B[e] stars are stars of spectral type B that display in their optical spectra
strong Balmer line emission and emission of permitted and forbidden
transitions of predominantly low-ionized and neutral metals. In
addition, B[e] stars exhibit a dense, dusty environment witnessed by their
infrared excess emission. In a nutshell, the abbreviation "B[e]" stands for
spectral type B with emission lines (e), and following the notation of
forbidden emission lines given by the square brackets .
Since their classification is purely based on their optical appearance, which
is dominated by the stars' circumstellar material rather than by the stellar
photosphere itself, it is obvious that the group of B[e] stars can contain
stars in quite different evolutionary stages. And in fact, detailed
investigations by Lamers et al. (1998) revealed that the group of B[e] stars
can be split into several B[e] classes:
Herbig Ae/B[e] stars
These are young, unevolved pre-main sequence stars. They
are still surrounded by dense, dusty accretion disks from which the infrared
excess and the Balmer line emission originate. Also, they are often surrounded
by reflection nebulae giving rise to the forbidden emission lines.
Compact planetary nebulae
These are evolved low-mass stars just after the asymptotic giant branch (AGB)
phase during which the star had blown off its outer shells, while the core of
the star starts to develop into a white dwarf. The central stars of such
compact planetary nebulae are thus not necessarily of spectral type B but can
be O stars as well. The dense and cool circumstellar remnant material from the
AGB phase hides and disguises the probably hot core. Only few such compact
planetary nebulae are known to show the B[e] phenomenon like the enigmatic object
Hen 2-90; they usually show
indications for dense circumstellar disks.
These are evolved massive stars surrounded by large and dense dusty disks or
rings. B[e] supergiants are the most puzzling class of B[e] stars. Especially
the presence of dusty disks around these hot and luminous objects is far from
These are interacting binary stars consisting of a cool red giant and a hot
compact object. Dust is usually formed in the accretion disk surrounding the
hot component. This accretion disk is created during the epoch(s) of binary
interaction and is rather short-lived. This is why basically every symbiotic
object shows the B[e] phenomenon at least once in its lifetime.
Unclassified B[e] stars
Despite the classification of B[e] stars according to their evolutionary phase,
we are left over with about half of all previously known B[e] stars for which
either no or no unambiguous classification could be achieved so far. These stars
are grouped in the bag of unclassified B[e] stars.
While Herbig objects, compact planetary nebula, as well as symbiotic objects
are reasonably well understood, the situation is completely different (but not
hopeless !) for the B[e] supergiants and the unclassified B[e] stars. Both
groups are worth being studied in great detail and this is what we are
We study these two groups of stars by means of:
To study the non-spherically symmetric winds and the dusty disks
we make use of several facilities:
- High-resolution optical spectroscopy performed with the ESO 2.2m
telescope in Chile (FEROS) and with the 2m telescope in Ondrejov,
Czech Republic, to determine (if possible) the spectral type of the
stars based on the photospheric absorption lines, and to study the
wind conditions, based on the huge bunch of emission lines.
- Interferometric data with the VLTI/AMBER and VLTI/MIDI instruments
to study (at least for the bright galactic targets) the dust
composition and the inclination angle and size of the dusty disks.
- Medium- and high-resolution near-infrared spectroscopy as provided
by the ESO/SINFONI, GEMINI/PHOENIX, GEMINI/GNIRS, LBT/LUCIFER
spectrographs, with special focus on the K-band at 2.23 μm.
These K-band spectra cover the first-overtone bands of the CO molecule
which are ideal tracers for the kinematics as well as the density and
temperature structure of the molecular parts of the disks.
For the modeling, we have developped several computer codes, which allow
- to calculate the latitude-dependent ionization structure in
non-spherically symmetric winds,
- to model especially the forbidden emission lines from the wind and
from the disk, to study the non-spherical mass loss history of B[e]
stars and the kinematics of the circumstellar gas,
- to model the free-free and free-bound emission from the ionized wind
- to model the spectral energy distributions of their circumstellar
- to model the emission of the circumstellar molecular gas like, e.g., the CO
bands, to test various geometries and kinematics (e.g.,
Keplerian rotating disk, outflowing disk-forming wind).
The following links will direct you to a brief overview on our most recent
research results on both the B[e] supergiants
and the unclassified B[e] stars
Last modified 4.2.2011