Astron. Astrophys. 332, 643-650 (1998)

3. The uvby calibration

In the previous section we have derived relations between the emission in the H line, measured through the index, and the excesses in the photometric uvby indices of circumstellar origin. The existence of such relations opens the possibility to use the H photometry to correct the uvby indices for circumstellar emission, and then derive the interstellar reddening and the intrinsic indices of the underlying star. These intrinsic indices can be used to estimate the astrophysical parameters of the underlying star by means of the usual uvby calibrations.

3.1. Interstellar reddening and intrinsic colours

The procedure to obtain the intrinsic uvby indices of a B star, correcting for the effect of the interstellar reddening, has been given by Crawford (1978). In the case of a Be star, we have to correct for both interstellar reddening and circumstellar continuum emission. In this section we propose a procedure to determine the circumstellar excesses, the interstellar reddening and the intrinsic colours of the underlying star. The basic idea is to use the index as additional information to characterize the circumstellar emission, and then use the Crawford (1978) method to measure the interstellar reddening. As both effects are coupled, there is not an easy way to decouple the effects of circumstellar and interstellar reddening. We propose an iterative procedure to determine both of them, which consist in the following steps:

• Make a first aproximation of = - 2.63. Substitute this value in Eqs. (10) and (11) to obtain and . Whit this values obtain and corrected for circumstellar excess.
• Whit the above and obtain the interstellar reddening and the intrinsic colours and indices, by means of the Crawford (1978) procedure.
• Use the intrinsic index to estimate the value of by means of Eq. (9).
• Make a new determination of = . Repeat all the procedure until convergence.

3.2. Luminosity calibration

The index is the main luminosity indicator for OB stars in the uvby photometric system. In the case of Be stars, however, the index is contaminated by the circumstellar emission, and is not useful to estimate the luminosity. Moreover, in the calibration method we are proposing, the index has been used to characterize the circumstellar emission and correct the other indices for circumstellar effects. There is not any other independent index which can be related with the stellar luminosity, and therefore there is not any way to directly determine the luminosity within our approach.

It has to be considered, however, that the present calibration is to be applied to a rather narrow range of luminosity classes. When studying absorption-line OB stars, there is a wide range of variation of the index for a given , originated by the differences in the H line strenght between supergiants and main sequence stars. Our calibration deals with Be stars, which are, by definition, restricted to luminosity classes III to V. As we stated in Sect. 2, within this range of luminosities the value of can be inferred from by means of Eq. (9). Then we propose to estimate the absolute magnitude of Be stars from the index alone, by means of the following procedure:

• Derive the value from by means of Eq. (9).
• Substitute the values of and in the Balona & Shobbrook (1984) calibration of absolute magnitude.

© European Southern Observatory (ESO) 1998

Online publication: March 23, 1998
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