294 research outputs found
Statistical Assessment of Shapes and Magnetic Field Orientations in Molecular Clouds through Polarization Observations
We present a novel statistical analysis aimed at deriving the intrinsic
shapes and magnetic field orientations of molecular clouds using dust emission
and polarization observations by the Hertz polarimeter. Our observables are the
aspect ratio of the projected plane-of-the-sky cloud image, and the angle
between the mean direction of the plane-of-the-sky component of the magnetic
field and the short axis of the cloud image. To overcome projection effects due
to the unknown orientation of the line-of-sight, we combine observations from
24 clouds, assuming that line-of-sight orientations are random and all are
equally probable. Through a weighted least-squares analysis, we find that the
best-fit intrinsic cloud shape describing our sample is an oblate disk with
only small degrees of triaxiality. The best-fit intrinsic magnetic field
orientation is close to the direction of the shortest cloud axis, with small
(~24 deg) deviations toward the long/middle cloud axes. However, due to the
small number of observed clouds, the power of our analysis to reject
alternative configurations is limited.Comment: 14 pages, 8 figures, accepted for publication in MNRA
Magnetic Fields in Star-Forming Molecular Clouds II. The Depolarization Effect in the OMC-3 Filament of Orion A
Polarized 850 micron thermal emission data of the region OMC-3 in the Orion A
molecular cloud are presented. These data, taken in 1998 with the SCUBA
polarimeter mounted on the James Clerk Maxwell Telescope, have been re-reduced
using improved software. The polarization pattern is not suggestive of a
uniform field structure local to OMC-3, nor does the orientation of the vectors
align with existing polarimetry maps of the OMC-1 core 20' to the south. The
depolarization toward high intensity regions cannot be explained by uniform
field geometry except in the presence of changing grain structure, which is
most likely to occur in regions of high density or temperature (i.e. the
embedded cores). The depolarization in fact occurs along the length of the
filamentary structure of OMC-3 and is not limited to the vicinity of the bright
cores. Such a polarization pattern is predicted by helical field models for
filamentary clouds. We present three scenarios to explain the observed
polarization pattern of OMC-3 in terms of a helical field geometry. Qualitative
models incorporating a helical field geometry are presented for two cases.Comment: 57 pages, 12 figures, 3 tables; accepted for publication in Ap
Structuring and support by Alfven waves around prestellar cores
Observations of molecular clouds show the existence of starless, dense cores,
threaded by magnetic fields. Observed line widths indicate these dense
condensates to be embedded in a supersonically turbulent environment. Under
these conditions, the generation of magnetic waves is inevitable. In this
paper, we study the structure and support of a 1D plane-parallel,
self-gravitating slab, as a monochromatic, circularly polarized Alfven wave is
injected in its central plane. Dimensional analysis shows that the solution
must depend on three dimensionless parameters. To study the nonlinear,
turbulent evolution of such a slab, we use 1D high resolution numerical
simulations. For a parameter range inspired by molecular cloud observations, we
find the following. 1) A single source of energy injection is sufficient to
force persistent supersonic turbulence over several hydrostatic scale heights.
2) The time averaged spatial extension of the slab is comparable to the
extension of the stationary, analytical WKB solution. Deviations, as well as
the density substructure of the slab, depend on the wave-length of the injected
wave. 3) Energy losses are dominated by loss of Poynting-flux and increase with
increasing plasma beta. 4) Good spatial resolution is mandatory, making similar
simulations in 3D currently prohibitively expensive.Comment: 13 pages, 8 figures, accepted for publication in A&A. The manuscript
with full color, high-resolution, figures can be downloaded from
http://www.astro.phys.ethz.ch/papers/folini/folini_p_nf.htm
Rotation and X-ray emission from protostars
The ASCA satellite has recently detected variable hard X-ray emission from
two Class I protostars in the rho Oph cloud, YLW15 (IRS43) and WL6, with a
characteristic time scale ~20h. In YLW15, the X-ray emission is in the form of
quasi-periodic energetic flares, which we explain in terms of strong magnetic
shearing and reconnection between the central star and the accretion disk. In
WL6, X-ray flaring is rotationally modulated, and appears to be more like the
solar-type magnetic activity ubiquitous on T Tauri stars. We find that YLW15 is
a fast rotator (near break-up), while WL6 rotates with a significantly longer
period. We derive a mass M_\star ~ 2 M_\odot and \simlt 0.4 M_\odot for the
central stars of YLW15 and WL6 respectively. On the long term, the interactions
between the star and the disk results in magnetic braking and angular momentum
loss of the star. On time scales t_{br} ~ a few 10^5 yrs, i.e., of the same
order as the estimated duration of the Class~I protostar stage. Close to the
birthline there must be a mass-rotation relation, t_{br} \simpropto M_\star,
such that stars with M_\star \simgt 1-2 M_\odot are fast rotators, while their
