Kevin Heng

Universität Bern, Berna, Bern, Switzerland

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Publications (53)353.99 Total impact

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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.
    09/2014;
  • 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;
  • Kevin Heng, Brice-Olivier Demory
    The Astrophysical Journal 03/2014; 785(1):80. · 6.73 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). · 16.24 Impact Factor
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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). · 6.73 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. · 6.35 Impact Factor
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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 10/2013; Submitted. · 2.97 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). · 6.73 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.
    07/2013;
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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). · 6.35 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; · 6.73 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;
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    ABSTRACT: Supernova remnants are among the most spectacular examples of astrophysical pistons in our cosmic neighborhood. The gas expelled by the supernova explosion is launched with velocities ~1000 kilometers per second into the ambient, tenuous interstellar medium, producing shocks that excite hydrogen lines. We have used an optical integral-field spectrograph to obtain high-resolution spatial-spectral maps that allow us to study in detail the shocks in the northwestern rim of supernova 1006. The two-component Hα line is detected at 133 sky locations. Variations in the broad line widths and the broad-to-narrow line intensity ratios across tens of atomic mean free paths suggest the presence of suprathermal protons, the potential seed particles for generating high-energy cosmic rays. (1 data file).
    VizieR Online Data Catalog. 04/2013;
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    ABSTRACT: Supernova remnants are among the most spectacular examples of astrophysical pistons in our cosmic neighborhood. The gas expelled by the supernova explosion is launched with velocities ∼1000 kilometers per second into the ambient, tenuous interstellar medium, producing shocks that excite hydrogen lines. We have used an optical integral-field spectrograph to obtain high-resolution spatial-spectral maps that allow us to study in detail the shocks in the northwestern rim of supernova 1006. The two-component Hα line is detected at 133 sky locations. Variations in the broad line widths and the broad-to-narrow line intensity ratios across tens of atomic mean free paths suggest the presence of suprathermal protons, the potential seed particles for generating high-energy cosmic-rays.
    Science 02/2013; · 31.20 Impact Factor
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    ABSTRACT: The WASP (Wide Angle Search for Planets) consortium has identified a rare gas giant transiting a red dwarf. The combination of the size ratio and brightness contrast, along with the luminosity of the host star means that WASP-80b is one of the most practical target on which we can carry out an atmospheric characterisation. In addition, it gives us an opportunity to study a Jovian planet whose atmosphere is much less irradiated than the usual hot Jupiters. In order to confront theoretical predictions that have been made about heat redistribution and flux penetration in a temperature regime where they have not been tested yet, we aim to use the Spitzer Space telescope to observe and detect a full orbital phase curve. While its very deep transit is favourable to carry out transmission spectroscopy, the contrast between the planet and the star means we will have no issue detecting its emission spectrum. Our Spitzer observations mark the starting point of our campaign to characterise the atmosphere of a warm Jupiter which will lead to the construction of a full spectrum and a measure of its chemical abundances. This data will also be used to seek an additional transiting companion down to an inner Mars-sized planet.
    Spitzer Proposal. 12/2012;
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    Kevin Heng
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    ABSTRACT: Supersonic flows are expected to exist in the atmospheres of irradiated exoplanets, but the question of whether shocks develop lingers. Specifically, it reduces to whether continuous flow in a closed loop may become supersonic and if some portions of the supersonic flow steepen into shocks. We first demonstrate that continuous, supersonic flow may exist in two flavors: isentropic and non-isentropic, with shocks being included in the latter class of solutions. Supersonic flow is a necessary but insufficient condition for shocks to develop. The development of a shock requires the characteristics of neighboring points in a flow to intersect. We demonstrate that the intersection of characteristics may be quantified via knowledge of the Mach number. Finally, we examine 3D simulations of hot Jovian atmospheres and demonstrate that shock formation is expected to occur mostly on the dayside hemisphere, upstream of the substellar point, because the enhanced temperatures near the substellar point provide a natural pressure barrier for the returning flow. Understanding the role of shocks in irradiated exoplanetary atmospheres is relevant to correctly modeling observables such as the peak offsets of infrared phase curves.
    The Astrophysical Journal Letters 10/2012; 761(1). · 6.35 Impact Factor
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    ABSTRACT: A dedicated mission to investigate exoplanetary atmospheres represents a major milestone in our quest to understand our place in the universe by placing our Solar System in context and by addressing the suitability of planets for the presence of life. EChO -the Exoplanet Characterisation Observatory- is a mission concept specifically geared for this purpose. EChO will provide simultaneous, multi-wavelength spectroscopic observations on a stable platform that will allow very long exposures. EChO will build on observations by Hubble, Spitzer and groundbased telescopes, which discovered the first molecules and atoms in exoplanetary atmospheres. EChO will simultaneously observe a broad enough spectral region -from the visible to the mid-IR- to constrain from one single spectrum the temperature structure of the atmosphere and the abundances of the major molecular species. The spectral range and resolution are tailored to separate bands belonging to up to 30 molecules to retrieve the composition and temperature structure of planetary atmospheres. The target list for EChO includes planets ranging from Jupiter-sized with equilibrium temperatures Teq up to 2000 K, to those of a few Earth masses, with Teq ~300 K. We have baselined a dispersive spectrograph design covering continuously the 0.4-16 micron spectral range in 6 channels (1 in the VIS, 5 in the IR), which allows the spectral resolution to be adapted from several tens to several hundreds, depending on the target brightness. The instrument will be mounted behind a 1.5 m class telescope, passively cooled to 50 K, with the instrument structure and optics passively cooled to ~45 K. EChO will be placed in a grand halo orbit around L2. We have also undertaken a first-order cost and development plan analysis and find that EChO is easily compatible with the ESA M-class mission framework.
    Experimental Astronomy 10/2012; 34(2):311-353. · 2.97 Impact Factor
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    Kevin Heng
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    ABSTRACT: Characterizing atmospheres beyond the Solar System is an endeavor no longer confined to the realm of science fiction.
    06/2012;
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    Kevin Heng, Pushkar Kopparla
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    ABSTRACT: We investigate the stability of super Earth atmospheres around M stars using a 7-parameter, analytical framework. We construct stability diagrams in the parameter space of exoplanetary radius versus semi-major axis and elucidate the regions in which the atmospheres are stable against the condensation of their major constituents, out of the gas phase, on their permanent nightside hemispheres. We find that super Earth atmospheres which are nitrogen-dominated ("Earth-like") occupy a smaller region of allowed parameter space, compared to hydrogen-dominated atmospheres, because of the dual effects of diminished advection and enhanced radiative cooling. Furthermore, some super Earths which reside within the habitable zones of M stars may not possess stable atmospheres, depending on the mean molecular weight and infrared photospheric pressure of their atmospheres. We apply our stability diagrams to GJ 436b and GJ 1214b, and demonstrate that atmospheric compositions with high mean molecular weights are disfavoured if these exoplanets possess solid surfaces and shallow atmospheres. Finally, we construct stability diagrams tailored to the Kepler dataset, for G and K stars, and predict that about half of the exoplanet candidates are expected to habour stable atmospheres if Earth-like conditions are assumed. We include 55 Cancri e and CoRoT-7b in our stability diagram for G stars.
    The Astrophysical Journal 03/2012; 754(1). · 6.73 Impact Factor

Publication Stats

369 Citations
353.99 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