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

The galaxy luminosity function and the Local Hole

In a previous study Whitbourn & Shanks have reported evidence for a local void underdense by ≈15 per cent extending to 150–300 h−1 Mpc around our position in the Southern Galactic Cap (SGC). Assuming a local luminosity function they modelled K- and r-limited number counts and redshift distributions in the 6dFGS/2MASS and SDSS redshift surveys and derived normalized n(z) ratios relative to the standard homogeneous cosmological model. Here we test further these results using maximum likelihood techniques that solve for the galaxy density distributions and the galaxy luminosity function simultaneously. We confirm the results from the previous analysis in terms of the number density distributions, indicating that our detection of the ‘Local Hole’ in the SGC is robust to the assumption of either our previous, or newly estimated, luminosity functions. However, there are discrepancies with previously published K- and r-band luminosity functions. In particular the r-band luminosity function has a steeper faint end slope than the r0.1 results of Blanton et al. but is consistent with the r0.1 results of Montero-Dorta & Prada and Loveday et al