64,542 research outputs found
Close Approach during Hard Binary--Binary Scattering
We report on an extensive series of numerical experiments of binary--binary
scattering, analysing the cross--section for close approach during interactions
for a range of hard binary parameters of interest in globular cluster cores. We
consider the implied rate for tidal interactions for different globular
clusters and compare our results with previous, complementary estimates of
stellar collision rates in globular clusters. We find that the collision rate
for binary--binary encounters dominates in low density clusters if the binary
fraction in the cluster is larger than for wide main--sequence binaries.
In dense clusters binary--single interactions dominate the collision rate and
the core binary fraction must be \ltorder 0.1 per decade in semi--major axis
or too many collisions take place compared to observations. The rates are
consistent if binaries with semi--major axes are overabundant in
low density clusters or if breakup and ejection substantially lowers the binary
fraction in denser clusters. Given reasonable assumptions about fractions of
binaries in the cores of low density clusters such as NGC~5053, we cannot
account for all the observed blue stragglers by stellar collisions during
binary encounters, suggesting a substantial fraction may be due to coalescence
of tight primordial binaries.Comment: 13 pages including 13 ps figures. MNRAS in pres
Condensation risk: comparison of steady-state and transient methods
Accurate assessment of both surface and interstitial condensation risk is important not only to reduce the damaging effect of moisture within the structure of buildings, but also to provide a healthy environment free from mould growth. The current British Standard (BS EN ISO 13788: 2002) contains an assessment procedure based on the assumption of a steady-state heat flow through the building envelope, neglecting the transient nature of the problem. This paper compares and evaluates numerical results of the condensation risk calculation under both steady-state and transient conditions using the existing numerical codes. Significant differences are apparent between the predictions of the simple (steady-state) and complex (transient) methods for all construction details modelled
An evaluation of the hygrothermal performance of 'standard' and 'as built' construction details using steadystate and transient modelling
Accurate assessment of both surface and interstitial condensation risk at the design stage of buildings is of great importance - not just to minimise the damaging effects moisture can cause to building envelopes, but also to contribute to the provision of adequate indoor air quality. Guidance certainly does exist with regards to limiting thermal bridging in order to prevent condensation occurring on new constructions. However, a recent study has provided clear evidence that the reality, both in translating the available guidance into a specific design and in construction on site is often rather different from the 'ideal'. This paper reports on that study and compares and evaluates the hygrothermal performance of construction details for different phases during the building life cycle. The results of both the surface and interstitial condensation risk simulations under both steady-state and transient conditions are presented and discussed. Significant differences in the hygrothermal performance of 'standard' and 'as built' construction details are observed
Shifts in hexapod diversification and what Haldane could have said
Data on species richness and taxon age are assembled for the extant hexapod orders (insects and their six-legged relatives). Coupled with estimates of phylogenetic relatedness, and simple statistical null models, these data are used to locate where, on the hexapod tree, significant changes in the rate of cladogenesis (speciation-minus-extinction rate) have occurred. Significant differences are found between many successive pairs of sister taxa near the base of the hexapod tree, all of which are attributable to a shift in diversification rate after the origin of the Neoptera (insects with wing flexion) and before the origin of the Holometabola (insects with complete metamorphosis). No other shifts are identifiable amongst supraordinal taxa. Whilst the Coleoptera have probably diversified faster than either of their putative sister lineages, they do not stand out relative to other closely related clades. These results suggest that any Creator had a fondness for a much more inclusive clade than the Coleoptera, definitely as large as the Eumetabola (Holometabola plus bugs and their relatives), and possibly as large as the entire Neoptera. Simultaneous, hence probable causative events are discussed, of which the origin of wing flexion has been the focus of much attention
The detection of FIR emission from high redshift star-forming galaxies in the ECDF-S
ABRIDGED: We have used the LABOCA Survey of the ECDF-S (LESS) to investigate
rest-frame FIR emission from typical SF systems (LBGs) at redshift 3, 4, and 5.
