Why the peculiar δ Scuti star HD 187547 is a superstar

V. Antoci, M. S. Cunha, G. Houdek

Abstract
The δ Scuti pulsators occupy a region in the Hertzsprung-Russell diagram where several physical processes occur: the subsurface convection layers change from being deep and vigorous to being shallow and ineffective to transport energy. This transition has a large impact not only on pulsational stability but also on stellar evolution, activity, transport of angular momentum, mixing processes, etc.. It is therefore of great interest to understand how exactly the stellar structure changes with increasing temperature and mass. Theoretical models (Houdek et al. 1999; Samadi et al. 2002) predicted that the convection in the outer layers of δ Scuti stars is still efficient enough to excite solar-like oscillations. The Kepler target, HD 187547 (a.k.a. Superstar), was the first δ Scuti star to suggest that solar-like oscillations are indeed present in this type of stars (Antoci et al. 2011). There were several reasons to conclude that HD 187547 is a δ Scuti/solar-like hybrid pulsator. (1) The peaks at high frequencies are modes of pulsations approximately equidistantly spaced, as expected for high radial order pressure modes; these peaks are not combination frequencies as it is sometimes observed in δ Scuti stars. (2) The opacity mechanism cannot excite a continuous frequency range as observed in HD 187547 (Pamyatnykh 2000). (3) The identification as an Am star consistent with the low v sini, makes it very unlikely to be a δ Scuti/roAp hybrid, because strong large-scale magnetic fields, a necessity for roAp pulsators, have never been detected in Am stars (Auriere et al. 2010). (4) Although a large number of Am stars are found in binary systems, we find no evidence in the observed spectra for a companion, i.e. no significant RV shift over 170 days can be detected and the absorption lines can perfectly be reproduced by assuming a slowly-rotating chemically peculiar Am star. This means that the peaks at high frequencies are unlikely to be from a companion, because such a star would be an A or F-type star and would be visible in the spectrum and this is not the case. (5) The large separation Δν, which is the frequency difference between consecutive radial orders of the same degree l, predicts the frequency of maximum power (ν_max) to be exactly where the mode with the highest amplitude in the supposed stochastic region is observed. Note that the Δν and ν_max scaling relation is valid only for stochastic oscillations (e.g. Huber et al. 2011). (6) The statistical properties (Chang & Gough 1998) of the modes interpreted to be solar-like were very similar to what is expected for stochastic signals in general.

New Advances in Stellar Physics: From Microscopic to Macroscopic Processes
(Eds.) G. Alecian, Y. Lebreton, O. Richard, G. Vauclair

Cambridge University Press
Vol. 63, Page 135
2013

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