New insights into the nature of the peculiar star $theta$~Carinae

S. Hubrig$^1$, M. Briquet$^2$, T. Morel$^2,3$, M. Sch"oller$^1$, J. F. Gonz'alez$^4$, and P. De Cat$^5$

1 - European Southern Observatory, Casilla 19001, Santiago 19, Chile;
2 - Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200 D, B-3001 Leuven, Belgium;
3 - Institut d'Astrophysique et de G'eophysique, Universit'e de Li`ege, All'ee du 6 Ao^ut, B^at. B5c, 4000 Li`ege, Belgium;
4 - Complejo Astron'omico El Leoncito, Casilla 467, 5400 San Juan, Argentina;
5 - Koninklijke Sterrenwacht van Belgi"e, Ringlaan 3, B-1180 Brussel, Belgium

$theta$~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.

A study of the physical nature of this hot massive binary which is furthermore
a well-known blue straggler lying $sim$2,mag above the turnoff of the young
open cluster IC~2602 is important to understand the origin of its strong
chemical anomalies.

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--0.6,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 $sim$1.2,h are rather
inconclusive: only few measurements have a significance level of 3$sigma$.
Although we detect a periodicity of the order of $sim$8.8,min in the dataset
involving the measurements on all hydrogen Balmer lines with the exception of
the H$alpha$ and H$beta$ lines, these results have to be confirmed by
additional time-resolved magnetic field observations.

Reference: accepted for publication in A&A
Status: Manuscript has been accepted