8,609 research outputs found
PSF calibration requirements for dark energy from cosmic shear
The control of systematic effects when measuring galaxy shapes is one of the
main challenges for cosmic shear analyses. In this context, we study the
fundamental limitations on shear accuracy due to the measurement of the Point
Spread Function (PSF) from the finite number of stars. In order to do that, we
translate the accuracy required for cosmological parameter estimation to the
minimum number of stars over which the PSF must be calibrated. We first derive
our results analytically in the case of infinitely small pixels (i.e.
infinitely high resolution). Then image simulations are used to validate these
results and investigate the effect of finite pixel size in the case of an
elliptical gaussian PSF. Our results are expressed in terms of the minimum
number of stars required to calibrate the PSF in order to ensure that
systematic errors are smaller than statistical errors when estimating the
cosmological parameters. On scales smaller than the area containing this
minimum number of stars, there is not enough information to model the PSF. In
the case of an elliptical gaussian PSF and in the absence of dithering, 2
pixels per PSF Full Width at Half Maximum (FWHM) implies a 20% increase of the
minimum number of stars compared to the ideal case of infinitely small pixels;
0.9 pixels per PSF FWHM implies a factor 100 increase. In the case of a good
resolution and a typical Signal-to-Noise Ratio distribution of stars, we find
that current surveys need the PSF to be calibrated over a few stars, which may
explain residual systematics on scales smaller than a few arcmins. Future
all-sky cosmic shear surveys require the PSF to be calibrated over a region
containing about 50 stars.Comment: 13 pages, 4 figures, accepted by A&
Sersiclets - A Matched Filter Extension of Shapelets for Weak Lensing Studies
The precision study of dark matter using weak lensing by large scale
structure is strongly constrained by the accuracy with which one can measure
galaxy shapes. Several methods have been devised but none have demonstrated the
ability to reach the level of precision required by future weak lensing
surveys. In this Letter we explore new avenues to the existing Shapelets
approach, combining a priori knowledge of the galaxy profile with the power of
orthogonal basis function decomposition. This Letter discusses the new issues
raised by this matched filter approach and proposes promising alternatives to
shape measurement techniques. In particular it appears that the use of a
matched filter (e.g. Sersic profile) restricted to elliptical radial fitting
functions resolves several well known Shapelet issues.Comment: 6 pages, 6 figures. MNRAS Accepte
Searching for "monogenic diabetes" in dogs using a candidate gene approach
BACKGROUND: Canine diabetes is a common endocrine disorder with an estimated breed-related prevalence ranging from 0.005% to 1.5% in pet dogs. Increased prevalence in some breeds suggests that diabetes in dogs is influenced by genetic factors and similarities between canine and human diabetes phenotypes suggest that the same genes might be associated with disease susceptibility in both species. Between 1-5% of human diabetes cases result from mutations in a single gene, including maturity onset diabetes of the adult (MODY) and neonatal diabetes mellitus (NDM). It is not clear whether monogenic forms of diabetes exist within some dog breeds. Identification of forms of canine monogenic diabetes could help to resolve the heterogeneity of the condition and lead to development of breed-specific genetic tests for diabetes susceptibility. RESULTS: Seventeen dog breeds were screened for single nucleotide polymorphisms (SNPs) in eighteen genes that have been associated with human MODY/NDM. Six SNP associations were found from five genes, with one gene (ZFP57) being associated in two different breeds. CONCLUSIONS: Some of the genes that have been associated with susceptibility to MODY and NDM in humans appear to also be associated with canine diabetes, although the limited number of associations identified in this study indicates canine diabetes is a heterogeneous condition and is most likely to be a polygenic trait in most dog breeds. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1186/2052-6687-1-8) contains supplementary material, which is available to authorized users
Wolf-Rayets in IC10: Probing the Nearest Starburst
IC10 is the nearest starburst galaxy, as revealed both by its Halpha surface
brightness and the large number of Wolf-Rayet stars (WRs) per unit area. The
relative number of known WC- to WN-type WRs has been thought to be unusually
high (~2), unexpected for IC10's metallicity. In this Letter we report the
first results of a new and deeper survey for WRs in IC10. We sucessfully
detected all of the spectroscopically known WRs, and based upon comparisons
with a neighboring control field, estimate that the total number of WRs in IC10
is about 100. We present spectroscopic confirmation of two of our WR
candidates, both of which are of WN type. Our photometric survey predicts that
the actual WC/WN ratio is ~0.3. This makes the WC/WN ratio of IC 10 consistent
with that expected for its metallicity, but greatly increases the already
unusually high number of WRs, resulting in a surface density that is about 20
times higher than in the LMC. If the majority of these candidates are
spectroscopically confirmed, IC10 must have an exceptional population of high
mass stars.Comment: Accepted by ApJL; only minor correction in this versio
The Discovery of a Twelfth Wolf-Rayet Star in the Small Magellanic Cloud
We report the discovery of a relatively faint (V=15.5) early-type WN star in
the SMC. The line strength and width of He II lambda 4686 emission is similar
to that of the other SMC WNs, and the presense of N V lambda 4603,19 emission
(coupled with the lack of N III) suggests this star is of spectral type
WN3-4.5, and thus is similar in type to the other SMC WRs. Also like the other
