1 Introduction

$\lambda$ Bootis stars have been defined as late B to mid F, metal-weak, Population I objects (Paunzen 2000). The nature and evolutionary state of these stars is still a matter of debate and different hypothesis can be found in the literature.

1.1 Very young stars which have not reached the Main Sequence

This theory, suggested by Venn & Lambert (1990), supports the idea of a star surrounded by a shell of gas and dust, the volatile elements remaining in the gaseous phase while the refractory elements, with a higher condensation temperature, are locked in the dust grains. The depleted gas is accreted by the star whereas the dust grains are swept away from the shell due to radiation pressure. Waters et al. (1992) and Andrievsky & Paunzen (2000) have proposed scenarios where the dust-gas decoupling is always present in regions where the temperature drops to a value less than the condensation temperature of the heavy elements.

Observational results supporting the accretion theory are the high [C/Si] and [O/Si] abundance ratios found in some $\lambda$ Bootis stars which can be used as indicators of gas-dust separation (Paunzen et al. 1999a): carbon and oxygen have low condensation temperatures and tends to remain in the gas phase of the interstellar medium. Silicon, on the other side, has a high condensation temperature representing the elements locked up in the grains. Any preferential accretion of gas will lead to a [C/Si] or [O/Si] larger than solar. The correlation with Si of other elements of high condensation temperature also fits nicely with the accretion scenario (Stürenburg 1993). Charbonneau (1991) combined the accretion hypothesis with the diffusion theory concluding that, with an accretion rate of the order of 10 $^{-13}~M_{\odot}$ yr^-1, many peculiar characteristics of the $\lambda$ Bootis stars (including their restriction to the above-mentioned spectral types) are reproduced quite naturally. Turcotte & Charbonneau (1993) investigated the effect of mixing through meridional circulation, concluding that the abundance anomalies would disappear in 10⁶ years, so all of the $\lambda$ Bootis stars should be essentially young objects on or just arriving to the ZAMS. The scarcity of $\lambda$ Bootis will be also explained by the strict requirements for the accretion rate. The fact that all $\lambda$ Bootis stars are young objects is further strengthened by the lack of $\lambda$ Bootis stars in open clusters older than 10⁷ years (Gray & Corbally 1988) and the discovery of $\lambda$ Bootis stars in the young Orion OB1 association and NGC 2264 (Paunzen & Gray 1997).

A critical point in the accretion scenario is the necessary existence of gas and dust shells around $\lambda$ Bootis stars. Some $\lambda$ Bootis stars show clear indications of both gas (circumstellar lines) and dust (IR excess) shells (Holweger & Rentzsch-Holm 1995; Holweger et al. 1999). However, the fact that some $\lambda$ Bootis stars do not show evidence of shells does not necessary rule out this hypothesis. King (1994) concluded that the amount of depleted gas required to cause underabundances in $\lambda$ Bootis stars is small enough that any circumstellar dust associated with this gas is not necessarily detectable in the IR or submillimetre regions.

1.2 Stars at the end of their Main Sequence lifetime

Michaud & Charland (1986) investigated the effect on abundances in stellar atmospheres of the diffusion mechanism operating in the presence of mass loss. They found that, after about 10⁸-10⁹ years, mass-loss rates of only 10 $^{-13}~M_{\odot}$ yr^-1 can reduce the extreme overabundances predicted by diffusion theory for the Am stars to underabundances of many elements, the degree of underabundance of an element being a function of both gravity and time. According to this, $\lambda$ Bootis stars would be rather old and at the end of their Main Sequence.

There are, however, some points that cannot be explained with theory. One is the moderate underabundances predicted (only a factor of five less solar), very far from the strong underabundances found by Venn & Lambert (1990) in the three classical $\lambda$ Bootis stars ($\lambda$ Boo, $\pi^{1}$ Ori and 29 Cyg). Also it is difficult to understand how diffusion could operate in the presence of the meridional circulation which would likely be generated by rapid rotation. Turcotte & Charbonneau (1993) showed that even an equatorial rotational velocity of 50 kms^-1  suppresses the appearance, at any epoch of Main Sequence evolution, of the characteristic $\lambda$ Bootis abundance pattern.

1.3 Spectroscopic binary theory

Faraggiana & Bonifacio (1999) proposed this alternative to explain the underabundances of, at least, some $\lambda$ Bootis stars. The composite spectra of a binary system with disentangled components not very dissimilar produces a veiling effect with apparent underabundances. The authors pointed out that the lack of a uniform pattern in the chemical composition of the $\lambda$ Bootis stars can be easily reconciled with the binary hypothesis. Moreover, they claimed that the hypothesis of all $\lambda$ Bootis stars being very young objects in the late phase of their PMS evolution is highly improbable on the basis of the rapid evolution and the number of bright $\lambda$ Bootis candidates which would imply that the star formation process is still very active in the solar neighborhood, leading to an unexpected large number of Main Sequence B stars in a similar volume.

1.4 Contact binary theory

Andrievsky (1997) proposed a complementary scenario in which $\lambda$ Bootis stars would be the result of the coalescence of contact binaries of W UMa type. The system would be formed by two main-sequence components of approximately equal spectral types. This scenario would lead to an age for $\lambda$ Bootis stars of 1 Gyr explaining the rather evolved nature of some $\lambda$ Bootis stars and the origin of the material around them.

A class definition of the $\lambda$ Bootis group is essential on one hand to distinguish these stars from other groups of stars populating the same region of the HR diagram and, on the other hand, to shed light into the nature of the $\lambda$ Bootis phenomenon. Ideally, a stellar class should be formed by a homogeneous sample of stars showing common properties originated by the same astrophysical processes. Historically, this has not been the case for the $\lambda$ Bootis group since the use in the past of classification criteria not unique to the group (weakness of the Mg II 4481 Å line, presence of spectral features at 1600 Å  and 3040 Å, IR excess, ...) has led to the inclusion of spurious members (horizontal branch stars, Ap stars, shell stars, He-weak stars, ...). Although the problem of the class definition can be efficiently alleviated making use of unambiguous criteria defined in the ultraviolet range (Solano & Paunzen 1998, 1999), the chemical composition-based definition of the class makes it necessary to perform an accurate determination of the stellar parameters for the final decision on the membership of a potential candidate to the $\lambda$ Bootis group. The observed sample was selected from the list of suspected $\lambda$ Bootis stars given in Paunzen (2000). HD 68758 and HD 184190 were also included as possible members of the class.