377 research outputs found
What Sets the Radial Locations of Warm Debris Disks?
The architectures of debris disks encode the history of planet formation in
these systems. Studies of debris disks via their spectral energy distributions
(SEDs) have found infrared excesses arising from cold dust, warm dust, or a
combination of the two. The cold outer belts of many systems have been imaged,
facilitating their study in great detail. Far less is known about the warm
components, including the origin of the dust. The regularity of the disk
temperatures indicates an underlying structure that may be linked to the water
snow line. If the dust is generated from collisions in an exo-asteroid belt,
the dust will likely trace the location of the water snow line in the
primordial protoplanetary disk where planetesimal growth was enhanced. If
instead the warm dust arises from the inward transport from a reservoir of icy
material farther out in the system, the dust location is expected to be set by
the current snow line. We analyze the SEDs of a large sample of debris disks
with warm components. We find that warm components in single-component systems
(those without detectable cold components) follow the primordial snow line
rather than the current snow line, so they likely arise from exo-asteroid
belts. While the locations of many warm components in two-component systems are
also consistent with the primordial snow line, there is more diversity among
these systems, suggesting additional effects play a role
Infrared Emission by Dust Around lambda Bootis Stars: Debris Disks or Thermally Emitting Nebulae?
We present a model that describes stellar infrared excesses due to heating of
the interstellar (IS) dust by a hot star passing through a diffuse IS cloud.
This model is applied to six lambda Bootis stars with infrared excesses.
Plausible values for the IS medium (ISM) density and relative velocity between
the cloud and the star yield fits to the excess emission. This result is
consistent with the diffusion/accretion hypothesis that lambda Bootis stars (A-
to F-type stars with large underabundances of Fe-peak elements) owe their
characteristics to interactions with the ISM. This proposal invokes radiation
pressure from the star to repel the IS dust and excavate a paraboloidal dust
cavity in the IS cloud, while the metal-poor gas is accreted onto the stellar
photosphere. However, the measurements of the infrared excesses can also be fit
by planetary debris disk models. A more detailed consideration of the
conditions to produce lambda Bootis characteristics indicates that the majority
of infrared-excess stars within the Local Bubble probably have debris disks.
Nevertheless, more distant stars may often have excesses due to heating of
interstellar material such as in our model.Comment: 10 pages, 5 figures, 4 tables, accepted by ApJ, emulateap
Relative photometry of HAT-P-1b occultations
We present HST STIS observations of two occultations of the transiting
exoplanet HAT-P-1b. By measuring the planet to star flux ratio near opposition,
we constrain the geometric albedo of the planet, which is strongly linked to
its atmospheric temperature gradient. An advantage of HAT-P-1 as a target is
its binary companion ADS 16402 A, which provides an excellent photometric
reference, simplifying the usual steps in removing instrumental artifacts from
HST time-series photometry. We find that without this reference star, we would
need to detrend the lightcurve with the time of the exposures as well as the
first three powers of HST orbital phase, and this would introduce a strong bias
in the results for the albedo. However, with this reference star, we only need
to detrend the data with the time of the exposures to achieve the same
per-point scatter, therefore we can avoid most of the bias associated with
detrending. Our final result is a 2 sigma upper limit of 0.64 for the geometric
albedo of HAT-P-1b between 577 and 947 nm.Comment: 8 pages, 2 figures, 3 table
The impact of suspension control on the controllability of the lateral vehicle dynamics
International audienceSince there is a coupling between lateral and vertical dynamics, the interactions between control components must be taken into consideration. The paper presents the effects of vertical load variations on the controlled invariant set of the steering system. In the model the nonlinear characteristics of the tire force are approximated by the polynomial form. The analysis is based on Sum-of-Squares programming method and parameter-dependent polynomial control Lyapunov functions. The Maximum Controlled Invariant Sets of the steering as a function of vertical loads are illustrated through a simulation example. The results of the analysis are built into the control design of the suspension system. A semi-active suspension system using preview control is applied. The operation of the controller is illustrated through simulation examples
SPITZER/IRAC-MIPS Survey of NGC2451A and B: Debris Disks at 50-80 million years
