Kevin Heng

Universität Bern, Berna, Bern, Switzerland

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Publications (63)342.18 Total impact

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    ABSTRACT: We present new {\it Hubble Space Telescope} images of high-velocity H-$\alpha$ and Lyman-$\alpha$ emission in the outer debris of SN~1987A. The H-$\alpha$ images are dominated by emission from hydrogen atoms crossing the reverse shock. For the first time we observe emission from the reverse shock surface well above and below the equatorial ring, suggesting a bipolar or conical structure perpendicular to the ring plane. Using the H$\alpha$ imaging, we measure the mass flux of hydrogen atoms crossing the reverse shock front, in the velocity intervals ($-$7,500~$<$~$V_{obs}$~$<$~$-$2,800 km s$^{-1}$) and (1,000~$<$~$V_{obs}$~$<$~7,500 km s$^{-1}$), $\dot{M_{H}}$ = 1.2~$\times$~10$^{-3}$ M$_{\odot}$ yr$^{-1}$. We also present the first Lyman-$\alpha$ imaging of the whole remnant and new $Chandra$ X-ray observations. Comparing the spatial distribution of the Lyman-$\alpha$ and X-ray emission, we observe that the majority of the high-velocity Lyman-$\alpha$ emission originates interior to the equatorial ring. The observed Lyman-$\alpha$/H-$\alpha$ photon ratio, $\langle$$R(L\alpha / H\alpha)$$\rangle$ $\approx$~17, is significantly higher than the theoretically predicted ratio of $\approx$ 5 for neutral atoms crossing the reverse shock front. We attribute this excess to Lyman-$\alpha$ emission produced by X-ray heating of the outer debris. The spatial orientation of the Lyman-$\alpha$ and X-ray emission suggests that X-ray heating of the outer debris is the dominant Lyman-$\alpha$ production mechanism in SN 1987A at this phase in its evolution.
    02/2015;
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    ABSTRACT: Exoplanet phase curves provide a wealth of information about atmospheric dynamics, energetics, and chemistry. Phase curves have been observed for relatively few planets, yet the current small sample already hints at the inadequacy of current atmospheric models. Our ultimate goal of understanding the global circulation patterns and their relation to atmospheric chemistry requires a larger and more homogenous sample. Here, we propose to more than double the sample of hot Jupiters with high S/N phase observations by targeting seven bright systems. Combined with the powerful new technique of high-resolution infrared Doppler spectroscopy, our observations will enable an unprecedented comparative study to relate global circulation patterns to atmospheric chemistry, and ultimately to facilitate retrieval of global abundance and temperature maps of extrasolar planets. The planets in our sample represent the best objects to leverage both space-based phase curves and ground-based spectroscopy in a combined analysis. Spectroscopic observations break the inclination degeneracy that plagued earlier non-transiting phase variations, while phase curves provide crucial information about the planetary thermal continuum that is lost in the inherently relative spectroscopic analysis. Our program uses Spitzer's recently-validated snapshot-phase curve mode to obtain high-precision photometry on long timescales with low data volumes and high scheduling flexibility, and our new retrieval approach will become a critical capability in an era of measurements at higher S/N and spectral resolution with JWST and Extremely Large ground-based telescopes.
    11/2014;
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    ABSTRACT: PLATO 2.0 is a mission candidate for ESA's M3 launch opportunity (2022/24). It addresses fundamental questions such as: How do planetary systems form and evolve? Are there other systems with planets like ours, able to develop life? The PLATO 2.0 instrument consists of 34 small aperture telescopes providing a wide field-of-view and a large photometric magnitude range. It targets bright stars in wide fields to detect and characterize planets down to Earth-size by photometric transits, whose masses can then be determined by ground-based radial-velocity follow-up measurements. Asteroseismology will be performed for stars <=11mag to obtain highly accurate stellar parameters, including masses and ages. The combination of bright targets and asteroseismology results in high accuracy for the bulk planet parameters: 2%, 4-10% and 10% for planet radii, masses and ages, respectively. The foreseen baseline observing strategy includes two long pointings (2-3 years) to detect and bulk characterize planets reaching into the habitable zone (HZ) of solar-like stars and an additional step-and-stare phase to cover in total about 50% of the sky. PLATO 2.0 will observe up to 1,000,000 stars and detect and characterize hundreds of small planets, and thousands of planets in the Neptune to gas giant regime out to the HZ. It will therefore provide the first large-scale catalogue of bulk characterized planets with accurate radii, masses, mean densities and ages. This catalogue will include Earth-like planets at intermediate orbital distances, where surface temperatures are moderate. Coverage of this parameter range with statistical numbers of bulk characterized planets is unique to PLATO 2.0. ...
