The Luminosities of the Beta Canis Majoris Variables, the Zero Age Main Sequence and the Distance of the Sco-cen Association

The Hγ-luminosity calibration of Balona & Crampton is used to derive the zero age main sequence. This is about $${0}^\text{m}\cdot2$$ brighter than Blaauw's. The Hγ calibration is applied to the β CMa variables. The results are consistent with a theoretical P–L–C relation. The pulsation constant, Q, is $$0\cdot027\,\pm \,0\cdot001$$ implying that the variables pulsate in the first overtone. The variables fall in the S-bend region of evolution. Contrary to previous work the slope of the instability strip is not inclined to the S-bend locus in the $${M}_\text{bol}-\text{log}\,{T}_\text{e}$$ diagram. The calibration is consistent with the interferometric absolute magnitude of α Vir and the astrometric distances of β CMa stars in Sco–Cen. It is brighter by 0^m·3 than the absolute magnitude calibration of Jones & Shobbrook. There is a serious discrepancy between the mean astrometric distance modulus of Sco–Cen (5^m·78) and the Hγ modulus (6^m·46). It seems possible that the cut-off applied to eliminate poor proper motions may have significantly biased the astrometric distance. A difference of o^m·24 in the modulus of Sco–Cen derived from Hγ and from the Strömgren β index may be partly due to inclusion of a temperature term in the Hγ calibration but not in the β calibration.