Christopher M. Brunt

University of Exeter, Exeter, England, United Kingdom

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Publications (122)

  • Source
    [Show abstract] [Hide abstract] ABSTRACT: (Abridged) We present the first public release of high-quality data products (DR1) from Hi-GAL, the {\em Herschel} infrared Galactic Plane Survey. Hi-GAL is the keystone of a suite of continuum Galactic Plane surveys from the near-IR to the radio, and covers five wavebands at 70, 160, 250, 350 and 500 micron, encompassing the peak of the spectral energy distribution of cold dust for 8 < T < 50K. This first Hi-GAL data release covers the inner Milky Way in the longitude range 68{\deg} > l > -70{\deg} in a |b|<1{\deg} latitude strip. Photometric maps have been produced with the ROMAGAL pipeline, that optimally capitalizes on the excellent sensitivity and stability of the bolometer arrays of the {\em Herschel} PACS and SPIRE photometric cameras, to deliver images of exquisite quality and dynamical range, absolutely calibrated with {\em Planck} and {\em IRAS}, and recovering extended emission at all wavelengths and all spatial scales. The compact source catalogues have been generated with the CuTEx algorithm, specifically developed to optimize source detection and extraction in the extreme conditions of intense and spatially varying background that are found in the Galactic Plane in the thermal infrared. Hi-GAL DR1 images will be accessible via a dedicated web-based image cutout service. The DR1 Compact Source Catalogues are delivered as single-band photometric lists containing, in addition to source position, peak and integrated flux and source sizes, a variety of parameters useful to assess the quality and reliability of the extracted sources, caveats and hints to help this assessment are provided. Flux completeness limits in all bands are determined from extensive synthetic source experiments and depend on the specific line of sight along the Galactic Plane. Hi-GAL DR1 catalogues contain 123210, 308509, 280685, 160972 and 85460 compact sources in the five bands, respectively.
    Full-text Article · Apr 2016
  • Julia Roman-Duval · Mark Heyer · Christopher M. Brunt · [...] · Rahul Shetty
    [Show abstract] [Hide abstract] ABSTRACT: Emission from carbon monoxide (CO) is ubiquitously used as a tracer of dense star forming molecular clouds. There is, however, growing evidence that a significant fraction of CO emission originates from diffuse molecular gas. Quantifying the contribution of diffuse CO-emitting gas is vital for understanding the relation between molecular gas and star formation. We examine the Galactic distribution of two CO-emitting gas components, a high column density component detected in 13CO and 12CO, and a low column density component detected in 12CO, but not in 13CO. The "diffuse" and "dense" components are identified using a combination of smoothing, masking, and erosion/dilation procedures, making use of three large-scale 12CO and 13CO surveys of the Inner and Outer Milky Way. The diffuse component, which globally represents 25 (1.5x1e8 Mo) of the total molecular gas mass (6.5x1e8 Mo), is more extended perpendicular to the Galactic plane. The fraction of diffuse gas increases from 15% at a galactocentric radius of 3 kpc to 50% at 15 kpc, and increases with decreasing surface density. In the Inner Galaxy, a yet denser component traced by CS emission represents 14% of the total molecular gas mass traced by 12CO emission. Only 14% of the molecular gas mass traced by 12CO emission is identified as part of molecular clouds in 12CO surveys by cloud identification algorithms. This study indicates that CO emission not only traces star forming clouds, but also a significant diffuse molecular ISM component.