    Experimental Astronomy 10/2014; 38:249. · 2.66 Impact Factor
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    ABSTRACT: We present high resolution transmission spectra of giant planet atmospheres from a coupled 3-D atmospheric dynamics and transmission spectrum model that includes Doppler shifts which arise from winds and planetary motion. We model jovian planets covering more than two orders of magnitude in incident flux, corresponding to planets with 0.9 to 55 day orbital periods around solar-type stars. The results of our 3-D dynamical models reveal certain aspects of high resolution transmission spectra that are not present in simple 1-D models. We find that the hottest planets experience strong substellar to anti-stellar (SSAS) winds, resulting in transmission spectra with net blue shifts of up to 3 km s$^{-1}$, whereas less irradiated planets show almost no net Doppler shifts. Compared to 1-D models, peak line strengths are significantly reduced for the hottest atmospheres owing to Doppler broadening from a combination of rotation (which is faster for close-in planets under the assumption of tidal locking) and atmospheric winds. Finally, high resolution transmission spectra may be useful in studying the atmospheres of exoplanets with optically thick clouds since line cores for very strong transitions should remain optically thick to very high altitude. High resolution transmission spectra are an excellent observational test for the validity of 3-D atmospheric dynamics models, because they provide a direct probe of wind structures and heat circulation. Ground-based exoplanet spectroscopy is currently on the verge of being able to verify some of our modeling predictions, most notably the dependence of SSAS winds on insolation. We caution that interpretation of high resolution transmission spectra based on 1-D atmospheric models may be inadequate, as 3-D atmospheric motions can produce a noticeable effect on the absorption signatures.
    The Astrophysical Journal 09/2014; 795(1). · 6.28 Impact Factor
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    Kevin Heng, Adam P. Showman
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    ABSTRACT: The characterization of exoplanetary atmospheres has come of age in the last decade, as astronomical techniques now allow for albedos, chemical abundances, temperature profiles and maps, rotation periods and even wind speeds to be measured. Atmospheric dynamics sets the background state of density, temperature and velocity that determines or influences the spectral and temporal appearance of an exoplanetary atmosphere. Hot exoplanets are most amenable to these characterization techniques; in the present review, we focus on highly-irradiated, large exoplanets (the "hot Jupiters"), as astronomical data begin to confront theoretical questions. We summarize the basic atmospheric quantities inferred from the astronomical observations. We review the state of the art by addressing a series of current questions and look towards the future by considering a separate set of exploratory questions. Attaining the next level of understanding will require a concerted effort of constructing multi-faceted, multi-wavelength datasets for benchmark objects. Understanding clouds presents a formidable obstacle, as they introduce degeneracies into the interpretation of spectra, yet their properties and existence are directly influenced by atmospheric dynamics. Confronting general circulation models with these multi-faceted, multi-wavelength datasets will help us understand these and other degeneracies. The coming decade will witness a decisive confrontation of theory and simulation by the next generation of astronomical data.
