1,207 research outputs found
SN 1987A's Circumstellar Envelope, II: Kinematics of the Three Rings and the Diffuse Nebula
We present several different measurements of the velocities of structures
within the circumstellar envelope of SN 1987A, including the inner, equatorial
ring (ER), outer rings (ORs), and the diffuse nebulosity at radii < 5 pc, based
on CTIO 4m and HST data. A comparison of STIS and WFPC2 [N II]6583 loci for the
rings show that the ER is expanding in radius at 10.5+-0.3 km/s, with the
northern OR expanding along the line of sight at about 26 km/s, and for the
southern OR, about 23 km/s. Similar results are found with CTIO 4m data.
Accounting for inclination, the best fit to all data show both ORs with an
expansion from the SN of 26 km/s. The ratio of the ER to OR velocities is
nearly equal to the ratio of ER to OR radii, so the rings are roughly
homologous, all having kinematic ages corresponding to about 20,000 yr before
the SN explosion. This makes previously reported, large compositional
differences between the ER and ORs difficult to understand. Additionally, a
grid of longslit 4m/echelle spectra centered on the SN shows two velocity
components over a region roughly coextensive with the outer circumstellar
envelope extending about 5 pc (20 arcsec) from the SN. One component is
blueshifted and the other redshifted from the SN centroid by about 10 km/s
each. These features may represent a bipolar flow expanding from the SN, in
which the ORs are propelled 10-15 km/s faster than that of the surrounding
envelope into which they propogate. The kinematic timescale for the entire
nebula is at least about 350,000 yr. The kinematics of these various structures
constrain possible models for the evolution of the progenitor and its formation
of a mass loss nebula.Comment: 25 pages AASTeX text plus 12 figures. ApJ, in pres
Donut: measuring optical aberrations from a single extra-focal image
We propose a practical method to calculate Zernike aberrations from analysis
of a single long-exposure defocused stellar image. It consists in fitting the
aberration coefficients and seeing blur directly to a realistic image binned
into detector pixels. This "donut" method is different from curvature sensing
in that it does not make the usual approximation of linearity. We calculate the
sensitivity of this technique to detector and photon noise and determine
optimal parameters for some representative cases. Aliasing of high-order
un-modeled aberrations is evaluated and shown to be similar to a low-order
Shack-Hartmann sensor. The method has been tested with real data from the SOAR
and Blanco 4m telescopes.Comment: 13 pages, 9 figures. Accepted at PAS
The Three-Dimensional Circumstellar Environment of SN 1987A
We present the detailed construction and analysis of the most complete map to
date of the circumstellar environment around SN 1987A, using ground and
space-based imaging from the past 16 years. PSF-matched difference-imaging
analyses of data from 1988 through 1997 reveal material between 1 and 28 ly
from the SN. Careful analyses allows the reconstruction of the probable
circumstellar environment, revealing a richly-structured bipolar nebula. An
outer, double-lobed ``Peanut,'' which is believed to be the contact
discontinuity between red supergiant and main sequence winds, is a prolate
shell extending 28 ly along the poles and 11 ly near the equator. Napoleon's
Hat, previously believed to be an independent structure, is the waist of this
Peanut, which is pinched to a radius of 6 ly. Interior to this is a cylindrical
hourglass, 1 ly in radius and 4 ly long, which connects to the Peanut by a
thick equatorial disk. The nebulae are inclined 41\degr south and 8\degr east
of the line of sight, slightly elliptical in cross section, and marginally
offset west of the SN. From the hourglass to the large, bipolar lobes, echo
fluxes suggest that the gas density drops from 1--3 cm^{-3} to >0.03 cm^{-3},
while the maximum dust-grain size increases from ~0.2 micron to 2 micron, and
the Si:C dust ratio decreases. The nebulae have a total mass of ~1.7 Msun. The
geometry of the three rings is studied, suggesting the northern and southern
rings are located 1.3 and 1.0 ly from the SN, while the equatorial ring is
elliptical (b/a < 0.98), and spatially offset in the same direction as the
hourglass.Comment: Accepted for publication in the ApJ Supplements. 38 pages in
apjemulate format, with 52 figure
Observations and simulations of recurrent novae: U Sco and V394 CrA
Observations and analysis of the Aug. 1987 outburst of the recurrent nova V394 CrA are presented. This nova is extremely fast and its outburst characteristics closely resemble those of the recurrent nova U Sco. Hydrodynamic simulations of the outbursts of recurrent novae were performed. Results as applied to the outbursts of V394 CrA and U Sco are summarized
