C. M. Brunt

University of Exeter, Exeter, England, United Kingdom

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Publications (35)89.28 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: We present CO, H2, HI and HISA distributions from a set of simulations of grand design spirals including stellar feedback, self-gravity, heating and cooling. We replicate the emission of the 2nd Galactic Quadrant by placing the observer inside the modelled galaxies and post process the simulations using a radiative transfer code, so as to create synthetic observations. We compare the synthetic datacubes to observations of the 2nd Quadrant of the Milky Way to test the ability of the current models to reproduce the basic chemistry of the Galactic ISM, as well as to test how sensitive such galaxy models are to different recipes of chemistry and/or feedback. We find that models which include feedback and self-gravity can reproduce the production of CO with respect to H2 as observed in our Galaxy, as well as the distribution of the material perpendicular to the Galactic plane. While changes in the chemistry/feedback recipes do not have a huge impact on the statistical properties of the chemistry in the simulated galaxies, we find that the inclusion of both feedback and self-gravity are crucial ingredients, as our test without feedback failed to reproduce all of the observables. Finally, even though the transition from H2 to CO seems to be robust, we find that all models seem to underproduce molecular gas, and have a lower molecular to atomic gas fraction than is observed. Nevertheless, our fiducial model with feedback and self-gravity has shown to be robust in reproducing the statistical properties of the basic molecular gas components of the ISM in our Galaxy.
    12/2014;
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    C. M. Brunt, C. Federrath
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    ABSTRACT: We introduce a new method for observationally estimating the fraction of momentum density (${\rho}{\mathbf{v}}$) power contained in solenoidal modes (for which $\nabla \cdot {\rho}{\mathbf{v}} = 0$) in molecular clouds. The method is successfully tested with numerical simulations of supersonic turbulence that produce the full range of possible solenoidal/compressible fractions. At present the method assumes statistical isotropy, and does not account for anisotropies caused by (e.g.) magnetic fields. We also introduce a framework for statistically describing density--velocity correlations in turbulent clouds.
    05/2014; 442(2).
  • C. M. Brunt, C. Federrath
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    ABSTRACT: We introduce a new method for observationally estimating the fraction of momentum density (${\rho}{\mathbf{v}}$) power contained in solenoidal modes (for which $\nabla \cdot {\rho}{\mathbf{v}} = 0$) in molecular clouds. The method is successfully tested with numerical simulations of supersonic turbulence that produce the full range of possible solenoidal/compressible fractions. At present the method assumes statistical isotropy, and does not account for anisotropies caused by (e.g.) magnetic fields. We also introduce a framework for statistically describing density--velocity correlations in turbulent clouds.
    04/2014;
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    C. M. Brunt, M. H. Heyer
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    ABSTRACT: Principal component analysis is a powerful statistical system to investigate the structure and dynamics of the molecular interstellar medium, with particular emphasis on the study of turbulence, as revealed by spectroscopic imaging of molecular line emission. To-date, the method to retrieve the power law index of the velocity structure function or power spectrum has relied on an empirical calibration and testing with model turbulent velocity fields, while lacking a firm theoretical basis. In this paper, we present an analytic formulation that reveals the detailed mechanics of the method and confirms previous empirical calibrations of its recovery of the scale dependence of turbulent velocity fluctuations.
    Monthly Notices of the Royal Astronomical Society 05/2013; 433(1). · 5.52 Impact Factor
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    ABSTRACT: Infrared stellar photometry from 2MASS and spectral line imaging observations of 12CO and 13CO J = 1-0 line emission from the FCRAO 14m telescope are analysed to assess the variation of the CO abundance with physical conditions throughout the Orion A and Orion B molecular clouds. Three distinct Av regimes are identified in which the ratio between the 13CO column density and visual extinction changes corresponding to the photon dominated envelope, the strongly self-shielded interior, and the cold, dense volumes of the clouds. Within the strongly self-shielded interior of the Orion A cloud, the 13CO abundance varies by 100% with a peak value located near regions of enhanced star formation activity. The effect of CO depletion onto the ice mantles of dust grains is limited to regions with AV > 10 mag and gas temperatures less than 20 K as predicted by chemical models that consider thermal-evaporation to desorb molecules from grain surfaces. Values of the molecular mass of each cloud are independently derived from the distributions of Av and 13CO column densities with a constant 13CO-to-H2 abundance over various extinction ranges. Within the strongly self-shielded interior of the cloud (Av > 3 mag), 13CO provides a reliable tracer of H2 mass with the exception of the cold, dense volumes where depletion is important. However, owing to its reduced abundance, 13CO does not trace the H2 mass that resides in the extended cloud envelope, which comprises 40-50% of the molecular mass of each cloud. The implied CO luminosity to mass ratios, M/L_{CO}, are 3.2 and 2.9 for Orion A and Orion B respectively, which are comparable to the value (2.9), derived from gamma-ray observations of the Orion region. Our results emphasize the need to consider local conditions when applying CO observations to derive H2 column densities.
