1,888 research outputs found
Distances to Cepheid Open Clusters Via Optical and K-Band Imaging
We investigate the reddening and Main Sequence fitted distances to eleven
young, Galactic open clusters that contain Cepheids. Each cluster contains or
is associated with at least one Cepheid variable star. Reddening to the
clusters is estimated using the U-B:B-V colours of the OB stars and the
distance modulus to the cluster is estimated via B-V:V and V-K:V
colour-magnitude diagrams. By main-sequence fitting we proceed to calibrate the
Cepheid P-L relation and find M_V=-2.81xlogP-1.33 +/-0.32 and
M_K=-3.44xlogP-2.20 +/-0.29 and a distance modulus to the LMC of 18.55+/-0.32
in the V-band and 18.47+/-0.29 in the K-band giving an overall distance modulus
to the LMC of 18.51+/-0.3.
In the case of two important clusters we find that the U-B:B-V diagram in
these clusters is not well fitted by the standard Main Sequence line. In one
case, NGC7790, we find that the F stars show a UV excess which if caused by
metallicity would imply Fe/H ~ -1.5; this is anomalously low compared to what
is expected for young open clusters. In a second case, NGC6664, the U-B:B-V
diagram shows too red U-B colours for the F stars which in this case would
imply a higher than solar metallicity. If these effects are due to metallicity
then it would imply that the Cepheid PL(V) and PL(K) zeropoints depend on
metallicity according to delta(M)/delta(Fe/H) ~0.66 in the sense that lower
metallicity Cepheids are intrinsically fainter. Medium-high resolution
spectroscopy for the main-sequence F stars in these two clusters is needed to
determine if metallicity really is the cause or whether some other explanation
applies.Comment: Submitted to MNRAS. Due to large size of paper, please see
http://star-www.dur.ac.uk:80/~fhoyle/papers.html for a version with the
figures correctly inserte
Cosmic Needles versus Cosmic Microwave Background Radiation
It has been suggested by a number of authors that the 2.7K cosmic microwave
background (CMB) radiation might have arisen from the radiation from Population
III objects thermalized by conducting cosmic graphite/iron needle-shaped dust.
Due to lack of an accurate solution to the absorption properties of exceedingly
elongated grains, in existing literature which studies the CMB thermalizing
process they are generally modelled as (1) needle-like spheroids in terms of
the Rayleigh approximation; (2) infinite cylinders; and (3) the antenna theory.
We show here that the Rayleigh approximation is not valid since the Rayleigh
criterion is not satisfied for highly conducting needles. We also show that the
available intergalactic iron dust, if modelled as infinite cylinders, is not
sufficient to supply the required opacity at long wavelengths to obtain the
observed isotropy and Planckian nature of the CMB. If appealing to the antenna
theory, conducting iron needles with exceedingly large elongations (10^4)
appear able to provide sufficient opacity to thermalize the CMB within the iron
density limit. But the applicability of the antenna theory to exceedingly thin
needles of nanometer/micrometer in thickness needs to be justified.Comment: 13 pages, 4 figures; submitted to ApJ
Modeling Repulsive Gravity with Creation
There is a growing interest in the cosmologists for theories with negative
energy scalar fields and creation, in order to model a repulsive gravity. The
classical steady state cosmology proposed by Bondi, Gold and Hoyle in 1948, was
the first such theory which used a negative kinetic energy creation field to
invoke creation of matter. We emphasize that creation plays very crucial role
in cosmology and provides a natural explanation to the various explosive
phenomena occurring in local (z<0.1) and extra galactic universe. We exemplify
this point of view by considering the resurrected version of this theory - the
quasi-steady state theory, which tries to relate creation events directly to
the large scale dynamics of the universe and supplies more natural explanations
of the observed phenomena. Although the theory predicts a decelerating universe
at the present era, it explains successfully the recent SNe Ia observations
(which require an accelerating universe in the standard cosmology), as we show
in this paper by performing a Bayesian analysis of the data.Comment: The paper uses an old SNeIa dataset. With the new improved data, for
example the updated gold sample (Riess et al, astro-ph/0611572), the fit
improves considerably (\chi^2/DoF=197/180 and a probability of
goodness-of-fit=18%
The Detection of Cold Dust in Cas A: Evidence for the Formation of Metallic Needles in the Ejecta
Recently, Dunne et al. (2003) obtained 450 and 850 micron SCUBA images of
CasA, and reported the detection of 2-4 M_sun of cold, 18K, dust in the
remnant. Here we show that their interpretation of the observations faces
serious difficulties. Their inferred dust mass is larger than the mass of
refractory material in the ejecta of a 10 to 30 M_sun star. The cold dust model
faces even more difficulties if the 170 micron observations of the remnant are
included in the analysis, decreasing the cold dust temperature to ~ 8K, and
increasing its mass to > 20 M_sun. We offer here a more plausible
interpretation of their observation, in which the cold dust emission is
generated by conducting needles with properties that are completely determined
by the combined submillimeter and X-ray observations of the remnant. The
needles consist of metallic whiskers with <1% of embedded impurities that may
have condensed out of blobs of material that were expelled at high velocities
from the inner metal-rich layers of the star in an asymmetric explosion. The
needles are collisionally heated by the shocked gas to a temperature of 8K.
Taking the destruction of needles into account, a dust mass of only 1E-4 to
1E-3M_sun is needed to account for the observed SCUBA emission. Aligned in the
magnetic field, needles may give rise to observable polarized emission. The
detection of submillimeter polarization will therefore offer definitive proof
for a needle origin for the cold dust emission. Supernovae may still be proven
to be important sources of interstellar dust, but the evidence is still
inconclusive.Comment: 18 pages including 4 figures. Accepted for publication in the ApJ.
