Sterrewacht Leiden

Leiden, Netherlands

Sterrewacht Leiden

Leiden, Netherlands
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Nidever D.L.,University of Virginia | Majewski S.R.,University of Virginia | Burton W.B.,Sterrewacht Leiden | Burton W.B.,U.S. National Radio Astronomy Observatory | Nigra L.,University of Wisconsin - Madison
Astrophysical Journal | Year: 2010

We establish with certainty that the Magellanic Stream (MS) is some 40> longer than previously known and that the entire MS and Leading Arm system is thus at least 200° long. With the Green Bank Telescope (GBT), we conducted a ∼200 deg2, 21 cm survey at the tip of the MS to substantiate the continuity of the MS between the Hulsbosch & Wakker data and the MS-like emission reported by Braun & Thilker. Our survey, in combination with the Arecibo survey by Stanimirović et al., shows that the MS gas is continuous in this region and that the MS is at least ∼140° long. The MS tip is composed of a multitude of forks and filaments. We identify a new filament on the eastern side of the MS that significantly deviates from the equator of the MS coordinate system for more than ∼45°. Additionally, we find a previously unknown velocity inflection in the MS tip near MS longitude Lms ≈ -120° at which the velocity reaches a minimum and then starts to increase. We find that five compact high-velocity clouds cataloged by de Heij et al. as well as Wright's Cloud are plausibly associated with the MS because they match the MS in position and velocity. The mass of the newly confirmed ∼40° extension of the MS tip is ∼2 × 107M ⊙ (d/120 kpc)2 (including Wright's Cloud increases this by ∼50%) and increases the total mass of the MS by ∼4%. However, projected model distances of the MS at the tip are generally quite large and, if true, indicate that the mass of the extension might be as large as ∼10 8 M⊙. From our combined map of the entire MS, we find that the total column density (integrated transverse to the MS) drops markedly along the MS and follows an exponential decline with Lms of N Hi = 5.9 × 1021 exp(LMS/19°.3)CM -2. Under the assumption that the observed sinusoidal velocity pattern of the LMC filament of the MS is due to the origin of the MS from a rotating LMC, we estimate that the age of the ∼140° long MS is ∼2.5Gyr. This coincides with bursts of star formation in the Magellanic Clouds and a possible close encounter of these two galaxies with each other that could have triggered the formation of the MS. These newly observed characteristics of the MS offer additional constraints for MS simulations. In the Appendix, we describe a previously little discussed problem with a standing wave pattern in GBT H i data and detail a method for removing it. © 2010. The American Astronomical Society.

Meshkat T.,Sterrewacht Leiden | Kenworthy M.A.,Sterrewacht Leiden | Quanz S.P.,ETH Zurich | Amara A.,ETH Zurich
Astrophysical Journal | Year: 2014

We present the results of a study to optimize the principal component analysis (PCA) algorithm for planet detection, a new algorithm complementing angular differential imaging and locally optimized combination of images (LOCI) for increasing the contrast achievable next to a bright star. The stellar point spread function (PSF) is constructed by removing linear combinations of principal components, allowing the flux from an extrasolar planet to shine through. The number of principal components used determines how well the stellar PSF is globally modeled. Using more principal components may decrease the number of speckles in the final image, but also increases the background noise. We apply PCA to Fomalhaut Very Large Telescope NaCo images acquired at 4.05 μm with an apodized phase plate. We do not detect any companions, with a model dependent upper mass limit of 13-18 M Jup from 4-10 AU. PCA achieves greater sensitivity than the LOCI algorithm for the Fomalhaut coronagraphic data by up to 1 mag. We make several adaptations to the PCA code and determine which of these prove the most effective at maximizing the signal-to-noise from a planet very close to its parent star. We demonstrate that optimizing the number of principal components used in PCA proves most effective for pulling out a planet signal. © 2014. The American Astronomical Society. All rights reserved..

