N. Murray

University of Toronto, Toronto, Ontario, Canada

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Publications (21)194.86 Total impact

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    ABSTRACT: Micron-size extrasolar dust particles have been convincingly detected by satellites. Larger extrasolar meteoroids (5-35 μm) have most likely been detected by ground-based radar at Arecibo and New Zealand. We present estimates of the minimum detectable particle sizes and the collecting areas for both radar systems. We show that particles larger than ~10 μm can propagate for tens of parsecs through the interstellar medium, opening up the possibility that ground-based radar systems can detect AGB stars, young stellar objects such as T Tauri stars, and debris disks around Vega-like stars. We provide analytical and numerical estimates of the ejection velocity in the case of a debris disk interacting with a Jupiter-mass planet. We give rough estimates of the flux of large micrometeoroids from all three classes of sources. Current radar systems are unlikely to detect significant numbers of meteors from debris disks such as β Pictoris. However, we suggest improvements to radar systems that should allow for the detection of multiple examples of all three classes.
    The Astrophysical Journal 12/2008; 600(2):804. · 6.73 Impact Factor
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    E. Harper-Clark, N. Murray
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    ABSTRACT: We have tested the two main theoretical models of bubbles around massive star clusters, Castor et al. and Chevalier & Clegg, against observations of the well studied Carina Nebula. The Castor et al. theory over-predicts the X-ray luminosity in the Carina bubble by a factor of 60 and expands too rapidly, by a factor of 4; if the correct radius and age are used, the predicted X-ray luminosity is even larger. In contrast, the Chevalier & Clegg model under-predicts the X-ray luminosity by a factor of 10. We modify the Castor et al. theory to take into account lower stellar wind mass loss rates, radiation pressure, gravity, and escape of or energy loss from the hot shocked gas. We argue that energy is advected rather than radiated from the bubble. We undertake a parameter study for reduced stellar mass loss rates and for various leakage rates and are able to find viable models. The X-ray surface brightness in Carina is highest close to the bubble wall, which is consistent with conductive evaporation from cold clouds. The picture that emerges is one in which the hot gas pressure is far below that found by dividing the time-integrated wind luminosity by the bubble volume; rather, the pressure in the hot gas is set by pressure equilibrium with the photoionized gas at T=10^4 K. It follows that the shocked stellar winds are not dynamically important in forming the bubbles.
    The Astrophysical Journal 12/2008; 693(2). · 6.73 Impact Factor
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    John E. Everett, Norm Murray
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    ABSTRACT: We build and test Parker-wind models to apply to observations of large-scale (of order 100 pc) outflows from Active Galactic Nuclei (AGNs). These models include detailed photoionization simulations, the observed radially varying mass profile, adiabatic cooling, and approximations for clouds dragged along in the wind and the interaction of the wind with the circumnuclear ISM of the galaxy. We test this model against recent HST/STIS observations of [O III] emission-line kinematics (in particular, we test against those observed in NGC 4151, but approximately the same kinematics is observed in NGC 1068 and Mrk 3) to constrain the viability of large-scale thermal winds in AGNs. We find that adiabatic cooling dominates in these outflows, decelerating Parker winds on large scales, making them highly unlikely as explanations of the observed kinematics.
