2010/01/05 by N. S. Brickhouse, S. R. Cranmer, A. K. Dupree +2 · 5 citations
Physics and Astronomy · #Accretion (finance) #Accretion disc #Astronomy and Astrophysical Research #Astrophysical Phenomena and Observations #Astrophysics and Star Formation Studies #Emission spectrum #Intermediate polar #Magnetic field #Shock wave #Spectral density #Spectral line #Stellar atmosphere #astro-ph.SR
paper · pdf · doi:10.1088/0004-637x/710/2/1835
13 pages (emulateapj style), 10 figures, ApJ, in press
arxiv created 2010/01/05 · openalex publication_date 2010/02/03 · arxiv updated 2015/05/14 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/05
We present X-ray spectral analysis of the accreting young star TW Hydrae from a 489 ks observation using the Chandra High Energy Transmission Grating. The spectrum provides a rich set of diagnostics for electron temperature T e , electron density N e , hydrogen column density N H , relative elemental abundances, and velocities, and reveals its source in three distinct regions of the stellar atmosphere: the stellar corona, the accretion shock, and a very large extended volume of warm postshock plasma. The presence of Mg xii , Si xiii , and Si xiv emission lines in the spectrum requires coronal structures at ∼10 MK. Lower temperature lines (e.g., from O viii , Ne ix , and Mg xi ) formed at 2.5 MK appear more consistent with emission from an accretion shock. He-like Ne ix line ratio diagnostics indicate that T e = 2.50 ± 0.25 MK and N e = 3.0 ± 0.2 × 10 12 cm −3 in the shock. These values agree well with standard magnetic accretion models. However, the Chandra observations significantly diverge from current model predictions for the postshock plasma. This gas is expected to cool radiatively, producing O vii as it flows into an increasingly dense stellar atmosphere. Surprisingly, O vii indicates N e = 5.7 +4.4 −1.2 × 10 11 cm −3 , 5 times lower than N e in the accretion shock itself and ∼7 times lower than the model prediction. We estimate that the postshock region producing O vii has roughly 300 times larger volume and 30 times more emitting mass than the shock itself. Apparently, the shocked plasma heats the surrounding stellar atmosphere to soft X-ray emitting temperatures and supplies this material to nearby large magnetic structures—which may be closed magnetic loops or open magnetic field leading to mass outflow. Our model explains the soft X-ray excess found in many accreting systems as well as the failure to observe high N e signatures in some stars. Such accretion-fed coronae may be ubiquitous in the atmospheres of accreting young stars.