    Monthly Notices of the Royal Astronomical Society 02/2013; 431(2). · 5.52 Impact Factor
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    ABSTRACT: An essential step in the formation of new stars is the condensation of ambient neutral atomic hydrogen (HI) into the molecular phase (H2). It is well known that molecular clouds collapse to form protostars, but less understood is how molecular clouds themselves begin to form. The process is difficult to study because the transition from HI to H2 is not very energetic, which limits direct observations. We study this process indirectly, by examining the interstellar dust within these H2-forming clouds. The dust is readily observed via infrared thermal emission. We use HIRES IRAS and Spitzer IRAC and MIPS imaging photometry to investigate a target cloud in the Perseus spiral arm in the which the HI-to-H2 transition appears to be underway, as evidenced by strong HI self-absorption, variable CO emission, and significant ``excess'' infrared emission. We have sampled the dust spectral energy distribution (SED) at many positions on and off this cloud in all IRAS and Spitzer photometric bands. We interpret these data by fitting the SEDs with the DustEM infrared emission model and infer the grain population composition and evolutionary status in this H2-forming cloud and others like it.
    American Astronomical Society Meeting Abstracts #219; 01/2012
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    J. C. Mottram, C. M. Brunt
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    ABSTRACT: We present $^{12}$CO, $^{13}$CO and C$^{18}$O (J=3$-$2) observations of a new cluster of outflows in the Vulpecula Rift with HARP-B on the JCMT. The mass associated with the outflows, measured using the $^{12}$CO HARP-B observations and assuming a distance to the region of 2.3 kpc, is 129 \msol{}, while the mass associated with the dense gas from C$^{18}$O observations is 458 \msol{} and the associated sub-millimeter core has a mass of 327 $\pm$ 112 \msol{} independently determined from Bolocam 1.1mm data. The outflow-to-core mass ratio is therefore $\sim$0.4, making this region one of the most efficient observed thus far with more than an order of magnitude more mass in the outflow than would be expected based on previous results. The kinetic energy associated with the flows, 94$\times10^{45}$ ergs, is enough to drive the turbulence in the local clump, and potentially unbind the local region altogether. The detection of SiO (J=8$-$7) emission toward the outflows indicates that the flow is still active, and not simply a fossil flow. We also model the SEDs of the four YSOs associated with the molecular material, finding them all to be of mid to early B spectral type. The energetic nature of the outflows and significant reservoir of cold dust detected in the sub-mm suggest that these intermediate mass YSOs will continue to accrete and become massive, rather than reach the main sequence at their current mass.
