High-resolution X-ray spectroscopy of Theta Car

TL;DR

This study presents high-resolution X-ray spectroscopy of Theta Car, revealing a very soft spectrum with narrow lines, anomalous abundances, and long-term variability, providing insights into its energetic processes and composition.

Contribution

First detailed high-resolution X-ray spectral analysis of Theta Car, highlighting its soft spectrum, narrow lines, and peculiar abundances, with implications for its emission mechanisms.

Findings

Soft X-ray spectrum with multiple temperature components (1-6 MK)

Narrow, unshifted X-ray lines similar to beta Cru and tau Sco

Anomalous abundances: depleted C, enriched N, slightly depleted O, and possible Ne and Fe depletion

Abstract

Context : The peculiar hot star Theta Car in the open cluster IC2602 is a blue straggler as well as a single-line binary of 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 50ks 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--6MK) 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 beta Cru and tau 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 (solar-like?). While no significant changes occur during the XMM-Newton observation, variability in the X-ray domain is detected on the long-term range. The formation radius of the X-ray emission is loosely constrained to <5 R_sol, 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.