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

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)

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

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

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

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

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.

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

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

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

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

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!

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 (?!).

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

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)

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

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 3.2 1021 cm-2 Brickhouse et al. 2012

Accretion Model Variations Brickhouse et al. 2012

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

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

H-beta shows similar behavior as H-alpha

Helium D3 exhibits abrupt red side enhancement

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

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

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.