Characterizing the role of the preâSMA in the control of speed/accuracy tradeâoff with directed functional connectivity mapping and multiple solution reduction
Several plausible theories of the neural implementation of speed/accuracy tradeâoff (SAT), the phenomenon in which individuals may alternately emphasize speed or accuracy during the performance of cognitive tasks, have been proposed, and multiple lines of evidence point to the involvement of the preâsupplemental motor area (preâSMA). However, as the nature and directionality of the preâSMAâs functional connections to other regions involved in cognitive control and task processing are not known, its precise role in the topâdown control of SAT remains unclear. Although recent advances in crossâsectional path modeling provide a promising way of characterizing these connections, such models are limited by their tendency to produce multiple equivalent solutions. In a sample of healthy adults (Nâ=â18), the current study uses the novel approach of Group Iterative Multiple Model Estimation for Multiple Solutions (GIMMEâMS) to assess directed functional connections between the preâSMA, other regions previously linked to control of SAT, and regions putatively involved in evidence accumulation for the decision task. Results reveal a primary role of the preâSMA for modulating activity in regions involved in the decision process but suggest that this region receives topâdown input from the DLPFC. Findings also demonstrate the utility of GIMMEâMS and solutionâreduction methods for obtaining valid directional inferences from connectivity path models.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/149347/1/hbm24493.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/149347/2/hbm24493_am.pd
Dynamics of charge-displacement channeling in intense laser-plasma interactions
The dynamics of transient electric fields generated by the interaction of
high intensity laser pulses with underdense plasmas has been studied
experimentally with the proton projection imaging technique. The formation of a
charged channel, the propagation of its front edge and the late electric field
evolution have been characterised with high temporal and spatial resolution.
Particle-in-cell simulations and an electrostatic, ponderomotive model
reproduce the experimental features and trace them back to the ponderomotive
expulsion of electrons and the subsequent ion acceleration.Comment: 5 figures, accepted for publication in New Journal of Physic
On the Hydrodynamic Interaction of Shock Waves with Interstellar Clouds. II. The Effect of Smooth Cloud Boundaries on Cloud Destruction and Cloud Turbulence
The effect of smooth cloud boundaries on the interaction of steady planar
shock waves with interstellar clouds is studied using a high-resolution local
AMR technique with a second-order accurate axisymmetric Godunov hydrodynamic
scheme. A 3D calculation is also done to confirm the results of the 2D ones. We
consider an initially spherical cloud whose density distribution is flat near
the cloud center and has a power-law profile in the cloud envelope. When an
incident shock is transmitted into a smooth cloud, velocity gradients in the
cloud envelope steepen the smooth density profile at the upstream side,
resulting in a sharp density jump having an arc-like shape. Such a ``slip
surface'' forms immediately when a shock strikes a cloud with a sharp boundary.
For smoother boundaries, the formation of slip surface and therefore the onset
of hydrodynamic instabilities are delayed. Since the slip surface is subject to
the Kelvin-Helmholtz and Rayleigh-Taylor instabilities, the shocked cloud is
eventually destroyed in cloud crushing times. After complete cloud
destruction, small blobs formed by fragmentation due to hydrodynamic
instabilities have significant velocity dispersions of the order of 0.1 ,
where is the shock velocity in the ambient medium. This suggests that
turbulent motions generated by shock-cloud interaction are directly associated
with cloud destruction. The interaction of a shock with a cold HI cloud should
lead to the production of a spray of small HI shreds, which could be related to
the small cold clouds recently observed by Stanimirovic & Heiles (2005). The
linewidth-size relation obtained from our 3D simulation is found to be
time-dependent. A possibility for gravitational instability triggered by shock
compression is also discussed.Comment: 62 pages, 16 figures, submitted to Ap
Nonlinear Aeroelastic Framework Based on Vortex-Lattice Method and Corotational Shell Finite Element
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/97077/1/AIAA2012-1976.pd
- âŠ