Photometry of 40 LMC Cepheids

We present V and I_c CCD photometry for 40 LMC Cepheids at 1 to 3 epochs. This represents a significant increase in the number of LMC Cepheids with $I$-band data, and, as we show, is a useful addition to the sample which can be used to calibrate the period--luminosity relations in these important bands

GRBs as Probes of the IGM

Gamma-ray Bursts (GRBs) are the most powerful explosions known, capable of outshining the rest of gamma-ray sky during their short-lived prompt emission. Their cosmological nature makes them the best tool to explore the final stages in the lives of very massive stars up to the highest redshifts. Furthermore, studying the emission from their low-energy counterparts (optical and infrared) via rapid spectroscopy, we have been able to pin down the exact location of the most distant galaxies as well as placing stringent constraints on their host galaxies and intervening systems at low and high-redshift (e.g. metallicity and neutral hydrogen fraction). In fact, each GRB spectrum contains absorption features imprinted by metals in the host interstellar medium (ISM) as well as the intervening intergalactic medium (IGM) along the line of sight. In this chapter we summarize the progress made using a large dataset of GRB spectra in understanding the nature of both these absorbers and how GRBs can be used to study the early Universe, in particular to measure the neutral hydrogen fraction and the escape fraction of UV photons before and during the epoch of re-ionization.Comment: 18 pages; 5 Figures. Accepted for publication in Space Science Review

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