2,515 research outputs found
Extensions of tempered representations
Let be irreducible tempered representations of an affine Hecke
algebra H with positive parameters. We compute the higher extension groups
explicitly in terms of the representations of analytic
R-groups corresponding to and . The result has immediate
applications to the computation of the Euler-Poincar\'e pairing ,
the alternating sum of the dimensions of the Ext-groups. The resulting formula
for is equal to Arthur's formula for the elliptic pairing of
tempered characters in the setting of reductive p-adic groups. Our proof
applies equally well to affine Hecke algebras and to reductive groups over
non-archimedean local fields of arbitrary characteristic. This sheds new light
on the formula of Arthur and gives a new proof of Kazhdan's orthogonality
conjecture for the Euler-Poincar\'e pairing of admissible characters.Comment: This paper grew out of "A formula of Arthur and affine Hecke
algebras" (arXiv:1011.0679). In the second version some minor points were
improve
Extending the Canada-France brown Dwarfs Survey to the near-infrared: first ultracool brown dwarfs from CFBDSIR
We present the first results of the ongoing Canada-France Brown Dwarfs
Survey-InfraRed, hereafter CFBDSIR, a Near InfraRed extension to the optical
wide-field survey CFBDS. Our final objectives are to constrain ultracool
atmosphere physics by finding a statistically significant sample of objects
cooler than 650K and to explore the ultracool brown dwarf mass function
building on a well defined sample of such objects. Candidates are identified in
CFHT/WIRCam J and CFHT/MegaCam z' images using optimised psf-fitting, and we
follow them up with pointed near infrared imaging with SOFI at NTT. We finally
obtain low resolution spectroscopy of the coolest candidates to characterise
their atmospheric physics. We have so far analysed and followed up all
candidates on the first 66 square degrees of the 335 square degrees survey. We
identified 55 T-dwarfs candidates with z'-J > 3:5 and have confirmed six of
them as T-dwarfs, including 3 that are strong later-than-T8 candidates, based
on their far-red and NIR colours. We also present here the NIR spectra of one
of these ultracool dwarfs, CFBDSIR1458+1013 which confirms it as one of the
coolest brown dwarf known, possibly in the 550-600K temperature range. From the
completed survey we expect to discover 10 to 15 dwarfs later than T8, more than
doubling the known number of such objects. This will enable detailed studies of
their extreme atmospheric properties and provide a stronger statistical base
for studies of their luminosity function.Comment: A&A, Accepte
The ultracool-field dwarf luminosity-function and space density from the Canada-France Brown Dwarf Survey
Context. Thanks to recent and ongoing large scale surveys, hundreds of brown
dwarfs have been discovered in the last decade. The Canada-France Brown Dwarf
Survey is a wide-field survey for cool brown dwarfs conducted with the MegaCam
camera on the Canada-France-Hawaii Telescope telescope. Aims. Our objectives
are to find ultracool brown dwarfs and to constrain the field brown-dwarf
luminosity function and the mass function from a large and homogeneous sample
of L and T dwarfs. Methods. We identify candidates in CFHT/MegaCam i' and z'
images and follow them up with pointed near infrared (NIR) imaging on several
telescopes. Halfway through our survey we found ~50 T dwarfs and ~170 L or
ultra cool M dwarfs drawn from a larger sample of 1400 candidates with typical
ultracool dwarfs i' - z' colours, found in 780 square degrees. Results. We have
currently completed the NIR follow-up on a large part of the survey for all
candidates from mid-L dwarfs down to the latest T dwarfs known with utracool
dwarfs' colours. This allows us to draw on a complete and well defined sample
of 102 ultracool dwarfs to investigate the luminosity function and space
density of field dwarfs. Conclusions. We found the density of late L5 to T0
dwarfs to be 2.0pm0.8 x 10-3 objects pc-3, the density of T0.5 to T5.5 dwarfs
to be 1.4pm0.3 x 10-3 objects pc-3, and the density of T6 to T8 dwarfs to be
5.3pm3.1 x 10-3 objects pc-3 . We found that these results agree better with a
flat substellar mass function. Three latest dwarfs at the boundary between T
and Y dwarfs give the high density 8.3p9.0m5.1 x 10-3 objects pc-3. Although
the uncertainties are very large this suggests that many brown dwarfs should be
found in this late spectral type range, as expected from the cooling of brown
dwarfs, whatever their mass, down to very low temperature.Comment: Accepted for publication in A&
Spitzer Mid-Infrared Photometry of 500 - 750 K Brown Dwarfs
Mid-infrared data, including Spitzer warm-IRAC [3.6] and [4.5] photometry, is
critical for understanding the cold population of brown dwarfs now being found,
objects which have more in common with planets than stars. As effective
temperature (T_eff) drops from 800 K to 400 K, the fraction of flux emitted
beyond 3 microns increases rapidly, from about 40% to >75%. This rapid increase
makes a color like H-[4.5] a very sensitive temperature indicator, and it can
be combined with a gravity- and metallicity-sensitive color like H-K to
constrain all three of these fundamental properties, which in turn gives us
mass and age for these slowly cooling objects. Determination of mid-infrared
color trends also allows better exploitation of the WISE mission by the
community. We use new Spitzer Cycle 6 IRAC photometry, together with published
data, to present trends of color with type for L0 to T10 dwarfs. We also use
the atmospheric and evolutionary models of Saumon & Marley to investigate the
masses and ages of 13 very late-type T dwarfs, which have H-[4.5] > 3.2 and
T_eff ~ 500 K to 750 K.Comment: To be published in the on-line version of the Proceedings of Cool
Stars 16 (ASP Conference Series). This is an updated version of Leggett et
al. 2010 ApJ 710 1627; a photometry compilation is available at
