Stellar Groups, VIII. The Structure of the Sirius Group

One hundred stars with ( U , V )-loci that pass within 4 km/sec of ( U , V ) = (−14.2, −0.4) are assigned to the Sirius group. The most note-worthy features of the resulting ( |MV, B-V| )-diagram are ( a ) the main sequence of the Sirius group is displaced from that of the Hyades group, |Δ MV| (Sirius-Hyades) = |+0m. 2| , and the main sequence stars with |+0m.3 B-V< + 0m.6| show an ultra-violet excess of |δ(U-B) = + 0m.04| with respect to the Hyades stars, ( b ) the small amplitude, short period light and velocity variable, δ Del, is a group member with luminosity and colour similar to those found for the prototype variable δ Scu in the Hyades group, ( c ) the presence of bluer main sequence stars and brighter G-type giants in the Sirius group indicates that this group is younger than the Hyades group, and ( d ) there are probably very many undiscovered group members near the Sun. The spatial distribution of the group members gives some indication of elongation in the direction of galactic rotation but because of the smallness of the motion and the resulting incompleteness of the group membership, no definite conclusion can be drawn. The mass-luminosity relation for the visual binaries in the group with |MV> +3^ m| deviates from the normal relation in the same way as the Hyades group binaries in this luminosity range; the Sun is one magnitude fainter than Hyades and Sirius group members of the same mass. The binaries with components brighter than |MV = +3^ m| , including the eclipsing binaries β Aur and α CrB, indicate that there may not be a unique mass-luminosity relation for group stars of this luminosity.