EDP Sciences
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Volume 444, Number 3, December IV 2005
Page(s) 941 - 946
Section Stellar atmospheres
DOI http://dx.doi.org/10.1051/0004-6361:20053546

A&A 444, 941-946 (2005)
DOI: 10.1051/0004-6361:20053546

An empirical temperature calibration for the $\Delta a$ photometric system

I. The B-type stars
E. Paunzen, A. Schnell and H. M. Maitzen

Institut für Astronomie der Universität Wien, Türkenschanzstr. 17, 1180 Wien, Austria
    e-mail: Ernst.Paunzen@univie.ac.at

(Received 31 May 2005 / Accepted 21 August 2005)

We establish an empirical effective temperature calibration of main sequence, luminosity class V to III B-type stars for the $\Delta a$ photometric system which was originally developed to detect magnetic chemically peculiar objects of the upper main sequence (early B-type to early F-type) at 5200 Å. However, this system provides the index (g1-y) which shows an excellent correlation with (B-V) as well as (b-y) and can be used as an indicator of the effective temperature. This is supplemented by a very accurate color-magnitude diagram, y or V versus (g1-y), which can be used, for example, to determine the reddening, distance and age of an open cluster. This makes the $\Delta a$ photometric system an excellent tool to investigate the Hertzsprung-Russell-Diagram (HRD) in more detail. Using the reddening-free parameters and already established calibrations within the Strömgren $uvby\beta$, Geneva 7-color and Johnson UBV systems, a polynomial fit of third degree for the averaged effective temperatures to the individual (g1-y)0 values was derived. For this purpose, data from the literature as well as new observations were taken resulting in 225 suitable bright normal B-type objects. The statistical mean of the error for this sample is 238 K which is sufficient to investigate the HRD of distant galactic open clusters as well as extragalactic aggregates in the future.

Key words: stars: chemically peculiar -- stars: early-type -- techniques: photometric

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