25,445 research outputs found
Collisional Grooming Models of the Kuiper Belt Dust Cloud
We modeled the 3-D structure of the Kuiper Belt dust cloud at four different
dust production rates, incorporating both planet-dust interactions and
grain-grain collisions using the collisional grooming algorithm. Simulated
images of a model with a face-on optical depth of ~10^-4 primarily show an
azimuthally-symmetric ring at 40-47 AU in submillimeter and infrared
wavelengths; this ring is associated with the cold classical Kuiper Belt. For
models with lower optical depths (10^-6 and 10^-7), synthetic infrared images
show that the ring widens and a gap opens in the ring at the location of of
Neptune; this feature is caused by trapping of dust grains in Neptune's mean
motion resonances. At low optical depths, a secondary ring also appears
associated with the hole cleared in the center of the disk by Saturn. Our
simulations, which incorporate 25 different grain sizes, illustrate that
grain-grain collisions are important in sculpting today's Kuiper Belt dust, and
probably other aspects of the Solar System dust complex; collisions erase all
signs of azimuthal asymmetry from the submillimeter image of the disk at every
dust level we considered. The model images switch from being dominated by
resonantly-trapped small grains ("transport dominated") to being dominated by
the birth ring ("collision dominated") when the optical depth reaches a
critical value of tau ~ v/c, where v is the local Keplerian speed.Comment: 31 pages, including 9 figure
Normal Nearby Galaxies
Following on from IRAS, ISO has provided a huge advancement in our knowledge
of the phenomenology of the infrared (IR) emission of normal galaxies and the
underlying physical processes. Highlights include: the discovery of an extended
cold dust emission component, present in all types of gas-rich galaxies and
carrying the bulk of the dust luminosity; the definitive characterisation of
the spectral energy distribution in the IR, revealing the channels through
which stars power the IR light; the derivation of realistic geometries for
stars and dust from ISO imaging; the discovery of cold dust associated with HI
extending beyond the optical body of galaxies; the remarkable similarity of the
near-IR (NIR)/ mid-IR (MIR) SEDs for spiral galaxies, revealing the importance
of the photo-dissociation regions in the energy budget for that wavelength
range; the importance of the emission from the central regions in shaping up
the intensity and the colour of the global MIR luminosity; the discovery of the
``hot'' NIR continuum emission component of interstellar dust; the predominance
of the diffuse cold neutral medium as the origin for the main interstellar
cooling line, [CII] 158 micron, in normal galaxies.Comment: 47 pages, 15 figures, to be published in the ISO Special Issue of
Space Science Reviews: "ISO science legacy - a compact review of ISO major
achievements", Springer 2005. See http://www.iso.vilspa.esa.es/science/SSR/
for a higher resolution version and for all papers in the volum
Probing Gluonic Spin-Orbit Correlations in Photon Pair Production
We consider photon pair production in hadronic collisions at large mass and
small transverse momentum of the pair, assuming that factorization in terms of
transverse momentum dependent parton distributions applies. The unpolarized
cross section is found to have azimuthal angular dependencies that are
generated by a gluonic version of the Boer-Mulders function. In addition, the
single-transversely polarized cross section is sensitive to the gluon Sivers
function. We present simple numerical estimates for the Boer-Mulders and Sivers
effects in diphoton production at RHIC and find that the process would offer
unique opportunities for exploring transverse momentum dependent gluon
distributions.Comment: 5 pages, 2 figures, minor changes, final version, to be published in
PR
Detection of Close-In Extrasolar Giant Planets Using the Fourier-Kelvin Stellar Interferometer
We evaluate the direct detection of extrasolar giant planets with a
two-aperture nulling infrared interferometer, working at angles
, and using a new `ratio-of-two-wavelengths' technique.
