6. Results

 

We have divided this section into two parts; in the first part we present our newly discovered and confirmed Bootis stars and in the second part we discuss certain candidate Bootis stars. The latter have a relatively high probability of being either bona-fide Bootis or related stars, but follow-up observations (e.g. with higher resolution and/or higher S/N spectroscopy) are required to confirm their spectral types.

6.1. New and confirmed Bootis stars

 

Based on our observational data we are able to identify six new Bootis stars and confirm the membership of six stars (see Table 1 (click here) in Paunzen et al. 1997). Table 2 (click here) lists all stars with a description of their spectra. Furthermore, we have listed Strömgren photometric indices from Hauck & Mermilliod (1990) as well as values from Uesugi & Fukuda (1982) and Abt & Morrell (1995). In the column (C) we give the classification of the hydrogen lines (normal or peculiar) according to Gray (1988). Classifications in parentheses - e.g. (Boo) indicate stars in which the Bootis characteristics are mild. The last column (Status) indicates if a star is newly discovered (N) or confirmed (C) as Bootis type.

   

HD HR ADS Spectral Type V b-y m₁ c₁ C Status

23392 8.70 0.014 0.154 0.975 2.917 NHL N

36726 4156 8.81 0.043 0.164 0.975 2.922 NHL N

64491 3083 6.23 0.196 0.132 0.669 2.734 75¹ (PHL) C 15²

74873 3481 5.87 0.064 0.188 0.934 2.890 85¹ NHL N 10²

90821 9.20 0.068 0.176 1.092 2.865 NHL N

91130 4124 7813 5.93 0.073 0.158 1.035 2.854 130¹ (PHL) C 190²

105058 8.88 0.129 0.124 0.993 130¹ (PHL) C

120500 6.59 0.069 0.171 1.060 2.871 NHL N

170680 6944 11411 5.14 0.006 0.140 1.052 2.892 305¹ NHL C 200²

171948 11498 6.71 NHL C

290492 4211 9.27D 0.084 0.133 0.931 2.851 NHL N

294253 9.69 0.023 0.133 0.926 2.904 NHL C

: ¹ Uesugi & Fukuda (1982), ² Abt & Morrell (1995).
HD23392: Mild Bootis star, hydrogen lines show broad wings.
HD36726: Spectrum very similar to HD31295 (see Gray 1988), member of the Orion OB1 association, classified as A0V (Warren & Hesser 1978) and as Am (kA1hA1mA5) by Abt & Levato (1977).
HD64491: Cool Bootis star similar to HD142703 with peculiar hydrogen lines, classified as A9Vp(Boo) by Abt & Morrell (1995).
HD74873: Classified as A1Vp (wk 4481) by Abt & Morrell (1995).
HD90821: Normal hydrogen lines.
HD91130: Shallow and broad hydrogen lines, classified as A0Vp (Boo) by Abt & Morrell (1995).
HD105058: This star was first classified as A2pec(Boo) by Slettebak et al. (1968). Observations in the UV confirmed the Bootis character (Faraggiana et al. 1990). Spectrum shows a very weak 4481Å line with slightly peculiar hydrogen lines.
HD120500: Mild Bootis star.
HD170680: Classified as A0Vp (Ca, Mg wk) by Abt (1984b), confirmed as Bootis stars in the UV by Baschek & Slettebak (1988).
HD171948: Classified as A0Vp (Boo) by Abt (1985).
HD290492: Mild Bootis character, star belongs to the Orion OB1 belt (Guetter 1981), classified therein as A0V. Close binary system (, d=2) which was resolved without any contamination.
HD294253: Classified as Bootis star by Levato et al. (1994), member of the Orion OB1 association.
It is one of the hottest as well as youngest Bootis star.

6.2. Candidate Bootis stars

 

Besides the above confirmed Bootis and "normal" type stars, we have also found some interesting stars which could not be classified as Bootis stars at first sight. Further observations are needed to unambiguously establish their spectral type and to avoid a misclassification. As a first step, a detailed abundance analysis is presently being carried out for three stars (HD84123, HD84948 and HD101108A). The italicized spectral types are derived from our spectra.

HD23258: A0Vb (sl wk metals), classified as A0Vp
( Boo) by Abt & Morrell (1995). This star is a borderline case: the metallic line spectrum is only marginally weak, the hydrogen lines are very broad.

HD39421: A1Va (wk 4481), classified as A2Vp
(wk 4481) by Abt & Morrell (1995).

HD66684: B9.5Va (sl wk metals), classified as B9.5Vp ( Boo) by Abt & Morrell (1995). Again a borderline case: SiII 4128-30 is present and of normal strength, MgII 4481 is only slightly weak.

HD74911: A2IV (wk 4481)

HD84123 and HD84948: Both stars show a very similar spectrum: kA6hF1mA6V and are perhaps field horizontal branch stars. HD84123 was found as a photometric candidate by Hauck (1986) and HD84948 was classified as F0Vwl(Boo) by Abt (1984a).

HD101108A: Classified as A5pec(Boo) by Slettebak et al. (1968) in the optical and was confirmed in the UV by Faraggiana et al. (1990). Our spectrum, on the other hand, result in: A3IV^- (wk 4481), very similar to the MK standard Eridani.

HD105199: kA0.5hF0mA3V Boo:, classified as A2V by Oblak et al. (1976). This would be the first Bootis star where the K-line type is in gross disagreement with the metallic-line spectrum.

HD149303A: A3IV-V (wk 4481), this star is classified as A2Vp(Boo) by Abt (1985).
Beside these galactic field stars, we have also observed stars in young open clusters. Observations in NGC2232 and NGC2301 yielded a null result. Two stars of NGC2264 turned out to be good candidates for membership in the Bootis group. Both stars are members of NGC2264 according to Walker (1956).

NGC2264#87W: A1.5IV (wk 4481), hydrogen line cores are shallow, maybe due to rotation.

NGC2264#138W: A0.5V (wk 4481), confirmed member of NGC2264 according to Vasilevskis et al. (1965).
Figures 1 (click here) and 2 (click here) show the location of all known Bootis stars in the `standard' Strömgren diagrams. Well established Bootis stars were taken from Paunzen et al. (1997), normal type stars are from Gray & Garrison (1987, 1989a,b). Although m₁ is not a good indicator of the metallicity for stars hotter than A2, the overlap (at the main sequence band, b-y<0.1) of the Bootis distribution with the normal stars might be taken to suggest that there exists a continuous transition from the normal stars to the Bootis stars (Fig. 1 (click here)). On the other hand, it should be noted that this diagram is essentially degenerate in this region, in that even quite extreme Bootis stars (such as Bootis itself) are found amongst the normal A-type stars. No difference between normal dwarf and Bootis stars shows up for the surface gravity in these photometric diagrams (Fig. 2 (click here)).

  
Figure 2: c₁ versus b-y. Symbols are the same as in Fig. 1 (click here). Standard line is taken from Philip & Egret (1980)