    Monthly Notices of the Royal Astronomical Society 09/2011; · 5.52 Impact Factor
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    ABSTRACT: Using a model Smoothed Particle Hydrodynamics spiral galaxy and a radiative transfer code, we have created a synthetic HI Galactic Plane Survey, with data cubes covering 90°
    12/2010;
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    Joseph C. Mottram, Chris M. Brunt
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    ABSTRACT: The Exeter FCRAO CO Galactic Plane Survey consists of 12CO and 13CO (J=1-0) observations over the galactic plane covering 55 degrees <= l <= 102 degrees, |b| >= 1 degree and 141 degrees <= l <= 195 degrees, -3.5 degrees <= b <= 5.5 degrees with a spatial resolution of ~45" and a spectral resolution of ~0.15km/s. We will present the methodology of a threshold-based cloud and clump determination method which retains hierarchical information, then discuss associating sources with clouds in the catalogue. Once complete, this catalogue of clouds and clumps will encompass the majority of the Northern Galactic Plane, providing knowledge of the molecular structure of the galaxy and the starting point for studies of the variation in star formation efficiency. In addition, it will allow us to identify clouds that have no or little star formation taking place inside them, which are often overlooked in the study of the conditions required for star formation to take place. Comment: 7 pages, 2 figures, proceeding of 'The Dynamic ISM: A celebration of the Canadian Galactic Plane Survey' in Naramata, Canada from 6-10 June 2010 to be published in ASP Conf.Series
    07/2010;
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    C. M. Brunt
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    ABSTRACT: Supersonic turbulence in molecular clouds is a key agent in generating density enhancements that may subsequently go on to form stars. The stronger the turbulence - the higher the Mach number - the more extreme the density fluctuations are expected to be. Numerical models predict an increase in density variance with rms Mach number of the form: sigma^{2}_{rho/rho_{0}} = b^{2}M^{2}, where b is a numerically-estimated parameter, and this prediction forms the basis of a large number of analytic models of star formation. We provide an estimate of the parameter b from 13CO J=1-0 spectral line imaging observations and extinction mapping of the Taurus molecular cloud, using a recently developed technique that needs information contained solely in the projected column density field to calculate sigma^{2}_{rho/rho_{0}}. We find b ~ 0.48, which is consistent with typical numerical estimates, and is characteristic of turbulent driving that includes a mixture of solenoidal and compressive modes. More conservatively, we constrain b to lie in the range 0.3-0.8, depending on the influence of sub-resolution structure and the role of diffuse atomic material in the column density budget. We also report a break in the Taurus column density power spectrum at a scale of ~1pc, and find that the break is associated with anisotropy in the power spectrum. The break is observed in both 13CO and dust extinction power spectra, which, remarkably, are effectively identical despite detailed spatial differences between the 13CO and dust extinction maps. [ abridged ] Comment: 8 pages, 9 figures. Accepted for publication in A&A
    Astronomy and Astrophysics 02/2010; · 5.08 Impact Factor
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    C. M. Brunt, C. Federrath, D. J. Price
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    ABSTRACT: We introduce and test an expression for calculating the variance of a physical field in three dimensions using only information contained in the two-dimensional projection of the field. The method is general but assumes statistical isotropy. To test the method we apply it to numerical simulations of hydrodynamic and magnetohydrodynamic turbulence in molecular clouds, and demonstrate that it can recover the 3D normalised density variance with ~10% accuracy if the assumption of isotropy is valid. We show that the assumption of isotropy breaks down at low sonic Mach number if the turbulence is sub-Alfvenic. Theoretical predictions suggest that the 3D density variance should increase proportionally to the square of the Mach number of the turbulence. Application of our method will allow this prediction to be tested observationally and therefore constrain a large body of analytic models of star formation that rely on it. Comment: 8 pages, 9 figures, accepted for publication in MNRAS
    01/2010;
  • C. M. Brunt, C. Federrath, D. J. Price
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    ABSTRACT: We introduce and test an expression for calculating the variance of a physical field in three dimensions using only information contained in the two-dimensional projection of the field. The method is general but assumes statistical isotropy. To test the method we apply it to numerical simulations of hydrodynamic and magnetohydrodynamic turbulence in molecular clouds, and demonstrate that it can recover the three-dimensional (3D) normalized density variance with ~10 per cent accuracy if the assumption of isotropy is valid. We show that the assumption of isotropy breaks down at low sonic Mach number if the turbulence is sub-Alfvénic. Theoretical predictions suggest that the 3D density variance should increase proportionally to the square of the Mach number of the turbulence. Application of our method will allow this prediction to be tested observationally and therefore constrain a large body of analytic models of star formation that rely on it.
    Monthly Notices of the Royal Astronomical Society 01/2010; · 5.52 Impact Factor
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    ABSTRACT: From diffuse interstellar cirrus to dense atomic and molecular clouds, from protostellar to post-AGB envelopes, from super-shells to supernovae remnants, the Herschel Hi-GAL survey offer an unprecedented snapshot of all the different phases of the Galactic ISM, its evolution and interactions. I will present early results on a variety of topics including the lifetime of massive pre-stellar phases, the fragmentation and collapse of extended structures, the timeline for massive star formation, dust properties in cirrus and molecular clouds.
    01/2010; 38:2488.
