P. C. Myers

McMaster University, Hamilton, Ontario, Canada

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Publications (392)1593.59 Total impact

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    ABSTRACT: We present Spitzer 3.6, 4.5, 5.8, 8.0, and 24 micron images of the Mon OB1 East giant molecular cloud, which contains the young star forming region NGC 2264, as well as more extended star formation. With Spitzer data and 2MASS photometry, we identify and classify young stellar objects (YSOs) with dusty circumstellar disks and/or envelopes in Mon OB1 East by their infrared-excess emission and study their distribution with respect to cloud material. We find a correlation between the local surface density of YSOs and column density of molecular gas as traced by dust extinction that is roughly described as a power law in these quantities. NGC 2264 follows a power law index of ~2.7, exhibiting a large YSO surface density for a given gas column density. Outside of NGC 2264 where the surface density of YSOs is lower, the power law is shallower and the region exhibits a larger gas column density for a YSO surface density, suggesting the star formation is more recent. In order to measure the fraction of cloud members with circumstellar disks/envelopes, we estimate the number of diskless pre-main sequence stars by statistical removal of background star detections. We find that the disk fraction of the NGC 2264 region is 45%, while the surrounding more distributed regions show a disk fraction of 19%. This may be explained by the presence an older, more dispersed population of stars. In total, the Spitzer observations provide evidence for heterogenous, non-coeval star formation throughout the Mon OB1 cloud.
    09/2014;
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    ABSTRACT: Stars form from the gravitational collapse of dense molecular cloud cores. In the protostellar phase, mass accretes from the core onto a protostar, likely through an accretion disk, and it is during this phase that the initial masses of stars and the initial conditions for planet formation are set. Over the past decade, new observational capabilities provided by the Spitzer Space Telescope and Herschel Space Observatory have enabled wide-field surveys of entire star-forming clouds with unprecedented sensitivity, resolution, and infrared wavelength coverage. We review resulting advances in the field, focusing both on the observations themselves and the constraints they place on theoretical models of star formation and protostellar evolution. We also emphasize open questions and outline new directions needed to further advance the field.
    01/2014;
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    Philip C. Myers
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    ABSTRACT: In an improved model of protostar mass functions (PMFs), protostars gain mass from isothermal cores in turbulent clumps. Their mass accretion rate is similar to Shu accretion at low mass, and to reduced Bondi accretion at high mass. Accretion durations follow a simple expression in which higher-mass protostars accrete for longer times. These times are set by ejections, stellar feedback, and gravitational competition, which terminate accretion and reduce its efficiency. The mass scale is the mass of a critically stable isothermal core. In steady state, the PMF approaches a power law at high mass due to competition between clump accretion and accretion stopping. The power law exponent is the ratio of the time scales of accretion and accretion stopping. The luminosity function (PLF) peaks near 1 L_Sun, due to inefficient accretion of core gas. Models fit observed PLFs in four large embedded clusters. These indicate that their underlying PMFs may be top-heavy compared to the IMF, depending on the model of protostar radius.
    The Astrophysical Journal 12/2013; 781(1). · 6.73 Impact Factor
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    ABSTRACT: First phases of the process of star formation are characterized by excess in infrared and high X-ray emission. With Spitzer and XMM-Newton we have surveyed the Orion A part relative to the filamentary cloud Lynds 1641 (L1641). Furthermore, an extended spectroscopic survey has been realized to better constraint the cluster membership of stars without IR excess. We find that:
    11/2013;
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    ABSTRACT: This paper presents the results of millimeter to sub-millimeter observations of CO, HCN, N2H+, and HCO+ lines in the dense molecular cloud L328, which harbors L328-IRS, a Very Low Luminosity Object (VeLLO). Our analysis of the line width finds that 13CO and N2H+ lines are broadened right over the smallest sub-core S2 where L328-IRS is located, while they are significantly narrower in other regions of L328. Thus, L328-IRS has a direct association with the sub-core. CO observations show a bipolar outflow from this VeLLO with an extent of ~0.08 pc. The outflow momentum flux and efficiency are much less than those of low-mass protostars. The most likely mass accretion rate (~3.6 × 10–7 M ☉ yr–1) inferred from the analysis of the CO outflow is an order of magnitude smaller than the canonical value for a protostar. If the main accretion lasts during the typical Class 0 period of a protostar, L328-IRS will accrete the mass of a brown dwarf, but not that of a star. Given that its envelope mass is small (~0.09 M ☉) and 100% star formation rate is unlikely, we suggest that L328-IRS is likely a proto-brown dwarf. Inward motions are found in global scale in the L328 cloud and its sub-cores with a typical infall speed found in starless cores. L328 is found to be fairly well isolated from other nearby clouds and seems to be forming three sub-cores simultaneously through a gravitational fragmentation process. Altogether, these all leave L328-IRS as the best example supporting the idea that a brown dwarf forms like a normal star.
