477 research outputs found
Une nouvelle mise en oeuvre de la méthode IIM pour les équations de Navier-Stokes en présence d'une force singulière
Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal
Resection of the contrast-enhancing tumor in diffuse gliomas bordering eloquent areas using electrophysiology and 5-ALA fluorescence: evaluation of resection rates and neurological outcome—a systematic review and meta-analysis
A4-03: A surgical predictive risk model of in-hospital mortality for primary resectable lung cancer: statistical analysis of 10,205 patients from a nationally representative database
Resolving the Stellar Populations in a z=4 Lensed Galaxy
We present deep near-infrared Keck/NIRC imaging of a recently-discovered
z=4.04 galaxy (Frye & Broadhurst 1998). This is lensed by the rich foreground
cluster Abell~2390 (z~0.23) into highly-magnified arcs 3-5arcsec in length. Our
H- and K'-band NIRC imaging allows us to map the Balmer+4000Ang break
amplitude. In combination with high-quality archival HST/WFPC2 data, we can
spatially resolve stellar populations along the arcs. The WFPC2 images clearly
reveal several bright knots, which correspond to sites of active star
formation. However, there are considerable portions of the arcs are
significantly redder, consistent with being observed >100Myr after star
formation has ceased. Keck/LRIS long-slit spectroscopy along the arcs reveals
that the Ly-alpha emission is spatially offset by ~1arcsec from the rest-UV
continuum regions. We show that this line emission is most probably powered by
star formation in neighboring HII regions, and that the z=4 system is unlikely
to be an AGN.Comment: Accepted for publication in the Astrophysical Journal. Uses
emulateapj.sty and graphics.sty (included). 34 pages - has 5 tables and 21
encapsulated postscript figures, 4 in colour mail (B&W versions also
provided
A Critical Reassessment of Q7 and Q8 Matrix Elements
We compare recent theoretical determinations of weak matrix elements of the
electroweak penguin operators Q7 and Q8. We pay special attention to the
renormalization scheme dependence of these determinations as well as to the
influence of higher dimension operators in the different approaches.Comment: 11 pages, latex; minor change
A new paradigm for space astrophysics mission design
Pursuing ground breaking science in a highly cost-constrained environment presents new challenges to the development of future space astrophysics missions. Within the conventional cost models for large observatories, executing a flagship “mission after next” appears to be unstainable. To achieve our nation’s science ambitions requires a new paradigm of system design, development and manufacture. This paper explores the nature of the current paradigm and proposes a series of steps to guide the entire community to a sustainable future
Episodic mass loss in binary evolution to the Wolf-Rayet phase: Keck and HST proper motions of RY Scuti's nebula
Binary mass transfer via Roche-lobe overflow (RLOF) is a key channel for
producing stripped-envelope Wolf-Rayet (WR) stars and may be critical to
account for SN Ib/c progenitors. RY Scuti is an extremely rare example of a
massive binary star caught in this brief but important phase. Its toroidal
nebula indicates equatorial mass loss during RLOF, while the mass-gaining star
is apparently embedded in an opaque accretion disk. RY Scuti's toroidal nebula
has two components: an inner ionised double-ring system, and an outer dust
torus that is twice the size of the ionised rings. We present two epochs of
Lband Keck NGS-AO images of the dust torus, plus three epochs of HST images of
the ionised gas rings. Proper motions show that the inner ionised rings and the
outer dust torus came from two separate ejection events roughly 130 and 250 yr
ago. This suggests that RLOF in massive contact binaries can be accompanied by
eruptive and episodic burst of mass loss, reminiscent of LBVs. We speculate
that the repeating outbursts may arise in the mass gainer from instabilities
associated with a high accretion rate. If discrete mass-loss episodes in other
RLOF binaries are accompanied by luminous outbursts, they might contribute to
the population of extragalactic optical transients. When RLOF ends for RY
Scuti, the overluminous mass gainer, currently surrounded by an accretion disk,
will probably become a B[e] supergiant and may outshine the hotter mass-donor
star that should die as a Type Ib/c supernova.Comment: 15 pages, 7 figures, submitted to MNRA
GREAT3 results I: systematic errors in shear estimation and the impact of real galaxy morphology
We present first results from the third GRavitational lEnsing Accuracy
Testing (GREAT3) challenge, the third in a sequence of challenges for testing
methods of inferring weak gravitational lensing shear distortions from
simulated galaxy images. GREAT3 was divided into experiments to test three
