Universities of Bonn and Cologne

Köln, Germany

Universities of Bonn and Cologne

Köln, Germany

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Britzen S.,Max Planck Institute for Radio Astronomy | Kudryavtseva N.A.,Max Planck Institute for Radio Astronomy | Kudryavtseva N.A.,University College Cork | Kudryavtseva N.A.,Universities of Bonn and Cologne | And 10 more authors.
Astronomy and Astrophysics | Year: 2010

Context: BL Lac objects show core-jet structures with features moving outwards along the jet. We present a kinematic analysis of jet component motion in the pc-scale jet of the BL Lac object S5 1803+784, which does not reveal long-term outward motion for most of the components. Aims: S5 1803+784 shows complex kinematic phenomena; understanding these provides new insights into the emission processes in BL Lac objects and possibly into the differences between quasars and BL Lac objects. Methods: The blazar S5 1803+784 has been studied with VLBI at ? = 1.6, 2.3, 5, 8.4, and 15 GHz between 1993.88 and 2005.68 in 26 observing runs. We (re)analyzed the data and present Gaussian model-fits. We collected the already published kinematic information for this source from the literature and re-identified the components according to the new scenario presented in this paper. Altogether, 94 epochs of observations have been investigated. Results: A careful study of the long-term kinematics reveals a new picture for jet component motion in S5 1803+784. In contrast to previously discussed motion scenarios, we find that the jet structure within 12 mas of the core can most easily be described by the coexistence of several bright jet features that remain on the long-term at roughly constant core separations (in addition to the already known "stationary" jet component ∼1.4 mas) and one faint component moving with an apparent superluminal speed (∼19c, based on 3 epochs). While most of the components maintain long-term roughly constant distances from the core, we observe significant, smooth changes in their position angles. We report on an evolution of the whole jet ridge line with time over the almost 12 years of observations. The width of the jet changes periodically with a period of ∼8-9 years. We find a correlation between changes in the position angle and maxima in the total flux-density light-curves. We present evidence for a geometric origin of the observed phenomena and discuss possible models. Conclusions: We find evidence for a significantly different scenario of jet component motion in S5 1803+784 compared to the generally accepted one of outwardly moving jet components, and conclude that the observed phenomena (evolution of the jet ridge line, roughly constant component core separations but with significant position angle changes) can most easily be explained within a geometric model. © 2010 ESO.


Kreplin A.,Max Planck Institute for Radio Astronomy | Kraus S.,University of Michigan | Hofmann K.-H.,Max Planck Institute for Radio Astronomy | Schertl D.,Max Planck Institute for Radio Astronomy | And 3 more authors.
Astronomy and Astrophysics | Year: 2012

Aims. We study the AU-scale circumstellar environment of the unclassified B[e] star V921 Sco in the near-infrared. For interpreting the observations, we employ temperature-gradient disk models. Methods. Using the near-infrared beam combiner instrument AMBER, we recorded spectrally dispersed (spectral resolution R = 35) interferograms in the H and K bands. To obtain an improved calibration of the visibilities, we developed a method that is able to equalize the histograms of the optical path difference of target and calibrator. We fit temperature-gradient disk models to the visibilities and spectral energy distribution (SED) to analyze the circumstellar dust geometry. Results. We derived a geometric ring-fit radius of 2.10 ± 0.16 mas in the K band. If we adopt the distance of 1150 ± 150 pc reported elsewhere, we obtain a ring-fit radius of ∼2.4 AU, which is slightly smaller than the 3.5 AU dust sublimation radius predicted by the size-luminosity relation. The fitted H-band radius of 1.61 ± 0.23 mas (1.85 AU) is found to be more compact than the K-band radius. The best-fit temperature-gradient disk model has an inner disk radius of ∼1.45 AU, an inner-edge disk temperature T 0 = 1533 K, and a temperature-gradient exponent q = 0.46 suggesting a flared disk geometry. Conclusions. The distance and luminosity of V921 Sco are not well known. If we assume a distance of 1150 ± 150 pc, we derive a ring-fit radius of ∼2.4 AU, which is approximately consistent with the computed temperature-gradient disk model with inner and outer ring radii of 1.45 and 8.5 AU, respectively. If the inner radius of V921 Sco is more compact than the sublimation radius, this compact observed size can be explained by emitting material (e.g., a gaseous disk) inside the dust sublimation radius, as suggested for several other B[e] stars. © 2012 ESO.


