New Estimates of the Scale Height and Surface Density of OB Stars in the Solar Neighborhood

Galactic latitudes and photographic magnitudes for large samples ( N ∼ 2000–4000) of OB stars within 10° of the Galactic plane and 4 kpc of the Sun are used to estimate the exponential scale height and surface density of these objects assuming a barometric extinction model. Depending upon the scale height and differential extinction of the absorbing layer, estimates of the OB-star scale height h OB vary from ∼ 25 to 65 pc and of the surface density from ∼ 6 to 80 stars per 10 5 pc 2 . Reasons for the wide spread in this latter figure are discussed. For an absorbing layer of scale height σ = 100 pc and B -band extinction of 2 mag kpc -1 , we find h OB ∼ 45 pc and ∼ 26 stars per 10 5 pc 2 . The total Galactic population of these stars is estimated at ∼ 60,000, and their formation rate at one every 40 years. Estimates of the scale height are found to be essentially independent of the assumed mean absolute magnitude for OB stars.