    Article · Jan 2016 · The Astrophysical Journal
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    [Show abstract] [Hide abstract] ABSTRACT: We present the 13CO/C18O (J = 3 → 2) Heterodyne Inner Milky Way Plane Survey (CHIMPS) which has been carried out using the Heterodyne Array Receiver Program on the 15 m James Clerk Maxwell Telescope (JCMT) in Hawaii. The high-resolution spectral survey currently covers |b| ≤ 0 $_{.}^{\circ}$5 and $28\deg \lesssim l \lesssim 46\deg$, with an angular resolution of 15 arcsec in 0.5 km s−1 velocity channels. The spectra have a median rms of ∼0.6 K at this resolution, and for optically thin gas at an excitation temperature of 10 K, this sensitivity corresponds to column densities of NH2 ∼ 3 × 1020 cm−2 and NH2 ∼ 4 × 1021 cm−2 for 13CO and C18O, respectively. The molecular gas that CHIMPS traces is at higher column densities and is also more optically thin than in other publicly available CO surveys due to its rarer isotopologues, and thus more representative of the three-dimensional structure of the clouds. The critical density of the J = 3 → 2 transition of CO is ≳104 cm−3 at temperatures of ≤20 K, and so the higher density gas associated with star formation is well traced. These data complement other existing Galactic plane surveys, especially the JCMT Galactic Plane Survey which has similar spatial resolution and column density sensitivity, and the Herschel infrared Galactic Plane Survey. In this paper, we discuss the observations, data reduction and characteristics of the survey, presenting integrated-emission maps for the region covered. Position–velocity diagrams allow comparison with Galactic structure models of the Milky Way, and while we find good agreement with a particular four-arm model, there are some significant deviations.
    Full-text Article · Dec 2015 · Monthly Notices of the Royal Astronomical Society
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    [Show abstract] [Hide abstract] ABSTRACT: We have mapped cold atomic gas in 21cm line HI self-absorption (HISA) at arcminute resolution over more than 90% of the Milky Way's disk. To probe the formation of H2 clouds, we have compared our HISA distribution with CO J=1-0 line emission. Few HISA features in the outer Galaxy have CO at the same position and velocity, while most inner-Galaxy HISA has overlapping CO. But many apparent inner-Galaxy HISA-CO associations can be explained as chance superpositions, so most inner-Galaxy HISA may also be CO-free. Since standard equilibrium cloud models cannot explain the very cold HI in many HISA features without molecules being present, these clouds may instead have significant CO-dark H2.
    Full-text Article · Oct 2015
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    [Show abstract] [Hide abstract] ABSTRACT: We present early results from the JCMT (James Clerk Maxwell Telescope) Plane Survey (JPS), which has surveyed the northern inner Galactic plane between longitudes ℓ = 7° and ℓ = 63° in the 850-μm continuum with SCUBA-2 (Submm Common-User Bolometer Array 2), as part of the JCMT Legacy Survey programme. Data from the ℓ = 30° survey region, which contains the massive-star-forming regions W43 and G29.96, are analysed after approximately 40 per cent of the observations had been completed. The pixel-to-pixel noise is found to be 19 mJy beam−1 after a smooth over the beam area, and the projected equivalent noise levels in the final survey are expected to be around 10 mJy beam−1. An initial extraction of compact sources was performed using the FellWalker method, resulting in the detection of 1029 sources above a 5σ surface-brightness threshold. The completeness limits in these data are estimated to be around 0.2 Jy beam−1 (peak flux density) and 0.8 Jy (integrated flux density) and are therefore probably already dominated by source confusion in this relatively crowded section of the survey. The flux densities of extracted compact sources are consistent with those of matching detections in the shallower APEX (Atacama Pathfinder Experiment) Telescope Large Area Survey of the Galaxy (ATLASGAL) survey. We analyse the virial and evolutionary state of the detected clumps in the W43 star-forming complex and find that they appear younger than the Galactic-plane average.
    Full-text Article · Sep 2015 · Monthly Notices of the Royal Astronomical Society
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    [Show abstract] [Hide abstract] ABSTRACT: We present CO, H2, HI and HISA distributions from a set of simulations of grand design spirals including stellar feedback, self-gravity, heating and cooling. We replicate the emission of the 2nd Galactic Quadrant by placing the observer inside the modelled galaxies and post process the simulations using a radiative transfer code, so as to create synthetic observations. We compare the synthetic datacubes to observations of the 2nd Quadrant of the Milky Way to test the ability of the current models to reproduce the basic chemistry of the Galactic ISM, as well as to test how sensitive such galaxy models are to different recipes of chemistry and/or feedback. We find that models which include feedback and self-gravity can reproduce the production of CO with respect to H2 as observed in our Galaxy, as well as the distribution of the material perpendicular to the Galactic plane. While changes in the chemistry/feedback recipes do not have a huge impact on the statistical properties of the chemistry in the simulated galaxies, we find that the inclusion of both feedback and self-gravity are crucial ingredients, as our test without feedback failed to reproduce all of the observables. Finally, even though the transition from H2 to CO seems to be robust, we find that all models seem to underproduce molecular gas, and have a lower molecular to atomic gas fraction than is observed. Nevertheless, our fiducial model with feedback and self-gravity has shown to be robust in reproducing the statistical properties of the basic molecular gas components of the ISM in our Galaxy.
