Results for "P. A. M. van Hoof"

total 393380took 0.19s
CKVul: evolving nebula and three curious background starsDec 20 2013We analyse the remnants of CK Vul (Nova Vul 1670) using optical imaging and spectroscopy. The imaging, obtained between 1991 and 2010, spans 5.6% of the life-time of the nebula. The flux of the nebula decreased during the last 2 decades. The central source ... More
The Evolution of NGC 7027 at Radio Frequencies: A New Determination of the Distance and Core MassJan 22 2008We present the results of a 25-year program to monitor the radio flux evolution of the planetary nebula NGC7027. We find significant evolution of the spectral flux densities. The flux density at 1465 MHz, where the nebula is optically thick, is increasing ... More
IRAS08281-4850 and IRAS14325-6428: two A-type post-AGB stars with s-process enrichmentJun 14 2007One of the puzzling findings in the study of the chemical evolution of (post-)AGB stars is why very similar stars (in terms of metallicity, spectral type, infrared properties, etc...) show a very different photospheric composition. We aim at extending ... More
Near-infrared and Br_gamma observations of post-AGB starsAug 01 2000We investigate the infrared properties of the IRAS sources not confirmed as PNe in Van de Steene & Pottasch (1993). We observed 28 objects in the N-band of which 20 were detected and 5 were resolved. We obtained medium resolution Br_gamma spectra and ... More
Molecular Hydrogen in Star-forming regions: implementation of its micro-physics in CloudyJan 22 2005Much of the baryonic matter in the Universe is in the form of H2 which includes most of the gas in galactic and extragalactic interstellar clouds. Molecular hydrogen plays a significant role in establishing the thermal balance in many astrophysical environments ... More
The collision strength of the [Ne V] infrared fine-structure linesDec 13 1999The calculation of accurate collision strengths for atomic transitions has been a long standing problem in quantitative spectroscopy. Most modern calculations are based on the R-matrix method and problems pertaining to the use of this method have led ... More
Current Development of the Photo-Ionization Code CloudyJan 11 2000This paper describes the most important differences between Cloudy 90 and Cloudy 94, the most recent upgrade of the code which has now been posted on the WWW. It also describes the upgrade of the grain model in Cloudy which is currently underway.
Dust-Bounded ULIRGs? Model Predictions for Infrared Spectroscopic SurveysMar 26 2009The observed faintness of infrared fine-structure line emission along with the warm far-infrared (FIR) colors of ultraluminous infrared galaxies (ULIRGs) is a long-standing problem. In this work, we calculate the line and continuum properties of a cloud ... More
Grain Size Distributions and Photo-Electric Heating in Ionized RegionsOct 08 2003In this paper we present results obtained with the new grain code in Cloudy which underline the strong effect of photo-electric heating by grains in photo-ionized regions. We study the effect that the distribution of grain sizes has on the magnitude of ... More
The H II Region/PDR Connection: Self-Consistent Calculations of Physical Conditions in Star-Forming RegionsJun 21 2005We have performed a series of calculations designed to reproduce infrared diagnostics used to determine physical conditions in star forming regions. We self-consistently calculate the thermal and chemical structure of an H II region and photodissociation ... More
Grain size distributions and photo-electric heating in ionized mediaFeb 16 2004Mar 24 2004Ever since the pioneering study of Spitzer, it has been widely recognized that grains play an important role in the heating and cooling of photo-ionized environments. A detailed code is necessary to model grains in a photo-ionized medium. In this paper ... More
Grains in Photo-Ionized EnvironmentsJul 10 2001Jul 12 2001Ever since the pioneering study of Spitzer, it has been widely recognized that grains play an important role in the heating and cooling of photo-ionized environments. This includes the diffuse ISM, as well as H II regions, planetary nebulae, and photo-dissociation ... More
