2,969 research outputs found
The effects of a background potential in star cluster evolution: a delay in the relaxation time-scale and runaway collision processes
Runaway stellar collisions in dense star clusters are invoked to explain the
presence of very massive stars or blue stragglers in the center of those
systems. This process has also been explored for the first star clusters in the
Universe and shown to yield stars that may collapse at some points into an
intermediate mass black hole. Although the early evolution of star clusters
requires the explicit modeling of the gas out of which the stars form, these
calculations would be extremely time-consuming and often the effects of the gas
can be accurately treated by including a background potential to account for
the extra gravitational force. We apply this approximation to model the early
evolution of the first dense star clusters formed in the Universe by performing
-body simulations, our goal is to understand how the additional
gravitational force affects the growth of a very massive star through stellar
mergers in the central parts of the star cluster. Our results show that the
background potential increases the velocities of the stars, causing an overall
delay in the evolution of the clusters and in the runaway growth of a massive
star at the center. The population of binary stars is lower due to the
increased kinetic energy of the stars, initially reducing the number of stellar
collisions, and we show that relaxation processes are also affected. Despite
these effects, the external potential enhances the mass of the merger product
by a factor 2 if the collisions are maintained for long times.Comment: 16 pages. Accepted for publication in Astronomy and Astrophysic
Balancing the Zoning Budget
The politics of urban land use frustrate even the best intentions. A number of cities have made strong political commitments to increasing their local housing supply in the face of a crisis of affordability and availability in urban housing. However decisions to engage in up-zoning or increasing the areas in which new housing can be built, are often offset by even more down-zoning laws that decrease the ability of residents in a designated area to build new housing as-of-right. The result is that housing availability does not increase by anywhere near the amount that elected officials promised In this Article, we argue that the difficulty cities face in increasing local housing supply is a result of the seriatim nature of local land use decisions. Because each down-zoning decision has only a small effect on the housing supply, citywide forces spend little political capital fighting them, leaving the field to neighborhood groups who care deeply. Further, because down-zoning decisions are made in advance of any proposed new development, the most active interest group in favor of new housing-developers-takes a pass on lobbying. The result is an uneven playing field that favors downzoning
Chemical and mineralogical properties of atmospheric particulate matter in the megacity Beijing
Water in the Near IR spectrum of Comet 8P/Tuttle
High resolution spectra of Comet 8P/Tuttle were obtained in the frequency
range 3440.6-3462.6 cm-1 on 3 January 2008 UT using CGS4 with echelle grating
on UKIRT. In addition to recording strong solar pumped fluorescent (SPF) lines
of H2O, the long integration time (152 miutes on target) enabled eight weaker
H2O features to be assigned, most of which had not previously been identified
in cometary spectra. These transitions, which are from higher energy upper
states, are similar in character to the so-called 'SH' lines recorded in the
post Deep Impact spectrum of comet Tempel 1 (Barber et al., 2007). We have
identified certain characteristics that these lines have in common, and which
in addition to helping to define this new class of cometary line, give some
clues to the physical processes involved in their production. Finally, we
derive an H2O rotational temperature of 62+/- K and a water production rate of
(1.4+/-0.3)E28 molecules/s.Comment: Paper has been accepted for publication by MNRAS (11/06/09
âPolitics as Marketsâ Reconsidered: Economic Theory, Competitive Democracy and Primary Ballot Access
Optical metrology alignment and impact on the measurement performance of the LISA Technology Package
Increased glycation and oxidative damage to apolipoprotein B100 of LDL cholesterol in patients with type 2 diabetes and effect of metformin
OBJECTIVE The aim of this study was to investigate whether apolipoprotein B100 of LDL suffers increased damage by glycation, oxidation, and nitration in patients with type 2 diabetes, including patients receiving metformin therapy.
RESEARCH DESIGN AND METHODS For this study, 32 type 2 diabetic patients and 21 healthy control subjects were recruited; 13 diabetic patients were receiving metformin therapy (median dose: 1.50 g/day). LDL was isolated from venous plasma by ultracentrifugation, delipidated, digested, and analyzed for protein glycation, oxidation, and nitration adducts by stable isotopic dilution analysis tandem mass spectrometry.
RESULTS Advanced glycation end product (AGE) content of apolipoprotein B100 of LDL from type 2 diabetic patients was higher than from healthy subjects: arginine-derived AGE, 15.8 vs. 5.3 mol% (P < 0.001); and lysine-derived AGE, 2.5 vs. 1.5 mol% (P < 0.05). Oxidative damage, mainly methionine sulfoxide residues, was also increased: 2.5 vs. 1.1 molar equivalents (P < 0.001). 3-Nitrotyrosine content was decreased: 0.04 vs. 0.12 mol% (P < 0.05). In diabetic patients receiving metformin therapy, arginine-derived AGE and methionine sulfoxide were lower than in patients not receiving metformin: 19.3 vs. 8.9 mol% (P < 0.01) and 2.9 vs. 1.9 mol% (P < 0.05), respectively; 3-nitrotyrosine content was higher: 0.10 vs. 0.03 mol% (P < 0.05). Fructosyl-lysine residue content correlated positively with fasting plasma glucose. Arginine-derived AGE residue contents were intercorrelated and also correlated positively with methionine sulfoxide.
CONCLUSIONS Patients with type 2 diabetes had increased arginine-derived AGEs and oxidative damage in apolipoprotein B100 of LDL. This was lower in patients receiving metformin therapy, which may contribute to decreased oxidative damage, atherogenicity, and cardiovascular disease
Electron Microscopy of Clay Minerals in Mudrocks from the San Andreas Fault Observatory at Depth (SAFOD)
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