J. C. Raymond

Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, United States

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Publications (685)2066.16 Total impact

  • C. Shen, J.C. Raymond, N.A. Murphy, J. Lin
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    ABSTRACT: Time-dependent ionization is important in astrophysical environments where the thermodynamical time scale is shorter than the ionization or recombination time scales. In this work, we report a FORTRAN program that performs fast non-equilibrium ionization calculations in post-processing based on hydrodynamics(HD) or magnetohydrodynamics(MHD) simulation results. Using HD or MHD simulation results, we track the movement of plasma in a Lagrangian framework, and obtain the evolutionary history of temperature and electron density. The time-dependent ionization equations are then solved by the Eigenvalue method. For any complex temperature and electron density histories, we introduce an adaptive time-step strategy to improve the computational efficiency. Our tests show that this program has advantages of high numerical stability and high accuracy. In addition, it is also easy to extend this solver to other HD and MHD simulations. This code is freely available for download from the Web.
    04/2015; 12. DOI:10.1016/j.ascom.2015.04.003
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    ABSTRACT: Recent observations by the Large Area Telescope (LAT) onboard the Fermi satellite have revealed bright gamma-ray emission from middle-aged supernova remnants (SNRs) inside our Galaxy. These remnants, which also possess bright non-thermal radio shells, are often found to be interacting directly with surrounding gas clouds. We explore the non-thermal emission mechanism at these dynamically evolved SNRs by constructing a hydrodynamical model. Two scenarios of particle acceleration, either a re-acceleration of Galactic cosmic rays (CRs) or an efficient nonlinear diffusive shock acceleration (NLDSA) of particles injected from downstream, are considered. Using parameters inferred from observations, our models are contrasted with the observed spectra of SNR W44. For the re-acceleration case, we predict a significant enhancement of radio and GeV emission as the SNR undergoes a transition into the radiative phase. If sufficiently strong magnetic turbulence is present in the molecular cloud, the re-acceleration scenario can explain the observed broadband spectral properties. The NLDSA scenario also succeeds in explaining the $\gamma$-ray spectrum but fails to reproduce the radio spectral index. Efficient NLDSA also results in a significant post-shock non-thermal pressure that limits the compression during cooling and prevents the formation of a prominent dense shell. Some other interesting differences between the two models in hydrodynamical behavior and resulting spectral features are illustrated.
    The Astrophysical Journal 04/2015; 806(1). DOI:10.1088/0004-637X/806/1/71 · 6.28 Impact Factor
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    ABSTRACT: Observations of SN1006 have shown that ions and electrons in the plasma behind fast supernova remnant shock waves are far from equilibrium, with the electron temperature much lower than the proton temperature and ion temperatures approximately proportional to ion mass. In the ~360 km/s shock waves of the Cygnus Loop, on the other hand, electron and ion temperatures are roughly equal, and there is evidence that the oxygen kinetic temperature is not far from the proton temperature. In this paper we report observations of the He II lambda 1640 line and the C IV lambda 1550 doublet in a 360 km/s shock in the Cygnus Loop. While the best fit kinetic temperatures are somewhat higher than the proton temperature, the temperatures of He and C are consistent with the proton temperature and the upper limits are 0.5 and 0.3 times the mass-proportional temperatures, implying efficient thermal equilibration in this collisionless shock. The equilibration of helium and hydrogen affects the conversion between proton temperatures determined from H alpha line profiles and shock speeds, and that the efficient equilibration found here reduces the shock speed estimates and the distance estimate to the Cygnus Loop of Medina et al. (2014) to about 800 pc.