Sensitivity of PDR Calculations to Microphysical DetailsAug 19 2008Our understanding of physical processes in Photodissociation regions or Photon Dominated Regions (PDRs) largely depends on the ability of spectral synthesis codes to reproduce the observed infrared emission-line spectrum. In this paper, we explore the ... More
The spectral evolution of post-AGB starsJun 07 1999A parameter study of the spectral evolution of a typical post-AGB star, with particular emphasis on the evolution of the infrared colours, is presented. The models are based on the latest evolutionary tracks for hydrogen burning post-AGB stars. For such ... More
The Planetary Nebula population of the Sagittarius Dwarf Spheroidal GalaxyMar 16 2006The identification of two new Planetary Nebulae in the Sagittarius Dwarf Spheroidal Galaxy (Sgr) is presented. This brings the total number to four. The first, StWr 2-21, belongs to the main body of Sgr. The second, the halo PN BoBn 1, has a location, ... More
Physical Conditions in Barnard's Loop, Components of the Orion-Eridanus Bubble, and Implications for the WIM Component of the ISMMar 14 2011We have supplemented existing spectra of Barnard's Loop with high accuracy spectrophotometry of one new position. Cloudy photoionization models were calculated for a variety of ionization parameters and stellar temperatures and compared with the observations. ... More
New Planerary Nebulae towards the Galactic bulgeMay 03 2006New Planetary Nebulae (PNe) were discovered through an [O III] 5007 A emission line survey in the Galactic bulge region with l>0 deg. We detected 240 objects, including 44 new PNe. Deep Halpha+[N II] CCD images as well as low resolution spectra were obtained ... More
The Very Fast Evolution of the VLTP Object V4334 SgrOct 03 2014V4334 Sgr (Sakurai's object) is an enigmatic evolved star that underwent a very late thermal pulse a few years before its discovery in 1996. It ejected a new, hydrogen-deficient nebula in the process. Emission lines from the newly ejected gas were first ... More
Kinematics and H_2 morphology of the multipolar Post-AGB star IRAS 16594-4656Jan 04 2008context: The spectrum of IRAS 16594-4656 shows shock excited H_2 emission and collisionally excited emission lines such as[O I],[C I],and [Fe II]. aim: The goal is to determine the location of the H_2 and [Fe II] shock emission, to determine the shock ... More
Evolution of the central stars of young planetary nebulaeNov 10 2014The evolution of central stars of planetary nebulae was so far documented in just a few cases. However, spectra collected a few decades ago may provide a good reference for studying the evolution of central stars using the emission line fluxes of their ... More
The evolving spectrum of the planetary nebula Hen 2-260May 05 2014We analysed the planetary nebula Hen 2-260 using optical spectroscopy and photometry. We compared our observations with the data from literature to search for evolutionary changes. The nebular line fluxes were modelled with the Cloudy photoionization ... More
On the enhanced cosmic-ray ionization rate in the diffuse cloud towards Zeta PerseiNov 02 2007The spatial distribution of the cosmic-ray flux is important in understanding the Interstellar Medium (ISM) of the Galaxy. This distribution can be analyzed by studying different molecular species along different sight lines whose abundances are sensitive ... More
Near-infrared photometry of Galactic planetary nebulae with the VVV SurveyApr 13 2013Planetary nebulae (PNe) are powerful tracers of evolved stellar populations. Among the 3000 known PNe in the Galaxy, about 600 are located within the 520 square-degree area covered by the VVV survey. The VVV photometric catalogue provides an important ... More
H-, He-like recombination spectra III: $n$-changing collisions in highly-excited Rydberg states and their impact on the radio, IR and optical recombination linesMar 13 2019Mar 18 2019At intermediate to high densities, electron (de-)excitation collisions are the dominant process for populating or depopulating high Rydberg states. In particular, the accurate knowledge of the energy changing ($n$-changing) collisional rates is determinant ... More
H-, He-like recombination spectra III: $n$-changing collisions in highly-excited Rydberg states and their impact on the radio, IR and optical recombination linesMar 13 2019At intermediate to high densities, electron (de-)excitation collisions are the dominant process for populating or depopulating high Rydberg states. In particular, the accurate knowledge of the energy changing ($n$-changing) collisional rates is determinant ... More