    07/2014;
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    Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series; 06/2014
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    ABSTRACT: We present a comprehensive analytical study of radiative transfer using the method of moments and include the effects of non-isotropic scattering in the coherent limit. Within this unified formalism, we derive the governing equations and solutions describing two-stream radiative transfer (which approximates the passage of radiation as a pair of outgoing and incoming fluxes), flux-limited diffusion (which describes radiative transfer in the deep interior) and solutions for the temperature-pressure profiles. Generally, the problem is mathematically under-determined unless a set of closures (Eddington coefficients) is specified. We demonstrate that the hemispheric (or hemi-isotropic) closure naturally derives from the radiative transfer equation if energy conservation is obeyed, while the Eddington closure produces spurious enhancements of both reflected light and thermal emission. We further demonstrate that traditional non-isothermal treatments of each atmospheric layer lead to unphysical contributions to the fluxes. We concoct recipes for implementing two-stream radiative transfer in stand-alone calculations and general circulation models. We use our two-stream solutions to construct toy models of the runaway greenhouse effect. We present a new solution for temperature-pressure profiles with a non-constant optical opacity and elucidate the effects of non-isotropic scattering in the optical and infrared. We derive generalized expressions for the spherical and Bond albedos and the photon deposition depth. We demonstrate that the value of the optical depth corresponding to the photosphere is not always 2/3 (Milne's solution) and depends on a combination of stellar irradiation, internal heat and the properties of scattering both in optical and infrared. Finally, we derive generalized expressions for the total, net, outgoing and incoming fluxes in the convective regime.
    The Astrophysical Journal Supplement Series 04/2014; 215(1). · 14.14 Impact Factor
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    Kevin Heng
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    ABSTRACT: Is numerical mimicry a third way of establishing truth?
    American Scientist 04/2014; 102(3). · 0.64 Impact Factor
  • Kevin Heng, Brice-Olivier Demory
    The Astrophysical Journal 03/2014; 785(1):80. · 6.28 Impact Factor
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    Kevin Heng, Jared Workman
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    ABSTRACT: Within the context of exoplanetary atmospheres, we present a comprehensive linear analysis of forced, damped, magnetized shallow water systems, exploring the effects of dimensionality, geometry (Cartesian, pseudo-spherical and spherical), rotation, magnetic tension and hydrodynamic and magnetic sources of friction. Across a broad range of conditions, we find that the key governing equation for atmospheres and quantum harmonic oscillators are identical, even when forcing (stellar irradiation), sources of friction (molecular viscosity, Rayleigh drag and magnetic drag) and magnetic tension are included. The global atmospheric structure is largely controlled by a single, key parameter that involves the Rossby and Prandtl numbers. This near-universality breaks down when either molecular viscosity or magnetic drag varies significantly across latitude or a poloidal magnetic field is present, suggesting that these effects will introduce qualitative changes to the familiar chevron-shaped feature witnessed in simulations of atmospheric circulation. We also find that hydrodynamic and magnetic sources of friction have dissimilar phase signatures and affect the flow in fundamentally different ways, implying that using Rayleigh drag to mimic magnetic drag is inaccurate. We exhaustively lay down the theoretical formalism (dispersion relations, governing equations and time-dependent wave solutions) for a broad suite of models. In all situations, we derive the steady state of an atmosphere, which is relevant to interpreting infrared phase and eclipse maps of exoplanetary atmospheres. We elucidate a pinching effect that confines the atmospheric structure to be near the equator. Our suite of analytical models may be used to decisively develop physical intuition and as a reference point for three-dimensional, magnetohydrodynamic (MHD) simulations of atmospheric circulation.
    The Astrophysical Journal Supplement Series 01/2014; 213(2). · 14.14 Impact Factor
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    ABSTRACT: Recent transmission spectrum observations of hot Jupiter exoplanets at high spectral resolution have led to reports of direct detection of orbital motion, as well as detection of molecular spectral features and high altitude winds. Motivated by these studies, we present high resolution transmission spectra of giant planet atmospheres from a coupled 3-D atmospheric dynamics and transmission spectrum model. Our model spectra allow us to explore the range of observational constraints that can be placed on physical properties of giant planet atmospheres using high resolution transmission spectra. These properties include day-to-night winds, planetary rotation speeds, atmospheric structure, and the presence of clouds. The detection of Doppler shifted spectral lines can reveal the nature of atmospheric circulation in the upper atmospheres of exoplanets as well as constraining whether these planets are truly tidally locked as many models would predict. We conclude that high resolution transmission spectra are a useful atmospheric diagnostic tool that is most strongly limited by the ability to obtain high signal-to-noise spectra. Observations with future 30-meter class telescopes will be able to produce the required high SNR observations to fully take advantage of the information provided by high resolution exoplanet transmission spectra.
