We have presented an updated and complete calibration scheme of the Geneva
photometric system in terms of effective temperature, surface gravity and
metallicity for all B to mid-G stars of the main sequence or just above it.
This calibration can be applied to giant B, A and F stars but not to red
giants, and it will not give a reliable estimate of the surface gravity of
giant F stars . It cannot be used either for B to G supergiants, since it is
based on LTE atmosphere models and no supergiant has been included in our set
of standard stars. Reddened B stars can be dealt with (provided the reddening
is not too large, i.e. not greater than about 
and the
reddening law is standard). For cool stars (
K),
metallicities can be safely estimated in the range 
, and the upper limit of this range extends to about +0.6 in the
case of Am stars, which are hotter (7000 - 8000 K).
The new features of this calibration are:
vs. B2-V1 diagram
K. We have tested
different ways of treating the convection, but found no simple way to
reproduce the change of slope of the cluster sequences.
We also checked whether a change of the microturbulent velocity 
could account for that; this is a reasonable assumption, since it is known
empirically that 
increases with 
up to about
8000 K (EAGLNT93, Coupry & Burkhart 1992). But,
the increase of 
from cool (5500 K) to hot (
)
stars being observationally both small (about 
) and
smooth, it cannot account for the observed sequence of the Hyades in the
d/B2-V1 diagram, since this sequence has a slope which increases abruptly
around 7000 K. One would have to make both 
increase e.g. from
2 to 
and overshooting disappear at
K, to reproduce the observed sequence, but this would appear extremely ad hoc.
Therefore, the cureseems far from straightforward.
As a result, the photometrically
estimated surface gravities of cool stars are not so reliable as one would
expect from the accuracy and homogeneity of the Geneva data. The most reliable
values are those obtained for unevolved, solar-metallicity stars.
The calibration we have just presented is not complete in the sense that it
does not give explicitly the absolute magnitude, bolometric correction, mass,
colour excess and distance of the star. In particular, we have dropped the
determination of the mass which was offered by NN90 and KN90; the reason is
that the Barcelona group (Prof. F. Figueras and co-workers) has devised a code
which interpolates the mass and age of a star from its effective temperature
and gravity, in evolutionary tracks from various authors including Schaller
et al. (1992). We had no reason to duplicate their work. The other
physical parameters can be found using calibrations published by other authors.
For the intrinsic colours (hence interstellar reddening) of O, B and early A
stars (hereafter ``hot stars''), see Cramer (1993), who updated an
earlier work (Cramer 1982). Intrinsic colours of B2 to M0 stars have
also been estimated by Hauck (1993). The bolometric correction of
the hot stars can be obtained from a formula given in Appendix by Cramer
(1984a); a formula giving 
as a function of X is also
given in that paper, but it should be considered as superseded by our work. A
calibration of the X and Y parameters in terms of Crawford's 
index is
worth mentioning too (Cramer 1984b): it allows to detect 
emission when both Geneva and 
photometric data are available. The
absolute magnitude of hot stars can be obtained from a recent work by Cramer
(1994), which supersedes an earlier calibration (Cramer & Maeder
1979). The absolute magnitude of A and F stars (excluding supergiants) can
be obtained from the calibration of Hauck (1973). The intrinsic
colours of A and F supergiants have been estimated by Meynet & Hauck
(1985). Finally, the Geneva system has been calibrated for G, K and M-type
stars essentially by Grenon (1978, 1982) and Grenon & Golay
(1979), as mentioned above in Sect. 4.
A fortran code has been written, which applies our calibration to stars measured in the Geneva system. This code is available by anonymous ftp at the Centre de Données de Strasbourg (CDS), following the instructions given in A&A 280, E1-E2 (1993). This code uses several ascii files containing the inverted grids, which are, of course, also available. The (uncorrected) Geneva colours of the Kurucz models are also available at the CDS.
Table 2: Eclipsing binaries and other stars used as surface gravity standards
(hot stars)
Table 3: Stars of the Orion association used as surface gravity standard
(hot stars)
Table 4: Standard stars of effective temperature (intermediate stars)
Table 5: Eclipsing binaries used as surface gravity standards for intermediate stars
Table 6: Stars of the Orion association used as surface gravity standards
(intermediate stars)
Table 7: Stars of the Pleiades used as surface gravity standards
(intermediate stars)
Table 8: Stars of IC 2391 used as surface gravity standards
(intermediate stars)
Table 9: Standard stars of effective temperature (cool stars)
Table 10: Stars of the Hyades used as surface gravity standards (cool stars)
Table 10: continued
Table 10: continued
Table 11: Stars of IC 2391 used as surface gravity standards (cool stars)
Table 12: Coefficients a, b and c of Eq. (15), for the correction
of log
in the case of cool stars
Table 13: Stars of Edvardsson et al. (1993) used as metallicity and surface gravity standards
(cool stars)
Table 13: continued
Table 13: continued
Table 14: Additional standard stars for metallicity (cool stars)
Acknowledgements
This work was supported in part by the Fonds National de la Recherche Scientifique. We thank Mr. David Bersier (Geneva Observatory) for having computed the colours of the corrected models of cool stars.