H, He-like recombination spectra I: $l$-changing collisions for hydrogenApr 13 2016Hydrogen and helium emission lines in nebulae form by radiative recombination. This is a simple process which, in principle, can be described to very high precision. Ratios of He I and H I emission lines can be used to measure the He$^+$/H$^+$ abundance ... More
H, He-like recombination spectra II: $l$-changing collisions for He Rydberg statesAug 31 2016Cosmological models can be constrained by determining primordial abundances. Accurate predictions of the He I spectrum are needed to determine the primordial helium abundance to a precision of $< 1$% in order to constrain Big Bang Nucleosynthesis models. ... More
A Far-Infrared Spectral Sequence of Galaxies: Trends and ModelsSep 08 2014We present a framework for the interpretation of the far-infrared spectra of galaxies in which we have expanded the model parameters compared with previous work by varying the ionization parameter $U$, column density $N$(H), and gas density at the cloud ... More
Morphologies of the Nebulae around "born-again" Central Stars of Planetary NebulaeApr 25 2008While in the past spherodicity was assumed, and still is used in modeling of most nebulae, we know now that only a small number of planetary nebulae (PNe) are really spherical or at least nearly round. Round planetary nebulae are the minority of objects. ... More
Radiative Cooling II: Effects of Density and MetallicityMar 12 2014May 08 2014This work follows Lykins et al. discussion of classic plasma cooling function at low density and solar metallicity. Here we focus on how the cooling function changes over a wide range of density (n_H<10^12 cm^(-3)) and metallicity (Z<30Z _sun ). We find ... More
The Belgian repository of fundamental atomic data and stellar spectra (BRASS) II. Quality assessment of atomic data for unblended lines in FGK starsFeb 11 2019May 09 2019Fundamental atomic transition parameters, such as oscillator strengths and wavelengths, play a key role in modelling and understanding the chemical composition of stars in the universe. Despite the significant work under way to produce these parameters ... More
The Belgian repository of fundamental atomic data and stellar spectra (BRASS) II. Quality assessment of atomic data for unblended lines in FGK starsFeb 11 2019Fundamental atomic transition parameters, such as oscillator strengths and wavelengths, play a key role in modelling and understanding the chemical composition of stars in the universe. Despite the significant work under way to produce these parameters ... More
Symmetry and Asymmetry in "born again" Planetary NebulaeApr 25 2008While in the past spheroidicity was assumed, and still is used in modeling of most nebulae, we know now that only a small number of planetary nebulae (PNe) are really spherical or at least nearly round. Round planetary nebulae are the minority of objects. ... More
The onset of photoionization in Sakurai's Object (V4334 Sgr)Jun 26 2007We investigate the reheating of the very late thermal pulse (VLTP) object V4334 Sgr (Sakurai's Object) using radio observations from the Very Large Array, and optical spectra obtained with the Very Large Telescope. We find a sudden rise of the radio flux ... More
A Herschel study of Planetary NebulaeSep 20 2011We present Herschel PACS and SPIRE images of the dust shells around the planetary nebulae NGC 650, NGC 6853, and NGC 6720, as well as images showing the dust temperature in their shells. The latter shows a rich structure, which indicates that internal ... More
Properties of Dust Grains in Planetary Nebulae - I. The Ionized Region of NGC 6445Oct 21 1999In this paper we study new infrared spectra of the evolved planetary nebula NGC 6445 obtained with ISO. These data show that the thermal emission from the grains is very cool and has a low flux compared to H beta. A model of the ionized region is constructed, ... More
Stout: Cloudy's Atomic and Molecular DatabaseJun 04 2015We describe a new atomic and molecular database we developed for use in the spectral synthesis code Cloudy. The design of Stout is driven by the data needs of Cloudy, which simulates molecular, atomic, and ionized gas with kinetic temperatures 2.8 K < ... More
