11,941 research outputs found
Discovery of a Shell of Neutral Atomic Hydrogen Surrounding the Carbon Star IRC+10216
We have used the Robert C. Byrd Green Bank Telescope to perform the most
sensitive search to date for neutral atomic hydrogen (HI) in the circumstellar
envelope (CSE) of the carbon star IRC+10216. Our observations have uncovered a
low surface brightness HI shell of diameter ~1300" (~0.8 pc), centered on
IRC+10216. The HI shell has an angular extent comparable to the far
ultraviolet-emitting astrosphere of IRC+10216 previously detected with the
GALEX satellite, and its kinematics are consistent with circumstellar matter
that has been decelerated by the local interstellar medium. The shell appears
to completely surround the star, but the highest HI column densities are
measured along the leading edge of the shell, near the location of a previously
identified bow shock. We estimate a total mass of atomic hydrogen associated
with IRC+10216 CSE of M_HI~3x10e-3 M_sun. This is only a small fraction of the
expected total mass of the CSE (<1%) and is consistent with the bulk of the
stellar wind originating in molecular rather than atomic form, as expected for
a cool star with an effective temperature T_eff<~2200 K. HI mapping of a 2 deg
x 2 deg region surrounding IRC+10216 has also allowed us to characterize the
line-of-sight interstellar emission in the region and has uncovered a link
between diffuse FUV emission southwest of IRC+10216 and the Local Leo Cold
Cloud.Comment: Accepted to MNRAS. 16 pages. A version with full resolution figures
may be found at
http://www.haystack.mit.edu/hay/staff/lmatthew/Matthews_etal_IRC+10216.pd
Communication costs in a multi-tiered MPSoC
The amount of digital processing required for phased array beamformers is very large. It requires many parallel processors, which can be organized in a multi-tiered structure. Communication costs differ for each of the stages in such an architecture. For example, communication costs from the antenna front-end to the first processing stages is costly because of the amount of connections and data rate. Furthermore there is a trade-off between sequential processing exploiting locality of reference versus exploiting parallelism but adding communication costs. Thus, the optimal architecture depends on the importance that is given to the different measures.\ud
\ud
A model is presented to determine the partitioning of a (beamforming) system based on communication costs. It is shown that different solutions can be explored based on the cost model and the incorporated quantitative and qualitative measures. Determining the importance of each measure is subjective to the situation and application. In this work a simple beamforming application is used optimised for energy efficiency
Soft X-ray observations of pre-main sequence stars in the chamaeleon dark cloud
Einstein IPC observations of the nearby Chamaeleon I star forming cloud show 22 well-resolved soft X-ray sources in a 1x2 deg region. Twelve are associated with H-alpha emission line pre-main sequence (PMS) stars, and four with optically selected PMS stars. Several X-ray sources have two or more PMS stars in their error circles. Optical spectra were obtained at CTIO of possible stellar counterparts of the remaining X-ray sources. They reveal 5 probable new cloud members, K7-MO stars with weak or absent emission lines. These naked X-ray selected PMS stars are similar to those found in the Taurus-Auriga cloud. The spatial distributions and H-R diagrams of the X-ray and optically selected PMS stars in the cloud are very similar. Luminosity functions indicate the Chamaeleon stars are on average approximately 5 times more X-ray luminous than Pleiad dwarfs. A significant correlation between L sub x and optical magnitude suggests this trend may continue within the PMS phase of stellar evolution. The relation of increasing X-ray luminosity with decreasing stellar ages is thus extended to stellar ages as young as 1 million years
Deep Remix: Remixing Musical Mixtures Using a Convolutional Deep Neural Network
Audio source separation is a difficult machine learning problem and
performance is measured by comparing extracted signals with the component
source signals. However, if separation is motivated by the ultimate goal of
re-mixing then complete separation is not necessary and hence separation
difficulty and separation quality are dependent on the nature of the re-mix.
Here, we use a convolutional deep neural network (DNN), trained to estimate
'ideal' binary masks for separating voice from music, to perform re-mixing of
the vocal balance by operating directly on the individual magnitude components
of the musical mixture spectrogram. Our results demonstrate that small changes
in vocal gain may be applied with very little distortion to the ultimate
re-mix. Our method may be useful for re-mixing existing mixes
SAWdoubler: a program for counting self-avoiding walks
This article presents SAWdoubler, a package for counting the total number
Z(N) of self-avoiding walks (SAWs) on a regular lattice by the length-doubling
method, of which the basic concept has been published previously by us. We
discuss an algorithm for the creation of all SAWs of length N, efficient
storage of these SAWs in a tree data structure, and an algorithm for the
computation of correction terms to the count Z(2N) for SAWs of double length,
removing all combinations of two intersecting single-length SAWs.
We present an efficient numbering of the lattice sites that enables
exploitation of symmetry and leads to a smaller tree data structure; this
numbering is by increasing Euclidean distance from the origin of the lattice.
Furthermore, we show how the computation can be parallelised by distributing
the iterations of the main loop of the algorithm over the cores of a multicore
architecture. Experimental results on the 3D cubic lattice demonstrate that
Z(28) can be computed on a dual-core PC in only 1 hour and 40 minutes, with a
speedup of 1.56 compared to the single-core computation and with a gain by
using symmetry of a factor of 26. We present results for memory use and show
how the computation is made to fit in 4 Gbyte RAM. It is easy to extend the
SAWdoubler software to other lattices; it is publicly available under the GNU
LGPL license.Comment: 29 pages, 3 figure
An Independent Calibration of Stellar Ages: HST Observations of White Dwarfs at V=25
The white dwarf luminosity function of a stellar cluster will have a sharp
truncation at a luminosity which is determined by the time since formation of
the first white dwarfs in that cluster. Calculation of the dependence of this
limiting luminosity on age requires relatively well-understood physics and is
independent of stellar evolutionary models. Thus, measurement of the
termination of the white dwarf luminosity function provides an independent
method to determine the age of a cluster, and thereby to calibrate stellar
evolutionary ages. We have obtained HST WFPC2 data in two open clusters,
identified the white dwarf sequence, and proved the feasibility of this
approach, by detecting white dwarfs to V=25. Much deeper data are feasible.
From our present limited data, we show that degenerate cooling ages are not
consistent with some published isochrone ages for clusters with ages of order
1Gyr.Comment: 5 pages plus 3 figures ps format, paper in press in MNRAS: previous
attempt lost the tex
- …