18,766 research outputs found
The Star Formation Histories and Efficiencies of Two Giant HII Regions in M33
photometry is used to re-identify the OB associations which power the
two most luminous HII regions in M33, NGC 604 and NGC 595. There is a
significant difference (2-3 Myr) in the ages of the most recent star formation
episode in these two regions, while NGC 595 also has undergone a prior episode
of star formation (10-15 Myr ago). These data, combined with the presence of
molecular clouds in the heart of NGC 604, suggest that molecular clouds may
survive at least one intense episode of massive star formation. The star
formation efficiencies (mass of stars per mass of gas) of these two HII regions
are up to a factor of 3 larger than the average efficiency in the inner disk of
M33 or in Galactic molecular clouds, but are still only 2-5\%.Comment: 26 pages including 2 figures, uuencoded compressed postscript file
(Figure 1 not available electronically). Accepted to Ap
Rational Construction of Stochastic Numerical Methods for Molecular Sampling
In this article, we focus on the sampling of the configurational
Gibbs-Boltzmann distribution, that is, the calculation of averages of functions
of the position coordinates of a molecular -body system modelled at constant
temperature. We show how a formal series expansion of the invariant measure of
a Langevin dynamics numerical method can be obtained in a straightforward way
using the Baker-Campbell-Hausdorff lemma. We then compare Langevin dynamics
integrators in terms of their invariant distributions and demonstrate a
superconvergence property (4th order accuracy where only 2nd order would be
expected) of one method in the high friction limit; this method, moreover, can
be reduced to a simple modification of the Euler-Maruyama method for Brownian
dynamics involving a non-Markovian (coloured noise) random process. In the
Brownian dynamics case, 2nd order accuracy of the invariant density is
achieved. All methods considered are efficient for molecular applications
(requiring one force evaluation per timestep) and of a simple form. In fully
resolved (long run) molecular dynamics simulations, for our favoured method, we
observe up to two orders of magnitude improvement in configurational sampling
accuracy for given stepsize with no evident reduction in the size of the
largest usable timestep compared to common alternative methods
Recommended from our members
Body dissatisfaction, appearance investment and wellbeing: how older obese men orient to 'aesthetic health' [forthcoming]
Most research on male body image to date has focused on young men using quantitative methods. The study reported here is based on qualitative interviews with a sample of older men (n = 30) on a weight management programme, and we asked them about body-related feelings. The interviews were all transcribed and analysed using thematic analysis. Our results indicate that although body weight was typically minimised, body image was a key concern, with many examples of body consciousness and body dissatisfaction evident. On the other hand, post-programme weight loss was associated with a transformative shift in body image, with the men emphasising enhanced body confidence, self-esteem and psychological wellbeing. We conclude by highlighting the need to recognise and address appearance issues and/as health concerns for middle-aged and older men
Recommended from our members
Citation and peer review of data: moving towards formal data publication
This paper discusses many of the issues associated with formally publishing data in academia, focusing primarily on the structures that need to be put in place for peer review and formal citation of datasets. Data publication is becoming increasingly important to the scientific community, as it will provide a mechanism for those who create data to receive academic credit for their work and will allow the conclusions arising from an analysis to be more readily verifiable, thus promoting transparency in the scientific process. Peer review of data will also provide a mechanism for ensuring the quality of datasets, and we provide suggestions on the types of activities one expects to see in the peer review of data. A simple taxonomy of data publication methodologies is presented and evaluated, and the paper concludes with a discussion of dataset granularity, transience and semantics, along with a recommended human-readable citation syntax
Coplanar Circumbinary Debris Disks
We present resolved Herschel images of circumbinary debris disks in the alpha
CrB (HD139006) and beta Tri (HD13161) systems. We find that both disks are
consistent with being aligned with the binary orbital planes. Though secular
perturbations from the binary can align the disk, in both cases the alignment
