330 research outputs found
The scattering from generalized Cantor fractals
We consider a fractal with a variable fractal dimension, which is a
generalization of the well known triadic Cantor set. In contrast with the usual
Cantor set, the fractal dimension is controlled using a scaling factor, and can
vary from zero to one in one dimension and from zero to three in three
dimensions. The intensity profile of small-angle scattering from the
generalized Cantor fractal in three dimensions is calculated. The system is
generated by a set of iterative rules, each iteration corresponding to a
certain fractal generation. Small-angle scattering is considered from
monodispersive sets, which are randomly oriented and placed. The scattering
intensities represent minima and maxima superimposed on a power law decay, with
the exponent equal to the fractal dimension of the scatterer, but the minima
and maxima are damped with increasing polydispersity of the fractal sets. It is
shown that for a finite generation of the fractal, the exponent changes at
sufficiently large wave vectors from the fractal dimension to four, the value
given by the usual Porod law. It is shown that the number of particles of which
the fractal is composed can be estimated from the value of the boundary between
the fractal and Porod regions. The radius of gyration of the fractal is
calculated analytically.Comment: 8 pages, 4 figures, accepted for publication in J. Appl. Crys
Thermoelastic Properties of Ringwoodite [Fe_x,Mg_(1-x)]_2SiO_4: Its Relationship to the 520 km Seismic Discontinuity
We combine density functional theory (DFT) within the local density
approximation (LDA), the quasiharmonic approximation (QHA), and a model
vibrational density of states (VDoS) to calculate elastic moduli and sound
velocities of gamma-[Fe_x,Mg_(1-x)]_2SiO_4 (ringwoodite), the most abundant
mineral of the lower Earth's transition zone (TZ). Comparison with experimental
values at room-temperature and high pressure or ambient-pressure and high
temperature shows good agreement with our first-principles findings. Then, we
investigate the contrasts associated with the
beta-to-gamma-[Fe_x,Mg_(1-x)]_2SiO_4 transformation at pressures and
temperatures relevant to the TZ. This information offers clearly defined
reference values to advance the understanding of the nature of the 520 km
seismic discontinuity.Comment: 29 pages, 6 figures, 2 tables. Under Revie
Orbits and Masses in the T Tauri System
We investigate the binary star T Tauri South, presenting the orbital
parameters of the two components and their individual masses. We combined
astrometric positions from the literature with previously unpublished VLT
observations. Model fits yield the orbital elements of T Tau Sa and Sb. We use
T Tau N as an astrometric reference to derive an estimate for the mass ratio of
Sa and Sb. Although most of the orbital parameters are not well constrained, it
is unlikely that T Tau Sb is on a highly elliptical orbit or escaping from the
system. The total mass of T Tau S is rather well constrained to 3.0 +0.15/-0.24
M_sun. The mass ratio Sb:Sa is about 0.4, corresponding to individual masses of
M_Sa = 2.1+/-0.2 M_sun and M_Sb = 0.8+/-0.1 M_sun. This confirms that the
infrared companion in the T Tauri system is a pair of young stars obscured by
circumstellar material.Comment: 10 pages, 11 figures, accepted by Astronomy and Astrophysic
Annular substructures in the transition disks around LkCa 15 and J1610
We present high resolution millimeter continuum ALMA observations of the
disks around the T Tauri stars LkCa 15 and J1610. These disks host
dust-depleted inner regions, possibly carved by massive planets, and are of
prime interest to study the imprints of planet-disk interactions. While at
moderate angular resolution they appear as a broad ring surrounding a cavity,
the continuum emission resolves into multiple rings at a resolution of
~6040 mas (~7.5 au for LkCa 15, ~6 au for J1610) and ~Jy
beam rms at 1.3 mm. In addition to a broad extended component, LkCa 15
and J1610 host 3 and 2 narrow rings, respectively, with two bright rings in
LkCa 15 being radially resolved. The rings look marginally optically thick,
with peak optical depths of ~0.5 (neglecting scattering), in agreement with
high angular resolution observations of full disks. We perform hydrodynamical
simulations with an embedded, sub-Jovian-mass planet and show that the observed
multi-ringed substructure can be qualitatively explained as the outcome of the
planet-disk interaction. We note however that the choice of the disk cooling
timescale alone can significantly impact the resulting gas and dust
distributions around the planet, leading to different numbers of rings and gaps
and different spacings between them. We propose that the massive outer disk
regions of transition disks are favorable places for planetesimals and possibly
second generation planet formation of objects with a lower mass than the
planets carving the inner cavity (typically few ), and that the
annular substructures observed in LkCa 15 and J1610 may be indicative of
planetary core formation within dust-rich pressure traps. Current observations
are compatible with other mechanisms being at the origin of the observed
substructures, in particular with narrow rings generated at the edge of the CO
and N snowlines.Comment: 17 pages, accepted for publication in Astronomy & Astrophysic
Candidate Coronagraphic Detections of Protoplanetary Disks around Four Young Stars
We present potential detections of H-band scattered light emission around
four young star, selected from a total sample of 45 young stars observed with
the CIAO coronagraph of the Subaru telescope. Two CTTS, CI Tau and DI Cep, and
two WTTS, LkCa 14 and RXJ 0338.3+1020 were detected. In all four cases, the
extended emission is within the area of the residual PSF halo, and is revealed
only through careful data reduction. We compare the observed extended emission
with simulations of the scattered light emission, to evaluate the plausibility
and nature of the detected emission.Comment: 9 Figures, 40 page
Spatially resolved H2 emission from the disk around T Tau N
We report the detection of quiescent H2 emission in a spatially resolved
ring-like structure within 100 AU of T Tau N. We present evidence to show that
the emission most likely arises from shocks in the atmosphere of a nearly
face-on disk around T Tau N. Using high spatial resolution 3D spectroscopic
K-band data, we trace the spatial distribution of several H2 NIR rovibrational
lines in the vicinity of T Tau N. We detect weak H2 emission from the v=1-0
S(0), S(1), Q(1) lines and the v=2-1 S(1) line in a ring-like structure around
T Tau N between 0.1'' (~15 AU) and 0.7'' (~100AU) from the star. The v=1-0 S(0)
and v=2-1 S(1) lines are detected only in the outer parts of the ring
structure. Closer to the star, the strong continuum limits our sensitivity to
these lines. The total flux of the v=1-0 S(1) line is 1.8 *10^{-14} ergs
s^{-1}cm^{-2}, similar to previous measurements of H2 in circumstellar disks.
