arXiv subjects

S. Pfalzner

Publications and source records attributed to S. Pfalzner.

33 records · Page 2Linked to original sources

arXiv2007

Quasi-binarity of massive stars in young dense clusters - the case of the ONC

Observations indicate that in young stellar clusters the binary fraction for massive stars is higher than for solar mass stars. For the Orion Nebula Cluster (ONC) there is a binary frequency of ~ 50% for solar-mass stars compared to 70-100% for the massive O- and B-stars. We explore the reasons for this discrepancy and come up with two possible answers: a) a primordially higher binarity of massive stars could be inherent to the star formation process or b) the primordial binary rate might be the same for solar-mass and massive stars, but the higher capture cross section of the massive stars possibly leads to the formation of additional massive binaries in the early cluster development. Here we investigate the likelihood of the latter using the ONC as an example. N-body simulations are performed to track the capture events in an ONC-like cluster. We find that whereas low-mass stars rarely form bound systems through capture, the dynamics of the massive stars - especially in the first 0.5 Myrs - is dominated by a rapid succession of ``transient binary or multiple systems''. In observations the transient nature of these systems would not be apparent, so that they would be rated as binaries. At 1-2 Myrs, the supposed age of the ONC, the ``transient'' massive systems become increasingly stable, lasting on average several 10^6 yrs. Despite the ONC being so young, the observed binary frequency for massive stars -- unlike that of solar-mass stars -- is not identical to the primordial binary frequency but is increased by at least 10-15% through dynamical interaction processes. This value might be increased to at least 20-25% by taking disc effects into account. The primordial binary frequency could well be the same for massive and solar mass stars because the observed difference can be explained by capture processes alone.

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arXiv2007

The nearby QSO host I Zw 1: The stellar disk and adjacent objects

The relation between tidal interactions, starbursts, and the onset and/or fueling of active galactic nuclei (AGN) is a matter of debate. I Zw 1 is considered as the prototypical narrow-line Seyfert 1 galaxy (NLS1) and as one of the closest quasi-stellar objects (QSOs). With a clear spiral host and a small companion galaxy to the west, I Zw 1 is a possible example of minor-merger-related nuclear activity. This study investigates possible signs of a relation between merger process, star formation activity, and AGN properties in the case of I Zw 1. The morphology of I Zw 1 and nearby sources is investigated via high-resolution NIR images. Color trends in the host galaxy of I Zw 1 are discussed by means of optical-to-NIR color composites. Long-slit spectra of the QSO nucleus of I Zw 1 and of the two nearby sources to the north and the west of the I Zw 1 disk are analyzed. The data support the scenario of a tidal interaction between I Zw 1 and the small companion galaxy to the west. A concentration of blue color in the western part of the I Zw 1 host galaxy might be the manifestation of merger-induced star formation activity. Previous findings that the likely companion has an old evolved stellar population are substantiated by the new data. An extension to the west of the putative companion emerges as a separate source. The source to the north of the I Zw 1 disk is reconfirmed as a late-type foreground star. Lines in the nuclear K-band spectrum of I Zw 1 are discussed in comparison to data prior to this article and line fluxes are reported.

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arXiv2006

Cluster-assisted accretion for massive stars

Gravitational interactions in very young high-density stellar clusters can to some degree change the angular momentum in the circumstellar discs surrounding initially the majority of stars. However, for most stars the cluster environment alters the angular momentum only slightly. For example, in simulations of the Orion Nebula cluster (ONC) encounters reduce the angular momentum of the discs on average at most by 3-5% and in the higher density region of the Trapezium %where encounters are more likely, the disc angular momentum is on average lowered by 15-20% - still a minor loss process. However, in this paper it is demonstrated that the situation is very different if one considers high-mass stars (M* > 10 M(solar) only. Assuming an age of 2 Myr for the ONC, their discs have on average a 50-90% lower angular momentum than primordially. This enormous loss in angular momentum in the disc should result in an equivalent increase in accretion, implying that the cluster environment boosts accretion for high-mass stars, thus %in the cluster center, making them even more massive.

