148 research outputs found
Velocity Field Statistics in Star-Forming Regions. I. Centroid Velocity Observations
The probability density functions (pdfs) of molecular line centroid velocity
fluctuations and fluctuation differences at different spatial lags are
estimated for several nearby molecular clouds with active internal star
formation. The data consist of over 75,000 CO line profiles divided
among twelve spatially and/or kinematically distinct regions. Although three
regions (all in Mon R2) appear nearly Gaussian, the others show strong evidence
for non-Gaussian, often nearly exponential, centroid velocity pdfs, possibly
with power law contributions in the far tails. Evidence for nearly exponential
centroid pdfs in the neutral HI component of the ISM is also presented, based
on older optical and radio observations. These results are in contrast to pdfs
found in isotropic incompressible turbulence experiments and simulations.
Furthermore, no evidence is found for the scaling of difference pdf kurtosis
with Reynolds number which is seen in incompressible turbulence, and the
spatial distribution of high-amplitude velocity differences shows little
indication of the filamentary appearance predicted by decay simulations
dominated by vortical interactions. The variation with lag of the difference
pdf moments is presented as a constraint on future simulations.Comment: LaTeX, 23 pages, with 15 Figures included separately as gif image
files. Refereed/revised version accepted to the Astrophysical Journal. A
complete (but much larger) postscript version is available from
http://ktaadn.gsfc.nasa.gov/~miesc
A Uniform CO Survey of the Molecular Clouds in Orion and Monoceros
We report the results of a new large scale survey of the Orion-Monoceros
complex of molecular clouds made in the J = 1->0 line of CO with the
Harvard-Smithsonian 1.2m millimetre-wave telescope. The survey consists of
52,288 uniformly spaced spectra that cover an area of 432 square degrees on the
sky and is the most sensitive large-scale survey of the region to date.
Distances to the constituent molecular clouds of the complex, estimated from an
analysis of foreground and background stars, have provided information on the
three dimensional structure of the entire complex.Comment: Accepted for publication in Astronomy and Astrophysics. 19 pages with
17 colour figures - 39 if you count the sub-figures separately. The figures
here have been bit-mapped with some loss of quality and beauty. The paper
version in A&A will be in greyscale with the on-line version in colour. In
the meantime the colour version can be obtained by following links at
http://www.star.bris.ac.uk/mrwm . The 9MB PostScript is recommended if you
have appropriate bandwidth or otherwise the 2.3MB PDF is usabl
Astrophysical turbulence modeling
The role of turbulence in various astrophysical settings is reviewed. Among
the differences to laboratory and atmospheric turbulence we highlight the
ubiquitous presence of magnetic fields that are generally produced and
maintained by dynamo action. The extreme temperature and density contrasts and
stratifications are emphasized in connection with turbulence in the
interstellar medium and in stars with outer convection zones, respectively. In
many cases turbulence plays an essential role in facilitating enhanced
transport of mass, momentum, energy, and magnetic fields in terms of the
corresponding coarse-grained mean fields. Those transport properties are
usually strongly modified by anisotropies and often completely new effects
emerge in such a description that have no correspondence in terms of the
original (non coarse-grained) fields.Comment: 88 pages, 26 figures, published in Reports on Progress in Physic
Time-Distance Helioseismology of Deep Meridional Circulation
A key component of solar interior dynamics is the meridional circulation
(MC), whose poleward component in the surface layers has been well observed.
