Redshift in the X-ray emission of TW~Hya: indication of a low-latitude accretion spot | Costanza Argiroffi (Dip. di Fisica e Chimica, Universita’ di Palermo)

High resolution spectroscopy, providing constraints on plasma motions and temperatures, is a powerful means to investigate the structure of accretion streams in Classical T Tauri stars (CTTS). In particular the accretion-shock region, where the accreting material is heated to temperatures of a few million degrees, can be probed by X-ray spectroscopy. MHD models predict that this hot post-shock plasma should have an inward bulk motion, with v_post ~ 100 km/s, with respect to the surrounding stellar atmosphere. To verify this prediction we searched for a Doppler shift in the deep Chandra/HETG observation of the CTTS TW Hya, perfectly suited for this task because of the excellent S/N and spectral resolution of the dataset, and because of the ideal target inclination. This test should allow us to constrain definitively the nature of this X-ray emitting plasma component in CTTS, and infer constraints on the accretion stream geometry. We searched for a Doppler shift in the X-ray emission from TW Hya by measuring the position of a selected sample of emission lines. To check the absolute wavelength calibration of the Chandra gratings, and to check whether or not bulk motions with respect to the photosphere are observed in coronal plasma, we also analyzed a sample of Chandra/HETG spectra of non-accreting active stars. We found that the soft part of the X-ray spectrum of TW Hya is significantly red-shifted by ~40 km/s with respect to the known radial velocity of the stellar photosphere. Conversely no X-ray redshift is observed in the X-ray emission of non-accreting active stars. The evidence that the X-ray emitting plasma on TW Hya is moving inward with respect to the stellar surface definitively confirms that it originates in the post-shock region, at the base of the accretion stream, and not in coronal structures. The observed radial velocity indicates that the base of the accretion stream on TW Hya is located at low latitudes on the stellar surface. Moreover the observed velocity of the soft X-ray emitting plasma is very similar to the velocity of the narrow component of the CIV resonance doublet at 1550 A, suggesting that they both originate from the same post-shock regions, that the hypothesis of free-fall regime in the accretion streams holds, and that complex magnetic field geometries, as that of TW Hya, allow low latitude accretion spots.