EI Eridani

EI Eridani = HD26337 (G5IV, = 1.945 days, $V$ = 7$.\!\!^{\rm m}$1) is a rapidly rotating ($v\sin i = 51$  ) single-lined active RS CVn star. RS CVn stars are close but detached binary systems that rotate synchronously due to tidal forces. The fast rotation is believed to be responsible for the high activity level found on these stars.

The CaII H and K emission of EI Eri was first noted by Bidelman & MacConnell (1973) and confirmed later by Fekel (1980) who classified it as moderate in strength, class C on the qualitative emission scale by Hearnshaw (1979). Fekel et al. (1982) classified EI Eri as RS CVn star when they detected its light variability, with an amplitude being almost 2$.\!\!^{\rm m}$0 in $V$ which was later confirmed by Bopp et al. (1983). Consequently, Kholopov et al. (1985) assigned the new variable star designation ``EI Eridani''. Hall et al. (1987) detected a photometric period of 1.945 days from $UBV$ photometry, interpreted it as rotation period and concluded that the primary component rotates synchronously.

Not surprisingly, H$\alpha$ is, as in most RSCVn stars, in absorption but highly filled in with chromospheric emission (see Smith & Bopp, 1982) and quite variable in strength (Fekel et al., 1986). More than likely, these variations are reproduced by rotational modulation of the chromospheric H$\alpha$-emission, presumably due to evolving plage regions on the stellar surface moving in and out of sight.

EI Eri has been detected at microwave frequencies (Slee et al., 1988); in the infrared (at 12$\mu m$, see Busso et al., 1988) and with IRAS (Infrared Astronomical Satellite; see Beichman et al., 1988); in the ultraviolet (Fekel et al. 1987 detected strong emission lines), with EUVE and with IUE (International Ultraviolet Explorer; see 1996IUEML.C......0I); at X-ray frequencies with the Einstein satellite (reference: 1994HEAO2.C......0M) and with the ROSAT All-Sky Survey; and at radio frequencies (Mutel & Lestrade, 1985);

Hall et al. (1987) already noted season-to-season changes in the photometric period ($\sim$1%), the light-curve amplitude (0$.\!\!^{\rm m}$07 - 0$.\!\!^{\rm m}$20) and the mean brightness ($\sim$10%), likely indicating latitude and/or longitude changes in the location of the starspots. Furthermore, EI Eridani shows an long-term activity cycle ...

Unfortunately, the rotation period is only slightly shorter than exactly two days. As a result, one needs at least 20 days of consecutive observing nights to cover a full rotation cycle. In practice, 14 consecutive nights are sufficient to give a good-quality Doppler image.

Table 1.1: Names of EI Eridani

V* EI Eri	BD-08 801	CABS 35
CCABS 31	CMC 202464	CMC 130994
2E 943	2E 0407.2-0801	1ES 0407-08.0
EUVE J0409-07.8	2EUVE J0409-07.8	GC 5012
GJ 3264	GSC 05312-01791	HD 26337
$[$HFE83$]$ 281	HIC 19431	HIP 19431
IRAS 04072-0801	N30 857	PPM 186606
RE J0409-075	RE J040939-075336	2RE J040940-075336
2RE J0409-075	1RXS J040940.8-075327	SAO 130994
SKY# 6350	TYC 5312 1791 1	uvby98 100026337 V
YZ 98 1031

Fig. 1.2 shows an image of EI Eri in an sky area of 14'1 $\times$ 14'1. For a plot of the sky around EI Eri from Simbad (FK5) see Fig. 1.3; radius is 20 arcmin.

Figure 1.2: Blue image of EI Eri from the Science and Engineering Research Council Survey (SERC). Size of frame: 14'1 x 14'1. - Source: Space Telescope Science Institute (STScI)

Figure 1.3: Sky plot around EI Eri; from Simbad (FK5, radius=20 arcmin, epoch and equinox = 2000.0)