As mentioned above the physical parameters are related to different photometric indices depending on the region of the HR diagram. So, it is necessary to establish empirical calibrations for each spectral range and luminosity class. The usual way to deal with the problem has been to define standard relations among photometric indices for unreddened and non-evolved stars with a metallicity of reference and then to consider corrections which depend on their evolutionary status and specific metallicity. The series of papers by Crawford (1975, 1978 and 1979) describing the B-, A- and F-type stars provides a clear discussion on how to build these standard relations.
In this work we adopted calibrations from the literature that cover a wide range of the HR diagram, i.e. from about O-K spectral types and V-Ia luminosity classes, although the range of luminosities depends on the spectral type. The zones of the HR diagram not covered by the calibrations are the subdwarfs and the G and K giants. The calibrations used to obtain absolute magnitudes are more restrictive than those used to obtain intrinsic colour. The calibrations are valid for stars of Pop I and non-extreme Pop II and are the most widely used at present.
The calibrations used to compute intrinsic colours and absolute magnitudes have been described in Figueras et al. (1991) and Jordi et al. (1992) and are summarized in Table 7 (click here).
The following comments on these calibrations are of interest:
For the intermediate region, Hilditch et al. (1983)
proposed using 
as the
free parameter to compute E(b-y), but 
is sensitive to differences
in chemical composition as it was designed to measure the blanketing.
Hilditch et al.'s procedure, slightly modified by Moon (1985), usually
overestimates the colour excess.
This overestimation of reddening
creates empty zones in a colour-colour diagram (see Fig. 4 (click here))
which are translated onto a
histogram of ages as a "deficiency" of stars in certain age ranges.
These empty zones are not present in the observed colour-colour diagrams
and as such, they are probably an artifact of using 
as the free parameter.
In the present work we dereddened the stars using Grosbøl (1978),
which does not produce this effect.
In the standard relation for F stars (Crawford 1975) we adopted
the values modified by Olsen (1988), corresponding to 
and
.
We adopted the dereddening expressions of Moon (1985) based on Olsen's
(1984) calibration for the region 5 (from G3 onwards).
The fact that the
calibration by Olsen is only preliminary means that the results
for this region should be regarded as being less accurate.
The stars with 
greater than 0.65 are not dealt with by our algorithm.
Figure 4: Intermediate stars in the 
plane. Solid line is the
standard relation Hilditch et al. (1983).
a) Observed diagram. b) Intrinsic colours using the procedure proposed by
Hilditch et al. (1983) modified by Moon (1985).
c) Intrinsic colours using Grosbøl (1978)
A preliminary calibration of the intrinsic colours of early supergiants was
conducted
by Zhang (1983) on 157 B-type stars with luminosity classes Ia,
Iab, Ib and II. Kilkenny & Whittet (1985) enlarged Zhang's sample up
to about 250 O, B and early A supergiants, and standard relations among
, 
and 
were given for each luminosity class.
An iterative process, similar to that of main sequence stars, was introduced in the algorithm to determine intrinsic from observed colours.
Gray (1991, 1992) discussed the problem of standard calibrations for A, F and G supergiants. He used a sample of supergiants belonging to open clusters, binary systems or near supergiants with published Strömgren photometry to built a new calibration. Arellano Ferro & Parrao (1990) proposed another calibration that partially covers the same spectral types. Here, we chose Gray's calibrations as they cover the full spectral range and, according to the author, were built to be continuous with Kilkenny & Whittet's (1985) calibration for early supergiants.
Dambis (1991) established an absolute magnitude calibration for
stars with 
(about A4-F3) while Arellano Ferro & Parrao
(1990) established a calibration for non-cepheid F0-G8. Both calibrations were
built from supergiants belonging to open clusters and associations.
For the range of 
, where both calibrations were suitable (spectral types
F0-F3), we
adopted that proposed by Arellano Ferro & Parrao.
Absolute magnitudes for the early supergiant stars were computed following
Balona & Shobbrook (1984).
Table 8 (click here) summarizes the calibrations used for the supergiants.
Table 7: Calibrations used in each main sequence photometric region
Table 8: Calibrations used in each supergiant photometric region