1,383 research outputs found

### Radiative torques alignment in the presence of pinwheel torques

We study the alignment of grains subject to both radiative torques and
pinwheel torques while accounting for thermal flipping of grains. By pinwheel
torques we refer to all systematic torques that are fixed in grain body axes,
including the radiative torques arising from scattering and absorption of
isotropic radiation. We discuss new types of pinwheel torques, which are
systematic torques arising from infrared emission and torques arising from the
interaction of grains with ions and electrons in hot plasma. We show that both
types of torques are long-lived, i.e. may exist longer than gaseous damping
time. We compare these torques with the torques introduced by E. Purcell,
namely, torques due to H$_2$ formation, the variation of accommodation
coefficient for gaseous collisions and photoelectric emission. Furthermore, we
revise the Lazarian & Draine model for grain thermal flipping. We calculate
mean flipping timescale induced by Barnett and nuclear relaxation for both
paramagnetic and superparamagnetic grains, in the presence of stochastic
torques associated with pinwheel torques, e.g. the stochastic torques arising
from H$_2$ formation, and gas bombardment. We show that the combined effect of
internal relaxation and stochastic torques can result in fast flipping for
sufficiently small grains and, because of this, they get thermally trapped,
i.e. rotate thermally in spite of the presence of pinwheel torques. For
sufficiently large grains, we show that the pinwheel torques can increase the
degree of grain alignment achievable with the radiative torques by increasing
the magnitude of the angular momentum of low attractor points and/or by driving
grains to new high attractor points.Comment: 23 pages and 15 figures emulated ApJ style. Thermal flipping and
trapping revised; paper accepted to Ap

### Thermal Flipping and Thermal Trapping -- New Elements in Dust Grain Dynamics

Since the classical work by Purcell (1979) it has been generally accepted
that most interstellar grains rotate suprathermally. Suprathermally rotating
grains would be nearly perfectly aligned with the magnetic field by
paramagnetic dissipation if not for ``crossovers'', intervals of low angular
velocity resulting from reversals of the torques responsible for suprathermal
rotation; during crossovers grains are susceptible to disalignment by random
impulses.
Lazarian and Draine (1997) identified thermal fluctuations within grain
material as an important component of crossover dynamics. For grains of size
less than 0.1 micron, these fluctuations ensure good correlation of angular
momentum before and after crossover resulting in good alignment, in accord with
observations of starlight polarization. In the present paper we discuss two new
processes which are important for the dynamics of grains with a<0.1 micron. The
first -- ``thermal flipping'' -- offers a way for small grains to bypass the
period of greatly reduced angular momentum which would otherwise take place
during a crossover, thereby enhancing the alignment of small grains. The second
effect -- ``thermal trapping'' -- arises when thermal flipping becomes rapid
enough to prevent the systematic torques from driving the grain to suprathermal
rotation. This effect acts to reduce the alignment of small grains.
The observed variation of grain alignment with grain size would then result
from a combination of the thermal flipping process -- which suppresses
suprathermal rotation of small grains -- and due to molecular hydrogen
formation and starlight -- which drive large grains to suprathermal rotation
rates.Comment: 16 pages, 2 figures, submitted ApJ

### Radiative torque alignment: Essential Physical Processes

We study the physical processes that affect the alignment of grains subject
to radiative torques (RATs). To describe the action of RATs, we use the
analytical model (AMO) of RATs introduced in Paper I. We focus our discussion
on the alignment by anisotropic radiation flux with respect to magnetic field,
which defines the axis of grain Larmor precession. Such an alignment does not
invoke paramagnetic dissipation (i.e. Davis-Greenstein mechanism), but,
nevertheless, grains tend to be aligned with long axes perpendicular to the
magnetic field. When we account for thermal fluctuations within grain material,
we show that for grains, which are characterized by a triaxial ellipsoid of
inertia, the zero-$J$ attractor point obtained in our earlier study develops
into a low-$J$ attractor point. We study effects of stochastic gaseous
bombardment and show that gaseous bombardment can drive grains from low-$J$ to
high-$J$ attractor points in cases when the high-$J$ attractor points are
present. As the alignment of grain axes with respect to angular momentum is
higher for higher values of $J$, counter-intuitively, gaseous bombardment can
increase the degree of grain alignment in respect to the magnetic field. We
also study the effects of torques induced by H$_2$ formation and show that they
can change the value of angular momentum at high-$J$ attractor point, but
marginally affect the value of angular momentum at low-$J$ attractor points. We
compare the AMO results with those obtained using the direct numerical
calculations of RATs acting upon irregular grains and validate the use of the
AMO for realistic situations of RAT alignment.Comment: 31 pages. MNRAS 2007, in press, typos are corrected