    Full-text Article · Dec 2014 · Monthly Notices of the Royal Astronomical Society
  • Source
    Tyler J. Foster · Christopher M. Brunt
    [Show abstract] [Hide abstract] ABSTRACT: We present a new catalogue of spectrophotometric distances and line-of-sight systemic velocities to 103 HII regions between 90$^{\circ}\leq \ell \leq~$195$^{\circ}$ (longitude quadrants II and part of III). Two new velocities for each region are independently measured using 1-arcminute resolution 21~cm HI and 2.6~mm $^{12}$CO line maps (from the Canadian Galactic Plane Survey and FCRAO Outer Galaxy Surveys) that show where gaseous shells are observed around the periphery of the ionized gas. Known and neighbouring OB-type stars with published UBV photometry and MK classifications are overlaid onto 21~cm continuum maps, and those stars observed within the boundary of the HII emission (and whose distance is not more than 3 times the standard deviation of the others) are used to calculate new mean stellar distances to each of the 103 nebulae. Using this approach of excluding distance outliers from the mean distance to a group of many stars in each HII region lessens the impact of anomalous reddening for certain individuals. Final mean distances of 9 common objects with VLBI parallax distances show a 1:1 correspondence. Further, comparison with previous catalogues of HII regions in these quadrants shows a 50% reduction in scatter for the distance to Perseus spiral arm objects in the same region, and a reduction by $\sim$1/$\sqrt{2}$ in scatter around a common angular velocity relative to the Sun $\Omega-\Omega_0$(km~s$^{-1}$~kpc$^{-1}$). The purpose of the catalogue is to provide a foundation for more detailed large-scale Galactic spiral structure and dynamics (rotation curve, density wave streaming) studies in the 2$^{\textrm{nd}}$ and 3$^{\textrm{rd}}$ quadrants, which from the Sun's location is the most favourably viewed section of the Galaxy.
    Full-text Article · May 2014 · The Astronomical Journal
  • C. M. Brunt · C. Federrath
    [Show abstract] [Hide abstract] ABSTRACT: We introduce a new method for observationally estimating the fraction of momentum density (${\rho}{\mathbf{v}}$) power contained in solenoidal modes (for which $\nabla \cdot {\rho}{\mathbf{v}} = 0$) in molecular clouds. The method is successfully tested with numerical simulations of supersonic turbulence that produce the full range of possible solenoidal/compressible fractions. At present the method assumes statistical isotropy, and does not account for anisotropies caused by (e.g.) magnetic fields. We also introduce a framework for statistically describing density--velocity correlations in turbulent clouds.
    Article · May 2014 · Monthly Notices of the Royal Astronomical Society
  • C. M. Brunt · C. Federrath
    [Show abstract] [Hide abstract] ABSTRACT: We introduce a new method for observationally estimating the fraction of momentum density (${\rho}{\mathbf{v}}$) power contained in solenoidal modes (for which $\nabla \cdot {\rho}{\mathbf{v}} = 0$) in molecular clouds. The method is successfully tested with numerical simulations of supersonic turbulence that produce the full range of possible solenoidal/compressible fractions. At present the method assumes statistical isotropy, and does not account for anisotropies caused by (e.g.) magnetic fields. We also introduce a framework for statistically describing density--velocity correlations in turbulent clouds.