    01/2014;
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    ABSTRACT: Rayleigh scattering by condensates characterises the transmission spectrum of HD 189733b at wavelengths shortward of 1 $\mu$m. We retrieve a range of condensate distributions consistent with transmission spectroscopy between 0.3-24 $\mu$m that were recently re-analyzed by Pont et al.(2013). We suggest that a vertically-confined condensate layer with a monodisperse particle size of about 0.06 $\mu$m and an optical depth of about 0.6 at wavelength 1 $\mu$m provides the best atmospheric scenario for the terminator regions of HD 189733b. Generally, we find that both vertically-confined and uniform condensate layers suggest plausible fits to the data if the optical depth is 0.1-3 and the particle size is smaller than 0.1 $\mu$m. Strong constraints on the condensate properties are provided by spectra at wavelengths shortward of 1 $\mu$m as well as longward of 8 $\mu$m. We show that these are the optimal wavelengths for quantifying the effects of condensates, which may guide the design of future space observations. The investigations in the present study indicate that the current data offer sufficient information to constrain the condensate properties of HD189733b, but the chemistry in the terminator regions remains highly uncertain.
    The Astrophysical Journal 10/2013; 789(1). · 6.28 Impact Factor
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    ABSTRACT: We present new visible and infrared observations of the hot Jupiter Kepler-7b to determine its atmospheric properties. Our analysis allows us to 1) refine Kepler-7b's relatively large geometric albedo of Ag=0.35+-0.02, 2) place upper limits on Kepler-7b thermal emission that remains undetected in both Spitzer bandpasses and 3) report a westward shift in the Kepler optical phase curve. We argue that Kepler-7b's visible flux cannot be due to thermal emission or Rayleigh scattering from H2 molecules. We therefore conclude that high altitude, optically reflective clouds located west from the substellar point are present in its atmosphere. We find that a silicate-based cloud composition is a possible candidate. Kepler-7b exhibits several properties that may make it particularly amenable to cloud formation in its upper atmosphere. These include a hot deep atmosphere that avoids a cloud cold trap, very low surface gravity to suppress cloud sedimentation, and a planetary equilibrium temperature in a range that allows for silicate clouds to potentially form in the visible atmosphere probed by Kepler. Our analysis does not only present evidence of optically thick clouds on Kepler-7b but also yields the first map of clouds in an exoplanet atmosphere.
    The Astrophysical Journal Letters 10/2013; 776(2):L25. · 5.60 Impact Factor
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    ABSTRACT: Exoplanet phase curves provide a wealth of information about atmospheric dynamics, energetics, and chemistry. Phase curves have been observed for relatively few planets, yet the current small sample already hints at the inadequacy of current atmospheric models. Our ultimate goal of understanding the global circulation patterns and their relation to atmospheric chemistry requires a larger and more homogenous sample. Here, we propose to more than double the sample of hot Jupiters with high S/N phase observations by targeting five bright non-transiting systems. Combined with the powerful new technique of high-resolution infrared Doppler spectroscopy, our observations will enable an unprecedented comparative study to relate global circulation patterns and atmospheric chemistry, and ultimately facilitate retrieval of the first global abundance and temperature maps of extrasolar planets. The planets in our sample represent the best objects to leverage both space-based phase curves and ground-based spectroscopy in a combined analysis. Spectroscopic observations break the inclination degeneracy that plagued earlier non-transiting phase variations, while phase curves provide crucial information about the planetary thermal continuum that is lost in the inherently relative spectroscopic analysis. Our program uses Spitzer's recently-validated observing mode to obtain high-precision photometry on long timescales with low data volumes and high scheduling flexibility, and our new retrieval approach will become a critical capability in an era of measurements at higher S/N and spectral resolution with JWST and Extremely Large ground-based telescopes.
