New whole earth telescope observations of cd-247599 : steps towards delta scuti star seismology

Abstract

92 h of new Whole Earth Telescope observations have been acquired for the δ Scuti star CD -24 7599. All the seven pulsation modes reported by Handler et al. are confirmed. However, significant amplitude variations which are not caused by beating of closely spaced frequencies occurred within two years. Analysing the combined data of both WET runs, we detect six further pulsation modes, bringing the total number up to 13. We also examine our data for high-frequency pulsations similar to those exhibited by rapidly oscillating Ap stars, but we do not find convincing evidence for variability in this frequency domain. From new colour photometry and spectroscopy we infer that CD-24 7599 is a hot mainsequence δ Scuti star with approximately solar metallicity and ν sin i = 52 ± 2 km s -1. We cannot yet propose a definite pulsation mode identification, but we report the detection of a characteristic frequency spacing between the different modes. We ascribe it to the simultaneous presence of l = 1 and l = 2 modes of consecutive radial order. A comparison of this frequency spacing with frequencies of solar-metallicity models, as well as stability analysis, allows us to constrain tightly the evolutionary state of CD - 24 7599. It is in the first half of its main-sequence evolution, and has a mass of 1.85 ± 0.05 M̛ and a mean density of p = 0.246 ± 0.020 P̛. This yields a seismological distance of 650 ± 70 pc, which i~ as accurate as distance determinations for 0 Scuti stars observed in clusters. Most of the pulsation modes are pure p modes of radial order k = 4-6, but the gl mode of l = 2 is likely to be excited and observed as well. Since a significant contribution to this mode's kinetic energy comes from the outer part of the convective core, CD-247599 becomes particularly interesti1)g for testing convective overshooting theories.