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X-ray Absorption as a Probe of Accretion Physics in Young Stars

This study examines the use of X-ray absorption as a powerful tool to investigate the accretion physics in young stars. The researchers analyze high-resolution X-ray spectra from the Chandra Workshop 2015 to study the accretion shock models and the impact of shock on the stellar atmosphere. The results show that X-ray absorption can provide valuable insights into the accretion process in young stars.

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X-ray Absorption as a Probe of Accretion Physics in Young Stars

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  1. X-ray Absorption as a Powerful New Probe of the Accretion Physics in Young StarsNancy S. BrickhouseHarvard-Smithsonian Center for AstrophysicsCollaborators: Steve Cranmer, Andrea Dupree, Juan Luna, Moritz Guenther, and Scott Wolk The Universe in High-resolution X-ray Spectra Chandra Workshop 2015 Cambridge, MA 19 August 2015

  2. Accretion shock models → Te and Ne for given Macc ● • Kastner et al. (2002) find high Ne at low Te (~ 3 MK) on TW Hya, using Chandra High Energy Transmission Grating (HETG) for 50 ksec • ChandraHETG Large Observing Program for 500 ksec (Brickhouse et al. 2010, ApJ, 710, 1835)

  3. Testing the Accretion Shock Model 2GM* R* Vff = [ (1 – R*/rt )]1/2 ~ 510 km/s for rt = 4.5 R* →Te = 3.4 MK ● Macc ~ f A*ρpre vff (Konigl 1991; Calvet & Gullbring 1998; Guenther et al. 2007; Cranmer 2008) Brickhouse et al. 2010

  4. TW Hya • Classical T Tauri Star • M = 0.8 MSun • R = 0.7 RSun • Distance 57 pc • 10 million yr old • Making planets? • High Neon abundance (Kastner et al. 2002; Drake, Testa, & Hartmann 2005) Romanova et al. 2004

  5. Diverse X-ray Spectra from Young Stars Observed with Chandra HETG

  6. TW Hya Is Viewed Face-on CO (3-2) indicates i=7o (Qi et al. 2004)

  7. Neon Region of HETG Spectrum Spectrum shows strong H-like Ne X and He-like Ne IX, up to n=7 or 8 in Ne X. Series lines are sensitive to absorption.

  8. Resonance Line Scattering Is Ruled Out Optical Depth Tau of Strongest Line Series lines scale as Tau ~ g f λ

  9. Photoelectric absorption Assume a neutral/near-neutral absorber to derive column density NH O VII: NH = 4.1 x 1020 cm-2 Ne IX: NH = 1.8 x 1021 cm-2

  10. Accurate Atomic Theory Benchmarked by Experiment Chen et al. 2006 Ne IX G-ratio (Te) Diagnostic Smith et al. 2009

  11. He-like Line Ratio Diagnostics O VII Ne IX Mg XI He-like Energy Levels Ne and Te Diagnostic Ratios (Smith et al. 2009)

  12. X-Ray Line Ratio Diagnostics for Density and Temperature Ne = 6 x 1012 cm-3 Mg XI 3 x 1012 Ne IX 6 x 1011 O VII Te = 2.50 ± 0.25 MK As predicted by accretion shock models!

  13. The Shock Structure • Te and Ne from Ne IX (the shock front) agree with the standard shock model. • But standard model predicts Ne at O VII 7 times larger than observed. • And EM analysis indicates that the post-shock region (O VII) has 30 x more mass than the shock (?!).

  14. Soft X-ray Excess (OVII) Ubiquitous Among Accreting Stars Gudel & Telleschi 2007 also see Robrade & Schmitt 2007

  15. OVII behaves strangely Too much of it? Gives the wrong accretion rate MAYBE an absorption effect? (Low NH) Ne IX behaves like we expect Reasonable Te and Ne reasonable accretion rate: ● Macc = 5 x 10-9 MSun/yr (Batalha et al. 2002) to 4 x 10-10 MSun/yr (Muzerolle et al. 2000)

  16. AccretionandCorona Light curve Emission Measure vs Te Emission measure distribution and variabilityallow us to isolate the accretion shock.

  17. Accretion Variation: Te, NH, Ne from Ne IX Te from 1.9 to 3.1 MK • 3 segments ~150 ksec each Te and NH differ. Ne varies slightly. • Variable Te means rt changes. • Assuming NH is from pre-shock gas, we can get path length <l> and thus the filling factor. • Observed diagnostics constrain model Macc, B, f, rin and rout ● NH from 0.9 to 3.2 1021 cm-2 Brickhouse et al. 2012

  18. Accretion Model Variations Brickhouse et al. 2012

  19. A TW Hya accretion event … X-Ray accretion lines: N VII, O VIII, Ne IX, Fe XVII, Mg XI 10 hours

  20. H-alpha asymmetry change 9 minutes later increased inflow for 1.5 hours Dupree et al. 2012

  21. H-beta shows similar behavior as H-alpha

  22. Helium D3 exhibits abrupt red side enhancement

  23. Accretion X-rays Delay after X-ray event H-alpha 9 minutes 9+ minutes H-beta 30 minutes Broad component He D3 Veiling ~2 hours

  24. Coronal enhancements follow increase in veiling Best correlation: 2.5 hours !!

  25. Conclusions • High S/N HETG spectrum derives from 3 regions: a hot 10 MK corona, an accretion shock, and a cool post-shock region. • Ne IX diagnostics show excellent agreement with simple models of the shock itself. • Standard 1D models of the post-shock cooling plasma don’t fit the O VII observations. ● • Te and NH vary: observations give Macc, B, f, and rin and rout . X-ray absorption is key. • The shock impacts the stellar atmosphere as observed in optical diagnostics.

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