Astron. Astrophys. 350, 571-581 (1999)

2. Observations and data reduction

Ultraviolet observations of V 711 Tau were obtained by the IUE satellite over 6 days from 12 to 18 December 1992 during a Multi-Site Continuous Spectroscopic (MUSICOS) campaign (Huang et al. 1995). The data set consists of 76 high-dispersion LWP spectra of the Mg II h (2803.53 Å) and k (2796.35 Å) lines, covering more than two contiguous orbital/rotational periods. We have analysed both the spectra reduced using the IUESIPS (IUE Spectral Image Processing System) procedure (cf. also Busà et al. 1996), and the subsequent Final Archive spectra reduced by the NEWSIPS (New Spectral Image Processing System) procedure. We discuss in Sect. 3.3 the results of the NEWSIPS data analysis, and comment on the difference between the NEWSIPS and IUESIPS results.

Orbital phases were computed using the ephemeris HJD 2,442,766.080 + 2.83774E (Strassmeier et al., 1993), with an accuracy .

The physical characteristic and orbital elements of V 711 Tau are listed in Table 1.

Table 1. Properties of the close binary system V 711 Tau (=HR 1099).
Notes:
a) Fekel, 1983
b) Vogt & Penrod, 1983
c) Donati et al. 1992
d) Strassmeier et al. 1993
e) Hipparcos Catalogue

The spectral resolution is about 0.2 Å for the Mg II region. The instrumental profile is Gaussian with FWHM=0.25 Å. Since the Mg II k line was saturated in most of the IUESIPS spectra, we have concentrated our analysis on the Mg II h line. However, the NEWSIPS reduction removed the saturation in the core of the k line (see Sect. 3.3), allowing us to measure the ratio between the h and k lines that is 0.800.08. It is worth noting that the ratio between the h and the saturated k lines in IUESIPS data is 0.790.06, implying that the saturation effect, in the k line core, was not too severe in the IUESIPS and that only a few spectra were actually affected. A summary of the observations is given in Table 2. For unknown reasons the high-dispersion spectra LWP24464, LWP24481, LWP24496 have been degraded to low-dispersion in the NEWSIPS reduction procedure, and, therefore, these spectra were not included in the present high-resolution study. The NEWSIPS spectra LWP24499 and LWP24538 show peculiar flux drops which were not present in the original IUESIPS data. Therefore, these spectra have been discarded as well. Finally, the NEWSIPS spectra LWP24466, LWP24467, LWP24482, LWP24494, LWP24495, LWP24520 and LWP24530 have been discarded because, as IUESIPS spectra (see Neff et al., 1992), they are obviously affected by scattered light, and the flux calibration leads to "drop outs" in coincidence with scattered light background peaks. The NEWSIPS reduction does not appear to have improved this calibration problem (see Fig. 1, as a representative example).

Fig. 1. Comparison between spectra obtained at the same phase, the lower spectrum (LWP 24482) in coincidence with a scattered light background peak (left panel : IUESIPS reduction; right panel : NEWSIPS reduction). It is evident that the NEWSIPS procedure reduction improves the quality of the spectra, but does not solve the flux calibration problem in presence of scattered light.

Table 2. Summary of IUE observations

Table 2. (continued)

In Fig. 2 the Mg II h fluxes, integrated between 2798 and 2808 Å, are plotted versus the orbital phases. Random variability of the integrated flux is evident, but no rotational modulation is visible. Marked enhancement in several transition region UV lines have been detected during the monitoring time at phases 0.84, 0.97, 0.18, 0.55, and 0.89, listed in order of acquisition time (see Neff et al. 1995). Of these flaring episodes, the flare at =0.55 during the second orbit (LWP24490 spectrum on Dec 14 at 10:30 UT) shows the most marked flux enhancement of the Mg II line emission.

Fig. 2. Integrated Mg II h line flux versus orbital phase. See text for details.

In Fig. 3 the Mg II h line profile at (when the component stars are at their maximum separation) is shown. Arrows mark the expected radial velocities of the G5 and K1 stellar components. The composite line profile is strongly asymmetric indicating that the emission flux from the G5 star is weaker than from the K1 star.

Fig. 3. The Mg II h line profile at when the K1 and G5 stars are at their maximum separation. Arrows indicate the expected central wavelengths of the Mg II h lines from the two stars.

The spectra were fitted adopting Gaussian profiles. We used the "ICUR Spectral Analysis Package" fitting program (version 3.1) based upon CURFIT (Bevington, 1969), which uses the Marquadrat algorithm to minimise the .

© European Southern Observatory (ESO) 1999

Online publication: October 4, 1999
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