New insights into the nature of the peculiar star theta Carinae

Abstract

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. 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 (0.4-0.6dex). 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 FORS1 at the VLT consisting of 26 measurements over a time span of ~1.2h are rather inconclusive: only few measurements have a significance level of 3sigma. Although we detect a periodicity of the order of ~8.8min in the dataset involving the measurements on all hydrogen Balmer lines with the exception of the Halpha and Hbeta lines, these results have to be confirmed by additional time-resolved magnetic field observations.

10 pages, 9 figures, 6 tables, accepted for publication in A&A

References in corpus (9)

• The surprising magnetic topology of tauSco: fossil remnant or dynamo output?
• The VLT-FLAMES survey of massive stars: Evolution of surface N abundances and effective temperature scales in the Galaxy and Magellanic Clouds
• Abundance analysis of prime B-type targets for asteroseismology I. Nitrogen excess in slowly-rotating beta Cephei stars
• Discovery of magnetic fields in the beta Cephei star xi^1 CMa and in several Slowly Pulsating B stars
• Nitrogen enrichment, boron depletion and magnetic fields in slowly-rotating B-type dwarfs
• Hydrogen and helium line formation in OB dwarfs and giants. A hybrid non-LTE approach
• The magnetic field and confined wind of the O star ${\theta }^1$~Orionis~C
• Rotation and massive close binary evolution
• Towards an understanding of the Of?p star HD191612: Phase-resolved multiwavelength observations

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