The Relationship of Spectral Type to Period among Variable Stars

VOL. 13, 1927

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THE RELATIONSHIP OF SPECTRAL TYPE TO PERIOD AMONG VARIABLE STARS By WALThR S. ADAMS AND ALFRED H. JOY MOUNT WILSON OBSZRVATORY, CARNJ&GII INSTITUTION OF WASHINGTON, PASAD1NA Read before the Academy April 25, 1927

Although it is well known that among the variable stars of the Cepheid class the spectral type is, in general, more advanced the longer the period of light variation, the faintness of many of the stars has prevented the derivation of any definite correlation between these two important quantities. Such a relationship, however, is vital in considerations of the luminosity of these stars and their connection with other types of variables. We have recently devoted considerable time with the 100-inch telescope to the observation of the radial velocities and spectral types of Cepheid variables, extending our results to stars of about the tenth magnitude visually. Sixty-four stars have been included, the observations in the majority of cases covering the maximum and minimum of light and some intermediate phases. In addition to the Cepheids we have secured spectrograms during past years of 27 short-period cluster-type variables which in the form of their light-curves resemble the Cepheids closely. There is, however, a marked interval between the longest period of the clustertype variables and the shortest period of the Cepheids within which no stars are known, and this gap separates the two classes definitely. The spectral tvpes of these stars have been determined by the aid of standard spectra selected from among the stars themselves, These have been chosen after a careful study of the spectral criteria of -the successive types of the Harvard system and are as follows:. SHORT-PERIOD VARIABLES

CEPHEID VARIABLES PHASE

PHASE

VX Herculis Z Can. Ven. RX Eridani

RS Bootis SW Androm.

Max. Max. Max. Max.

Min.

A3 A5 FO F2 F5

RT Aurigae RS Orionis SV Persei TT Aquilae

RS Puppis S Vulpeculae V Vulpeculae

Max. Max. Max. Min. Min. Max.

Min.

cF5 cF8 cGO cG4 cG6 eKO cK2

All of the spectrograms of the stars were compared with these standard spectra under a Hartmann spectrocomparator and estimates were made in the case of intermediate values. Since the variation of spectral type with phase of light is well known, the results in the case of each star were reduced to a median value by applying a positive correction of two units to the spectral type at maximum of light and a negative correction of the same amount at minimum. The final result for each star is the mean of

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all these reduced values. This simple method has the advantage of being based on a uniform system and is especially suitable when a differential comparison is desired. The relationship of spectral type to period for these stars proved of sufficient interest to suggest the possibility of the inclusion in the investigation of the other principal type of variable stars, the long-period red variables. AFlist of 44 of these stars was selected from among those observed by Dr. Merrill, and determinations of spectral type were made from standard spectra, Merrill's values and our own being assigned equal weight. The observations of these stars were all near maximum of light; and, since little is known regarding the spectrum of most of them at other phases, the spectral types have been used without correction. Their periods range between 90 and 444 days.

Sp*r-TI

'Npe

The spectral types of the individual stars are shown in figure 1 plotted against the logarithms of their periods of light variation. The principal features are the rather surprising way in which the majority of the stars fall on a smooth curve not very different from a straight line, and, on the other hand, the outstanding deviations of many of the short-period variables and of a few of the Cepheids. The short-period stars whose periods, with but few exceptions, group closely about 0.5 day show a considerable range in spectral type. Among the Cepheids there is a small group with the longest periods of any stars of this class, all of which have spectral types considerably less advanced than would be expected from the behavior ofithe majority. One of these stars, SX Herculis, has bright hydrogen lines, and Luyten's observations have indicated the presence of bands near minimum. The spectra of at least two of the others are somewAat peculiar. The periods of these stars all fall above the gap between 27

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and 42 days (within which few Cepheids are known at present), tend toward irregularity and in some cases are none too certain. It is possible that these stars form a separate group, perhaps related to the irregular M-type variables. The brightest and best known Cepheids show a correlation between spectral type and logarithm of period which is practically linear within the errors of observation. That the long-period variables and the mean values for the short-period cluster-type variables should be so nearly represented by the curve for the Cepheids is an unexpected feature of the results, and one which is probably significant in the interpretation of the relationship of these different classes.

HIGH-DISPERSION STELLAR SPECTRA AND SOME RESULTS OF A STUDY OF y CYGNI By WALTUR S. ADAMS AND ALFRZD H. Joy MOUNT WILSON OBSZRVATORY, CARNSGIS INSTITUTION OF WASHINGTON, PASAD}NA

Read before the Academy April 25, 1927

One of the chief advantages of the coud6 form of mounting as applied to a large telescope is the opportunity it affords for the use of much more powerful spectroscopic apparatus than could be employed if it were attached to a moving instrument. The original design of the 100-inch reflector at Mount Wilson provided for a spectrograph of large size to be mounted on a pier in an enclosed room south of the telescope, the light passing into the room through the hollow polar axis. ILast year a spectrograph of 15 feet focal length and 6 inches aperture was completed and installed, and a considerable number of photographs of the spectra of bright state has been obtained during the past few months. The spectrograph is of the auto-collimating type with a dense flint-glass prism of 530 angle through which the light is returned by a mirror. Two 10-inch plates of different photographic emulsions are used regularly in the plateholder, one for the blue, and the other for the green, yellow and red portions of the spectrum. The focal curve of the lens is such that with a suitable inclination of each plate within the holder good definition is obtained throughout the range from X 4100 to X 6700. The linear scale of these spectrograms, about 2.9 angstroms to the millimeter at Hy, is the largest ever used for stellar spectra, if exception is made of photographs of two or three of the brightest stars observed at Mount Wilson and elsewhere some years ago. The resolving power and definition are such that these spectra are especially suitable for studies of the intensity and identification of lines, and the long range of spectrum