Kepler photometry and optical spectroscopy of the ZZ Lep central star of the Planetary Nebula NGC 6826: rotational and wind variability

TL;DR

This study uses Kepler photometry and optical spectroscopy to analyze the central star of NGC 6826, revealing rotational modulation and wind variability, and ruling out pulsations and close binary scenarios.

Contribution

It provides the first detailed analysis combining long-term Kepler photometry with spectroscopy to characterize variability mechanisms in a planetary nebula central star.

Findings

Detected a 1.238-day rotational modulation in the star's light curve.

Attributed short-term variability to a variable stellar wind, not pulsations.

Ruled out close binary companion with orbital period less than seven days.

Abstract

We present three years of long-cadence and over one year of short-cadence photometry of the central star of the Planetary Nebula NGC 6826 obtained with the Kepler spacecraft, and temporally coinciding optical spectroscopy. The light curves are dominated by incoherent variability on time scales of several hours, but contain a lower-amplitude periodicity of 1.23799 d. The temporal amplitude and shape changes of this signal are best explicable with a rotational modulation, and are not consistent with a binary interpretation. We argue that we do not observe stellar pulsations within the limitations of our data, and show that a binary central star with an orbital period less than seven days could only have escaped our detection in the case of low orbital inclination. Combining the photometric and spectroscopic evidence, we reason that the hourly variations are due to a variable stellar wind, and are global in nature. The physical cause of the wind variability of NGC 6826 and other ZZ Leporis stars is likely related to the mechanism responsible for wind variations in massive hot stars.