Rotation Effects, Interstellar Absorption, and Certain Dynamical Constants of the Galaxy Determined from Cepheid Variables.
Data-The radial velocities of 156 variable stars of the ~ Cephei type are available for a determination of the rotation of the galaxy, the coefficient of interstellar absorp- tion, and certain galactic constants. Distribution-The ~ Cepheids are situated close to the plane of the galaxy and, as seen from the earth, they are well distributed in longitude. The known stars of this type are considerably clustered about the sun. Interstellar absorption.-The distances used are determined from apparent magni- tudes and absolute magnitudes derived from the period-luminosity curve. In order that the rotation effect may show a linear relationship with distance, it is found necessary to correct the distances by postulating the presence of interstellar absorption. The coefficient of absorption is estimated to be 0.85 mag/kpc and the absorbing material is assumed to be uniformly distributed about the galactic plane in a layer having a total thickness of 0.4 kpc. Rotational effects and galactic constants-For a study of galactic rotation the stars are divided into four distance groups. Solutions by Oort's method give for lo, the longi- tude of the direction to the center of rotation, 325?3 ± i ~3, and for A, the rotation effect at one kioparsec, 20.9 ± o.8 km/sec. The distance to the center is estimated to be io kpc. By the use of Bottlinger's formula the circular orbital velocity of the sun is found to be 296 km/sec and the period 207,000,000 years. Residual radial velocities-After taking out the effect of a solar motion of 20 km/sec and circular rotational effects, the average residual radial velocity is io.8 km/sec