2,048 research outputs found
Constraints on the Space Density of Methane Dwarfs and the Substellar Mass Function from a Deep Near-Infrared Survey
We report preliminary results of a deep near-infrared search for
methane-absorbing brown dwarfs; almost five years after the discovery of Gl
229b, there are only a few confirmed examples of this type of object. New J
band, wide-field images, combined with pre-existing R band observations, allow
efficient identification of candidates by their extreme (R-J) colours.
Follow-up measurements with custom filters can then confirm objects with
methane absorption. To date, we have surveyed a total of 11.4 square degrees to
J~20.5 and R~25. Follow-up CH_4 filter observations of promising candidates in
1/4 of these fields have turned up no methane absorbing brown dwarfs. With 90%
confidence, this implies that the space density of objects similar to Gl 229b
is less than 0.012 per cubic parsec. These calculations account for the
vertical structure of the Galaxy, which can be important for sensitive
measurements. Combining published theoretical atmospheric models with our
observations sets an upper limit of alpha <= 0.8 for the exponent of the
initial mass function power law in this domain.Comment: 11 pages + 2 figures To be published in Astrophysical Journal Letter
Improved Si:As BIBIB (Back-Illuminated Blocked-Impurity-Band) hybrid arrays
Results of a program to increase the short wavelength (less than 10 microns) detective quantum efficiency, eta/beta, of Si:As Impurity Band Conduction arrays are presented. The arrays are epitaxially grown Back-Illuminated Blocked (BIB) Impurity-Band (BIBIB) 10x50 detectors bonded to switched-FET multiplexers. It is shown that the 4.7 microns detective quantum efficiency increases proportionately with the thickness of the infrared active layer. A BIB array with a thick active layer, designed for low dark current, exhibits eta/beta = 7 to 9 percent at 4.7 microns for applied bias voltages between 3 and 5 V. The product of quantum efficiency and photoelectric gain, etaG, increases from 0.3 to 2.5 as the voltage increases from 3 to 5 V. Over this voltage range, the dark current increases from 8 to 120 e(-)s(-1) at a device temperature of 4.2 K and is under 70 e(-)s(-1) for all voltages at 2 K. Because of device gain, the effective dark current (equivalent photon rate) is less than 3 e(-)s(-1) under all operating conditions. The effective read noise (equivalent photon noise) is found to be less than 12 electrons under all operating conditions and for integration times between 0.05 and 100 seconds
A Quasi-Conforming Embedded Reproducing Kernel Particle Method for Heterogeneous Materials
We present a quasi-conforming embedded reproducing kernel particle method
(QCE-RKPM) for modeling heterogeneous materials that makes use of techniques
not available to mesh-based methods such as the finite element method (FEM) and
avoids many of the drawbacks in current embedded and immersed formulations
which are based on meshed methods. The different material domains are
discretized independently thus avoiding time-consuming, conformal meshing. In
this approach, the superposition of foreground (inclusion) and background
(matrix) domain integration smoothing cells are corrected by a quasi-conforming
quadtree subdivision on the background integration smoothing cells. Due to the
non-conforming nature of the background integration smoothing cells near the
material interfaces, a variationally consistent (VC) correction for domain
integration is introduced to restore integration constraints and thus optimal
convergence rates at a minor computational cost. Additional interface
integration smoothing cells with area (volume) correction, while
non-conforming, can be easily introduced to further enhance the accuracy and
stability of the Galerkin solution using VC integration on non-conforming
cells. To properly approximate the weak discontinuity across the material
interface by a penalty-free Nitsche's method with enhanced coercivity, the
interface nodes on the surface of the foreground discretization are also shared
with the background discretization. As such, there are no tunable parameters,
such as those involved in the penalty type method, to enforce interface
compatibility in this approach. The advantage of this meshfree formulation is
that it avoids many of the instabilities in mesh-based immersed and embedded
methods. The effectiveness of QCE-RKPM is illustrated with several examples
Evolution of optically faint AGN from COMBO-17 and GEMS
We have mapped the AGN luminosity function and its evolution between z=1 and
z=5 down to apparent magnitudes of . Within the GEMS project we have
analysed HST-ACS images of many AGN in the Extended Chandra Deep Field South,
enabling us to assess the evolution of AGN host galaxy properties with cosmic
time.Comment: to appear in proceedings 'Multiwavelength AGN Surveys', Cozumel 200
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