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Studies of c-type RR Lyrae stars Mendes de Oliveira, Cláudia Lúcia

Abstract

Three studies of c-type RR Lyrae stars in the field of the Galaxy and in the Ursa Minor dwarf spheroidal galaxy have been completed. They include a study of the secular period behaviour of the star RU Psc; a determination of new radial velocities for 28 field c-type RR Lyrae stars; and a study of seven variables in the Ursa Minor dwarf galaxy, five of which are c-type RR Lyrae stars. RU Psc (period P=0[sup d].3904) is known for irregular cycle-to-cycle brightness variations and a possible 28 day Blazhko period (Tremko 1964). Using data from the literature, from 1956 to 1984, the long period behaviour of the star was analysed and, in particular, the possibility of RU Psc being a double-mode star was investigated. It is concluded that the peculiarities seen in the photometry of RU Psc are not due to secondary oscillations of the sort seen in RRd stars. Radial velocities for 20 field c-type RR Lyrae stars, whose radial velocities previously were unknown, are presented. The typical uncertainties are ± 15 km/s. Seven variable stars in the Ursa Minor dwarf galaxy have been studied using new photometry from 38 CCD frames. Combining these data with photographic photometry from van Agt (1967, 1968), and new photographic data from Nemec, Wehlau and Mendes de Oliveira, (1988, hereafter NWO), very accurate periods and colours were derived for these seven variables. With accurate mean colours, the blue and red edges of the instability strip could be set with little uncertainty. A distance modulus of (ra — M)[sub o]=19.23 ± 0.27 was derived for Ursa Minor from the mean magnitude of the c-type RR Lyrae stars. Analysing the new photographic data (NWO), four new anomalous Cepheids were found in Ursa Minor bringing to seven the total number of anomalous Cepheids now known. None of the c-type RR Lyrae stars in Ursa Minor, for which sufficient data are available, was found to be a double-mode RR Lyrae star.

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