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Up: Determination of physical parameters

1 Introduction

The determination of physical parameters such as effective temperature and superficial gravity can be done through the Strömgren photometric data reduced to the standard system for B-F stars, once corrected for interstellar extinction. This procedure has been done previously by the authors for $\delta$ scuti variables and for stars in open cluster which lie within these spectral; its description can be found in Peniche et al. (1990), for example.

We must emphasize that for these fast pulsators, some of which are multi-periodic or with unexplained light curve variations, the data gathered in the present work has the advantage that it need not be adjusted in phase as it would have been with a one-channel photometer. This also allows an extensive time coverage along the cycle, in particular if the star is faint. On the other hand, the simultaneous measurements in the different filters that define the Strömgren system let us avoid problems due to interpolation which worsen if more than one frequency is present since it is well known (Breger 1998) that many high amplitude $\delta$ Scuti stars (HADS) are double mode radial pulsators. Hence, the analysis of the photometry presented in the current work shows the real variation of the physical characteristics of the stars along their pulsation cycles.

If the photometric system is well-defined and calibrated, it will provide an efficient way to investigate physical conditions. A comparison with theoretical models, such as those of Relyea & Kurucz (1978), allows a direct comparison with intermediate or wide band photometry measured from the stars with those obtained theoretically for early type stars. Relyea & Kurucz (1978) calculated grids for stellar atmospheres for G, F, A, B and O stars for the solar abundance $\rm [Fe/H] = 0.00$ in a temperature range from 5500 K up to 50 000 K. They also considered abundances of 0.1 and 0.001. A comparison of the photometric unreddened indexes (b-y)0 and c0 obtained for each star with such models permits the determination of the effective temperature Te and superficial gravity $\log g$ along the cycle of pulsation. Therefore, the research goals for $\delta$ Scuti stars should be to try to determine not only the frequency content but also the maximum number of physical characteristics of as many stars as possible, along each star's cycle variation. Furthermore, there still exists the question of the real physical differences between the $\delta$ Scuti stars and Dwarf Cepheids; Breger (1980) stated that the majority of dwarf Cepheids mimic Pop I $\delta$ Scuti stars in basically all aspects, except for a few stars in the subgroup of Dwarf Cepheids known as the SX Phe that show low metallicity, high space velocities and low luminosities and that do not conform the theoretical period gravity relation in the direction of low mass. Hence motivation of the present work.

Table 1: Uncertainties in the color indexes V, (b-y), m1, c1 and $\beta$

Season & star & $\delta V$\...
 ...0.015 & 0.013 & 0.017 & 0.018 & 0.005\\ \noalign{\smallskip}

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