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arXiv2006

Gravitational Instabilities induced by Cluster Environment? - The encounter-induced angular momentum transfer in discs

AIM:The aim of this work is to understand to what extend gravitational interactions between the stars in high-density young stellar clusters, like the Orion Nebula Cluster (ONC), change the angular momentum in their protoplanetary discs. METHOD:Two types of simulations were combined -- N-body simulations of the dynamics of the stars in the ONC, and angular momentum loss results from simulations of star-disc encounters. RESULTS:It is shown that in a star-disc encounter the angular momentum loss is usually larger than the mass loss, so that the disc remnant has a lower specific angular momentum. Assuming an age of 1-2 Myr for the ONC, the disc angular momentum in the higher density region of the Trapezium is reduced by 15-20% on average. Encounters therefore play an important part in the angular momentum transport in these central regions but are not the dominant process. More importantly, even in the outer cluster regions the angular momentum loss is on average 3-5%. Here it is shown that a 3-5% loss in angular momentum might be enough to trigger gravitational instabilities even in low-mass discs - a possible prerequisite for the formation of planetary systems.

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arXiv2006

The Fate of Discs around Massive Stars in Young Clusters

AIMS: The aim of this work is to understand whether there is a difference in the dispersion of discs around stars in high-density young stellar clusters like the Orion Nebula Cluster (ONC) according to the mass of the star. METHODS: Two types of simulations were combined -- N-body simulations of the dynamics of the stars in the ONC and mass loss results from simulations of star-disc encounters, where the disc mass loss of all stars is determined as a function of time. RESULTS: We find that in the Trapezium, the discs around high-mass stars are dispersed much more quickly and to a larger degree by their gravitational interaction than for intermediate-mass stars. This is consistent with the very recent observations of IC 348, where a higher disc frequency was found around solar mass stars than for more massive stars, suggesting that this might be a general trend in large young stellar clusters.

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arXiv2006

Encounter-Triggered Disc Mass Loss in the ONC

The relevance of encounters on the destruction of protoplanetary discs in the Orion Nebula Cluster (ONC) is investigated by combining two different types of numerical simulation. First, star-cluster simulations are performed to model the stellar dynamics of the ONC, the results of which are used to investigate the frequency of encounters, the mass ratio and separation of the stars involved, and the eccentricity of the encounter orbits. The results show that interactions that could influence the star-surrounding disc are more frequent than previously assumed in the core of the ONC, the so-called Trapezium cluster. Second, a parameter study of star-disc encounters is performed to determine the upper limits of the mass loss of the discs in encounters. For simulation times of $\sim$ 1-2 Myr (the likely age of the ONC) the results show that gravitational interaction might account for a significant disc mass loss in dense clusters. Disc destruction is dominated by encounters with high-mass stars, especially in the Trapezium cluster, where the fraction of discs destroyed due to stellar encounters can reach 10-15%. These estimates are in accord with observations of (Lada et al. 2000) who determined a stellar disc fraction of 80-85%. Thus, it is shown that in the ONC - a typical star-forming region - stellar encounters do have a significant effect on the mass of protoplanetary discs and thus affect the formation of planetary systems.

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arXiv2005

Disc-Disc Encounters between Low-Mass Protoplanetary Accretion Discs

Simulations of the collapse and fragmentation of turbulent molecular clouds and dense young clusters show that encounters between disc-surrounded stars are relatively common events which should significantly influence the resulting disc structure. In turn this should alter the accretion rate of disc matter onto the star and the conditions under which planet formation occurs. Although the effects of star-disc encounters have been previously investigated, very little is known about encounters where both stars are surrounded by discs. In this paper encounters of such disc-disc systems are studied quantitatively. It is found that for low-mass discs ($M_D$= 0.01 $M_\sun$) the results from star-disc encounters can be straightforwardly generalized to disc-disc encounters as long as there is no mass transport between the discs. Differences to star-disc encounters occur naturally where significant amounts of matter are transported between the discs. In this case it is found that although the mass distribution does not change significantly, matter caught onto highly eccentric orbits is transported surprisingly far inside the disc. The captured mass partly replenishes the disc, but has a much lower angular momentum. This can lead to a reduction of the angular momentum in the entire disc and thus considerably increased accretion shortly after the encounter as well as in the long term.

