Stellar X-ray accretion signatures

Accretion is observed in a wide range of objects with partially overlapping properties. In this paper, we focus on accretion in young stars. Material impacting the stellar surface is shock-heated to temperatures of a few 10(6) K (MK), which is ideally suited for observations in the X-ray regime. Indeed, young, accreting stars show a surplus of cool plasma, thought to be somehow related to accretion, the so-called soft excess. High-resolution grating spectroscopy with instruments like the Reflection Grating Spectrometer (RGS) on board XMM-Newton allows us to infer the properties of this cool plasma. We present results from our recent 250-ks XMM-Newton/Chandra program targeting the prototypical T Tau system and compare them with other accreting systems, focusing on potentially different accretion modes. We find a strong cool excess in T Tau but line ratios indicative of low densities. Thus, the cool excess in T Tau, and likely in other accreting stars, is not caused directly by postshock emission from accretion spots, which are expected in magnetic stars like T Tau. Rather, the coolest plasma might be linked to other processes in accreting stars, such as an interaction of the corona with postshock plasma.