Hydrogen Two-Photon Continuum Emission from the Horseshoe Filament in NGC 1275Jun 27 2012Far ultraviolet emission has been detected from a knot of Halpha emission in the Horseshoe filament, far out in the NGC 1275 nebula. The flux detected relative to the brightness of the Halpha line in the same spatial region is very close to that expected ... More
The origin of molecular hydrogen emission in cooling-flow filamentsFeb 18 2008The optical filaments found in many cooling flows in galaxy clusters consist of low density ($\sim 10^3 \pcc$) cool ($\sim 10^3$ K) gas surrounded by significant amounts of cosmic-ray and magnetic-field energy. Their spectra show anomalously strong low-ionization ... More
Collisional excitation of [C II], [O I] and CO in Massive GalaxiesJan 06 2015Many massive galaxies at the centres of relaxed galaxy clusters and groups have vast reservoirs of cool (~10,000 K) and cold (~100 K) gas. In many low redshift brightest group and cluster galaxies this gas is lifted into the hot ISM in filamentary structures, ... More
Rotationally Warm Molecular Hydrogen in the Orion BarJun 12 2009The Orion Bar is one of the nearest and best-studied photodissociation or photon-dominated regions (PDRs). Observations reveal the presence of H2 lines from vibrationally or rotationally excited upper levels that suggest warm gas temperatures (400 to ... More
Properties of post-AGB stars with IRAS colors typical of planetary nebulaeOct 26 2000We have investigated the infrared properties of a sample of objects with IRAS colors typical of planetary nebulae. The selected objects have not yet evolved to the planetary nebula stage since they have no detectable radio continuum emission and they ... More
Collisional heating as the origin of filament emission in galaxy clustersOct 29 2008[Abridged] Photoionization, whether by starlight or other sources, has difficulty in accounting for the observed spectra of the optical filaments that often surround central galaxies in large clusters. Our first paper examined whether heating by energetic ... More
Expanded Iron UTA spectra -- probing the thermal stability limits in AGN cloudsMar 01 2013The Fe unresolved transition array (UTAs) produce prominent features in the 15-17?A wavelength range in the spectra of Active Galactic Nuclei (AGN). Here we present new calculations of the energies and oscillator strengths of inner- shell lines from Fe ... More
H alpha emission line profiles of selected post-AGB starsOct 01 1999Dec 16 1999We present H alpha emission line spectra of 7 post-AGB stars. Typically, they have a P-Cygni type profile with a very strong emission component and a relatively narrow absorption component. These lines are formed in the fast post-AGB wind which in most ... More
Accurate determination of the free-free Gaunt factor. II - relativistic Gaunt factorsFeb 24 2015When modelling an ionised plasma, all spectral synthesis codes need the thermally averaged free-free Gaunt factor defined over a very wide range of parameter space in order to produce an accurate prediction for the spectrum. Until now no data set exists ... More
Accurate determination of the free-free Gaunt factor; I - non-relativistic Gaunt factorsJul 18 2014Aug 17 2014Modern spectral synthesis codes need the thermally averaged free-free Gaunt factor defined over a very wide range of parameter space in order to produce an accurate prediction for the spectrum emitted by an ionized plasma. Until now no set of data exists ... More
Radio and Infrared Emission from a [WC]-Type Planetary Nebula in the LMCMar 17 1994Radio continuum emission has been detected from a planetary nebula in the Large Magellanic Cloud: this is the first radio continuum detection for any planetary nebula outside our galaxy. The radio flux density is a factor of two lower than predicted from ... More
Ultraviolet emission lines of Si II in cool star and solar spectraOct 29 2015Recent atomic physics calculations for Si II are employed within the Cloudy modelling code to analyse Hubble Space Telescope (HST) STIS ultraviolet spectra of three cool stars, Beta-Geminorum, Alpha-Centauri A and B, as well as previously published HST/GHRS ... More
Introducing EMILI: Computer Aided Emission Line IdentificationJul 02 2003The identification of spectral lines can be a tedious process requiring the interrogation of large spectroscopic databases, but it does lend itself to software algorithms that can determine the characteristics of candidate line identifications. We present ... More