    Spitzer Proposal. 10/2013;
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    Kevin Heng, Brice-Olivier Demory
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    ABSTRACT: Unlike previously explored relationships between the properties of hot Jovian atmospheres, the geometric albedo and the incident stellar flux do not exhibit a clear correlation, as revealed by our re-analysis of Q0 to Q14 Kepler data. If the albedo is primarily associated with the presence of clouds in these irradiated atmospheres, a holistic modeling approach needs to relate the following properties: the strength of stellar irradiation (and hence the strength and depth of atmospheric circulation), the geometric albedo (which controls both the fraction of starlight absorbed and the pressure level at which it is predominantly absorbed) and the properties of the embedded cloud particles (which determine the albedo). The anticipated diversity in cloud properties renders any correlation between the geometric albedo and the stellar flux to be weak and characterized by considerable scatter. In the limit of vertically uniform populations of scatterers and absorbers, we use an analytical model and scaling relations to relate the temperature-pressure profile of an irradiated atmosphere and the photon deposition layer and to estimate if a cloud particle will be lofted by atmospheric circulation. We derive an analytical formula for computing the albedo spectrum in terms of the cloud properties, which we compare to the measured albedo spectrum of HD 189733b by Evans et al. (2013). Furthermore, we show that whether an optical phase curve is flat or sinusoidal depends on whether the particles are small or large as defined by the Knudsen number. This may be an explanation for why Kepler-7b exhibits evidence for the longitudinal variation in abundance of condensates, while Kepler-12b shows no evidence for the presence of condensates, despite the incident stellar flux being similar for both exoplanets.
    The Astrophysical Journal 09/2013; 777(2). · 6.28 Impact Factor
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    ABSTRACT: We present integral-field spectroscopic observations with the VIMOS-IFU at the VLT of fast (2000-3000 km/s) Balmer-dominated shocks surrounding the northwestern rim of the remnant of supernova 1006. The high spatial and spectral resolution of the instrument enable us to show that the physical characteristics of the shocks exhibit a strong spatial variation over few atomic scale lengths across 133 sky locations. Our results point to the presence of a population of non-thermal protons (10-100 keV) which might well be the seed particles for generating high-energy cosmic rays. We also present observations of Tycho's supernova remnant taken with the narrow-band tunable filter imager OSIRIS at the GTC and the Fabry-Perot interferometer GHaFaS at the WHT to resolve respectively the broad and narrow H\alpha\ lines across a large part of the remnant.
    Proceedings of the International Astronomical Union 07/2013; 9(S296).
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    ABSTRACT: We present a secondary eclipse observation for the hot Jupiter HD189733b across the wavelength range 290-570nm made using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. We measure geometric albedos of Ag = 0.40 \pm 0.12 across 290-450nm and Ag < 0.12 across 450-570nm at 1-sigma confidence. The albedo decrease toward longer wavelengths is also apparent when using six wavelength bins over the same wavelength range. This can be interpreted as evidence for optically thick reflective clouds on the dayside hemisphere with sodium absorption suppressing the scattered light signal beyond ~450nm. Our best-fit albedo values imply that HD189733b would appear a deep blue color at visible wavelengths.
