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Astron. Astrophys. 330, 619-625 (1998)

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3. Results

The results are given in Table 1 and are summarized in Figs. 1 and 2. Fig. 1 provides a graphical representation for the main sequence, whereas in Fig. 2 are plotted all the objects of classes V to I. An examination of Fig. 1 shows that the main sequence is not a line, but rather a wide band, whose width can be calculated from those spectral types where we have at least three dwarf standards of the same spectral type (B2, B3, B8, A0, A1, A3, F3). The respective widths are 1.2, 0.9, 1.9, 0.8, 1.9, 0.6 and 0.4 with an average of 1.1 magnitudes. In the figure we have plotted the Schmidt-Kaler main sequence. It can be seen that the mean calibration curve runs through widely scattered points, so that the assumption of a dispersion of 0.3 mag is an overoptimistic estimate. The scatter is especially large around A0. An examination of Fig. 2 shows that in general giants lie above dwarfs, as it should be. There is however no gap between class V and class III in the domain of B-type stars, but rather a continuum. From A2 on, the separation between classes III and V becomes clearer. To examine in more detail the situation, we have plotted in Fig. 3 the different absolute magnitudes for a given spectral type, specifying for each point its luminosity class. It can be seen that in general a sequence exists, although at some points the natural order is not kept. For instance at B5 the order is reversed (giant less luminous than the subgiant), at B7 the class IV falls below the V, in B8 the class II below the III. At A2, A7 and F2 we have abnormally positioned class IV objects. At F2 we have four points of class IV, IV-III and III, which coincide within 0.3 mag. At F5 we also have an inversion of luminosity classes. It is clear that a (large) number of problems are related to class IV stars, so that the old saying that luminosity class IV is really well defined only for late-type stars regains its actuality. What will be needed is a careful cleaning up of the MK standards in this region. It is excellent that many standards were defined for the same type, so that it suffices to leave out some of them without having to redefine the whole system. We have looked into the question if the conflicting points come from a particular list of standards or can be attributed to high or low rotational velocities (vsini). The disagreeing objects are listed in Table 2, together with comments on their position in the HR diagram and with the name of the list of standards they came from and the rotational velocity. It can be seen that there exist stars with problems in all the three lists of standards (see Table 2), although C standards predominate. As far as rotation is concerned, nothing clear comes out either.

[FIGURE] Fig. 1. The main sequence standards. Abscissa: spectral type. Ordinate: absolute magnitude [FORMULA]. The dashed curve represents Schmidt-Kaler's main sequence.

[FIGURE] Fig. 2. All MK standards. Filled circle, class V; cross, class IV; empty diamond, class IV-V; empty square, class III; empty triangle right, class III-IV; filled triangle up, classes II and II-III; +, class I. Abscissa: spectral type. Ordinate: absolute magnitude [FORMULA].

[FIGURE] Fig. 3a and b. Luminosity class and absolute magnitude. Relation between luminosity class and absolute magnitude [FORMULA] at a given spectral type. The symbols for luminosity class are: filled circle, class V; cross, class IV; empty diamond, class IV-V; empty square, class III; empty triangle right, class III-IV; filled triangle up, class II. The length of the vertical line at each point denotes the error of [FORMULA].


Table 2. List of early type MK standards whose absolute magnitude disagrees with the luminosity class

The number of stars listed in Table 2 seems large, but there are only 11 disagreeing stars in a list of about 100. Similar inconsistencies were found when we examined the infrared region -there we found among 110 standards observed, 13 stars which presented some discrepancies between the blue and the infrared (Jaschek & Andrillat, 1997). All this shows that the list of MK standards needs some cleaning, by rejecting standards which behave either abnormally in the infrared or in which the luminosity class disagrees with the absolute magnitude.

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© European Southern Observatory (ESO) 1998

Online publication: January 16, 1998