A 21-cm investigation of the Pleiades

21-cm investigation of the Pleiades. Observations of the 21-cm line of hydrogen have been made within an area of 100 square degrees, centered on the Pleiades. The line profile in the direction of the Pleiades has two peaks, separated by 6 km/sec. We interpret these as being due to two concentrations of hydrogen with different radial velocities. On this basis, one concentration has a radial velocity -2 km/sec relative to the local standard of rest, the other +6 km/sec. Neither velocity can be due to galactic rotation, which is negligible in this direction, 1 = 134?5, b = - 22?3. The +6 km/sec radial velocity of one concentration agrees well with the mean radial velocity of the interstellar lines observed in the Pleiades (Adams, 1949). It is suggested that the interstellar lines are formed in this concentration. An analysis of all the observed profiles shows that the positive radial velocity concentration of hydrogen extends over the entire region observed, while the negative velocity concentration is present, in strength, only within 20 to 30 of the cluster. Furthermore, the radial velocity of the cluster is about the same as that of the hydrogen concentration with negative velocity. It seems therefore that the negative velocity concentration may be associated, both in space and in motion, with the cluster. The increase, near the cluster, in the amount of hydrogen with negative radial velocity is accompanied by a corresponding decrease in the amount of hydrogen with positive radial velocity. It would appear that the hydrogen associated with the Pleiades is there as the result of an interaction between the Pleiades and the interstellar gas. Adams, W. S. `949, Ap. J. Io~, 354. Harvard College Observatory, Cambridge, Mass.