    Article · Apr 2014
  • L. Chavarria · L. Allen · C. Brunt · [...] · G. Fazio
    [Show abstract] [Hide abstract] ABSTRACT: We present Spitzer, near-IR (NIR) and millimetre observations of the massive star-forming regions W5-east, S235, S252, S254-S258 and NGC 7538. Spitzer data is combined with NIR observations to identify and classify the young population while 12CO and 13CO observations are used to examine the parental molecular cloud. We detect in total 3021 young stellar objects (YSOs). Of those, 539 are classified as Class I, and 1186 as Class II sources. YSOs are distributed in groups surrounded by a more scattered population. Class I sources are more hierarchically organized than Class II and associated with the most dense molecular material. We identify in total 41 embedded clusters containing between 52 and 73 per cent of the YSOs. Clusters are in general non-virialized, turbulent and have star formation efficiencies between 5 and 50 per cent. We compare the physical properties of embedded clusters harbouring massive stars (MEC) and low-mass embedded clusters (LEC) and find that both groups follow similar correlations where the MEC are an extrapolation of the LEC. The mean separation between MEC members is smaller compared to the cluster Jeans length than for LEC members. These results are in agreement with a scenario where stars are formed in hierarchically distributed dusty filaments where fragmentation is mainly driven by turbulence for the more massive clusters. We find several young OB-type stars having IR-excess emission which may be due to the presence of an accretion disc.
    Article · Mar 2014 · Monthly Notices of the Royal Astronomical Society
  • C. M. Brunt · M. H. Heyer
    [Show abstract] [Hide abstract] ABSTRACT: Principal component analysis is a powerful statistical system to investigate the structure and dynamics of the molecular interstellar medium, with particular emphasis on the study of turbulence, as revealed by spectroscopic imaging of molecular line emission. To-date, the method to retrieve the power law index of the velocity structure function or power spectrum has relied on an empirical calibration and testing with model turbulent velocity fields, while lacking a firm theoretical basis. In this paper, we present an analytic formulation that reveals the detailed mechanics of the method and confirms previous empirical calibrations of its recovery of the scale dependence of turbulent velocity fluctuations.
    Article · May 2013 · Monthly Notices of the Royal Astronomical Society
  • F. Ripple · M. H. Heyer · R. Gutermuth · [...] · C. M. Brunt
    [Show abstract] [Hide abstract] ABSTRACT: Infrared stellar photometry from the Two Micron All-Sky Survey (2MASS) and spectral line imaging observations of 12CO and 13CO J = 1–0 line emission from the Five College Radio Astronomy Observatory (FCRAO) 14-m telescope are analysed to assess the variation of the CO abundance with physical conditions throughout the Orion A and Orion B molecular clouds. Three distinct Av regimes are identified in which the ratio between the 13CO column density and visual extinction changes corresponding to the photon-dominated envelope, the strongly self-shielded interior, and the cold, dense volumes of the clouds. Within the strongly self-shielded interior of the Orion A cloud, the 13CO abundance varies by 100 per cent with a peak value located near regions of enhanced star formation activity. The effect of CO depletion on to the ice mantles of dust grains is limited to regions with Av > 10 mag and gas temperatures less than ∼20 K as predicted by chemical models that consider thermal evaporation to desorb molecules from grain surfaces. Values of the molecular mass of each cloud are independently derived from the distributions of Av and 13CO column densities with a constant 13CO-to-H2 abundance over various extinction ranges. Within the strongly self-shielded interior of the cloud (Av> 3 mag), 13CO provides a reliable tracer of H2 mass with the exception of the cold, dense volumes where depletion is important. However, owing to its reduced abundance, 13CO does not trace the H2 mass that resides in the extended cloud envelope, which comprises 40–50 per cent of the molecular mass of each cloud. The implied CO luminosity to mass ratios, M/LCO, are 3.2 and 2.9 for Orion A and Orion B, respectively, which are comparable to the value (2.9), derived from γ-ray observations of the Orion region. Our results emphasize the need to consider local conditions when applying CO observations to derive H2 column densities.
    Article · Feb 2013 · Monthly Notices of the Royal Astronomical Society
  • [Show abstract] [Hide abstract] ABSTRACT: Formation of new stars requires the condensation of ambient neutral atomic hydrogen into the molecular phase. It is well known that stars form from molecular hydrogen clouds; less understood is how molecular clouds themselves begin to form. We use AKARI (MIR and FIS), HIRES (IRAS) and Spitzer (IRAC and MIPS) imaging photometry to probe thermal dust emission in a target cloud in the Perseus spiral arm, where the H1 to H2 transition appears to be underway. H2 formation in this region is indicated by strong HI self-absorption, variable CO emission, and significant ``excess'' infrared emission. We have sampled the dust spectral energy distribution (SED) at many positions on and off this cloud using AKARI, IRAS, and SST data. We present these SED plots with fits generated by the DustEM modeling program to provide some insight as to the nature of the dust composition, size distribution, and dominant heating mechanisms within this target and possibly other H2 forming regions.
