5 Chromospheric activity indicators

The echelle spectra analysed in this paper allow us to study the behaviour of the different optical chromospheric activity indicators from the Ca  II H & K to the Ca  II IRT lines, formed at different atmospheric heights. As shown in our previous work (Montes et el. 2000b, and references therein) with the simultaneous analysis of the different optical chromospheric activity indicators and using the spectral subtraction technique, it is possible to study in detail the chromosphere, discriminating between the different structures: plages, prominences, flares and microflares.

The chromospheric contribution in these features has been determined using the spectral subtraction technique described in detail by Montes et al. (1995, 1997, 1998, 2000b). The synthesized spectrum was constructed using the program STARMOD developed at Penn State University (Barden 1985) and modified by us. The inactive stars used as reference stars in the spectral subtraction were observed during the same observing run as the active stars. Spectra of representative reference stars in the Ca  II H & K, H$\beta$, H$\alpha$, and Ca  II IRT line regions are plotted in Fig. 4. In Table 4 we give the excess emission equivalent width (EW) (measured in the subtracted spectra) for the Ca  II H & K, H$\epsilon$, H$\delta$, H$\gamma$, H$\beta$, H$\alpha$, and Ca  II IRT ($\lambda$8498, $\lambda$8542, $\lambda$8662) lines, as well as the reference star used in the subtraction technique for each observation. We have estimated the errors in the measured EW taking into account the typical internal precisions of STARMOD (0.5-2 km s^-1 in velocity shifts, and $\pm$5 km s^-1 in $v\sin{i}$), the rms obtained in the fit between observed and synthesized spectra in the regions outside the chromospheric features (typically in the range 0.01-0.03) and the standard deviations resulting in the EW measurements. The estimated errors are in the range of 10-20%. For low active stars errors are larger and we have considered as a clear detection of excess emission or absorption in the chromospheric lines only when these features in the difference spectrum are larger than 3$\sigma$. Errors in the chromospheric features of the blue spectral region are larger due to the lower S/N of the spectra in this region. As an indication of the accuracy of the data, we give in Table 1 the S/N in the Ca  II H & K, and H$\alpha$ line regions. The excess emission EW have been converted to absolute chromospheric flux at the stellar surface by using the calibration of Hall (1996) as a function of (B-V). In Table 5 we give the absolute flux at the stellar surface (log$F_{\rm S}$) for the lines listed in Table 4.

Representative spectra in the H$\alpha$and Ca  II IRT ($\lambda$8498, $\lambda$8542) line regions of the star sample have been plotted in Figs. 5 and 6. For each star we have plotted the observed spectrum (solid-line) and the synthesized spectrum (dashed-line) in the left panel and the subtracted spectrum in the right panel. H$\alpha$ emission above the continuum is detected in PW And, V834 Tau, and LQ Hya; in the rest of the stars excess H$\alpha$ emission is detected in the subtracted spectra except GJ 3706. Filled-in absorption in other Balmer lines is also detected in many of the stars. Ca  II H & K emission is observed in all the stars in which these lines are included in our spectra. Emission reversal in the Ca  II IRT lines is observed in PW And, V368 Cep, V383 Lac, DX Leo, EK Dra, V834 Tau, and LQ Hya, in the rest of the stars a filled-in absorption line profile is observed.

Figure 4: Spectra of representative reference stars in the Ca  II H & K, H$\beta$, H$\alpha$, and Ca  II IRT lines region.

Figure 5: Spectra in the H$\alpha$ line region for our star sample. Observed and synthetic spectra are shown in the left panel and subtracted spectra in the right panel.