    The Astrophysical Journal Letters 07/2013; 772(2). · 5.60 Impact Factor
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    ABSTRACT: Directly-imaged exoplanets are unexplored laboratories for the application of the spectral and temperature retrieval method, where the chemistry and composition of their atmospheres are inferred from inverse modeling of the available data. As a pilot study, we focus on the extrasolar gas giant HR 8799b for which more than 50 data points are available. We upgrade our non-linear optimal estimation retrieval method to include a phenomenological model of clouds that requires the cloud optical depth and monodisperse particle size to be specified. Previous studies have focused on forward models with assumed values of the exoplanetary properties; there is no consensus on the best-fit values of the radius, mass, surface gravity and effective temperature of HR 8799b. We show that cloudfree models produce reasonable fits to the data if the atmosphere is of super-solar metallicity and non-solar elemental abundances. Intermediately cloudy models with moderate values of the cloud optical depth and micron-sized particles provide an equally reasonable fit to the data and require a lower mean molecular weight. We report our best-fit values for the radius, mass, surface gravity and effective temperature of HR 8799b. The mean molecular weight is about 3.8, while the carbon-to-oxygen ratio is about unity due to the prevalence of carbon monoxide. Our study emphasizes the need for robust claims about the nature of an exoplanetary atmosphere to be based on analyses involving both photometry and spectroscopy and inferred from beyond a few photometric data points, such as are typically reported for hot Jupiters.
    The Astrophysical Journal 07/2013; · 6.28 Impact Factor
  • Kevin Heng, Matej Malik
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    ABSTRACT: Motivated by the reported dearth of debris discs around M stars, we use survival models to study the occurrence of planetesimal discs around them. These survival models describe a planetesimal disc with a small number of parameters, determine if it may survive a series of dynamical processes and compute the associated infrared excess. For the Wide-field Infrared Survey Explorer (WISE) satellite, we demonstrate that the dearth of debris discs around M stars may be attributed to the small semimajor axes generally probed if either: (1) the dust grains behave like blackbodies emitting at a peak wavelength coincident with the observed one; (2) or the grains are hotter than predicted by their blackbody temperatures and emit at peak wavelengths that are shorter than the observed one. At these small distances from the M star, planetesimals are unlikely to survive or persist for time-scales of 300 Myr or longer if the disc is too massive. Conversely, our survival models allow for the existence of a large population of low-mass debris discs that are too faint to be detected with current instruments. We gain further confidence in our interpretation by demonstrating the ability to compute infrared excesses for Sun-like stars that are broadly consistent with reported values in the literature. However, our interpretation becomes less clear and large infrared excesses are allowed if only one of these scenarios holds: (3) the dust grains are hotter than blackbody and predominantly emit at the observed wavelength; (4) or are blackbody in nature and emit at peak wavelengths longer than the observed one. Both scenarios imply that the parent planetesimals reside at larger distances from the star than inferred if the dust grains behaved like blackbodies. In all scenarios, we show that the infrared excesses detected at 22 μm (via WISE) and 70 μm (via Spitzer) from AU Mic are easily reconciled with its young age (12 Myr). Conversely, the existence of the old debris disc (2-8 Gyr) from GJ 581 is due to the large semimajor axes probed by the Herschel PACS instrument. We elucidate the conditions under which stellar wind drag may be neglected when considering dust populations around M stars. The WISE satellite should be capable of detecting debris discs around young M stars with ages ˜10 Myr.
    Monthly Notices of the Royal Astronomical Society 07/2013; 432(3):2562-2572. · 5.23 Impact Factor
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    Kevin Heng
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    ABSTRACT: The ubiquity of worlds beyond our Solar System confounds us.
    04/2013;

Publication Stats

641 Citations
342.18 Total Impact Points

Institutions

  • 2013–2014
    • Universität Bern
      • Center for Space and Habitability
      Berna, Bern, Switzerland
    • European Southern Observatory
      Arching, Bavaria, Germany
    • University of Oxford
      Oxford, England, United Kingdom
  • 2010
    • Hochschule für Technik Zürich
      Zürich, Zurich, Switzerland
  • 2004–2008
    • University of Colorado at Boulder
      • • Center for Astrophysics and Space Astronomy
      • • Department of Astrophysical and Planetary Sciences
      Boulder, Colorado, United States
  • 2007
    • Max Planck Institute for Extraterrestrial Physics
      Arching, Bavaria, Germany