P. C. Myers

University of Michigan, Ann Arbor, Michigan, United States

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Publications (260)744.64 Total impact

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    ABSTRACT: We analyze the spatial distribution of dusty young stellar objects (YSOs) identified in the Spitzer Survey of the Orion Molecular clouds, augmenting these data with Chandra X-ray observations to correct for incompleteness in dense clustered regions. We also devise a scheme to correct for spatially varying incompleteness when X-ray data are not available. The local surface densities of the YSOs range from 1 pc$^{-2}$ to over 10,000 pc$^{-2}$, with protostars tending to be in higher density regions. This range of densities is similar to other surveyed molecular clouds with clusters, but broader than clouds without clusters. By identifying clusters and groups as continuous regions with surface densities $\ge10$ pc$^{-2}$, we find that 59% of the YSOs are in the largest cluster, the Orion Nebular Cluster (ONC), while 13% of the YSOs are found in a distributed population. A lower fraction of protostars in the distributed population is evidence that it is somewhat older than the groups and clusters. An examination of the structural properties of the clusters and groups show that the peak surface densities of the clusters increase approximately linearly with the number of members. Furthermore, all clusters with more than 70 members exhibit asymmetric and/or highly elongated structures. The ONC becomes azimuthally symmetric in the inner 0.1 pc, suggesting that the cluster is only $\sim 2$ Myr in age. We find the star formation efficiency (SFE) of the Orion B cloud is unusually low, and that the SFEs of individual groups and clusters are an order of magnitude higher than those of the clouds. Finally, we discuss the relationship between the young low mass stars in the Orion clouds and the Orion OB 1 association, and we determine upper limits to the fraction of disks that may be affected by UV radiation from OB stars or by dynamical interactions in dense, clustered regions.
    Full-text · Article · Nov 2015 · The Astronomical Journal
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    ABSTRACT: We present mid-IR (19 - 37 microns) imaging observations of S106 from SOFIA/FORCAST, complemented with IR observations from Spitzer/IRAC (3.6 - 8.0 microns), IRTF/MIRLIN (11.3 and 12.5 microns), and Herschel/PACS (70 and 160 microns). We use these observations, observations in the literature, and radiation transfer modeling to study the heating and composition of the warm (~ 100 K) dust in the region. The dust is heated radiatively by the source S106 IR, with little contributions from grain-electron collisions and Ly-alpha radiation. The dust luminosity is >~ (9.02 +/- 1.01) x 10^4 L_sun, consistent with heating by a mid- to late-type O star. We find a temperature gradient (~ 75 - 107 K) in the lobes, which is consistent with a dusty equatorial geometry around S106 IR. Furthermore, the SOFIA observations resolve several cool (~ 65 - 70 K) lanes and pockets of warmer (~ 75 - 90 K) dust in the ionization shadow, indicating that the environment is fragmented. We model the dust mass as a composition of amorphous silicates, amorphous carbon, big grains, very small grains, and PAHs. We present the relative abundances of each grain component for several locations in S106.
    No preview · Article · Oct 2015 · The Astrophysical Journal
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    ABSTRACT: Column density (N) PDFs serve as a powerful tool to characterize the physical processes that influence the structure of molecular clouds. Star-forming clouds can best be characterized by lognormal PDFs for the lower N range and a power-law tail for higher N, commonly attributed to turbulence and self-gravity and/or pressure, respectively. We report here on PDFs obtained from observations of 12CO, 13CO, C18O, CS, and N2H+ in the Cygnus X North region and compare to a PDF derived from dust observations with the Herschel satellite. The PDF of 12CO is lognormal for Av~1-30, but is cut for higher Av due to optical depth effects. The PDFs of C18O and 13CO are mostly lognormal up for Av~1-15, followed by excess up to Av~40. Above that value, all CO PDFs drop, most likely due to depletion. The high density tracers CS and N2H+ exhibit only a power law distribution between Av~15 and 400, respectively. The PDF from dust is lognormal for Av~2-15 and has a power-law tail up to Av~500. Absolute values for the molecular line column densities are, however, rather uncertain due to abundance and excitation temperature variations. Taken the dust PDF face value, we 'calibrate' the molecular line PDF of CS to the one of the dust and determined an abundance [CS]/[H2] of 10^-9. The slopes of the power-law tails of the CS, N2H+, and dust PDFs are -1.6, -1.4, and -1.9, respectively, and are thus consistent with free-fall collapse of filaments and clumps. A quasi static configuration of filaments and clumps can possibly also account for the observed N-pdfs, as long as they have a sufficiently condensed density structure and external ram pressure by gas accretion is provided. The somehow flatter slopes of N2H+ and CS can reflect an abundance change and/or subthermal excitation at low column densities.
