Argelander Institute For Astronomie Aifa

Schönau-Berzdorf, Germany

Argelander Institute For Astronomie Aifa

Schönau-Berzdorf, Germany
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Kupper A.H.W.,Argelander Institute For Astronomie Aifa | Kupper A.H.W.,European Southern Observatory | Maschberger T.,Argelander Institute For Astronomie Aifa | Maschberger T.,CNRS Grenoble Institute for Particle Astrophysics and Cosmology Laboratory | And 2 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2011

By analysing models of the young massive cluster R136 30 Doradus, set-up using the herewith introduced and publicly made available code McLuster, we investigate and compare different methods for detecting and quantifying mass segregation and substructure non-seeing limited N-body data. For this purpose we generate star cluster models with different degrees of mass segregation and fractal substructure and analyse them. We quantify mass segregation by measuring, from the projected 2D model data, the mass function slope radial annuli, by looking for colour gradients radial colour profiles, by measuring Allison's Λ parameter and by determining the local stellar surface density around each star. We find that these methods for quantifying mass segregation often produce ambiguous results. Most reliable for detecting mass segregation is the mass function slope method, whereas the colour-gradient method is the least practical an R136-like configuration. The other two methods are more sensitive to low degrees of mass segregation but are computationally much more demanding. We also discuss the effect of binaries on these measures. Moreover, we quantify substructure by looking at the projected radial stellar density profile, by comparing projected azimuthal stellar density profiles and by determining Cartwright & Whitworth's Q parameter. We find that only high degrees of substructure affect the projected radial density profile, whereas the projected azimuthal density profile is very sensitive to substructure. The Q parameter is also sensitive to substructure but its absolute value shows a dependence on the radial density gradient of the cluster and is strongly influenced by binaries. Thus, terms of applicability and comparability for large sets of N-body data, the mass function slope method and the azimuthal density profile method seem to be the best choices for quantifying the degree of mass segregation and substructure, respectively. The other methods are computationally too demanding to be practically feasible for large data sets. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.


Lane R.R.,University of Concepción | Kupper A.H.W.,Argelander Institute For Astronomie Aifa | Kupper A.H.W.,European Southern Observatory | Heggie D.C.,University of Edinburgh
Monthly Notices of the Royal Astronomical Society | Year: 2012

The Galactic globular cluster 47 Tucanae (47 Tuc) shows a rare increase in its velocity dispersion profile at large radii, indicative of energetic, yet bound, stars at large radii dominating the velocity dispersion and, potentially, of ongoing evaporation. Escaping stars will form tidal tails, as seen with several Galactic globular clusters; however, the tidal tails of 47 Tuc are yet to be uncovered. We model these tails of 47 Tuc using the most accurate input data available, with the specific aim of determining their locations, as well as the densities of the epicyclic overdensities within the tails. The overdensities from our models show an increase of 3-4 per cent above the Galactic background and, therefore, should be easily detectable using matched filtering techniques. We find that the most influential parameter with regard to both the locations and densities of the epicyclic overdensities is the heliocentric distance to the cluster. Hence, uncovering these tidal features observationally will contribute greatly to the ongoing problem of determining the distance to 47 Tuc, tightly constraining the distance of the cluster independent of other methods. Using our streakline method for determining the locations of the tidal tails and their overdensities, we show how, in principle, the shape and extent of the tidal tails of any Galactic globular cluster can be determined without resorting to computationally expensive N-body simulations. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.


Kupper A.H.W.,Argelander Institute For Astronomie Aifa | Kupper A.H.W.,European Southern Observatory | Lane R.R.,University of Concepción | Heggie D.C.,University of Edinburgh
Monthly Notices of the Royal Astronomical Society | Year: 2012

We investigate the epicyclic motion of stars escaping from star clusters. Using streaklines, we visualize the path of escaping stars and show how epicyclic motion leads to over- and underdensities in tidal tails of star clusters moving on circular and eccentric orbits about a galaxy. Additionally, we investigate the effect of the cluster mass on the tidal tails, by showing that their structure is better matched when the perturbing effect of the cluster mass is included. By adjusting streaklines to results of N-body computations we can accurately and quickly reproduce all observed substructure, especially the streaky features often found in simulations which may be interpreted in observations as multiple tidal tails. Hence, we can rule out tidal shocks as the origin of such substructures. Finally, from the adjusted streakline parameters we can verify that for the star clusters we studied escape mainly happens from the tidal radius of the cluster, given by x L= (GM/(Ω 2-∂ 2Φ/∂R 2)) 1/3. We find, however, that there is another limiting radius, the 'edge' radius, which gives the smallest radius from which a star can escape during one cluster orbit about the galaxy. For eccentric cluster orbits the edge radius shrinks with increasing orbital eccentricity (for fixed apocentric distance) but is always significantly larger than the respective perigalactic tidal radius. In fact, the edge radii of the clusters we investigated, which are extended and tidally filling, agree well with their (fitted) King radii, which may indicate a fundamental connection between these two quantities. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.


