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Astron. Astrophys. 318, 60-72 (1997)

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6. Results

The spectral standard stars chosen to form a match for the ER Vulpeculae system were 26 Dra (G0V) and HD 186427 (G5V). Agreement between the wavelength calibrations of spectral standards and target spectra were checked by measuring the positions of several atmospheric lines visible in all spectra. The maximum deviation was no more than 0.01Å or approximately 0.5 km s-1 at H [FORMULA].

Spectral synthesis was performed for the active orders for all ER Vul spectra. The lines analysed were H [FORMULA] ([FORMULA] 6562.85), one of the Ca II IRT lines ([FORMULA] 8498.02), Na I D1 and D2 ([FORMULA] 5889.95 and [FORMULA] 5895.92), He I D3 ([FORMULA] 5875.62) and Mg I b ([FORMULA] 5167.33, [FORMULA] 5172.68 and [FORMULA] 5183.61). Fig. 2 shows examples for each of these orders of the synthetic spectra formed for one ER Vul spectrum. Bold lines are the observed spectra and thin lines the synthetic spectra. Also plotted on these diagrams are the subtracted spectra. In orders containing no activity-sensitive features good cancellation of lines was achieved. As can be seen the H [FORMULA] line, Ca II IRT and Mg I b lines all show excess emission features. This indicates that these lines have significant filling-in of the global absorption profiles over and above that expected for non-active stars. The small difference in normalisation for the Mg I b order is not thought to effect the degree and position of the excess emission. The Na I D lines show no excess features above the variation due to small atmospheric absorption features. However it is clear that He I D3 has a high degree of excess absorption in this system. Subsequent analysis concerns the evolution of these excess features throughout the observations.

[FIGURE] Fig. 2. Observed, synthetic and subtracted spectra for spectral orders containing activity-sensitive lines for a single ER Vulpeculae spectrum (phase 0.939). Lines shown are H [FORMULA] a, the Ca II IRT line at [FORMULA] 8498.02 b, the Mg I b lines c and the order containing the Na I D1 and He I D3 lines d. Bold lines indicate the observed spectra with overlying thin lines showing the synthetic spectra. The zero-continuum data are the resulting subtracted spectra showing excess emission/absorption features.

Fig. 3 shows the results of measurements on the subtracted H [FORMULA] emission line. There is an indication of a small amount of excess absorption on the blue edge of the emission profile during the eclipse spectra. Although this feature is not significantly above the noise level it may be real since the fluxes for the continuum level across this order are remarkably consistent throughout the observations whilst the absorption feature is seen to undergo more significant variations. If this feature is real it is absorbing at a velocity of [FORMULA] 200 km s-1 on the blue edge of the secondary star.

[FIGURE] Fig. 3. Results of the analysis of the subtracted H [FORMULA] emission line from ER Vulpeculae. The upper panel shows the variation in EW, the middle panel the variation in FWHM and the bottom panel the radial velocity of the emission compared to the RV curves of each component.

Fig. 3a shows the equivalent width (EW) measurements for the excess H [FORMULA] emission. As can be seen these values remain essentially constant throughout the observations implying that the total excess emission from both components is not varying with phase. However the FWHM of the emission (shown in Fig. 3b) varies quite substantially being lower during the eclipse. This implies that the emission is indeed originating in both components since it is the relative positions of the features which changes the overall width of the profile. Finally Fig. 3c shows the measured radial velocity of the emission feature (diamonds) against phase with the actual orbital velocity of the primary and secondary components plotted as solid lines. Clearly the majority of the excess emission is associated with the secondary (cool) component of ER Vul.

Fig. 4 shows results for the Ca II IRT ([FORMULA] 8498.02) excess emission profiles from ER Vul. Again the EW is essentially constant while the FWHM varies in a way consistent with emission arising on both components. Measurements of the velocities of these profiles (shown in Fig. 4c) show an almost constant velocity centred around zero. This indicates that the Ca II IRT line from each component are of similar intensity so that the resulting velocity is an average of the orbital velocities of the two components. The small variation with phase indicates that the secondary may be slightly more luminous in Ca II IRT excess emission than the primary.

[FIGURE] Fig. 4. Results of the analysis of the subtracted Ca II IRT emission line at 8498.02Å from ER Vulpeculae. EW, FWHM and radial velocities are shown.

Fig. 5 shows the results obtained for two of the Mg I b lines for ER Vul. For both these lines the EW remains almost constant while the FWHM varies. Again emission in these lines appears to be originating on both components, although significantly more from the secondary.

[FIGURE] Fig. 5. Results of the analysis of the subtracted Mg I b emission lines from ER Vulpeculae. EW, FWHM and radial velocities are shown for the [FORMULA] 5183.61 line (filled squares) and the [FORMULA] 5172.68 line (open squares).

Results for the He I D3 line excess absorption are shown in Fig. 6. This line is very weak in ER Vul and presented some difficulties for the measurement process. However it was possible to determine values for the EW, FWHM and velocity. It appears that He I D3 absorption is occurring in the atmospheres of both components since the EW remains constant while the FWHM decreases to a minimum during conjunction.

[FIGURE] Fig. 6. Results of the analysis of the subtracted He I D3 absorption line from ER Vulpeculae.
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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998