    The Astrophysical Journal 04/2015; 805(2). DOI:10.1088/0004-637X/805/2/152 · 6.28 Impact Factor
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    ABSTRACT: We use the best available X-ray data from the intermediate polar EX Hydrae to study the cooling-flow model often applied to interpret the X-ray spectra of these accreting magnetic white dwarf binaries. First, we resolve a long-standing discrepancy between the X-ray and optical determinations of the mass of the white dwarf in EX Hya by applying new models of the inner disk truncation radius. Our fits to the X-ray spectrum now agree with the white dwarf mass of 0.79 M$_{\odot}$sun determined using dynamical methods through spectroscopic observations of the secondary. We use a simple isobaric cooling flow model to derive the emission line fluxes, emission measure distribution, and H-like to He-like line ratios for comparison with the 496 ks Chandra High Energy Transmission Grating observation of EX Hydrae. We find that the H/He ratios are not well reproduced by this simple isobaric cooling flow model and show that while H-like line fluxes can be accurately predicted, fluxes of lower-Z He-like lines are significantly underestimated. This discrepancy suggests that some extra heating mechanism plays an important role at the base of the accretion column, where cooler ions form. We thus explored more complex cooling models including the change of gravitational potential with height in the accretion column and a magnetic dipole geometry. None of these modifications to the standard cooling flow model are able to reproduce the observed line ratios. While a cooling flow model with subsolar (0.1 $\odot$) abundances is able to reproduce the line ratios by reducing the cooling rate at temperatures lower than $\sim 10^{7.3}$ K, the predicted line-to-continuum ratios are much lower than observed. We discuss and discard mechanisms such as photoionization, departures from constant pressure, resonant scattering, different electron-ion temperatures, and Compton cooling. [Abridged]
    Astronomy and Astrophysics 04/2015; 578. DOI:10.1051/0004-6361/201525755 · 4.48 Impact Factor
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    Salvatore Mancuso, John C. Raymond
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    ABSTRACT: We report the first detection of long-period, slowly decaying Doppler-shift oscillations in the H i Lyα (1215.67 Å) coronal emission line with the Ultraviolet Coronagraph Spectrometer (UVCS) onboard the Solar and Heliospheric Observatory (SOHO) satellite. The UV spectral data were collected at 1.43 R⊙ above the eastern limb of the Sun during a special high-cadence sit-and-stare observation on 1997 December 14. Time-series analyses with different spectral techniques clearly show highly significant Doppler-shift oscillations in a portion with a size of 154′′ of the UVCS slit that lasted for several cycles. A period of P = 14.3 ± 0.4 min was established with a confidence of better than 99.9% in the Lomb-Scargle periodogram. On average, the Doppler-shift amplitude of 3.7 ± 0.7 km s-1 was estimated for the most significant oscillations, roughly corresponding to a displacement of 800 ± 150 km. The origin of the regular H i Lyα Doppler-shift oscillations is most probably due to the excitation of propagating fast magnetoacoustic kink waves along a narrow, jet-like ejection observed higher up in the white-light corona. However, different mechanisms, such as low-amplitude coherent kink oscillations of a bundle of loops along the line of sight or quasi-periodic outflows caused by oscillatory magnetic reconnection in the low corona cannot be ruled out.
    Astronomy and Astrophysics 01/2015; 573(A33). DOI:10.1051/0004-6361/201424898 · 4.48 Impact Factor
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    ABSTRACT: Plasma in the Sun's hot corona expands into the heliosphere as a supersonic and highly magnetized solar wind. This paper provides an overview of our current understanding of how the corona is heated and how the solar wind is accelerated. Recent models of magnetohydrodynamic turbulence have progressed to the point of successfully predicting many observed properties of this complex, multi-scale system. However, it is not clear whether the heating in open-field regions comes mainly from the dissipation of turbulent fluctuations that are launched from the solar surface, or whether the chaotic "magnetic carpet" in the low corona energizes the system via magnetic reconnection. To help pin down the physics, we also review some key observational results from ultraviolet spectroscopy of the collisionless outer corona.
    Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences 12/2014; 373(2041). DOI:10.1098/rsta.2014.0148 · 2.86 Impact Factor
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    ABSTRACT: Magnetic reconnection is believed to be the dominant energy release mechanism in solar flares. The standard flare model predicts both downward and upward outflow plasmas with speeds close to the coronal Alfv\'{e}n speed. Yet, spectroscopic observations of such outflows, especially the downflows, are extremely rare. With observations of the newly launched Interface Region Imaging Spectrograph (IRIS), we report the detection of greatly redshifted ($\sim$125 km s$^{-1}$ along line of sight) Fe {\sc{xxi}} 1354.08\AA{} emission line with a $\sim$100 km s$^{-1}$ nonthermal width at the reconnection site of a flare. The redshifted Fe {\sc{xxi}} feature coincides spatially with the loop-top X-Ray source observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). We interpret this large redshift as the signature of downward-moving reconnection outflow/hot retracting loops. Imaging observations from both IRIS and the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) also reveal the eruption and reconnection processes. Fast downward-propagating blobs along these loops are also found from cool emission lines (e.g., Si {\sc{iv}}, O {\sc{iv}}, C {\sc{ii}}, Mg {\sc{ii}}) and images of AIA and IRIS. Furthermore, the entire Fe {\sc{xxi}} line is blueshifted by $\sim$260 km s$^{-1}$ at the loop footpoints, where the cool lines mentioned above all exhibit obvious redshift, a result that is consistent with the scenario of chromospheric evaporation induced by downward-propagating nonthermal electrons from the reconnection site.
