8,234 research outputs found
Photometric Accretion Signatures Near the Substellar Boundary
Multi-epoch imaging of the Orion equatorial region by the Sloan Digital Sky
Survey has revealed that significant variability in the blue continuum persists
into the late-M spectral types, indicating that magnetospheric accretion
processes occur below the substellar boundary in the Orion OB1 association. We
investigate the strength of the accretion-related continuum veiling by
comparing the reddening-invariant colors of the most highly variable stars
against those of main sequence M dwarfs and evolutionary models. A gradual
decrease in the g band veiling is seen for the cooler and less massive members,
as expected for a declining accretion rate with decreasing mass. We also see
evidence that the temperature of the accretion shock decreases in the very low
mass regime, reflecting a reduction in the energy flux carried by the accretion
columns. We find that the near-IR excess attributed to circumstellar disk
thermal emission drops rapidly for spectral types later than M4. This is likely
due to the decrease in color contrast between the disk and the cooler stellar
photosphere. Since accretion, which requires a substantial stellar magnetic
field and the presence of a circumstellar disk, is inferred for masses down to
0.05 Msol we surmise that brown dwarfs and low mass stars share a common mode
of formation.Comment: 37 pages, 14 figures, accepted by A
A method for space-variant deblurring with application to adaptive optics imaging in astronomy
Images from adaptive optics systems are generally affected by significant
distortions of the point spread function (PSF) across the field of view,
depending on the position of natural and artificial guide stars. Image
reduction techniques circumventing or mitigating these effects are important
tools to take full advantage of the scientific information encoded in AO
images. The aim of this paper is to propose a method for the deblurring of the
astronomical image, given a set of samples of the space-variant PSF. The method
is based on a partitioning of the image domain into regions of isoplanatism and
on applying suitable deconvolution methods with boundary effects correction to
each region. The effectiveness of the boundary effects correction is proved.
Moreover, the criterion for extending the disjoint sections to partially
overlapping sections is validated. The method is applied to simulated images of
a stellar system characterized by a spatially variable PSF. We obtain good
photometric quality, and therefore good science quality, by performing aperture
photometry on the deblurred images. The proposed method is implemented in IDL
in the Software Package "Patch", which is available on
http://www.airyproject.eu.Comment: 11 pages, 9 figures, 7 tables, accepted by A&
The Clustering of Luminous Red Galaxies in the Sloan Digital Sky Survey Imaging Data
We present the 3D real space clustering power spectrum of a sample of
\~600,000 luminous red galaxies (LRGs) measured by the Sloan Digital Sky Survey
(SDSS), using photometric redshifts. This sample of galaxies ranges from
redshift z=0.2 to 0.6 over 3,528 deg^2 of the sky, probing a volume of 1.5
(Gpc/h)^3, making it the largest volume ever used for galaxy clustering
measurements. We measure the angular clustering power spectrum in eight
redshift slices and combine these into a high precision 3D real space power
spectrum from k=0.005 (h/Mpc) to k=1 (h/Mpc). We detect power on gigaparsec
scales, beyond the turnover in the matter power spectrum, on scales
significantly larger than those accessible to current spectroscopic redshift
surveys. We also find evidence for baryonic oscillations, both in the power
spectrum, as well as in fits to the baryon density, at a 2.5 sigma confidence
level. The statistical power of these data to constrain cosmology is ~1.7 times
better than previous clustering analyses. Varying the matter density and baryon
fraction, we find \Omega_M = 0.30 \pm 0.03, and \Omega_b/\Omega_M = 0.18 \pm
0.04, The detection of baryonic oscillations also allows us to measure the
comoving distance to z=0.5; we find a best fit distance of 1.73 \pm 0.12 Gpc,
corresponding to a 6.5% error on the distance. These results demonstrate the
ability to make precise clustering measurements with photometric surveys
(abridged).Comment: 23 pages, 27 figures, submitted to MNRA
The Faintest X-ray Sources from z=0-8
We use the new 4 Ms exposure of the CDF-S field obtained with the Chandra
X-ray satellite to investigate the properties of the faintest X-ray sources
over a wide range of redshifts. We use an optimized averaging procedure to
investigate the weighted mean X-ray fluxes of optically selected sources in the
CDF-S over the redshift range z=0-8 and down to 0.5-2 keV fluxes as low as
5e-19 erg/cm^2/s. None of the samples of sources at high redshifts (z>5) show
any significant flux, and at z=6.5 we place an upper limit on the X-ray
luminosity of 4e41 erg/s in the rest-frame 3.75-15 keV band for the sample of
Bouwens et al. (2006). This is consistent with any X-ray production in the
galaxies being solely due to star formation. At lower redshifts we find
significant weighted mean X-ray fluxes in many samples of sources over the
redshift range z=0-4. We use these to argue that (1) the relation between star
formation and X-ray production remains invariant over this redshift range, (2)
X-ray sources below the direct detection threshold in the CDF-S are primarily
star-forming, and (3) there is full consistency between UV and X-ray
estimations of the star formation history.Comment: 13 pages, ApJ, in press. This accepted version includes a new figure
on the star formation history determined from the X-ray dat
Image Segmentation Using Weak Shape Priors
The problem of image segmentation is known to become particularly challenging
in the case of partial occlusion of the object(s) of interest, background
clutter, and the presence of strong noise. To overcome this problem, the
present paper introduces a novel approach segmentation through the use of
"weak" shape priors. Specifically, in the proposed method, an segmenting active
contour is constrained to converge to a configuration at which its geometric
parameters attain their empirical probability densities closely matching the
corresponding model densities that are learned based on training samples. It is
shown through numerical experiments that the proposed shape modeling can be
regarded as "weak" in the sense that it minimally influences the segmentation,
which is allowed to be dominated by data-related forces. On the other hand, the
priors provide sufficient constraints to regularize the convergence of
segmentation, while requiring substantially smaller training sets to yield less
biased results as compared to the case of PCA-based regularization methods. The
main advantages of the proposed technique over some existing alternatives is
demonstrated in a series of experiments.Comment: 27 pages, 8 figure
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