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Astron. Astrophys. 333, 125-140 (1998)

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6. Transient high-velocity absorption components

During the precursor phase, but also at the beginning of an outburst, transient high velocity absorption components may develop. They only appear at RVs more negative than [FORMULA] sin i, i.e. only on the blue side and at super-rotational values. When they are present, they can be seen in all stronger He i lines (Table 4 and Fig. 5). However they were not detected in hydrogen or other lines. In µ Cen, such an event was first observed by Peters (1986) in March 1985 when it was also followed by an episode of enhanced [FORMULA] emission.


[TABLE]

Table 4. Velocities and equivalent widths of the temporary high velocity absorption components during 3 events described in Sect.  6for different He i lines. For the He i [FORMULA] 3820,4026,4471 lines the average profile of the 1996 run has been subtracted before fitting a Gaussian to the additional absorption. Therefore, larger errors are to be expected especially for the central position of the Gaussian [FORMULA] in these lines. The typical errors are [FORMULA] for [FORMULA], and [FORMULA] mÅ for the Gaussian's equivalent width [FORMULA]


In 1996, the increase of the emission wings of the major outburst began around MJD 50 185 with the appearance of the high-velocity absorptions (the dotted line in Fig. 3, 1996 panel). A similar but weaker high-velocity absorption event was observed in January 1997 on MJD 50 457. In Peters' observations, [FORMULA] was in emission already at least one hour prior to the helium absorption event. In 1997, another event happened on MJD 50 461, i.e. only 4 days after the one described above. Interestingly, a single spectrum taken in between, namely on MJD 50 459, showed no trace of additional absorption.

In all three cases the lifetime of the high-velocity absorption features is unfortunately only ill constrained by the sampling of the data. In 1996 it was at most one day and in 1997 at most two days. For the same reason, nothing can be said about any possible migration or not of the high-velocity features. But from Table 4 it can be seen that for a given spectral line all three events occurred within a narrow range of only [FORMULA] 20 [FORMULA]. Even the most pronounced event had only a depth of about 2% of the continuum flux.

Additionally, we also observe manyfold transient line profile variability, described e.g. as "dimples" by Smith et al. (1996). However for the HEROS data neither the spectral nor the temporal resolution is high enough to allow a study of these features.

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© European Southern Observatory (ESO) 1998

Online publication: April 15, 1998
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