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arXiv2005

Parameter Study of Star-Discs Encounters

Interactions between disc-surrounded stars might play a vital role in the formation of planetary systems. Here a first parameter study of the effects of encounters on low-mass discs is presented. The dependence of the mass and angular momentum transport on the periastron distance, the relative mass of the encountering stars and eccentricity of the encounter is investigated in detail. This is done for prograde and retrograde coplanar encounters as well as non-coplanar encounters. For distant coplanar encounters our simulation results agree with the analytical approximation of the angular momentum loss by Ostriker(1994). However, for close or high-mass encounters, significant differences to this approximation are found. This is especially so in the case of retrograde encounters, where the analytical result predict no angular momentum loss regardless of the periastron distance whereas the simulations find up to ~ 20% loss for close encounters. For the non-coplanar case a more complex dependency on the inclination between orbital path and disc plane is found than for distant encounters. For the coplanar prograde case new fitting formulae for the mass and angular momentum loss are obtained, which cover the whole range from grazing to distant encounters. In addition, the final disc size and the mass exchange between discs is examined, demonstrating that for equal mass stars in encounters as close as 1.5 the disc radius, the disc size only is reduced by approximately 10%.

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arXiv2005

Weighing the cusp at the Galactic Centre

This paper has been withdrawn temporarily by the authors. As stars close to the galactic centre have short orbital periods it has been possible to trace large fractions of their orbits in the recent years. Previously the data of the orbit of the star S2 have been fitted with Keplerian orbits corresponding to a massive black hole (MBH) with a mass of M$_{BH}$=3-4$\times10^6$M_sun implying an insignificant cusp mass.However, it has also been shown that the central black hole resides in a ~1'' diameter stellar cluster of a priori unknown mass. In a spherical potential which is neither Keplerian nor harmonic, orbits will precess resulting in inclined rosetta shaped trajectories on the sky. In this case, the assumption of non-Keplerian orbits is a more physical approach. It is also the only approach through which cusp mass information can be obtained via stellar dynamics of the cusp members. This paper presents the first exemplary modelling efforts in this direction. Using positional and radial data of star S2, we find that there could exist an unobserved extended mass component of several 10$^5$M$_{\odot}$ forming a so-called 'cusp' centred on the black hole position. Considering only the fraction of the cusp mass M$_{S2_{apo}}$ within the apocenter of the S2 orbit we find as an upper limit that M$_{S2_{apo}}$/(M$_{BH}$ + M$_{S2_{apo}}$) $\le$ 0.05. A large extended cusp mass, if present, is unlikely to be composed of sub-solar mass constituents, but could be explained rather well by a cluster of high M/L stellar remnants, which we find to form a stable configuration.

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arXiv2003

The Merger QSO 3C 48 and its Host Galaxy in the Near Infrared

In this contribution we present new near-infrared (NIR) data on the quasar 3C 48 and its host galaxy, obtained with ISAAC at the Very Large Telescope (ESO, Chile). The NIR images and spectra reveal a reddening of several magnitudes caused by extinction due to molecular material and dust within the host galaxy. For the first time we clearly identify the highly reddened potential second nucleus 3C 48A about 100 northeast of the quasar position in the NIR. Its reddening can be accounted for by warm dust, heated by star formation or an interaction of the 3C 48 radio jet with the interstellar medium, or both. The NIR colors and the CO(6-3) absorption feature both give a stellar contribution of about 30 percent to the QSO-dominated light. These results will contribute to the question of how the nuclear activity and the apparent merger process are influencing the host galaxy properties and they will improve existing models.

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arXiv2003

The Nearby QSO Host I Zw 1: NIR Probing of Structural Properties and Stellar Populations

The likely merger process and the properties of the stellar populations in the I Zw 1 host galaxy are analyzed on the basis of multi-wavelength observations (with the ISAAC camera at the Very Large Telescope (VLT/UT1) of the European Southern Observatory (ESO), Chile (Paranal), with the interferometer of the Berkeley-Illinois-Maryland Association (BIMA), USA (Hat Creek/California), and with the IRAM Plateau de Bure Interferometer (PdBI), France) and N-body simulations. The data give a consistent picture of I Zw 1, with properties between those of ultra-luminous infrared galaxies (ULIRGs) and QSOs as displayed by transition objects in the evolutionary sequence of active galaxies.

