Shction 111., 1909. [ 215 ] Tbans. R. S. 0. 



XIII. — The Spectroscopic Binary /3 Orionis 

 By J. S. Plaskett, B.A. 

 Presented by Dr. W. F. King. 



It was mentioned in my report to tlie Chief Astronomer of 1907-1908, 

 under a description and discussion of the effect of slit width on the errors 

 of setting, that this star showed such a difference in the mean velocities 

 obtained on two nights (mean of 10 plates on March 20, 24.9 km.; mean 

 of 12 plates on Marcli 24, 20.6 km.) as to lead to a strong suspicion of 

 the variability of its velocity. This suspicion was strengthened by plates 

 obtained on other nights up to April 13, 1908, and it was decided on 

 account of its brightness and its interesting history to follow it closely 

 a? soon as it again came into position v/here it could be observed. 



The radial velocity of ^Orionis was first determined at Potsdam 

 by Vogel and Scheiner ^ in the years 1888-1891 in the beginning of 

 photographic determinations of radial velocity. From their measures of 

 the 14 plates secured, velocities varying between about + 3 and + 34 

 kms. per second were obtained. They suspected a variation in the star's 

 velocity due to orbital motion, but were unable to obtain evidence of its 

 periodicity, and the accuracy of these early measures was scarce!}^ sufh- 

 ciently high to decide the question. The next published measures of the 

 star's velocit}^ were by Frost and Adams ^ from plates obtained in 1901- 

 1902. They obtained values ranging between + 14.9 and + 23.4 km., 

 but they attributed this range to the character of the lines in the star's 

 spectrum and concluded that their results showed no indication of vari- 

 ability in velocity. The measures of 5 plates of /? Orionis obtained at 

 the Lick Observatory ^ indicate a range of 10 km. from + 15 to + 25 

 km. in its velocity, but Campbell and Curtis in discussing those mea- 

 sures attribute this range to the small number of lines available, to their 

 poor quality, and to overexposure of some of the negatives. They con- 

 sider that proper exposure would considerably reduce the observed range 

 and conclude that their results do not give any evidence of variability 

 of velocitv. However, a recent personal communication from Prof. 

 Campbell informs me that they have suspected variation but, owing to 

 press of their regular programme, have not followed up the matter. 



There seemed to be no question of the smallness of the range in 

 velocity, if any, and it was evident that the only hope of obtaining any- 

 thing definite, considering tliis and the fairly large accidental errors of a 

 velocity determination owing to the character of the lines of the spec- 



1 Potsr'am Publications Band ATI, p. 146. 



- Publications of the Yerkes Observatory 2, 61. 



s Lick Observatory Bulletin, No. 70. 