Simple arguments suggest that interferometric detection and characterization
should be quite possible for planets much closer than the conventional inner
working angle, or angular resolution limit. We show that the peak signal from a
nulling infrared interferometer of baseline ( meters) will often
occur `inside the null', and that the signal variations from path-difference
fluctuations will cancel to first order in the ratio of two wavelengths. Using
a new interferometer simulation code, we evaluate the detectability of all the
known extrasolar planets as observed using this two-color method with the
proposed {\it Fourier Kelvin Stellar Interferometer (FKSI)}. In its minimum
configuration {\it FKSI} uses two 0.5-meter apertures on a 12.5-meter baseline,
and a field-of-regard. We predict that known
extrasolar planets are directly detectable using {\it FKSI}, with
low-resolution spectroscopy () being possible in the most favorable
cases. Spaceborne direct detection of extrasolar giant planets is possible with
meter baselines, and does not require the much longer baselines
provided by formation flying.Comment: Accepted for publication in ApJ Letter
Multi-epoch Sub-arcsecond [Fe II] Spectroimaging of the DG Tau Outflows with NIFS. II. On the Nature of the Bipolar Outflow Asymmetry
The origin of bipolar outflow asymmetry in young stellar objects (YSOs)
remains poorly understood. It may be due to an intrinsically asymmetric outflow
launch mechanism, or it may be caused by the effects of the ambient medium
surrounding the YSO. Answering this question is an important step in
understanding outflow launching. We have investigated the bipolar outflows
driven by the T Tauri star DG Tauri on scales of hundreds of AU, using the
Near-infrared Integral Field Spectrograph (NIFS) on Gemini North. The
approaching outflow consists of a well-collimated jet, nested within a
lower-velocity disc wind. The receding outflow is composed of a
single-component bubble-like structure. We analyse the kinemat- ics of the
receding outflow using kinetic models, and determine that it is a
quasi-stationary bubble with an expanding internal velocity field. We propose
that this bubble forms because the receding counterjet from DG Tau is
obstructed by a clumpy ambient medium above the circumstellar disc surface,
based on similarities between this structure and those found in the modeling of
active galactic nuclei outflows. We find evidence of interaction between the
obscured counterjet and clumpy ambient material, which we attribute to the
large molecular envelope around the DG Tau system. An analytical model of a
momentum-driven bubble is shown to be consistent with our interpretation. We
conclude that the bipolar outflow from DG Tau is intrinsically symmetric, and
the observed asymmetries are due to environmental effects. This mechanism can
potentially be used to explain the observed bipolar asymmetries in other YSO
outflows.Comment: 16 pages, 10 figures, accepted for publication in MNRA
Energy Conversion Using New Thermoelectric Generator
During recent years, microelectronics helped to develop complex and varied
technologies. It appears that many of these technologies can be applied
successfully to realize Seebeck micro generators: photolithography and
deposition methods allow to elaborate thin thermoelectric structures at the
micro-scale level. Our goal is to scavenge energy by developing a miniature
power source for operating electronic components. First Bi and Sb micro-devices
on silicon glass substrate have been manufactured with an area of 1cm2
including more than one hundred junctions. Each step of process fabrication has
been optimized: photolithography, deposition process, anneals conditions and
metallic connections. Different device structures have been realized with
different micro-line dimensions. Each devices performance will be reviewed and
discussed in function of their design structure.Comment: Submitted on behalf of TIMA Editions
(http://irevues.inist.fr/tima-editions
The M87 Black Hole Mass From Gas-Dynamical Models Of Space Telescope Imaging Spectrograph Observations
The supermassive black hole of M87 is one of the most massive black holes known and has been the subject of several stellar and gas-dynamical mass measurements; however, the most recent revision to the stellar-dynamical black hole mass measurement is a factor of about two larger than the previous gas-dynamical determinations. Here, we apply comprehensive gas-dynamical models that include the propagation of emission-line profiles through the telescope and spectrograph optics to new Space Telescope Imaging Spectrograph observations from the Hubble Space Telescope. Unlike the previous gas-dynamical studies of M87, we map out the complete kinematic structure of the emission-line disk within similar to 40 pc from the nucleus, and find that a small amount of velocity dispersion internal to the gas disk is required to match the observed line widths. We examine a scenario in which the intrinsic velocity dispersion provides dynamical support to the disk, and determine that the inferred black hole mass increases by only 6%. Incorporating this effect into the error budget, we ultimately measure a mass of M-BH = (3.5(-0.7)(+0.9)) x 10(9)M circle dot (68% confidence). Our gas-dynamical black hole mass continues to differ from the most recent stellar-dynamical mass by a factor of two, underscoring the need for carrying out more cross-checks between the two main black hole mass measurement methods.NSF Astronomy and Astrophysics Postdoctoral Fellowship 1102845Space Telescope Science Institute 12162NASA NAS 5-26555NSF AST-1108835Astronom
Conceptual design of a lunar base thermal control system
Space station and alternate thermal control technologies were evaluated for lunar base applications. The space station technologies consisted of single-phase, pumped water loops for sensible and latent heat removal from the cabin internal environment and two-phase ammonia loops for the transportation and rejection of these heat loads to the external environment. Alternate technologies were identified for those areas where space station technologies proved to be incompatible with the lunar environment. Areas were also identified where lunar resources could enhance the thermal control system. The internal acquisition subsystem essentially remained the same, while modifications were needed for the transport and rejection subsystems because of the extreme temperature variations on the lunar surface. The alternate technologies examined to accommodate the high daytime temperatures incorporated lunar surface insulating blankets, heat pump system, shading, and lunar soil. Other heat management techniques, such as louvers, were examined to prevent the radiators from freezing. The impact of the geographic location of the lunar base and the orientation of the radiators was also examined. A baseline design was generated that included weight, power, and volume estimates
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