    The Astrophysical Journal 11/2006; · 6.73 Impact Factor
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    ABSTRACT: Asa result of deephard X-rayobservations byChandraandXMM-Newton,asignificant fraction of the CXRB has been resolved into individual sources. These objects are almost all AGNs, and optical follow-up observations find that they are mostly obscured type 2 AGNs, have Seyfert-like X-ray luminosities, and peak in redshift at z � 0:7. Since this redshift is similar to the peak in the cosmic star formation rate, this paper proposes that the obscuring material required for AGN unification is regulated by star formation within the host galaxy. We test this idea by computing CXRB synthesis models with a ratio of type 2 to type 1 AGNs that is a function of both z and 2-10 keV X-rayluminosity,LX.Theevolutionarymodelsareconstrainedbyparameterizingtheobservedtype1AGNfractions from the recent work by Barger et al. The parameterization that simultaneously best accounts for Barger's data, the CXRB spectrum, and the X-ray number counts has a local, low-LX type 2/type 1 ratio of 4 and predicts a type 2 AGN fraction that evolves as (1 þ z)0:3. This particular evolution predicts a type 2/type 1 ratio of 1-2 for log LX > 44, and thus the deep X-ray surveys are missing about half the obscured AGNs with these luminosities. These objects are likely to be Compton thick. Overall, these calculations show that the current data strongly support a change to the AGN unification scenario in which the obscuration is connected with star formation in the host galaxy rather than a moleculartorus alone.Theevolutionoftheobscurationimplies aclose relationshipbetween star formationandAGN fueling. Subject headinggs: galaxies: active — galaxies: evolution — galaxies: formation — galaxies: Seyfert — X-rays: diffuse background
    The Astrophysical Journal 01/2006; 639(2):740-752. · 6.73 Impact Factor
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    ABSTRACT: Hubble Space Telescope images of MCG-6-30-15 show a dust lane crossing the galaxy just below the nucleus. In this paper, we argue that this dust lane is responsible for the observed reddening of the nuclear emission and the Fe I edge hinted at in the Chandra spectrum of MCG-6-30-15. We further suggest that the gas within the dust lane can comprise much of the low ionization component (i.e., the one contributing the O VII edge) of the observed warm absorber. Moreover, placing the warm absorbing material at such distances (hundreds of pc) can account for the small outflow velocities of the low ionization absorption lines as well as the constancy of the O VIII edge. Photoionization models of a dusty interstellar gas cloud (with a column appropriate for the reddening toward MCG-6-30-15) using a toy Seyfert 1 spectral energy distribution show that it is possible to obtain a significant O VII edge (\tau~0.2) if the material is ~150 pc from the ionizing source. For MCG-6-30-15, such a distance is consistent with the observed dust lane. The current data on MCG-6-30-15 is unable to constrain the dust composition within the warm absorber. Astronomical silicate is a viable candidate, but there are indications of a very low O abundance in the dust, which is inconsistent with a silicate origin. If true, this may indicate that there were repeated cycles of grain destruction and growth from shocks in the interstellar medium of MCG-6-30-15. Pure iron grains are an unlikely dust constituent due to the limit on their abundance in the Galaxy, yet they cannot be ruled out. The high column densities inferred from the highly ionized zone of the warm absorber implies that this gas is dust-free.
    Astronomy & Astrophysics - ASTRON ASTROPHYS. 01/2003; 409(2):503-509.
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    E. I. Chiang, N. Murray
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    ABSTRACT: The orbits of the outer two known planets orbiting Upsilon Andromedae are remarkably eccentric. Planet C possesses an orbital eccentricity of e1 = 0.253. For the more distant planet D, e2 = 0.308. Previous dynamical analyses strongly suggest that the two orbits are nearly co-planar and are trapped in an apsidal resonance in which the difference between their longitudes of periastron undergoes a bounded oscillation about 0 degrees. Here we elucidate the origin of these large eccentricities and of the apsidal alignment. Resonant interactions between a remnant circumstellar disk of gas lying exterior to the orbits of both planets can smoothly grow e2. Secular interactions between planets D and C can siphon off the eccentricity of the former to grow that of the latter. Externally amplifying e2 during the phase of the apsidal oscillation when e2/e1 is smallest drives the oscillation amplitude towards zero. Thus, the substantial eccentricity of planet C and the locking of orbital apsides are both consequences of externally pumping the eccentricity of planet D over timescales exceeding apsidal precession periods of order 1e4 yr. We explain why the recently detected stellar companion to Upsilon Andromedae is largely dynamically decoupled from the planetary system. Comment: accepted to ApJ
    The Astrophysical Journal 05/2002; · 6.73 Impact Factor
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    N. Murray, M. Paskowitz, M. Holman
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    ABSTRACT: We examine the eccentricity evolution of a system of two planets locked in a mean motion resonance, in which either the outer or both planets lose energy and angular momentum. The sink of energy and angular momentum could be a gas or planetesimal disk. We analytically calculate the eccentricity damping rate in the case of a single planet migrating through a planetesimal disk. When the planetesimal disk is cold (the average eccentricity is much less than 1), the circularization time tauc is comparable to the inward migration time taum, as previous calculations have found for the case of a gas disk. If the planetesimal disk is hot, tauc can be an order of magnitude shorter than taum. We show that the eccentricity of both planetary bodies can grow to large values, particularly if the inner body does not directly exchange energy or angular momentum with the disk. We present the results of numerical integrations of two migrating resonant planets showing rapid growth of eccentricity. We also present integrations in which a Jupiter-mass planet is forced to migrate inward through a system of 5-10 roughly Earth-mass planets. The migrating planet can eject or accrete the smaller bodies; roughly 5% of the mass (averaged over all the integrations) accretes onto the central star. The results are discussed in the context of the currently known extrasolar planetary systems.
