Spectrographic observations of short-period Cepheids.

Abstract

view Abstract Citations (1) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Spectrographic observations of short-period Cepheids. Struve, O. Abstract During the summer of 1947 a series of 390 spectrograms of RR Lyrae, whose principal period is 13.6 hours, was obtained at the Mc- Donald Observatory in order to study the effect upon the velocity curve of the long period of 41 days, which is known from the light curve. The spectrograms were measured by Dr. A. Blaauw and the writer. On June 5 and again on July 17 the velocity curve was retarded by 20 minutes-a conspicuous phenomenon for a velocity curve in which a change of 2okm/sec takes place in 24 minutes. Approximately half- way between, on June 26, the velocity curve was advanced by 20 minutes. This shift along the time-axis of the short-period curve agrees exactly with the shift of the light-curve as observed by Walraven at Leiden in 1947. After the short-period velocity curve was corrected for this oscillation it disclosed only relatively insignificant, though probably real, changes in shape, within the ~I-day cycle, when the metallic lines were used. The hydrogen lines, H~, H~ and H~ however, differ in a remarkable manner from the metallic lines. For them the descending branch of the curve is steeper and the range larger than for the metallic lines. The minimum velocity for H is about - I I5km!sec while for the metals it is about - Iookm/sec. The higher members of the Balmer series agree with the metallic lines. In the case of H~ alone, the minimum velocity is nearly - I2okm!sec, so that we are concerned with a phenomenon which depends upon the quantum number of the upper state. The maxima are less strikingly different. The H velocity curves show an interesting connection with the 41-day period: when the velocity curve is most retarded, that is, near June 5 and July 17, the curve rises to a peaked maximum at about -25km!sec. When the velocity curve is most advanced the maximum is about -45km!sec. The peaked curves correspond to the greatest intensity of the sharp H emission lines which are observed at about median decreasing velocity. It is possible that the sharp H absorption line which dominates the spectrum at minimum light continues to increase in velocity until it fades out, while at the same time a broad line with Stark wings is already present at a shorter wave length. The transition from peaked maximum to very nearly minimum velocity occurs in about 40 minutes. The roundeff minimum of the A-type lines is probably real, in the sense that the deeper layers at first have a moderate velocity of recession, reaching minimum at phase o.4oP. But there is no such rounding off at the peaked maxima, so that it would seem that the fading line from the upper level continues to show inward acceleration until it has disappeared. When the emission lines are absent, the transition is a more gradual one since there are then no peaked maxima. During the present summer Dr. A. van Hoof and the writer have been observing the cluster- type variable XZ Cygni-suggested to us by Dr. E. C. Yowell, who has made an exhaustive study of its light curve. There are also two periods, a shorter one of I I hours, and a longer one of the order of 57 days. These observations are now being discussed. Thus far, we have found no definite evidence of bright H lines, but near the phase of median increasing brightness there are indications of two H components. The red component is sharp and becomes rapidly weaker, while the violet component is broad and at first very shallow, becoming rapidly strong enough to obliterate the weakening sharp component. In this respect we have an exact analogue of RR Lyrae. We conclude that during the rise in brightness the motions are not uniform throughout the pulsating atmosphere and that in both stars there are indications of the simultaneous existence of an inner, expanding, and an outer, contracting layer. Verkes Observatory, Williams Bay, Wis., and McDonald Observatory, Fort Davis, Tex. Publication: The Astronomical Journal Pub Date: October 1948 DOI: 10.1086/106192 Bibcode: 1948AJ.....54...50S full text sources ADS |