### Tsallis statistics as a tool for studying interstellar turbulence

We used magnetohydrodynamic (MHD) simulations of interstellar turbulence to
study the probability distribution functions (PDFs) of increments of density,
velocity, and magnetic field. We found that the PDFs are well described by a
Tsallis distribution, following the same general trends found in solar wind and
Electron MHD studies. We found that the PDFs of density are very different in
subsonic and supersonic turbulence. In order to extend this work to ISM
observations we studied maps of column density obtained from 3D MHD
simulations. From the column density maps we found the parameters that fit to
Tsallis distributions and demonstrated that these parameters vary with the
sonic and Alfv\'en Mach numbers of turbulence. This opens avenues for using
Tsallis distributions to study the dynamical and perhaps magnetic states of
interstellar gas.Comment: 8 pages, 5 figures, 1 table. Accepted for publication in the
Astrophysical Journa

### Electric dipole moments and disalignment of interstellar dust grains

The degree to which interstellar grains align with respect to the
interstellar magnetic field depends on disaligning as well as aligning
mechanisms. For decades, it was assumed that disalignment was due primarily to
the random angular impulses a grain receives when colliding with gas-phase
atoms. Recently, a new disalignment mechanism has been considered, which may be
very potent for a grain that has a time-varying electric dipole moment and
drifts across the magnetic field. We provide quantitative estimates of the
disalignment times for silicate grains with size > approximately 0.1 micron.
These appear to be shorter than the time-scale for alignment by radiative
torques, unless the grains contain superparamagnetic inclusions.Comment: 12 pages, 9 figures, submitted to MNRA

### Interstellar Sonic and Alfv\'enic Mach Numbers and the Tsallis Distribution

In an effort to characterize the Mach numbers of ISM magnetohydrodynamic
(MHD) turbulence, we study the probability distribution functions (PDFs) of
patial increments of density, velocity, and magnetic field for fourteen ideal
isothermal MHD simulations at resolution 512^3. In particular, we fit the PDFs
using the Tsallis function and study the dependency of fit parameters on the
compressibility and magnetization of the gas. We find that the Tsallis function
fits PDFs of MHD turbulence well, with fit parameters showing sensitivities to
the sonic and Alfven Mach numbers. For 3D density, column density, and
position-position-velocity (PPV) data we find that the amplitude and width of
the PDFs shows a dependency on the sonic Mach number. We also find the width of
the PDF is sensitive to global Alfvenic Mach number especially in cases where
the sonic number is high. These dependencies are also found for mock
observational cases, where cloud-like boundary conditions, smoothing, and noise
are introduced. The ability of Tsallis statistics to characterize sonic and
Alfvenic Mach numbers of simulated ISM turbulence point to it being a useful
tool in the analysis of the observed ISM, especially when used simultaneously
with other statistical techniques.Comment: 20 pages, 16 figures, ApJ submitte

