Chromospheric Activity of HAT-P-11: an Unusually Active Planet-Hosting K Star

TL;DR

HAT-P-11 exhibits an unusually strong and possibly long activity cycle, with chromospheric emission levels comparable to solar maximum, likely influenced by its close-in planet, making it a unique case among similar stars.

Contribution

This study identifies and characterizes an unusually active chromospheric cycle in HAT-P-11, highlighting its exceptional activity levels compared to similar planet-hosting stars.

Findings

HAT-P-11 shows a >10 year activity cycle with a prolonged maximum phase.

Its chromospheric emission is stronger than similar stars, possibly due to planetary tidal effects.

The star's spot distribution resembles that of the Sun near solar maximum.

Abstract

Kepler photometry of the hot Neptune host star HAT-P-11 suggests that its spot latitude distribution is comparable to the Sun's near solar maximum. We search for evidence of an activity cycle in the CaII H & K chromospheric emission $S$-index with archival Keck/HIRES spectra and observations from the echelle spectrograph on the ARC 3.5 m Telescope at APO. The chromospheric emission of HAT-P-11 is consistent with a $\gtrsim 10$ year activity cycle, which plateaued near maximum during the Kepler mission. In the cycle that we observed, the star seemed to spend more time near active maximum than minimum. We compare the $\log R^\prime_{HK}$ normalized chromospheric emission index of HAT-P-11 with other stars. HAT-P-11 has unusually strong chromospheric emission compared to planet-hosting stars of similar effective temperature and rotation period, perhaps due to tides raised by its planet.