The VELA Star Cloud. III. The B8 to A0 stars and interstellar reddening.

Intermediate-band and Hβ observations of 360 HD stars of types B8 - A0 in a 10°×10° region in Vela, and all the CPD stars in a small adjacent region, are discussed. The interstellar absorption in this and in a neighboring region can be understood as a combination of three absorbing clouds with A_V(1 kpc) = 1.50, 0.50, and 0.17 mag. The density of B8 to A0 stars is near 13×10^-5pc^-3 and slightly increases at the distance of the "Vela Sheet" (350 - 450 pc), where these objects are apparently coeval at 2 to 3×10⁸yr. The stars of mid B type and earlier in the Sheet are considerably younger at 2 to 5×10⁷yr, a situation that is also obtained in the Pleiades and α Persei clusters that border the Taurus dark cloud. The major difference in the two environments is the presence of numerous T Tauri stars in Taurus. The B8 to A0 stars more distant then the Vela Sheet (550 - 700 pc) show a very large range of ages. Several clusters in the Vela region are discussed: Cr 173, NGC 2451, NGC 2547, and Tr 10.