    The Astrophysical Journal 01/2002; 565(1):608-620. · 6.73 Impact Factor
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    ABSTRACT: The physical basis of chaos in the solar system is now better understood: in all cases investigated so far, chaotic orbits result from overlapping resonances. Perhaps the clearest examples are found in the asteroid belt. Overlapping resonances account for its Kirkwood gaps and were used to predict and find evidence for very narrow gaps in the outer belt. Further afield, about one new ``short-period'' comet is discovered each year. They are believed to come from the ``Kuiper Belt'' (at 40 AU or more) via chaotic orbits produced by mean-motion and secular resonances with Neptune. Finally, the planetary system itself is not immune from chaos. In the inner solar system, overlapping secular resonances have been identified as the possible source of chaos. For example, Mercury, in 10^{12} years, may suffer a close encounter with Venus or plunge into the Sun. In the outer solar system, three-body resonances have been identified as a source of chaos, but on an even longer time scale of 10^9 times the age of the solar system. On the human time scale, the planets do follow their orbits in a stately procession, and we can predict their trajectories for hundreds of thousands of years. That is because the mavericks, with shorter instability times, have long since been ejected. The solar system is not stable; it is just old! Comment: 65 pages, 27 figures
    Annual Review of Astronomy and Astrophysics 11/2001; · 23.33 Impact Factor
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    N. Murray, B. Chaboyer
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    ABSTRACT: We compare the metallicities of stars with radial velocity planets to the metallicity of a sample of field dwarfs. We confirm recent work indicating that the stars-with-planet sample as a whole is iron rich. However, the lowest mass stars tend to be iron poor, with several having [Fe/H]<-0.2, demonstrating that high metallicity is not required for the formation of short period Jupiter-mass planets. We show that the average [Fe/H] increases with increasing stellar mass (for masses below 1.25 solar masses) in both samples, but that the increase is much more rapid in the stars-with-planet sample. The variation of metallicity with stellar age also differs between the two samples. We examine possible selection effects related to variations in the sensitivity of radial velocity surveys with stellar mass and metallicity, and identify a color cutoff (B-V>0.48) that contributes to but does not explain the mass-metallicity trend in the stars-with-planets sample. We use Monte Carlo models to show that adding an average of 6.5 Earth masses of iron to each star can explain both the mass-metallicity and the age-metallicity relations of the stars-with-planets sample. However, for at least one star, HD 38529, there is good evidence that the bulk metallicity is high. We conclude that the observed metallicities and metallicity trends are the result of the interaction of three effects; accretion of about 6 Earth masses of iron rich material, selection effects, and in some cases, high intrinsic metallicity. Comment: 19 pages 11 figures
    The Astrophysical Journal 06/2001; · 6.73 Impact Factor
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    N Murray, M Holman
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    ABSTRACT: Our understanding of the Solar System has been revolutionized over the past decade by the finding that the orbits of the planets are inherently chaotic. In extreme cases, chaotic motions can change the relative positions of the planets around stars, and even eject a planet from a system. Moreover, the spin axis of a planet-Earth's spin axis regulates our seasons-may evolve chaotically, with adverse effects on the climates of otherwise biologically interesting planets. Some of the recently discovered extrasolar planetary systems contain multiple planets, and it is likely that some of these are chaotic as well.