Tasse C.,University Paris Diderot | Tasse C.,Rhodes University | Van Der Tol S.,Sterrewacht Leiden | Van Zwieten J.,Netherlands Institute for Radio Astronomy | And 2 more authors.
Astronomy and Astrophysics | Year: 2013

The required high sensitivities and large fields of view of the new generation of radio interferometers impose high dynamic ranges, e.g., ~1:10 6 to 1:108 for the Square Kilometre Array (SKA). The main problem for achieving these high ranges is the calibration and correction of direction dependent effects (DDE) that can affect the electro-magnetic field (antenna beams, ionosphere, Faraday rotation, etc.). It has already been shown that the A-Projection is a fast and accurate algorithm that can potentially correct for any given DDE in the imaging step. With its very wide field of view, low operating frequency (~30-250 MHz), long baselines, and complex station-dependent beam patterns, the LOw Frequency ARray (LOFAR) is certainly the most complex SKA pathfinder instrument. In this paper we present a few implementations of the A-Projection in LOFAR which can deal nondiagonal Mueller matrices. The algorithm is designed to correct for all DDE, including individual antennas, projection of the dipoles on the sky, beam forming, and ionospheric effects. We describe a few important algorithmic optimizations related to LOFAR's architecture that allowed us to build a fast imager. Based on simulated datasets we show that A-Projection can dramatically improve the dynamic range for both phased array beams and ionospheric effects. However, certain problems associated with the calibration of DDE remain (especially ionospheric effects), and the effect of the algorithm on real LOFAR survey data still needs to be demonstrated. We will be able to use this algorithm to construct the deepest extragalactic surveys, comprising hundreds of days of integration. © ESO, 2013.

Quanz S.P.,ETH Zurich | Amara A.,ETH Zurich | Meyer M.R.,ETH Zurich | Girard J.H.,European Southern Observatory | And 2 more authors.
Astrophysical Journal | Year: 2015

We present the first multi-wavelength, high-contrast imaging study confirming the protoplanet embedded in the disk around the Herbig Ae/Be star HD 100546. The object is detected at L′ (∼3.8 μm) and M′ (∼4.8 μm), but not at Ks (∼2.1 μm), and the emission consists of a point source component surrounded by spatially resolved emission. For the point source component we derive apparent magnitudes of L′ = 13.92 ± 0.10 mag, M′ = 13.33 ± 0.16 mag, and Ks > 15.43 ± 0.06 mag (3σ limit), and a separation and position angle of (0.457 ± 0.014)″ and (8.4 ± 1.4)°, and (0.472 ± 0.014)″ and (9.2 ± 1.4)° in L′ and M′, respectively. We demonstrate that the object is co-moving with HD 100546 and can reject any (sub-)stellar fore-/background object. Fitting a single-temperature blackbody to the observed fluxes of the point source component yields an effective temperature of Teff = 932+193-202K and a radius for the emitting area of R = 6.9+2.7-2.9RJupiter. The best-fit luminosity is L = (2.3+0.6-0.4) · 10-4 L⊙. We quantitatively compare our findings with predictions from evolutionary and atmospheric models for young, gas giant planets, discuss the possible existence of a warm, circumplanetary disk, and note that the deprojected physical separation from the host star of (53 ± 2) AU poses a challenge to standard planet formation theories. Considering the suspected existence of an additional planet orbiting at ∼13-14 AU, HD 100546 appears to be an unprecedented laboratory to study the formation of multiple gas giant planets empirically. © 2015. The American Astronomical Society. All rights reserved.