lower-mass counterparts have had the time to spin down. The rapid rotation and
strong star-disk magnetic interactions of YLW15 also naturally explain the
observation of X-ray ``superflares''. In the case of YLW15, and perhaps also of
other protostars, a hot coronal wind (T~10^6 K) may be responsible for the VLA
thermal radio emission. This paper thus proposes the first clues to the
rotation status and evolution of protostars.Comment: 13 pages with 6 figures. To be published in ApJ (April 10, 2000 Part
1 issue
Magnetic Fields in Star-Forming Molecular Clouds III. Submillimeter Polarimetry of Intermediate Mass Cores and Filaments in Orion B
Using the imaging polarimeter for the Submillimeter Common User Bolometric
Array at the James Clerk Maxwell Telescope, we have detected polarized thermal
emission at 850 micron from dust toward three star-forming core systems in the
Orion B molecular cloud: NGC 2071, NGC 2024 and LBS 23N (HH 24). The
polarization patterns are not indicative of those expected for magnetic fields
dominated by a single field direction, and all exhibit diminished polarization
percentages toward the highest intensity peaks. NGC 2024 has the most organized
polarization pattern which is centered consistently along the length of a chain
of 7 far-infrared sources. We have modeled NGC 2024 using a helical field
geometry threading a curved filament and also as a magnetic field swept up by
the ionization front of the expanding HII region. In the latter case, the field
is bent by the dense ridge, which accounts for both the polarization pattern
and existing measurements of the line-of-sight field strength toward the
northern cores FIR 1 to FIR 4. The direction of the net magnetic field
direction within NGC 2071 is perpendicular to the dominant outflow in that
region. Despite evidence that line contamination exists in the 850 micron
continuum, the levels of polarization measured indicate that the polarized
emission is dominated by dust.Comment: 39 pages, 9 postscript figures (6 color), 3 tables; accepted for
publication in Ap
The G11.11-0.12 Infrared-Dark Cloud: Anomalous Dust and a Non-Magnetic Isothermal Model
The G11.11-0.12 Infrared-Dark Cloud has a filamentary appearance, both in
absorption against the diffuse 8micron Galactic background, and in emission
from cold dust at 850micron. Detailed comparison of the dust properties at
these two wavelengths reveals that standard models for the diffuse interstellar
dust in the Galaxy are not consistent with the observations. The ratio of
absorption coefficients within the cloud is kappa_8/kappa_850 <= 1010, which is
well below that expected for the diffuse ISM where kappa_8/kappa_850 ~ 1700.
This may be due to the formation of ice mantles on the dust and grain
coagulation, both of which are expected within dense regions of molecular
clouds. The 850micron emission probes the underlying radial structure of the
filament. The profile is well represented by a marginally resolved central
region and a steeply falling envelope, with Sigma(r) proportional to r^(-a),
where a <= 3, indicating that G11.11-0.12 is the first observed filament with a
profile similar to that of a non-magnetic isothermal cylinder.Comment: 13 pages, 4 figures, accepted for publication by ApJ Letter
Dynamics of an Intruder in Dense Granular Fluids
We investigate the dynamics of an intruder pulled by a constant force in a
dense two-dimensional granular fluid by means of event-driven molecular
dynamics simulations. In a first step, we show how a propagating momentum front
develops and compactifies the system when reflected by the boundaries. To be
closer to recent experiments \cite{candelier2010journey,candelier2009creep}, we
then add a frictional force acting on each particle, proportional to the
particle's velocity. We show how to implement frictional motion in an
event-driven simulation. This allows us to carry out extensive numerical
simulations aiming at the dependence of the intruder's velocity on packing
fraction and pulling force. We identify a linear relation for small and a
nonlinear regime for high pulling forces and investigate the dependence of
these regimes on granular temperature
Screening of melon genotypes for resistance to vegetable leafminer and your phenotypic correlations with colorimetry.
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Formation of Small-Scale Condensations in the Molecular Clouds via Thermal Instability
A systematic study of the linear thermal instability of a self-gravitating
magnetic molecular cloud is carried out for the case when the unperturbed
background is subject to local expansion or contraction. We consider the
ambipolar diffusion, or ion-neutral friction on the perturbed states. In this
way, we obtain a non-dimensional characteristic equation that reduces to the
prior characteristic equation in the non-gravitating stationary background. By
parametric manipulation of this characteristic equation, we conclude that there
are, not only oblate condensation forming solutions, but also prolate solutions
according to local expansion or contraction of the background. We obtain the
conditions for existence of the Field lengths that thermal instability in the
molecular clouds can occur. If these conditions establish, small-scale
condensations in the form of spherical, oblate, or prolate may be produced via
thermal instability.Comment: 16 page, accepted by Ap&S
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