7,946 research outputs found
The Wardle Instability in Interstellar Shocks: I. Nonlinear Dynamical Evolution
The nonlinear evolution of unstable C-type shocks in weakly ionized plasmas
is studied by means of time-dependent magnetohydrodynamical simulations. This
study is limited to shocks in magnetically dominated plasmas (in which the
Alfven speed in the neutrals greatly exceeds the sound speed), and
microphysical processes such as ionization and recombination are not followed.
Both two-dimensional simulations of planar perpendicular and oblique C-type
shocks, and fully three-dimensional simulation of a perpendicular shock are
presented.Comment: 20 pages, 7 Postscript figures, LaTeX, accepted by Ap.
A Feasibility Study of Liquid Sterile Insertion
Technique for sterile insertion of liquids into previously sterilized spacecraf
Handling uncertainties in background shapes: the discrete profiling method
A common problem in data analysis is that the functional form, as well as the
parameter values, of the underlying model which should describe a dataset is
not known a priori. In these cases some extra uncertainty must be assigned to
the extracted parameters of interest due to lack of exact knowledge of the
functional form of the model. A method for assigning an appropriate error is
presented. The method is based on considering the choice of functional form as
a discrete nuisance parameter which is profiled in an analogous way to
continuous nuisance parameters. The bias and coverage of this method are shown
to be good when applied to a realistic example.Comment: Accepted by J.Ins
Effect of environment on biological burden during spacecraft assembly
Determining effects of environment on accumulation of biological burden on spacecraft during assembl
Quartz crystal microbalance use in biological studies
Design, development, and applications of quartz crystal microbalance are discussed. Two types of crystals are used. One serves as reference and other senses changes in mass. Specific application to study of bacterial spores is described
Mapping warm molecular hydrogen with Spitzer's Infrared Array Camera (IRAC)
Photometric maps, obtained with Spitzer's Infrared Array Camera (IRAC), can
provide a valuable probe of warm molecular hydrogen within the interstellar
medium. IRAC maps of the supernova remnant IC443, extracted from the Spitzer
archive, are strikingly similar to spectral line maps of the H2 pure rotational
transitions that we obtained with the Infrared Spectrograph (IRS) instrument on
Spitzer. IRS spectroscopy indicates that IRAC Bands 3 and 4 are indeed
dominated by the H2 v=0-0 S(5) and S(7) transitions, respectively. Modeling of
the H2 excitation suggests that Bands 1 and 2 are dominated by H2 v=1-0 O(5)
and v=0-0 S(9). Large maps of the H2 emission in IC433, obtained with IRAC,
show band ratios that are inconsistent with the presence of gas at a single
temperature. The relative strengths of IRAC Bands 2, 3, and 4 are consistent
with pure H2 emission from shocked material with a power-law distribution of
gas temperatures. CO vibrational emissions do not contribute significantly to
the observed Band 2 intensity. Assuming that the column density of H2 at
temperatures T to T+dT is proportional to T raised to the power -b for
temperatures up to 4000 K, we obtained a typical estimate of 4.5 for b. The
power-law index, b, shows variations over the range 3 to 6 within the set of
different sight-lines probed by the maps, with the majority of sight-lines
showing b in the range 4 to 5. The observed power-law index is consistent with
the predictions of simple models for paraboloidal bow shocks.Comment: 27 pages, including 11 figures. Accepted for publication in Ap
Nonlinear Development and Observational Consequences of Wardle C-Shock Instabilities
We compute the nonlinear development of the instabilities in C-shocks first
described by Wardle, using a version of the ZEUS code modified to include a
semi-implicit treatment of ambipolar diffusion. We find that, in three
dimensions, thin sheets parallel to the shock velocity and perpendicular to the
magnetic field lines form. High resolution, two-dimensional models show that
the sheets are confined by the Brandenburg & Zweibel ambipolar diffusion
singularity, forcing them to numerically unresolvable thinness. Hot and cold
regions form around these filaments, disrupting the uniform temperature
structure characteristic of a steady-state C-shock. This filamentary region
steadily grows as the shock progresses. We compare steady-state to unstable
C-shocks, showing excitation diagrams, line ratios, and line profiles for
molecular hydrogen lines visible in the K-band, with the Infrared Space
Observatory, and with NICMOS on the Hubble Space Telescope.Comment: 24 pages, 18 figures, uses aaspp4.sty, submitted to ApJ, 26 March
1997 Larger figures are jpegs; original postscript is available from
http://www.mpia-hd.mpg.de/MPIA/Projects/THEORY/maclow/papers/cshk/p2.htm
Shocked molecular hydrogen towards the Tornado nebula
We present near-infrared and millimetre-line observations of the Tornado
nebula (G357.7-0.1). We detected 2.12 micron_m H2 1-0 S(1) line emission
towards the suspected site of interaction with a molecular cloud revealed by
the presence of an OH(1720 MHz) maser. The distribution of the H2 emission is
well correlated with the nonthermal radio continuum emission from the Tornado,
and the velocity of the H2 emission spans over 100 km/s, which both imply that
the H2 emission is shock excited. We also detected millimetre-lines from 12CO
and 13CO transitions at the velocity of the maser, and mapped the distribution
of the molecular cloud in a 2 x 2 arcmin^2 region around the maser. The peak of
the molecular cloud aligns well with an indentation in the nebula's radio
continuum distribution, suggesting that the nebula's shock is being decelerated
at this location, which is consistent with the presence of the OH(1720 MHz)
maser and shocked H2 emission at that location.Comment: 10 pages, 8 figures, minor changes, accepted to MNRA
- …