### Studying Turbulence using Doppler-broadened lines: Velocity Coordinate Spectrum

We discuss a new technique for studying astrophysical turbulence that
utilizes the statistics of Doppler-broadened spectral lines. The technique
relates the power Velocity Coordinate Spectrum (VCS), i.e. the spectrum of
fluctuations measured along the velocity axis in Position-Position-Velocity
(PPV) data cubes available from observations, to the underlying power spectra
of the velocity/density fluctuations. Unlike the standard spatial spectra, that
are function of angular wavenumber, the VCS is a function of the velocity wave
number k_v ~ 1/v. We show that absorption affects the VCS to a higher degree
for small k_v and obtain the criteria for disregarding the absorption effects
for turbulence studies at large k_v. We consider the retrieval of turbulence
spectra from observations for high and low spatial resolution observations and
find that the VCS allows one to study turbulence even when the emitting
turbulent volume is not spatially resolved. This opens interesting prospects
for using the technique for extragalactic research. We show that, while thermal
broadening interferes with the turbulence studies using the VCS, it is possible
to separate thermal and non-thermal contributions. This allows a new way of
determining the temperature of the interstellar gas using emission and
absorption spectral lines.Comment: 27 page, 3 figures, content extended and presentation reorganized to
correspond to the version accepted to Ap

### Velocity Field of Compressible MHD Turbulence: Wavelet Decomposition and Mode Scalings

We study compressible MHD turbulence, which holds key to many astrophysical
processes, including star formation and cosmic ray propagation. To account for
the variations of the magnetic field in the strongly turbulent fluid we use
wavelet decomposition of the turbulent velocity field into Alfven, slow and
fast modes, which presents an extension of the Cho & Lazarian (2003)
decomposition approach based on Fourier transforms. The wavelets allow to
follow the variations of the local direction of magnetic field and therefore
improve the quality of the decomposition compared to the Fourier transforms
which are done in the mean field reference frame. For each resulting component
we calculate spectra and two-point statistics such as longitudinal and
transverse structure functions, as well as, higher order intermittency
statistics. In addition, we perform the Helmholtz-Hodge decomposition of the
velocity field into the incompressible and compressible parts and analyze these
components. We find that the turbulence intermittency is different for
different components and we show that the intermittency statistics depend on
whether the phenomenon was studied in the global reference frame related to the
mean magnetic field or it was studied in the frame defined by the local
magnetic field. The dependencies of the measures we obtained are different for
different components of velocity, for instance, we show that while the Alfven
mode intermittency changes marginally with the Mach number the intermittency of
the fast mode is substantially affected by the change.Comment: 16 pages, 9 figures, 2 table

### Galactic foregrounds: Spatial fluctuations and a procedure of removal

Present day cosmic microwave background (CMB) studies require more accurate
removal of Galactic foreground emission. In this paper, we consider a way of
filtering out the diffuse Galactic fluctuations on the basis of their
statistical properties, namely, the power-law spectra of fluctuations. We focus
on the statistical properties of two major Galactic foregrounds that arise from
magnetized turbulence, namely, diffuse synchrotron emission and thermal
emission from dust and describe how their power laws change with the Galactic
latitude. We attribute this change to the change of the geometry of the
emission region and claim that the universality of the turbulence spectrum
provides a new way of removing Galactic foregrounds. We discuss and demonstrate
how we can make use of our findings to remove Galactic foregrounds using a
template of spatial fluctuations. In particular, we consider examples of
spatial filtering of a foreground at small scales, when the separation into CMB
signal and foregrounds is done at larger scales. We demonstrate that the new
technique of spatial filtering of foregrounds may be promising for recovering
the CMB signal in a situation when foregrounds are known at a scale different
from the one under study. It can also improve filtering by combining
measurements obtained at different scales.Comment: 24 pages, 12 figures, accepted to Ap

### Magnetic reconnection as the cause of cosmic ray excess from the heliospheric tail

The observation of a broad excess of sub-TeV cosmic rays compatible with the
direction of the heliospheric tail (Nagashima et al. 1998) and the discovery of
two significant localized excess regions of multi-TeV cosmic rays by the
MILAGRO collaboration (Abdo et al. 2008), also from the same region of the sky,
have raised questions on their origin. In particular, the coincidence of the
most significant localized region with the direction of the heliospheric tail
and the small angular scale of the observed anisotropy (~ 10deg) is suggestive
a local origin and of a possible connection to the low energy broad excess.
Cosmic ray acceleration from magnetic reconnection in the magnetotail is
proposed as a possible source of the energetic particles.Comment: 10 pages, 5 figures, accepted for publication in Ap

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