Aims. Our goal is to explore the chemical pattern of early-type stars with planets, searching for a possible signature of planet formation. In particular, we study a likely relation between the λ Boötis chemical pattern and the presence of giant planets. Methods. We performed a detailed abundance determination in a sample of early-type stars with and without planets via spectral synthesis. Fundamental parameters were initially estimated using Strömgren photometry or literature values and then refined by requiring excitation and ionization balances of Fe lines. We derived chemical abundances for 23 different species by fitting observed spectra with an iterative process. Synthetic spectra were calculated using the program SYNTHE together with local thermodynamic equilibrium ATLAS12 model atmospheres. We used specific opacities calculated for each star, depending on the individual composition and microturbulence velocity vmicro through the opacity sampling method. The complete chemical pattern of the stars were then compared to those of λ Boötis stars and other chemically peculiar stars. Results. We compared the chemical pattern of the stars in our sample (13 stars with planets and 24 stars without detected planets) with those of λ Boötis and other chemically peculiar stars. We have found four λ Boötis stars in our sample, two of which present planets and circumstellar disks (HR 8799 and HD 169142) and one without planets detected (HD 110058). We have also identified the first λ Boötis star orbited by a brown dwarf (ζ Del). This interesting pair, the λ Boötis star and brown dwarf, could help to test stellar formation scenarios. We found no unique chemical pattern for the group of early-type stars bearing giant planets. However, our results support, in principle, a suggested scenario in which giant planets orbiting pre-main-sequence stars possibly block the dust of the disk and result in a λ Boötis-like pattern. On the other hand, we do not find a λ Boötis pattern in different hot-Jupiter planet host stars, which does not support the idea of possible accretion from the winds of hot-Jupiters, recently proposed in the literature. As a result, other mechanisms should account for the presence of the λ Boötis pattern between main-sequence stars. Finally, we suggest that the formation of planets around λ Boötis stars, such as HR 8799 and HD 169142, is also possible through the core accretion process and not only gravitational instability.

Aims. In an effort trying to improve spectroscopic methods of stellar parameters determination, we implemented non-solar-scaled opacities in a simultaneous derivation of fundamental parameters and abundances. We want to compare the results with the usual solar-scaled method using a sample of solar-type and evolved stars. Methods. We carried out a high-precision stellar parameters and abundance determination by applying non-solar-scaled opacities and model atmospheres. Our sample is composed by 20 stars (including main-sequence and evolved objects), with six stars belonging to binary systems. The stellar parameters were determined by imposing ionization and excitation equilibrium of Fe lines, with an updated version of the FUNDPAR program, together with plane-parallel ATLAS12 model atmospheres and the MOOG code. Opacities for an arbitrary composition and v micro were calculated through the OS (Opacity Sampling) method. Detailed abundances were derived using equivalent widths and spectral synthesis with the MOOG program. We applied the full line-by-line differential technique using the Sun as reference star, both in the derivation of stellar parameters and in the abundance determination. We start using solar-scaled models in a first step, and then continue the process but scaling to the abundance values found in the previous step (i.e. non-solar-scaled). The process finish when the stellar parameters of one step are the same of the previous step, i.e. we use a doubly-iterated method. Results. We obtained a small difference in stellar parameters derived with non-solar-scaled opacities compared to classical solarscaled models. The differences in T eff , log g and [Fe/H] amount up to 26 K, 0.05 dex and 0.020 dex for the stars in our sample. These differences could be considered as the first estimation of the error due to the use of classical solar-scaled opacities to derive stellar parameters with solar-type and evolved stars. We note that some chemical species could also show an individual variation higher than those of the [Fe/H] (up to ∼0.03 dex) and varying from one specie to another, obtaining a chemical pattern difference between both methods. This means that condensation temperature T c trends could also present a variation. We include an example showing that using non-solar-scaled opacities, the solution found with the classical solar-scaled method indeed cannot always verify the excitation and ionization balance conditions required for a model atmosphere. We discuss in the text the significance of the differences obtained when using solar-scaled vs non-solar-scaled methods. Conclusions. We consider that the use of the non-solar-scaled opacities is not mandatory e.g. in every statistical study with large samples of stars. However, for those high-precision works whose results depend on the mutual comparison of different chemical species (such as the analysis of condensation temperature T c trends), we consider that it is whortwhile its aplication. To date, this is probably one of the more precise spectroscopic ...