potentially invoke a wide variety of physical phenomena, the most important ones being emission of gravitational waves (GWs), formation of X-ray binaries, and modification of the dynamical evolution of the cluster. Here we propose, for the first time, that rapid removal of stars from the outer parts of a cluster by the strong tidal field in the inner region of our Galaxy can unveil its BH sub-cluster, which appears as a star cluster that is gravitationally bound by an invisible mass. We study the formation and properties of such systems through direct N-body computations and estimate that they can be present in significant numbers in the inner region of the Milky Way. We call such objects "dark star clusters" (DSCs) as they appear dimmer than normal star clusters of similar mass and they comprise a predicted, new class of entities. The finding of DSCs will robustly cross-check BH retention; they will not only constrain the uncertain natal kicks of BHs, thereby the widely debated theoretical models of BH formation, but will also pinpoint star clusters as potential sites for GW emission for forthcoming ground-based detectors such as the Advanced LIGO. Finally, we also discuss the relevance of DSCs for the nature of IRS 13E.

astro-ph.GA↗

arXiv2011

Dynamical population synthesis: Constructing the stellar single and binary contents of galactic field populations

[abridged] The galactic field's late-type stellar single and binary population is calculated on the supposition that all stars form as binaries in embedded star clusters. A recently developed tool (Marks, Kroupa & Oh) is used to evolve the binary star distributions in star clusters for a few Myr so that a particular mixture of single and binary stars is achieved. On cluster dissolution the population enters the galactic field with these characteristics. The different contributions of single stars and binaries from individual star clusters which are selected from a power-law embedded star cluster mass function are then added up. This gives rise to integrated galactic field binary distribution functions (IGBDFs) resembling a galactic field's stellar content (Dynamical Population Synthesis). It is found that the binary proportion in the galactic field of a galaxy is larger the lower the minimum cluster mass, the lower the star formation rate, the steeper the embedded star cluster mass function and the larger the typical size of forming star clusters in the considered galaxy. In particular, period-, mass-ratio- and eccentricity IGBDFs for the Milky Way are modelled. The afore mentioned theoretical IGBDFs agree with independently observed distributions. Of all late-type binaries, 50% stem from M<300M_sun clusters, while 50% of all single stars were born in M>10^4M_sun clusters. Comparison of the G-dwarf and M-dwarf binary population indicates that the stars formed in mass-segregated clusters. In particular it is pointed out that although in the present model all M-dwarfs are born in binary systems, in the Milky Way's Galactic field the majority ends up being single stars. This work predicts that today's binary frequency in elliptical galaxies is lower than in spiral and in dwarf-galaxies. The period and mass-ratio distributions in these galaxies are explicitly predicted.

astro-ph.GA↗

arXiv2011

Using dwarf satellite proper motions to determine their origin

The highly organised distribution of satellite galaxies surrounding the Milky Way is a serious challenge to the concordance cosmological model. Perhaps the only remaining solution, in this framework, is that the dwarf satellite galaxies fall into the Milky Way's potential along one or two filaments, which may or may not plausibly reproduce the observed distribution. Here we test this scenario by making use of the proper motions of the Fornax, Sculptor, Ursa Minor and Carina dwarf spheroidals, and trace their orbits back through several variations of the Milky Way's potential and account for dynamical friction. The key parameters are the proper motions and total masses of the dwarf galaxies. Using a simple model we find no tenable set of parameters that can allow Fornax to be consistent with filamentary infall, mainly because the 1 sigma error on its proper motion is relatively small. The other three must walk a tightrope between requiring a small pericentre (less than 20 kpc) to lose enough orbital energy to dynamical friction and avoiding being tidally disrupted. We then employed a more realistic model with host halo mass accretion, and found that the four dwarf galaxies must have fallen in at least 5 Gyrs ago. This time interval is longer than organised distribution is expected to last before being erased by the randomisation of the satellite orbits.

astro-ph.CO↗

arXiv2011

An analytical description of the evolution of binary orbital-parameter distributions in N-body computations of star clusters

A new method is presented to describe the evolution of the orbital-parameter distributions for an initially universal binary population in star clusters by means of the currently largest existing library of N-body models. It is demonstrated that a stellar-dynamical operator exists, which uniquely transforms an initial orbital parameter distribution function for binaries into a new distribution depending on the initial cluster mass and half-mass radius, after some time of dynamical evolution. For the initial distribution the distribution functions derived by Kroupa (1995a,b) are used, which are consistent with constraints for pre-main sequence and Class I binary populations. Binaries with a lower energy and a higher reduced-mass are dissolved preferentially. The stellar-dynamical operator can be used to efficiently calculate and predict binary properties in clusters and whole galaxies without the need for further N-body computations. For the present set of N-body models it is found that the binary populations change their properties on a crossing time-scale such that the stellar dynamical operator can be well parametrized as a function of the initial cluster density. Furthermore it is shown that the binary-fraction in clusters with similar initial velocity dispersions follows the same evolutionary tracks as a function of the passed number of relaxation-times. Present-day observed binary populations in star clusters put constraints on their initial stellar densities which are found to be in the range 10^2 - 2x10^5 M_sun pc^-3 for open clusters and a few x 10^3 - 10^8 M_sun pc^-3 for globular clusters, respectively.

astro-ph.GA↗

arXiv2011

The initial period function of late-type binary stars and its variation

The variation of the period distribution function of late-type binaries is studied. It is shown that the Taurus--Auriga pre-main sequence population and the main sequence G dwarf sample do not stem from the same parent period distribution with better than 95 per cent confidence probability. The Lupus, Upper Scorpius A and Taurus--Auriga populations are shown to be compatible with being drawn from the same initial period function (IPF), which is inconsistent with the main sequence data. Two possible IPF forms are used to find parent distributions to various permutations of the available data which include Upper Scorpius B (UScB), Chameleon and Orion Nebula Cluster pre-main sequence samples. All the pre-main sequence samples studied here are consistent with the hypothesis that there exists a universal IPF which is modified through binary-star disruption if it forms in an embedded star cluster leading to a general decline of the observed period function with increasing period. The pre-main sequence data admit a log-normal IPF similar to that arrived at by Duquennoy & Mayor (1991) for main sequence stars, provided the binary fraction among pre-main sequence stars is significantly higher. But, for consistency with proto-stellar data, the possibly universal IPF ought to be flat in log-P or log-semi-major axis and must be similar to the K1 IPF form derived through inverse dynamical population synthesis, which has been shown to lead to the main sequence period function if most stars form in typical embedded clusters.

astro-ph.SR↗