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Astron. Astrophys. 337, 757-771 (1998)

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6. Analysis of the H[FORMULA] variations

Figs. 2 - 4 were used as a guide to trace significant features in the individual ratio spectra. Each of them was investigated by using the gaussian-fitting method that was previously followed in the case of HK Aqr (Byrne, Eibe & Rolleston 1996). Due to the complexity of the profiles, deblending fits were often necessary to resolve the contribution of different components. Results from this analysis are presented in this section.

6.1. Absorption features

As shown in Sect. 5.1, several absorption transients were detected in the course of the observations. They will be labeled with letters A, B and C for quick reference in the text. Transient A is the event that was seen to recur on the three nights between phases [FORMULA]0.2 and [FORMULA]0.3. Transient B is the strong absorption feature seen near [FORMULA]0.5, just prior to the major emission event on June 27. Finally, transient C corresponds to the very deep absorption detected near the start of the series on June 25, at phases [FORMULA]0.9-0.1. The possible recurrence of transients B and C on the three nights will be discussed below. Subscripts 25-27 will be used to indicate the corresponding night in each case.

The three absorption transients, which have been identified using the technique of dynamic spectra, are now analysed in more detail to check whether they can be explained in terms of prominence condensations of neutral material.

6.1.1. Transient A

Fig. 9 shows the results from gaussian fits to the well-defined absorption transient centred at phase [FORMULA]0.25. Data from different nights are overplotted using different symbols: + for June 25, [FORMULA] for June 26, and [FORMULA] for June 27. The two horizontal lines in the RV diagram (bottom panel) indicate the projected rotational velocity, [FORMULA]sini ([FORMULA] sin[FORMULA]61 km s-1). Changes in the radial velocity (RV), equivalent width (EW) and full width at half maximum (FWHM) appear to be correlated as the transient moves across the H[FORMULA] profile. Details of these variations are specified below:

  1. The feature is seen at similar phases during at least three nights ([FORMULA]0.20-0.29 on the night of June 25, [FORMULA]0.18-0.29 on the night of June 26, [FORMULA]0.23-0.28 on the night of June 27). The phase coverage in June 27 made it impossible to detect the complete transit on that night.

  2. Agreement between different nights is especially good in RV, except for the slightly more redshifted velocities measured on June 26 between [FORMULA]0.18 and [FORMULA]0.21. According to the pattern of the EW variations, however, the absorption appears stronger on the first night.

  3. The EW increases rapidly as the feature appears in the blue wing of the profile and continues to move redwards. Absorption gets again weaker at larger positive velocities.

  4. A sharp peak of EW occurs near [FORMULA]0.21. At this time the central position of the feature is shifted to the blue. The pattern of the EW variations is therefore asymmetric, increasing more quickly than it is seen to decline.

  5. The FWHM variations show a larger scatter. The feature is generally narrow ([FORMULA]1-1.5) but broadens considerably near [FORMULA]0.21, coinciding with maximum EW. In any case, it is much less than expected for rotational broadening, [FORMULA]2.31Å.

  6. The central position of the feature varies linearly with phase, moving from blue to red, in an interval of phase [FORMULA]0.1.

  7. When observed, the extreme velocities are not perfectly symmetric about zero but systematically blueshifted by [FORMULA]20 km s-1.

[FIGURE] Fig. 9. Results from the gaussian fits to the transient A seen between phases 0.2 and 0.3 on the nights 25 June (+), 26 June ([FORMULA]), 27 June ([FORMULA]). The two horizontal lines in the RV diagram (bottom panel) correspond to the maximum Doppler shift associated to the projected rotational velocity

6.1.2. Transient B

This feature is seen clearly in Fig. 4 (B27). It appears at [FORMULA]0.36 as a relatively weak (EW[FORMULA]0.03Å), narrow absorption (FWHM[FORMULA]1.05Å), blueshifted by about -20 km s-1 with respect to the stellar rest frame. It rapidly propagates towards the red, increasing in depth and width before disappearing with the start of a strong flare. Its mean EW and FWHM during the last phases ([FORMULA]0.54-0.62) are, respectively, 0.10Å and 1.90Å, substantially higher than when seen in the blue. Fig. 4 suggests the presence of two maxima of absorption at this time. One appears to be fixed at 65 km s -1 while the other is slightly more to the blue but is more variable and propagates slowly to the red until both features finally merge together. Individual ratio profiles for these phases are shown in Fig. 10. It can be seen that the broad absorption component has often two peaks suggesting a possible blend between two individual features whose contributions are difficult to assess. In addition, simultaneous emission is seen in the blue at these phases with important effects in the profile, which may result in larger uncertainties in the velocities measured from gaussian fits.