    Nature 05/2001; 410(6830):773-9. · 38.60 Impact Factor
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    ABSTRACT: We study spectroscopically determined iron abundances of 642 solar-type stars to search for the signature of accreted iron-rich material. We find that the metallicity [Fe/H] of a subset of 466 main sequence stars, when plotted as a function of stellar mass, mimics the pattern seen in lithium abundances in open clusters. Using Monte Carlo models we find that, on average, these stars have accreted about 0.4 Earth masses of iron while on the main sequence. A much smaller sample of 19 stars in the Hertzsprung gap, which are slightly evolved and whose convection zones are significantly more massive, have lower average [Fe/H], and their metallicity shows no clear variation with stellar mass. These findings suggest that terrestrial-type material is common around solar type stars. Comment: 33 pages, 11 figures. Submitted to ApJ
    The Astrophysical Journal 11/2000; · 6.73 Impact Factor
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    ABSTRACT: believe that most or all are real. It is difficult to understand how such planets could form in place. Although it is difficult to form planets at such small radii, once in place they can survive (10). Thus, it is natural to ask whether giant planets can form at orbital radii of a few astronomical units and then migrate inward. One proposed migration mechanism involves the generation by the planet of density waves in the gaseous protoplanetary disk, which cause the planet to spiral inward (10, 11). The movement of the planet might be halted by short-range tidal or magnetic effects from the central star (10); however, short-range stopping mechanisms cannot easily explain the objects in Table 1 with semimajor axes a p * 0.2 AU. Another migration scenario involves interactions between two or more Jupiter-mas
    07/2000;
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    N Murray, M Holman
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    ABSTRACT: Classical analytic theories of the solar system indicate that it is stable, but numerical integrations suggest that it is chaotic. This disagreement is resolved by a new analytic theory. The theory shows that the chaos among the jovian planets results from the overlap of the components of a mean motion resonance among Jupiter, Saturn, and Uranus, and provides rough estimates of the Lyapunov time (10(7) years) and the dynamical lifetime of Uranus (10(18) years). The jovian planets must have entered the resonance after all the gas and most of the planetesimals in the protoplanetary disk were removed.
    Science 04/1999; 283(5409):1877-81. · 31.20 Impact Factor
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    N. Murray, J. Chiang
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    ABSTRACT: The disk-wind model suggests that the broad emission lines of a quasar are emitted from the surface of the accretion disk feeding the central black hole. In this paper, we assume that a line-driven wind emerges from the illuminated face of the disk and calculate the line emission produced by this wind. The location and mass-loss rate of the wind are determined by the physics of the line driving. We calculate line ratios and the radius at which each line forms using a photoionization code. From the source functions, together with simple wind-radiative transfer calculations, we then calculate line profiles. The model reproduces the observed line ratios with a degree of success comparable to standard "cloud" models, underpredicting the flux in low-ionization lines and in N V λ1240. Using C IV to fix all the model parameters except for the line ratios, we find that the model successfully predicts the variety of line profiles seen in two PG quasars. We also present line profiles for Ne VIII that are very broad (FWHM ~ 17,000 km s-1) and predict that Mg X, while unlikely to be seen in emission, will be seen in absorption (in broad absorption line quasars). As suggested by previous authors, we find that the very broad feature seen around λ2800 and usually attributed to Mg II is actually a blend of that line, which is relatively narrow, and Fe II emission.
    The Astrophysical Journal 02/1998; 494(1):125. · 6.73 Impact Factor
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    N. Murray, M. Holman, M. Potter
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    ABSTRACT: We consider the effect of gravitational perturbations from Jupiter on the dynamics of asteroids, when Jupiter is itself perturbed by Saturn. The presence of Saturn introduces a number of additional frequencies into Jupiters orbit. These frequencies in turn produce chaos in narrow regions on either side of the chaotic zones associated with the mean motion resonances between the asteroids and Jupiter. The resonant arguments of these three-body resonances contain the longitudes of Jupiter and the asteroid together with either the secular frequency 9-6, or the longitude of Saturn. Resonances involving the longitude of Saturn are analogs of the Laplace resonance in the Jovian satellite system. We show that many three-body resonances involving the longitude of Saturn are chaotic. We give simple expressions for the width of the chaotic region and the associated Lyapunov time. In some cases the chaos can produce a diffusive growth in the 4 eccentricity of the asteroid that leads to ejection of the asteroid on times shorter than the age of the solar system. We give simple estimates for the diffusion time. Finally, we present the results of numerical integrations testing the theory.
    The Astronomical Journal 02/1998; · 4.97 Impact Factor
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    ABSTRACT: A planet orbiting in a disk of planetesimals can experience an instability in which it migrates to smaller orbital radii. Resonant interactions between the planet and planetesimals remove angular momentum from the planetesimals, increasing their eccentricities. Subsequently, the planetesimals either collide with or are ejected by the planet, reducing the semimajor axis of the planet. If the surface density of the planetesimals exceeds a critical value, corresponding to approximately 0.03 solar mass of gas inside the orbit of Jupiter, the planet will migrate inward a large distance. This instability may explain the presence of Jupiter-mass objects in small orbits around nearby stars.