    11/2014; 797(2). DOI:10.1088/2041-8205/797/2/L14
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    ABSTRACT: We investigate seven eruptive plasma observations by Hinode/XRT. Their corresponding EUV and/or white light CME features are visible in some events. Five events are observed in several passbands in X-rays, which allows the determination of the eruptive plasma temperature using a filter ratio method. We find that the isothermal temperatures vary from 1.6 to 10 MK. These temperatures are an average weighted toward higher temperature plasma. We determine the mass constraints of eruptive plasmas by assuming simplified geometrical structures of the plasma with isothermal plasma temperatures. This method provides an upper limit to the masses of the observed eruptive plasmas in X-ray passbands since any clumping causes the overestimation of the mass. For the other two events, we assume the temperatures are at the maximum temperature of the XRT temperature response function, which gives a lower limit of the masses. We find that the masses in XRT, ~3x10 13 - 5x10 14 g, are smaller in their upper limit than total masses obtained by LASCO, ~1x10 15 g. In addition, we estimate the radiative loss, thermal conduction, thermal, and kinetic energies of the eruptive plasma in X-rays. For four events, we find that the thermal conduction time scales are much shorter than the duration of eruption. This result implies that additional heating during the eruption may be required to explain the plasma observations in X-rays for the four events.
    The Astrophysical Journal 11/2014; 798(2). DOI:10.1088/0004-637X/798/2/106 · 6.28 Impact Factor
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    ABSTRACT: Comet C/2002 S2, a member of the Kreutz family of Sungrazing comets, was discovered in white light images of the SOHO/LASCO coronagraph on 2002 September 18 and observed in \hi\, \lya\, emission by the SOHO/UVCS instrument at four different heights as it approached the Sun. The \hi\, \lya\, line profiles detected by UVCS are analyzed to determine the spectral parameters: line intensity, width and Doppler shift with respect to the coronal background. Two dimensional comet images of these parameters are reconstructed at the different heights. A novel aspect of the observations of this sungrazing comet data is that, whereas the emission from the most of the tail is blue--shifted, that along one edge of the tail is red--shifted. We attribute these shifts to a combination of solar wind speed and interaction with the magnetic field. In order to use the comet to probe the density, temperature and speed of the corona and solar wind through which it passes, as well as to determine the outgassing rate of the comet, we develop a Monte Carlo simulation of the \hi\, \lya\, emission of a comet moving through a coronal plasma. From the outgassing rate, we estimate a nucleus diameter of about 9 meters. This rate steadily increases as the comet approaches the Sun while the optical brightness decreases by more than a factor of ten and suddenly recovers. This indicates that the optical brightness is determined by the lifetimes of the grains, sodium atoms and molecules produced by the comet.
    The Astrophysical Journal 11/2014; 798(1). DOI:10.1088/0004-637X/798/1/47 · 6.28 Impact Factor
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    ABSTRACT: We present an observational study using high-resolution echelle spectroscopy of collisionless shocks in the Cygnus Loop supernova remnant. Measured H alpha line profiles constrain pre-shock heating processes resulting in narrow component broadening, cosmic-ray acceleration, and electron-proton equilibration. The shocks produce faint H alpha emission line profiles, which are characterized by narrow and broad components. The narrow component is representative of the pre-shock conditions, while the broad component is produced after charge transfer between neutrals entering the shock and protons in the post-shock gas, thus reflecting the properties of the post-shock gas. We observe a diffuse H alpha region extending about 2.5 arcmin ahead of the shock with line width about 29 km/s, while the H alpha profile of the shock itself consists of a broader than expected narrow (36 km/s) and a broad (250 km/s) component. The observed diffuse emission arises in a photoionization precursor heated to about 18,000 K by He I and He II emission from the shock, with additional narrow component broadening originating from a thin cosmic-ray precursor. Broad to narrow component intensity ratios of about 1.0 imply full electron-proton temperature equilibration (equal ion and electron temperatures) in the post-shock region. Broad component line widths indicate shock velocities of about 400 km/s. Combining the shock velocities with proper motions suggests the distance to the Cygnus Loop is about 890 pc, significantly greater than the generally accepted upper limit of 637 pc. This work is supported in part by the NSF REU and DOD ASSURE programs under NSF grant no. 1262851 and by the Smithsonian Institution. This work was partially supported by the grant HST-60-12885 to the Smithsonian Institution.
