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

Abstract

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 hOB vary from ~ 25 to 65 pc and of the surface density from ~ 6 to 80 stars per 105 pc2. 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 hOB ~ 45 pc and ~ 26 stars per 105 pc2. 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.