    Science 02/1998; 279(5347):69-72. · 31.20 Impact Factor
  • N. Murray, M. Potter, M. Holman
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    ABSTRACT: Numerical integrations show that orbits in the outer asteroid belt are chaotic, with Lyapunov times that vary dramatically with the initial semimajor axis of the orbit. In earlier work we showed, in the context of a model where Jupiter was the only planet, that the orbits with the shortest Lyapunov times are chaotic because high order mean motion resonances overlap. In this contribution we show that much of the rest of the chaos seen in complete solar system models is the result of variations in the orbital elements of Jupiter forced by Saturn. These orbital variations produce sidebands to the mean motion resonances, and motion at the location in semimajor axis where the sidebands lie is chaotic.
    12/1997;
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    N. Murray, M. Holman
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    ABSTRACT: We present an analytic theory of motion near resonances in the planar elliptic restricted three-body problem. The theory predicts the location and extent in semimajor axis and eccentricity (a,e) space of the chaotic motion, the Lyapunov time, and the time for objects on chaotic orbits to be removed from the system. The latter is given by the time for test bodies with small initial eccentricities to diffuse to the eccentricity at which they suffer close encounters with the perturbing body. The theory predicts gaps in the outer asteroid belt similar to the Kirkwood gaps seen in the inner belt, in agreement with our recent numerical results. It also predicts that asteroids in a number of high order mean motion resonances will possess very short Lyapunov times ( ~ 10,000 years) but removal times comparable or longer than the life time of the solar system; Helga, Ulla, and Wingolfia may afford examples of such bodies. Finally, we explore the relationship between the Lyapunov time and the removal time. We explain the simple power law relation found in previous numerical work, and show where it does and does not apply.
    The Astronomical Journal 08/1997; 114:1246-1259. · 4.97 Impact Factor
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    ABSTRACT: The warm absorber observed in the Seyfert 1 galaxy MCG–6-30-15 is known to consist of at least two zones and very likely contains dust. Hubble Space Telescope images of MCG–6-30-15 show a dust lane crossing the galaxy just below the nucleus. In this paper, we argue that this dust lane is responsible for the observed reddening of the nuclear emission and the Fe i edge hinted at in the Chandra spectrum of MCG–6-30-15. We further suggest that the gas within the dust lane can comprise much of the low ionization component (i.e., the one contributing the O vii edge) of the observed warm absorber. Moreover, placing the warm absorbing material at such distances (hundreds of pc) can account for the small outflow velocities of the low ionization absorption lines as well as the constancy of the O vii edge. Photoionization models of a dusty interstellar gas cloud (with a column appropriate for the reddening toward MCG–6-30-15) using a toy Seyfert 1 spectral energy distribution show that it is possible to obtain a significant O vii edge ($\tau \sim 0.2$) if the material is ~150 pc from the ionizing source. For MCG–6-30-15, such a distance is consistent with the observed dust lane. We emphasize the point first made by Kraemer et al.: dusty interstellar material will likely contribute to the warm absorber, and should be included in spectral modeling. The current data on MCG–6-30-15 is unable to constrain the dust composition within the warm absorber. Astronomical silicate is a viable candidate, but there are indications of a very low O abundance in the dust, which is inconsistent with a silicate origin. If true, this may indicate that there were repeated cycles of grain destruction and growth from shocks in the interstellar medium of MCG–6-30-15. Pure iron grains are an unlikely dust constituent due to the limit on their abundance in the Galaxy, yet they cannot be ruled out. The high column densities inferred from the highly ionized zone of the warm absorber implies that this gas is dust-free.
    http://dx.doi.org/10.1051/0004-6361:20031166.
  • N. Murray, M. Holman, M. Potter

Publication Stats

722 Citations
194.86 Total Impact Points

Institutions

  • 1998–2008
    • University of Toronto
      • Canadian Institute for Theoretical Astrophysics
      Toronto, Ontario, Canada
  • 2006
    • University of Wisconsin–Madison
      Madison, Wisconsin, United States
  • 1999
    • Harvard-Smithsonian Center for Astrophysics
      Cambridge, Massachusetts, United States