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Analysis and Interpretation of Astronomical Spectra 76

attributed to non-radial pulsations of the star or the passage of a close binary star com-
ponent in the periastron of the orbit.
– Such discs can arise within a short time but also disappear. This phenomenon may pass
through overall three stages: Common B star, Be–star and Be–shell star [33].
Classical example for that behaviour is Plejone (Plejades M45), which has passed all
three phases within a few decades [147]. Due to its viscosity, the disk material moves
outwards during the rotation [141].
– With increasing distance from the star, the thickness of the disk is growing and the den-
sity is fading.
– If the mass loss of the star exceeds that of the disc, the material is collected close to the
star. In the opposite case it can form a ring.
– The X-ray and infrared radiation is strongly increased.
If a B star mutates within a relatively short time to a Be star (eg scorpii δ), the Hα absorp-
tion from the stellar photosphere is changing to an emission line, now generated in the cir-
cumstellar disk. At the same time it becomes the most intense spectral feature (extensive
example see [30] sect. 22). It represents now the kinematic state of the ionised gas disk. It
is, similar to the absorption lines of ordinary stars, broadened by Doppler Effects, but here
due to the rotating disk of gas and additional, non-kinematic effects. Therefore the FWHM
value of the emission line is now a measure for the typical rotation velocity of the disc ma-
terial. For δ scorpii in [146] it is impressively demonstrated, that since about 2000 the
FWHM and EW values of the Hα emission line are subject to strong long-period fluctua-
tions. Between the first outbursts in 2000 to 2011, the Doppler velocity of the FWHM value
fluctuated between about 100–350 km/s and the EW value from –5 to –25 Å, indicating
highly dynamic processes in the disk formation process. Furthermore the chronological pro-
file of FWHM and EW values is strikingly phase-shifted.

Formula for the Rotation Velocity of the Disk Material:

Several formulas have been published, which allow to estimate the rotation speed of the
disc material from Be stars. In most cases is calculated with values at the

Hα line.

Following a formula by Dachs et al which Soria used in [145]: It expresses explicitly the
value, based on the [km/s] at the Hα emission line, combined with the

(negative) equivalent width EW [Å].

s

Example: The Hα Linie of Soria‘s Be–star yields , corresponding to a
.
Doppler Velocity of 278 km/s and . This results to

In [30] sect. 22.3 this formula is applied to a DADOS spectrum of δ scorpii. The accuracy of
this method is limited to ±30 km/s. Therefore "estimate", is here probably the better ex-
pression than "calculate".

Hanuschik [127] shows a simple linear formula, which expresses just with the

[km/s] of the Hα emission line. It corresponds to the median fit of a strongly

scattering sample with 115 Be stars, excluding those for which an underestimation of the
value was assumed; [127], {1b}.

With Soria’s (see above) results a different