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Mordecaimark Mac Low - One of the best experts on this subject based on the ideXlab platform.
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the energy dissipation rate of supersonic magnetohydrodynamic turbulence in Molecular Clouds
The Astrophysical Journal, 1999Co-Authors: Mordecaimark Mac LowAbstract:Molecular Clouds have broad line widths, which suggests turbulent supersonic motions in the Clouds. These motions are usually invoked to explain why Molecular Clouds take much longer than a free-fall time to form stars. Classically, it was thought that supersonic hydrodynamical turbulence would dissipate its energy quickly but that the introduction of strong magnetic fields could maintain these motions. A previous paper has shown, however, that isothermal, compressible MHD and hydrodynamical turbulence decay at virtually the same rate, requiring that constant driving occur to maintain the observed turbulence. In this paper, direct numerical computations of uniform, randomly driven turbulence with the ZEUS astrophysical MHD code are used to derive the value of the energy-dissipation coefficient, which is found to be with ηv = 0.21/π, where vrms is the root-mean-square (rms) velocity in the region, Ekin is the total kinetic energy in the region, m is the mass of the region, and is the driving wavenumber. The ratio τ of the formal decay time Ekin/kin of turbulence to the free-fall time of the gas can then be shown to be where Mrms is the rms Mach number, and κ is the ratio of the driving wavelength to the Jeans wavelength. It is likely that κ < 1 is required for turbulence to support gas against gravitational collapse, so the decay time will probably always be far less than the free-fall time in Molecular Clouds, again showing that turbulence there must be constantly and strongly driven. Finally, the typical decay time constant of the turbulence can be shown to be where is the driving wavelength.
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the energy dissipation rate of supersonic magnetohydrodynamic turbulence in Molecular Clouds
arXiv: Astrophysics, 1998Co-Authors: Mordecaimark Mac LowAbstract:Molecular Clouds have broad linewidths suggesting turbulent supersonic motions in the Clouds. These motions are usually invoked to explain why Molecular Clouds take much longer than a free-fall time to form stars. It has classically been thought that supersonic hydrodynamical turbulence would dissipate its energy quickly, but that the introduction of strong magnetic fields could maintain these motions. In a previous paper it has been shown, however, that isothermal, compressible, MHD and hydrodynamical turbulence decay at virtually the same rate, requiring that constant driving occur to maintain the observed turbulence. In this paper direct numerical computations of uniformly driven turbulence with the ZEUS astrophysical MHD code are used to derive the absolute value of energy dissipation as a function of the driving wavelength and amplitude. The ratio of the formal decay time of turbulence E_{kin}/\dot{E}_{kin} to the free-fall time of the gas can then be derived as a function of the ratio of driving wavelength to Jeans wavelength and rms Mach number, and shown to be most likely far less than unity, again showing that turbulence in Molecular Clouds must be constantly and strongly driven. (abridged)
Mark H. Heyer - One of the best experts on this subject based on the ideXlab platform.
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Molecular Clouds in the milky way
Annual Review of Astronomy and Astrophysics, 2015Co-Authors: Mark H. Heyer, T M DameAbstract:In the past twenty years, the reconnaissance of 12 CO and 13 CO emission in the Milky Way by single-dish millimeter-wave telescopes has expanded our view and understanding of interstellar Molecular gas. We enumerate the major surveys of CO emission along the Galactic plane and summarize the various approaches that leverage these data to determine the large-scale distribution of Molecular gas: its radial and vertical distributions, its concentration into Clouds, and its relationship to spiral structure. The integrated properties of Molecular Clouds are compiled from catalogs derived from the CO surveys using uniform assumptions regarding the Galactic rotation curve, solar radius, and the CO-to-H2 conversion factor. We discuss the radial variations of cloud surface brightness, the distributions of cloud mass and size, and scaling relations between velocity dispersion, cloud size, and surface density that affirm that the larger Clouds are gravitationally bound. Measures of density structure and gas kinematics within nearby, well-resolved Clouds are examined and attributed to the effects of magnetohydrodynamic turbulence. We review the arguments for short, intermediate, and long Molecular lifetimes based on the observational record. The review concludes with questions that shall require further observational attention.
