2,430 research outputs found
A method for detection of structure
Context. In order to understand the evolution of molecular clouds it is
important to identify the departures from self-similarity associated with the
scales of self-gravity and the driving of turbulence.
Aims. A method is described based on structure functions for determining
whether a region of gas, such as a molecular cloud, is fractal or contains
structure with characteristic scale sizes.
Methods. Using artificial data containing structure it is shown that
derivatives of higher order structure functions provide a powerful way to
detect the presence of characteristic scales should any be present and to
estimate the size of such structures. The method is applied to observations of
hot H2 in the Kleinman-Low nebula, north of the Trapezium stars in the Orion
Molecular Cloud, including both brightness and velocity data. The method is
compared with other techniques such as Fourier transform and histogram
techniques.
Results. It is found that the density structure, represented by H2 emission
brightness in the K-band (2-2.5micron), exhibits mean characteristic sizes of
110, 550, 1700 and 2700AU. The velocity data show the presence of structure at
140, 1500 and 3500AU. Compared with other techniques such as Fourier transform
or histogram, the method appears both more sensitive to characteristic scales
and easier to interpret.Comment: Astronomy and Astrophysics, in pres
The Turbulent Interstellar Medium
An overview is presented of the main properties of the interstellar medium.
Evidence is summarized that the interstellar medium is highly turbulent, driven
on different length scales by various energetic processes. Large-scale
turbulence determines the formation of structures like filaments and shells in
the diffuse interstellar medium. It also regulates the formation of dense, cold
molecular clouds. Molecular clouds are now believed to be transient objects
that form on timescales of order 1e7 yrs in regions where HI gas is compressed
and cools. Supersonic turbulence in the compressed HI slab is generated by a
combination of hydrodynamical instabilities, coupled with cooling. Turbulent
dissipation is compensated by the kinetic energy input of the inflow. Molecular
hydrogen eventually forms when the surface density in the slab reaches a
threshold value of 1e21 cm^-2 at which point further cooling triggers the onset
of star formation by gravitational collapse. A few Myrs later, the newly formed
stars and resulting supernovae will disperse their molecular surrounding and
generate new expanding shells that drive again turbulence in the diffuse gas
and trigger the formation of a next generation of cold clouds. Although a
consistent scenario of interstellar medium dynamics and star formation is
emerging many details are still unclear and require more detailed work on
microphysical processes as well as a better understanding of supersonic,
compressible turbulence.Comment: 13 pages, 4 figures, to appear in "Statistical Mechanics of
Non-Extensive Systems", eds. F. Combes and R. Robert (Elsevier
Isotropically Driven versus Outflow Driven Turbulence: Observational Consequences for Molecular Clouds
Feedback from protostellar outflows can influence the nature of turbulence in
star forming regions even if they are not the primary source of velocity
dispersion for all scales of molecular clouds. For the rate and power expected
in star forming regions, we previously (Carroll et al. 2009) demonstrated that
outflows could drive supersonic turbulence at levels consistent with the
scaling relations from Matzner 2007 although with a steeper velocity power
spectrum than expected for an isotropically driven supersonic turbulent
cascade. Here we perform higher resolution simulations and combine simulations
of outflow driven turbulence with those of isotropically forced turbulence. We
find that the presence of outflows within an ambient isotropically driven
turbulent environment produces a knee in the velocity power spectrum at the
outflow scale and a steeper slope at sub-outflow scales than for a purely
isotropically forced case. We also find that the presence of outflows flattens
the density spectrum at large scales effectively reducing the formation of
large scale turbulent density structures. These effects are qualitatively
independent of resolution. We have also carried out Principal Component
Analysis (PCA) for synthetic data from our simulations. We find that PCA as a
tool for identifying the driving scale of turbulence has a misleading bias
toward low amplitude large scale velocity structures even when they are not
necessarily the dominant energy containing scales. This bias is absent for
isotropically forced turbulence but manifests strongly for collimated outflow
driven turbulence.Comment: 30 pages, 10 figures, Submitted to Ap
A new radiative cooling curve based on an up to date plasma emission code
This work presents a new plasma cooling curve that is calculated using the
SPEX package. We compare our cooling rates to those in previous works, and
implement the new cooling function in the grid-adaptive framework `AMRVAC'.
Contributions to the cooling rate by the individual elements are given, to
allow for the creation of cooling curves tailored to specific abundance
requirements. In some situations, it is important to be able to include
radiative losses in the hydrodynamics. The enhanced compression ratio can
trigger instabilities (such as the Vishniac thin-shell instability) that would
otherwise be absent. For gas with temperatures below 10,000 K, the cooling time
becomes very long and does not affect the gas on the timescales that are
generally of interest for hydrodynamical simulations of circumstellar plasmas.
However, above this temperature, a significant fraction of the elements is
ionised, and the cooling rate increases by a factor 1000 relative to lower
temperature plasmas.Comment: 11 pages, 6 figures. Typos fixed to match version on A&A
'forthcoming' website. Tables in text format online available at
http://www.phys.uu.nl/~schure/coolin
Scaling laws in decaying helical hydromagnetic turbulence
We study the evolution of growth and decay laws for the magnetic field
coherence length xi, energy E_M and magnetic helicity H in freely decaying 3D
MHD turbulence. We show that with certain assumptions, self-similarity of the
magnetic power spectrum alone implies that xi~t^{1/2}. This in turn implies
that magnetic helicity decays as H~t^{-2s}, where s=(xi_{diff}/xi_H)^2, in
terms of xi_{diff}, the diffusion length scale, and xi_H, a length scale
defined from the helicity power spectrum. The relative magnetic helicity
remains constant, implying that the magnetic energy decays as E_M~t^{-1/2-2s}.
