312 research outputs found
Photo-assisted shot noise in Coulomb interacting systems
We consider the fluctuations of the electrical current (shot noise) in the
presence of a voltage time-modulation. For a non-interacting metal, it is known
that the derivative of the photo-assisted noise has a staircase behavior. In
the presence of Coulomb interactions, we show that the photo-assisted noise
presents a more complex profile, in particular for the two following systems:
1) a two-dimensional electron gas in the fractional quantum Hall regime for
which we have obtained evenly spaced singularities in the noise derivative,
with a spacing related to the filling factor and, 2) a carbon nanotube for
which a smoothed staircase in the noise derivative is obtained.Comment: Proceedings of the 6th Rencontres du Vietnam, Hanoi (2006
An electronic Mach-Zehnder interferometer in the Fractional Quantum Hall effect
We compute the interference pattern of a Mach-Zehnder interferometer
operating in the fractional quantum Hall effect. Our theoretical proposal is
inspired by a remarkable experiment on edge states in the Integer Quantum Hall
effect (IQHE). The Luttinger liquid model is solved via two independent
methods: refermionization at nu=1/2 and the Bethe Ansatz solution available for
Laughlin fractions. The current differs strongly from that of single electrons
in the strong backscattering regime. The Fano factor is periodic in the flux,
and it exhibits a sharp transition from sub-Poissonian (charge e/2) to
Poissonian (charge e) in the neighborhood of destructive interferences
The Planetary Mass Companion 2MASS1207-3932 B: Temperature, Mass and Evidence for an Edge-On Disk
We present J-band imaging and H+K-band low-resolution spectroscopy of
2MASS1207-3932 AB, obtained with VLT NACO. For the putative planetary mass
secondary, we find J = 20.0+/-0.2 mag. The HK spectra of both components imply
low gravity, and a dusty atmosphere for the secondary. Comparisons to synthetic
spectra yield Teff_A ~ 2550+/-150K, and Teff_B ~ 1600+/-100K, consistent with
their late-M and mid-to-late L types. For these Teff, and an age of 5-10 Myrs,
evolutionary models imply M_A ~ 24+/-6 M_Jup and M_B ~ 8+/-2 M_Jup. Independent
comparisons of these models to the observed colors, spanning ~I to L', also
yield the same masses and temperatures. Our primary mass agrees with other
recent analyses; however, our secondary mass, while still in the planetary
regime, is 2-3 times larger than claimed previously. This discrepancy can be
traced to the luminosities: while the absolute photometry and Mbol of the
primary agree with theoretical predictions, the secondary is ~ 2.5+/-0.5 mag
fainter than expected in all bands from I to L' and in Mbol. This accounts for
the much lower secondary mass (and temperature) derived earlier. We argue that
this effect is highly unlikely to result from a variety of model-related
problems, and is instead real. This conclusion is bolstered by the absence of
any luminosity problems in either the primary, or in AB Pic B which we also
analyse. We therefore suggest grey extinction in 2M1207B, due to occlusion by
an edge-on circum-secondary disk. This is consistent with the observed
properties of edge-on disks around T Tauri stars, and with the known presence
of a high-inclination evolved disk around the primary. Finally, the system's
implied mass ratio of ~0.3 suggests a binary-like formation scenario.
(abridged)Comment: Accepted by The Astrophysical Journal, 43 pages text + 16 figs + 1
tabl
Deep near-infrared luminosity function of a cluster of galaxies at z=0.3
The deep near-infrared luminosity function of AC118, a cluster of galaxies at
z=0.3, is presented. AC118 is a bimodal cluster, as evidenced both by our
near-infrared images of lensed galaxies, by public X-ray Rosat images and by
the spatial distribution of bright galaxies. Taking advantage of the extension
and depth of our data, which sample an almost unexplored region in the depth
vs. observed area diagram, we derive the luminosity function (LF), down to the
dwarf regime (M*+5), computed in several cluster portions. The overall LF,
computed on a 2.66 Mpc2 areas (H_0=50 km/s/Mpc), has an intermediate slope
(alpha=-1.2). However, the LF parameters depend on the surveyed cluster region:
the central concentration has 2.6^{+5.1}_{-1.7} times more bright galaxies and
5.3^{+7.2}_{-2.3} times less dwarfs per typical galaxy than the outer region,
which includes galaxies at an average projected distance of ~580 kpc (errors
are quoted at the 99.9 % confidence level). The LF in the secondary AC118 clump
is intermediate between the central and outer one. In other words, the
near-infrared AC118 LF steepens going from high to low density regions. At an
average clustercentric distance of ~580 kpc, the AC118 LF is statistically
indistinguishable from the LF of field galaxies at similar redshift, thus
suggesting that the hostile cluster environment plays a minor role in shaping
the LF at large clustercentric distances, while it strongly affects the LF at
higher galaxy density.Comment: ApJ, in press. The whole paper with all high resolution images is
available at http://www.na.astro.it/~andreon/listapub.htm
Size segregation and convection
The size segregation of granular materials in a vibrating container is
investigated using Molecular Dynamics. We find that the rising of larger
particles is accompanied by the existence of convection cells even in the case
of the lowest possible frequencies. The convection can, however, also be
triggered by the larger particle itself. The possibility of rising through this
mechanism strongly depends on the depth of the larger particle.Comment: 7 pages, 4 figure
Searching for sub-stellar companion into the LkCa15 proto-planetary disk
Recent sub-millimetric observations at the Plateau de Bure interferometer
evidenced a cavity at ~ 46 AU in radius into the proto-planetary disk around
the T Tauri star LkCa15 (V1079 Tau), located in the Taurus molecular cloud.