    The Astrophysical Journal 11/2013; 777(1):50-. · 6.73 Impact Factor
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    ABSTRACT: We have detected bright HC7N J = 21-20 emission toward multiple locations in the Serpens South cluster-forming region using the K-Band Focal Plane Array at the Robert C. Byrd Green Bank Telescope. HC7N is seen primarily toward cold filamentary structures that have yet to form stars, largely avoiding the dense gas associated with small protostellar groups and the main central cluster of Serpens South. Where detected, the HC7N abundances are similar to those found in other nearby star forming regions. Toward some HC7N `clumps', we find consistent variations in the line centroids relative to NH3 (1,1) emission, as well as systematic increases in the HC7N non-thermal line widths, which we argue reveal infall motions onto dense filaments within Serpens South with minimum mass accretion rates of M ~ 2-5 M_sun Myr^-1. The relative abundance of NH3 to HC7N suggests that the HC7N is tracing gas that has been at densities n ~ 10^4 cm^-3, for timescales t < 1-2 x 10^5 yr. Since HC7N emission peaks are rarely co-located with those of either NH3 or continuum, it is likely that Serpens South is not particularly remarkable in its abundance of HC7N, but instead the serendipitous mapping of HC7N simultaneously with NH3 has allowed us to detect HC7N at low abundances in regions where it otherwise may not have been looked for. This result extends the known star-forming regions containing significant HC7N emission from typically quiescent regions, like the Taurus molecular cloud, to more complex, active environments.
    Monthly Notices of the Royal Astronomical Society 09/2013; 436(2). · 5.52 Impact Factor
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    The Astrophysical Journal 07/2013; 773(1):80. · 6.73 Impact Factor
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    ABSTRACT: Cyanopolyynes are carbon-chain molecules of the form HC2n+1N. At higher n, these molecules are among the longest and heaviest molecules found in the interstellar medium, and to date have been primarily seen toward several nearby, low-mass star forming regions, and in the atmospheres of AGB stars. We have detected bright HC7N J = 21-20 emission toward multiple locations in the Serpens South cluster-forming region using the K-Band Focal Plane Array at the Robert C. Byrd Green Bank Telescope. HC7N is seen primarily toward cold filamentary structures that have yet to form stars, largely avoiding the dense gas associated with small protostellar groups and the main central cluster of Serpens South. Toward some HC7N 'clumps', we find consistent variations in the line centroids relative to NH3 (1,1) emission, as well as systematic increases in the HC7N non-thermal line widths, which we argue reveal infall motions onto dense filaments within Serpens South with mass accretion rates of M ~ 2 - 5 M⊙ Myr-1. This result extends the known star-forming regions containing significant HC7N emission from typically quiescent regions, like the Taurus molecular cloud, to more complex, active environments.