The parameter s is inversely proportional to the magnetic Reynolds number Re_M,
which is constant in the self-similar regime.Comment: 7 pages, 3 figures, accepted by Astron Nach
Dependence of the Star Formation Efficiency on the Parameters of Molecular Cloud Formation Simulations
We investigate the response of the star formation efficiency (SFE) to the
main parameters of simulations of molecular cloud formation by the collision of
warm diffuse medium (WNM) cylindrical streams, neglecting stellar feedback and
magnetic fields. The parameters we vary are the Mach number of the inflow
velocity of the streams, Msinf, the rms Mach number of the initial background
turbulence in the WNM, and the total mass contained in the colliding gas
streams, Minf. Because the SFE is a function of time, we define two estimators
for it, the "absolute" SFE, measured at t = 25 Myr into the simulation's
evolution (sfeabs), and the "relative" SFE, measured 5 Myr after the onset of
star formation in each simulation (sferel). The latter is close to the "star
formation rate per free-fall time" for gas at n = 100 cm^-3. We find that both
estimators decrease with increasing Minf, although by no more than a factor of
2 as Msinf increases from 1.25 to 3.5. Increasing levels of background
turbulence similarly reduce the SFE, because the turbulence disrupts the
coherence of the colliding streams, fragmenting the cloud, and producing
small-scale clumps scattered through the numerical box, which have low SFEs.
Finally, the SFE is very sensitive to the mass of the inflows, with sferel
decreasing from ~0.4 to ~0.04 as the the virial parameter in the colliding
streams increases from ~0.15 to ~1.5. This trend is in partial agreement with
the prediction by Krumholz & McKee (2005), since the latter lies within the
same range as the observed efficiencies, but with a significantly shallower
slope. We conclude that the observed variability of the SFE is a highly
sensitive function of the parameters of the cloud formation process, and may be
the cause of significant scatter in observational determinations.Comment: 19 pages, submitted to MNRA
Evolution of the nucleus
Under a Creative Commons license.The nucleus represents a major evolutionary transition. As a consequence of separating translation from transcription many new functions arose, which likely contributed to the remarkable success of eukaryotic cells. Here we will consider what has recently emerged on the evolutionary histories of several key aspects of nuclear biology; the nuclear pore complex, the lamina, centrosomes and evidence for prokaryotic origins of relevant players.Work in our laboratories was supported by the following agencies, and which is gratefully acknowledged; MRC and Wellcome Trust (MR/K008749/1 and 090007/Z/09/Z respectively, to MCF), C2A Junta de Andalucia to DPD and DFG GR1642/4-1 to RG.Open Access funded by Wellcome Trust.Peer Reviewe
Magnetic helicity in primordial and dynamo scenarios of galaxies
Some common properties of helical magnetic fields in decaying and driven
turbulence are discussed. These include mainly the inverse cascade that
produces fields on progressively larger scales. Magnetic helicity also
restricts the evolution of the large scale field: the field decays less rapidly
than a non-helical field, but it also saturates more slowly, i.e. on a
resistive time scale if there are no magnetic helicity fluxes. The former
effect is utilized in primordial field scenarios, while the latter is important
for successfully explaining astrophysical dynamos that saturate faster than
resistively. Dynamo action is argued to be important not only in the galactic
dynamo, but also in accretion discs in active galactic nuclei and around
protostars, both of which contribute to producing a strong enough seed magnetic
field. Although primordial magnetic fields may be too weak to compete with
these astrophysical mechanisms, such fields could perhaps still be important in
producing polarization effects in the cosmic background radiation.Comment: 8 pages, 5 figures, talk given in Bologna, August 2006, proceedings
of "The Origin and Evolution of Cosmic Magnetism", Astron. Nachr. (in press
A Phenomenological Analysis of Gluon Mass Effects in Inclusive Radiative Decays of the and $\Upsilon
The shapes of the inclusive photon spectra in the processes \Jp \to \gamma
X and \Up \to \gamma X have been analysed using all available experimental
data.
Relativistic, higher order QCD and gluon mass corrections were taken into
account in the fitted functions. Only on including the gluon mass corrections,
were consistent and acceptable fits obtained. Values of
GeV and GeV were found for the
effective gluon masses (corresponding to Born level diagrams) for the \Jp and
\Up respectively. The width ratios \Gamma(V \to {\rm hadrons})/\Gamma(V \to
\gamma+ {\rm hadrons}) V=\Jp, \Up were used to determine and . Values consistent with the current world
average were obtained only when gluon mass correction factors,
calculated using the fitted values of the effective gluon mass, were applied. A
gluon mass GeV, as suggested with these results, is consistent with
previous analytical theoretical calculations and independent phenomenological
estimates, as well as with a recent, more accurate, lattice calculation of the
gluon propagator in the infra-red region.Comment: 50 pages, 11 figures, 15 table
Models of Giant Planet formation with migration and disc evolution
We present a new model of giant planet formation that extends the
core-accretion model of Pollack etal (1996) to include migration, disc
evolution and gap formation. We show that taking into account these effects can
lead to a much more rapid formation of giant planets, making it compatible with
the typical disc lifetimes inferred from observations of young circumstellar
discs. This speed up is due to the fact that migration prevents the severe
depletion of the feeding zone as observed in in situ calculations. Hence, the
growing planet is never isolated and it can reach cross-over mass on a much
shorter timescale. To illustrate the range of planets that can form in our
model, we describe a set of simulations in which we have varied some of the
initial parameters and compare the final masses and semi-major axes with those
inferred from observed extra-solar planets.Comment: Accepted in Astronomy & Astrophysic
- …