    Conference Paper · Jan 2013
  • D. Acreman · C. Brunt · C. Dobbs · K. Douglas
    [Show abstract] [Hide abstract] ABSTRACT: We present synthetic Galactic H I surveys derived from simulations of spiral galaxies, which we compare with the Canadian Galactic Plane Survey. We find that when including stellar feedback in the simulations, it is possible to reproduce the vertical scale height and structure of the observed H I emission.
    Article · Jul 2012
  • David M. Acreman · Clare L. Dobbs · Christopher M. Brunt · Kevin A. Douglas
    [Show abstract] [Hide abstract] ABSTRACT: We generate synthetic H I Galactic plane surveys from spiral galaxy simulations which include stellar feedback processes. Compared to a model without feedback we find an increased scale height of H I emission (in better agreement with real observations) and more realistic spatial structure (including supernova blown bubbles). The synthetic data show H I self-absorption with a morphology similar to that seen in observations. The density and temperature of the material responsible for H I self-absorption is consistent with observationally determined values, and is found to be only weakly dependent on absorption strength and star formation efficiency.
    Article · May 2012 · Monthly Notices of the Royal Astronomical Society
  • Source
    [Show abstract] [Hide abstract] ABSTRACT: The Gould Belt Legacy Survey will map star-forming regions within 500 pc, using HARP (Heterodyne Array Receiver Programme), SCUBA-2 (Submillimetre Common-User Bolometer Array 2) and POL-2 (Polarimeter 2) on the James Clerk Maxwell Telescope (JCMT). This paper describes HARP observations of the J = 3-2 transitions of 13CO and C18O towards Orion A. The 1500-resolution observations cover 5 pc of the Orion filament, including OMC1 (inc. BN-KL and Orion Bar), OMC 2/3 and OMC 4, and allow a comparative study of the molecular gas properties throughout the star-forming cloud. The filament shows a velocity gradient of ~1 km/s /pc between OMC 1, 2 and 3, and high velocity emission is detected in both isotopologues. The Orion Nebula and Bar have the largest masses and line widths, and dominate the mass and energetics of the high velocity material. Compact, spatially resolved emission from CH3CN, 13CH3OH, SO, HCOOCH3, C2H5OH, CH3CHO and CH3OCHO is detected towards the Orion Hot Core. The cloud is warm, with a median excitation temperature of ~24 K; the Orion Bar has the highest excitation temperature gas, at >80 K. The C18O excitation temperature correlates well with the dust temperature (to within 40%). The C18O emission is optically thin, and the 13CO emission is marginally optically thick; despite its high mass, OMC 1 shows the lowest opacities. A virial analysis indicates that Orion A is too massive for thermal or turbulent support, but is consistent with a model of a filamentary cloud that is threaded by helical magnetic fields. The variation of physical conditions across the cloud is reflected in the physical characteristics of the dust cores....continued
    Full-text Article · Jan 2012 · Monthly Notices of the Royal Astronomical Society
  • Source
    David M. Acreman · Clare L. Dobbs · Christopher M. Brunt · Kevin A. Douglas
    [Show abstract] [Hide abstract] ABSTRACT: We generate synthetic HI Galactic plane surveys from spiral galaxy simulations which include stellar feedback processes. Compared to a model without feedback we find an increased scale height of HI emission (in better agreement with observations) and more realistic spatial structure (including supernova blown bubbles). The synthetic data show HI self-absorption with a morphology similar to that seen in observations. The density and temperature of the material responsible for HI self-absorption is consistent with observationally determined values, and is found to be only weakly dependent on absorption strength and star formation efficiency.
    Full-text Article · Jan 2012
  • [Show abstract] [Hide abstract] ABSTRACT: An essential step in the formation of new stars is the condensation of ambient neutral atomic hydrogen (HI) into the molecular phase (H2). It is well known that molecular clouds collapse to form protostars, but less understood is how molecular clouds themselves begin to form. The process is difficult to study because the transition from HI to H2 is not very energetic, which limits direct observations. We study this process indirectly, by examining the interstellar dust within these H2-forming clouds. The dust is readily observed via infrared thermal emission. We use HIRES IRAS and Spitzer IRAC and MIPS imaging photometry to investigate a target cloud in the Perseus spiral arm in the which the HI-to-H2 transition appears to be underway, as evidenced by strong HI self-absorption, variable CO emission, and significant ``excess'' infrared emission. We have sampled the dust spectral energy distribution (SED) at many positions on and off this cloud in all IRAS and Spitzer photometric bands. We interpret these data by fitting the SEDs with the DustEM infrared emission model and infer the grain population composition and evolutionary status in this H2-forming cloud and others like it.
