Low-frequency variations of unknown origin in the Kepler Delta Scuti star KIC 5988140 = HD 188774

TL;DR

This study analyzes Kepler photometry and spectroscopy of the star KIC 5988140, revealing multiple pulsation frequencies and complex variability that cannot be fully explained by common scenarios like binarity or rotation.

Contribution

It provides detailed frequency analysis of KIC 5988140, identifying multiple Delta Scuti pulsation modes and highlighting unexplained low-frequency variations.

Findings

Detected two dominant frequencies at 0.688 and 0.344 c/d.

Identified nine additional Delta Scuti pulsation frequencies.

Low-frequency variations remain unexplained by current models.

Abstract

We used high-quality Kepler photometry and spectroscopic data to investigate the Kepler binary candidate KIC 5988140. Using the spectrum synthesis method, we derived the fundamental parameters Teff, log g, [M/H], and v.sini and the abundances. Frequency analyses of both the photometric and the spectroscopic data were performed, revealing the same two dominant frequencies (F_1=0.688 and F_2=0.344 c/d). We also detected in the photometry the signal of nine more, significant frequencies located in the typical range of Delta Scuti pulsation. The light and radial velocity curves follow a similar, stable double-wave pattern which are not exactly in anti-phase but show a relative phase shift of about 0.1 period between the moment of minimum velocity and that of maximum light. We considered three different scenarios: binarity, co-existence of both Gamma Doradus and Delta Scuti pulsations and rotation of the stellar surface with an axisymmetric intensity distribution. However, none of these scenarios is capable of explaining all of the characteristics of the observed variations. We confirm the occurrence of various independent Delta Scuti-type pressure modes in the Kepler light curve. With respect to the low-frequency content, however, we argue that the physical cause of the remaining light and radial velocity variations of this late A-type star remains unexplained by any of the presently considered scenarios.