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

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2. Observations

All spectroscopic observations were obtained at Kitt Peak National Observatory (KPNO) with the coudé feed telescope during March 1994 and February-March 1995. The 6420-Å data in this paper are from March 1994 while the H [FORMULA] spectra are from February-March 1995. Both data sets were obtained with a 800 [FORMULA] 800 TI CCD (TI-5 chip, 15 [FORMULA] pixels) with grating A, camera 5, and the long collimator enabling a resolving power of 38,000 and a useful wavelength range of 80 Å . Table 1 is a summary of the spectroscopic observations.


Table 1. Spectroscopic log and radial velocity data ([FORMULA]). "P" and "S" indicate the primary and secondary component, respectively.

All data were reduced in the same standard fashion using IRAF and included bias subtraction, flat fielding, cosmic-ray removal and optimal aperture extraction. Th-Ar comparison spectra and spectra of bright radial-velocity standards were obtained several times throughout the night to ensure an accurate wavelength calibration. Radial velocities were derived from cross correlating the IL Hya spectra with spectra of the IAU velocity standards 16 Vir (K0.5III, [FORMULA] km s-1) and HR 3145 (K2III, [FORMULA] km s-1) and are listed in Table 1 along with the corresponding standard errors. The exposure level for both wavelength regions corresponds to a signal-to-noise ratio of approximately 200:1. Usually twenty flat-field exposures with a Tungsten reference lamp were taken at the beginning of the night and again at the end of the night. These fourty flat fields were co-added and used to remove the pixel-to-pixel variations in the stellar spectra. The TI CCD does not show obvious signs of fringing near 6420 Å or H [FORMULA] and no attempts were made to correct for it other than the standard flat-field division.

The new photometric data were obtained with the Fairborn Observatory T7 0.75-m automatic photoelectric telescope (APT), then still on Mt. Hopkins, Arizona and now part of the University of Vienna twin APTs at the new Fairborn Observatory at Washington Camp in Southern Arizona (Strassmeier et al., 1997b). The 211 observations were made differentially with respect to HD 81904 as the comparison star ([FORMULA] mag, [FORMULA] mag, [FORMULA] mag) and HD 80991 as the check star. All photometry has been transformed to match the Johnson-Cousins [FORMULA] system. Observations started on JD 2,449,022 on the basis of one observation per night and cover 51 nights in 1992/93, 101 nights in 1993/94 and 59 nights in 1994/95. This data and additional photometry for IL Hydrae, plus data for other stars, was presented in Strassmeier et al. (1997a).

Fig. 1 shows the V -light curve and the [FORMULA] and [FORMULA] color curves of IL Hydrae in 1992/93, 1993/94 and 1994/95 as well as the periodogram from the combined 1992-1995 V -band data.

[FIGURE] Fig. 1. Left: Periodogram from the entire 1992-1995 V -band APT data (panel a) and the window function showing strong aliasing towards multiples of one day (panel b). The largest reduction of the residuals is achieved with a period near the orbital period at 12.730 [FORMULA] 0.004 days (frequency [FORMULA]). The other indicated frequencies are just aliases of the true [FORMULA] frequency. Right: Light and color curves for the three seasons from 1992 to 1995 phased with the elements in Table 2 (panel c). Notice that most of the scatter in the data is due to intrinsic variations of the starspot distribution and not observational scatter.

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© European Southern Observatory (ESO) 1998

Online publication: January 27, 1998