Spectroscopic investigations of classical Cepheids and main-sequence stars in galactic open clusters and associations

I. Association Cas OB2 and the small-amplitude Cepheid SU Cassiopeae

¹ Astronomical Observatory of Odessa State University, Odessa 65014, Ukraine e-mail: val@deneb.odessa.ua
² Isaac Newton Institute of Chile, Odessa Branch
³ Special Astrophysical Observatory, Russian Academy of Sciences, Nizhny Arkhyz, Stavropol Territory, 369167, Russia e-mail: valenta@sao.ru; panchuk@sao.ru; ermak@sao.ru
⁴ Isaac Newton Institute of Chile, SAO RAS Branch

Corresponding author: I. A. Usenko, igus@deneb.odessa.ua

Received: 13 July 2000
Accepted: 17 November 2000

Abstract

The small-amplitude Cepheid SU Cas and four members of the association Cas OB2 (HD 16893, HD 17327a and b, HD 17443) were investigated, using high-resolution CCD spectra. The following results were obtained: 1) All these objects have the same metallicity values, close to that of the Sun; 2) Elemental abundance indicates that SU Cas is a post first dredge-up star with an age from 1 10⁸ to 1.45 10⁸ yr, and it is not crossing the Cepheid instability strip for the first time. The mean value of = 2.35 corresponds to pulsations in the fundamental tone, although errors in gravity estimations provide overtone pulsations. The questions about its pulsational mode and membership in Cas OB2 remained open; 3) HD 17327a is a slowly rotating HgMn-star with the highest helium content among such objects, while HD 16893 also has a manganese overabundance and might be classified as an Am-star; 4) HD 17327b and HD 17443 are rapidly rotating main-sequence stars, while HD 17443 has a helium content comparable with that of the Sun.