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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