The Cape rapidly oscillating Ap star survey - I. First results

Abstract

We present the first results of a systematic survey of the roAp phenomenon in the southern hemisphere. The scientific goals of the survey are; (i) to increase the number of known roAp stars so that the extent of the roAp phenomenon can be better determined; (ii) to apply the techniques of asteroseismology to these new roAp stars; (iii) to study the relationship between the roAp stars and other pulsating stars, particularly the |$\delta$| Scuti stars which lie in the same region of the HR diagram. Our initial goal is to obtain a complete catalogue of Strömgren uvby and Hβ photometry of the Ap SrCrEu stars listed in the four currently available volumes of the Michigan Spectral Catalogue of the HD stars. The survey currently extends as far north as |$\delta=-12$|˚. In this declination range we have identified 464 Ap SrCrEu stars which comprise our primary list of candidates. This list is supplemented by a further 71 candidates with |$\delta\gt-12$|˚ drawn from the list of Ap stars by Bidelman & MacConnell. Thus far we have obtained at least one |$u\nu by\beta$| observation of 51 per cent of our candidates. Stars are then selected for follow-up observations with high-speed photometry on the basis of the similarity of their |$b-y,m_1,c_1$| and β indices to those of the known roAp stars. The efficacy of our technique is such that we are able to present here five new rapidly oscillating Ap stars which were discovered on 10 photometric nights spanning one month. These are as follows, (i) HD 161459. which oscillates with several periods near 12.0 min although the severity of the aliases in our amplitude spectra is such that we are able only to determine the principal oscillation frequency |$\nu_1=1.39$| mHz. (ii) HD 190290 which shows oscillatory behaviour reminiscent of that seen in the rich multi-model pulsator HR 1217. We demonstrate the presence of oscillatioas with frequencies at |$\nu_1=2.27$| mHz (p = 7.34 min) and |$\nu_2=2.23$| (P = 7.47 min) and we argue that these must be two excited p-modes. Other oscillation frequencies are also indicated. (iii) HD 196470, an equatorial Ap SrEu(Cr) star which oscillates with a 10.79-min period, (iv) HD 193756, which oscillates with a 12.98-min period, (v) HD 218495, a hot (β = 2.87) roAp star which oscillates with a period of 7.44 min. There is also an indication of the presence of the first harmonic of the principal oscillation frequency.