The chromospherically active binary star EI Eridani: II. Long‐term Doppler imaging

Abstract

Data from 11 years of continuous spectroscopic observations of the active RS CVn-type binary star EI Eridani -gained at NSO/McMath-Pierce, KPNO/Coudé Feed and during the MUSICOS 98 campaign -were used to obtain 34 Doppler maps in three spectroscopic lines for 32 epochs, 28 of which are independent of each other. Various parameters are extracted from our Doppler maps: average temperature, fractional spottedness, and longitudinal and latitudinal spot-occurrence functions. We find that none of these parameters show a distinct variation nor a correlation with the proposed activity cycle as seen from photometric long-term observations. This suggests that the photometric brightness cycle may not necessarily be due to just a cool spot cycle. The general morphology of the spot pattern remains persistent over the whole period of 11 years. A large cap-like polar spot was recovered from all our images. A high degree of variable activity was noticed near latitudes of ≈60-70 • where the appendages of the polar spot emerged and dissolved.

Doppler images of DI Piscium during 2004–2006

“…Doppler images based on observation with larger phase gaps than this have been published before, e.g. in the studies of II Peg (Berdyugina et al 1998), EI Eri (Washuettl et al 2009), and HD 171488 (Jeffers et al 2011). The largest phase gap in the observations from 2006 corresponds to roughly 160…”

Variability of a stellar corona on a time scale of days

“…Long term photometric monitoring of EI Eri seemed to indicate a ∼11 yr brightness cycle that was conjectured to be an analog of the solar activity cycle (Oláh & Strassmeier 2002), but unlike the sun, long term Doppler imaging of EI Eri by Washuettl et al (2009b) shows little variability of its cool starspots over 11 yr. As for the hot corona, Osten et al (2002) observed EI Eri with ASCA (and EUVE) and fitted its spectrum with a 2T model of kT = 0.72 keV and 2.2 keV, but the abundances could not be tightly constrained with ASCA. The XMM-Newton CCD spectra of a short flare on EI Eri on 2009-08-06 during the present campaign was analyzed by Pandey & Singh (2012), who suggested the flare originated from a loop the size of ∼R * (primary), which is about a tenth of the binary separation distance.…”