Time-distance helioseismic studies of the deep structure of MC, however, have
yielded conflicting inferences. Here, following a summary of existing results
we show how a large center-to-limb systematics (CLS) in the measured travel
times of acoustic waves affect the inferences through an analysis of frequency
dependence of CLS, using data from the Helioseismic and Doppler Imager (HMI)
onboard Solar Dynamics Observatory (SDO). Our results point to the residual
systematics in travel times as a major cause of differing inferences on the
deep structure of MC.Comment: 6 pages, 3 figures, to appear in the Springer series Astrophysics and
Space Science Proceedings of "Dynamics of the Sun & Stars: Honoring the Life
& Work of Michael Thompson" (2020
Turbulent magnetic Prandtl number and magnetic diffusivity quenching from simulations
Forced turbulence simulations are used to determine the turbulent kinematic
viscosity, nu_t, from the decay rate of a large scale velocity field. Likewise,
the turbulent magnetic diffusivity, eta_t, is determined from the decay of a
large scale magnetic field. In the kinematic regime, when the field is weak,
the turbulent magnetic Prandtl number, nu_t/eta_t, is about unity. When the
field is nonhelical, eta_t is quenched when magnetic and kinetic energies
become comparable. For helical fields the quenching is stronger and can be
described by a dynamical quenching formula.Comment: 7 pages, 6 figure
Seismic Monitoring of the Sun's Far Hemisphere: A Crucial Component in Future Space Weather Forecasting (A White Paper Submitted to the Decadal Survey for Solar and Space Physics (Heliophysics) -- SSPH 2024-2033)
The purpose of this white paper is to put together a coherent vision for the
role of helioseismic monitoring of magnetic activity in the Sun's far
hemisphere that will contribute to improving space weather forecasting as well
as fundamental research in the coming decade. Our goal fits into the broader
context of helioseismology in solar research for any number of endeavors when
helioseismic monitors may be the sole synoptic view of the Sun's far
hemisphere. It is intended to foster a growing understanding of solar activity,
as realistically monitored in both hemispheres, and its relationship to all
known aspects of the near-Earth and terrestrial environment. Some of the
questions and goals that can be fruitfully pursued through seismic monitoring
of farside solar activity in the coming decade include: What is the
relationship between helioseismic signatures and their associated magnetic
configurations, and how is this relationship connected to the solar EUV
irradiance over the period of a solar rotation?; How can helioseismic
monitoring contribute to data-driven global magnetic-field models for precise
space weather forecasting?; What can helioseismic monitors tell us about
prospects of a flare, CME or high-speed stream that impacts the terrestrial
environment over the period of a solar rotation?; How does the inclusion of
farside information contribute to forecasts of interplanetary space weather and
the environments to be encountered by human crews in interplanetary space?
Thus, it is crucial for the development of farside monitoring of the Sun be
continued into the next decade either through ground-based or space-borne
observations
The quest for the solar g modes
Solar gravity modes (or g modes) -- oscillations of the solar interior for
which buoyancy acts as the restoring force -- have the potential to provide
unprecedented inference on the structure and dynamics of the solar core,
inference that is not possible with the well observed acoustic modes (or p
modes). The high amplitude of the g-mode eigenfunctions in the core and the
evanesence of the modes in the convection zone make the modes particularly
sensitive to the physical and dynamical conditions in the core. Owing to the
existence of the convection zone, the g modes have very low amplitudes at
photospheric levels, which makes the modes extremely hard to detect. In this
paper, we review the current state of play regarding attempts to detect g
modes. We review the theory of g modes, including theoretical estimation of the
g-mode frequencies, amplitudes and damping rates. Then we go on to discuss the
techniques that have been used to try to detect g modes. We review results in
the literature, and finish by looking to the future, and the potential advances
that can be made -- from both data and data-analysis perspectives -- to give
unambiguous detections of individual g modes. The review ends by concluding
that, at the time of writing, there is indeed a consensus amongst the authors
that there is currently no undisputed detection of solar g modes.Comment: 71 pages, 18 figures, accepted by Astronomy and Astrophysics Revie
Control of star formation by supersonic turbulence
Understanding the formation of stars in galaxies is central to much of modern
astrophysics. For several decades it has been thought that stellar birth is
primarily controlled by the interplay between gravity and magnetostatic
support, modulated by ambipolar diffusion. Recently, however, both
observational and numerical work has begun to suggest that support by
supersonic turbulence rather than magnetic fields controls star formation. In
this review we outline a new theory of star formation relying on the control by
turbulence. We demonstrate that although supersonic turbulence can provide
global support, it nevertheless produces density enhancements that allow local
collapse. Inefficient, isolated star formation is a hallmark of turbulent
support, while efficient, clustered star formation occurs in its absence. The
consequences of this theory are then explored for both local star formation and
galactic scale star formation. (ABSTRACT ABBREVIATED)Comment: Invited review for "Reviews of Modern Physics", 87 pages including 28
figures, in pres
Statistical Properties of Turbulence: An Overview
We present an introductory overview of several challenging problems in the
statistical characterisation of turbulence. We provide examples from fluid
turbulence in three and two dimensions, from the turbulent advection of passive
scalars, turbulence in the one-dimensional Burgers equation, and fluid
turbulence in the presence of polymer additives.Comment: 34 pages, 31 figure
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