We initially concentrate on LBGs at z~3 and select three subsamples on stellar
mass, extinction corrected SF and rest-frame UV-magnitude. We produce composite
870micron images of the typical source in our subsamples, obtaining ~4sigma
detections and suggesting a correlation between FIR luminosity and stellar
mass. We apply a similar procedure to our full samples at z~3, 4, 4.5 and 5 and
do not obtain detections - consistent with a simple scaling between FIR
luminosity and stellar mass. In order to constrain the FIR SED of these systems
we explore their emission at multiple wavelengths spanning the peak of dust
emission at z~3 using the Herschel SPIRE observations of the field. We obtain
detections at multiple wavelengths for both our stellar mass and UV-magnitude
selected samples, and find a best-fit SED with T_dust in the ~33-41K range. We
calculate L_FIR, obscured SFRs and M_dust, and find that a significant fraction
of SF in these systems is obscured. Interestingly, our extinction corrected SFR
sample does not display the large FIR fluxes predicted from its red UV-spectral
slope. This suggests that the method of assuming an intrinsic UV-slope and
correcting for dust attenuation may be invalid for this sample - and that these
are not in fact the most actively SF systems. All of our z~3 samples fall on
the `main sequence' of SF galaxies at z~3 and our detected subsamples are
likely to represent the high obscuration end of LBGs at their epoch. We compare
the FIR properties of our subsamples with various other populations, finding
that our stellar mass selected sample shows similar FIR characteristics to SMGs
at the same epoch and therefore potentially represents the low L_FIR end of the
high redshift FIR luminosity function.Comment: 18 pages, 10 figure, MNRAS accepted, corrected typos,
acknowledgements adde
New Cosmological Structures on Medium Angular Scales Detected with the Tenerife Experiments
We present observations at 10 and 15 GHz taken with the Tenerife experiments
in a band of the sky at Dec.=+35 degrees. These experiments are sensitive to
multipoles in the range l=10-30. The sensitivity per beam is 56 and 20 microK
for the 10 and the 15 GHz data, respectively. After subtraction of the
prediction of known radio-sources, the analysis of the data at 15 GHz at high
Galactic latitude shows the presence of a signal with amplitude Delta Trms ~ 32
microK. In the case of a Harrison-Zeldovich spectrum for the primordial
fluctuations, a likelihood analysis shows that this signal corresponds to a
quadrupole amplitude Q_rms-ps=20.1+7.1-5.4 microK, in agreement with our
previous results at Dec.+=40 degrees and with the results of the COBE DMR.
There is clear evidence for the presence of individual features in the RA range
190 degrees to 250 degrees with a peak to peak amplitude of ~110 microK. A
preliminary comparison between our results and COBE DMR predictions for the
Tenerife experiments clearly indicates the presence of individual features
common to both. The constancy in amplitude over such a large range in frequency
(10-90 GHz) is strongly indicative of an intrinsic cosmological origin for
these structures.Comment: ApJ Letters accepted, 13 pages Latex (uses AASTEX) and 4 encapsulated
postscript figures
The effect of stellar-mass black holes on the structural evolution of massive star clusters
We present the results of realistic N-body modelling of massive star clusters
in the Magellanic Clouds, aimed at investigating a dynamical origin for the
radius-age trend observed in these systems. We find that stellar-mass black
holes, formed in the supernova explosions of the most massive cluster stars,
can constitute a dynamically important population. If a significant number of
black holes are retained (here we assume complete retention), these objects
rapidly form a dense core where interactions are common, resulting in the
scattering of black holes into the cluster halo, and the ejection of black
holes from the cluster. These two processes heat the stellar component,
resulting in prolonged core expansion of a magnitude matching the observations.
Significant core evolution is also observed in Magellanic Cloud clusters at
early times. We find that this does not result from the action of black holes,
but can be reproduced by the effects of mass-loss due to rapid stellar
evolution in a primordially mass segregated cluster.Comment: Accepted for publication in MNRAS Letters; 2 figures, 1 tabl
Handling uncertainties in background shapes: the discrete profiling method
A common problem in data analysis is that the functional form, as well as the
parameter values, of the underlying model which should describe a dataset is
not known a priori. In these cases some extra uncertainty must be assigned to
the extracted parameters of interest due to lack of exact knowledge of the
functional form of the model. A method for assigning an appropriate error is
presented. The method is based on considering the choice of functional form as
a discrete nuisance parameter which is profiled in an analogous way to
continuous nuisance parameters. The bias and coverage of this method are shown
to be good when applied to a realistic example.Comment: Accepted by J.Ins
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