393 research outputs found
Enrichment Activities and Materials for the Intermediate Grades\u27 Arithmetic Program
What are some worthwhile enrichment activities and materials for the intermediate grades\u27 arithmetic program
The evolution of M 2-9 from 2000 to 2010
M 2-9, the Butterfly nebula, is an outstanding representative of extreme
aspherical flows. It presents unique features such as a pair of high-velocity
dusty polar blobs and a mirror-symmetric rotating pattern in the inner lobes.
Imaging monitoring of the evolution of the nebula in the past decade is
presented. We determine the proper motions of the dusty blobs, which infer a
new distance estimate of 1.3+-0.2 kpc, a total nebular size of 0.8 pc, a speed
of 147 km/s, and a kinematical age of 2500 yr. The corkscrew geometry of the
inner rotating pattern is quantified. Different recombination timescales for
different ions explain the observed surface brightness distribution. According
to the images taken after 1999, the pattern rotates with a period of 92+-4 yr.
On the other hand, the analysis of images taken between 1952 and 1977 measures
a faster angular velocity. If the phenomenon were related to orbital motion,
this would correspond to a modest orbital eccentricity (e=0.10+-0.05), and a
slightly shorter period (86+-5 yr). New features have appeared after 2005 on
the west side of the lobes and at the base of the pattern. The geometry and
travelling times of the rotating pattern support our previous proposal that the
phenomenon is produced by a collimated spray of high velocity particles (jet)
from the central source, which excites the walls of the inner cavity of M 2-9,
rather than by a ionizing photon beam. The speed of such a jet would be
remarkable: between 11000 and 16000 km/s. The rotating-jet scenario may explain
the formation and excitation of most of the features observed in the inner
nebula, with no need for additional mechanisms, winds, or ionization sources.
All properties point to a symbiotic-like interacting binary as the central
source of M 2-9.Comment: Accepted for publication on Astronomy and Astrophysics (10 pages, 8
figures
HST NICMOS Observations of the Polarization of NGC 1068
We have observed the polarized light at 2 micron in the center of NGC 1068
with HST NICMOS Camera 2. The nucleus is dominated by a bright, unresolved
source, polarized at a level of 6.0 pm 1.2% with a position angle of 122degr pm
1.5degr. There are two polarized lobes extending up to 8'' northeast and
southwest of the nucleus. The polarized flux in both lobes is quite clumpy,
with the maximum polarization occurring in the southwest lobe at a level of 17%
when smoothed to 0.23'' resolution. The perpendiculars to the polarization
vectors in these two lobes point back to the intense unresolved nuclear source
to within one 0.076'' Camera 2 pixel, thereby confirming that this is the
illuminating source of the scattered light and therefore the probable AGN
central engine. Whereas the polarization of the nucleus is probably caused by
dichroic absorption, the polarization in the lobes is almost certainly caused
by scattering, with very little contribution from dichroic absorption. Features
in the polarized lobes include a gap at a distance of about 1'' from the
nucleus toward the southwest lobe and a ``knot'' of emission about 5''
northeast of the nucleus. Both features had been discussed by ground-based
observers, but they are much better defined with the high spatial resolution of
NICMOS. The northeast knot may be the side of a molecular cloud that is facing
the nucleus, which cloud may be preventing the expansion of the northeast radio
lobe at the head of the radio synchrotron-radiation-emitting jet. We also
report the presence of two ghosts in the Camera 2 polarizers. These had not
been detected previously (Hines et al. 2000) because they are relatively faint
and require observations of a source with a large dynamic range.Comment: 17 pages, 4 figure
Towards a New Standard Model for Black Hole Accretion
We briefly review recent developments in black hole accretion disk theory,
emphasizing the vital role played by magnetohydrodynamic (MHD) stresses in
transporting angular momentum. The apparent universality of accretion-related
outflow phenomena is a strong indicator that large-scale MHD torques facilitate
vertical transport of angular momentum. This leads to an enhanced overall rate
of angular momentum transport and allows accretion of matter to proceed at an
interesting rate. Furthermore, we argue that when vertical transport is
important, the radial structure of the accretion disk is modified at small
radii and this affects the disk emission spectrum. We present a simple model
demonstrating how energetic, magnetically-driven outflows modify the emergent
disk emission spectrum with respect to that predicted by standard accretion
disk theory. A comparison of the predicted spectra against observations of
quasar spectral energy distributions suggests that mass accretion rates
inferred using the standard disk model may severely underestimate their true
values.Comment: To appear in the Fifth Stromlo Symposium Proceedings special issue of
ApS
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