Suppression of Dielectronic Recombination Due to Finite Density Effects II: Analytical Refinement and Application to Density-dependent Ionization Balances and AGN Broad-line EmissionAug 22 2018We present improved fits to our treatment of suppression of dielectronic recombination at intermediate densities. At low densities, most recombined excited states eventually decay to the ground state, and therefore the total dielectronic recombination ... More
Pumping up the [N I] nebular linesAug 31 2012The optical [N I] doublet near 5200 {\AA} is anomalously strong in a variety of emission-line objects. We compute a detailed photoionization model and use it to show that pumping by far-ultraviolet (FUV) stellar radiation previously posited as a general ... More
The Real-Time Evolution of Sakurai's Star (V4334 Sgr) and other (V)LTP ObjectsMay 05 2006We report on the progress of our on-going campaign to monitor the evolution of the VLTP objects V4334 Sgr and V605 Aql, as well as the suspected (V)LTP object CK Vul. V4334 Sgr does not show signs of increased ionization compared to our previous observations ... More
Implications of IR continua for x-ray emission/reflection in AGNNov 22 2003Observations of infrared emission from AGN show that grains exist over a broad range of distances from the central object, extending to the point where they are destroyed by sublimation. These ~1000 K grains produce much of the 1 micron continuum. In ... More
Herschel observations of PNe in the MESS key programOct 20 2011In this paper we give a progress report on the Herschel imaging and spectroscopic observations of planetary nebulae that are carried out as part of the MESS guaranteed time key program. We present and discuss imaging and temperature maps of NGC 6720, ... More
The enigma of the oldest `nova': the central star and nebula of CK VulSep 24 2007CK Vul is classified as, amongst others, the slowest known nova, a hibernating nova, or a very late thermal pulse object. Following its eruption in AD 1670, the star remained visible for 2 years. A 15-arcsec nebula was discovered in the 1980's, but the ... More
Implications of Coronal Line Emission in NGC 4696Oct 16 2014We announce a new facility in the spectral code CLOUDY that enables tracking the evolution of a cooling parcel of gas with time. For gas cooling from temperatures relevant to galaxy clusters, earlier calculations estimated the [Fe XIV] {\lambda}5303 / ... More
The 2013 Release of CloudyFeb 18 2013This is a summary of the 2013 release of the plasma simulation code Cloudy. Cloudy models the ionization, chemical, and thermal state of material that may be exposed to an external radiation field or other source of heating, and predicts observables such ... More
Radiative cooling in collisionally and photo ionized plasmasDec 06 2012We discuss recent improvements in the calculation of the radiative cooling in both collisionally and photo ionized plasmas. We are extending the spectral simulation code Cloudy so that as much as possible of the underlying atomic data is taken from external ... More
The central stars of the planetary nebulae NGC 7027 and NGC 6543Jun 09 1999Infrared spectra of NGC 7027 and NGC 6543 ranging from 2.4 to 45 micron were obtained with the Short Wavelength Spectrometer on board the Infrared Space Observatory. A first analysis of these spectra, with the aid of photo-ionization models, is presented. ... More
Radio observations of planetary nebulae: no evidence for strong radial density gradientsJul 03 2018Radio continuum observations trace thermal emission of ionized plasma in planetary nebulae and bring useful information on nebular geometries. A model of homogeneous sphere or shell cannot fit the nebular spectra and brightness temperatures. Two alternative ... More
Ballistic electron transport through magnetic domain wallsOct 06 1998Electron transport limited by the rotating exchange-potential of domain walls is calculated in the ballistic limit for the itinerant ferromagnets Fe, Co, and Ni. When realistic band structures are used, the domain wall magnetoresistance is enhanced by ... More
