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

Dark Energy Survey Year 1 Results: redshift distributions of the weak-lensing source galaxies

We describe the derivation and validation of redshift distribution estimates and their uncertainties for the populations of galaxies used as weak-lensing sources in the Dark Energy Survey (DES) Year 1 cosmological analyses. The Bayesian Photometric Redshift (BPZ) code is used to assign galaxies to four redshift bins between z ≈ 0.2 and ≈1.3, and to produce initial estimates of the lensing-weighted redshift distributions n^i_(PZ)(z)∝dn^i/dz for members of bin i. Accurate determination of cosmological parameters depends critically on knowledge of n^i, but is insensitive to bin assignments or redshift errors for individual galaxies. The cosmological analyses allow for shifts n^i(z)=n^i_(PZ)(z−Δz^i) to correct the mean redshift of n^i(z) for biases in n^i_(PZ). The Δz^i are constrained by comparison of independently estimated 30-band photometric redshifts of galaxies in the Cosmic Evolution Survey (COSMOS) field to BPZ estimates made from the DES griz fluxes, for a sample matched in fluxes, pre-seeing size, and lensing weight to the DES weak-lensing sources. In companion papers, the Δziof the three lowest redshift bins are further constrained by the angular clustering of the source galaxies around red galaxies with secure photometric redshifts at 0.15 < z < 0.9. This paper details the BPZ and COSMOS procedures, and demonstrates that the cosmological inference is insensitive to details of the n^i(z) beyond the choice of Δzi. The clustering and COSMOS validation methods produce consistent estimates of Δz^i in the bins where both can be applied, with combined uncertainties of σΔ_(z^i_ = 0.015,0.013,0.011, and 0.022 in the four bins. Repeating the photo-z procedure instead using the Directional Neighbourhood Fitting algorithm, or using the n^i(z) estimated from the matched sample in COSMOS, yields no discernible difference in cosmological inferences

Cross-Newell equations for hexagons and triangles

The Cross-Newell equations for hexagons and triangles are derived for general real gradient systems, and are found to be in flux-divergence form. Specific examples of complex governing equations that give rise to hexagons and triangles and which have Lyapunov functionals are also considered, and explicit forms of the Cross-Newell equations are found in these cases. The general nongradient case is also discussed; in contrast with the gradient case, the equations are not flux-divergent. In all cases, the phase stability boundaries and modes of instability for general distorted hexagons and triangles can be recovered from the Cross-Newell equations.Comment: 24 pages, 1 figur

Multi-shocks in asymmetric simple exclusions processes: Insights from fixed-point analysis of the boundary-layers

The boundary-induced phase transitions in an asymmetric simple exclusion process with inter-particle repulsion and bulk non-conservation are analyzed through the fixed points of the boundary layers. This system is known to have phases in which particle density profiles have different kinds of shocks. We show how this boundary-layer fixed-point method allows us to gain physical insights on the nature of the phases and also to obtain several quantitative results on the density profiles especially on the nature of the boundary-layers and shocks.Comment: 12 pages, 8 figure

Entropic-acoustic instability of shocked Bondi accretion I. What does perturbed Bondi accretion sound like ?

In the radial flow of gas into a black hole (i.e. Bondi accretion), the infall of any entropy or vorticity perturbation produces acoustic waves propagating outward. The dependence of this acoustic flux on the shape of the perturbation is investigated in detail. This is the key process in the mechanism of the entropic-acoustic instability proposed by Foglizzo & Tagger (2000) to explain the instability of Bondi-Hoyle-Lyttleton accretion. These acoustic waves create new entropy and vorticity perturbations when they reach the shock, thus closing the entropic-acoustic cycle. With an adiabatic index 1<gamma<=5/3, the linearized equations describing the perturbations of the Bondi flow are studied analytically and solved numerically. The fundamental frequency of this problem is the cut-off frequency of acoustic refraction, below which ingoing acoustic waves are refracted out. This cut-off is significantly smaller than the Keplerian frequency at the sonic radius and depends on the latitudinal number l of the perturbations. When advected adiabatically inward, entropy and vorticity perturbations trigger acoustic waves propagating outward, with an efficiency which is highest for non radial perturbations l=1. The outgoing acoustic flux produced by the advection of vorticity perturbations is always moderate and peaks at rather low frequency. By contrast, the acoustic flux produced by an entropy wave is highest close to the refraction cut-off. It can be very large if gamma is close to 5/3. These results suggest that the shocked Bondi flow with gamma=5/3 is strongly unstable with respect to the entropic-acoustic mechanism.Comment: 14 pages, 11 figures, accepted for publication in A&

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%

Astrophysical implications of hypothetical stable TeV-scale black holes

We analyze macroscopic effects of TeV-scale black holes, such as could possibly be produced at the LHC, in what is regarded as an extremely hypothetical scenario in which they are stable and, if trapped inside Earth, begin to accrete matter. We examine a wide variety of TeV-scale gravity scenarios, basing the resulting accretion models on first-principles, basic, and well-tested physical laws. These scenarios fall into two classes, depending on whether accretion could have any macroscopic effect on the Earth at times shorter than the Sun's natural lifetime. We argue that cases with such effect at shorter times than the solar lifetime are ruled out, since in these scenarios black holes produced by cosmic rays impinging on much denser white dwarfs and neutron stars would then catalyze their decay on timescales incompatible with their known lifetimes. We also comment on relevant lifetimes for astronomical objects that capture primordial black holes. In short, this study finds no basis for concerns that TeV-scale black holes from the LHC could pose a risk to Earth on time scales shorter than the Earth's natural lifetime. Indeed, conservative arguments based on detailed calculations and the best-available scientific knowledge, including solid astronomical data, conclude, from multiple perspectives, that there is no risk of any significance whatsoever from such black holes.Comment: Version2: Minor corrections/fixed typos; updated reference