Astron. Astrophys. 341, 527-538 (1999)

7. H emission features

7.1. Observations on August 4

Evidence of large-scale motions is seen during almost 2 hrs at the beginning of the night (0-0.16). Fig. 5 (left ) shows emission out to velocities of -250 km s^-1 implying upward flows of the emitting material. While this is not necessarily a bulk velocity (because the H line is optically thick) it probably indicates large upward motion. For comparison, the escape velocity of the star at the surface is 600 km s^-1. Individual line profiles are displayed in Fig. 8. In order to mark the asymmetry of the profile both two lines are drawn at 69 km s^-1, the estimated vsini of the star. The intensity of the peak does not exhibit significant changes at those phases, while the red wing is seen to recede slightly. As a result, an overall decrease of both EW and FWHM is observed at these phases in Fig. 3. Note also that changes in the red wing occur mainly at velocities, 50 km s^-1 and that profiles shown are significantly shifted to the blue. This is perhaps the most extreme manifestation of the strong asymmetry presented in Sect. 5 and so confirms that this effect is real rather than just an artifact due to the method of analysis itself.

Fig. 8. Sequence of H line spectra of BD+ taken at the beginning of the night August 4 at an interval of 7 min, in the order -, - -, ..., -.- from 0 to 0.06. The central line marks the rest velocity of the stellar photosphere and the other two vertical lines indicate the maximum Doppler shifts associated to the rotational velocity, vsini. Extra emission is clearly seen in the blue wing up to -200 km s-1, away from the line centre. Signs of excess emission are also apparent more or less symmetrically in the red (v200 km s-1), as explained in Sect. 7.1

An emission transient event is seen to start at approx. 0.11 with considerable enhancement of the blue wing of the H profile. The phenomenon has been identified as a flare according to the fact that the He I D₃ line went into emission at the time the H EW and FWHM reached their respective maxima, 0.2. He I D₃ emission is known as a good indicator for explosive release of energy in connection to flares. However, contrary to what is commonly seen during flares there is no appreciable change in the red wing of the H line profile, and so the asymmetry we referred to before becomes more evident at these phases. The flare corresponds to the well distinguished peaks in both EW and FWHM that are shown in Fig. 3 (intervals B-C), at the time of maximum blueshift. This is illustrated by the sequence of profiles in Fig. 9. The line wings extend to much larger velocities on the blue side than on the red side.

Fig. 9. Series of H line profiles illustrating the evolution of the flare seen on August 4. Profiles occur in order of increasing peak, at phases 0.11 (-), 0.13 (...), 0.20 (- -). Note the strong asymmetry in the red wing

Extra emission reappears weakly in the far blue wing (vsin-240 km s^-1) at 0.34 on a short time scale (40 min). This feature is presumably due to less intense mass motions. Another possibility is that it results from the scattering of light by co-rotating clouds back into the line of sight. In this case, the observed velocities imply a projected cloud distance of 3.6sini (v = ), which may be compared to the stellar co-rotation radius, = 2.83. Evidence of a pedestal emission, that could be interpreted as the signature of co-rotating clouds is also clear in Fig. 8, is detected symmetrically in the far wings of the H profile (200 km s^-1).

Evidence of excess emission in the blue wing is found again at the end of the night (0.58-0.62) with velocities in the order of -200 km s^-1. This occurs at the time the H line is stronger and more symmetric, showing a weaker red-shifted absorption than during the rest of the night.

7.2. Observations on August 5

During this night the star displays a higher level of activity than in the previous night. The series of spectra shown in Fig. 10 were selected to illustrate the evolution of the H line profile during the main period of activity. This consisted of two contiguous episodes, within which the line was seen to enhance progressively and decay afterwards in a continuous way. Therefore, the whole sequence is segmented in four different sets of successive spectra, each of them corresponding to a monotonic increase/decrease of the H emission. Spectra in the same set are overplotted using different line styles. In order to help to follow the relative changes in the profile, the last spectrum of each set is also the first one in the next set. The two vertical lines mark the velocity range vsini and the corresponding phases are annotated in the right to allow comparison with Fig. 3.

Fig. 10. Series of H line profiles illustrating the evolution of the large flare seen on August 5. The spectra are grouped in different sets to represent the main stages of the whole event, as discussed in Sect. 7.2. Different line styles were used to distinguish them. The last spectra of each set (- -) is always the first one in the next set (-) and the corresponding phases are written on the right. While profiles in the first and third sets (from bottom to top) are in order of increasing line peak, in the second and fourth sets of profiles are in order of decreasing peak

A first brightening with simultaneous fast ejections of material was already developing when the observations started. Note the extra emission seen in Fig. 10 at velocities up to -200 km s^-1 as the flare evolves. The H line becomes gradually more symmetric and considerably stronger, up to an absolute maximum EW 0.97, reached at both 0.48 and 0.57. A considerable blueshift becomes evident at this time in spite of the fact that the H line seems to be inherently drifting to the red during the whole interval of phases 0.4-0.9 (see Fig. 3). Such discontinuity is actually due to significant mass motions in the blue together with an absorption of the red wing that causes a strong asymmetry in the profile (see Figs. 5 (right) and 10). This is the same absorption effect that was observed during the previous night but it is less important here and does not persist during the flare. A simple blueshift of the line centre would not explain the extreme asymmetry of the profile and would also be difficult to understand since the Doppler shift of the line tends to be positive at these phases according to the observed modulation of the RV parameter.

He I emission was also detected during the steep rise of the H EW curve in Fig. 3. This feature, together with a dramatic enhancement of the Balmer lines, are both characteristic of the solar flare spectrum. Thus, the phenomenon seen here is ascribed to a chromospheric flare in BD+.

The decay of the flare corresponds to the second set of spectra in Fig. 10, at phases 0.56-0.70, and leads to very low values of EW and FWHM (0.7). Apparently it consists of a progressive absorption in the blue wing with respect to the preceding spectra but this could be caused by the intrinsic modulation of the line as manifested by the temporal evolution of the RV parameter in Fig. 3.

A similar phenomenon follows at 0.72, with associated He I emission. As before, this second event is attributed to another flare. The corresponding profiles in Fig. 10 show the rapid rising of the flare and its posterior decay. However, its effects are only important in the core of the line and there is no sign of extra emission in the wings. It extends over a shorter interval of phases, corresponding to the narrower peaks of both EW and FWHM that follow the previous flare in Fig. 3. Note that the FWHM does not experiment such a large increase as the EW does, reflecting the fact that this second brightening enhances mainly the peak of the line.

In contrast with the first flare, this second event decays rapidly just after the time of the maximum at 0.8. The EW and FWHM of the line decrease dramatically as the left half of the profile is seen to recede. An absolute minimum of emission is reached at 0.92 before the line recovers again. At these phases rotational modulation effects could not be significantly affecting the appearance of the profile as the line centre keeps at velocities close to zero. The strong depression seen in the blue wing while the peak velocity does not shift significantly to the red argues for a real absorption component observed at negative velocities (v-50 km s^-1). A preflare spectrum is not available due to the complexity of the whole flare event and also because when the observations started the first part of the flare was already in progress. Therefore, an attempt to remove the rotational modulation effects and study in more detailed the intrinsic flare emission would be inappropriate.

© European Southern Observatory (ESO) 1999

Online publication: December 4, 1998
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