KIC 1718290: A Helium-rich V1093-Her-like Pulsator on the Blue Horizontal Branch

Østensen, R.H., Degroote, P., Telting, J.H., Vos, J., Aerts, C., Jeffery, C.S., Green, E.M., Reed, M.D., Heber, U.

Figure 1: (a) The discovery spectrum of KIC 1718290. (b) SED fit to broadband photometry. (c) Line-profile fit to the WHT spectrum.

Abstract

We introduce the first g–mode pulsator found to reside on the classical blue horizontal branch. One year of Kepler observations of KIC 1718290 reveals a rich spectrum of low-amplitude modes with periods between 1 and 12 hr, most of which follow a regular spacing of 276.3 s. This mode structure strongly resembles that of the V1093 Her pulsators, with only a slight shift toward longer periods. Our spectroscopy, however, reveals KIC 1718290 to be quite distinct from the sdB stars that show V1093 Her pulsations, which all have surface gravities higher than log g = 5.1 and helium abundances depleted by at least an order of magnitude relative to the solar composition. We find that KIC 1718290 has T_eff = 22,100 K, log g = 4.72, and a super-solar helium abundance (log N_He/N_H = òÈÒ0.45). This places it well above the extreme horizontal branch, rather on the very blue end of the classical horizontal branch, where shell hydrogen burning is present. We conclude that KIC 1718290 must have suffered extreme mass loss during its first giant stage, but not sufficient to reach the extreme horizontal branch.

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Last Revised: 2012 September 26th