The velocity of the H2 emitting gas around T Tau N is consistent with the rest
velocity of the star, and the H2 does not seem to be part of a collimated
outflow. Both shocks impinging on the surface of a disk and irradiation of a
disk by UV-photons and X-rays from the central star are plausible candidates
for the H2 excitation mechanism. However, irradiation should not create a large
degree of excitation at radii larger than 20 AU. Most likely the H2 emission
arises in the atmosphere of a flared disk with radius 85-100 AU and mass
0.005-0.5Msun, where the gas is excited by shocks created when a wide-angle
wind impinges on the disk. The H2 emission could also originate from shock
excitation in the cavity walls of an envelope, but this requires an unusually
high velocity of the wide-angle wind from T Tau N.Comment: Accepted by A&
Direct Imaging of Fine Structures in Giant Planet Forming Regions of the Protoplanetary Disk around AB Aurigae
We report high-resolution 1.6 \micron polarized intensity () images of
the circumstellar disk around the Herbig Ae star AB Aur at a radial distance of
22 AU () up to 554 AU (3.85), which have been obtained by the
high-contrast instrument HiCIAO with the dual-beam polarimetry. We revealed
complicated and asymmetrical structures in the inner part (140 AU) of
the disk, while confirming the previously reported outer ( 200 AU)
spiral structure. We have imaged a double ring structure at 40 and
100 AU and a ring-like gap between the two. We found a significant
discrepancy of inclination angles between two rings, which may indicate that
the disk of AB Aur is warped. Furthermore, we found seven dips (the typical
size is 45 AU or less) within two rings as well as three prominent
peaks at 40 AU. The observed structures, including a bumpy double ring, a
ring-like gap, and a warped disk in the innermost regions, provide essential
information for understanding the formation mechanism of recently detected
wide-orbit ( 20 AU) planets.Comment: 12 pages, 3 figure
Subaru Imaging of Asymmetric Features in a Transitional Disk in Upper Scorpius
We report high-resolution (0.07 arcsec) near-infrared polarized intensity
images of the circumstellar disk around the star 2MASS J16042165-2130284
obtained with HiCIAO mounted on the Subaru 8.2 m telescope. We present our
-band data, which clearly exhibits a resolved, face-on disk with a large
inner hole for the first time at infrared wavelengths. We detect the
centrosymmetric polarization pattern in the circumstellar material as has been
observed in other disks. Elliptical fitting gives the semimajor axis, semiminor
axis, and position angle (P.A.) of the disk as 63 AU, 62 AU, and -14
, respectively. The disk is asymmetric, with one dip located at P.A.s
of . Our observed disk size agrees well with a previous study
of dust and CO emission at submillimeter wavelength with Submillimeter Array.
Hence, the near-infrared light is interpreted as scattered light reflected from
the inner edge of the disk. Our observations also detect an elongated arc (50
AU) extending over the disk inner hole. It emanates at the inner edge of the
western side of the disk, extending inward first, then curving to the
northeast. We discuss the possibility that the inner hole, the dip, and the arc
that we have observed may be related to the existence of unseen bodies within
the disk.Comment: 21 pages, 3 figures, published 2012 November 7 by ApJL, typo
correcte
TLSim: A software for Electrical Public Transportation Systems
The Laboratory of Electrical Machines of the Swiss Federal Institute of Technology (Lausanne) has developed a software (TLSim) for the simulation of electrical urban transportation systems. This program can simulate the behaviour of the power supply network (current and voltage level at any point of the topology) comprising uni- and bi- directionnal sub-stations. In the network the vehicules (trolleybus) may run along any defined path through different electrical supply sectors. Several models of trolleybus are available, one of these allowing for the injection of current back in the supply network. The operating schedule of the buses as well as the geographical data of the network are provided by the user. This program is a perfect tool for system planning as ascertained by its use by a Swiss transportation operator. A comparison of simulation and measurements on a real network has been performed with satisfactory results. This software evolved from a specific study to a flexible program still being extended with new developments. The latest of such developments being the study of the influence of capacitors in helping the system to keep voltages over a specified level mainly by supplying the surge current during the acceleration of the trolleybuses
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