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2. Present status

2.1. Classification criteria

  The peculiarity of tex2html_wrap_inline1914Bootis 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 tex2html_wrap_inline1916Bootis 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 tex2html_wrap_inline1918Bootis 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: Baschek et al. (1984) found some strong absorption features at 1600Å and 3040Å in IUE spectra. These features are observed only for tex2html_wrap_inline1940Bootis stars and were used to define new candidates. Holweger et al. (1994) identified the 1600Å feature as a satellite in the Lyman tex2html_wrap_inline1942 profile due to perturbation by neutral hydrogen. Observations in the infrared and optical region gave some evidence for gas and dust shells around tex2html_wrap_inline1944Bootis stars (Gerbaldi & Faraggiana 1993; Bohlender & Walker 1994; Andrillat et al. 1995).
To what extent the lack of a measurable magnetic field larger than tex2html_wrap_inline1946 300tex2html_wrap_inline1948 (Bohlender & Landstreet 1990) is characteristic for tex2html_wrap_inline1950Bootis 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 tex2html_wrap_inline1952Bootis 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 tex2html_wrap_inline1956Bootis stars?
The membership problem is also reflected in the inflation of members in tex2html_wrap_inline1958Bootis 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 tex2html_wrap_inline1960Bootis stars in their catalogue.
For a consolidation of the catalogue we have to return to what are considered to be the intrinsic properties of tex2html_wrap_inline1962Bootis stars: Popi hydrogen burning A-type stars, which are, except of C, N, O and S, metal poor.
The degree of metal deficiency, tex2html_wrap_inline1964, and tex2html_wrap_inline1966 can be determined primarily by spectroscopic techniques, and in particular by time consuming abundance analyses.

2.2. Abundance analyses

  The first abundance analysis was made by Burbidge & Burbidge (1956). They investigated two tex2html_wrap_inline1968Bootis 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: 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.

  figure274
Figure 3: b-y versus tex2html_wrap_inline1972. The solid line is the standard relation after Philip & Egret (1980), the symbols are the same as in Fig. 1

2.3. Theories

Only recently, theories have been developed to explain the tex2html_wrap_inline1976Bootis phenomenon. First, Michaud & Charland (1986) advanced a diffusion/mass-loss theory, according to which the tex2html_wrap_inline1978Bootis\ 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 tex2html_wrap_inline1980Bootis\ phenomenon, what would result in young tex2html_wrap_inline1982Bootis 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 tex2html_wrap_inline1984Bootis star.

  figure290
Figure 4: b-y versus tex2html_wrap_inline1988. 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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