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|Title:||Redshifted X-rays from the material accreting onto TW Hydrae: Evidence of a low-latitude accretion spot||Authors:||Argiroffi, Costanza
Drake, J. J.
|Issue Date:||2017||Journal:||ASTRONOMY & ASTROPHYSICS||Number:||607||First Page:||A14||Abstract:||Context. 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 as it continues its inward bulk motion, can be probed by X-ray spectroscopy. <BR /> Aims: In an attempt to detect for the first time the motion of this X-ray-emitting post-shock material, we searched for a Doppler shift in the deep Chandra High Energy Transmission Grating observation of the CTTS TW Hya. This test should unveil the nature of this X-ray emitting plasma component in CTTS and constrain the accretion stream geometry. <BR /> Methods: We searched for a Doppler shift in the X-ray emission from TW Hya with two different methods: by measuring the position of a selected sample of emission lines and by fitting the whole TW Hya X-ray spectrum, allowing the line-of-sight velocity to vary. <BR /> Results: We found that the plasma at T 2 - 4 MK has a line-of-sight velocity of 38.3 ± 5.1 km s<SUP>-1</SUP> with respect to the stellar photosphere. This result definitively confirms that this X-ray-emitting material originates in the post-shock region, at the base of the accretion stream, and not in coronal structures. The comparison of the observed velocity along the line of sight, 38.3 ± 5.1 km s<SUP>-1</SUP>, with the inferred intrinsic velocity of the post shock of TW Hya, v<SUB>post</SUB> ≈ 110 - 120 km s<SUP>-1</SUP>, indicates that the footpoints of the accretion streams on TW Hya are located at low latitudes on the stellar surface. <BR /> Conclusions: Our results indicate that complex magnetic field geometries, such as those of TW Hya, permit low-latitude accretion spots. Moreover, since on TW Hya the redshift of the soft X-ray emission is very similar to that of the narrow component of the C iv resonance doublet at 1550 Å, then the plasma at 2 - 4 MK and that at 0.1 MK likely originate in the same post-shock regions.||URI:||http://hdl.handle.net/20.500.12386/30634||URL:||https://www.aanda.org/articles/aa/full_html/2017/11/aa31342-17/aa31342-17.html||ISSN:||0004-6361||DOI:||10.1051/0004-6361/201731342||Bibcode ADS:||2017A&A...607A..14A||Fulltext:||open|
|Appears in Collections:||1.01 Articoli in rivista|
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checked on Jun 15, 2021
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