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Astron. Astrophys. 355, 227-235 (2000)

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6. Conclusions

From the analysis of the observational data presented here, several conclusions can be drawn. First of all, the upper transition region of II Peg was in a phase of low activity when the observing programme was carried out. The transition region fluxes measured are amongst the lowest ever found in this system and no trace of rotational modulation of the ultraviolet emission is apparent. While the low activity affects mainly the upper transition region, lines originated in lower temperature regime such as C II and the essentially chromospheric Mg II lines are less affected. The low activity seems to contradict the fact found from the photometric light curve that in this epoch there was a relatively high global coverage by starspots (see Paper I). It should be noted that this discussion applies to the quiescent state of II Peg during the first weeks after one of the most energetic flares detected in this system. No rotational modulation is detected in any of the lines observed with IUE except Fe II and even in this case, the evidence is weak.

The comparison of the observations presented here with the previous observing campaigns shows no hints of cyclic behaviour in the C IV emission. It nevertheless suggests the possibility that the minimum flux within each observing campaign diminishes. The latest observations obtained with HST consist only of two spectra and are therefore of no help in order to check if the trend continues after 1992.

New emission measure distributions presented here differ from previous determinations mainly due to the decrease in the upper transition region line fluxes (Si IV , C IV and N V ). Therefore we find a steeper decrease in the slope of the high temperature range of the emission measure distribution.

The Si III ] density diagnostic gives a value of the electron density during quiescence around [FORMULA]11.6[FORMULA] in agreement with previous work. The enhancement of the density during the flare can only be assesed with the second spectrum (SWP 45532) and amounts to 0.3 dex.

The Si IV ([FORMULA]1394/1403)/C III ]([FORMULA]1909) line ratio, characteristic of a higher temperature range, yields values of [FORMULA]10.7, 10.9 and 11.2 for the mean quiescent state and the two flare spectra respectively. As shown by Fig. 3 in Paper I, the light curve of the flare has a complex time development which makes it difficult to interpret integrated observations as the SWP spectra used in the density determination.

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© European Southern Observatory (ESO) 2000

Online publication: March 17, 2000