8,140 research outputs found
Discovery of extreme asymmetry in the debris disk surrounding HD 15115
We report the first scattered light detection of a dusty debris disk
surrounding the F2V star HD 15115 using the Hubble Space Telescope in the
optical, and Keck adaptive optics in the near-infrared. The most remarkable
property of the HD 15115 disk relative to other debris disks is its extreme
length asymmetry. The east side of the disk is detected to ~315 AU radius,
whereas the west side of the disk has radius >550 AU. We find a blue optical to
near-infrared scattered light color relative to the star that indicates grain
scattering properties similar to the AU Mic debris disk. The existence of a
large debris disk surrounding HD 15115 adds further evidence for membership in
the Beta Pic moving group, which was previously argued based on kinematics
alone. Here we hypothesize that the extreme disk asymmetry is due to dynamical
perturbations from HIP 12545, an M star 0.5 degrees (0.38 pc) east of HD 15115
that shares a common proper motion vector, heliocentric distance, galactic
space velocity, and age.Comment: 4 pages, 3 figures, ApJ Letters, accepte
Theoretical Predictions for Surface Brightness Fluctuations and Implications for Stellar Populations of Elliptical Galaxies
(Abridged) We present new theoretical predictions for surface brightness
fluctuations (SBFs) using models optimized for this purpose. Our predictions
agree well with SBF data for globular clusters and elliptical galaxies. We
provide refined theoretical calibrations and k-corrections needed to use SBFs
as standard candles. We suggest that SBF distance measurements can be improved
by using a filter around 1 micron and calibrating I-band SBFs with the
integrated V-K galaxy color. We also show that current SBF data provide useful
constraints on population synthesis models, and we suggest SBF-based tests for
future models. The data favor specific choices of evolutionary tracks and
spectra in the models among the several choices allowed by comparisons based on
only integrated light. In addition, the tightness of the empirical I-band SBF
calibration suggests that model uncertainties in post-main sequence lifetimes
are less than +/-50% and that the IMF in ellipticals is not much steeper than
that in the solar neighborhood. Finally, we analyze the potential of SBFs for
probing unresolved stellar populations. We find that optical/near-IR SBFs are
much more sensitive to metallicity than to age. Therefore, SBF magnitudes and
colors are a valuable tool to break the age/metallicity degeneracy. Our initial
results suggest that the most luminous stellar populations of bright cluster
galaxies have roughly solar metallicities and about a factor of three spread in
age.Comment: Astrophysical Journal, in press (uses Apr 20, 2000 version of
emulateapj5.sty). Reposted version has a minor cosmetic change to Table
Impact of CO2 fertilization on maximum foliage cover across the globe's warm, arid environments
Satellite observations reveal a greening of the globe over recent decades. The role in this greening of the "CO2 fertilization" effect-the enhancement of photosynthesis due to rising CO2 levels-is yet to be established. The direct CO2 effect on vegetatio
STIS Coronagraphic Imaging of Fomalhaut: Main Belt Structure and the Orbit of Fomalhaut b
We present new optical coronagraphic data of the bright star Fomalhaut
obtained with the HST in 2010/2012 using STIS. Fomalhaut b is recovered at both
epochs to high significance. The observations include the discoveries of
tenuous nebulosity beyond the main dust belt detected to at least 209 AU
projected radius and a ~50 AU wide azimuthal gap in the belt northward of Fom
b. The morphology of Fomalhaut b appears elliptical in the STIS detections. We
show that residual noise in the processed data can plausibly result in point
sources appearing extended. A MCMC analysis demonstrates that the orbit of Fom
b is highly eccentric, with e=0.8+/-0.1, a=177+/-68 AU, and q = 32+/-24 AU. Fom
b is apsidally aligned with the belt and 90% of allowed orbits have mutual
inclination 36 deg or less. Fomalhaut b's orbit is belt-crossing in projection,
but only 12% of possible orbits have nodes within a 25 AU wide belt annulus
(133-158 AU). The high e invokes a dynamical history where Fom b may have
experienced a significant dynamical interaction with a hypothetical planet
Fomalhaut c, and the current orbital configuration may be relatively
short-lived. The new value for the periastron distance diminishes the Hill
radius of Fom b and any weakly bound satellite system surrounding a planet
would be sheared and dynamically heated at periapse. We argue that Fom b's
minimum mass is that of a dwarf planet in order for a circumplanetary satellite