http://www.gemini.edu/staff/slegget
Pressure measurement in 2D sloshing simulations with SPH
Sloshing for low filling level resonant pitch motion is studied experimentally and numerically using SPH. Special attention is paid to the pressure fields on the tanks. Comparisons are made with experimental data and with Particle Finite Element Method (PFEM) calculations.Postprint (published version
The discovery of a T6.5 subdwarf
We report the discovery of ULAS J131610.28+075553.0, an sdT6.5 dwarf in the UKIDSS Large Area Survey 2 epoch proper motion catalogue. This object displays significant spectral peculiarity, with the largest yet seen deviations from T6 and T7 templates in the Y and K bands for this subtype. Its large, similar to 1 arcsec yr(-1), proper motion suggests a large tangential velocity of V-tan approximate to 240-340 km s(-1), if we assume its M-J lies within the typical range for T6.5 dwarfs. This makes it a candidate for membership of the Galactic halo population. However, other metal-poor T dwarfs exhibit significant under luminosity both in specific bands and bolometrically. As a result, it is likely that its velocity is somewhat smaller, and we conclude it is a likely thick disc or halo member. This object represents the only T dwarf earlier than T8 to be classified as a subdwarf, and is a significant addition to the currently small number of known unambiguously substellar subdwarfs.Peer reviewe
The ultracool eld dwarfs luminosity function from the Canada-France Brown Dwarf Survey
The Canada-France Brown Dwarf Survey is a wide eld survey for cool brown
dwarfs conducted with the MegaCam camera on the CFHT telescope. Our objectives
are to nd ultracool brown dwarfs and to constrain the eld brown dwarf mass
function from a large and homogeneous sample of L and T dwarfs. We identify
candidates in CFHT/Megacam i' and z' images and follow them up with pointed NIR
imaging on several telescopes. Our survey has to date found 50 T dwarfs
candidates and 170 L or late M dwarf candidates drawn from a larger sample of
1300 candidates with typical ultracool dwarfs i'-z' colours, found in 900
square degrees. We currently have completed the NIR follow-up on a large part
of the survey for all candidates from the latest T dwarfs known to the late L
color range. This allows us to build on a complete and well de ned sample of
ultracool dwarfs to investigate the luminosity function of eld L and T dwarfs.Comment: Cool Stars XV conference. to appear in proceedings of Cool Stars XV
Conferenc
The SPHERE data center: a reference for high contrast imaging processing
The objective of the SPHERE Data Center is to optimize the scientific return
of SPHERE at the VLT, by providing optimized reduction procedures, services to
users and publicly available reduced data. This paper describes our motivation,
the implementation of the service (partners, infrastructure and developments),
services, description of the on-line data, and future developments. The SPHERE
Data Center is operational and has already provided reduced data with a good
reactivity to many observers. The first public reduced data have been made
available in 2017. The SPHERE Data Center is gathering a strong expertise on
SPHERE data and is in a very good position to propose new reduced data in the
future, as well as improved reduction procedures.Comment: SF2A proceeding
A Markovian event-based framework for stochastic spiking neural networks
In spiking neural networks, the information is conveyed by the spike times,
that depend on the intrinsic dynamics of each neuron, the input they receive
and on the connections between neurons. In this article we study the Markovian
nature of the sequence of spike times in stochastic neural networks, and in
particular the ability to deduce from a spike train the next spike time, and
therefore produce a description of the network activity only based on the spike
times regardless of the membrane potential process.
To study this question in a rigorous manner, we introduce and study an
event-based description of networks of noisy integrate-and-fire neurons, i.e.
that is based on the computation of the spike times. We show that the firing
times of the neurons in the networks constitute a Markov chain, whose
transition probability is related to the probability distribution of the
interspike interval of the neurons in the network. In the cases where the
Markovian model can be developed, the transition probability is explicitly
derived in such classical cases of neural networks as the linear
integrate-and-fire neuron models with excitatory and inhibitory interactions,
for different types of synapses, possibly featuring noisy synaptic integration,
transmission delays and absolute and relative refractory period. This covers
most of the cases that have been investigated in the event-based description of
spiking deterministic neural networks
Uncovering the Orbit of the Hercules Dwarf Galaxy
We present new chemo--kinematics of the Hercules dwarf galaxy based on Keck
II-- DEIMOS spectroscopy. Our 21 confirmed members have a systemic velocity of
kms and a velocity dispersion
kms. From the strength of
the Ca II triplet, we obtain a metallicity of [Fe/H]= dex and
dispersion of dex. This makes
Hercules a particularly metal--poor galaxy, placing it slightly below the
standard mass--metallicity relation. Previous photometric and spectroscopic
evidence suggests that Hercules is tidally disrupting and may be on a highly
radial orbit. From our identified members, we measure no significant velocity
gradient. By cross--matching with the second \textit{Gaia} data release, we
determine an uncertainty--weighted mean proper motion of
mas yr,
mas yr. This proper motion is slightly
misaligned with the elongation of Hercules, in contrast to models which suggest
that any tidal debris should be well aligned with the orbital path. Future
observations may resolve this tension.Comment: Submitted to MNRAS. 17 pages, 14 figure
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