    07/2013;
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    ABSTRACT: We present an XMM-Newton survey of the part of the Orion A cloud south of the Orion Nebula. This survey includes the Lynds 1641 (L1641) dark cloud, a region of the Orion A cloud with very few massive stars and hence a relatively low ambient UV flux, and the region around the O9 III star ι Orionis. In addition to proprietary data, we used archival XMM data of the Orion Nebula Cluster (ONC) to extend our analysis to a major fraction of the Orion A cloud. We have detected 1060 X-ray sources in L1641 and the ι Ori region. About 94% of the sources have Two Micron All Sky Survey and Spitzer counterparts, 204 and 23 being Class II and Class I or protostar objects, respectively. In addition, we have identified 489 X-ray sources as counterparts to Class III candidates, given they are bright in X-rays and appear as normal photospheres at mid-IR wavelengths. The remaining 205 X-ray sources are likely distant active galactic nuclei or other galactic sources not related to Orion A. We find that Class III candidates appear more concentrated in two main clusters in L1641. The first cluster of Class III stars is found toward the northern part of L1641, concentrated around ι Ori. The stars in this cluster are more evolved than those in the Orion Nebula. We estimate a distance of 300-320 pc for this cluster showing that it is in the foreground of the Orion A cloud. Another cluster rich in Class III stars is located in L1641 South and appears to be a slightly older cluster embedded in the Orion A cloud. Furthermore, other evolved Class III stars are found north of the ONC toward NGC 1977.
    The Astrophysical Journal 04/2013; 768(2):99. · 6.73 Impact Factor
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    ABSTRACT: One puzzle in understanding how stars form in clusters is the source of mass -- is all of the mass in place before the first stars are born, or is there an extended period when the cluster accretes material which can continuously fuel the star formation process? We use a multi-line spectral survey of the southern filament associated with the Serpens South embedded cluster-forming region in order to determine if mass is accreting from the filament onto the cluster, and whether the accretion rate is significant. Our analysis suggests that material is flowing along the filament's long axis at a rate of ~30Msol/Myr (inferred from the N2H+ velocity gradient along the filament), and radially contracting onto the filament at ~130Msol/Myr (inferred from HNC self-absorption). These accretion rates are sufficient to supply mass to the central cluster at a similar rate to the current star formation rate in the cluster. Filamentary accretion flows may therefore be very important in the ongoing evolution of this cluster.
    The Astrophysical Journal 01/2013; 766(2). · 6.73 Impact Factor
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    Philip C. Myers
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    ABSTRACT: Mass functions of starless dense cores (CMFs) may arise from contraction and dispersal of core-forming filaments. In an illustrative model, a filament contracts radially by self-gravity, increasing the mass of its cores. During this contraction, FUV photoevaporation and ablation by shocks and winds disperse filament gas and limit core growth. The stopping times of core growth are described by a waiting-time distribution. The initial filament column density profile and the resulting CMF each match recent Herschel observations in detail. Then low-mass cores have short growth ages and arise from the innermost filament gas, while massive cores have long growth ages and draw from more extended filament gas. The model fits the initial density profile and CMF best for mean core density 2 10^4 cm^-3 and filament dispersal time scale 0.5 Myr. Then the typical core mass, radius, mean column density, and contraction speed are respectively 0.8 solar masses, 0.06 pc, 6 10^21 cm^-2, and 0.07 km s^-1, also in accord with observed values.
    The Astrophysical Journal 01/2013; 764(2). · 6.73 Impact Factor
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    ABSTRACT: Most stars in the Galaxy, if not all, form within cold, dense, and filamentary molecular clouds. Many of these clouds are seen in extinction against the bright mid-infrared Galactic background and understanding star formation requires probing the physics within these infrared dark clouds (IRDCs). To tackle this problem, we are currently carrying out a systematic study of the principal characteristics of ~150 IRDCs in the Galaxy using archival data from Herschel and Spitzer satellites, as well as molecular line observations. Data from the Herschel satellite allows us to observe the large scale structure of the star forming infrared dark filaments, in emission, and with unprecedent sensitivity. This study provides the most complete dataset to date where to study the molecular and dusty structure associated to the earliest stages of clustered star formation. Deriving column densities, masses, temperatures, and the young-stellar content of a large sample of these filamentary clouds is a first crucial step to assess: i) the relevance that filamentary geometry has on star formation, determining the initial mass and luminosity distributions, ii) how pertinent are turbulence, large-scale shocks, magnetic fields, and/or tidal forces in shaping this particular morphology. We present the first results of our study, focusing on statistically significant determination of the main physical parameters of IRDCs in the Galaxy, and consistent theoretical interpretations.
