2. Photometric selection of candidate stars

 

When we started this project, there was considerable confusion in the literature over which stars should be considered as members of the Bootis group. A small sample of twenty well established members (Gray & Corbally 1993) gave a general indication of the region the Bootis stars occupied in the various photometric diagrams, but the boundaries of these regions were ill-defined. Since these stars are quite rare (they comprise about 1% of the field A-type stars), the main problem was to define photometric boxes which would allow the detection of Bootis stars with a high probability. These boxes had to be large enough to include the whole range of possible Bootis stars, but also small enough to make a spectroscopic survey feasible. We finally adopted the relations given in Table 1 (click here) as our "search box" to select candidates from their colours in the Strömgren photometric system. We have found it impractical to use derived indices such as and because of their insensitivity to the characteristics of Bootis stars. This is especially true for stars in the main sequence band (Fig. 1 (click here)). As an example we take Bootis itself. The calibrations taken from Hilditch et al. (1983) give E(b-y)=0 and which implies a solar abundance for Bootis, however, Venn & Lambert (1990) have found . The Hauck & Mermilliod (1990) catalogue was used as the database for our search. To avoid duplication of effort, we have removed from our candidate list stars already classified in Corbally (1984) and Gray & Garrison (1987, 1989a,b) as well as known MK standards. In addition we have searched for possible candidates in open clusters using the list of North (1993). Members of these clusters were dereddened and checked for their location in the photometric boxes. In total a list of 600 possible candidates was compiled for the northern and southern hemisphere. Up to now about 300 stars have been observed and 120 have been classified.

   

-0.025 < b-y < 0.3

0.13-0.3(b-y) < m₁ < 0.22-0.3(b-y)

c₁ < 1.4-2(b-y)

Table 1: Selection criteria for candidates

  
Figure 1: m₁ versus b-y. Crosses are normal stars from Gray & Garrison (1987, 1989a,b), filled triangles are Bootis stars from Paunzen et al. (1997), filled circles are Bootis stars discussed in Sect. 6.1 (click here), open circles are candidate Bootis stars discussed in Sect. 6.2 (click here). The standard line is taken from Philip & Egret (1980)