A RECALIBRATION OF THE ABSOLUTE MAGNITUDES OF EARLY TYPE STARS CLASSIFIED ON THE MK SYSTEM

Galactic clusters and OB stars frequently provide the data for estimates of the numerical values of galactic parameters. Conclusions drawn from such values are numerically correct only to the extent that the distance scale of the specific type of objects used for the discussion is correct. In particular, conclusions based on agreement or disagreement of galactic parameter values derived from clusters and field OB stars are significant only to the extent that the distance scales of the clusters and the field stars are consistent. The distance scale of the galactic clusters is now generally based upon the unevolved main sequence, the zero point of which has been established from accurate kinematic parallax determinations of members of the Hyades cluster. Johnson has discussed practical details of fitting observed cluster main sequences to the standard zero-age main sequence.1 In the procedure one assumption is implicit: variations in chemical composition or related effects are not sufficiently great to produce any significant displacement between the unevolved main sequence of the observed cluster and the standard zero-age main sequence. In what follows we shall take it as axiomatic that this assumption is fulfilled. Further, we accept the standard zeroage main sequence as being well established in the Mv, B - V plane, and assert that it provides a means of determining accurate cluster distances. We ask : Is the distance scale of the OB stars, derived from the currently used absolute magnitudes of the MK spectral types and luminosity classes, consistent with that of the galactic clusters, based upon the standard zero-age main sequence?