    01/2013;
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    ABSTRACT: The star formation characteristics of dense massive clusters are the subject of several observational and theoretical efforts, given their importance in unveiling fundamental aspects such as the slope and upper cutoff of the Initial Mass Function (IMF),and the nature of massive star formation. The degree of success of such methods is limited by the fact that most of the studied clusters are spatially blended or unresolved by current facilities at mid- and far-infrared wavelengths. We present a Bayesian method to study these unresolved clusters using a SED fitting approach that uses a library of pre-computed SED models and archival data from multi-wavelength surveys, making special emphasis on the different beam sizes of the observations. We apply the method to the star-forming complex W43, located in the inner spiral arm of the galaxy and analyze the results in terms of the Luminosity Function (LF), and census of massive stars in this region. We then compare our results to theoretical predictions on the shape of the IMF, and its relation with the young stellar objects LM.
    01/2013;
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    ABSTRACT: We present a survey of the Orion A and B molecular clouds undertaken with the IRAC and MIPS instruments on board Spitzer. In total, five distinct fields were mapped, covering 9 deg{sup 2} in five mid-IR bands spanning 3-24 {mu}m. The survey includes the Orion Nebula Cluster, the Lynds 1641, 1630, and 1622 dark clouds, and the NGC 2023, 2024, 2068, and 2071 nebulae. These data are merged with the Two Micron All Sky Survey point source catalog to generate a catalog of eight-band photometry. We identify 3479 dusty young stellar objects (YSOs) in the Orion molecular clouds by searching for point sources with mid-IR colors indicative of reprocessed light from dusty disks or infalling envelopes. The YSOs are subsequently classified on the basis of their mid-IR colors and their spatial distributions are presented. We classify 2991 of the YSOs as pre-main-sequence stars with disks and 488 as likely protostars. Most of the sources were observed with IRAC in two to three epochs over six months; we search for variability between the epochs by looking for correlated variability in the 3.6 and 4.5 {mu}m bands. We find that 50% of the dusty YSOs show variability. The variations are typically small ({approx}0.2 mag) with the protostars showing a higher incidence of variability and larger variations. The observed correlations between the 3.6, 4.5, 5.8, and 8 {mu}m variability suggests that we are observing variations in the heating of the inner disk due to changes in the accretion luminosity or rotating accretion hot spots.
    The Astronomical Journal 12/2012; 144:192. · 4.97 Impact Factor
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    ABSTRACT: We present a survey of the Orion A and B molecular clouds undertaken with the IRAC and MIPS instruments onboard Spitzer. In total, five distinct fields were mapped covering 9 sq. degrees in five mid-IR bands spanning 3-24 microns. The survey includes the Orion Nebula Cluster, the Lynds 1641, 1630 and 1622 dark clouds, and the NGC 2023, 2024, 2068 and 2071 nebulae. These data are merged with the 2MASS point source catalog to generate a catalog of eight band photometry. We identify 3479 dusty young stellar objects (YSOs) in the Orion molecular clouds by searching for point sources with mid-IR colors indicative of reprocessed light from dusty disks or infalling envelopes. The YSOs are subsequently classified on the basis of their mid-IR colors and their spatial distributions are presented. We classify 2991 of the YSOs as pre-main sequence stars with disks and 488 as likely protostars. Most of the sources were observed with IRAC in 2-3 epochs over 6 months; we search for variability between the epochs by looking for correlated variability in the 3.6 and 4.5 micron bands. We find that 50% of the dusty YSOs show variability. The variations are typically small (0.2 mag.) with the protostars showing a higher incidence of variability and larger variations. The observed correlations between the 3.6, 4.5, 5.8 and 8 micron variability suggests that we are observing variations in the heating of the inner disk due to changes in the accretion luminosity or rotating accretion hot spots.
