Diagnosing the Shock from Accretion onto a Young Star Nancy S. Brickhouse Harvard-Smithsonian Center...
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Transcript of Diagnosing the Shock from Accretion onto a Young Star Nancy S. Brickhouse Harvard-Smithsonian Center...
![Page 1: Diagnosing the Shock from Accretion onto a Young Star Nancy S. Brickhouse Harvard-Smithsonian Center for Astrophysics Collaborators: Steve Cranmer, Moritz.](https://reader036.fdocuments.us/reader036/viewer/2022062517/56649f125503460f94c250cd/html5/thumbnails/1.jpg)
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
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Outline• Collisionally ionized plasmas and their
X-ray spectra
• Young stars: coronae and accretion
• Case study: TW Hydrae (TW Hya)
• Implications
• Conclusions
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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”
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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)
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Other Young Stars
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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)
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TW Hya Campaign:Four Continents Plus Chandra
Dupree et al. 2012
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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
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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
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He-like Line Ratio Diagnostics
He-like Energy Levels(Smith et al. 2009)
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Atomic Theory and Benchmarks:Ne IX G-ratio Diagnostic
G-ratio vs Te
Chen et al. 2006Smith et al. 2009
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He-like Ions in TW Hya: O VII, Ne IX, and Mg XI
Diagnostics for Te and Ne
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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!
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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.
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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
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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)
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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”
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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
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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
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Soft X-ray Excess (OVII) Ubiquitous
Gudel & Telleschi 2007
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• 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
●
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Accretion Model Variations
Brickhouse et al. 2012
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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.
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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