New insights into the nature of the peculiar star θ Carinae

Context. theta Carinae belongs to a group of peculiar early-type stars (OBN) with enhanced nitrogen and carbon deficiency. It is also known as a binary system, but it is not clear yet whether the chemical anomalies can be explained by mass transfer between the two components. On the basis of the previously reported spectral variability of a few metal lines it may be expected that theta Car possesses a weak magnetic field. Aims. A study of the physical nature of this hot massive binary which is furthermore a well-known blue straggler lying similar to 2 mag above lthe turnoff of the young open cluster IC 2602 is important to understand the origin of its strong chemical anomalies. Methods. We acquired high resolution spectroscopic and low resolution spectropolarimetric observations to achieve the following goals: a) to improve the orbital parameters to allow a more in-depth discussion on the possibility of mass transfer in the binary system; b) to carry out a non-local thermodynamic equilibrium (NLTE) abundance analysis; and c) to search for the presence of a magnetic field. Results. The study of the radial velocities using CORALIE spectra allowed us to significantly improve the orbital parameters. A comparative NLTE abundance analysis was undertaken for theta Car and two other early B-type stars with recently detected magnetic fields, tau Sco and xi(1) CMa. The analysis revealed significantly different abundance patterns: a one-order-of-magnitude nitrogen overabundance and carbon depletion was found in theta Car, while the oxygen abundance is roughly solar. For the stars xi(1) CMa and tau Sco the carbon abundance is solar and, while an N excess is also detected, it is of much smaller amplitude (0.4-0.6 dex). Such an N overabundance is typical of the values already found for other slowly-rotating (magnetic) B-type dwarfs. For theta Car, we attribute instead the chemical peculiarities to a past episode of mass transfer between the two binary components. The results of the search for a magnetic field using FORS 1 at the VLT consisting of 26 measurements over a time span of similar to 1.2 h are rather inconclusive: only few measurements have a significance level of 3 sigma. Although we detect a periodicity of the order of similar to 8.8 min in the dataset involving the measurements on all hydrogen Balmer lines with the exception of the H alpha and H beta lines, these results have to be confirmed by additional time-resolved magnetic field observations.