Spectroscopic survey of field Type II Cepheids

Details Spectroscopic survey of field Type II Cepheids Spectroscopic survey of field Type II Cepheids

Jun 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998mnras.296..824v&link_type=abstract

Monthly Notices of the Royal Astronomical Society, Volume 296, Issue 4, pp. 824-838.

Astronomy and Astrophysics

Astronomy

20

Stars: Atmospheres, Stars: Oscillations, Cepheids

Scientific paper

A sample of relatively bright, short- and intermediate-period (P=1-10d) Type II Cepheids in the Galactic field have been observed spectroscopically with an intermediate-resolution (lambda/Deltalambda=11000) spectrograph. The wavelength region was 6500-6700A, including the Hα line and some photospheric iron lines. The signal-to-noise ratio (S/N) was usually between 50 and 100, depending on weather conditions and the brightness of target stars. Radial velocities were determined by cross-correlating the Cepheid spectra with those of selected IAU velocity standard stars having F-G spectral types. The internal error of the velocity determination process was calculated to be about 1 km s^-1. Hα emission and strong line splitting were observed in BL Her during the expansion phase, but no similar phenomenon was detected in any other stars in this programme, except for AU Peg which has an unusual Hα line showing a P Cygni-like profile. The velocity curve agrees well with recent CORAVEL measurements. The velocity gradients in Cepheid atmospheres are studied using the Hα minus metallic velocities. Similar data are collected from the literature. It seems that having large velocity differences (v_Hα-v_metal>40 km s^-1) is a characteristic feature of the very short-period (P<1.5d) and longer period (P>10d) Cepheids. Between these period regions the Cepheid atmospheres exhibit smaller velocity differences. Most of the Type II Cepheids observed in the present study fall into this latter category. There might be a tendency for classical Cepheids of intermediate period to have larger maximum velocity differences.

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