The Bootis stars are a class of metal-poor Populationi A-type stars.
Although the
prototype was already described by Morgan et al. (1943),
the definition as a separate class among
chemically peculiar (CP) stars is still controversial.
Bootis stars occupy the same
parameter space in a Hertzsprung-Russell-diagram
as do "normal" A-type stars, CP1
and cool CP2 stars. Figures1 (click here), 4 (click here) and 5 (click here) illustrate
this coincidence for Geneva and Strömgren colour indices.
The plotted colour indices were taken from Hauck & Mermilliod (1990),
Handler (1995) for the Strömgren photometric system and from the Geneva
database for the Geneva photometric system. The values for the
Bootis stars are
listed in Tables 2 and 3.
Figure 1: B2-V1 versus d. The solid lines are the standard
relations after Golay (1980), crosses are
"normal stars" from Gray & Garrison (1987, 1989a,b),
open squares are
CP-stars brighter than seventh magnitude from
Renson et al. (1991),
filled triangles are Bootis stars
The evolutionary status of Bootis stars is not clear yet.
The two theories discussed in the literature
exclude each other and observations are not yet conclusive
enough for a decision.
These theories involve either accretion of interstellar matter,
or a combination of diffusion and mass loss.
Bootis stars would
therefore provide excellent
tests for processes which play a considerable rôle in modern astrophysics.
Figure 2: B2-V1 versus . The solid line is the standard relation
after
Golay (1980). Symbols are the
same as in Fig. 1
In an attempt to compare properties of various stars
which were claimed to be Bootis\
stars and to determine unquestionable classification criteria,
it became obvious that
confusion exists frequently whether a given star belongs
to the group or not. When we started this project, there
was not even a sample of commonly accepted
Bootis stars available
which was large enough for a sound statistical analysis of group properties.
We felt it therefore necessary to define first a sample of unambiguously
identified
Bootis stars, based on a consistent and independent
classification, before any
further steps can be taken to increase the number of group
members and to define their
common properties, as well as to develop a consistent
Bootis evolutionary theory.