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arXiv2003

A multi-particle model of the 3C 48 host

The first successful multi-particle model for the host of the well-known quasi-stellar object (QSO) 3C 48 is reported. It shows that the morphology and the stellar velocity field of the 3C 48 host can be reproduced by the merger of two disk galaxies. The conditions of the interaction are similar to those used for interpreting the appearance of the ''Antennae'' (NGC 4038/39) but seen from a different viewing angle. The model supports the controversial hypothesis that 3C 48A is the second nucleus of a merging galaxy, and it suggests a simple solution for the problem of the missing counter tidal tail.

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arXiv2003

Angular Momentum Transfer in Star-Discs Encounters: The Case of Low-Mass Discs

A prerequisite for the formation of stars and planetary systems is that angular momentum is transported in some way from the inner regions of the accretion disc. Tidal effects may play an important part in this angular momentum transport. Here the angular momentum transfer in an star-disc encounter is investigated numerically for a variety of encounter parameters in the case of low mass discs. Although good agreement is found with analytical results for the entire disc, the loss {\it inside} the disc can be up to an order of magnitude higher than previously assumed. The differences in angular momentum transport by secondaries on a hyperbolic, parabolic and elliptical path are shown, and it is found that a succession of distant encounters might be equally, if not more, successful in removing angular momentum than single close encounter.

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arXiv2003

I Zw 1: Decomposition of the Nearby QSO Host

The paper presents an analysis of the nearby QSO host I Zw 1 based on new J-band imaging data, obtained with ISAAC at the Very Large Telescope (VLT) of the European Southern Observatory (ESO). As one of the best-studied QSOs with exceptional properties lying between those of QSOs, narrow-line Seyfert 1s, and ultraluminous infrared galaxies, I Zw 1 is a prime candidate for a detailed case study of a QSO host. With its high angular resolution and sensitivity, the new J-band image provides strong evidence for an interaction between I Zw 1 and the western companion galaxy. We present a procedure for subtracting the QSO nucleus from the I Zw 1 image to obtain an undisturbed view on the underlying host galaxy. Based on the derived surface brightness profile and the gas rotation curve, the host is classified as a high-surface-brightness Freeman type I galaxy with a central disk surface brightness significantly larger than that of normal spirals. The surface-brightness profile is decomposed into a Plummer bulge and a Kuzmin disk with similar luminosities and a bulge-to-disk scale length ratio of 0.29, comparable to that of nearby spiral hosts with active galactic nuclei (AGN). Different models for the decomposition of the gas rotation curve are discussed. The resulting J-band mass-to-light ratio (M/L) of (0.7+/-0.4) M_sun/L_sun for the disk component suggests a mean solar-type stellar population with a tendency towards young stars. The bulge exhibits a lower M/L of 0.4+/-0.2 M_sun/L_sun, which supports previous findings of enhanced starburst activity in the nuclear and circumnuclear region.

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arXiv1999

N-body calculations of cluster growth in proto-planetary disks

We investigated numerically the dust growth driven by Brownian motion in a proto-planetary disc around a solar-type young stellar object. This process is considered as the first stage in the transformation of the initially micron-sized solid particles to a planetary system. In contrast to earlier studies the growth was investigated at the small particle number densities typical for the conditions in a proto-planetary disc. Under such circumstances, the mean particle distance exceeds the typical aggregate diameter by orders of magnitude, and a collision will be a very rare event. We derived a criterion which allows an efficient detection of candidates for imminent collisions. The N-particle-method we used is based upon an adaptive time step scheme respecting the individual dynamical states of the aggregates. Its basic concept is to perform on average constant ``length steps'', instead of using constant time steps. The numerical cost of the algorithm scales with the particle number better than N log N. In order to minimise the influence of the decreasing number of particles within the simulation box, a new rescaling method is used throughout the aggregation process. Our numerical results indicate that at very low number densities, the growth process is influenced by spatial number density fluctuations.

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