In this paper we present the results of a detailed investigation on the intrinsic accuracy of radius estimates obtained by adopting both the revised CORS method and the pure BW method. In order to avoid systematic errors the numerical experiments were performed by adopting theoretical observables -light, color, and radial velocity variations- predicted by nonlinear, convective models of classical Cepheids at solar chemical composition. The main findings of this analysis are the following:
1) the revised CORS method and the pure BW method applied to NIR data provide radius estimates characterized on average by the same accuracy. However, the former method supplies more accurate radius determinations than the latter one when applied to optical bands.
2) In agreement with current empirical evidence (Laney & Stobie 1995) the PR relations obtained by adopting theoretical predictions are affected by the "photometric drift", i.e. the slope becomes steeper when moving from optical to NIR bands. Thus suggesting that at fixed period radius determinations based on NIR/optical bands overestimate/underestimate "true" radii.
At the same time, in order to develop a method which can be applied to a large sample of Cepheids, we provided a new theoretical calibration of the term included in the revised CORS method. On the basis of this new calibration we find that the computed radii are affected by a discrepancy when compared with theoretical radii that is 7%. Moreover and even more importantly, we also find that computed radii based on optical bands do not show any systematic difference with theoretical radii.
Obviously before any firm conclusion on the accuracy of the current Cepheid PR relations can be reached, this method should be applied directly to empirical data. However, the main interesting feature of the current calibration is that it only relies on theoretical models. Therefore the comparison between theory and observations can allow us to supply tight constraints on the systematic uncertainties which affect radius estimates such as metallicity, reddening, and microturbulence velocity.
Finally, we mention that direct measurements of Cepheid angular diameters through optical interferometry are becoming available (Nordgren et al. 2000, and references therein). In the near future, new and more accurate interferometric data can allow us to assess on a firm physical basis the calibration of the CORS method. At the same time, it is worth emphasizing that the development of a homogeneous theoretical framework to be compared with new empirical data can also supply sound suggestions on the plausibility and the accuracy of the physical assumptions adopted for constructing both pulsation and atmosphere models.
© European Southern Observatory (ESO) 2000
Online publication: January 31, 2000