Astron. Astrophys. 355, 1003-1008 (2000)

1. Introduction

One of the basic tools in modern astrophysics is the calibration of stellar absolute magnitudes which enables conclusions on luminosity, age, distance and evolution of stars. More than 40 years ago, Crawford (1958) introduced a powerful method for this purpose: the photoelectric measurement of the H absorption line in two interference filters (a narrow and a broad one, both centered at H) which allows to determine absolute magnitudes of stars of early and intermediate spectral types (B - F). However, it took more than two decades until Crawford (1975, 1978, 1979) and Hilditch et al. (1983) have published a reliable - calibration for normal main sequence, in the above mentioned spectral type range. On the other hand, the question whether this calibration is also valid for peculiar stars remains to a large extent open till now.

25 years ago Bidelman & MacConnell (1973) published a list of nearly 800 southern Ap-stars which they had identified on objective prism plates collected at the Cerro Tololo Interamerican Observatory as kind of a precursor work for the huge Michigan project of two-dimensional spectral classification.

This list has been used as basis for three different photometric projects carried out at ESO-La Silla with a limiting magnitude V = 8.5 mag:

1. uvby photometry of 341 Ap stars by Vogt & Faúndez (1979);

2. -photometry of 338 Ap stars by Maitzen & Vogt (1983) in the system of Maitzen (1976) demonstrating the very high agreement of photometric Ap-detections with the spectroscopic results of Bidelman & MacConnell (1973);

3. H photometry of 233 objects which were observed at the Danish 50cm telescope on La Silla in 1982 (this paper).

The latter work was intended not only to formally complete Strömgren-Crawford data for a significantly large set of chemically peculiar stars (excluding Am and HgMn objects) but also to yield their galactic locations. Publishing these data has been delayed for some years because we wanted to have an independent data set to compare with in order to check how far the application of a normal star calibration to our Ap-sample deviates from reality. Such an independent set has become available by the meanwhile published Hipparcos catalogue (Perryman et al. 1997) which, fortunately, contains high precision parallaxes for two thirds of our sample.

After a brief description of observing and reduction procedures, we give the results of our H photometry (Sect. 2) and compare the photometric absolute magnitude calibrations with those based on trigonometric parallaxes of Hipparcos (Sect. 3). We discuss these results for different spectral types as well as for two different domains of the accuracy of trigonometric parallaxes (Sect. 4).

© European Southern Observatory (ESO) 2000

Online publication: March 21, 2000
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