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

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
Chandra HETG 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 Ne IX: NH = 1.8 x 1021 cm-2

10. Accurate Atomic Theory Benchmarked by Experiment
Ne IX G-ratio (Te) Diagnostic
Chen et al. 2006
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 cm-3 Mg XI
3 x Ne IX
6 x 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 = x MSun/yr (Batalha et al. 2002)
to 4 x MSun/yr (Muzerolle et al. 2000)

16. Accretion and Corona
Light
curve
Emission Measure
vs Te
Emission measure distribution and variability allow us to isolate the accretion shock.

17. Accretion Variation: Te, NH, Ne from Ne IX
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
Te from
1.9 to 3.1 MK ●
NH from
0.9 to 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.