Calibration of Strömgren-Crawford photometry for Ap-stars compared to Hipparcos results


Nikolaus Vogt, Santiago de Chile / Sonneberg, Ernst Paunzen, Hans M. Maitzen, Vienna


Complete text (abstract only):

25 years ago Bidelman and MacConnell (1973) published a list of nearly 800 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 3 different photometric projects carried out at ESO-La Silla with limiting magnitude V = 8.5:
1. uvby photometry by Vogt and Faúndez (1979);
2. {delta}a photometry by Maitzen and Vogt (1983) in the system of Maitzen (1976) demonstrating the very high agreement of photometric Ap-detections with the spectroscopic results of Bidelman and MacConnell;
3. H{beta} photometry of 226 objects which were observed at the Danish 50cm telescope on La Silla in 1982.

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. Reddening corrections mattered only for the hot peculiars. Absolute magnitudes were derived according to the calibrations worked out by Crawford two decades ago for normal main sequence AB stars.

The Hipparcos catalogue which appeared in 1997 offers an independent way to check whether these calibrations derived for normal stars do apply also for chemically peculiar stars. Fortunately, we identified Hipparcos parallaxes for two thirds of our sample.

We divided our objects according to 3 calibration groups: early (= B type stars), intermediate (A0-A2 type stars), late (other A type stars). First of all we consider normal stars with published {beta}-values in order to compare their photometric absolute magnitudes to those based on the Hipparcos catalogue.

The degree of correlation between both quantities is not excellent for the normal B-type stars, but no systematic trend is visible. For the stars around the Balmer maximum (intermediate group) we also do not recognize a significant systematic structure in the distribution around the 45-line. Only in the case of normal late type stars one does note that part of the stars have obviously fainter photometric absolute magnitudes as compared to their Hipparcos based values.

Turning now to our Ap-sample - bearing in mind that assigning the proper calibration group is not always trivial - we do in fact notice systematics: The photometric calibration for the early group stars yields too bright absolute magnitudes which may be related to the well known fact that Bp stars have too small Balmer jumps as well as Balmer lines. Statistically, this effect would lead to an overestimate of the distances by about 25 %.

The situation is reversed for the intermediate and late group of Ap stars: the lower the temperature, the fainter becomes the photometrically derived absolute magnitude in comparison to the value calculated from the Hipparcos parallaxes.

While in the case of intermediate Ap-stars the apparent luminosity difference only rarely surpasses one magnitude, the late type peculiars are photometrically underluminous by roughly 1.5 magnitudes on the average. The outstanding systematic behaviour of the late type Ap-stars is reminiscent of the nonsymmetric distribution of normal late A-type stars (mentioned before) and could be interpreted by the presence of non-detected peculiar stars in the sample of normal stars.

In the light of these findings the Strömgren-Crawford calibrations will be approximately reliable only if applied to CP2 stars at the transition from the early to the intermediate group. Endeavours should concentrate on appropriate peculiarity corrections for stars outside this domain.



Bidelman, W.P., MacConnell, D.J.: 1973, AJ 78, 678
Maitzen, H.M.: 1976, A&A 51, 223
Maitzen, H.M., Vogt, N.: 1983, A&A 123, 48
Vogt, N., Faúndez, M.: 1979, A&AS 36, 4772

Bibliographical details:

Nikolaus Vogt, Ernst Paunzen, Hans M. Maitzen: Calibration of Strömgren-Crawford photometry for Ap-stars compared to Hipparcos results. In: Peter Brosche, Wolfgang R. Dick, Oliver Schwarz, Roland Wielen (Eds.): The Message of the Angles - Astrometry from 1798 to 1998. Proceedings of the International Spring Meeting of the Astronomische Gesellschaft, Gotha, May 11-15, 1998. (Acta Historica Astronomiae; 3). Thun ; Frankfurt am Main : Deutsch, 1998, p. 204-206.