Quanz S.P.,ETH Zurich | Meyer M.R.,ETH Zurich | Kenworthy M.A.,Sterrewacht Leiden | Girard J.H.V.,European Southern Observatory | And 8 more authors.
Astrophysical Journal Letters | Year: 2010

Direct imaging of exoplanets requires both high contrast and h igh spatial resolution. Here, we present the first scientific results obtained with the newly commissioned apodizing phase plate coronagraph (APP) on VLT/NACO.We detected the exoplanet β Pictoris b in the narrowband filter centered at 4.05 μm (NB4.05). The position angle (209. ° 13 ± 2. ° 12) and the projected separation to its host star (0.″.354 ± 0. ″.012, i.e., 6.8 ± 0.2 AU at a distance of 19.3 pc) are in good agreement with the recently presented data from Lagrange et al. Comparing the observed NB4.05 magnitude of 11.20 ± 0.23 mag to theoretical atmospheric models, we find a best fit with a 7-10 M Jupiter object for an age of 12 Myr, again in agreement with previous estimates. Combining our results with published L′photometry, we can compare the planet's [L ′ - NB4.05] color to that of cool field dwarfs of higher surface gravity suggesting an effective temperature of ∼ 1700 K. The best-fit theo retical model predicts an effective temperature of ∼ 1470 K, but this difference is not significant given our photometric uncertainties. Our results demonstrate the potential of NACO/APP for future planet searches and provide independent confirmation as well as complementary data for β Pic b. © 2010. The American Astronomical Society. All rights reserved.

Quanz S.P.,ETH Zurich | Amara A.,ETH Zurich | Meyer M.R.,ETH Zurich | Kenworthy M.A.,Sterrewacht Leiden | And 2 more authors.
Astrophysical Journal Letters | Year: 2013

We present high-contrast observations of the circumstellar environment of the Herbig Ae/Be star HD 100546. The final 3.8 μm image reveals an emission source at a projected separation of 0.″48 ± 0.″04 (corresponding to ∼47 ± 4 AU) at a position angle of 8.9 ± 0.9. The emission appears slightly extended with a point source component with an apparent magnitude of 13.2 ± 0.4 mag. The position of the source coincides with a local deficit in polarization fraction in near-infrared polarimetric imaging data, which probes the surface of the well-studied circumstellar disk of HD 100546. This suggests a possible physical link between the emission source and the disk. Assuming a disk inclination of ∼47, the de-projected separation of the object is ∼68 AU. Assessing the likelihood of various scenarios, we favor an interpretation of the available high-contrast data with a planet in the process of forming. Follow-up observations in the coming years can easily distinguish between the different possible scenarios empirically. If confirmed, HD 100546 "b" would be a unique laboratory to study the formation process of a new planetary system, with one giant planet currently forming in the disk and a second planet possibly orbiting in the disk gap at smaller separations. © 2013 The American Astronomical Society. All rights reserved.

Narayanan D.,Harvard - Smithsonian Center for Astrophysics | Hayward C.C.,Harvard - Smithsonian Center for Astrophysics | Cox T.J.,Harvard - Smithsonian Center for Astrophysics | Hernquist L.,Harvard - Smithsonian Center for Astrophysics | And 4 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2010

We describe a model for the formation of z ∼ 2 submillimetre galaxies (SMGs) which simultaneously accounts for both average and bright SMGs while providing a reasonable match to their mean observed spectral energy distributions (SEDs). By coupling hydrodynamic simulations of galaxy mergers with the high-resolution 3D polychromatic radiative transfer code sunrise, we find that a mass sequence of merger models which use observational constraints as physical input naturally yield objects which exhibit black hole, bulge and H2 gas masses similar to those observed in SMGs. The dominant drivers behind the 850 μm flux are the masses of the merging galaxies and the stellar birth cloud covering fraction. The most luminous (S850 ≳ 15 mJy) sources are recovered by ∼1013 M⊙ 1:1 major mergers with a birth cloud covering fraction close to unity, whereas more average SMGs (S850 ∼ 5-7 mJy) may be formed in lower mass haloes (∼5 × 1012 M⊙). These models demonstrate the need for high spatial resolution hydrodynamic and radiative transfer simulations in matching both the most luminous sources as well as the full SEDs of SMGs. While these models suggest a natural formation mechanism for SMGs, they do not attempt to match cosmological statistics of galaxy populations; future efforts along this line will help ascertain the robustness of these models. © 2009 RAS.