Kupper A.H.W.,Argelander Institute For Astronomie Aifa | Kupper A.H.W.,European Southern Observatory | Mieske S.,European Southern Observatory | Kroupa P.,Argelander Institute For Astronomie Aifa
Monthly Notices of the Royal Astronomical Society | Year: 2011

We investigate the dynamical status of the low-mass globular cluster Palomar 13 by means of N-body computations to test whether its unusually high mass-to-light ratio of about 40 and its peculiarly shallow surface density profile can be caused by tidal shocking. Alternatively, we test - by varying the assumed proper motion - if the orbital phase of Palomar 13 within its orbit about the Milky Way can influence its appearance and thus may be the origin of these peculiarities, as has been suggested by Küpper et al. We find that, of these two scenarios, only the latter can explain the observed mass-to-light ratio and surface density profile. We note, however, that the particular orbit that best reproduces those observed parameters has a proper motion inconsistent with the available literature value. We discuss this discrepancy and suggest that it may be caused by an underestimation of the observational uncertainties in the proper motion determination. We demonstrate that Palomar 13 is most likely near apogalacticon, which makes the cluster appear supervirial and blown-up due to orbital compression of its tidal debris. Since the satellites of the Milky Way are on average closer to apogalacticon than perigalacticon, their internal dynamics may be influenced by the same effect, and we advocate that this needs to be taken into account when interpreting their kinematical data. Moreover, we briefly discuss the influence of a possible binary population on such measurements. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.


Dabringhausen J.,Argelander Institute For Astronomie Aifa | Kroupa P.,Argelander Institute For Astronomie Aifa | Pflamm-Altenburg J.,Argelander Institute For Astronomie Aifa | Mieske S.,European Southern Observatory
Astrophysical Journal | Year: 2012

It has been shown before that the high mass-to-light ratios of ultracompact dwarf galaxies (UCDs) can be explained if their stellar initial mass function (IMF) was top-heavy, i.e., that the IMF was skewed toward high-mass stars. In this case, neutron stars (NSs) 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 a 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 NS accretes matter from an evolving low-mass star. We find that LMXBs are indeed up to 10times 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 that 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 in the starburst galaxy Arp220 suggests a top-heavy IMF in that system. This finding is consistent with the notion that starburst 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 M ⊙ yr-1 pc-3 in pc-scale regions. © 2012. The American Astronomical Society. All rights reserved.


Kupper A.H.W.,Argelander Institute For Astronomie Aifa | Kupper A.H.W.,European Southern Observatory | Kroupa P.,Argelander Institute For Astronomie Aifa | Baumgardt H.,Argelander Institute For Astronomie Aifa | And 2 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2010

Based on our recent work on tidal tails of star clusters we investigate star clusters of a few 104M⊙ by means of velocity dispersion profiles and surface density profiles. We use a comprehensive set of N-body computations of star clusters on various orbits within a realistic tidal field to study the evolution of these profiles with time, and ongoing cluster dissolution. From the velocity dispersion profiles we find that the population of potential escapers, i.e. energetically unbound stars inside the Jacobi radius, dominates clusters at radii above about 50 per cent of the Jacobi radius. Beyond 70 per cent of the Jacobi radius nearly all stars are energetically unbound. The velocity dispersion therefore significantly deviates from the predictions of simple equilibrium models in this regime. We furthermore argue that for this reason this part of a cluster cannot be used to detect a dark matter halo or deviations from the Newtonian gravity. By fitting templates to about 104 computed surface density profiles we estimate the accuracy which can be achieved in reconstructing the Jacobi radius of a cluster in this way. We find that the template of King works well for extended clusters on nearly circular orbits, but shows significant flaws in the case of eccentric cluster orbits. This we fix by extending this template with three more free parameters. Our template can reconstruct the tidal radius over all fitted ranges with an accuracy of about 10 per cent, and is especially useful in the case of cluster data with a wide radial coverage and for clusters showing significant extra-tidal stellar populations. No other template that we have tried can yield comparable results over this range of cluster conditions. All templates fail to reconstruct tidal parameters of concentrated clusters, however. Moreover, we find that the bulk of a cluster adjusts to the mean tidal field which it experiences and not to the tidal field at perigalacticon as has often been assumed in other investigations, i.e. a fitted tidal radius is a cluster,s time average mean tidal radius and not its perigalactic one. Furthermore, we study the tidal debris in the vicinity of the clusters and find it to be well represented by a power law with a slope of -4 to -5. This steep slope we ascribe to the epicyclic motion of escaped stars in the tidal tails. Star clusters close to apogalacticon show a significantly shallower slope of up to -1, however. We suggest that clusters at apogalacticon can be identified by measuring this slope. © 2010 The Authors. Journal compilation. © 2010 RAS.