    Full-text · Article · Sep 2015 · Astronomy and Astrophysics
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    ABSTRACT: We have detected bright HC7N J = 21 − 20 emission towards 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 towards 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. Towards 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 on to dense filaments within Serpens South with minimum mass accretion rates of M ∼ 2–5 M⊙ Myr−1. The relative abundance of NH3 to HC7N suggests that the HC7N is tracing gas that has been at densities n ∼ 104 cm−3 for time-scales t ≲ 1–2 × 105 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.
    Full-text · Article · Sep 2013 · Monthly Notices of the Royal Astronomical Society
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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.
    No preview · Article · Jan 2013
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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.
    No preview · Article · Jan 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.
    No preview · Article · Dec 2012 · The Astronomical Journal
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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.
    Full-text · Article · Sep 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.
    Full-text · Article · Apr 2012 · The Astrophysical Journal
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    ABSTRACT: We identify protostars in Spitzer surveys of nine star-forming (SF) molecular clouds within 1 kpc: Serpens, Perseus, Ophiuchus, Chamaeleon, Lupus, Taurus, Orion, Cep OB3, and Mon R2, which combined host over 700 protostar candidates. These clouds encompass a variety of SF environments, including both low-mass and high-mass SF regions, as well as dense clusters and regions of sparsely distributed star formation. Our diverse cloud sample allows us to compare protostar luminosity functions in these varied environments. We combine near- and mid-infrared photometry from the Two Micron All Sky Survey and Spitzer to create 1-24 μm spectral energy distributions (SEDs). Using protostars from the c2d survey with well-determined bolometric luminosities, we derive a relationship between bolometric luminosity, mid-IR luminosity (integrated from 1-24 μm), and SED slope. Estimations of the bolometric luminosities 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 SF clouds (Orion, Mon R2, and Cep OB3) peak near 1 L ☉ and show a tail extending toward luminosities above 100 L ☉. The luminosity functions of the low-mass SF clouds (Serpens, Perseus, Ophiuchus, Taurus, Lupus, and Chamaeleon) do not exhibit a common peak, however the combined luminosity function of these regions peaks below 1 L ☉. Finally, we examine the luminosity functions as a function of the local surface density of young stellar objects. In the Orion molecular clouds, 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 that invoke competitive accretion, although we do not find strong agreement between the high-mass SF clouds and any of the models.
    Full-text · Article · Apr 2012 · The Astronomical Journal
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    ABSTRACT: We report the discovery and characterization of a power law correlation between the local surface densities of Spitzer-identified, dusty young stellar objects and the column density of gas (as traced by near-IR extinction) in eight molecular clouds within 1 kpc and with 100 or more known YSOs. This correlation, which appears in data smoothed over size scales of ~1 pc, varies in quality from cloud to cloud; those clouds with tight correlations, MonR2 and Ophiuchus, are fit with power laws of slope 2.67 and 1.87, respectively. The spread in the correlation is attributed primarily to local gas disruption by stars that formed there or to the presence of very young sub-regions at the onset of star formation. We explore the ratio of the number of Class II to Class I sources, a proxy for the star formation age of a region, as a function of gas column density; this analysis reveals a declining Class II to Class I ratio with increasing column density. We show that the observed star-gas correlation is consistent with a star formation law where the star formation rate per area varies with the gas column density squared. We also propose a simple picture of thermal fragmentation of dense gas in an isothermal, self-gravitating layer as an explanation for the power law. Finally, we briefly compare the star gas correlation and its implied star formation law with other recent proposed of star formation laws at similar and larger size scales from nearby star forming regions.
