On the frequency and amplitude variations of the δ Scuti star CD−24 7599 (=XX Pyx) |
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| Authors: | GHandler A APamyatnykh WZima D JSullivan NAudard ANitta |
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| Institution: | Institut für Astronomie, Universitat Wien, Turkenschanzstrae 17, A-1180 Wien, Austria; Copernicus Astronomical Center, ul. Bartycka 18, 00-716 Warsaw, Poland; Institute of Astronomy, Pyatnitskaya Street 48, Moscow 109017, Russia; Physics Department, Victoria University of Wellington, PO Box 600, Wellington, New Zealand; Observatoire de Nice, BP 4229, F-06304 Nice Cedex 4, France; Institute for Astronomy, Madingley Road, Cambridge CB3 0HA; Department of Astronomy and McDonald Observatory, University of Texas, Austin, TX 78712, USA |
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| Abstract: | We present 132 h of new time-series photometric observations of the δ Scuti star CD?24 7599 acquired during 86 nights from 1993 to 1996 to study its frequency and amplitude variations. By using all published observations we demonstrate that the three dominating pulsation modes of the star can change their photometric amplitudes within one month at certain times, while the amplitudes can remain constant within the measurement errors at other times. CD?24 7599 also exhibits frequency variations, which do not show any correspondence between the different modes. The typical time-scale for the amplitude variations is found to be several hundred days, which is of the same order of magnitude as the inverse linear growth rates of a selected model. We find no evidence for periodic amplitude modulation of two of the investigated modes ( f 2 and f 3), but f 1 may exhibit periodic modulation. The latter result could be spurious and requires confirmation. The observed frequency variations may either be continuous or reflect sudden frequency jumps. No evidence for cyclical period changes is obtained. We exclude precession of the pulsation axis and oblique pulsation for the amplitude variations. Beating of closely spaced frequencies cannot explain the amplitude modulations of two of the modes, while it is possible for the third. Evolutionary effects, binarity, magnetic field changes or avoided crossings cannot be made responsible for the observed period changes. Only resonance between different modes may be able to explain the observations. However, at this stage a quantitative comparison is not possible. More observations, especially data leading to a definite mode identification and further measurements of the temporal behaviour of the amplitudes and frequencies of CD?24 7599, are required. |
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| Keywords: | techniques: photometric stars: individual: CD?24 7599 stars: individual: XX Pyx stars: oscillations δ Scuti |
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