Habibi M.,University of Bonn | Habibi M.,Universities of Bonn and Cologne | Stolte A.,University of Bonn | Harfst S.,TU Berlin
Astronomy and Astrophysics | Year: 2014

Recently, more than 100 Wolf-Rayet and OB stars were identified in the Galactic center. About a third of these sources are not spatially associated with any of the known star clusters in this region. We probe the distribution of drifted sources in numerical models of the massive clusters in the Galactic center and compare it to the observed distribution of isolated massive sources in this region. We find that stars as massive as 100 M⊙ drift away from the center of each cluster by up to ~ 60 pc using the cluster models. Our best model reproduces ~ 60% of the known isolated massive stars out to 80 pc from the center of the Arches cluster. This number increases to 70-80% when we only consider the region that is ~ 20 pc from the Arches cluster. © ESO, 2014.


Marks M.,University of Bonn | Marks M.,Max Planck Institute for Radio Astronomy | Marks M.,Universities of Bonn and Cologne | Kroupa P.,University of Bonn
EPJ Web of Conferences | Year: 2012

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 [1]). © Owned by the authors, published by EDP Sciences, 2012.


Shi X.,University of Bonn | Shi X.,Universities of Bonn and Cologne | Schneider P.,University of Bonn | Joachimi B.,University of Edinburgh
Astronomy and Astrophysics | Year: 2011

With the growing interest in and ability of using weak lensing studies to probe the non-Gaussian properties of the matter density field, there is an increasing need for the study of suitable statistical measures, e.g. shear three-point statistics. In this paper we establish the relations between the three-point configuration space shear and convergence statistics, which are an important missing link between different weak lensing three-point statistics and provide an alternative way of relating observation and theory. The method we use also allows us to derive the relations between other two-and three-point correlation functions. We show the consistency of the relations obtained with already established results and demonstrate how they can be evaluated numerically. As a direct application, we use these relations to formulate the condition for E/B-mode decomposition of lensing three-point statistics, which is the basis for constructing new three-point statistics which allow for exact E/B-mode separation. Our work applies also to other two-dimensional polarization fields such as that of the cosmic microwave background. © 2011 ESO.


Shi X.,University of Bonn | Shi X.,Universities of Bonn and Cologne | Joachimi B.,University of Bonn | Schneider P.,University of Bonn
Astronomy and Astrophysics | Year: 2010

Three-point weak lensing statistics provide cosmic information that complements two-point statistics. However, both statistics suffer from intrinsic-shear alignment, which is one of their limiting systematics. The nulling technique is a model-independent method developed to eliminate intrinsic-shear alignment at the two-point level. In this paper we demonstrate that the nulling technique can also be naturally generalized to the three-point level, thereby controlling the corresponding GGI systematics. We show that under the assumption of exact redshift information the intrinsic-shear alignment contamination can be completely eliminated. To show how well the nulling technique performs on data with limited redshift information, we apply the nulling technique to three-point weak lensing statistics from a fictitious survey analogous to a typical future deep imaging survey, in which the three-point intrinsic-shear alignment systematics is generated from a power-law toy model.Using 10 redshift bins, the nulling technique leads to a factor of 10 suppression of the GGI/GGG ratio and reduces the bias on cosmological parameters to less than the original statistical error. More detailed redshift information allowing for finer redshift bins leads to better bias reduction performance. The information loss during the nulling procedure doubles the statistical error on cosmological parameters. A comparison of the nulling technique with an unconditioned compression of the data suggests that part of the information loss can be retained by considering higher order nulling weights during the nulling procedure. A combined analysis of two- and three-point statistics confirms that the information contained in them is of comparable size and is complementary, both before and after nulling. © 2010 ESO.


Gomez L.,Max Planck Institute for Radio Astronomy | Gomez L.,National Autonomous University of Mexico | Gomez L.,Universities of Bonn and Cologne | Luis L.,National Autonomous University of Mexico | And 6 more authors.
Astrophysical Journal, Supplement Series | Year: 2010

We present Very Large Array (VLA) observations of the 70-6 1A+ methanol maser transition at 44 GHz toward NGC 6334F, G8.67-0.36, and M17. These arcsecond resolution observations complete a previous, larger VLA survey of this maser transition in high-mass, star-forming regions reported by Kurtz et al. We confirm the presence of 44 GHz methanol maser emission in all 3 sources, detecting 8 distinct maser components in NGC 6334F, 12 components in G8.67-0.36, and 1 in M17. © 2010.