    09/2012;
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    ABSTRACT: We present Spitzer IRAC and MIPS observations of the star-forming region containing intermediate-mass young stellar object (YSO) AFGL 490. We supplement these data with near-IR 2MASS photometry and with deep SQIID observations off the central high extinction region. We have more than doubled the known membership of this region to 57 Class I and 303 Class II YSOs via the combined 1-24 um photometric catalog derived from these data. We construct and analyze the minimum spanning tree of their projected positions, isolating one locally over-dense cluster core containing 219 YSOs (60.8% of the region's members). We find this cluster core to be larger yet less dense than similarly analyzed clusters. Although the structure of this cluster core appears irregular, we demonstrate that the parsec-scale surface densities of both YSOs and gas are correlated with a power law slope of 2.8, as found for other similarly analyzed nearby molecular clouds. We also explore the mass segregation implications of AFGL 490's offset from the center of its core, finding that it has no apparent preferential central position relative to the low-mass members.
    The Astrophysical Journal 04/2012; 752(2). · 6.73 Impact Factor
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    ABSTRACT: We identify protostars in Spitzer surveys of nine star-forming molecular clouds within 1 kpc: Serpens, Perseus, Ophiuchus, Chamaeleon, Lupus, Taurus, Orion, Cep OB3, and Mon R2, which combined host over 700 protostar candidates. Our diverse cloud sample allows us to compare protostar luminosity functions in these varied environments. We combine photometry from 2MASS J, H, and Ks bands and Spitzer IRAC and MIPS 24 micron bands to create 1 - 24 micron spectral energy distributions (SEDs). Using protostars from the c2d survey with well-determined bolometric luminosities (Lbol), we derive a relationship between Lbol, L_MIR (integrated from 1 - 24 microns), and SED slope. Estimations of Lbol for protostar candidates are combined to create luminosity functions for each cloud. Contamination due to edge-on disks, reddened Class II sources, and galaxies is estimated and removed from the luminosity functions. We find that luminosity functions for high mass star forming clouds peak near 1 Lsun and show a tail extending toward luminosities above 100 Lsun. The luminosity functions of the low mass star forming clouds do not exhibit a common peak, however the combined luminosity function of these regions peaks below 1 Lsun. Finally, we examine the luminosity functions as a function of the local surface density of YSOs. In the Orion molecular cloud, we find a significant difference between the luminosity functions of protostars in regions of high and low stellar density, the former of which is biased toward more luminous sources. This may be the result of primordial mass segregation, although this interpretation is not unique. We compare our luminosity functions to those predicted by models and find that our observed luminosity functions are best matched by models which invoke competitive accretion, although we do not find strong agreement of the high mass star forming clouds with any of the models.
    The Astronomical Journal 04/2012; 144(2). · 4.97 Impact Factor
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    Philip C. Myers
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    ABSTRACT: A model of protostar mass and luminosity evolution in clusters gives new estimates of cluster age, protostar birthrate, accretion rate and mean accretion time. The model assumes constant protostar birthrate, core-clump accretion, and equally likely accretion stopping. Its parameters are set to reproduce the initial mass function, and to match protostar luminosity distributions in nearby star-forming regions. It obtains cluster ages and birthrates from the observed numbers of protostars and pre-main sequence (PMS) stars, and from the modal value of the protostar luminosity. In 31 embedded clusters and complexes the global cluster age is 1-3 Myr, matching available estimates based on optical spectroscopy and evolutionary tracks. This method of age estimation is simpler than optical spectroscopy, and is more useful for young embedded clusters where optical spectrocopy is not possible. In the youngest clusters, the protostar fraction decreases outward from the densest gas, indicating that the local star-forming age increases outward from a few 0.1 Myr in small protostar-dominated zones to a few Myr in large PMS-dominated zones.
