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Lane R.R.,University of Concepcion | 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. Source


Kupper A.H.W.,Argelander Institute For Astronomie Aifa | Kupper A.H.W.,European Southern Observatory | Lane R.R.,University of Concepcion | 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. Source


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. Source


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. Source


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. Source

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