Spectral Variations of 121 Faint Long-Period Variable Stars.

Spectral variations of 121 long-period variables of spectral type M in Groningen-Palomar Field 2 (a=l7hO7rn,6=~l9?4:l~~40 b11=+120) were studied. Schmidt objective-prism spectra covering the interval from 6800-8800 A, with a dispersion of 1800 A/mm, were used. Spectral types of each star were determined at from 7 to 18 distinct epochs, with the standard error of a single classification being +0.7 subtypes for classes M2 to M6, and +0.4 subtypes for classes M6 and later. The variables having photographic light ranges greater than two magnitudes show spectral changes averaging four or five subtypes, and all reach class M6.5 or later. Variables having ranges of 1.5 mag or less show no significant spectral variation. Periods were determined for 60 of the larger- amplitude variables. For periods longer than 200 days the resulting period-spectrum relation, using spectral type at maximum, agrees well with the relation for more nearby stars (Keenan, Astrophys. J. Snppl. 13, 333, 1966: Campbell and Cannon, Harvard Bull. No. 862,1928). For periods from 150 to 200 days the maximum spectral types are earlier than those of the Keenan and Harvard relations, although there is considerable scatter. Preston (Publ. Astron. Soc. Pacific. 79,125,1967) has found that in this period interval there is a correlation at a given period between the space velocity and spectral type at maximum, stars with higher velocities having earlier maximum spectral types. The majority of the variables in my study are at distances of 4-7 kpc with z distances of 800-1500 pc. At such heights many of the variables may be of intermediate Population II, with higher velocities than nearby variables, as is indicated by their earlier maximum spectral types. The period vs spectral type at minimum relation is nearly parallel to the relation for maximum type. The range of spectral variation and temperature variation are therefore almost independent of period. The spectral class varies, on the average, from Ml to M6.5 at P= 150 days and from M6 to M10 at P=350 days. Plots of spectral type vs phase for 39 variables show that the variables remain near minimum spectral type during much of their light cycle.