Short Period Type II Cepheids whose Spectra Exhibit Carbon Star Characteristics

Ultraviolet spectra with the International Ultraviolet Explorer of three Type II Cepheid variable stars which exhibit carbon stars characteristics are obtained

Blazhko Effect

The cause of the Blazhko effect, the long-term modulation of the light and radial velocity curves of some RR Lyr stars, is still not understood. The observational characteristics of the Blazhko effect are discussed. Some preliminary results are presented from two recent campaigns to observe RR Lyr, using the International Ultraviolet Explorer along with ground-based spectroscopy and photometry, throughout a pulsation cycle, at a variety of Blazhko phases. A set of ultraviolet light curves have been generated from low dispersion IUE spectra. In addition, the (visual) light curves from IUE's Fine Error Sensor are analyzed using the Fourier decomposition technique. The values of the parameters Psi(sub 21) and R(sub 21) at different Blazhko phases of RR Lyr span the range of values found for non-Blazhko variables of similar period

Cepheid temperature and the Blazhko effect

Two separate research projects were covered under this contract. The first project was to study the temperatures of Cepheid variable stars, while the second was a study of the Blazhko effect in RR Larae, both of them using IUE data. They will be reported on separately, in what follows

The anomalous Cepheid XZ Ceti

XZ Ceti is the only known anomalous Cepheid in the Galactic field. Being the nearest and brightest such variable star, a detailed study of XZ Ceti may shed light on the behaviour of anomalous Cepheids whose representatives have been mostly detected in external galaxies. CCD photometric and radial velocity observations have been obtained. The actual period and amplitude of pulsation were determined by Fourier analysis. The long time scale behaviour of the pulsation period was studied by the method of the O-C diagram using the archival Harvard photographic plates and published photometric data. XZ Ceti differs from the ordinary classical Cepheids in several respects. Its most peculiar feature is cycle-to-cycle variability of the light curve. The radial velocity phase curve is not stable either. The pulsation period is subjected to strong changes on various time scales including a very short one. The ratio of amplitudes determined from the photometric and radial velocity observations indicates that this Cepheid performs an overtone pulsation, in accord with the other known anomalous Cepheid in our Galaxy, BL Boo (V19 in the globular cluster NGC 5466). Continued observations are necessary to study the deviations from regularity, to determine their time scale, as well as to confirm binarity of XZ Ceti and to study its role in the observed peculiar behaviour.Comment: 7 pages, 4 figures. accepted for Astron. Astrophy

The Unusual Pulsating Variable XZ Ceti

The peculiar pulsating variable XZ Ceti has a period of 0.8231 day, suggesting that it is a Bailey type ab RR Lyrae star, but it has a light curve shape and amplitude which is like that of the Bailey type c variables. Fourier decomposition analysis verifies that the parameters of its light curve match those of the type c variables, except for its unacceptably long period. We have obtained a new photometric light curve which verifies this long period. We have also obtained energy distributions from spectrum scans, which are used to derive a temperature and approximate surface gravity of this star. These results are used to construct pulsation models so as to attempt to identify the nature of XZ Ceti. The models suggest the possibility that XZ Ceti is an anomalous Cepheid or, perhaps, an overtone BL Her star

The Light Curves of RR Lyrae Field Stars

Fourier decompositions have been made of the light curves of a large sample of RR Lyrae field stars. The coefficients have been tabulated. Following the scheme of an earlier investigation of classical Cepheids, certain combinations of the low-order coefficients - φ21, R21, and φ31 - are plotted against period. The Bailey-type c pulsators stand out from the type ab stars, particularly on the R21 plot which is found to be a more sensitive discriminator of Bailey type than is the traditionally employed amplitude-period diagram. We compare the RR Lyrae plots of φ21, R21, and φ31 with those previously obtained for classical Cepheids. It is noted that, while the Cepheid plots display a tightly defined progression with period, reflecting the influence of a modal resonance, in the RR Lyrae case there is much more scatter. However, some evidence is shown to exist for a Cepheid-like progression appearing among the longer period RR Lyrae pulsators and culminating in the unique small-amplitude variable XZ Ceti. New observations will be required to confirm the reality of such a progression

The Structural Properties of Cepheid Velocity Curves

Fourier decompositions are performed on the velocity curves of 11 classical Cepheids. The progression of curve shape with period is described in terms of combinations of the lower order Fourier coefficients. These quantities are shown to change with pulsation period in a manner similar to that already demonstrated for Cepheid light variations (Simon and Lee). We recommend further velocity observations, particularly in the period range 10 ≤ P ≤ 16 days

Cephid Velocity Curves

Fourier decompositions are made for~ number of observed velocity curves of classical Cepheids. The observations are fit with Fourier series of the form vobs = A0 - Ai sin(iwt + ϕi) , where the index i runs from 1 to 4 or 1 to 8 depending upon the requirements of the data. Although the sample of stars is small, we show that the Hertzsprung progression expresses itself quantitatively in terms of the low-order Fourier coefficients, particularly the quantity ϕ21 = ϕ2 - 2ϕ1. This result complements a similar finding for the light curves. When the Fourier decompositions of the velocity curves are compared with those of some theoretical models (Vermury and Stothers 1978, Ap. J. 225, 939), new evidence is uncovered favoring a resonance explanation for the bump sequence