Massive luminous early type stars in the LMC

I. The reddening of individual stars and the LMC reddening law

¹ Institut für Technische Betriebswirtschaft, Fachhochschule Münster, Stegerwaldstraße 39, 48565 Steinfurt, Germany
² Astronomisches Institut der Ruhr-Universität Bochum, 44780 Bochum, Germany
³ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
⁴ Georg-Büchner-Straße 37, 97276 Margetshöchheim, Germany

Corresponding author: J. Gochermann, gochermann@fh-muenster.de

Received: 21 December 2001
Accepted: 5 April 2002

Abstract

In order to construct a comprehensive HRD of early type stars in the Large Magellanic Cloud (LMC) in the first step the reddenings of individual stars and the LMC reddening law have been investigated. 1942 LMC member stars with good photometries from the Bochum photometry data base have been first corrected individually for galactic foreground reddening. From stars with good spectral classification the slope of the LMC internal reddening line was calculated for each spectral subclass between O3 and A4. A remarkable difference to the galactic reddening law was found. The slope of the reddening line first decreases for stars from O3 to B0, and then increases rapidly between B0 and B3 from ~0.7 to ~1.1. For later type stars it remains higher than for early type stars. This effect has important consequences for all extinction corrections. We checked this using different methods. Because no evidence for systematically wrong classifications was found, the differences in the reddening slopes must be caused by the ISM of the LMC itself. Four possible causes are considered.