Herschel imaging of the dust in the Helix Nebula (NGC 7293)Nov 17 2014Dec 19 2014In our series of papers presenting the Herschel imaging of evolved planetary nebulae, we present images of the dust distribution in the Helix nebula (NGC 7293). Images at 70, 160, 250, 350, and 500 micron were obtained with the PACS and SPIRE instruments ... More
Herschel/PACS observations of the 69 $μm$ band of crystalline olivine around evolved starsMar 25 2014We present 48 Herschel/PACS spectra of evolved stars in the wavelength range of 67-72 $\mu$m. This wavelength range covers the 69 $\mu$m band of crystalline olivine ($\text{Mg}_{2-2x}\text{Fe}_{(2x)}\text{SiO}_{4}$). The width and wavelength position ... More
A Herschel study of NGC 650Aug 12 2013As part of the Herschel Guaranteed Time Key Project MESS (Mass loss of Evolved StarS) we have imaged a sample of planetary nebulae. In this paper we present the PACS and SPIRE images of the classical bipolar planetary nebula NGC 650. We used these images ... More
Herschel SPIRE and PACS observations of the red supergiant VY CMa: analysis of the molecular line spectraOct 10 2013We present an analysis of the far-infrared and submillimetre molecular emission line spectrum of the luminous M-supergiant VY CMa, observed with the SPIRE and PACS spectrometers aboard the Herschel Space Observatory. Over 260 emission lines were detected ... More
Herschel spectral-mapping of the Helix Nebula (NGC 7293): Extended CO photodissociation and OH+ emissionApr 08 2014The Helix Nebula (NGC 7293) is the closest planetary nebulae. Therefore, it is an ideal template for photochemical studies at small spatial scales in planetary nebulae. We aim to study the spatial distribution of the atomic and the molecular gas, and ... More
A PDR-Code Comparison StudyFeb 08 2007We present a comparison between independent computer codes, modeling the physics and chemistry of interstellar photon dominated regions (PDRs). Our goal was to understand the mutual differences in the PDR codes and their effects on the physical and chemical ... More
Carbon Monoxide in the Cold Debris of Supernova 1987AJul 24 2013We report spectroscopic and imaging observations of rotational transitions of cold CO and SiO in the ejecta of SN1987A, the first such emission detected in a supernova remnant. In addition to line luminosities for the CO J=1-0, 2-1, 6-5, and 7-6 transitions, ... More
Implications of infalling Fe II - emitting clouds in active galactic nuclei: anisotropic propertiesNov 06 2009Nov 19 2009We investigate consequences of the discovery that Fe II emission in quasars, one of the spectroscopic signatures of "Eigenvector 1", may originate in infalling clouds. Eigenvector 1 correlates with the Eddington ratio L/L_Edd so that Fe II/Hbeta increases ... More
Imaging Planetary Nebulae with Herschel-PACS and SPIRESep 27 2010In this paper we will discuss the images of Planetary Nebulae that have recently been obtained with PACS and SPIRE on board the Herschel satellite. This comprises results for NGC 650 (the little Dumbbell nebula), NGC 6853 (the Dumbbell nebula), and NGC ... More
Herschel Planetary Nebula Survey (HerPlaNS) - First Detection of OH+ in Planetary NebulaeApr 09 2014May 14 2014We report the first detections of OH$^+$ emission in planetary nebulae (PNe). As part of an imaging and spectroscopy survey of 11 PNe in the far-IR using the PACS and SPIRE instruments aboard the Herschel Space Observatory, we performed a line survey ... More
Information Systems Playground - The Target Infrastructure, Scaling Astro-WISE into the Petabyte rangeOct 05 2011The Target infrastructure has been specially built as a storage and compute infrastructure for the information systems derived from Astro-WISE. This infrastructure will be used by several applications that collaborate in the area of information systems ... More
From Fano resonances to bound states in the continuum in dipole arrays at THz frequenciesJan 10 2019Fano resonances and bound states in the continuum (BICs) exhibit a rich phenomenology stemming from, respectively, their asymmetric line shapes and infinite quality factors. Here, we show experimentally and theoretically that rod dimer metasurfaces exhibit ... More
Space Telescope and Optical Reverberation Mapping Project. X. The Hidden Life of "the Obscurer" in NGC 5548 and Understanding the Absorption-Line HolidayDec 30 2018Jan 02 2019The flux variations in the emission lines in active galactic nuclei (AGNs) are driven by variations in the ionizing continuum flux --which are usually reflected in the observable UV-optical continuum. The "Reverberation mapping" technique measures the ... More
