Diagnosing the Shock from Accretion onto a Young Star

Nancy S. Brickhouse

Harvard-Smithsonian Center for Astrophysics

Collaborators: Steve Cranmer, Moritz GuentherAndrea Dupree, Juan Luna, and Scott Wolk

H2012 ADAS WorkshopCadarache, France

24-25 Sept 2012

Outline• Collisionally ionized plasmas and their

X-ray spectra

• Young stars: coronae and accretion

• Case study: TW Hydrae (TW Hya)

• Implications

• Conclusions

Collisionally Ionized Plasmas and Their X-ray Spectra

• ATOMDB (Smith et al. 2001; Foster et al. 2012)

• Collisionally ionized X-ray sources include: - Hot gas in galaxies - Hot gas in clusters of galaxies - Hot gas in the interstellar medium - Ejecta and shocks in supernova remnants - Shocks in hot star winds and binary colliding winds - Shocks from magnetically controlled accretion - Stellar coronae

• 13 years of Chandra and XMM-Newton gratings for “point sources”

Emission Measure (Ne2 V) of Stars

(Kastner et al. 2002)

Stellar coronae, but accretion shock in TW Hydrae? (Kastner et al. 2002)

log Ne2V

(cm-3)

log Te (K)

Other Young Stars

Accretion or Corona?

• Original argument for accretion shock based on high density

• Additional diagnostics needed to test accretion-shock model

Chandra Large Observing Program TW Hya

500 ks with High Energy Transmission Grating

(Brickhouse et al. 2010)

TW Hya Campaign:Four Continents Plus Chandra

Dupree et al. 2012

TW Hya

• Classical T Tauri star (accreting)• i=7o (pole-on)

• M = 0.8 MSun

• R = 0.7 RSun

• Distance 57 pc• 10 million yr old • Poised to make planets

Romanova et al. 2004

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

Neon Region of HETG Spectrum

He-like Line Ratio Diagnostics

He-like Energy Levels(Smith et al. 2009)

Atomic Theory and Benchmarks:Ne IX G-ratio Diagnostic

G-ratio vs Te

Chen et al. 2006Smith et al. 2009

He-like Ions in TW Hya: O VII, Ne IX, and Mg XI

Diagnostics for Te and Ne

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

This looks like the accretion shock!

Accretion and a Corona

Emission Measure vs Te

Lightcurve

Hot “coronal” lines exhibit a large flare. The “accretion” lines do NOT flare.Variability occurs in both.

Complex absorption

Use photoelectric absorption model

• O VII: NH = 4.1 x 1020 cm-2

• Ne IX: NH = 1.8 x 1021 cm-2

Not resonance scattering:

Tau ~ g f λ, for a given ion

Series line ratios rule out

Accretion shock cools radiatively

Vff = (1 – R*/rt )1/2

~ 510 km/s

Te = 3.4 MK

Macc = f A* ρpre vff

2GM*

R*

●

(Konigl 1991;Cranmer 2008)

Accretion shock cools radiatively

Vff = (1 – R*/rt )1/2

~ 510 km/s

Te = 3.4 MK

Macc = f A* ρpre vff

2GM*

R*

●

(Konigl 1991;Cranmer 2008)

“Settling”

Te and Ne from Ne IX agree with the shock model.

Standard model predicts Ne at O VII 7 times larger than observed.

Post-shock region has 30 x more mass than the shock!

The Splash:A New Accretion-Fed Post-Shock Structure

Definitely not “settling”

Te and Ne from Ne IX agree with the shock model.

Standard model predicts Ne at O VII 7 times larger than observed.

Post-shock region has 30 x more mass than the shock!

The Splash:A New Accretion-Fed Post-Shock Structure

Soft X-ray Excess (OVII) Ubiquitous

Gudel & Telleschi 2007

• 3 segments ~150 ksec each

Te and NH differ.

Ne varies only 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

Accretion Variation: Te, NH, Ne from Ne IX

Brickhouse et al. 2012

Te from 1.9 to 3.1 MK

NH from 0.9 to 3.2 1021 cm-2

●

Accretion Model Variations

Brickhouse et al. 2012

Conclusions

• Diagnostics show excellent agreement with simple models of the shock itself.

• Diagnostics show that standard, one-dimensional models of the post-shock cooling plasma don’t explain all the data.

• The shock heats and ionizes stellar material, potentially feeding open and closed field lines.

• Without accurate diagnostics, studies such as this cannot take advantage of the potential of Chandra.

Implications• How good is the dipole assumption?• How does the magnetic field evolve?• Do turbulent “hot spots” develop on

more massive accretors?• What MHD processes drive stellar and/or

disk outflows?• How does the magnetic field connect

star and disk?

Donati et al. 2008BP Tau