    The Astrophysical Journal 04/2012; 752(1). · 6.73 Impact Factor
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    ABSTRACT: We present findings from an ammonia molecular line study of the Serpens South star-forming cluster, within the Serpens molecular cloud, performed using the K-band Focal Plane Array at the GBT. We have mapped a 40' by 30' area in the NH3 (1,1), (2,2) and (3,3) inversion transitions, as well as in HC7N 21-20. The NH3 (1,1) and (2,2) emission was used to derive kinetic temperatures across the region, which range from 7.5 K to 17.5 K. Despite a 10 K range of temperature for NH3, all of the HC7N was found within the 2 K range of 9.5 K and 11.5 K. We find the distribution of both molecules throughout the cloud is different, although the line-of-sight velocities agree well. The HC7N emission peaks do not coincide with peaks in NH3 emission, and we additionally find HC7N in areas with very low NH3 abundance. Where present, the abundance of HC7N is broadly consistent with previous studies of other carbon chain producing regions. Finally, we find self-absorbed HC7N emission towards several dense cores, with line profiles suggesting both infall and outflow motions. We fit the line profiles to determine infall and outflow velocities for these regions.
    01/2012;
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    ABSTRACT: We present high spatial (<300 AU) and spectral (0.07 km/s) resolution Submillimeter Array observations of the dense starless cluster core Oph A-N6, in the 1 mm dust continuum and the 3-2 line of N2H+ and N2D+. The dust continuum observations reveal a compact source not seen in single-dish observations, of size ~1000 AU and mass 0.005-0.01 M\odot. The combined line and single-dish observations reveal a core of size 3000 \times 1400 AU elongated in a NW-SE direction, with almost no variation in either line width or line center velocity across the map, and very small non-thermal motions. The deuterium fraction has a peak value of ~0.15 and is >0.05 over much of the core. The N2H+ column density profile across the major axis of Oph A-N6 is well represented by an isothermal cylinder, with temperature 20 K, peak density 7.1 \times 10^6 cm^{-3}, and N2H+ abundance 2.7 \times 10^{-10}. The mass of Oph A-N6 is estimated to be 0.29 M\odot, compared to a value of 0.18 M\odot from the isothermal cylinder analysis, and 0.63 M\odot for the critical mass for fragmentation of an isothermal cylinder. Compared to isolated low-mass cores, Oph A-N6 shows similar narrow line widths and small velocity variation, with a deuterium fraction similar to "evolved" dense cores. It is significantly smaller than isolated cores, with larger peak column and volume density. The available evidence suggests Oph A-N6 has formed through the fragmentation of the Oph A filament and is the precursor to a low-mass star. The dust continuum emission suggests it may already have begun to form a star.
    The Astrophysical Journal 11/2011; 745(2). · 6.73 Impact Factor

Publication Stats

9k Citations
1,593.59 Total Impact Points

Institutions

  • 2013
    • McMaster University
      Hamilton, Ontario, Canada
  • 1987–2013
    • Harvard-Smithsonian Center for Astrophysics
      • Smithsonian Astrophysical Observatory
      Cambridge, Massachusetts, United States
  • 2009–2010
    • University of Toledo
      • Department of Physics and Astronomy
      Toledo, Ohio, United States
  • 2004–2010
    • University of Michigan
      • Department of Astronomy
      Ann Arbor, Michigan, United States
    • National Institute of Astrophysics
      Roma, Latium, Italy
  • 2006–2009
    • National Research Council Canada
      Ottawa, Ontario, Canada
    • University of Maryland, College Park
      • Department of Astronomy
      Maryland, United States
  • 2007
    • Seoul National University
      • Department of Physics and Astronomy
      Sŏul, Seoul, South Korea
  • 1997–2007
    • Harvard University
      • Department of Astronomy
      Cambridge, Massachusetts, United States
  • 2005
    • Xavier University
      • Department of Physics
      Cincinnati, Ohio, United States
  • 2002
    • University of Rochester
      • Department of Physics and Astronomy
      Rochester, NY, United States
  • 2001
    • University of California, Berkeley
      • Radio Astronomy Laboratory
      Berkeley, California, United States
  • 1993
    • CFA Institute
      Charlottesville, Virginia, United States
    • University of Kentucky
      • Department of Physics & Astronomy
      Lexington, KY, United States
  • 1978–1993
    • University of Massachusetts Amherst
      • Department of Astronomy
      Amherst Center, Massachusetts, United States
  • 1975–1982
    • Massachusetts Institute of Technology
      Cambridge, Massachusetts, United States