[FIGURE] Fig. 10. Series of individual ratio profiles between [FORMULA]0.5-0.6 on the night of June 25. The first spectrum is the one shown at the bottom and the following are shifted upward for clarity. The time between subsequent spectra is [FORMULA]5min ([FORMULA]0.007)

Evidence of absorption at these phases is also found on the other two nights (B25 and B26 in Figs. 2 and 3, respectively) but with different behaviour. B25 is seen as strong absorption (EW[FORMULA]0.2Å) at [FORMULA]-50 km s-1 and shows little variation with phase. Its appearance, however, may be influenced by the presence of additional emission in the red wing due to the effect of an emission transient as described in Sect. 6.2. The observed behaviour of B26 is more similar as in this case the absorption is seen to cross the profile from blue to red. The feature is not so clear and measurements are less accurate due to the poorer signal-to-noise. It is interesting that the extreme radial velocities at the beginning and end of the transit ([FORMULA]0.4 and [FORMULA]0.67, respectively) are symmetric with respect to zero velocity, being the amplitude of the variations close to 50 km s-1.

6.1.3. Transient C

From Fig. 2 it is seen that transient C25 is one of the strongest absorption events observed in RE 1816+541. It shows a complex morphology due to the concurrence of several features in the profile, making an unique interpretation of its evolution more difficult.

Two discrete components are seen during the onset of the event ([FORMULA]0.9): an already present redshifted absorption at an average velocity of about 10 km s-1 and a blueshifted absorption by [FORMULA]-60 km s-1, that is moving towards the red. Both features are strongly blended at [FORMULA]0.96, resulting in a very broad and deep absorption centred at about 10 km s-1. A sudden brightening is seen in only one spectrum at [FORMULA]0.98 and is followed by the appearance of a weak blueshifted absorption. Note that a bright transient (see below) is developing at this time so that the blueshifted absorption is seen superimposed on the blue emission wing. The absorption component in the red becomes more prominent for about 30 min ([FORMULA]0.04) with an average width of about 2.3Å, close to the expected rotational broadening. It fades at [FORMULA]0.1 but it remains visible in the red, before merging into transient A at [FORMULA]0.3.

Unfortunately, differences in phase coverage do not allow to confirm the same kind of variations on the other two nights. At the end of June 27 ([FORMULA]0.93), however, there is evidence for a well defined absorption at -80 km s-1 that propagates rapidly to the red (C27 in Fig. 4). On the other hand, weak absorption in the red between [FORMULA]0.1-0.2 was also observed on June 26 (C26 in Fig. 3). If associated to the same event, the main part of its evolution could have been missed because the series of observations on this night started in fact at [FORMULA]0.06.

6.2. Emission features

The bright emission transients seen in the H[FORMULA] line have also been analysed in order to investigate their origin and their possible connection to the absorptions. From Figs. 2 - 4 two major features can be distinguished: a discrete emission seen between phases [FORMULA]0-0.5 on the three nights (transient a) and a very strong emission event that develops between [FORMULA]0.6-0.9 on June 27.

6.2.1. Transient a

This feature is seen most clearly on the night of June 25 (a25 in Fig. 2). Although in less detail, it can also be identified on the other two nights (a26 and a27 in Figs. 3 and 4, respectively). On June 27 only the closing phases were covered. Results from a gaussian-fitting analysis are presented in Fig. 11. A summary of the most important properties revealed by this method is given below:

  1. The whole event covers at least a phase interval of [FORMULA]0.5.

  2. The feature disappears temporarily between phases 0.2 and 0.3, coinciding with the passage of the absorption transient A.

  3. Its width is narrower than expected from the rotationally broadened stellar profile, its FWHM being of the order of 1.22Å.

  4. The RV changes from blue to red within the entire episode and behaves almost linearly with time. Large deviations from linearity occurred near [FORMULA]0.2 and also at the red end of the curve obtained on June 25.

  5. The velocity amplitude exceeds [FORMULA]sini of the star.

  6. Extreme velocities are not symmetric with respect to zero velocity. On June 25 the RV was seen to change from [FORMULA] -82 km s-1 to [FORMULA]50 km s-1. The extreme velocities observed on June 26 were [FORMULA] -67 km s-1 in the blue and [FORMULA]75 km s-1 in the red, but at this night the opening phases of the event were missed.

  7. Variations in EW are less pronounced. There is a trend of brightening at the closing phases on June 25 ([FORMULA]0.4). A sudden brightening of the feature at the end of its passage across the disk is also observed on June 27. Unfortunately, earlier phases of the event were not covered on this night.

  8. The phenomenon persists during three nights.

[FIGURE] Fig. 11. Results from the gaussian fits to the emission feature a , seen between phases 0.0 and 0.5 on the nights of 25 June (+), 26 June ([FORMULA]), 27 June ([FORMULA]). The two horizontal lines in the RV diagram (bottom panel) correspond to the maximum Doppler shift associated to the projected rotational velocity

6.2.2. The strong emission on June 27

An extremely strong emission feature is clearly seen in the H[FORMULA] dynamic spectra of June 27 (see Fig. 4), starting near [FORMULA]0.6 but extending over [FORMULA]3.7hrs. This is associated with a large flare that manifests dramatically in all the other chromospheric lines selected for study (see Figs. 5 - 8), implying it affected both above and below the temperature-minimum region. One of its most conspicuous aspects is the way it grows rapidly towards the red after emerging in the blue wing of the H[FORMULA] profile. This flare will be the subject of further investigation in future work.

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© European Southern Observatory (ESO) 1998

Online publication: August 27, 1998