7,574 research outputs found
Shearing box simulations of accretion disk winds
The launching process of a magnetically driven outflow from an accretion disk
is investigated in a local, shearing box model which allows a study of the
feedback between accretion and angular momentum loss. The mass-flux instability
found in previous linear analyses of this problem is recovered in a series of
2D (axisymmetric) simulations in the MRI-stable (high magnetic field strength)
regime. At low field strengths that are still sufficient to suppress MRI, the
instability develops on a short radial length scale and saturates at a modest
amplitude. At high field strengths, a long-wavelength "clump" instability of
large amplitude is observed, with growth times of a few orbits. As speculated
before, the unstable connection between disk and outflow may be relevant for
the time dependence observed in jet-producing disks. The success of the
simulations is due in a large part to the implementation of an effective
wave-transmitting upper boundary condition.Comment: 10 pages, 13 figure, accepted for publication in A&A, complimentary
movies at http://ucolick.org/~rainer/pascheibe
Photoacoustic detection of stimulated emission pumping in p-difluorobenzene
Photoacoustic detection has been used to monitor a stimulated emission pumping process in pâdifluorobenzene. Using the Ă^(1)B_(2u)5^1 state as an intermediate, several vibrational levels of the ground electronic state were populated. The photoacoustic method is an attractive alternative to other detection techniques because of its sensitivity, simplicity, and its ability to differentiate between stimulated emission pumping and excited state absorption. An example of excited state absorption in aniline is given
Double-diffusive erosion of the core of Jupiter
We present Direct Numerical Simulations of the transport of heat and heavy
elements across a double-diffusive interface or a double-diffusive staircase,
in conditions that are close to those one may expect to find near the boundary
between the heavy-element rich core and the hydrogen-helium envelope of giant
planets such as Jupiter. We find that the non-dimensional ratio of the buoyancy
flux associated with heavy element transport to the buoyancy flux associated
with heat transport lies roughly between 0.5 and 1, which is much larger than
previous estimates derived by analogy with geophysical double-diffusive
convection. Using these results in combination with a core-erosion model
proposed by Guillot et al. (2004), we find that the entire core of Jupiter
would be eroded within less than 1Myr assuming that the core-envelope boundary
is composed of a single interface. We also propose an alternative model that is
more appropriate in the presence of a well-established double-diffusive
staircase, and find that in this limit a large fraction of the core could be
preserved. These findings are interesting in the context of Juno's recent
results, but call for further modeling efforts to better understand the process
of core erosion from first principles.Comment: Accepted for publication in Ap
A map on the space of rational functions
We describe dynamical properties of a map defined on the space
of rational functions. The fixed points of are classified and
the long time behavior of a subclass is described in terms of Eulerian
polynomials
Conditioning Expressive Language in a Nonverbal Child
The problem of treating the child with delayed speech acquisition has generated a considerable amount of research and these studies have pointed out the need for further work in this area. The purpose of this study is to test the procedures of one particularly effective study, that of Dr. Burl B. Gray at the Monterey Institute for Speech and Hearing in Monterey, California. By removing it from the clinical setting of the Institute, with its unusually well-trained and highly specialized staff, and by successfully employing its procedures using comparatively untrained personnel, it was felt that this program might be applied to more general use. This study, then, investigated the effectiveness of Gray\u27s language acquisition program on a nonverbal five and one half year old boy
Large jets from small-scale magnetic fields
We consider the conditions under which a rotating magnetic object can produce
a magnetically powered outflow in an initially unmagnetized medium stratified
under gravity. 3D MHD simulations are presented in which the footpoints of
localized, arcade-shaped magnetic fields are put into rotation. It is shown how
the effectiveness in producing a collimated magnetically powered outflow
depends on the rotation rate, the strength and the geometry of the field. The
flows produced by uniformly rotating, non-axisymmetric fields are found to
consist mainly of buoyant plumes heated by dissipation of rotational energy.
Collimated magnetically powered flows are formed if the field and the rotating
surface are arranged such that a toroidal magnetic field is produced. This
requires a differential rotation of the arcades' footpoints. Such jets are
well-collimated; we follow their propagation through the stratified atmosphere
over 100 times the source size. The magnetic field is tightly wound and its
propagation is dominated by the development of non-axisymmetric instabilities.
We observe a Poynting flux conversion efficiency of over 75% in the longest
simulations. Applications to the collapsar model and protostellar jets are
discussed.Comment: 9 pages, 12 figures, accepted for publication in A&A, complementary
movies at http://www.mpa-garching.mpg.de/~rmo/pap3/index.htm
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