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physical properties and galactic distribution of Molecular Clouds identified in the galactic ring survey
The Astrophysical Journal, 2010Co-Authors: Mark H. Heyer, Julia Romanduval, J M Jackson, J M Rathborne, R SimonAbstract:We derive the physical properties of 580 Molecular Clouds based on their 12CO and 13CO line emission detected in the University of Massachusetts-Stony Brook (UMSB) and Galactic Ring surveys. We provide a range of values of the physical properties of Molecular Clouds, and find a power-law correlation between their radii and masses, suggesting that the fractal dimension of the interstellar medium is around 2.36. This relation, M = (228 ± 18) R 2.36 ± 0.04, allows us to derive masses for an additional 170 Galactic Ring Survey (GRS) Molecular Clouds not covered by the UMSB survey. We derive the Galactic surface mass density of Molecular gas and examine its spatial variations throughout the Galaxy. We find that the azimuthally averaged Galactic surface density of Molecular gas peaks between Galactocentric radii of 4 and 5 kpc. Although the Perseus arm is not detected in Molecular gas, the Galactic surface density of Molecular gas is enhanced along the positions of the Scutum-Crux and Sagittarius arms. This may indicate that Molecular Clouds form in spiral arms and are disrupted in the inter-arm space. Finally, we find that the CO excitation temperature of Molecular Clouds decreases away from the Galactic center, suggesting a possible decline in the star formation rate with Galactocentric radius. There is a marginally significant enhancement in the CO excitation temperature of Molecular Clouds at a Galactocentric radius of about 6 kpc, which in the longitude range of the GRS corresponds to the Sagittarius arm. This temperature increase could be associated with massive star formation in the Sagittarius spiral arm.
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physical properties and galactic distribution of Molecular Clouds identified in the galactic ring survey
arXiv: Astrophysics of Galaxies, 2010Co-Authors: Mark H. Heyer, Julia Romanduval, J M Jackson, J M Rathborne, R SimonAbstract:We derive the physical properties of 580 Molecular Clouds based on their 12CO and 13CO line emission detected in the University of Massachusetts-Stony Brook (UMSB) and Galactic Ring surveys. We provide a range of values of the physical properties of Molecular Clouds, and find a power-law correlation between their radii and masses, suggesting that the fractal dimension of the ISM is around 2.36. This relation, M = (228 +/- 18) R^{2.36+/-0.04}, allows us to derive masses for an additional 170 GRS Molecular Clouds not covered by the UMSB survey. We derive the Galactic surface mass density of Molecular gas and examine its spatial variations throughout the Galaxy. We find that the azimuthally averaged Galactic surface density of Molecular gas peaks between Galactocentric radii of 4 and 5 kpc. Although the Perseus arm is not detected in Molecular gas, the Galactic surface density of Molecular gas is enhanced along the positions of the Scutum-Crux and Sagittarius arms. This may indicate that Molecular Clouds form in spiral arms and are disrupted in the inter-arm space. Last, we find that the CO excitation temperature of Molecular Clouds decreases away from the Galactic center, suggesting a possible decline in the star formation rate with Galactocentric radius. There is a marginally significant enhancement in the CO excitation temperature of Molecular Clouds at a Galactocentric radius of about 6 kpc, which in the longitude range of the GRS corresponds to the Sagittarius arm. This temperature increase could be associated with massive star formation in the Sagittarius spiral arm.
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Turbulent driving scales in Molecular Clouds
Astronomy and Astrophysics, 2009Co-Authors: Christopher M. Brunt, Mark H. HeyerAbstract:Context. 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. Aims. 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 models and Molecular spectral line observations of real Molecular Clouds. Methods. We use principal component analysis, applied to both models and observational data, to extract a quantitative measure of the driving scale of turbulence. Results. 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. Conclusions. Astrophysical processes acting on large scales, including supernova-driven turbulence, magneto-rotational 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.
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kinematic distances to Molecular Clouds identified in the galactic ring survey
The Astrophysical Journal, 2009Co-Authors: Julia Romanduval, Mark H. Heyer, J M Jackson, J M Rathborne, A M Johnson, Ronak Y Shah, R SimonAbstract:Kinematic distances to 750 Molecular Clouds identified in the 13 CO J = 1 → 0 Boston University–Five College Radio Astronomy Observatory Galactic Ring Survey (GRS) are derived assuming the Clemens rotation curve of the Galaxy. The kinematic distance ambiguity is resolved by examining the presence of Hi self-absorption toward the 13 CO emission peak of each cloud using the Very Large Array Galactic Plane Survey. We also identify 21 cm continuum sources embedded in the GRS Clouds in order to use absorption features in the Hi 21 cm continuum to distinguish between near and far kinematic distances. The Galactic distribution of GRS Clouds is consistent with a four-arm model of the Milky Way. The locations of the Scutum-Crux and Perseus arms traced by GRS Clouds match star-count data from the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire star-count data. We conclude that Molecular Clouds must form in spiral arms and be short-lived (lifetimes < 10 7 yr) in order to explain the absence of massive, 13 CO bright Molecular Clouds in the interarm space.
