On the double-mode RR Lyrae variables of the Sculptor dwarf galaxy

Free access article

Issue		A&A Volume 375, Number 2, August IV 2001


Page(s)		469 - 475
Section		Formation, structure and evolution of stars
DOI		http://dx.doi.org/10.1051/0004-6361:20010880

A&A 375, 469-475 (2001)
DOI: 10.1051/0004-6361:20010880

On the double-mode RR Lyrae variables of the Sculptor dwarf galaxy

G. Kovács

Konkoly Observatory, PO Box 67, 1525, Budapest, Hungary

(Received 22 February 2001 / Accepted 12 June 2001)

Abstract
Frequency analysis of more than 300 stars of the OGLE database on Sculptor galaxy has led to the discovery of 18 double-mode RR Lyrae (RRd) variables. This yields a 20% incidence rate for double-mode pulsation among the variables previously classified as first overtone RR Lyrae stars in this galaxy. Most of the RRd stars cover the period range of $0\fd47< P_0< 0\fd49$ but there are two stars with longer periods of ${\approx} 0\fd54$ . All variables fit well in the pattern of the $P_0\rightarrow P_1/P_0$ diagram, spanned by the RRd stars of the Galactic globular clusters and those of the Large Magellanic Cloud (LMC). It follows from our previous investigations that the luminosities and masses of the RRd stars in Galactic globular clusters and in the LMC are almost independent of metallicity. By assuming that the Sculptor RRd variables also obey this rule, with the aid of the pulsation equations we estimate their metallicities. For most of the stars we get ${\rm [Fe/H]}\approx-1.5$ , which is the same value as that obtained from a semi-empirical method for the average metallicity of the fundamental mode (RRab) stars. Two RRd stars have considerably lower metallicities, but even those are within the range corresponding to the RRab stars. The narrower metallicity range of the RRd stars is in agreement with their observed luminosity range, which is about three times smaller than that of the RRab stars.

Key words: stars: fundamental parameters -- stars: variables: RR Lyr -- stars: oscillations -- stars: horizontal branch -- globular clusters: -- galaxies: individual: Sculptor dwarf spheriodal

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