000001981 001__ 1981
000001981 005__ 20160701171656.0
000001981 0247_ $$2DOI$$a10.1051/0004-6361:200809972
000001981 037__ $$aASTROimport-138
000001981 100__ $$aHubrig, S.
000001981 245__ $$aNew insights into the nature of the peculiar star θ Carinae
000001981 260__ $$c2008
000001981 520__ $$aContext: θ 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 θ 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 ~2 mag above the 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 θ Car and two other early B-type stars with recently detected magnetic fields, τ Sco and ξ1 CMa. The analysis revealed significantly different abundance patterns: a one-order-of-magnitude nitrogen overabundance and carbon depletion was found in θ Car, while the oxygen abundance is roughly solar. For the stars ξ1 CMa and τ 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 θ 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 ~1.2 h are rather inconclusive: only few measurements have a significance level of 3s. Although we detect a periodicity of the order of ~8.8 min in the dataset involving the measurements on all hydrogen Balmer lines with the exception of the Ha and Hß lines, these results have to be confirmed by additional time-resolved magnetic field observations. Based on observations collected with the CORALIE spectrograph attached to the Euler Telescope of the Geneva Observatory located at La Silla (Chile), at the European Southern Observatory, Paranal, Chile (ESO programmes 67.D-0239(A), 072.D-0377(A), 078.D-0080(A) and 278.D-5056(A)), and at the Complejo Astronómico El Leoncito (CASLEO), Argentina. 
000001981 700__ $$a Briquet, M.
000001981 700__ $$a Morel, T.
000001981 700__ $$a Schöller, M.
000001981 700__ $$a González, J. F.
000001981 700__ $$a De Cat, P.
000001981 773__ $$c287-296$$i1$$pAstronomy and Astrophysics$$v488$$y2008
000001981 85642 $$ahttp://esoads.eso.org/abs/2008A%26A...488..287H
000001981 905__ $$apublished in
000001981 980__ $$aREFERD