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We have revisited and updated the $uvby$ Str\"omgren colour and colour-indexdistributions of A, F and early G-type main sequence stars. For this aim, weselected 7054 dwarf stars along with 65 MK standard stars within the samespectral range but covering all luminosity classes. The standard stars wereselected following the MK mandate strictly, using spectra taken atclassification resolution recorded on photographic plates. We used the coloursof these stars to determine the effective temperature and surface gravity.After correcting for systematic offsets using fundamental parameters andconsidering a few exceptions, we find an one-to-one correspondence, among MKspectral types, Str\"omgren photometry, and their associated physicalproperties.  We have applied a principal component analysis to the mean Str\"omgrenindices for dwarf stars complemented by MK standards for higher luminosityclasses. We have used the projections to introduce three new photometricmetaindices, namely SM1, SM2, and SM3. We have defined a 3D-box, which allowedus to segregate dwarf stars from bright giants and supergiant stars, with theaid of the metaindices. Two of the planes show that the projections of dwarfsand supergiants are ordered by temperature; however, the temperature dependencefor the supergiants is not as well defined as for the dwarfs. Following the MKProcess, we were able to form an automatic classifier. We present someapplications and assigned synthetical spectral types. We suggest that ourmetaindices formalism allows the extension of Str\"omgren photometric outsideits original mandate (i.e., later types), without requiring the introduction ofadditional photometric filters.

monthly notices of the royal astronomical society

The Average Physical Properties of A-G Stars Derived from uvby-H$\beta$  Str\"omgren-Crawford Photometry as the Basis for a Spectral-Classification  Synthetical Approach