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Astron. Astrophys. 330, 1029-1040 (1998)

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1. Introduction

IL Hydrae = HD 81410 ([FORMULA], [FORMULA], 2000.0, V = 7.25-7.9 mag) is an evolved double-lined spectroscopic binary with very strong CaII H & K emission. This and the fact that the photometric period is very close to the orbital period of 12.9 days (Raveendran et al., 1982) makes IL Hydrae a typical RS CVn star (Fekel et al., 1986). It was also found to be a radio source (Slee et al., 1984; Mutel & Lestrade, 1985), a X-ray source (Dempsey et al., 1993) and an emitter of microwave radiation (Mitrou et al., 1996). Randich et al. (1993) performed a spectrum synthesis in the 6700-Å region and found an iron abundance of 0.5 dex below solar and a moderate Lithium abundance of [FORMULA]. The secondary star can not be seen in near-ultraviolet spectra (La Dous & Gimenez, 1994) but from the difference of the measured [FORMULA] colors of IL Hya to standard values Cutispoto (1995) estimated the secondary to be a G8V star. Just recently, the secondary component was detected by Donati et al. (1997) in two spectra at optical wavelengths.

The large amplitude of the light and color curves of IL Hydrae - discovered by Eggen (1973) - already suggested that a large fraction of the stellar photosphere must be covered with spots. Simply looking at the observed line profiles one can already identify the changing profile asymmetries due to pseudo-emission "bumps" from cool spots. Even though the rotational velocity and thus the rotational broadening of the line profiles is small, the strength of these bumps is comparable to those observed in more rapidly rotating RS CVn stars.

In this paper we present a series of twelve moderately high-resolution spectra in the 6420-Å region taken in 1994 and nine in the H [FORMULA] region taken in 1995. Each data set was obtained within a single stellar rotation. [FORMULA] photometry was gathered from fall 1992 until spring 1995. The combination of these spectroscopic and photometric data is used to study the spatial distribution of the photospheric activity on IL Hydrae in 1994 and 1995. The instrumentation and the data reductions are described in Sect.  2, the stellar properties relevant for the Doppler-imaging analysis are determined in Sect.  3. Doppler maps from several spectral regions are derived in Sect.  4, and the H [FORMULA] spectra collected in 1995 are presented in Sect.  5. A description of the long-term goals of our Doppler-imaging program has already been presented in the previous papers of this series, i.e. paper I on the RS CVn binary UZ Librae (Strassmeier, 1996), paper II on the weak-lined T Tauri star V410 Tau (Rice & Strassmeier, 1996), paper III on the RS CVn binary IN Vir (Strassmeier, 1997) and paper IV on the rapidly-rotating G5III-IV star IN Comae (Strassmeier et al., 1997c).

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© European Southern Observatory (ESO) 1998

Online publication: January 27, 1998
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