Measurement of acoustic glitches in solar-type stars from oscillation frequencies observed by Kepler

A. Mazumdar, M. J. P. F. G. Monteiro, J. Ballot, H. M. Antia, S. Basu, G. Houdek, S. Mathur, M. S. Cunha, V. Silva Aguirre, R. A. Garcia, D. Salabert, G. A. Verner, J. Christensen-Dalsgaard, T. S. Metcalfe, D. T. Sanderfer, S. E. Seader, J. C. Smith, W. J. Chaplin

Published 2013-12-17Version 1

For the very best and brightest asteroseismic solar-type targets observed by Kepler, the frequency precision is sufficient to determine the acoustic depths of the surface convective layer and the helium ionization zone. Such sharp features inside the acoustic cavity of the star, which we call acoustic glitches, create small oscillatory deviations from the uniform spacing of frequencies in a sequence of oscillation modes with the same spherical harmonic degree. We use these oscillatory signals to determine the acoustic locations of such features in 19 solar-type stars observed by the Kepler mission. Four independent groups of researchers utilized the oscillation frequencies themselves, the second differences of the frequencies and the ratio of the small and large separation to locate the base of the convection zone and the second helium ionization zone. Despite the significantly different methods of analysis, good agreement was found between the results of these four groups, barring a few cases. These results also agree reasonably well with the locations of these layers in representative models of the stars. These results firmly establish the presence of the oscillatory signals in the asteroseismic data and the viability of several techniques to determine the location of acoustic glitches inside stars.