Habibi M.,University of Bonn | Habibi M.,Universities of Bonn and Cologne | Stolte A.,University of Bonn | Brandner W.,Max Planck Institute for Astronomy | And 2 more authors.
Astronomy and Astrophysics | Year: 2013

The Galactic center is the most active site of star formation in the Milky Way, where particularly high-mass stars have formed very recently and are still forming today. However, since we are looking at the Galactic center through the Galactic disk, knowledge of extinction is crucial when studying this region. The Arches cluster is a young, massive starburst cluster near the Galactic center. We observed the Arches cluster out to its tidal radius using Ks-band imaging obtained with NAOS/CONICA at the VLT combined with Subaru/CISCO J-band data to gain a full understanding of the cluster mass distribution. We show that the determination of the mass of the most massive star in the Arches cluster, which had been used in previous studies to establish an upper mass limit for the star formation process in the Milky Way, strongly depends on the assumed slope of the extinction law. Assuming the two regimes of widely used infrared extinction laws, we show that the difference can reach up to 30% for individually derived stellar masses and ΔAKs ∼ 1 magnitude in acquired Ks-band extinction, while the present-day mass function slope changes by ∼ 0.17 dex. The present-day mass function slope derived assuming the more recent extinction law increases from a flat slope of αNishi = -1.50 ± 0.35 in the core (r < 0.2 pc) to αNishi = -2.21 ± 0.27 in the intermediate annulus (0.2 < r < 0.4 pc), where the Salpeter slope is -2.3. The mass function steepens to αNishi = -3.21 ± 0.30 in the outer annulus (0.4 < r < 1.5 pc), indicating that the outer cluster region is depleted of high-mass stars. This picture is consistent with mass segregation owing to the dynamical evolution of the cluster. © ESO, 2013.


Liu F.-C.,Max Planck Institute for Radio Astronomy | Liu F.-C.,Universities of Bonn and Cologne | Parise B.,Max Planck Institute for Radio Astronomy | Wyrowski F.,Max Planck Institute for Radio Astronomy | And 2 more authors.
Astronomy and Astrophysics | Year: 2013

Context.Water is an essential molecule in oxygen chemistry and the main constituent of grain icy mantles. The formation of water can be studied through the HDO/H2O ratio. Thanks to the launch of the Herschel satellite and the advance of sensitive submillimeter receivers on ground telescopes, many H2O and HDO transitions can now be observed, enabling more accurate studies of the level of water fractionation. Aims. Using these new technologies, we aim at revisiting the water fractionation studies toward massive star-forming regions. We present here a detailed study toward G34.26+0.15, a massive star-forming region associated with compact HII regions. Methods.We present observations of five HDO lines obtained with the APEX telescope. Two of those transitions are ground-state transitions. Two of the three high-excitation lines were additionally observed at higher angular resolution with the SMA.We analyzed these observations using the 1D radiative transfer code RATRAN and adopting different physical profiles from two different models. Results. Although the inner and outer fractional abundances relative to H2 can be best constrained to be XHDO in (T > 100 K) = (5-7) × 10-8(3σ) and XHDO out (T = 100 K) = (0.3-2) × 10-11(3σ), the line profile of the 893 GHz ground transition cannot be well reproduced. This line profile is shown to be very sensitive to the velocity field. To better constrain the velocity field, it is necessary to observe the HDO line at 893 GHz with high angular resolution. The H2O abundance is deduced from one high-excitation and one ground transition H18 2 O line. The D/H ratios of water are 3.0 × 10-4 in the inner region and (1.9-4.9) × 10-4 in the outer region of the core. The HDO fractional abundance in the inner and outer regions are different by more than four orders, which implies that the sublimation is very similar in low-and high-mass protostars. The D/H ratios of water in G34.26 + 0.15 are close to the value obtained for the same source in a previous study, and similar to those in other high-mass sources, but lower than those in low-mass protostars, suggesting the possibility that the dense and cold pre-collapse phase is shorter for high-mass star-forming regions. © 2013 ESO.

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