Physics – Optics
Scientific paper
Aug 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997phdt........18k&link_type=abstract
Thesis (PHD). HARVARD UNIVERSITY , Source DAI-B 58/02, p. 750, Aug 1997, 133 pages.
Physics
Optics
Baade Wesselink
Scientific paper
We observed 11 Cepheids with the Advanced Fiber Optics Echelle (AFOE) spectrograph. We measured the effective temperatures of Cepheids using spectral line depth ratios and Kurucz's model atmospheres. For the Cepheids we used the fact that the reddening is constant as a function of phase as an additional constraint. We found uncertainties in the mean temperature as small as 10 K for the best sampled Cepheids. Our temperatures and surface brightnesses disagree with the results of the Barnes-Evans method. We showed that the velocity is best measured from the line centroid because the centroid is insensitive to the turbulent broadening of the line. We demonstrated that the projection factor for the centroid velocity is 1.36 with an uncertainty of about 1% due to the limb darkening. We measured the pulsational velocities, line asymmetries and line level effects for 11 Cepheids and RRLyr. The line level effects are small for δCep and lead to an error in the Baade-Wesselink radius of 1%. Line level effects are more prominent in SVVul and RRLyr. For these stars the uncertainty in the Baade-Wesselink radius due to these effects is at least 5%. We developed a new formulation of the Baade-Wesselink method which uses the Fourier coefficients of the observables. We derived an explicit, analytic expression to determine the mean radius from each Fourier order. The simplicity of this method allowed us to derive the uncertainty in the mean radius due to measurement errors. Using simulations and a recent dataset we demonstrated that the precision of the radius measurement with optical magnitudes is in most cases limited by the accuracy of the measurement of the phase difference between the light and the color index curve. In this case it is advantageous to determine the inverse radius, because it has normal errors. We used the Baade-Wesselink radii and temperatures from model atmospheres to calibrate the Cepheid Period-Luminosity relation with 10 well studied Galactic Cepheids ranging in period from 2.5 to 45 days. We found radii and distances that are smaller than previous Baade-Wesselink measurements. Unlike previous Galactic calibrations, the slope of our Period-Luminosity relation is in good agreement with the slope found for Cepheids in the LMC and the SMC. We found M[V] = (-3.98 ± 0.07)-(2.79 ± 0.20)[&log P-1]. Without a metallicity correction we found a distance to the LMC of 46 ± 2 kpc. With the best available metallicity correction the LMC distance becomes 4±2-1+1 kpc.
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