    Full-text · Article · Oct 2011 · The Astrophysical Journal
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    ABSTRACT: We report the detection of a compact (of order 5 arcsec; about 1800 AU projected size) CO outflow from L1148-IRS. This confirms that this Spitzer source is physically associated with the nearby (about 325 pc) L1148 dense core. Radiative transfer modeling suggests an internal luminosity of 0.08 to 0.13 L_sun. This validates L1148-IRS as a Very Low Luminosity Object (VeLLO; L < 0.1 L_sun). The L1148 dense core has unusually low densities and column densities for a star-forming core. It is difficult to understand how L1148-IRS might have formed under these conditions. Independent of the exact final mass of this VeLLO (which is likely < 0.24 M_sun), L1148-IRS and similar VeLLOs might hold some clues about the isolated formation of brown dwarfs.
    Full-text · Article · Jun 2011 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: Mid-infrared observations, obtained by the Cores-to-Disks Spitzer Legacy team, revealed several protostars and young stellar objects in the core DC274.2-0.4. With evidence for a nearby distance (130-200 pc), one of these sources qualifies as a Very Low Luminosity Object (VeLLO) and appears to be among the lowest luminosity embedded sources known, with an internal luminosity of 0.01-0.03 Lsun. If this source is indeed a VeLLO, then its discovery is exciting as it may represent the earliest stage protostar or proto-brown dwarf yet detected. However, the internal luminosity critically depends on the distance and some evidence suggests that DC274.2-0.4 is more distant (>400 pc), making this core a more typical site of low-mass star formation. We review the different distance determinations, and present new optical spectra obtained with Magellan/IMACS to help settle the ambiguity. Finally, we discuss the star-forming environment of DC274.2-0.4 in light of additional infrared observations from CTIO/ISPI, Magellan/MMIRS, and HST/WFC3.
    No preview · Article · Jan 2011
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    ABSTRACT: A fundamental problem in astrophysics is how do stars form in clusters of 100 or more, in a region of size 1 pc, in a time of 1 Myr, with a mass distribution following the IMF. Stars form within dense gas cores, and recent work suggests the initial conditions provided by those cores are a key to the problem. We have recently mapped the distribution of dense gas in 5 nearby cluster forming regions via N2H+ 1-0 observations with Mopra, to determine their physical state. Here we propose to measure the kinetic temperature of the cores identified in the N2H+ map, through mapping observations of the (1,1) and (2,2) lines of ammonia, to determine whether thermal or non-thermal motions dominate, and whether they are bound and so likely to form new stars.
    No preview · Article · Oct 2010
  • C. W. Lee · P. C. Myers · R. Plume
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    ABSTRACT: We observed both the CS(3-2) and the DCO+(2-1) transiti the NRAO 12m telescope in two seasons, 1998 June and October. (1 data file).
    No preview · Article · Sep 2010
  • Y.-S. Park · C. W. Lee · P. C. Myers
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    ABSTRACT: We carried out observations in 12CO and 13CO J= toward a fraction of starless cores cataloged in Lee & Myers (1999, Cat. J/ApJS/123/233) between 1998 December and 1999 February, using the 14m telescope at Taeduk Radio Astronomy Observatory (TRAO) in Korea. (1 data file).
    No preview · Article · Sep 2010
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    Preview · Article · Aug 2010 · The Astrophysical Journal Supplement Series
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    ABSTRACT: We present N2D+ 3-2 (IRAM), and H2D+ 111-110 and N2H+ 4-3 (JCMT) maps of the small cluster-forming Ophiuchus B2 core in the nearby Ophiuchus molecular cloud. In conjunction with previously published N2H+ 1-0 observations, the N2D+ data reveal the deuterium fractionation in the high-density gas across Oph B2. The average deuterium fractionation RD = N(N2D+)/N(N2H+) ~ 0.03 over Oph B2, with several small scale RD peaks and a maximum RD = 0.1. The mean RD is consistent with previous results in isolated starless and protostellar cores. The column density distributions of both H2D+ and N2D+ show no correlation with total H2 column density. We find, however, an anticorrelation in deuterium fractionation with proximity to the embedded protostars in Oph B2 to distances 0.04 pc. Destruction mechanisms for deuterated molecules require gas temperatures greater than those previously determined through NH3 observations of Oph B2 to proceed. We present temperatures calculated for the dense core gas through the equating of non-thermal line widths for molecules (i.e., N2D+ and H2D+) expected to trace the same core regions, but the observed complex line structures in B2 preclude finding a reasonable result in many locations. This method may, however, work well in isolated cores with less complicated velocity structures. Finally, we use RD and the H2D+ column density across Oph B2 to set a lower limit on the ionization fraction across the core, finding a mean x e,lim few × 10–8. Our results show that care must be taken when using deuterated species as a probe of the physical conditions of dense gas in star-forming regions.
