EDP Sciences
Free access
Volume 377, Number 1, October I 2001
Page(s) 156 - 160
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20011081

A&A 377, 156-160 (2001)
DOI: 10.1051/0004-6361:20011081

FN Aquilae -an unusual Cepheid with anomalous CNO abundances

I. A. Usenko1, 2, V. V. Kovtyukh1, 2 and V. G. Klochkova3, 4

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

(Received 23 May 2001 / Accepted 23 July 2001)

Spectroscopic analysis of the classical Cepheid FN Aql from three high-resolution CCD spectra near minimum and maximum light gives the following results: 1) Mean $T_{\rm eff}=5483$ K; log g=1.25 and $V_{\rm t}=4.0$ km s-1; 2) Metallicities derived for each phase are consistent with each other and close to the solar value; 3) The Cepheid is crossing the instability strip for at least the third time. 4) It has anomalously low carbon ( $[{\rm C}/{\rm H}]=-0.93$ dex) and nitrogen ( $[{\rm N}/{\rm H}]=+0.09$ dex) abundances, which is unusual for a Cepheid not crossing the instability strip for the first time. Since FN Aql is an IRAS object and candidate protoplanetary nebulae according to Volk & Kwok (1989), we assume that the star's peculiarities are caused either by envelope loss after the second dredge-up or that it is an unusual Cepheid for which existing nucleosynthesis and mixing models are unable to reproduce the CNO abundance pattern.

Key words: stars: abundances -- stars: variables: Cepheids

Offprint request: I. A. Usenko, igus@deneb.odessa.ua

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