Missing reference adde
Clustering in the 2dF QSO Redshift Survey
We present clustering results from the 2dF QSO Redshift Survey (2QZ) which
currently contains over 20,000 QSOs at z<3. The two-point correlation function
of QSOs averaged over the entire survey (~1.5) is found to be similar to
that of local galaxies. When sub-dividing the sample as a function of redshift,
we find that for an Einstein-de Sitter universe QSO clustering is constant (in
comoving coordinates) over the entire redshift range probed by the 2QZ, while
in a universe with Omega_0=0.3 and Lambda_0=0.7 there is a marginal increase in
clustering with redshift. Sub-dividing the 2QZ on the basis of apparent
magnitude we find only a slight difference between the clustering of QSOs of
different apparent brightness, with the brightest QSOs having marginally
stronger clustering. We have made a first measurement of the redshift space
distortion of QSO clustering, with the goal of determining the value of
cosmological parameters (in partcular Lambda_0) from geometric distortions. The
current data do not allow us to discriminate between models, however, in
combination with constraints from the evolution of mass clustering we find
Omega_0=1-Lambda_0=0.23 +0.44-0.13 and beta(z~1.4)=0.39 +0.18-0.17. The full
2QZ data set will provide further cosmological constraints.Comment: 5 pages, 7 figures. Contributed to the 'Where's the Matter'
conference in Marseille 25-29 June 200
The 2dF QSO Redshift Survey - 10K@2K!
With ~10000 QSO redshifts, the 2dF QSO Redshift Survey (2QZ) is already the
biggest individual QSO survey. The aim for the survey is to have ~25000 QSO
redshifts, providing an order of magnitude increase in QSO clustering
statistics. We first describe the observational parameters of the 2dF QSO
survey. We then describe several highlights of the survey so far; we present
new estimates of the QSO luminosity function and the QSO correlation function.
We also present the first estimate of the QSO power spectrum from the 2QZ
catalogue, probing the form of the fluctuation power-spectrum out to the
\~1000h-1Mpc scales only previously probed by COBE. We find a power spectrum
which is steeper than the prediction of standard CDM and more consistent with
the prediction of Lambda-CDM. The best-fit value for the power spectrum shape
parameter for a range of cosmologies is Gamma=0.1+-0.1. Finally, we discuss how
the complete QSO survey will be able to constrain the value of Omega_Lambda by
combining results from the evolution of QSO clustering and from a geometric
test of clustering isotropy.Comment: 11 pages, 6 figures, latex, eso and springer sty files included. To
appear in the proceedings of the MPA/ESO/MPA conference "Mining the Sky",
Garching, July 31 - August 4 2000, eds. A.J. Banday et a
First Results from the 2dF QSO redshift survey
We present some initial results from the 2dF QSO redshift survey. The aim of
the survey is to produce an optically-selected catalogue of 25000 QSOs over the
redshift range 0<z<3 using the 2-degree field at the Anglo-Australian
Telescope.Comment: 7 pages, 6 figures, submitted to proceedings of ESO Deep Fields
conferenc
A model-based assessment of the cost-utility of strategies to identify Lynch syndrome in early-onset colorectal cancer patients.
This is a freely-available open access publication. Please cite the published version which is available via the DOI link in this record.BACKGROUND: Lynch syndrome is an autosomal dominant cancer predisposition syndrome caused by mutations in the DNA mismatch repair genes MLH1, MSH2, MSH6 and PMS2. Individuals with Lynch syndrome have an increased risk of colorectal cancer, endometrial cancer, ovarian and other cancers. Lynch syndrome remains underdiagnosed in the UK. Reflex testing for Lynch syndrome in early-onset colorectal cancer patients is proposed as a method to identify more families affected by Lynch syndrome and offer surveillance to reduce cancer risks, although cost-effectiveness is viewed as a barrier to implementation. The objective of this project was to estimate the cost-utility of strategies to identify Lynch syndrome in individuals with early-onset colorectal cancer in the NHS. METHODS: A decision analytic model was developed which simulated diagnostic and long-term outcomes over a lifetime horizon for colorectal cancer patients with and without Lynch syndrome and for relatives of those patients. Nine diagnostic strategies were modelled which included microsatellite instability (MSI) testing, immunohistochemistry (IHC), BRAF mutation testing (methylation testing in a scenario analysis), diagnostic mutation testing and Amsterdam II criteria. Biennial colonoscopic surveillance was included for individuals diagnosed with Lynch syndrome and accepting surveillance. Prophylactic hysterectomy with bilateral salpingo-oophorectomy (H-BSO) was similarly included for women diagnosed with Lynch syndrome. Costs from NHS and Personal Social Services perspective and quality-adjusted life years (QALYs) were estimated and discounted at 3.5% per annum. RESULTS: All strategies included for the identification of Lynch syndrome were cost-effective versus no testing. The strategy with the greatest net health benefit was MSI followed by BRAF followed by diagnostic genetic testing, costing £5,491 per QALY gained over no testing. The effect of prophylactic H-BSO on health-related quality of life (HRQoL) is uncertain and could outweigh the health benefits of testing, resulting in overall QALY loss. CONCLUSIONS: Reflex testing for Lynch syndrome in early-onset colorectal cancer patients is predicted to be a cost-effective use of limited financial resources in England and Wales. Research is recommended into the cost-effectiveness of reflex testing for Lynch syndrome in other associated cancers and into the impact of prophylactic H-BSO on HRQoL.NIH
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