11,448 research outputs found
The Discrete Sources of Cosmic Radio Emission
One of the principal problems in the radio astronomy of the
discrete sources is their identification with visible objects.
Identification is necessary in . most instances to determine the distance
of a source and thus its intrinsic luminosity. It is of assistance
in deciding the probable mechanism for the radio emission.
Identification of a reasonable sample of radio sources should be made
prior to speculations from statistical studies of the radio sources
as a class
Bored pile design in stiff clay II:Mechanisms and uncertainty
The soil mechanics related to pile design in clay has been the subject of substantial engineering research. In a companion paper, various codes of practice were reviewed showing the effect on pile capacity of the different global factors of safety that emerge from the various partial factor combinations for the ultimate limit state. Factors of safety are generally specified based on the opinions of experts. In this paper an assessment will be made of various objective procedures that can be used to reduce uncertainty in the design process, especially regarding the adoption of a pile resistance model and the selection of a soil strength profile as part of a ultimate limit state check, and the estimation of pile head settlement in the context of a serviceability limit state check. It is shown that both total stress and effective stress calculation methods are applicable in London Clay. Estimates of settlement using a non-linear soil stress–strain relationship are made and compared with published data. It is shown that the compression of the concrete dominates the settlement of long piles. Given the low settlements observed, recommendations are made for a reduction in standard factors of safety for bored pile design in stiff clays. </jats:p
B2 and G2 Toda systems on compact surfaces: a variational approach
We consider the B2 and G2 Toda systems on compact surfaces. We attack the
problem using variational techniques. We get existence and multiplicity of
solutions under a topological assumption on the surface and some generic
conditions on the parameters. We also extend some of the results to the case of
general systems.Comment: 28 pages, accepted on Journal of Mathematical Physic
How neutral is the intergalactic medium surrounding the redshift z=7.085 quasar ULAS J1120+0641?
The quasar ULAS J1120+0641 at redshift z=7.085 has a highly ionised near zone
which is smaller than those around quasars of similar luminosity at z~6. The
spectrum also exhibits evidence for a damping wing extending redward of the
systemic Lya redshift. We use radiative transfer simulations in a cosmological
context to investigate the implications for the ionisation state of the
inhomogeneous IGM surrounding this quasar. Our simulations show that the
transmission profile is consistent with an IGM in the vicinity of the quasar
with a volume averaged HI fraction of f_HI>0.1 and that ULAS J1120+0641 has
been bright for 10^6--10^7 yr. The observed spectrum is also consistent with
smaller IGM neutral fractions, f_HI ~ 10^-3--10-4, if a damped Lya system in an
otherwise highly ionised IGM lies within 5 proper Mpc of the quasar. This is,
however, predicted to occur in only ~5 per cent of our simulated sight-lines
for a bright phase of 10^6--10^7 yr. Unless ULAS J1120+0641 grows during a
previous optically obscured phase, the low age inferred for the quasar adds to
the theoretical challenge of forming a 2x10^9 M_sol black hole at this high
redshift.Comment: 5 pages, 4 figures, accepted to MNRAS letter
The Effect of Temperature on the Surface Tension of Sap of Thuja Plicata Heartwood
Capillary rise and drop volume methods were applied to the measurement of heartwood sap surface tension. Increase in sap temperature appeared to cause a substantial decrease in surface tension as measured by the former method. Since with the latter method surface tension values only decreased from 58.1 to 56.8 dyne/cm over the temperature range 25 to 80 C, it was concluded that changes in the contact angle of sap on glass were likely to have affected the results obtained by the capillary rise method. The absence of a substantial decrease in sap surface tension with increasing temperature is of considerable relevance to lumber drying
A Search for Nitrogen Enriched Quasars in the Sloan Digital Sky Survey Early Data Release
A search for nitrogen-rich quasars in the Sloan Digital Sky Survey Early Data
Release (SDSS EDR) catalog has yielded 16 candidates, including five with very
prominent emission, but no cases with nitrogen emission as strong as in
Q0353-383. The quasar Q0353-383 has long been known to have extremely strong
nitrogen intercombination lines at lambda 1486 and lambda 1750 Angstroms,
implying an anomalously high nitrogen abundance of about 15 times solar. It is
still the only one of its kind known. A preliminary search through the EDR
using the observed property of the weak C IV emission seen in Q0353-383
resulted in a sample of 23 objects with unusual emission or absorption-line
