SPECTROSCOPIC SURVEY OF FIELD TYPE-II CEPHEIDS

A sample of relatively bright, short-and intermediate-period (P = 1-10 d) Type II Cepheids in the Galactic field have been observed spectros copically with an intermediate-resolution (lambda/Delta lambda = 11000 ) spectrograph. The wavelength region was 6500-6700 Angstrom, includin g the H alpha line and some photospheric iron lines, The signal-to-noi se ratio (S/N) was usually between 50 and 100, depending on weather co nditions and the brightness of target stars. Radial velocities were de termined by cross-correlating the Cepheid spectra with those of select ed IAU velocity standard stars having F-G spectral types. The internal error of the velocity determination process was calculated to be abou t 1 km s(-1). H alpha emission and strong line splitting were observed in BL Her during the expansion phase, but no similar phenomenon was d etected in any other stars in this programme, except for AU Peg which has an unusual H alpha line showing a P Cygni-like profile. The veloci ty curve agrees well with recent CORAVEL measurements. The velocity gr adients in Cepheid atmospheres are studied using the H alpha minus met allic velocities. Similar data an collected from the literature, It se ems that having large velocity differences (nu(H alpha) - nu(metal) > 40 km s(-1)) is a characteristic feature of the very short-period (P < 1.5 d) and longer period (P > 10 d) Cepheids. Between these period re gions the Cepheid atmospheres exhibit smaller velocity differences. Mo st of the Type IT Cepheids observed in the present study fall into thi s latter category, There might be a tendency for classical Cepheids of intermediate period to have larger maximum velocity differences.