M Ostrowski - One of the best experts on this subject based on the ideXlab platform.
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supernova remnants in Molecular Clouds on cosmic ray electron spectra
Astronomy and Astrophysics, 1999Co-Authors: M OstrowskiAbstract:The particle acceleration process at a shock wave, in the presence of the second-order Fermi acceleration in the turbulent medium near the shock, is discussed as an alterna- tive explanation for the observed flat synchrotron spectra of supernova remnants (SNRs) in Molecular Clouds. We argue that medium AlfvMach number shocks considered by Chevalier (1999) for such SNRs can naturally lead to the observed spectral indices.
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Supernova remnants in Molecular Clouds: on cosmic ray electron spectra
Astronomy & Astrophysics, 1999Co-Authors: M OstrowskiAbstract:The particle acceleration process at a shock wave, in the presence of the second-order Fermi acceleration in the turbulent medium near the shock, is discussed as an alternative explanation for the observed flat synchrotron spectra of supernova remnants (SNRs) in Molecular Clouds. We argue that medium Alfv\'en Mach number shocks considered by Chevalier (1999, ApJ, in press) for such SNRs can naturally lead to the observed spectral indices.
Åke Nordlund - One of the best experts on this subject based on the ideXlab platform.
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The structure and characteristic scales of Molecular Clouds
Astronomy and Astrophysics - A&A, 2020Co-Authors: Sami Dib, D. Elia, N. Schneider, D. Arzoumanian, Sylvain Bontemps, Volker Ossenkopf-okada, Mohsen Shadmehri, Frédérique Motte, Mark Heyer, Åke NordlundAbstract:The structure of Molecular Clouds holds important clues regarding the physical processes that lead to their formation and subsequent dynamical evolution. While it is well established that turbulence imprints a self-similar structure onto the Clouds, other processes, such as gravity and stellar feedback, can break their scale-free nature. The break of self-similarity can manifest itself in the existence of characteristic scales that stand out from the underlying structure generated by turbulent motions. In this work, we investigate the structure of the Cygnus-X North and Polaris Flare Molecular Clouds, which represent two extremes in terms of their star formation activity. We characterize the structure of the Clouds using the delta-variance (Δ-variance) spectrum. In the Polaris Flare, the structure of the cloud is self-similar over more than one order of magnitude in spatial scales. In contrast, the Δ-variance spectrum of Cygnus-X North exhibits an excess and a plateau on physical scales of ≈0.5−1.2 pc. In order to explain the observations for Cygnus-X North, we use synthetic maps where we overlay populations of discrete structures on top of a fractal Brownian motion (fBm) image. The properties of these structures, such as their major axis sizes, aspect ratios, and column density contrasts with the fBm image, are randomly drawn from parameterized distribution functions. We are able to show that, under plausible assumptions, it is possible to reproduce a Δ-variance spectrum that resembles that of the Cygnus-X North region. We also use a “reverse engineering” approach in which we extract the compact structures in the Cygnus-X North cloud and reinject them onto an fBm map. Using this approach, the calculated Δ-variance spectrum deviates from the observations and is an indication that the range of characteristic scales (≈0.5−1.2 pc) observed in Cygnus-X North is not only due to the existence of compact sources, but is a signature of the whole population of structures that exist in the cloud, including more extended and elongated structures.
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Scaling Relations of Supersonic Turbulence in Molecular Clouds
Astrophysics and Space Science, 2004Co-Authors: S. Boldyrev, R. Jiménez, Paolo Padoan, Åke NordlundAbstract:We discuss a model for driven supersonic, super-Alfvénic MHD turbulence that is believed to govern the structure of Molecular Clouds. Such turbulence is highly intermittent; we describe its statistical properties by obtaining scaling of velocity-difference structure functions. This scaling was analytically predicted in Boldyrev (2002), confirmed in numerical simulations by Boldyrev et al. (2002), and discovered in observations by Padoan et al. (2003).