    Conference Paper · Jan 2012
  • Source
    Julia Roman-Duval · Christoph Federrath · Christopher Brunt · [...] · and Ralf S. Klessen
    [Show abstract] [Hide abstract] ABSTRACT: Turbulence plays a major role in the formation and evolution of molecular clouds. Observationally, turbulent velocities are convolved with the density of an observed region. To correct for this convolution, we investigate the relation between the turbulence spectrum of model clouds, and the statistics of their synthetic observations obtained from principal component analysis (PCA). We apply PCA to spectral maps generated from simulated density and velocity fields, obtained from hydrodynamic simulations of supersonic turbulence, and from fractional Brownian motion (fBm) fields with varying velocity, density spectra, and density dispersion. We examine the dependence of the slope of the PCA pseudo-structure function, αPCA, on intermittency, on the turbulence velocity (βv ) and density (βn ) spectral indexes, and on density dispersion. We find that PCA is insensitive to βn and to the log-density dispersion σs , provided σs ≤ 2. For σs > 2, αPCA increases with σs due to the intermittent sampling of the velocity field by the density field. The PCA calibration also depends on intermittency. We derive a PCA calibration based on fBm structures with σs ≤ 2 and apply it to 367 13CO spectral maps of molecular clouds in the Galactic Ring Survey. The average slope of the PCA structure function, αPCA = 0.62 ± 0.2, is consistent with the hydrodynamic simulations and leads to a turbulence velocity exponent of βv = 2.06 ± 0.6 for a non-intermittent, low density dispersion flow. Accounting for intermittency and density dispersion, the coincidence between the PCA slope of the GRS clouds and the hydrodynamic simulations suggests βv 1.9, consistent with both Burgers and compressible intermittent turbulence.
    Full-text Article · Oct 2011 · The Astrophysical Journal
  • Mark H. Heyer · Christopher M. Brunt
    [Show abstract] [Hide abstract] ABSTRACT: Magnetically aligned velocity anisotropy over varying physical conditions and environments within the Taurus molecular cloud is evaluated from analysis of wide field spectroscopic imaging of 12CO and 13CO J= 1–0 emission. Such anisotropy is a result of magnetohydrodynamic turbulence in the strong magnetic field regime and provides an indirect measure of the role of magnetic fields upon the gas. Velocity anisotropy aligned with the local, projected mean magnetic field direction is limited to fields with low surface brightness 12CO emission corresponding to regions of low visual extinction and, presumably, low gas volume density. The more optically thin 13CO J= 1–0 emission shows little evidence for velocity anisotropy. We compare our results with computational simulations with varying degrees of magnetic field strength and Alfvénic Mach numbers. In the diffuse, molecular envelope of the cloud, a strong magnetic field and sub-Alfvénic turbulent motions are inferred. Super-Alfvénic motions are present within the high column density filaments of the Taurus cloud. From this trans-Alfvénic flow, we constrain the scaling exponent, κ, of the magnetic field density relation (B∼nκ) to be near zero as expected for ambipolar diffusion or material loading of magnetic flux tubes.
    Article · Oct 2011 · Monthly Notices of the Royal Astronomical Society

Publication Stats

2k Citations

Institutions

  • 2008-2014
    • University of Exeter
      • Department of Physics and Astronomy
      Exeter, England, United Kingdom
  • 1998-2009
    • University of Massachusetts Amherst
      • Department of Astronomy
      Amherst Center, Massachusetts, United States
  • 2000-2008
    • The University of Calgary
      • Department of Physics and Astronomy
      Calgary, Alberta, Canada
  • 2006
    • Harvard-Smithsonian Center for Astrophysics
      • Smithsonian Astrophysical Observatory
      Cambridge, Massachusetts, United States