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    ABSTRACT: New observations with ensuremath Herschelensuremath<?iensuremath> allow accurate measurement of the equilibrium temperature of large dust grains heated by the interstellar radiation field (ISRF), which is critical in deriving dust column density and masses. We present temperature maps derived from the ensuremath Herschelensuremath<?iensuremath> SPIRE and PACS data in two fields along the Galactic plane, obtained as part of the Hi-GAL survey during the ensuremath Herschelensuremath<?iensuremath> science demonstration phase (SDP). We analyze the distribution of the dust temperature spatially, as well as along the two lines-of-sight (LOS) through the Galaxy. The zero-level offsets in the ensuremath Herschelensuremath<?iensuremath> maps were established by comparison with the IRAS and ensuremath Planckensuremath<?iensuremath> data at comparable wavelengths. We derive maps of the dust temperature and optical depth by adjusting a detailed model for dust emission at each pixel. The dust temperature maps show variations in the ISRF intensity and reveal the intricate mixture of the warm dust heated by massive stars and the cold filamentary structures of embedded molecular clouds. The dust optical depth at 250 ensuremath Î?ensuremath<?iensuremath>m is well correlated with the gas column density, but with a significantly higher dust emissivity than in the solar neighborhood. We correlate the optical depth with 3-D cubes of the dust extinction to investigate variations in the ISRF strength and dust abundance along the line of sight through the spiral structure of the Galaxy. We show that the warmest dust along the LOS is located in the spiral arms of the Galaxy, and we quantify their respective IR contribution.
    http://dx.doi.org/10.1051/0004-6361/201014540. 01/2010;
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    C. M. Brunt, M. H. Heyer, M. -M. Mac Low
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    ABSTRACT: Supersonic turbulence in molecular clouds is a dominant agent that strongly affects the clouds' evolution and star formation activity. Turbulence may be initiated and maintained by a number of processes, acting at a wide range of physical scales. By examining the dynamical state of molecular clouds, it is possible to assess the primary candidates for how the turbulent energy is injected. The aim of this paper is to constrain the scales at which turbulence is driven in the molecular interstellar medium, by comparing simulated molecular spectral line observations of numerical magnetohydrodynamic (MHD) models and molecular spectral line observations of real molecular clouds. We use principal component analysis, applied to both models and observational data, to extract a quantitative measure of the driving scale of turbulence. We find that only models driven at large scales (comparable to, or exceeding, the size of the cloud) are consistent with observations. This result applies also to clouds with little or no internal star formation activity. Astrophysical processes acting on large scales, including supernova-driven turbulence, magnetorotational instability, or spiral shock forcing, are viable candidates for the generation and maintenance of molecular cloud turbulence. Small scale driving by sources internal to molecular clouds, such as outflows, can be important on small scales, but cannot replicate the observed large-scale velocity fluctuations in the molecular interstellar medium. Comment: 8 pages, 7 figures, accepted for publication in A&A
    Astronomy and Astrophysics 10/2009; · 5.08 Impact Factor
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    ABSTRACT: Using the radiative transfer code Torus, we produce spectral-line cubes of the predicted HI profile from global SPH simulations of spiral galaxies. Torus grids the SPH galaxy using Adaptive Mesh Refinement, then applies a ray-tracing method to infer the HI profile along the line(s) of sight. The gridded galaxy can be observed from any direction, which enables us to model the observed HI profile for galaxies of any orientation. We can also place the observer inside the galaxy, to simulate HI observations taken from the Earth's position in the Milky Way. Comment: 4 pages, 2 figures, conference proceedings for "Panoramic Radio Astronomy: 1-2 Ghz Research on Galaxy Evolution" June 2-5, 2009 Groningen
    09/2009;
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    ABSTRACT: We report the results of a 100 square degree survey of the Taurus Molecular Cloud region in the J = 1-0 transition of 12CO and 13CO. The image of the cloud in each velocity channel includes ~ 3 million Nyquist sampled pixels on a 20" grid. The high sensitivity and large linear dynamic range of the maps in both isotopologues reveal a very complex, highly structured cloud morphology. There are large scale correlated structures evident in 13CO emission having very fine dimensions, including filaments, cavities, and rings. The 12CO emission shows a quite different structure, with particularly complex interfaces between regions of greater and smaller column density defining the boundaries of the largest-scale cloud structures. The axes of the striations seen in the 