system to remain bound to a sufficient radius from the planet to be consistent
with the dust scattered light hypothesis. Fom b may be optically bright because
the recent passage through periapse and/or the ascending node has increased the
erosion rates of planetary satellites. In the coplanar case, Fomalhaut b will
collide with the main belt around 2032, and the subsequent emergent phenomena
may help determine its physical nature.Comment: 49 Pages, 33 Figures, 5 Tables; Submitted to ApJ, Dec. 31, 201
Modeling Self-Subtraction in Angular Differential Imaging: Application to the HD 32297 Debris Disk
We present a new technique for forward-modeling self-subtraction of spatially
extended emission in observations processed with angular differential imaging
(ADI) algorithms. High-contrast direct imaging of circumstellar disks is
limited by quasi-static speckle noise and ADI is commonly used to suppress
those speckles. However, the application of ADI can result in self-subtraction
of the disk signal due to the disk's finite spatial extent. This signal
attenuation varies with radial separation and biases measurements of the disk's
surface brightness, thereby compromising inferences regarding the physical
processes responsible for the dust distribution. To compensate for this
attenuation, we forward-model the disk structure and compute the form of the
self-subtraction function at each separation. As a proof of concept, we apply
our method to 1.6 and 2.2 micron Keck AO NIRC2 scattered-light observations of
the HD 32297 debris disk reduced using a variant of the "locally optimized
combination of images" (LOCI) algorithm. We are able to recover disk surface
brightness that was otherwise lost to self-subtraction and produce simplified
models of the brightness distribution as it appears with and without
self-subtraction. From the latter models, we extract radial profiles for the
disk's brightness, width, midplane position, and color that are unbiased by
self-subtraction. Our analysis of these measurements indicates a break in the
brightness profile power law at r~110 AU and a disk width that increases with
separation from the star. We also verify disk curvature that displaces the
midplane by up to 30 AU towards the northwest relative to a straight fiducial
midplane.Comment: Accepted for publication in ApJ, 20 pages, 10 figures, 1 tabl
The application of lagrangian vortex methods to the prediction of hydrodynamic damping of floating bodies
Lagrangian vortex methods of simulating the vortex shedding which occurs at the bilges and sharp edges of floating bodies under oscillatory flow conditions due to incident waves and motion of the body in response are presented. Local forces are taken from discrete vortex simulations of the flow around an isolated edge representing the bilge section of the hull. Both the classical, meshless, potential flow vortex method and a vortex-in-cell viscous simulation are used. The flow around the bilge of a typical long floating hull in beam waves is treated on a sectional basis and the isolated edge results are matched to the outer threedimensional
wave potential flow provided by a standard surface panel method. The advantage of this matching procedure is that the more computationally expensive vortex flow simulation is limited to the local bilge section for which universal results may be computed whereas the large scale wave-hull interaction which extends out many hull- or wave-lengths from the body is solved by the less computationally intensive panel method. This procedure
thus provides an efficient method replacing empirical vortex damping coefficients, as presently used, by a more rational method based on flow physics. Results for regular waves generating sinusoidal flows around right angle edges, edges fitted with flat plate bilge keels and rounded edges are presented and some comparisons made with measured data from laboratory wave tank tests and results of full Navier-Stokes simulations
Electron Optics
Contains research objectives and summary of research on one research project.Joint Services Electronics Program (Contract DAAB07-75-C-1346
A new scorpion genus representing a primitive taxon of tribe Stahnkeini, with a description of a new species from Sonora, Mexico (Scorpiones: Vaejovidae)
A new scorpion genus, Gertschius gen. nov., and species, Gertschius crassicorpus sp. nov., (Scorpiones: Vaejovidae) from Sonora, Mexico is described. Gertschius exhibits a reduced modification of the basal pectinal teeth in the female holotype and serration of the chelal finger denticles is limited to the median denticles (MD), thus we consider it a relict member of tribe Stahnkeini. Serradigitus agilis Sissom et Stockwell, 1991, is transferred to this new genus; Gertschius agilis, comb. nov
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