Doppler Imaging of EIEri

With its large rotational velocity and an intermediate inclination, EI Eri is, as Fekel et al. (1987) already noted, an ideal candidate for Doppler imaging. Consequently, EIEri has been a prime target since the first application of this technique to spotted late-type stars in 1982. Doppler images can be found in Strassmeier (1990) (epoch 1987), Strassmeier et al. (1991) (epoch 1988) and Hatzes & Vogt (1992) (epoch 1984-87).

This technique (see Rice, 1996) allows the reconstruction of the surface spot distribution of rapidly rotating stars by using the relation between wavelength position across an absorption line and spatial position across the stellar disk.

Observations of EIEri have now been carried out for 17 years. Six years of our long term synoptic observations of EIEri with the McMath-Pierce telescope (1989-1995), one dedicated KPNO/coudé feed run in 1988 and one each in 1995, 1996 and 1997 led to the following results: All surface maps of EIEri show high-latitude spots surrounding or covering the rotational pole as also observed by Hatzes & Vogt (1992), but in contradiction to what is seen on the Sun where spots occur only within $\pm 35^{\circ}$ of the equator. This high-latitude/polar spot seems to be long-lived (at least a decade) but changes its shape on comparatively short timescales (of the order of one month). From time to time spots along the stellar equator also occur, but their lifetimes tend to be relatively short and are not sufficiently well determined.

In one respect, EIEri is less complicated to Doppler map than other RSCVn stars, as it is a single-lined spectroscopic binary, and no correction for the contribution of the companion star has to be adopted.

Zeeman imaging of EIEri

On EI Eri, a magnetic field has been detected using Stokes V measurements (Donati et al., 1997). The Zeeman signatures observed indicate that the field structure is not simply dipolar.
A Zemann image of EIEri would allow to link the surface features and the magnetic polarities. So far, only very few stars have been mapped magnetically. AB Dor is one example where the magnetic field structure has been reconstructed in unprecedented detail (Donati & Collier Cameron, 1997). In this case it was shown that the magnetic field exhibits an important non-dynamo contribution. To date, we do not know how typical either AB Dor or the Sun are and how the evolutionary status of a star affects the magnetic polarity patterns observed. A Zeeman map of EI Eri would thus be an important extension of the so-far limited sample.

Zeeman Doppler imaging has even more stringent noise requirements (S/N $> 10^4$). However, by combining the information of more than 1000 lines, the signal-to-noise ratio can be improved by a factor of about 20 (Donati et al., 1997), so that ratios of more than $10^4$ can be obtained.

Requirements for maping EIEri

In Doppler imaging it is important to obtain high signal-to-noise (S/N $\approx$ 200) at moderate to high resolution ($\lambda$/ $\Delta\lambda\ \approx$ 30-40,000).

The wavelength region around 6420 Å provides up to four relatively unblended lines usable for Doppler imaging : CaI 6439, FeI 6430, FeI 6411, and FeI 6393.

Short 10-min exposures should be carried out continuously during the night and combined as appropriate. Expected total integration time per spectrum (for EIEri) is 40 min but can be up to 80 min in case of thin cirrus.

Because of the critical rotational period of EIEri (which amounts to 1.945 days, almost an integer multiple of the day/night cycle), one ideally needs 20 nights of continuous observations in order to achieve perfect phase coverage. It is possible to map just parts of EIEri's surface with a minimum of 14 nights, but one would lack other surface parts which results in artifacts in the Doppler map.