Space Telescope and Optical Reverberation Mapping Project. X. Understanding the Absorption-Line Holiday in NGC 5548Dec 30 2018Apr 25 2019The flux variations in the emission lines in active galactic nuclei (AGNs) are driven by variations in the ionizing continuum flux --which are usually reflected in the observable UV-optical continuum. The "Reverberation mapping" technique measures the ... More
Herschel Planetary Nebula Survey (HerPlaNS): Hydrogen Recombination Laser Lines in Mz 3May 16 2018The bipolar nebula Menzel 3 (Mz 3) was observed as part of the \textit{Herschel} Planetary Nebula Survey (\textit{HerPlaNS}), which used the PACS and SPIRE instruments aboard the \textit{Herschel Space Observatory} to study a sample of planetary nebulae ... More
MESS (Mass-loss of Evolved StarS), a Herschel Key ProgramDec 13 2010MESS (Mass-loss of Evolved StarS) is a Guaranteed Time Key Program that uses the PACS and SPIRE instruments on board the Herschel Space Observatory to observe a representative sample of evolved stars, that include asymptotic giant branch (AGB) and post-AGB ... More
A stubbornly large mass of cold dust in the ejecta of Supernova 1987ANov 26 2014We present new Herschel photometric and spectroscopic observations of Supernova 1987A, carried out in 2012. Our dedicated photometric measurements provide new 70 micron data and improved imaging quality at 100 and 160 micron compared to previous observations ... More
Stochastic Beams and Where to Find Them: The Gumbel-Top-k Trick for Sampling Sequences Without ReplacementMar 14 2019The well-known Gumbel-Max trick for sampling from a categorical distribution can be extended to sample $k$ elements without replacement. We show how to implicitly apply this 'Gumbel-Top-$k$' trick on a factorized distribution over sequences, allowing ... More
Attention, Learn to Solve Routing Problems!Mar 22 2018Feb 07 2019The recently presented idea to learn heuristics for combinatorial optimization problems is promising as it can save costly development. However, to push this idea towards practical implementation, we need better models and better ways of training. We ... More
The Herschel Planetary Nebula Survey (HerPlaNS) I. Data Overview and Analysis Demonstration with NGC 6781Mar 11 2014This is the first of a series of investigations into far-IR characteristics of 11 planetary nebulae (PNs) under the Herschel Space Observatory Open Time 1 program, Herschel Planetary Nebula Survey (HerPlaNS). Using the HerPlaNS data set, we look into ... More
The Herschel Planetary Nebula Survey (HerPlaNS) - a comprehensive dusty photoionization model of NGC6781Jul 20 2017We perform a comprehensive analysis of the planetary nebula (PN) NGC6781 to investigate the physical conditions of each of its ionized, atomic, and molecular gas and dust components and the object's evolution, based on panchromatic observational data ... More
Herschel images of NGC 6720: H2 formation on dust grainsMay 10 2010Herschel PACS and SPIRE images have been obtained of NGC 6720 (the Ring Nebula). This is an evolved planetary nebula with a central star that is currently on the cooling track, due to which the outer parts of the nebula are recombining. From the PACS ... More
A symplectic covariant formulation of special Kahler geometry in superconformal calculusApr 08 1999Jun 15 1999We present a formulation of the coupling of vector multiplets to N=2 supergravity which is symplectic covariant (and thus is not based on a prepotential) and uses superconformal tensor calculus. We do not start from an action, but from the combination ... More
Vignetted-aperture correction for spectral cameras with integrated thin-film Fabry-Pérot filtersMay 14 2019Spectral cameras with integrated thin-film Fabry-P\'erot filters have become increasingly important in many applications. These applications often require the detection of spectral features at specific wavelengths or to quantify small variations in the ... More
Deep Generative Modeling of LiDAR DataDec 04 2018Dec 17 2018Building models capable of generating structured output is a key challenge for AI and robotics. While generative models have been explored on many types of data, little work has been done on synthesizing lidar scans, which play a key role in robot mapping ... More