Holden B.P.,University of California at Santa Cruz | Van Der Wel A.,Max Planck Institute for Astronomy | Kelson D.D.,Carnegie Institution of Washington | Franx M.,Sterrewacht Leiden | Illingworth G.D.,University of California at Santa Cruz
Astrophysical Journal | Year: 2010

We have measured velocity dispersions (σ) for a sample of 36 galaxies with J < 21.2 or Mr < -20.6 mag in MS 1054-03, a massive cluster of galaxies at z = 0.83. Our data are of uniformly high quality down to our selection limit, our 16 hr exposures typically yielding errors of only δ(σ) ∼ 10% for L* and fainter galaxies. By combining our measurements with data from the literature, we have 53 cluster galaxies with measured dispersions, and HST/ACS-derived sizes, colors and surface brightness. This sample is complete for the typical L* galaxy at z ∼ 1, unlike most previous z ∼ 1 cluster samples which are complete only for the massive cluster members (> 1011 M ⊙). We find no evidence for a change in the tilt of the fundamental plane (FP). Nor do we find evidence for evolution in the slope of the color-σ relation and M/LB-σ relations; measuring evolution at a fixed σ should minimize the impact of structural evolution found in other work. The M/LB at fixed σ evolves by Δlog10 M/LB = -0.50 ± 0.03 between z = 0.83 and z = 0.02 or d log10 M/LB = -0.60 ± 0.04 d z, and we find Δ(U - V)z = -0.24 ± 0.02 mag at fixed σ in the rest frame, matching the expected evolution in M/LB within 2.25 standard deviations. The implied formation redshift from both the color and M/LB evolution is z* = 2.0 ± 0.2 ± 0.3(sys), during the epoch in which the cosmic star formation activity peaked, with the systematic uncertainty showing the dependence of z* on the assumptions we make about the stellar populations. The lack of evolution in either the tilt of the FP or in the M/L-σ and color-σ relations imply that the formation epoch depends weakly on mass, ranging from z * = 2.3+1.3 -0.3 at σ = 300 km s-1 to z* = 1.7+0.3 -0.2 at σ = 160 km s-1 and implies that the initial mass function similarly varies slowly with galaxy mass. © 2010. The American Astronomical Society.

Portegies Zwart S.F.,Sterrewacht Leiden | McMillan S.L.W.,Drexel University | Van Elteren A.,Sterrewacht Leiden | Pelupessy F.I.,Sterrewacht Leiden | De Vries N.,Sterrewacht Leiden
Computer Physics Communications | Year: 2013

We introduce a general-purpose framework for interconnecting scientific simulation programs using a homogeneous, unified interface. Our framework is intrinsically parallel, and conveniently separates all component numerical modules in memory. This strict separation allows automatic unit conversion, distributed execution of modules on different cores within a cluster or grid, and orderly recovery from errors. The framework can be efficiently implemented and incurs an acceptable overhead. In practice, we measure the time spent in the framework to be less than 1% of the wall-clock time. Due to the unified structure of the interface, incorporating multiple modules addressing the same physics in different ways is relatively straightforward. Different modules may be advanced serially or in parallel. Despite initial concerns, we have encountered relatively few problems with this strict separation between modules, and the results of our simulations are consistent with earlier results using more traditional monolithic approaches. This framework provides a platform to combine existing simulation codes or develop new physical solver codes within a rich "ecosystem" of interchangeable modules. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.

Habing H.J.,Sterrewacht Leiden
Astronomy and Astrophysics | Year: 2016

Maser stars have been found with radial velocities up to +350 km s-1 and down to-350 km s-1 and exclusively within a few degrees from the Galactic centre. They form two spatially separated streams: one stream is at positive longitudes and consists of stars going away from us and the other is at negative longitudes consisting of stars approaching us. I show that closed orbits in a simple mass model for the bar explain quantitatively the existence of the two streams and the velocities observed. The mass of the bar is estimated on dynamical grounds: 3 × 1010 Msun. © ESO, 2016.

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