12CO emission from relatively diffuse gas are aligned with the direction of the magnetic field. Using a column density-dependent model for the CO fractional abundance, we derive the mass of the region mapped to be 24,000 solar masses, a factor of three greater than would be obtained with canonical CO abundance restricted to the high column density regions. We determine that half the mass of the cloud is in regions having column density below 2.1x10^{21} per square cm. The distribution of young stars in the region covered is highly nonuniform, with the probability of finding a star in a pixel with a specified column density rising sharply for N(H2) = 6x10^{21} cm^{-2}. We determine a relatively low star formation efficiency (mass of young stars/mass of molecular gas), between 0.3 and 1.2 %, and an average star formation rate during the past 3 Myr of 8x10^{-5} stars yr^{-1}. Comment: 53 pages, 21 figures
    The Astrophysical Journal 02/2008; · 6.73 Impact Factor
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    ABSTRACT: The Canadian Galactic Plane Survey (CGPS) is a project to combine radio, millimeter, and infrared surveys of the Galactic plane to provide arcminute-scale images of all major components of the interstellar medium over a large portion of the Galactic disk. We describe in detail the observations for the low-frequency component of the CGPS, the radio surveys carried out at the Dominion Radio Astrophysical Observatory (DRAO), and summarize the properties of the merged database of surveys that comprises the CGPS. The DRAO Synthesis Telescope surveys have imaged a 73° section of the Galactic plane, using ~85% of the telescope time between 1995 April and 2000 June. The observations provide simultaneous radio continuum images at two frequencies, 408 and 1420 MHz, and spectral-line images of the λ = 21 cm transition of neutral atomic hydrogen. In the radio continuum at 1420 MHz, dual-polarization receivers provide images in all four Stokes parameters. The surveys cover the region 742 < l < 1473, with latitude extent -36 < b < +56 at 1420 MHz and -67 < b < +87 at 408 MHz. By integration of data from single-antenna observations, the survey images provide complete information on all scales of emission structures down to the resolution limit, which is just below 1' × 1' csc δ at 1420 MHz and 34 × 34 csc δ at 408 MHz. The continuum images have a dynamic range of several thousand, yielding essentially noise-limited images with an rms of ~0.3 mJy beam-1 at 1420 MHz and ~3 mJy beam-1 at 408 MHz. The spectral-line data are noise limited with rms brightness temperature ΔTB ~ 3 K in a 0.82 km s-1 channel. The complete CGPS data set, including the DRAO surveys and data at similar resolution in 12CO (1–0) and in infrared emission from dust, all imaged to an identical Galactic coordinate grid and map projection, are being made publicly available through the Canadian Astronomy Data Centre.
    The Astronomical Journal 12/2007; 125(6):3145. · 4.97 Impact Factor
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    ABSTRACT: New high-resolution surveys reveal an abundance of cold H i features in the Galactic plane. These frequently trace spiral arm structure while failing to trace CO features as well as they should if the cold H i is primarily in molecular clouds.
    03/2006: pages 47-50;
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    ABSTRACT: Cold atomic gas contains about 1/3 of the mass of the interstellar medium (ISM) in clouds occupying only a few percent of the ISM volume. These clouds can be imaged as 21cm-line HI self-absorption (HISA) shadows against warmer background HI emission at the same radial velocity, allowing a simultaneous probe of the temperature and velocity structure of HI across the Galactic disk. A recent census of HISA in the Canadian Galactic Plane Survey (CGPS) found a wealth of intricate features over 71 degrees of longitude. The weak CGPS HISA is widely scattered and may arise from turbulent fluctuations of the ambient ISM, while stronger absorption is concentrated into discrete complexes. The CGPS HISA census has now been extended with CGPS-II and VLA Galactic Plane Survey (VGPS) data to cover 157 degrees of longitude in the first two Galactic quadrants. We present and discuss the findings of this composite HISA survey in the context of Galactic structure and spiral density waves. We also compare the HISA distribution with detected CO emission and discuss its relevance to the hypothesized atomic-to-molecular phase change in spiral arms prior to star formation. This work has been supported by a grant from the Natural Sciences and Engineering Research Council of Canada to the University of Calgary and by the National Astronomy and Ionosphere Center operated by Cornell University under Cooperative Agreement with the National Science Foundation.
    12/2005;

Publication Stats

210 Citations
89.28 Total Impact Points

Institutions

  • 2008–2014
    • University of Exeter
      Exeter, England, United Kingdom
    • California Institute of Technology
      • Jet Propulsion Laboratory
      Pasadena, California, United States
  • 2005
    • The University of Calgary
      • Department of Physics and Astronomy
      Calgary, Alberta, Canada