4. The recent system of the Garrison-Gray standards
In recent years Gray & Garrison (1987, 1989a, 1989b) and Garrison & Gray (1994) have proposed a calibration of B8-F2 stars with standards for both low and high rotational velocity. As remarked before this is a consequence of the use of higher dispersion than the one used originally in the MK system.
We have analysed their system, as we did with the original MK system, using only those stars for which the errors in absolute magnitudes were 0.3 mag. The additional stars are included as an appendix to Table 1. The procedure for corrections for binarity and for interstellar extinction were the same as for the previous sample.
Fig. 4 provides a plot of the standards recommended by the above mentioned authors, with different symbols for low and high rotational velocity standards. An inspection of the figure shows that we find again, as for the MK standards, a wide sequence. We find also no clear separation of giants and dwarfs before A2 and we find also a mixture and inversion of luminosity classes with absolute magnitude. In Fig. 5 we have plotted the absolute magnitudes for different spectral types, marking in each case the luminosity class. We find also now that in general giants lie above dwarfs, as it should be, but there are places where this relation is inverted. The B II standard has the same absolute magnitude as a B9 V and B9 III standard; at B9, among low rotation standards, classes V and IV are inverted. At A2 a class III high rotation standard lies between two class IV standards and at class A7 the standards IV and V practically coincide. One can also try to ascertain if low rotational velocity standards have lower or higher luminosity than high rotational velocity standards and the answer is that there is no clear tendency - sometimes they go in one sense, sometimes in the other. We have also looked into the matter of a possible influence of rotation for standards earlier than B0 and we have found no evidence. One could notice in by-passing that although we have fewer disagreements than with the MK system (where we found 11 cases -see Table 2), this is partly due to the fact that we have only 24 new standards, because the authors retained a number of old standards for their new system. The conclusion is thus that the new list of standards has improved the description of the spectral features, but that this does not show up the same way in the luminosities.
As a last point one should be aware that our absolute magnitudes need a systematic correction. If one wishes to apply the so called Lutz-Kelker correction (Lutz & Kelker, 1973), for 90% of the stars the absolute magnitudes should be brightened by less than 0.11 mag and for the remaining 10% the correction ranges between -0.11 and -0.24 mag. A more realistic correction has been derived by means of a complex simulation; the obtained systematic correction was of about -0.08 mag. Full details on the simulation will be provided in a forthcoming paper by Jaschek & Valbousquet (1997). This correction has not been applied in Table 2, since we were interested only in relative positioning.
© European Southern Observatory (ESO) 1998
Online publication: January 16, 1998