7 Discussion

In order to search for correlations providing clues about the nature of $\lambda$ Bootis stars, the abundances for our program stars have been compared with those of Procyon, a primary standard with solar metallicity (Steffen 1985). Chemical abundances of the class prototype, $\lambda$ Boo, were taken from the literature (Paunzen et al. 1999b). As can be deduced from Table 4, all the program stars but one (HD 184190) show a clear metal deficiency when compared to Procyon. Moreover, it is quite evident that the candidate $\lambda$ Bootis stars here analyzed do not resemble the abundance pattern of the class prototype, in particular for some chemical species (magnesium, titanium and iron). This result poses some concern with the idea of that $\lambda$ Bootis stars form a well separate, chemically homogeneous group of the stars. On the contrary, this result reinforces the hypothesis proposed by Stürenburg (1993) that $\lambda$ Bootis stars cover a continuous sequence of underabundances from very metal week to solar metallicities.

At this point, it would be necessary to reopen the question of the $\lambda$ Bootis class definition. If the definition by Baschek & Searle (1969) - "... $\lambda$ Bootis stars can be defined as stars whose composition resembles that of $\lambda$ Boo itself'' - is strictly adopted, then all the stars in our sample should be rejected as potential candidates to the $\lambda$ Bootis group. On the contrary, if the more conservative definition given by Paunzen et al. (1997a) is considered - "... $\lambda$ Bootis stars are Population I, metal-weak (except for C, N, O, S) stars'' -, all the stars (except HD 184190) could be catalogued as members of the $\lambda$ Bootis class provided that they show solar abundances for carbon, nitrogen, oxygen and sulphur. It is of fundamental importance to stress this point since the $\lambda$ Bootis phenomenon is not ascribed to an overall metal deficiency but to a mechanism able to produce underabundances of the heavy elements contrasting with the solar abundances of C, N, O and S. None of these species have been analyzed in this paper due to the lack of spectral lines fulfilling the requirements quoted in Sect. 6. NLTE abundances of nitrogen and sulphur for the program stars will be part of a subsequent paper (Kamp et al. 2001).

The observed distribution of the abundance values does not permit us to discriminate between the theories proposed to explain the $\lambda$ Bootis phenomenon. It could be explained in terms of the diffusion/mass-loss hypothesis on the basis of different stages of the diffusion process. Under the assumption of the accretion hypothesis, the observed differential deficiencies would reflect different scenarios in the gas-dust decoupling ascribed to different disk properties. Moreover, the binarity theories would be also supported. The relation between metallicity and $v \sin i$, $\log\,g$, $T_{\rm eff}$ has been plotted in Fig. 4. Whereas no obvious correlation between $\log\,g$and $T_{\rm eff}$with [Fe/H] exists, there is evidence for a connection between projected rotational velocities and metallicity in the sense that the metallicity is higher when $v \sin i$  increases, although the correlation coefficient ( $\rho = 0.59$) and the small number of measurements (n = 14) do not permit to obtain any statistically significant conclusion. A similar result was found by Holweger & Rentzsch-Holm (1995) using calcium abundances derived from the Ca II K line. If, after the determination of the C, N, O and S abundances the membership of the program stars to the $\lambda$ Bootis group is finally confirmed, this result would nicely fit with the accretion theory: for large $v \sin i$  the meridional circulation mixes material of solar composition from the stellar interior into the convection zone so that any surface contamination due accretion of circumstellar material should vanish.