10,861 research outputs found
The plumes of IO: A detection of solid sulfur dioxide particles
Spectra of Io obtained during eclipse show a narrow deep absorption feature at 4.871 microns, the wavelength of the Nu sub 1 + Nu sub 3 band of solid SO2. The 4 micron radiation comes from volcanic hot spots at a temperature too high for the existence of solid SO2. It is concluded that the spectral feature results from SO2 particles suspended in plumes above the hot spots. The derived abundance of approximately 0.0003 gm/sq cm may imply an SO2 solid-to-gas ratio of roughly one for the Loki plume, which would in turn suggest that it is driven by the SO2 rather than by sulfur
Experimental pressure distributions for a family of blunt bodies at Mach numbers from 2.49 to 4.63 and angles of attack from 0 deg to 15 deg
Pressure distributions for blunt body wind tunnel models at supersonic speeds and angles of attack from 0 to 15 degree
VV Pup in a low state: secondary-star irradiation or stellar activity?
Aims. Emission lines in polars show complex profiles with multiple components
that are typically ascribed to the accretion stream, threading region,
accretion spot, and the irradiated secondary-star. In low-state polars the
fractional contribution by the accretion stream, and the accretion spot is
greatly reduced offering an opportunity to study the effect of the
secondary-star irradiation or stellar activity. We observed VV Pup during an
exceptional low-state to study and constrain the properties of the line-forming
regions and to search for evidence of chromospheric activity and/or
irradiation. Methods. We obtained phase-resolved optical spectra at the ESO
VLT+FORS1 with the aim of analyzing the emission line profile and radial
velocity as a function of the orbital period. We also tailored irradiated
secondary-star models to compare the predicted and the observed emission lines
and to establish the nature of the line-forming regions. Results. Our
observations and data analysis, when combined with models of the irradiated
secondary-star, show that, while the weak low ionization metal lines (FeI and
MgI) may be consistent with irradiation processes, the dominant Balmer H
emission lines, as well as NaI and HeI, cannot be reproduced by the irradiated
secondary-star models. We favor the secondary-star chromospheric activity as
the main forming region and cause of the observed H, NaI, and He emission
lines, though a threading region very close to the L1 point cannot be excluded.Comment: 10 pages, 9 figures, in press on A&
Shape optimization of pressurized air bearings
Use of externally pressurized air bearings allows for the design of mechanical systems requiring extreme precision in positioning. One application is the fine control for the positioning of mirrors in large-scale optical telescopes. Other examples come from applications in robotics and computer hard-drive manufacturing. Pressurized bearings maintain a finite separation between mechanical components by virtue of the presence of a pressurized flow of air through the gap between the components. An everyday example is an air hockey table, where a puck is levitated above the table by an array of vertical jets of air. Using pressurized bearings there is no contact between “moving parts” and hence there is no friction and no wear of sensitive components.
This workshop project is focused on the problem of designing optimal static air bearings subject to given engineering constraints. Recent numerical computations of this problem, done at IBM by Robert and Hendriks, suggest that near-optimal designs can have unexpected complicated and intricate structures. We will use analytical approaches to shed some light on this situation and to offer some guides for the design process.
In Section 2 the design problem is stated and formulated as an optimization problem for an elliptic boundary value problem.
In Section 3 the general problem is specialized to bearings with rectangular bases.
Section 4 addresses the solutions of this problem that can be obtained using variational formulations of the problem.
Analysis showing the sensitive dependence to perturbations (in numerical computations or manufacturing constraints) of near-optimal designs is given in Section 5.
In Section 6, a restricted class of “groove network” designs motivated by the original results of Robert and Hendriks is examined.
Finally, in Section 7, we consider the design problem for circular axisymmetric air bearings
Adaptive Optics Images of Kepler Objects of Interest
All transiting planets are at risk of contamination by blends with nearby,
unresolved stars. Blends dilute the transit signal, causing the planet to
appear smaller than it really is, or produce a false positive detection when
the target star is blended with eclipsing binary stars. This paper reports on
high spatial-resolution adaptive optics images of 90 Kepler planetary
candidates. Companion stars are detected as close as 0.1 arcsec from the target
star. Images were taken in the near-infrared (J and Ks bands) with ARIES on the
MMT and PHARO on the Palomar Hale 200-inch. Most objects (60%) have at least
one star within 6 arcsec separation and a magnitude difference of 9. Eighteen
objects (20%) have at least one companion within 2 arcsec of the target star; 6
companions (7%) are closer than 0.5 arcsec. Most of these companions were
previously unknown, and the associated planetary candidates should receive
additional scrutiny. Limits are placed on the presence of additional companions
for every system observed, which can be used to validate planets statistically
using the BLENDER method. Validation is particularly critical for low-mass,
potentially Earth-like worlds, which are not detectable with current-generation
radial velocity techniques. High-resolution images are thus a crucial component
of any transit follow-up program.Comment: 9 pages, 4 figures, accepted to A
Spitzer Space Telescope Observations of Circumbinary Dust Disks around Polars
We present Spitzer Space Telescope IRAC photometry of the magnetic cataclysmic variables EF Eri, MR Ser, VV Pup, V834 Cen, GG Leo and V347 Pav. When we combine our results with the 2MASS data, we find that at least five of the polars have flux densities in the mid-IR in excess of the emission expected from the stellar components alone. We are unable to model this mid-IR excess with cyclotron emission, but we can recreate the observed spectral energy distributions with the inclusion of a simple circumbinary dust disk model. Importantly, we find that the masses of our modelled disks are approximately 12 orders of magnitude lower than required to significantly affect CV evolution. The accretion disk-less polars are ideal places to search for these disks, since the luminous accretion disk in most CVs would drown out the faint IR signature of the cooler, dimmer circumbinary disks
Time-resolved, multi-color photometry and spectroscopy of Virgo 4 (OU Vir): a high orbital inclination, short orbital period dwarf nova
We present multi-color photometry and time resolved spectroscopy of OU Vir.
