12,930 research outputs found
Traffic at the Edge of Chaos
We use a very simple description of human driving behavior to simulate
traffic. The regime of maximum vehicle flow in a closed system shows
near-critical behavior, and as a result a sharp decrease of the predictability
of travel time. Since Advanced Traffic Management Systems (ATMSs) tend to drive
larger parts of the transportation system towards this regime of maximum flow,
we argue that in consequence the traffic system as a whole will be driven
closer to criticality, thus making predictions much harder. A simulation of a
simplified transportation network supports our argument.Comment: Postscript version including most of the figures available from
http://studguppy.tsasa.lanl.gov/research_team/. Paper has been published in
Brooks RA, Maes P, Artifical Life IV: ..., MIT Press, 199
Non-LTE spectral models for the gaseous debris-disk component of Ton 345
For a fraction of single white dwarfs with debris disks, an additional
gaseous disk was discovered. Both dust and gas are thought to be created by the
disruption of planetary bodies. The composition of the extrasolar planetary
material can directly be analyzed in the gaseous disk component, and the disk
dynamics might be accessible by investigating the temporal behavior of the Ca
II infrared emission triplet, hallmark of the gas disk.
We obtained new optical spectra for the first helium-dominated white dwarf
for which a gas disk was discovered (Ton 345) and modeled the non-LTE spectra
of viscous gas disks composed of carbon, oxygen, magnesium, silicon, sulfur,
and calcium with chemical abundances typical for solar system asteroids. Iron
and its possible line-blanketing effects on the model structure and spectral
energy distribution was still neglected. A set of models with different radii,
effective temperatures, and surface densities as well as chondritic and
bulk-Earth abundances was computed and compared with the observed line profiles
of the Ca II infrared triplet.
Our models suggest that the Ca II emission stems from a rather narrow gas
ring with a radial extent of R=0.44-0.94 Rsol, a uniform surface density
Sigma=0.3 g/cm2, and an effective temperature of Teff=6000 K. The often assumed
chemical mixtures derived from photospheric abundances in polluted white dwarfs
- similar to a chondritic or bulk-Earth composition - produce unobserved
emission lines in the model and therefore have to be altered. We do not detect
any line-profile variability on timescales of hours, but we confirm the
long-term trend over the past decade for the red-blue asymmetry of the
double-peaked lines.Comment: 7 pages, 6 figures, 2 table
Asymmetric Line Profiles in Spectra of Gaseous Metal Disks Around Single White Dwarfs
Around several single DAZ and DBZ white dwarfs metal-rich disks have been
observed, which are mostly believed to originate from disruption of smaller
rocky planetesimals. In some cases the material does not (only) form a dusty
but gaseous disk. In the case of SDSS J122859.93+104032.9 the double peaked
infrared Ca II triplet at about 8500 angstrom, one of only two emission
features of the spectra, exhibits a strong red/violet asymmetry. Assuming a
composition similar to a chondrite-like asteroid, being the most prominent type
in our own solar system, we calculated the spectrum and vertical structure of
the disk using the Tuebingen NLTE accretion disk code "AcDc". Modified to
simulate different non axis-symmetrical disk geometries, the first preliminary
results are in good agreement with the observed asymmetric line profile.Comment: 6 pages, 5 figures, proceeding of the "17th European White Dwarf
Workshop", Tuebingen, Germany, 201
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