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Subsections

   
4 AU Mic multi-wavelength observations

The value of using multi-wavelength observations is evident from the above. Unfortunately it is difficult to obtain simultaneous observations with different satellites, and most authors merge observations that can be even years apart. This procedure has some limitations, since active stars are known to present high variability in the XUV on all time scales, from minutes (due to flaring) to years (possibly due to stellar cycles). In this paper we consider FUSE, STIS and EUVE observations of the dMe star AU Mic as a benchmark study. AU Mic is a nearby (9.94 pc) dM1e star smaller than the Sun (0.59 $R_{\odot}$). We have selected AU Mic for two main reasons.

The first is that AU Mic is one of the brightest ultraviolet and X-ray sources, observed by several satellites. EUVE observed AU Mic in quiescent and flaring state on July 14-17, 1992. A detailed analysis of the main plasma parameters during both phases was presented by Monsignori Fossi et al. (1996). AU Mic was also observed on July 22-23, 1993 (Del Zanna et al. 1995) during a quiet phase, with count rates very similar to those measured during the quiescent phase of the July 1992 observation. We have therefore adopted the July 1993 EUVE observation as representative of AU Mic in quiescence. STIS observed AU Mic on September 6, 1998 in both quiescent and flaring state. An analysis of the quiescent part of this observation (totaling to 9200s) has been presented by Pagano et al. (2000), while Robinson et al. (2001) discusses the flaring part.

The second reason is that the spectroscopic diagnostics that have been applied to AU Mic by other authors present some of the problems that we highlight in this paper. DEM analyses and calculations of radiative losses for AU Mic in quiescence have been presented by Quin et al. (1993, using IUE spectra) and by Pagano et al. (2000). The latter measured densities using different methods and ions and found some of the commonly known inconsistencies. In particular, Pagano et al. found a quite substantial discrepancy between the density derived from O IV ratios ( $\log N_{\rm e} \simeq 10.8$) and that deduced with the emission measure loci method ( $\log N_{\rm e} = 11.5{-}12$). We use a new FUSE observation of AU Mic in order to complement those of STIS and EUVE, and revise the results of Quin et al. and Pagano et al. in terms of densities, DEM and elemental abundances.

   
4.1 FUSE observations of AU Mic

AU Mic was observed in August 2000 with the FUSE large square aperture (LWRS: $30\hbox{$^{\prime\prime}$ }\times 30\hbox{$^{\prime\prime}$ }$), starting at 19:35:47 UT on the 26th and ending at 09:21:52 UT on the 27th, with a total of 9 observations (one for each orbit) of about 30 min each. We have used the publicly available data, processed with the standard FUSE pipeline package (CALFUSE version 1.7.7). Among all the channels, the SiC 2A, LiF 1A, and LiF 2A present the highest effective areas and are those that have been considered here. The standard wavelength calibration required some corrections. Normally, the wavelength scale for a given observation has a variable zero point offset. The single observations did not require any wavelength adjustments. On the other hand, large wavelength shifts of -0.25, -0.3, and -0.15 Å were required to adjust the calibration of the SiC 2A, LiF 1A, and LiF 2A channels, respectively (similar to what found by others, see, e.g., Harper et al. 2001). These shifts produce a good representation of the lines observed, as can be seen from a comparison with the theoretical wavelengths obtained from the CHIANTI database, and shown in Table 3 (note that the theoretical wavelengths are given with three decimal places for easier comparison with the observations, although they are not always known with such accuracy).

  \begin{figure}
\par\includegraphics[angle=90,width=8cm,clip]{H3236_1.ps} \end{figure} Figure 1: Fluxes of a selection of lines observed by FUSE during the AU Mic observation. Note the presence of a flare during the last observation.

First, an analysis of the 9 observations has been performed. The S/N is such that only the fluxes of the brightest lines could be measured, for each observation. Figure 1 shows the time variation of the fluxes of a selection of lines during the 9 observations. The star was obviously in a quiescent state during the first 8 orbits, while during the last observation a large flare occurred. All the transition region lines increased their intensities by factors of 2 to 5. The C III, Si IV lines presented the largest increase, while lines emitted at lower (e.g. C II, Ly$_\gamma$) or higher (e.g. O VI) temperatures were only slightly affected. Lines emitted at coronal temperatures (Fe XVIII, Fe XIX) did not show any significant enhancement. This event cannot be classified as a classical stellar flare, but instead as a transition region explosive event. The paper by Robinson et al. (2001) focuses on brightenings with similar characteristics, although of much smaller amplitude. Something similar was also observed with STIS by Ayres (2001) during a multi-wavelength (EUVE, Chandra, STIS) campaign on the RS CVn binary HR 1099. It is possible that these types of stellar transition region events are very common in active stars. The lower sensitivity of previous instruments only allowed the observation of the long duration flares.

  \begin{figure}
\par\includegraphics[angle=90,width=12cm,clip]{H3236_2.ps}\end{figure} Figure 2: A selection of FUSE wavelength ranges, with the AU Mic observed spectra.

Aside from the flare, all the previous 8 observations present a remarkable constancy in fluxes. We therefore proceeded by averaging the first 8 spectra of each channel, for a total of 15686 s. In order to further increase the S/N, the spectra have also been rebinned. The line fluxes have been measured with multiple profile fitting and removal of the background. The errors have been calculated according to the S/N in the line, and derived directly from the extracted spectra in counts. The results are shown in Table 3.

Taking into account the relatively short exposure time, a considerable number of lines are clearly detected (see Fig. 2). All lines are formed in the chromosphere and transition region, with the exception of two coronal lines. In fact, we can positively identify not only the Fe XVIII 974.86 Å line (already observed in the Capella spectrum by Young et al. 2001), but also the bright Fe XIX 1118.06 Å line. The blending of C I lines with the Fe XIX line can be excluded, since C I lines are very weak in the spectrum. The identification of the Fe XIX line as well as the others is further confirmed by the DEM analysis (see Sect. 5.3 and Table 3).

4.2 Merging of the datasets

First, we have verified that the fluxes in the TR lines observed by STIS and FUSE during quiescence are consistent. In particular, the fluxes of the C III multiplet at 1175 Å as measured by both instruments are similar, with a deviation of only 7%, well within the statistical and calibration errors. Second, it is interesting to note that the fluxes of the TR lines observed by STIS and FUSE during quiescence are consistent with those obtained by IUE (Linsky et al. 1982; Ayres et al. 1983; Butler et al. 1987; Quin et al. 1993). This gives us confidence in the merging of the two datasets, that should be representative of the AU Mic quiescent "average transition region''. We have used the line fluxes of Pagano et al. (2000), assuming for the DEM analysis an indicative 20% error, since no error estimates were presented.

In this paper, we are mainly interested in the diagnostics applied to the transition region. However, we have complemented the STIS and FUSE data with the EUVE ones. We have verified that the Fe XXI 1354 Å emission observed by STIS is consistent with the Fe XXI emission measured by EUVE, and that the EUVE fluxes are broadly consistent with the FUSE data (see Sect. 5.3), thus giving us confidence in the similarity of the coronal activity during these observations years apart.


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