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

TL;DR

This paper reports the discovery of a helium-rich pulsator on the blue horizontal branch, exhibiting g-mode pulsations similar to V1093Her stars but with distinct surface properties, revealing new insights into stellar evolution.

Contribution

First identification of a g-mode pulsator on the classical blue horizontal branch with unique surface characteristics, expanding understanding of stellar pulsation and evolution.

Findings

Rich spectrum of low-amplitude modes with regular spacing

KIC 1718290 has T_eff = 22,100K, log g = 4.72, and high helium abundance

Resides on the blue end of the classical horizontal branch, not the extreme horizontal branch

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 one and twelve hours, most of which follow a regular spacing of 276.3 s. This mode structure strongly resembles that of the V1093Her pulsators, with only a slight shift towards longer periods. Our spectroscopy, however, reveals KIC 1718290 to be quite distinct from the sdB stars that show V1093Her 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 KIC1718290 has T_eff = 22 100K, log g = 4.72, and a super-solar helium abundance (log N(He)/N(H) = -0.45). This places it well above the extreme horizontal branch, and 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.