time at the distances at which the disk is resolved is greater than the stellar
age, so we conclude that the coplanarity was primordial. Neither disk can be
modelled as a narrow ring, requiring extended radial distributions. To satisfy
both the Herschel and mid-IR images of the alpha CrB disk, we construct a model
that extends from 1-300AU, whose radial profile is broadly consistent with a
picture where planetesimal collisions are excited by secular perturbations from
the binary. However, this model is also consistent with stirring by other
mechanisms, such as the formation of Pluto-sized objects. The beta Tri disk
model extends from 50-400AU. A model with depleted (rather than empty) inner
regions also reproduces the observations and is consistent with binary and
other stirring mechanisms. As part of the modelling process, we find that the
Herschel PACS beam varies by as much as 10% at 70um and a few % at 100um. The
70um variation can therefore hinder image interpretation, particularly for
poorly resolved objects. The number of systems in which circumbinary debris
disk orientations have been compared with the binary plane is now four. More
systems are needed, but a picture in which disks around very close binaries
(alpha CrB, beta Tri, and HD 98800, with periods of a few weeks to a year) are
aligned, and disks around wider binaries (99 Her, with a 50 yr period) are
misaligned, may be emerging. This picture is qualitatively consistent with the
expectation that the protoplanetary disks from which the debris emerged are
more likely to be aligned if their binaries have shorter periods.Comment: accepted to MNRA
Discovery of the Fomalhaut C debris disc
Fomalhaut is one of the most interesting and well studied nearby stars,
hosting at least one planet, a spectacular debris ring, and two distant
low-mass stellar companions (TW PsA and LP 876-10, a.k.a. Fomalhaut B & C). We
observed both companions with Herschel, and while no disc was detected around
the secondary, TW PsA, we have discovered the second debris disc in the
Fomalhaut system, around LP 876-10. This detection is only the second case of
two debris discs seen in a multiple system, both of which are relatively wide
(3000 AU for HD 223352/40 and 158 kAU [0.77 pc] for Fomalhaut/LP
876-10). The disc is cool (24K) and relatively bright, with a fractional
luminosity , and represents the rare
observation of a debris disc around an M dwarf. Further work should attempt to
find if the presence of two discs in the Fomalhaut system is coincidental,
perhaps simply due to the relatively young system age of 440 Myr, or if the
stellar components have dynamically interacted and the system is even more
complex than it currently appears.Comment: Published in MNRAS Letters. Merry Xma
On the long-time integration of stochastic gradient systems
This article addresses the weak convergence of numerical methods for Brownian dynamics. Typical analyses of numerical methods for stochastic differential equations focus on properties such as the weak order which estimates the asymptotic (stepsize h â 0) convergence behavior of the error of finite time averages. Recently it has been demonstrated, by study of Fokker-Planck operators, that a non-Markovian numerical method [Leimkuhler and Matthews, 2013] generates approximations in the long time limit with higher accuracy order (2nd order) than would be expected from its weak convergence analysis (finite-time averages are 1st order accurate). In this article we describe the transition from the transient to the steady-state regime of this numerical method by estimating the time-dependency of the coefficients in an asymptotic expansion for the weak error, demonstrating that the convergence to 2nd order is exponentially rapid in time. Moreover, we provide numerical tests of the theory, including comparisons of the efficiencies of the Euler-Maruyama method, the popular 2nd order Heun method, and the non-Markovian method
Heat transfer investigation of Langley Research Center transition models at a Mach number of 8, volume 2
The results are presented of a wind tunnel test program to determine aerodynamic heat transfer distributions on delta body and straight body transition models of the space shuttle. Heat transfer rates were determined by the phase-change paint technique on Stycast and RTV models using Tempilag as the surface temperature indicator. The nominal test conditions were: Mach 8, length Reynolds numbers of 5 million and 7.4 million, and angles of attack of 20, 40, and 60 deg. Model details, test conditions, and reduced heat transfer data are included. Data reduction of the phase-change paint photographs was performed by utilizing a new technique
Growth adaptation to temperature in N. crassa wild-type strains
Growth adaptation to temperature in N. crassa wild-type strain
- âŠ