Kupper A.H.W.,European Southern Observatory | Kupper A.H.W.,Argelander Institute For Astronomie Aifa | Kroupa P.,Argelander Institute For Astronomie Aifa
Astrophysical Journal | Year: 2010

We investigate the velocity dispersion of Pal 14, an outer Milky Way globular cluster at a Galactocentric distance of 71 kpc with a very low stellar density (central density 0.1-0.2 M pc-3). Due to this low stellar density the binary population of Pal 14 is likely to be close to the primordial binary population. Artificial clusters are generated with the observed properties of Pal 14, and the velocity dispersion within these clusters is measured as Jordi et al. have done with 17 observed stars of Pal 14. We discuss the effect of the binary population on these measurements and find that the small velocity dispersion of 0.38 km s-1, which has been found by Jordi et al., would imply a binary fraction of less than 0.1, even though from the stellar density of Pal 14 we would expect a binary fraction of more than 0.5. We also discuss the effect of mass segregation on the velocity dispersion as a possible explanation for this discrepancy, but find that it would increase the velocity dispersion further. Thus, either Pal 14 has a very unusual stellar population and its birth process was significantly different than we see in today's star-forming regions, or the binary population is regular and we would have to correct the observed 0.38 km s-1 for binarity. In this case, the true velocity dispersion of Pal 14 would be much smaller than this value and the cluster would have to be considered as "kinematically frigid," thereby possibly posing a challenge for Newtonian dynamics but in the opposite sense to modified Newtonian dynamics. © 2010. The American Astronomical Society. All rights reserved.


Winkel B.,Max Planck Institute for Radio Astronomy | Floer L.,Argelander Institute For Astronomie Aifa | Kraus A.,Max Planck Institute for Radio Astronomy
Astronomy and Astrophysics | Year: 2012

We report on a novel method to solve the basket-weaving problem. Basket-weaving is a technique that is used to remove scan-line patterns from single-dish radio maps. The new approach applies linear least-squares and works on gridded maps from arbitrarily sampled data, which greatly improves computational efficiency and robustness. It also allows masking of bad data, which is useful for cases where radio frequency interference is present in the data. We evaluate the algorithms using simulations and real data obtained with the Effelsberg 100-m telescope. © 2012 ESO.


Assmann P.,University of Concepción | Fellhauer M.,University of Concepción | Kroupa P.,Argelander Institute For Astronomie Aifa | Bruns R.C.,Argelander Institute For Astronomie Aifa | Smith R.,University of Concepción
Monthly Notices of the Royal Astronomical Society | Year: 2011

It is widely believed that star clusters form with low star formation efficiencies. With the onset of stellar winds by massive stars or finally when the first supernova blows off, the residual gas is driven out of the embedded star cluster. Due to this fact, a large number, if not all, of the stars become unbound and disperse in the gravitational potential of the galaxy. In this context, Kroupa suggested a new mechanism for the emergence of thickened galactic discs. Massive star clusters add kinematically hot components to the galactic field populations, building up, in this way, the Galactic thick disc as well. In this work, we perform, for the first time, numerical simulations to investigate this scenario for the formation of the Galactic discs of the Milky Way (MW). We find that a significant kinematically hot population of stars may be injected into the disc of a galaxy such that a thick disc emerges. For the MW, the star clusters that formed the thick disc must have had masses of about 106M⊙. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.


Kupper A.H.W.,Argelander Institute For Astronomie Aifa | Kroupa P.,Argelander Institute For Astronomie Aifa | Baumgardt H.,Argelander Institute For Astronomie Aifa | Heggie D.C.,University of Edinburgh
Monthly Notices of the Royal Astronomical Society | Year: 2010

Based on recent findings of a formation mechanism of substructure in tidal tails by Küpper et al., we investigate a more comprehensive set of N-body models of star clusters on orbits about a Milky Way like potential. We find that the predicted epicyclic overdensities arise in any tidal tail no matter which orbit the cluster follows as long as the cluster lives long enough for the overdensities to build up. The distance of the overdensities along the tidal tail from the cluster centre depends for circular orbits only on the mass of the cluster and the strength of the tidal field, and therefore decreases monotonically with time, while for eccentric orbits the orbital motion influences the distance, causing a periodic compression and stretching of the tails and making the distance oscillate with time. We provide an approximation for estimating the distance of the overdensities in this case. We describe an additional type of overdensity which arises in extended tidal tails of clusters on eccentric orbits, when the acceleration of the tidal field on the stellar stream is no longer homogeneous. Moreover, we conclude that a pericentre passage or a disc shock is not the direct origin of an overdensity within a tidal tail. Escape due to such tidal perturbations does not take place immediately after the perturbation but is rather delayed and spread over the orbit of the cluster. All observable overdensities are therefore of the mentioned two types. In particular, we note that substructured tidal tails do not imply the existence of dark matter substructures in the haloes of galaxies. © 2009 RAS.

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