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The peculiarity
of
Bootis itself was detected by Morgan et al. (1943)
and soon after this classification, other stars with similar
peculiarities were discovered
(Slettebak 1952, 1954). Slettebak et al. (1968) also used the space
velocity to distinguish
Bootis from PopulationII stars as well as their
"moderately large rotational velocity''.
Hauck & Slettebak (1983) investigated the group properties
in the Geneva and Strömgren
system. After the discovery of new
Bootis stars by
Abt (1984a, 1985), a list of criteria for accepting new
candidates was established by
Hauck (1986).
Gray (1988) investigated the hydrogen-line profiles and adopted
two subgroups with
normal and peculiar hydrogen-line profiles. He also proposed
the most recent definition for membership:
-
Bootis stars have K and metallic-line types within a few
temperature classes of A0, weak
4481 lines, hydrogen lines
with cores typical of early to late A type stars, and broad, but often shallow
wings, similar in extent to those of early AV a or V b stars. Relative
to a temperature type based on the hydrogen-line cores, the K- and
metallic-line types are too early, thus the spectrum as a whole appears metal
weak.
-
The following classes of stars should be excluded from the
Bootis\
even if they show weak
4481 lines: shell stars, protoshell stars,
He-weak stars (easily distinguished on the basis of their hydrogen-line
temperature types), and other CP stars. FHB and intermediate PopulationII\
stars may be distinguished from the
Bootis stars on the basis of
their hydrogen-line profiles. High-
stars should be considered as
Bootis candidates only if the weakening of
4481 is
obvious with respect to standards with high values of
.
-
The
Bootis stars fall into two distinct classes with normal (early A-dwarf)
hydrogen-line
profiles (NHL), and with peculiar hydrogen-line profiles (PHL) with weak cores
and broad but often shallow wings.
Baschek et al. (1984) found some strong absorption features
at 1600Å and 3040Å
in IUE spectra. These features are observed only
for
Bootis stars and were used to define new
candidates. Holweger et al. (1994) identified the 1600Å feature
as a satellite in the Lyman
profile due to perturbation by neutral
hydrogen.
Observations in the infrared and optical region gave some
evidence for gas and dust shells around
Bootis stars (Gerbaldi & Faraggiana 1993;
Bohlender & Walker 1994; Andrillat et al. 1995).
To what extent the lack of a measurable magnetic field larger
than
300
(Bohlender & Landstreet 1990) is characteristic for
Bootis stars
cannot yet be assessed due to the
limited number of polarimetrically investigated stars.
This brief review of the development of various classification
criteria explains the present
inhomogeneity of the group of
Bootis stars. Very few of
the members fulfill all the
photometric and spectroscopic criteria including the UV, visible and IR spectral
regions. But what about those candidates which match only a
subset, and which criteria are
unique to
Bootis stars?
The membership
problem is also reflected in the inflation of members
in
Bootis star lists. A critical analysis
of candidates known in the eighties resulted in
20 entries (Gray 1988), the same number
is given in Faraggiana et al. (1990). Renson et al. (1990) include already
101
Bootis stars in their catalogue.
For a consolidation of the catalogue we have to return to what
are considered to be the intrinsic properties of
Bootis stars:
Popi hydrogen burning A-type stars, which are, except
of C, N, O and S, metal poor.
The degree of metal deficiency,
, and
can be
determined primarily by
spectroscopic techniques, and in particular by time consuming
abundance analyses.
The first abundance analysis was made by Burbidge &
Burbidge (1956). They investigated two
Bootis stars and found metal
deficiencies by a factor of 20 relative to the Sun. Baschek & Searle (1969)
reported a metal deficiency by a factor of 3 in three stars, but they found
the oxygen abundance being almost normal. Venn & Lambert (1990)
confirmed the previous results and added C, N, and S as
near-solar abundant elements.
Stürenburg (1993) analysed extensively 13 stars and he
summarized the abundances
pattern:
- The light elements (C, N, O and S) have a solar abundance.
- The heavier elements (Mg, Al, Ca, Fe, ...) are underabundant by up to a
factor of 100.
Heiter (1996) confirmed the abundance values obtained for two stars
by Venn & Lambert (1990)
and by Stürenburg (1993) and extended for those two stars the
list of elements with determined abundances.

Figure 3: b-y versus
. The solid line is the standard relation after
Philip & Egret (1980), the symbols are the same as in Fig. 1
Only recently, theories have been developed to explain the
Bootis phenomenon.
First, Michaud &
Charland (1986) advanced a diffusion/mass-loss theory, according
to which the
Bootis\
stars are rather old and at the end of their main-sequence lifetime.
Venn & Lambert (1990) argue that accretion of metal-depleted
gas from the surrounding interstellar medium causes the
Bootis\
phenomenon, what would result in young
Bootis stars on the ZAMS.
Waters et al. (1992) described a selective depletion scenario
similar to post-AGB stars. Charbonneau (1991, 1993) and Turcotte & Charbonneau
(1993) presented numerical calculations describing the surface
and internal abundance evolution of a
Bootis star.

Figure 4: b-y versus
. The solid line is the standard relation after
Philip & Egret (1980). Symbols are the same as in Fig.1
Results from spectroscopy (Gray & Corbally 1993) and
asteroseismology (Weiss et al. 1994) are presently inadequate to decide between
these theories. The latter technique would be particularly powerful for a
discrimination between evolved and unevolved stars what motivated us to perform
simultaneously to our spectroscopic classification survey a
photometric survey for
variability which presently includes 2/3 of all catalogue members.

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