High-resolution X-ray spectroscopy of θ Carinae

Abstract

Context: The peculiar hot star θ Car in the open cluster IC 2602 is a blue straggler as well as a single-line binary with a short period (2.2d).
Aims: Its high-energy properties are not well known, though X-rays can provide useful constraints on the energetic processes at work in binaries as well as in peculiar, single objects.
Methods: We present the analysis of a 50 ks exposure taken with the XMM-Newton observatory. It provides medium as well as high-resolution spectroscopy.
Results: Our high-resolution spectroscopy analysis reveals a very soft spectrum with multiple temperature components (1-6 MK) and an X-ray flux slightly below the `canonical' value (log [L_X(0.1{-}10.)/L_BOL]∼-7). The X-ray lines appear surprisingly narrow and unshifted, reminiscent of those of β Cru and τ Sco. Their relative intensities confirm the anomalous abundances detected in the optical domain (C strongly depleted, N strongly enriched, O slightly depleted). In addition, the X-ray data favor a slight depletion in neon and iron, but they are less conclusive for the magnesium abundance, which may be solar-like. While no significant changes occur during the XMM-Newton observation, variability in the X-ray domain is detected in the long-term range. The formation radius of the X-ray emission is loosely constrained to < 5 R_⊙, which allows for a range of models (wind-shock, corona, magnetic confinement, ...) though not all of them can be reconciled with the softness of the spectrum and the narrowness of the lines.

Based on observations collected with XMM-Newton, an ESA Science Mission with instruments and contributions directly funded by ESA Member States and the USA (NASA).