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Analysis and Interpretation of Astronomical Spectra 79

17 The Measurement of the Expansion Velocity

Different types of stars repel, at certain stages of evolution, more or less strongly matter.
The speed range reaches from relatively slow 20–30 km/s, typical for planetary nebulae,
up to several 1000 km/s for Novae and Supernovae (SNR). This process manifests itself in
different spectral symptoms, dependent mainly on the density of repelled material.

17.1 P Cygni Profiles
The P Cygni profiles have been already introduced in
sect. 5.6, as examples for mixed absorption- and
emission line spectra. They are a common spectral
phenomenon which occurs in all spectral classes,
and are a reliable sign for expanding star material.
The evaluation of this effect with the Doppler formula
is demonstrated here at the expansion velocity of the
stellar envelope of P Cygni.

As a rough approximation the offset [Å] is
measured between the peaks of the emission- and
the blue-shifted absorption part of the P Cygni
profiles [5]. Anyway, where exactly the offset should
be measured, depends also on the particular profile
shape. In the example for the Hα line of P Cygni
itself, the measured Peak-Peak difference yields:

(single measurement of 07.16.2009, 2200 UTC, DADOS and 900L/mm grating)

with , , yields:

The accepted values lie in the range of –185 bis – 205 km/s ±10km/s.The heliocentric
correction is not necessary here, since the Doppler shift is measured not absolutely but
relatively, as the difference, visible in the spectrum. P Cygni forms not a separate class
of stars. Such profiles are a common spectral phenomenon, which can be found in all
spectral classes and are a reliable indication for expanding stellar matter.

17.2 Inverse P Cygni Profiles

In contrast to the normal P Cygni profiles, the inverse ones are al-
ways a reliable sign for a contraction process. The absorption kink
of this feature is here shifted to the red side of the emission line.
Textbook example is the Protostar T Tauri which is formed by accre-
tion from a circumstellar gas and dust disk. The forbidden [O I] and
[S II] lines show here clearly inverse P Cygni profiles, indicating
large-scale contraction movements within the accretion disk,
headed towards the protostar. The Doppler analysis showed here
contraction velocities of some 600 km/s. The excerpt of the T Tauri
spectrum is from [33], Table 18.