    The Astrophysical Journal 08/2014; 791(1). DOI:10.1088/0004-637X/791/1/30 · 6.28 Impact Factor
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    ABSTRACT: We obtained UV spectra of X-ray binary Scorpius X-1 in the 900-1200 A range with the Far Ultraviolet Spectroscopic Explorer over the full 0.79 day binary orbit. The strongest emission lines are the doublet of O VI at 1032,1038 A and the C III complex at 1175 A. The spectrum is affected by a multitude of narrow interstellar absorption lines, both atomic and molecular. Examination of line variability and Doppler tomograms suggests emission from both the neighborhood of the donor star and the accretion disk. Models of turbulence and Doppler broadened Keplerian disk lines Doppler shifted with the orbit of the neutron star added to narrow Gaussian emission lines with undetermined Doppler shift fit the data with consistent values of disk radius, inclination, and radial line brightness profile. The Doppler shift of the narrow component with the orbit suggests an association with the donor star. We test our line models with previously analyzed near UV spectra obtained with the Hubble Space Telescope Goddard High Resolution Spectrograph and archival spectra obtained with the HST Cosmic Origins Spectrograph.
    The Astrophysical Journal 07/2014; 793(1). DOI:10.1088/0004-637X/793/1/59 · 6.28 Impact Factor
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    ABSTRACT: We report results from Spitzer observations of SNR 0509-68.7, also known as N103B, a young Type Ia supernova remnant in the Large Magellanic Cloud that shows interaction with a dense medium in its western hemisphere. Our images show that N103B has strong IR emission from warm dust in the post-shock environment. The post-shock gas density we derive, 45 cm$^{-3}$, is much higher than in other Type Ia remnants in the LMC, though a lack of spatial resolution may bias measurements towards regions of higher than average density. This density is similar to that in Kepler's SNR, a Type Ia interacting with a circumstellar medium. Optical images show H$\alpha$ emission along the entire periphery of the western portion of the shock, with [O III] and [S II] lines emitted from a few dense clumps of material where the shock has become radiative. The dust is silicate in nature, though standard silicate dust models fail to reproduce the "18 $\mu$m" silicate feature that peaks instead at 17.3 $\mu$m. We propose that the dense material is circumstellar material lost from the progenitor system, as with Kepler. If the CSM interpretation is correct, this remnant would become the second member, along with Kepler, of a class of Type Ia remnants characterized by interaction with a dense CSM hundreds of years post-explosion. A lack of N enhancement eliminates symbiotic AGB progenitors. The white dwarf companion must have been relatively unevolved at the time of the explosion.
    The Astrophysical Journal 06/2014; 790(2). DOI:10.1088/0004-637X/790/2/139 · 6.28 Impact Factor
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    ABSTRACT: Extreme-ultraviolet images of Comet Lovejoy (C/2011 W3) from the Atmospheric Imaging Assembly show striations related to the magnetic field structure in both open and closed magnetic regions. The brightness contrast implies coronal density contrasts of at least a factor of six between neighboring flux tubes over scales of a few thousand kilometers. These density structures imply variations in the Alfvén speed on a similar scale. They will drastically affect the propagation and dissipation of Alfvén waves, and that should be taken into account in models of coronal heating and solar wind acceleration. In each striation, the cometary emission moves along the magnetic field and broadens with time. The speed and the rate of broadening are related to the parallel and perpendicular components of the velocities of the cometary neutrals when they become ionized. We use a magnetohydrodynamic model of the coronal magnetic field and the theory of pickup ions to compare the measurements with theoretical predictions, in particular with the energy lost to Alfvén waves as the cometary ions isotropize.
