New insights into the nature of the peculiar star theta Carinae

TL;DR

This study improves orbital parameters, analyzes chemical abundances, and searches for magnetic fields in the peculiar star theta Carinae, revealing signs of past mass transfer and chemical peculiarities, but with inconclusive magnetic field detection.

Contribution

It provides new orbital data, detailed NLTE abundance analysis, and magnetic field search results for theta Carinae, highlighting its chemical peculiarities and potential binary interaction history.

Findings

Improved orbital parameters for theta Carinae.

Detected significant nitrogen overabundance and carbon depletion.

Inconclusive magnetic field detection with hints of periodicity.

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.