Deep Generative Modeling of LiDAR DataDec 04 2018May 03 2019Building models capable of generating structured output is a key challenge for AI and robotics. While generative models have been explored on many types of data, little work has been done on synthesizing lidar scans, which play a key role in robot mapping ... More
Ray chaos in optical cavities based upon standard laser mirrorsJul 17 2003We present a composite optical cavity made of standard laser mirrors; the cavity consists of a suitable combination of stable and unstable cavities. In spite of its very open nature the composite cavity shows ray chaos, which may be either soft or hard, ... More
Decay and coherence of two-photon excited yellow ortho-excitons in Cu2OMay 15 2005Photoluminescence excitation spectroscopy has revealed a novel, highly efficient two-photon excitation method to produce a cold, uniformly distributed high density excitonic gas in bulk cuprous oxide. A study of the time evolution of the density, temperature ... More
On the mass distribution of neutron stars in HMXBsFeb 16 2005We present the results of a monitoring campaign of three eclipsing high-mass X-ray binaries (HMXBs: SMC X-1, LMC X-4 and Cen X-3). High-resolution VLT/UVES spectra are used to measure the radial velocities of these systems with high accuracy. We show ... More
Equilibrium spherically curved 2D Lennard-Jones systemsJul 06 2005To learn about basic aspects of nano-scale spherical molecular shells during their formation, spherically curved two-dimensional N-particle Lennard-Jones systems are simulated, studying curvature evolution paths at zero-temperature. For many N-values ... More
Covariant Quantization of Superstrings Without Pure Spinor ConstraintsDec 18 2001We construct a covariant quantum superstring, extending Berkovits' approach by introducing new ghosts to relax the pure spinor constraints. The central charge of the underlying Kac-Moody algebra, which would lead to an anomaly in the BRST charge, is treated ... More
On the origin of exponential growth in induced earthquakes in GroningenOct 19 2016The Groningen gas field shows exponential growth in earthquakes event counts around a magnitude M1 with a doubling time of 6-9 years since 2001. This behavior is identified with dimensionless curvature in land subsidence, which has been evolving at a ... More
Upscaling of dislocation walls in finite domainsAug 23 2013We wish to understand the macroscopic plastic behaviour of metals by upscaling the micro-mechanics of dislocations. We consider a highly simplified dislocation network, which allows our microscopic model to be a one dimensional particle system, in which ... More
The Peculiar Evolutionary History of IGR J17480-2446 in Terzan 5Dec 22 2011May 06 2012The low mass X-ray binary (LMXB) IGR J17480-2446 in the globular cluster Terzan 5 harbors an 11 Hz accreting pulsar. This is the first object discovered in a globular cluster with a pulsar spinning at such low rate. The accreting pulsar is anomalous because ... More
Coherent amplitudon generation in K_0.3MoO_3 through ultrafast inter-band quasi particle decayOct 10 2006The charge density wave system K_0.3MoO_3 has been studied using variable energy pump-probe spectroscopy, ellipsometry, and inelastic light scattering. The observed transient reflectivity response exhibits quite a complex behavior, containing contributions ... More
Evolution of induced earthquakes from dimensionless scalingOct 24 2014Event counts are a powerful diagnostic for earthquake analysis. We report on a bimodal distribution of earthquake event counts in the U.S. since 2013. The new peak is about a magnitude M1 with a distribution very similar to that of induced earthquakes ... More
On measuring the Tully-Fisher relation at $z > 1$Jan 13 2006The evolution of the line width - luminosity relation for spiral galaxies, the Tully-Fisher relation, strongly constrains galaxy formation and evolution models. At this moment, the kinematics of z>1 spiral galaxies can only be measured using rest frame ... More
Lagrange formalism of point-massesMay 27 2002We prove by symmetry properties that the Lagrangian of a free point-mass is a quadratic function of the speed in the non-relativistic case, and that the action of the free point-mass between two spacetime points is the proper time passed in the relativistic ... More