5,856 research outputs found
Near Infrared Spectroscopy of Young Brown Dwarfs in Upper Scorpius
Spectroscopic follow-up is a pre-requisite for studies of the formation and
early evolution of brown dwarfs. Here we present IRTF/SpeX near-infrared
spectroscopy of 30 candidate members of the young Upper Scorpius association,
selected from our previous survey work. All 24 high confidence members are
confirmed as young very low mass objects with spectral types from M5 to L1,
15-20 of them are likely brown dwarfs. This high yield confirms that brown
dwarfs in Upper Scorpius can be identified from photometry and proper motions
alone, with negligible contamination from field objects (<4%). Out of the 6
candidates with lower confidence, 5 might still be young very low mass members
of Upper Scorpius, according to our spectroscopy. We demonstrate that some very
low mass class II objects exhibit radically different near infrared (0.6 -
2.5micron) spectra from class III objects, with strong excess emission
increasing towards longer wavelengths and partially filled in features at
wavelengths shorter than 1.25micron. These characteristics can obscure the
contribution of the photosphere within such spectra. Therefore, we caution that
near infrared derived spectral types for objects with discs may be unreliable.
Furthermore, we show that the same characteristics can be seen to some extent
in all class II and even a significant fraction of class III objects (~40%),
indicating that some of them are still surrounded by traces of dust and gas.
Based on our spectra, we select a sample of objects with spectral types of M5
to L1, whose near-infrared emission represents the photosphere only. We
recommend the use of these objects as spectroscopic templates for young brown
dwarfs in the future.Comment: 12 pages, 9 figures, Accepted in MNRA
Hybrid Quantum Dot-2D Electron Gas Devices for Coherent Optoelectronics
We present an inverted GaAs 2D electron gas with self-assembled InAs quantum
dots in close proximity, with the goal of combining quantum transport with
quantum optics experiments. We have grown and characterized several wafers --
using transport, AFM and optics -- finding narrow-linewidth optical dots and
high-mobility, single subband 2D gases. Despite being buried 500 nm below the
surface, the dots are clearly visible on AFM scans, allowing precise
localization and paving the way towards a hybrid quantum system integrating
optical dots with surface gate-defined nanostructures in the 2D gas.Comment: 4 pages, 5 figures (color
Quand l'information échappe à ses créateurs : le cas de l'artificialisation des terres agricoles en Languedoc-Roussillon
International audienceThis article aims to describe how the quantification of a phenomenon in relation to a specific worldview spreads and is distorted. This project has been supported by a regional public authority in Languedoc-Roussillon (France), and consisted in the production of information so to quantify and qualify the farmland decrease due to urban sprawl. The paper is based on Latour's actor-network theory; it characterizes the process of enrolment, and the development of dissidences. Identifying contradictory logics and dissidences help questioning the complexity of ‘closing the black-box' regarding information production and of sensemaking between heterogeneous actants involved in a supposed-to-be public debate.Cet article vise à spécifier comment la quantification d'un phénomène fondée sur une certaine vision du monde se propage et se déforme. Ce projet a été porté par une direction déconcentrée de l'Etat soucieuse de l'emprise croissante du bâti sur les terres agricoles en Languedoc-Roussillon. Le cadre théorique mobilisé, la sociologie de la traduction, permet d'identifier les grandes étapes d'intéressement, d'enrôlement, ainsi que le développement de controverses. Les difficultés rencontrées interrogent sur la complexité de clôture de processus innovants s'agissant de production d'information et de co-construction de sens associant des actants hétérogènes impliqués dans des procédures de débat public
Evolution of displacements and strains in sheared amorphous solids
The local deformation of two-dimensional Lennard-Jones glasses under imposed
shear strain is studied via computer simulations. Both the mean squared
displacement and mean squared strain rise linearly with the length of the
strain interval over which they are measured. However, the
increase in displacement does not represent single-particle diffusion. There
are long-range spatial correlations in displacement associated with slip lines
with an amplitude of order the particle size. Strong dependence on system size
is also observed. The probability distributions of displacement and strain are
very different. For small the distribution of displacement has
a plateau followed by an exponential tail. The distribution becomes Gaussian as
increases to about .03. The strain distributions consist of
sharp central peaks associated with elastic regions, and long exponential tails
associated with plastic regions. The latter persist to the largest studied.Comment: Submitted to J. Phys. Cond. Mat. special volume for PITP Conference
on Mechanical Behavior of Glassy Materials. 16 Pages, 8 figure
Resolving asymmetries along the pulsation cycle of the Mira star X Hya
The mass-loss process in Mira stars probably occurs in an asymmetric way
where dust can form in inhomogeneous circumstellar molecular clumps. Following
asymmetries along the pulsation cycle can give us clues about these mass-loss
processes. We imaged the Mira star X Hya and its environnement at different
epochs to follow the evolution of the morphology in the continuum and in the
molecular bands. We observed X Hya with AMBER in J-H-K at low resolution at two
epochs. We modelled squared visibilities with geometrical and physical models.
We also present imaging reconstruction results obtained with MiRA and based on
the physical a priori images. We report on the angular scale change of X Hya
between the two epochs. 1D CODEX profiles allowed us to understand and model
the spectral variation of squared visibilities and constrain the stellar
parameters. Reconstructed model-dependent images enabled us to reproduce
closure phase signals and the azimuthal dependence of squared visibilities.
They show evidence for material inhomogeneities located in the immediate
environment of the star.Comment: Accepted for publication in A&A, 17 pages, 16 figure
Simulation study of spatio-temporal correlations of earthquakes as a stick-slip frictional instability
Spatio-temporal correlations of earthquakes are studied numerically on the
basis of the one-dimensional spring-block (Burridge-Knopoff) model. As large
events approach, the frequency of smaller events gradually increases, while,
just before the mainshock, it is dramatically suppressed in a close vicinity of
the epicenter of the upcoming mainshock, a phenomenon closely resembling the
``Mogi doughnut'
Spin-down evolution and radio disappearance of the magnetar PSR J16224950
We report on 2.4 yr of radio timing measurements of the magnetar PSR
J16224950 using the Parkes telescope, between 2011 November and 2014 March.
During this period the torque on the neutron star (inferred from the rotational
frequency derivative) varied greatly, though much less erratically than in the
2 yr following its discovery in 2009. During the last year of our measurements
the frequency derivative decreased in magnitude monotonically by 20\%, to a
value of s, a factor of 8 smaller than when
discovered. The flux density continued to vary greatly during our monitoring
through 2014 March, reaching a relatively steady low level after late 2012. The
pulse profile varied secularly on a similar timescale as the flux density and
torque. A relatively rapid transition in all three properties is evident in
early 2013. After PSR J16224950 was detected in all of our 87 observations
up to 2014 March, we did not detect the magnetar in our resumed monitoring
starting in 2015 January and have not detected it in any of the 30 observations
done through 2016 September.Comment: 8 pages, 5 figures, submitted to Ap
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