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Astron. Astrophys. 348, 831-842 (1999)

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4. Synopsis

4.1. Empirical conditions for the occurrence of CQE's

From the database described above, it appears that several conditions are important for the occurrence of CQE's:

  • The star possesses a circumstellar disk.

  • The orientation is such that the star is seen equator-on and the disk edge-on.

  • The continuum opacity of the disk is low, while the shell lines are opaque.

  • The outer radius of the disk is small.

  • The intrinsic width and turbulent broadening of the shell lines are low.

The observational evidence is, point by point, summarized in the following:

  • Orientation: All six stars showing CQE's are shell stars which are characterized by narrow absorption lines mostly of singly ionized metals superimposed on the spectrum of a broad-lined B star. This is now commonly accepted to be due to the combination of a rapidly rotating star with a circumstellar equatorial disk viewed edge-on. Sect. 6 elaborates more on the disk nature of the circumstellar envelope of Be stars.

  • Shell nature: The new observations show clearly that CQE's occur only when shell features are also present. Often they appear in typical shell lines ([FORMULA] Cap, [FORMULA] Car, o And), but also in photospheric lines with circumstellar contributions such as, e.g., emission components in the wings of HeI lines of [FORMULA] Cap in 1995. In 1998 September, the latter star provided an even more significant indication of the circumstellar connection, when the CQE's in HeI lines disappeared simultaneously with the circumstellar emission components.

  • Dimensions of the disk: At the time of visibility of CQE's in [FORMULA] Cap, [FORMULA] Cen, and o And, their H [FORMULA] emission was relatively weak, which is indicative of a comparatively small spatial extent of the disks from where it originates. For 4 Her it is known (Koubský et al. 1993, 1997), that CQE's are present only when the disk is in the beginning of its formation process, but they disappear later. For [FORMULA] Cen, Coté et al. (1996) conclude from the analysis of IRAS far-IR data, which were obtained when the H[FORMULA] emission probably was weak, that this star's envelope was at that time, too, small.

    The strength of the Balmer emission of [FORMULA] Car suggests that its disk is probably the most extended one of the observed sample of stars. However, either its intrinsic density or the column density along the line of sight (or both) do not seem to be high, judging from the strengths of the shell lines. Moreover, CQE's appear with a range of prominence in different shell lines, as is demonstrated in Fig. 4 for [FORMULA] Car. In this object the FeII lines, which are supposed to be formed over the greatest range of parameters among the metal shell lines, only show narrow, pure absorption cores. Among the FeII lines, these cores are strongest in the transitions belonging to multiplet 42, which is the one most commonly observed shell line in Be stars. By contrast, the lines of MgII , TiII , and CrII , which are probably formed in a more limited region, i.e. closer to the star, do exhibit CQE's.

  • Thermal width: The above description of the observations shows unambiguously that CQE's are seen more easily in lines with higher atomic weight, i.e. lower thermal width. This is demonstrated best for [FORMULA] Car and o And in Table 2, where the FWHM, and by implication also the potential visibility, of CQE's changes from HeI to FeII . That is, the FWHM of CQE's in HeI or SiII is larger by about 10 km/s than in FeII . Therefore, in HeI or SiII lines a CQE may appear in the form of a flat bottom of a profile rather than a local flux maximum separated clearly from the absorption wings. In a smaller number of lines this trend in FWHM can also be confirmed for [FORMULA] Cap. To some extent, such a behaviour can also be seen in Fig. 5 of Koubský et al. (1997), comparing MgII to the neighbouring TiII and FeII lines. When CQE's were reported in HI lines ([FORMULA] Cap, o And), it was always at the very beginning of a shell phase in these lines, when the density and radius of the disk were still rather low.

  • Line strength: Similarly, CQE's occur preferentially in the relatively strongest shell lines, i.e. when the line opacity is high.

It is evident from the above that either this set of conditions overconstraints the formation of CQE's or the available observations are still incomplete or both. But it does provide solid, empirical guidelines for an attempt to understand CQE's in the framework of shell line formation.

4.2. CQE's and disk phase transitions

Many shell stars are known for phase transitions from a shell- to a pure Be-, sometimes even plain B-, and then again shell-type appearance (e.g., Hubert-Deplace & Hubert 1979). The typical time scale for such cycles is close to a decade. But there are also less spectacular changes in the appearance of the circumstellar spectrum, often on shorter time scales. It is likely that many of these variations are linked to the replenishment and subsequent dilution of the circumstellar disk (see also Sect. 6).

From the rather scattered observations of CQE's it is difficult to establish a temporal profile of their life cycles. But the time scales of their variability are not incompatible with those of disk phase transitions. Moreover, the cases of [FORMULA] Cap, [FORMULA] Cen, o And, and 4 Her suggest that CQE's are more likely to occur around the early phases of the build-up of a new disk or when the inner disk is re-filled with matter. They would weaken and eventually disappear, once the disk has reached more sizeable dimensions and greater density. Therefore, the initial speculation (Baade 1983) that CQE's are early tracers of the recurrence of a shell is still only a conjecture but now better justifies a more systematic follow-up.

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© European Southern Observatory (ESO) 1999

Online publication: August 13, 199