The X-ray soft excess in classical T Tauri stars

TL;DR

This study investigates the unusually high X-ray flux ratio of OVII to OVIII in classical T Tauri stars, revealing a soft excess from cool plasma likely related to accretion and magnetic activity, distinct from jet emissions.

Contribution

It identifies the origin of the soft X-ray excess in CTTS as cool plasma emission, and distinguishes it from jet-related soft X-ray components, advancing understanding of stellar accretion processes.

Findings

All CTTS show an anomalous OVII/OVIII flux ratio.

The excess is due to cool plasma (<~2 MK), not hot plasma deficiency.

The soft excess correlates with stellar X-ray luminosity, not UV excess.

Abstract

We study an anomaly in the X-ray flux (or luminosity) ratio between the OVII 21.6-22.1A triplet and the OVIII Lya line seen in classical T Tauri stars (CTTS). This ratio is unusually high when compared with ratios for main-sequence and non-accreting T Tauri stars (Telleschi et al. 2007). We compare these samples to identify the source of the excess. A sample of recently discovered X-ray stars with a soft component attributed to jet emission is also considered. We discuss data obtained from the XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST) project, complemented by data from the published literature. We also present data from the CTTS RU Lup. All CTTS in the sample show an anomalous OVII/OVIII flux ratio when compared with WTTS or MS stars. The anomaly is due to an excess of cool, OVII emitting material rather than a deficiency of hotter plasma. The excess plasma must therefore have temperatures of <~2 MK. This soft excess does not correlate with UV excesses of CTTS, but seems to be related with the stellar X-ray luminosity. The spectra of the jet-driving TTS do not fit into this picture. The soft excess depends both on the presence of accretion streams in CTTS and on magnetic activity. The gas may be shock-heated near the surface, although it may also be heated in the magnetospheric accretion funnels. The soft component of the jet-driving sources is unlikely to be due to the same process.