Stellar Radial-Velocity Programs of the Lick Observatory

STELLAR RADIAL-VELOCITY PROGRAMS OF THE LICK OBSERVATORY George H. Herbig Lick Observatory, University of California The first director of the Lick Observatory, Edward S. Holden, fully recognized the importance of extensive radial-velocity infor- mation for the determination of the motion of the solar system in space. He regarded it as "the main [spectroscopic] research to be undertaken with the great telescope." In 1890, he drew up an extensive program of observations to be carried out visually at the 36-inch refractor by J. E. Keeler. The contribution of Holden to- ward starting radial-velocity work at the Lick Observatory is not generally appreciated. It is true that he played an administrative rather than a participating role, but it was an essential contribution nonetheless. Holden's attitude is indicated by the letter of instruction from him to Keeler, dated September 15, 1890, which reads in part : My dear Mr. Keeler : It is now time to begin the principal spectroscopic work for which the great telescope was designed; namely the observations of the motion of stars in the line of sight for the determination of the motion of the solar system in space. Your observations on the nebulae have shown that you can do this work better than it is done elsewhere . Such a work as this done properly will require several, perhaps many years, and it ought to be thoroughly thought out before it is begun and an adequate programme adopted and adhered to. I have therefore drawn up the following prelimi- nary scheme for you to look over and criticise . You have what I think to be by far the most important work with the great telescope, and I promise you on my part every official and personal assistance which it is in my power to give. Sincerely yours, E. S. Holden It is obvious to us now that the very extensive program pro- posed by Holden was unrealistic, considering the limitations of the visual method, but the approach was a sound one in the light of modern knowledge. Work by Keeler in 1890-91 with a spectroscope containing a plane Rowland grating gave good results for the radial velocities of three first-magnitude stars as well as for fourteen gaseous nebu- 191 © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 192 GEORGE H. HERBIG lae. Keeler's velocities were incomparably superior to those obtained by earlier visual observers, whose results seem to bear little relation to the true stellar velocities. Keeler's procedure, in observing the stars, consisted of measuring with a micrometer the velocity shift of the D lines of sodium in the star with respect to the same lines in a comparison spectrum. The visual observations were exceedingly difficult even under optimum conditions, but it was planned, nevertheless, to extend the work to all feasible objects. This extended program was to be carried out by Keeler with the assistance of W. W. Campbell (who had helped Keeler in the earlier work on a voluntary basis) and Henry Crew, both newly appointed to the staff in the summer of 1891. Keeler, how- ever, left the Lick Observatory in mid-1891 to assume the direc- torship of the Allegheny Observatory, and following the departure of Crew a year later, the work fell into Campbell's hands. In 1891 the work of Scheiner and Vogel of Potsdam on the determination of radial velocities by the photographic method ap- peared. The advantages of photography over the eye at the telescope were very apparent : the Potsdam observers, with an 11-inch refractor, had reached fainter stars than had Keeler, visually, with the 36-inch. The probable error of a single Potsdam observation was given as ±2.6 km/sec from the internal agreement of the observations, but comparison with modern velocities demon- strates that the Potsdam results were less dependable than this figure would indicate. Campbell realized that the day of visual work in astronomical spectroscopy was over, and he attempted to adapt Keeler's grating spectroscope to photography, but he found it to be so subject to mechanical flexure that the attempt was abandoned. A limited amount of work was done with it, with prismatic dispersion, until it was completely superseded in 1895 by the original Mills spectrograph. Not long thereafter, Campbell's interests, which had until then ranged over a broad field of astronomical subjects, con- verged on the problems of radial velocities. The Mills spectrograph was reminiscent, outwardly, of the Potsdam instrument, but in its details and optical arrangement it represented a considerable improvement. Campbell was generally responsible for the design, but the spectrograph also owed a © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System LICK RADIAL-VELOCITY PROGRAMS 193 great deal to Keeler, who initiated Campbeirs spectroscopic train- ing. The dispersion, produced by three dense flint-glass prisms, was 12.5 A/mm at Hy. The results, after some initial difficulties had been located and removed, fully justified the design : the probable error of a single observation of a bright star with good lines was reduced to ±0.5 km/sec, and for bright stars with the best lines, the probable error was even smaller.1 With this instrument Campbell and his colleagues ushered in a new era in radial-velocity work. One would certainly expect new developments in any field in which the accuracy of observation is suddenly improved by an entire order of magnitude, and in which the number of objects observable with such precision is increased perhaps fiftyfold. Successful as the Mills spectrograph proved to be, after it had been in operation for some time Campbell, Wright, and others at Lick recognized that increased efficiency would result if several modifications could be made. For example, the mechanical con- struction was such that a certain amount of internal flexure was present ; furthermore, the spectrograph operated in a wave-length region (centered at λ 4340) at which the transmission of the 36- inch objective and the flint-glass prisms was not high. The New Mills spectrograph, a completely new instrument except for the prisms, was put into operation in 1903, and the original Mills was retired, except as will be mentioned later. The New Mills incor- porated a superior support system, based on a suggestion of Wright's, by which internal flexure was reduced to undetectability. · The central ray was now a-t λ 4500, a choice that involved a com- promise between the opposing requirements of the failing transmission of the optics to shorter wave lengths, the decreasing sensitivity of ordinary photographic emulsions to longward, and consideration of the spectral lines that fell in that part of the spectrum in stars of various spectral types. Other mechanical improvements were also made. The dispersion of the new spectrograph is 10.9 A/mm at λ 4500, somewhat higher than the original Mills at Η γ, but no large improvement in the quality of the velocities of bright stars resulted on this score by replacement of the original Mills. Instead, the increased mechanical and ther- mal stability and somewhat higher efficiency of the new instrument enabled essentially the same standards of accuracy to be extended © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 194 GEORGE H. HERBIG to fainter stars. At the present time, through the use of coated optics and faster plates, the precision obtainable fifty years ago with the original Mills only for first- and second-magnitude stars is now attained for sixth-magnitude stars with the New Mills. Campbell had realized in the mid-'nineties, when the radial- velocity work was getting under way at Mount Hamilton, that regardless of how well the velocities of the stars observable from Lick were determined, the investigation of the solar motion would be seriously handicapped by the lack of data from the inaccessible southern third of the sky. In 1900, the necessary funds were obtained from D. O. Mills of San Francisco and New York, donor of the Mills spectrographs, to set up, equip, and maintain for two years an observing station in the southern hemisphere for the express purpose of determining the radial velocities of the brighter southern stars. In 1903, the expedition, under the direction of Wright, established an observatory near Santiago, Chile. The only telescope was a 37-inch Cassegrain reflector that was equipped, at first, with a three-prism spectrograph similar to the new Mills ; later, one- and two-prism instruments were provided. It was soon apparent that an extension of the life of the expedition was desirable ; support was provided by Mr. Mills until his death in 1910, and by his son, Ogden Mills, until 1927. From that year until 1929, the station was supported by gifts from friends of the Lick Observatory. In 1929, the station and its equipment were sold to the Catholic University of Chile. During the twenty-six years of its operation, about 11,000 spectrograms of planets, stars, and gaseous nebulae were obtained. The major velocity program at the two observatories was the determination of the radial motions of the brighter stars. For a period in the early years of the program, the limiting magnitude was taken to be 5.01 in the northern sky (the authority for the magnitudes was the Revised Harvard Photometry), but this was later extended to magnitude 5.51 for the entire sky. From 1896, when the program was initiated, until 1926, when the last spectrograms were taken, some 15,000 plates for the determination of radial velocity were obtained at Mount Hamilton and about 10,000 in Chile.

Load more