    The Astrophysical Journal 06/2014; 788(2):152. DOI:10.1088/0004-637X/788/2/152 · 6.28 Impact Factor
  • Salvatore Mancuso, John C. Raymond
    SOLAR AND STELLAR FLARES 2014 OBSERVATIONS, SIMULATIONS AND SYNERGIES, Prague; 06/2014
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    ABSTRACT: EUV images of Comet Lovejoy (C/2011 W3) from the AIA show striations related to the magnetic field structure in both open and closed magnetic regions. The brightness contrast implies coronal density contrasts of at least a factor of 6 between neighboring flux tubes over scales of a few thousand km. These density structures imply variations in the Alfven speed on a similar scale. They will drastically affect the propagation and dissipation of Alfven waves, and that should be taken into account in models of coronal heating and solar wind acceleration. In each striation, the cometary emission moves along the magnetic field and broadens with time. The speed and the rate of broadening are related to the parallel and perpendicular components of the velocities of the cometary neutrals when they become ionized. We use an MHD model of the coronal magnetic field and the theory of pickup ions to compare the measurements with theoretical predictions, in particular with the energy lost to Alfven waves as the cometary ions isotropize.
  • [Show abstract] [Hide abstract]
    ABSTRACT: EUV images of Comet Lovejoy (C/2011 W3) from the AIA show striations related to the magnetic field structure in both open and closed magnetic regions. The brightness contrast implies coronal density contrasts of at least a factor of 6 between neighboring flux tubes over scales of a few thousand km. These density structures imply variations in the Alfven speed on a similar scale. They will drastically affect the propagation and dissipation of Alfven waves, and that should be taken into account in models of coronal heating and solar wind acceleration. In each striation, the cometary emission moves along the magnetic field and broadens with time. The speed and the rate of broadening are related to the parallel and perpendicular components of the velocities of the cometary neutrals when they become ionized. We use an MHD model of the coronal magnetic field and the theory of pickup ions to compare the measurements with theoretical predictions, in particular with the energy lost to Alfven waves as the cometary ions isotropize.
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    ABSTRACT: We report on spectra of two positions in the XA region of the Cygnus Loop supernova remnant obtained with the InfraRed Spectrograph on the Spitzer Space Telescope. The spectra span the 10-35 micron wavelength range, which contains a number of collisionally excited forbidden lines. These data are supplemented by optical spectra obtained at the Whipple Observatory and an archival UV spectrum from the International Ultraviolet Explorer. Coverage from the UV through the IR provides tests of shock wave models and tight constraints on model parameters. Only lines from high ionization species are detected in the spectrum of a filament on the edge of the remnant. The filament traces a 180 km/s shock that has just begun to cool, and the oxygen to neon abundance ratio lies in the normal range found for Galactic H II regions. Lines from both high and low ionization species are detected in the spectrum of the cusp of a shock-cloud interaction, which lies within the remnant boundary. The spectrum of the cusp region is matched by a shock of about 150 km/s that has cooled and begun to recombine. The post-shock region has a swept-up column density of about 1.3E18 cm^-2, and the gas has reached a temperature of 7000 to 8000 K. The spectrum of the Cusp indicates that roughly half of the refractory silicon and iron atoms have been liberated from the grains. Dust emission is not detected at either position.
    The Astrophysical Journal 03/2014; 787(1). DOI:10.1088/0004-637X/787/1/3 · 6.28 Impact Factor
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    ABSTRACT: We have detected emission in C IV λλ1548,1551 from C atoms sputtered from dust in the gas behind a shock wave in the Cygnus Loop using COS on HST. The intensity agrees approximately with predictions from model calculations that match the Spitzer 24 μm and the X-ray intensity profiles. Thus these observations confirm the overall picture of dust destruction in SNR shocks and the sputtering rates assumed. However, the CIV intensity 10" behind the shock is too high compared to the intensities at the shock and 25" behind it. Projection effects and a complex geometry are probably responsible for the discrepancy.
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    ABSTRACT: The Cygnus Loop supernova remnant serves as an excellent laboratory for the study of radiative and non-radiative shocks with speeds in the 150-450 km s-1 range. We present results on shock-excited emission and dust destruction based on Spitzer Space Telescope observations of two well-studied regions in the remnant, (i) a non-radiative shock filament along the NE limb, and (ii) the XA region, characterized by emission from bright radiative shocks.
