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Recent progress on Baade–Wesselink (BW)-type techniques to determine the distances to classical Cepheids is reviewed. Particular emphasis is placed on the near-infrared surface-brightness (IRSB) version of the BW method. Its most recent calibration is described and shown to be capable of yielding individual Cepheid distances accurate to 6%, including systematic uncertainties. Cepheid distances from the IRSB method are compared to those determined from open cluster zero-age main-sequence fitting for Cepheids located in Galactic open clusters, yielding excellent agreement between the IRSB and cluster Cepheid distance scales. Results for the Cepheid period–luminosity (PL) relation in near-infrared and optical bands based on IRSB distances and the question of the universality of the Cepheid PL relation are discussed. Results from other implementations of the BW method are compared to the IRSB distance scale and possible reasons for discrepancies are identified.
The science of extra-solar planets is one of the most rapidly changing areas of astrophysics and since 1995 the number of planets known has increased by almost two orders of magnitude. A combination of ground-based surveys and dedicated space missions has resulted in 560-plus planets being detected, and over 1200 that await confirmation. NASA's Kepler mission has opened up the possibility of discovering Earth-like planets in the habitable zone around some of the 100,000 stars it is surveying during its 3 to 4-year lifetime. The new ESA's Gaia mission is expected to discover thousands of new planets around stars within 200 parsecs of the Sun. The key challenge now is moving on from discovery, important though that remains, to characterisation: what are these planets actually like, and why are they as they are?
In the past ten years, we have learned how to obtain the first spectra of exoplanets using transit transmission and emission spectroscopy. With the high stability of Spitzer, Hubble, and large ground-based telescopes the spectra of bright close-in massive planets can be obtained and species like water vapour, methane, carbon monoxide and dioxide have been detected. With transit science came the first tangible remote sensing of these planetary bodies and so one can start to extrapolate from what has been learnt from Solar System probes to what one might plan to learn about their faraway siblings. As we learn more about the atmospheres, surfaces and near-surfaces of these remote bodies, we will begin to build up a clearer picture of their construction, history and suitability for life.
The Exoplanet Characterisation Observatory, EChO, will be the first dedicated mission to investigate the physics and chemistry of Exoplanetary Atmospheres. By characterising spectroscopically more bodies in different environments we will take detailed planetology out of the Solar System and into the Galaxy as a whole.
EChO has now been selected by the European Space Agency to be assessed as one of four M3 mission candidates.
We apply the Barnes–Evans variant of the Baade–Wesselink method to Cepheids in the LMC and SMC in an attempt to determine the distance directly to individual stars in these galaxies and to determine the metallicity effect on the Cepheid period–luminosity relation. We now have K-band light curves for a sample of SMC stars as well as for many Cepheids in young clusters in the LMC. Using the FV, (V – K) calibration of Fouqué & Gieren (1997) we find preliminary evidence for a metallicity effect which makes metal poor Cepheids brighter. This is at odds with earlier results based on optical photometry and the reason is not entirely understood yet.
We review the process of determining the distances to Cepheid variables with the infrared surface brightness technique. We show that both versions of this technique (working with the V, V – K and the pure infrared K, J – K magnitude/color combinations) yield consistent distances to Cepheid variables. For a Cepheid with average-quality photometric and radial velocity data, the typical total uncertainty in the distance is ±5%. We use the distances of 34 Galactic variables to calibrate period-luminosity relations in the V, I, J, H, K bands and demonstrate that these relations lead to distance moduli of the LMC which agree to ±0.02 mag. The infrared surface brightness technique yields a best LMC distance modulus of 18.46 ±0.02. We discuss possible metallicity corrections, such as the ones determined by Sasselov et al. from EROS Cepheids comparison in the LMC and SMC, and conclude that our distance modulus is negligibly affected by these corrections. Finally, we present a program to obtain reddening- and metallicity-independent infrared surface brightness distances of Cepheid variables in LMC clusters which will yield an even more accurate mean LMC distance independent of any metallicity correction to Cepheid absolute magnitudes.
We have determined new Cepheid distances on a homogeneous system to nine nearby galaxies with Cepheid observations in the V bandpass, using the extensive calibration of the galactic Cepheid PL(V) relation of Gieren, Barnes & Moffett (1993). Absorption corrections to the observed distance moduli were taken from the RC3 catalog (de Vaucouleurs et al. 1991). For most of the galaxies, the galactic PL(V) relation of GBM gives a very good fit to the Cepheid data.
We compare the best determined Baade-Wesselink (BW) period-luminosity (PL) relation for 100 galactic Cepheids to the PL relation derived for 32 open cluster and association Cepheids by the ZAMS-fitting method. Eighteen stars in common lead to the conclusion that BW and ZAMS-fitting distances can agree to better than 0.1 mag, after proper allowance of systematic effects in both methods.
The absolute calibration of the Cepheid period-luminosity (PL) relation with galactic Cepheids is discussed. Various methods, most importantly the cluster ZAMS-fitting scale and the Baade-Wesselink scale are found to yield PL zero points which agree within ∼ ± 0.1 mag. The present Cepheid calibration sets the Large Magellanic Cloud at μ0 (LMC) = 18.6 ± 0.1 mag, in good agreement with the distance derived from SN 1987A and other methods except RR Lyrae stars which seem to give a shorter distance scale.
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