    Preview · Article · Jun 2010 · The Astrophysical Journal
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    ABSTRACT: We present Chandra X-ray data of the NGC 1333 embedded cluster, combining these data with existing Chandra data, Sptizer photometry and ground based spectroscopy of both the NGC 1333 & Serpens North clusters to perform a detailed study of the X-ray properties of two of the nearest embedded clusters to the Sun. In NGC 1333, a total of 95 cluster members are detected in X-rays, of which 54 were previously identified with Spitzer. Of the Spitzer sources, we detect 23% of the Class I protostars, 53% of the Flat Spectrum sources, 52% of the Class II, and 50% of the Transition Disk YSOs. Forty-one Class III members of the cluster are identified, bringing the total identified YSO population to 178. The X-ray Luminosity Functions (XLFs) of the NGC 1333 and Serpens clusters are compared to each other and the Orion Nebula Cluster. Based on this comparison, we obtain a new distance for the Serpens cluster of 360+22/-13 pc. The X-ray luminosity was found to depend on the bolometric luminosity as in previous studies of other clusters, and that Lx depends primarily on the stellar surface area. In the NGC 1333 cluster, the Class III sources have a somewhat higher X-ray luminosity for a given surface area. We also find evidence in NGC 1333 for a jump in the X-ray luminosity between spectral types of M0 and K7, we speculate that this may result from the presence of radiative zones in the K-stars. The gas column density vs. extinction in the NGC 1333 was found to be N_H = 0.89 +/- 0.13 x 10^22 A_K, this is lower than expected of the standard ISM but similar to that found previously in the Serpens Cloud Core. Comment: 58 pages, 14 figures, accepted by AJ
    Full-text · Article · May 2010 · The Astronomical Journal
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    ABSTRACT: The NGC 1999 reflection nebula features a dark patch with a size of ~10,000 AU, which has been interpreted as a small, dense foreground globule and possible site of imminent star formation. We present Herschel PACS far-infrared 70 and 160mum maps, which reveal a flux deficit at the location of the globule. We estimate the globule mass needed to produce such an absorption feature to be a few tenths to a few Msun. Inspired by this Herschel observation, we obtained APEX LABOCA and SABOCA submillimeter continuum maps, and Magellan PANIC near-infrared images of the region. We do not detect a submillimer source at the location of the Herschel flux decrement; furthermore our observations place an upper limit on the mass of the globule of ~2.4x10^-2 Msun. Indeed, the submillimeter maps appear to show a flux depression as well. Furthermore, the near-infrared images detect faint background stars that are less affected by extinction inside the dark patch than in its surroundings. We suggest that the dark patch is in fact a hole or cavity in the material producing the NGC 1999 reflection nebula, excavated by protostellar jets from the V 380 Ori multiple system. Comment: accepted for the A&A Herschel issue; 7 pages
    Full-text · Article · May 2010 · Astronomy and Astrophysics

Publication Stats

9k Citations
744.64 Total Impact Points

Institutions

  • 2015
    • University of Michigan
      Ann Arbor, Michigan, United States
  • 1991-2015
    • 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
  • 1988-2008
    • Harvard University
      • Department of Physics
      Cambridge, Massachusetts, United States
  • 2002
    • University of Rochester
      • Department of Physics and Astronomy
      Rochester, NY, United States
  • 1989
    • University of California, Berkeley
      Berkeley, California, United States
  • 1987-1989
    • Wellesley College
      • Department of Astronomy
      Уэлсли, Massachusetts, United States
  • 1979-1987
    • University of Massachusetts Amherst
      • Department of Astronomy
      Amherst Center, Massachusetts, United States
  • 1975-1983
    • Massachusetts Institute of Technology
      • Department of Physics
      Cambridge, Massachusetts, United States
  • 1980
    • Space Studies Institute
      MHV, California, United States
  • 1978
    • Columbia University
      New York, New York, United States