    Proceedings of the International Astronomical Union 12/2013; 9(S296). DOI:10.1017/S174392131300940X
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    ABSTRACT: The magnetic field plays a pivotal role in many fields of Astrophysics. This is especially true for the physics of the solar atmosphere. Measuring the magnetic field in the upper solar atmosphere is crucial to understand the nature of the underlying physical processes that drive the violent dynamics of the solar corona—that can also affect life on Earth. SolmeX, a fully equipped solar space observatory for remote-sensing observations, will provide the first comprehensive measurements of the strength and direction of the magnetic field in the upper solar atmosphere. The mission consists of two spacecraft, one carrying the instruments, and another one in formation flight at a distance of about 200 m carrying the occulter to provide an artificial total solar eclipse. This will ensure high-quality coronagraphic observations above the solar limb. SolmeX integrates two spectro-polarimetric coronagraphs for off-limb observations, one in the EUV and one in the IR, and three instruments for observations on the disk. The latter comprises one imaging polarimeter in the EUV for coronal studies, a spectro-polarimeter in the EUV to investigate the low corona, and an imaging spectro-polarimeter in the UV for chromospheric studies. SOHO and other existing missions have investigated the emission of the upper atmosphere in detail (not considering polarization), and as this will be the case also for missions planned for the near future. Therefore it is timely that SolmeX provides the final piece of the observational quest by measuring the magnetic field in the upper atmosphere through polarimetric observations.
    European Solar Physics Meeting (ESPM-13); 12/2013

Publication Stats

10k Citations
2,066.16 Total Impact Points

Institutions

  • 1979–2015
    • Harvard-Smithsonian Center for Astrophysics
      • Smithsonian Astrophysical Observatory
      Cambridge, Massachusetts, United States
  • 2014
    • Towson University
      • Department of Physics, Astronomy and Geosciences
      Maryland, United States
  • 2013
    • Max Planck Institute for Solar System Research
      Göttingen, Lower Saxony, Germany
  • 2012
    • Elmhurst College
      Elmhurst, Illinois, United States
  • 2011
    • Northeastern University
      • Department of Physics
      Boston, Massachusetts, United States
    • China University of Petroleum
      Ch’ang-p’ing-ch’ü, Beijing, China
  • 2010
    • United States Naval Observatory
      Washington, Maine, United States
  • 2008
    • Pennsylvania State University
      • Department of Astronomy and Astrophysics
      University Park, Maryland, United States
    • University of Groningen
      • Kapteyn Astronomical Institute
      Groningen, Groningen, Netherlands
  • 2006
    • University of Amsterdam
      • Astronomical Institute Anton Pannekoek
      Amsterdamo, North Holland, Netherlands
  • 2004–2006
    • Idenix Pharmaceuticals, Inc.
      Cambridge, Massachusetts, United States
    • Drew University
      Мадисон, New Jersey, United States
  • 2003
    • Massachusetts Institute of Technology
      Cambridge, Massachusetts, United States
  • 2001
    • Space Telescope Science Institute
      Baltimore, Maryland, United States
    • University of Central Lancashire
      Preston, England, United Kingdom
  • 2000
    • Rutgers, The State University of New Jersey
      • Department Physics and Astronomy
      New Brunswick, New Jersey, United States
  • 1997–2000
    • University of Florence
      Florens, Tuscany, Italy
    • University of Oregon
      Eugene, Oregon, United States
  • 1999
    • University of Maryland, College Park
      • Department of Astronomy
      Maryland, United States
  • 1988–1999
    • Johns Hopkins University
      • Department of Physics and Astronomy
      Baltimore, MD, United States
    • European Southern Observatory
      Arching, Bavaria, Germany
  • 1979–1996
    • Harvard University
      Cambridge, Massachusetts, United States
  • 1995
    • Utrecht University
      Utrecht, Utrecht, Netherlands
    • University of California, Berkeley
      Berkeley, California, United States
  • 1993–1994
    • University of Massachusetts Amherst
      Amherst Center, Massachusetts, United States
    • University of North Carolina at Chapel Hill
      North Carolina, United States
    • University of Colorado
      Denver, Colorado, United States
    • Lawrence Livermore National Laboratory
      Livermore, California, United States
  • 1992
    • University of Washington Seattle
      Seattle, Washington, United States
  • 1991
    • Columbia University
      New York, New York, United States