Astron. Astrophys. 348, 831-842 (1999)
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
The continuum opacity of the disk is low, while the shell lines are
The outer radius of the disk is small.
The intrinsic width and turbulent broadening of the shell lines are
The observational evidence is, point by point, summarized in the
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 ( Cap,
Car, o And), but also in
photospheric lines with circumstellar contributions such as, e.g.,
emission components in the wings of HeI lines of
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
Dimensions of the disk: At the time of visibility of CQE's
Cen, and o And, their
H 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
Cen, Coté et al. (1996)
conclude from the analysis of IRAS far-IR data, which were
obtained when the H emission probably
was weak, that this star's envelope was at that time, too, small.
The strength of the Balmer emission of
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
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 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 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
( 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
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
Cap, 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.
© European Southern Observatory (ESO) 1999
Online publication: August 13, 199