specific questions, and included simulated space- and ground-based data with
constant or cosmologically-varying shear fields. The simplest (control)
experiment included parametric galaxies with a realistic distribution of
signal-to-noise, size, and ellipticity, and a complex point spread function
(PSF). The other experiments tested the additional impact of realistic galaxy
morphology, multiple exposure imaging, and the uncertainty about a
spatially-varying PSF; the last two questions will be explored in Paper II. The
24 participating teams competed to estimate lensing shears to within systematic
error tolerances for upcoming Stage-IV dark energy surveys, making 1525
submissions overall. GREAT3 saw considerable variety and innovation in the
types of methods applied. Several teams now meet or exceed the targets in many
of the tests conducted (to within the statistical errors). We conclude that the
presence of realistic galaxy morphology in simulations changes shear
calibration biases by per cent for a wide range of methods. Other
effects such as truncation biases due to finite galaxy postage stamps, and the
impact of galaxy type as measured by the S\'{e}rsic index, are quantified for
the first time. Our results generalize previous studies regarding sensitivities
to galaxy size and signal-to-noise, and to PSF properties such as seeing and
defocus. Almost all methods' results support the simple model in which additive
shear biases depend linearly on PSF ellipticity.Comment: 32 pages + 15 pages of technical appendices; 28 figures; submitted to
MNRAS; latest version has minor updates in presentation of 4 figures, no
changes in content or conclusion
Star Formation from DLA Gas in the Outskirts of Lyman Break Galaxies at z~3
We present evidence for spatially extended low surface brightness emission
around Lyman break galaxies (LBGs) in the V band image of the Hubble Ultra Deep
Field, corresponding to the z~3 rest-frame FUV light, which is a sensitive
measure of star formation rates (SFRs). We find that the covering fraction of
molecular gas at z~3 is not adequate to explain the emission in the outskirts
of LBGs, while the covering fraction of neutral atomic-dominated hydrogen gas
at high redshift is sufficient. We develop a theoretical framework to connect
this emission around LBGs to the expected emission from neutral H I gas i.e.,
damped Lyman alpha systems (DLAs), using the Kennicutt-Schmidt (KS) relation.
Working under the hypothesis that the observed FUV emission in the outskirts of
LBGs is from in situ star formation in atomic-dominated hydrogen gas, the
results suggest that the SFR efficiency in such gas at z~3 is between factors
of 10 and 50 lower than predictions based on the local KS relation. The total
SFR density in atomic-dominated gas at z~3 is constrained to be ~10% of that
observed from the inner regions of LBGs. In addition, the metals produced by in
situ star formation in the outskirts of LBGs yield metallicities comparable to
those of DLAs, which is a possible solution to the "Missing Metals" problem for
DLAs. Finally, the atomic-dominated gas in the outskirts of galaxies at both
high and low redshifts has similar reduced SFR efficiencies and is consistent
with the same power law.Comment: 26 pages, 12 figures, 2 tables, appendix, accepted by ApJ, proof
corrections include
Mechanistic insight into acrylate metabolism and detoxification in marine dimethylsulfoniopropionate-catabolizing bacteria
Dimethylsulfoniopropionate (DMSP) cleavage, yielding dimethyl sulfide (DMS) and acrylate, provides vital carbon sources to marine bacteria, is a key component of the global sulfur cycle and effects atmospheric chemistry and potentially climate. Acrylate and its metabolite acryloyl-CoA are toxic if allowed to accumulate within cells. Thus, organisms cleaving DMSP require effective systems for both the utilization and detoxification of acrylate. Here, we examine the mechanism of acrylate utilization and detoxification in Roseobacters. We propose propionate-CoA ligase (PrpE) and acryloyl-CoA reductase (AcuI) as the key enzymes involved and through structural and mutagenesis analyses, provide explanations of their catalytic mechanisms. In most cases, DMSP lyases and DMSP demethylases (DmdAs) have low substrate affinities, but AcuIs have very high substrate affinities, suggesting that an effective detoxification system for acylate catabolism exists in DMSP-catabolizing Roseobacters. This study provides insight on acrylate metabolism and detoxification and a possible explanation for the high Km values that have been noted for some DMSP lyases. Since acrylate/acryloyl-CoA is probably produced by other metabolism, and AcuI and PrpE are conserved in many organisms across all domains of life, the detoxification system is likely relevant to many metabolic processes and environments beyond DMSP catabolism
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