We present a Spitzer IRAC and MIPS survey of NGC 2451 A and B, two open
clusters in the 50-80 Myr age range. We complement these data with extensive
ground-based photometry and spectroscopy to identify the cluster members in the
Spitzer survey field. We find only two members with 8 micron excesses. The
incidence of excesses at 24 microns is much higher, 11 of 31 solar-like stars
and 1 of 7 early-type (A) stars. This work nearly completes the debris disk
surveys with Spitzer of clusters in the 30-130 Myr range. This range is of inte
rest because it is when large planetesimal collisions may have still been
relatively common (as indicated by the one that led to the formation of the
Moon during this period of the evolution of the Solar System). We review the
full set of surveys and find that there are only three possible cases out of
about 250 roughly solar-mass stars where very large excesses suggest that such
collisions have occurred recently.Comment: Accepted for publication in ApJ. 25 pages 17 figure
The first CCD photometric study of the open cluster NGC 2126
We present the first CCD photometric observations of the northern open
cluster NGC 2126. Data were taken on eight nights in February and December 2002
with a total time span of ~57 hours. Almost 1000 individual V-band frames were
examined to find short-period variable stars. We discovered six new variable
stars, of which one is a promising candidate for an eclipsing binary with a
pulsating component. Two stars were classified as delta Scuti stars and one as
Algol-type eclipsing binary. Two stars are slow variables with ambiguous
classification. From absolute VRI photometry we have estimated the main
characteristics of the cluster: m-M=11.0+/-0.5, E(V-I)=0.4+/-0.1,
E(V-R)=0.08+/-0.06 (E(B-V)=0.2+/-0.15) and d=1.3+/-0.6 kpc. Cluster membership
is suggested for three variable stars from their positions on the
colour-magnitude diagram.Comment: 7 pages, 10 figures, accepted for publication in A&
The design of an H∞/LPV active braking control to improve vehicle roll stability
International audienceThe active braking control system is an active safety system designed to prevent accidents and to stabilize dynamic manoeuvers of a vehicle by generating an artificial yaw moment using differential braking forces. In this paper, the yaw-roll model of a single unit heavy vehicle is used for studying the active braking system by using the longitudinal braking force at each wheel. The grid-based LPV approach is used to synthesize the H ∞ /LPV controller by considering the parameter dependant weighting function for the lateral acceleration. The braking monitor designs are proposed to allow the active braking system to react when the normalized load transfer at the rear axle reaches the criteria of rollover ±1. The simulation results indicate that the active braking system satisfies the adaptation of vehicle rollover in an emergency situation, with low braking forces and improved handling performance of the vehicle
Breathing Spots in a Reaction-Diffusion System
A quasi-2-dimensional stationary spot in a disk-shaped chemical reactor is
observed to bifurcate to an oscillating spot when a control parameter is
increased beyond a critical value. Further increase of the control parameter
leads to the collapse and disappearance of the spot. Analysis of a bistable
activator-inhibitor model indicates that the observed behavior is a consequence
of interaction of the front with the boundary near a parity breaking front
bifurcation.Comment: 4 pages RevTeX, see also http://chaos.ph.utexas.edu/ and
http://t7.lanl.gov/People/Aric
Photoevaporation of Protoplanetary Disks
We present HST/NICMOS Paschen alpha images and low and high resolution IRS
spectra of photoevaporating disk-tail systems originally detected at 24 micron
near O stars. We find no Paschen alpha emission in any of the systems. The
resulting upper limits correspond to about 0.000002-0.000003 solar mass of mass
in hydrogen in the tails suggesting that the gas is severely depleted. The IRAC
data and the low resolution 5-12 micron IRS spectra provide evidence for an
inner disk while high resolution long wavelength (14-30 micron) IRS spectra
confirm the presence of a gas free ``tail'' that consists of ~ 0.01 to ~ 1
micron dust grains originating in the outer parts of the circumstellar disks.
Overall our observations support theoretical predictions in which
photoevaporation removes the gas relatively quickly (<= 100000 yrs) from the
outer region of a protoplanetary disk but leaves an inner more robust and
possibly gas-rich disk component of radius 5-10 AU. With the gas gone, larger
solid bodies in the outer disk can experience a high rate of collisions and
produce elevated amounts of dust. This dust is being stripped from the system
by the photon pressure of the O star to form a gas-free dusty tail.Comment: 9 pages 5 figures, accepted for publication in Ap
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