Additional Spitzer observations have corroborated this result possibly
explained by the presence of a massive (>= 5 MJup) planetary mass, a brown
dwarf or a low mass star companion at about 30 AU from the star. We used the
most recent developments of high angular resolution and high contrast imaging
to search directly for the existence of this putative companion, and to bring
new constraints on its physical and orbital properties. The NACO adaptive
optics instrument at VLT was used to observe LkCa15 using a four quadrant phase
mask coronagraph to access small angular separations at relatively high
contrast. A reference star at the same parallactic angle was carefully observed
to optimize the quasi-static speckles subtraction (limiting our sensitivity at
less than 1.0). Although we do not report any positive detection of a faint
companion that would be responsible for the observed gap in LkCa15's disk
(25-30 AU), our detection limits start constraining its probable mass,
semi-major axis and eccentricity. Using evolutionary model predictions, Monte
Carlo simulations exclude the presence of low eccentric companions with masses
M >= 6 M Jup and orbiting at a >= 100 AU with significant level of confidence.
For closer orbits, brown dwarf companions can be rejected with a detection
probability of 90% down to 80 AU (at 80% down to 60 AU). Our detection limits
do not access the star environment close enough to fully exclude the presence
of a brown dwarf or a massive planet within the disk inner activity (i.e at
less than 30 AU). Only, further and higher contrast observations should unveil
the existence of this putative companion inside the LkCa15 disk.Comment: 6 pages, 4 figures, accepted for publication in A&
Properties of mm galaxies: Constraints from K-band blank fields
We have used the IRAM Plateau de Bure mm interferometer to locate with
subarcsecond accuracy the dust emission of three of the brightest 1.2mm sources
in the NTT Deep Field (NDF) selected from our 1.2mm MAMBO survey at the IRAM
30m telescope. We combine these results with deep B to K imaging and VLA
interferometry. Strikingly, none of the three accurately located mm galaxies
MMJ120546-0741.5, MMJ120539-0745.4, and MMJ120517-0743.1 has a K-band
counterpart down to the faint limit of K>21.9. This implies that these three
galaxies are either extremely obscured and/or are at very high redshifts
(z>~4). We combine our results with literature data for 11 more (sub)mm
galaxies that are identified with similar reliability. In terms of their K-band
properties, the sample divides into three roughly equal groups: (i) undetected
to K~22, (ii) detected in the near-infrared but not the optical and (iii)
detected in the optical with the possibility of optical follow-up spectroscopy.
We find a trend in this sample between near-infrared to submm and submm to
radio spectral indices, which in comparison to spectral energy distributions
(SEDs) of low redshift infrared luminous galaxies suggests that the most
plausible primary factor causing the extreme near-infrared faintness of our
objects is their high redshift. We show that the near-infrared to radio SEDs of
the sample are inconsistent with SEDs that resemble local far-infrared cool
galaxies with moderate luminosities, which were proposed in some models of the
submm sky. We briefly discuss the implications of the results for our
understanding of galaxy formation.Comment: aastex, 5 figures. Accepted by Ap
ESO Imaging Survey: infrared observations of CDF-S and HDF-S
This paper presents infrared data obtained from observations carried out at
the ESO 3.5m New Technology Telescope (NTT) of the Hubble Deep Field South
(HDF-S) and the Chandra Deep Field South (CDF-S). These data were taken as part
of the ESO Imaging Survey (EIS) program, a public survey conducted by ESO to
promote follow-up observations with the VLT. In the HDF-S field the infrared
observations cover an area of ~53 square arcmin, encompassing the HST WFPC2 and
STIS fields, in the JHKs passbands. The seeing measured in the final stacked
images ranges from 0.79" to 1.22" and the median limiting magnitudes (AB
system, 2" aperture, 5sigma detection limit) are J_AB~23.0, H_AB~22.8 and
K_AB~23.0 mag. Less complete data are also available in JKs for the adjacent
HST NICMOS field. For CDF-S, the infrared observations cover a total area of
\~100 square arcmin, reaching median limiting magnitudes (as defined above) of
J_AB~23.6 and K_AB~22.7 mag. For one CDF-S field H-band data are also
available. This paper describes the observations and presents the results of
new reductions carried out entirely through the un-supervised, high-throughput
EIS Data Reduction System and its associated EIS/MVM C++-based image processing
library developed, over the past 5 years, by the EIS project and now publicly
available. The paper also presents source catalogs extracted from the final
co-added images which are used to evaluate the scientific quality of the survey
products, and hence the performance of the software. This is done comparing the
results obtained in the present work with those obtained by other authors from
independent data and/or reductions carried out with different software packages
and techniques. The final science-grade catalogs and co-added images are
available at CDS.Comment: Accepted for publication in A&A, 13 pages, 12 figures; a full
resolution version of the paper is available from
http://www.astro.ku.dk/~lisbeth/eisdata/papers/4528.pdf ; related catalogs
and images are available through http://www.astro.ku.dk/~lisbeth/eisdata
Angle of Repose and Angle of Marginal Stability: Molecular Dyanmics of Granular Particles
We present an implementation of realistic static friction in molecular
dynamics (MD) simulations of granular particles. In our model, to break
contacts between two particles, one has to apply a finite amount of force,
determined by the Coulomb criterion. Using a two dimensional model, we show
that piles generated by avalanches have a {\it finite} angle of repose
(finite slopes). Furthermore, these piles are stable under tilting
by an angle smaller than a non-zero tilting angle , showing that
is different from the angle of marginal stability ,
which is the maximum angle of stable piles. These measured angles are compared
to a theoretical approximation. We also measure by continuously
adding particles on the top of a stable pile.Comment: 14 pages, Plain Te
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