Time-series Doppler imaging of the red giant HD 208472. Active longitudes and differential rotation

TL;DR

This study used time-series Doppler imaging over three years to map the surface of the active red giant HD 208472, revealing high-latitude spots, a flip-flop event, and weak solar-like differential rotation.

Contribution

It provides the first detailed Doppler images of HD 208472's surface, linking photometric variations to spot distribution and measuring its differential rotation pattern.

Findings

Spots are mainly at high latitudes with no polar caps.

Detected a flip-flop event in spot activity between 2009 and 2010.

Star exhibits solar-like differential rotation, ~15 times weaker than the Sun.

Abstract

HD 208472 is among the most active RS~CVn binaries with cool starspots. Decade-long photometry has shown that the spots seem to change their longitudinal appearance with a period of about six years, coherent with brightness variations. Our aim is to spatially resolve the stellar surface of HD 208472 and relate the photometric results to the true longitudinal and latitudinal spot appearance. Furthermore, we investigate the surface differential rotation pattern of the star. We employed three years of high-resolution spectroscopic data with a high signal-to-noise ratio (S/N) from the STELLA robotic observatory and determined new and more precise stellar physical parameters. Precalculated synthetic spectra were fit to each of these spectra, and we provide new spot-corrected orbital elements. A sample of 34 absorption lines per spectrum was used to calculate mean line profiles with a S/N of several hundred. A total of 13 temperature Doppler images were reconstructed from these line profiles with the inversion code iMap. Differential rotation was investigated by cross-correlating successive Doppler images in each observing season. Spots on HD 208472 are distributed preferably at high latitudes and less frequently around mid-to-low latitudes. No polar-cap like structure is seen at any epoch. We observed a flip-flop event between 2009 and 2010, manifested as a flip of the spot activity from phase 0.0 to phase 0.5, while the overall brightness of the star continued to increase and reached an all-time maximum in 2014. Cross-correlation of successive Doppler images suggests a solar-like differential rotation that is ~15 times weaker than that of the Sun.