85 research outputs found
Magnetic fields in protoplanetary disks
Magnetic fields likely play a key role in the dynamics and evolution of
protoplanetary discs. They have the potential to efficiently transport angular
momentum by MHD turbulence or via the magnetocentrifugal acceleration of
outflows from the disk surface, and magnetically-driven mixing has implications
for disk chemistry and evolution of the grain population. However, the weak
ionisation of protoplanetary discs means that magnetic fields may not be able
to effectively couple to the matter. I present calculations of the ionisation
equilibrium and magnetic diffusivity as a function of height from the disk
midplane at radii of 1 and 5 AU. Dust grains tend to suppress magnetic coupling
by soaking up electrons and ions from the gas phase and reducing the
conductivity of the gas by many orders of magnitude. However, once grains have
grown to a few microns in size their effect starts to wane and magnetic fields
can begin to couple to the gas even at the disk midplane. Because ions are
generally decoupled from the magnetic field by neutral collisions while
electrons are not, the Hall effect tends to dominate the diffusion of the
magnetic field when it is able to partially couple to the gas.
For a standard population of 0.1 micron grains the active surface layers have
a combined column of about 2 g/cm^2 at 1 AU; by the time grains have aggregated
to 3 microns the active surface density is 80 g/cm^2. In the absence of grains,
x-rays maintain magnetic coupling to 10% of the disk material at 1 AU (150
g/cm^2). At 5 AU the entire disk thickness becomes active once grains have
aggregated to 1 micron in size.Comment: 11 pages, 11 figs, aastex.cls. Accepted for publication in
Astrophysics & Space Science. v3 corrects bibliograph
Dimensionless cosmology
Although it is well known that any consideration of the variations of
fundamental constants should be restricted to their dimensionless combinations,
the literature on variations of the gravitational constant is entirely
dimensionful. To illustrate applications of this to cosmology, we explicitly
give a dimensionless version of the parameters of the standard cosmological
model, and describe the physics of Big Bang Neucleosynthesis and recombination
in a dimensionless manner. The issue that appears to have been missed in many
studies is that in cosmology the strength of gravity is bound up in the
cosmological equations, and the epoch at which we live is a crucial part of the
model. We argue that it is useful to consider the hypothetical situation of
communicating with another civilization (with entirely different units),
comparing only dimensionless constants, in order to decide if we live in a
Universe governed by precisely the same physical laws. In this thought
experiment, we would also have to compare epochs, which can be defined by
giving the value of any {\it one} of the evolving cosmological parameters. By
setting things up carefully in this way one can avoid inconsistent results when
considering variable constants, caused by effectively fixing more than one
parameter today. We show examples of this effect by considering microwave
background anisotropies, being careful to maintain dimensionlessness
throughout. We present Fisher matrix calculations to estimate how well the fine
structure constants for electromagnetism and gravity can be determined with
future microwave background experiments. We highlight how one can be misled by
simply adding to the usual cosmological parameter set
On-disk coronal rain
Small and elongated, cool and dense blob-like structures are being reported
with high resolution telescopes in physically different regions throughout the
solar atmosphere. Their detection and the understanding of their formation,
morphology and thermodynamical characteristics can provide important
information on their hosting environment, especially concerning the magnetic
field, whose understanding constitutes a major problem in solar physics. An
example of such blobs is coronal rain, a phenomenon of thermal non- equilibrium
observed in active region loops, which consists of cool and dense chromospheric
blobs falling along loop-like paths from coronal heights. So far, only off-limb
coronal rain has been observed and few reports on the phenomenon exist. In the
present work, several datasets of on-disk H{\alpha} observations with the CRisp
Imaging SpectroPolarimeter (CRISP) at the Swedish 1-m Solar Telescope (SST) are
analyzed. A special family of on-disk blobs is selected for each dataset and a
statistical analysis is carried out on their dynamics, morphology and
temperatures. All characteristics present distributions which are very similar
to reported coronal rain statistics. We discuss possible interpretations
considering other similar blob-like structures reported so far and show that a
coronal rain interpretation is the most likely one. Their chromospheric nature
and the projection effects (which eliminate all direct possibility of height
estimation) on one side, and their small sizes, fast dynamics, and especially,
their faint character (offering low contrast with the background intensity) on
the other side, are found as the main causes for the absence until now of the
detection of this on-disk coronal rain counterpart.Comment: 18 pages, 10 figures. Accepted for Solar Physic
Panspermia, Past and Present: Astrophysical and Biophysical Conditions for the Dissemination of Life in Space
Astronomically, there are viable mechanisms for distributing organic material
throughout the Milky Way. Biologically, the destructive effects of ultraviolet
light and cosmic rays means that the majority of organisms arrive broken and
dead on a new world. The likelihood of conventional forms of panspermia must
therefore be considered low. However, the information content of dam-aged
biological molecules might serve to seed new life (necropanspermia).Comment: Accepted for publication in Space Science Review
Magnetic Field Amplification in Galaxy Clusters and its Simulation
We review the present theoretical and numerical understanding of magnetic
field amplification in cosmic large-scale structure, on length scales of galaxy
clusters and beyond. Structure formation drives compression and turbulence,
which amplify tiny magnetic seed fields to the microGauss values that are
observed in the intracluster medium. This process is intimately connected to
the properties of turbulence and the microphysics of the intra-cluster medium.
Additional roles are played by merger induced shocks that sweep through the
intra-cluster medium and motions induced by sloshing cool cores. The accurate
simulation of magnetic field amplification in clusters still poses a serious
challenge for simulations of cosmological structure formation. We review the
current literature on cosmological simulations that include magnetic fields and
outline theoretical as well as numerical challenges.Comment: 60 pages, 19 Figure
Physical Processes in Star Formation
© 2020 Springer-Verlag. The final publication is available at Springer via https://doi.org/10.1007/s11214-020-00693-8.Star formation is a complex multi-scale phenomenon that is of significant importance for astrophysics in general. Stars and star formation are key pillars in observational astronomy from local star forming regions in the Milky Way up to high-redshift galaxies. From a theoretical perspective, star formation and feedback processes (radiation, winds, and supernovae) play a pivotal role in advancing our understanding of the physical processes at work, both individually and of their interactions. In this review we will give an overview of the main processes that are important for the understanding of star formation. We start with an observationally motivated view on star formation from a global perspective and outline the general paradigm of the life-cycle of molecular clouds, in which star formation is the key process to close the cycle. After that we focus on the thermal and chemical aspects in star forming regions, discuss turbulence and magnetic fields as well as gravitational forces. Finally, we review the most important stellar feedback mechanisms.Peer reviewedFinal Accepted Versio
Plantas medicinais de um remascente de Floresta Ombrófila Mista Altomontana, Urupema, Santa Catarina, Brasil
Statewide treatment outcome assessment in Colorado: The Colorado Client Assessment Record (CCAR)
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