properties, including one very luminous redshift 2.5 star-forming galaxy. We
present descriptions, preliminary emission-line measurements, and spectra for
all the objects discussed here.Comment: 20 pages, 5 figures, submitted to AJ; final refereed versio
The impact of relative position and returns on sacrifice and reciprocity: an experimental study using individual decisions
We present a comprehensive experimental design that makes it possible to characterize other-regarding preferences and their relationship to the decision maker’s relative position. Participants are faced with a large number of decisions involving variations in the trade-offs between own and other’s payoffs, as well as in other potentially important factors like the decision maker’s relative position. We find that: (1) choices are responsive to the cost of helping and hurting others; (2) The weight a decision maker places on others’ monetary payoffs depends on whether the decision maker is in an advantageous or disadvantageous relative position; and (3) We find no evidence of reciprocity of the type linked to menu-dependence. The results of a mixture-model estimation show considerable heterogeneity in subjects’ motivations and confirm the absence of reciprocal motives. Pure selfish behavior is the most frequently observed behavior. Among the subjects exhibiting social preferences, social-welfare maximization is the most frequent, followed by inequality-aversion and by competitiveness
How cold is cold dark matter? Small scales constraints from the flux power spectrum of the high-redshift Lyman-alpha forest
We present constraints on the mass of warm dark matter (WDM) particles
derived from the Lyman-alpha flux power spectrum of 55 high- resolution HIRES
spectra at 2.0 < z < 6.4. From the HIRES spectra, we obtain a lower limit of
mwdm > 1.2 keV 2 sigma if the WDM consists of early decoupled thermal relics
and mwdm > 5.6 keV (2 sigma) for sterile neutrinos. Adding the Sloan Digital
Sky Survey Lyman-alpha flux power spectrum, we get mwdm > 4 keV and mwdm > 28
keV (2 sigma) for thermal relics and sterile neutrinos. These results improve
previous constraints by a factor two.Comment: Some issues clarified (especially resolution related). Conclusions
unchanged. Accepted version by PR
Closing in on the picture : analyzing interactions in video recordings
This paper provides a detailed account of the processing and analysing of data, obtained through video recording during reflective practitioner research. It sets out five stages in the analysis of video recordings of classroom interactions during a series of educational drama lessons: from decisions relating to the selection of data for close analysis, to the seeking of themes, and finally to the presentation of conclusions. The researcher adapted and synthesised several processes derived from discourse analysis (Wells, 2001; Spiers, 2004; Gee, 2005) to produce a range of instruments for use in transcription and analysis of verbal and non-verbal discourse. These include: a simple transcription key; classifications for verbal and non-verbal discourse; and a template for a transcription and analysis matrix
A first direct measurement of the intergalactic medium temperature around a quasar at z=6
The thermal state of the intergalactic medium (IGM) provides an indirect
probe of both the HI and HeII reionisation epochs. Current constraints on the
IGM temperature from the Lya forest are restricted to the redshift range
2<z<4.5, limiting the ability to probe the thermal memory of HI reionisation
toward higher redshift. In this work, we present the first direct measurement
of the IGM temperature around a z=6 quasar by analysing the Doppler widths of
Lya absorption lines in the proximity zone of SDSS J0818+1722. We use a high
resolution (R= 40000) Keck/HIRES spectrum in combination with detailed
numerical modelling to obtain the temperature at mean density,
T_0=23600\pm^5000_6900K (\pm^9200_9300K) at 68 (95) per cent confidence
assuming a prior probability 13500K<T_0<38500 K following HI and HeII
reionisation. This enables us to place an upper limit on the redshift of HI
reionisation, z_H, within 33 comoving Mpc of SDSS J0818+1722. If the quasar
reionises the HeII in its vicinity, then in the limit of instantaneous
reionisation we infer z_H<9.0 (11.0) at 68 (95) per cent confidence assuming
photoheating is the dominant heat source and that HI reionisation is driven by
ionising sources with soft spectra, typical of population II stars. If the HI
and HeII in the IGM around SDSS J0818+1722 are instead reionised simultaneously
by a population of massive metal-free stars, characterised by very hard
ionising spectra, we obtain a tighter upper limit of z_H<8.4 (9.4). Initiating
reionisation at higher redshifts produces temperatures which are too low with
respect to our constraint unless the HI ionising sources or the quasar itself
have spectra significantly harder than typically assumed.Comment: 15 pages, 9 figures, accepted to MNRA
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