The analysis of the quiescent light curve shows that OU Vir is characterized by
i) strong cycle-to-cycle brightness variations, and ii) hot spot modulated
light curve with grazing eclipse of the impact region. Colors are derived both
in- and out- of eclipse. The time-resolved spectroscopy allows us to produce
the radial velocity curve from the H accretion disk emission line which
possibly reveals only weak evidence for hot spot line emission. The hot spot is
believed to be a turbulent optically thick region, producing mostly continuum
emission.Comment: 8 pages (including figures), 7 figures. To Be published in A&
Infrared spectroscopy of cataclysmic variables: III. Dwarf novae below the period gap and novalike variables
We present K-band spectra of the short-period dwarf novae YZ Cnc, LY Hya, BK
Lyn, T Leo, SW UMa and WZ Sge, the novalike variables DW UMa, V1315 Aql, RW
Tri, VY Scl, UU Aqr and GP Com, and a series of field dwarf stars with spectral
types ranging from K2-M6.
The spectra of the dwarf novae are dominated by emission lines of HI and HeI.
The large velocity and equivalent widths of these lines, in conjunction with
the fact that the lines are double-peaked in the highest inclination systems,
indicate an accretion disc origin. In the case of YZ Cnc and T Leo, for which
we obtained time-resolved data covering a complete orbital cycle, the emission
lines show modulations in their equivalent widths which are most probably
associated with the bright spot (the region where the gas stream collides with
the accretion disc). There are no clear detections of the secondary star in any
of the dwarf novae below the period gap, yielding upper limits of 10-30% for
the contribution of the secondary star to the observed K-band flux. In
conjunction with the K-band magnitudes of the dwarf novae, we use the derived
secondary star contributions to calculate lower limits to the distances to
these systems.
The spectra of the novalike variables are dominated by broad, single-peaked
emission lines of HI and HeI - even the eclipsing systems we observed do not
show the double-peaked profiles predicted by standard accretion disc theory.
With the exception of RW Tri, which exhibits NaI, CaI and 12CO absorption
features consistent with a M0V secondary contributing 65% of the observed
K-band flux, we find no evidence for the secondary star in any of the novalike
variables. The implications of this result are discussed.Comment: 13 pages, 5 figures, to appear in MNRA
"Dark Matter" in Accretion Disks
Using Spitzer Space Telescope photometric observations of the eclipsing,
interacting binary WZ Sge, we have discovered that the accretion disk is far
more complex than previously believed. Our 4.5 and 8 micron time series
observations reveal that the well known gaseous accretion disk is surrounded by
an asymmetric disk of dusty material with a radius approximately 15 times
larger than the gaseous disk. This dust ring contains only a small amount of
mass and is completely invisible at optical and near-IR wavelengths, hence
consisting of "dark matter". We have produced a model dust ring using 1 micron
spherical particles with a density of 3 g/cm and with a temperature profile
ranging from 700-1500K. Our discovery about the accretion disk structure and
the presence of a larger, outer dust ring have great relevance for accretion
disks in general, including those in other interacting binary systems, pre-main
sequence stars, and active galaxies.Comment: 34 pages, 8 figures (3 in color). Accepted to Ap
Neutron-proton analyzing power at 12 MeV and inconsistencies in parametrizations of nucleon-nucleon data
We present the most accurate and complete data set for the analyzing power
Ay(theta) in neutron-proton scattering. The experimental data were corrected
for the effects of multiple scattering, both in the center detector and in the
neutron detectors. The final data at En = 12.0 MeV deviate considerably from
the predictions of nucleon-nucleon phase-shift analyses and potential models.
The impact of the new data on the value of the charged pion-nucleon coupling
constant is discussed in a model study.Comment: Six pages, four figures, one table, to be published in Physics
Letters
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