SMC WN stars, an early-type absorption spectrum is weakly present. The absolute
magnitude is comparable to that of other (single) Galactic early-type WNs. The
star is located in the Hodge 53 OB association, which is also the home of two
other SMC WNs. This star, which we designate SMC-WR12, was actually detected at
a high significance level in an earlier interference-filter survey, but the
wrong star was observed as part of a spectroscopic followup, and this case of
mistaken identity resulted in its Wolf-Rayet nature not being recognized until
now.Comment: Accepted by PASP (November 2003 issue
The Star Formation History and Mass Function of the Double Cluster h and Ï Persei
The h and Ï Per "double cluster" is examined using wide-field (0°.98 Ă 0°.98) CCD UBV imaging supplemented by optical spectra of several hundred of the brightest stars. Restricting our analysis to near the cluster nuclei, we find identical reddenings [E(B-V) = 0.56 ± 0.01], distance moduli (11.85 ± 0.05), and ages (12.8 ± 1.0 Myr) for the two clusters. In addition, we find an initial mass function slope for each of the cluster nuclei that is quite normal for high-mass stars, Î = -1.3 ± 0.2, indistinguishable from a Salpeter value. We derive masses of 3700 M_â (h) and 2800 M_â (Ï) integrating the present-day mass function from 1 to 120 M_â. There is evidence of mild mass segregation within the cluster cores. Our data are consistent with the stars having formed at a single epoch; claims to the contrary are very likely due to the inclusion of the substantial population of early-type stars located at similar distances in the Perseus spiral arm, in addition to contamination by G and K giants at various distances. We discuss the uniqueness of the double cluster, citing other examples of such structures in the literature but concluding that the nearly identical nature of the two cluster cores is unusual. We fail to settle the long-standing controversy regarding whether or not the double cluster is the core of the Per OB1 association and argue that this may be unanswerable with current techniques. We also emphasize the need for further work on the pre-main-sequence population of this nearby and highly interesting region
Spitzer SAGE-SMC Infrared Photometry of Massive Stars in the Small Magellanic Cloud
We present a catalog of 5324 massive stars in the Small Magellanic Cloud
(SMC), with accurate spectral types compiled from the literature, and a
photometric catalog for a subset of 3654 of these stars, with the goal of
exploring their infrared properties. The photometric catalog consists of stars
with infrared counterparts in the Spitzer, SAGE-SMC survey database, for which
we present uniform photometry from 0.3-24 um in the UBVIJHKs+IRAC+MIPS24 bands.
We compare the color magnitude diagrams and color-color diagrams to those of
the Large Magellanic Cloud (LMC), finding that the brightest infrared sources
in the SMC are also the red supergiants, supergiant B[e] (sgB[e]) stars,
luminous blue variables, and Wolf-Rayet stars, with the latter exhibiting less
infrared excess, the red supergiants being less dusty and the sgB[e] stars
being on average less luminous. Among the objects detected at 24 um are a few
very luminous hypergiants, 4 B-type stars with peculiar, flat spectral energy
distributions, and all 3 known luminous blue variables. We detect a distinct Be
star sequence, displaced to the red, and suggest a novel method of confirming
Be star candidates photometrically. We find a higher fraction of Oe and Be
stars among O and early-B stars in the SMC, respectively, when compared to the
LMC, and that the SMC Be stars occur at higher luminosities. We estimate
mass-loss rates for the red supergiants, confirming the correlation with
luminosity even at the metallicity of the SMC. Finally, we confirm the new
class of stars displaying composite A & F type spectra, the sgB[e] nature of
2dFS1804 and find the F0 supergiant 2dFS3528 to be a candidate luminous blue
variable with cold dust.Comment: 23 pages, 17 figures, 5 tables, accepted for publication in the
Astronomical Journa
Experiencing spaceâtime: the stretched lifeworlds of migrant workers in India
In the relatively rare instances when the spatialities of temporary migrant work, workersâ journeys, and labour-market negotiations have been the subject of scholarly attention, there has been little work that integrates time into the analysis. Building on a case study of low-paid and insecure migrant manual workers in the context of rapid economic growth in India, we examine both material and subjective dimensions of these workersâ spatiotemporal experiences. What does it mean to live life stretched out, multiplyattached to places across national space? What kinds of place attachments emerge for people temporarily sojourning in, rather than moving to, new places to reside and work? Our analysis of the spatiotemporalities of migrant workersâ experiences in India suggests that, over time, this group of workers use their own agency to seek to avoid the experience of humiliation and indignity in employment relations. Like David Harvey, we argue that money needs to be integrated into such analysis, along with space and time. The paper sheds light on processes of exclusion, inequality and diff erentiation, unequal power geometries, and social topographies that contrast with neoliberalist narratives of âIndian shining
Variable Star Candidates in an ACS Field of M31
A search for variable stars is performed using two epochs of Hubble Space
Telescope (HST) Advanced Camera for Surveys (ACS) imaging data for a 9.28
square arcminute portion of M31. This data set reveals 254 sources that vary by
at least 4-sigma between epochs. The positions and 2-epoch B-band (equivalent)
photometry of these sources are presented. The photometry suggests that this
catalog includes most of the RR Lyrae population of this portion of M31.Comment: 22 pages, 3 tables, 5 figures, accepted for publication in A
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