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Scaling Relations of Supersonic Turbulence in Molecular Clouds
Astrophysics and Space Science, 2004Co-Authors: S. Boldyrev, R. Jiménez, Paolo Padoan, Åke NordlundAbstract:We discuss a model for driven supersonic, super-Alfvenic MHD turbulence that is believed to govern the structure of Molecular Clouds. Such turbulence is highly intermittent; we describe its statistical properties by obtaining scaling of velocity-difference structure functions. This scaling was analytically predicted in Boldyrev (2002), confirmed in numerical simulations by Boldyrev et al. (2002), and discovered in observations by Padoan et al. (2003).
Javier Ballesterosparedes - One of the best experts on this subject based on the ideXlab platform.
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gravity or turbulence ii evolving column density probability distribution functions in Molecular Clouds
Monthly Notices of the Royal Astronomical Society, 2011Co-Authors: Javier Ballesterosparedes, Lee Hartmann, Enrique Vazquezsemadeni, Adriana Gazol, Fabian Heitsch, Pedro ColinAbstract:It has been recently shown that Molecular Clouds do not exhibit a unique shape for the column density probability distribution function (N-PDF). Instead, Clouds without star formation seem to possess a lognormal distribution, while Clouds with active star formation develop a powerlaw tail at high column densities. The lognormal behaviour of the N-PDF has been interpreted in terms of turbulent motions dominating the dynamics of the Clouds, while the power-law behaviour occurs when the cloud is dominated by gravity. In the present contribution, we use thermally bi-stable numerical simulations of cloud formation and evolution to show that, indeed, these two regimes can be understood in terms of the formation and evolution of Molecular Clouds: a very narrow lognormal regime appears when the cloud is being assembled. However, as the global gravitational contraction occurs, the initial density fluctuations are enhanced, resulting, first, in a wider lognormal N-PDF, and later, in a power-law N-PDF. We thus suggest that the observed N-PDF of Molecular Clouds are a manifestation of their global gravitationally contracting state. We also show that, contrary to recent suggestions, the exact value of the power-law slope is not unique, as it depends on the projection in which the cloud is being observed.
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rapid formation of Molecular Clouds and stars in the solar neighborhood
The Astrophysical Journal, 2001Co-Authors: Lee Hartmann, Javier Ballesterosparedes, Edwin A BerginAbstract:We show how Molecular Clouds in the solar neighborhood might be formed and produce stars rapidly enough to explain stellar population ages, building on results from numerical simulations of the turbulent interstellar medium and general considerations of Molecular gas formation. Observations of both star-forming regions and young, gas-free stellar associations indicate that most nearby Molecular Clouds form stars only over a short time span before dispersal; large-scale —ows in the diUuse interstellar medium have the potential for forming Clouds sufficiently rapidly and for producing stellar populations with ages much less than the lateral crossing times of their host Molecular Clouds. We identify four important factors for understanding rapid star formation and short cloud lifetimes. First, much of the accumulation and dispersal of Clouds near the solar circle might occur in the atomic phase; only the
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rapid formation of Molecular Clouds and stars in the solar neighborhood
arXiv: Astrophysics, 2001Co-Authors: Lee Hartmann, Javier Ballesterosparedes, Edwin A BerginAbstract:Observations of both star-forming regions and young, gas-free stellar associations indicate that most nearby Molecular Clouds form stars only over a short time span before dispersal; large-scale flows in the diffuse interstellar medium have the potential for forming Clouds sufficiently rapidly, and for producing stellar populations with ages much less than the lateral crossing times of their host Molecular Clouds. We identify four important factors for understanding rapid star formation and short cloud lifetimes. First, much of the accumulation and dispersal of Clouds near the solar circle might occur in the atomic phase; only the high-density portion of a cloud's lifecycle is spent in the Molecular phase, thus helping to limit Molecular cloud ``lifetimes''. Second, once a cloud achieves a high enough column density to form $\h2$ and CO, gravitational forces become larger than typical interstellar pressure forces; thus star formation can follow rapidly upon Molecular gas formation and turbulent dissipation in limited areas of each cloud complex. Third, typical magnetic fields are not strong enough to prevent rapid cloud formation and gravitational collapse. Fourth, rapid dispersal of gas by newly-formed stars, and reduction of shielding by a small expansion of the cloud after the first events of star formation, might limit the length of the star formation epoch and the lifetime of a cloud in its Molecular state. This picture emphasizes the importance of large-scale boundary conditions for understanding Molecular cloud formation, and implies that star formation is a highly dynamic, rather than quasi-static, process.
