H2012 ADAS Workshop
Cadarache, France
24-25 Sept 2012
Diagnosing the Shock from
Accretion onto a Young Star
Nancy S. Brickhouse
Harvard-Smithsonian Center for Astrophysics
Collaborators: Steve Cranmer, Moritz Guenther
Andrea Dupree, Juan Luna, and Scott Wolk
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
log Ne2V
(cm-3)
log Te (K)
Stellar coronae, but accretion shock in TW Hydrae?
(Kastner et al. 2002)
Other Young Stars
Accretion or Corona?
• Original argument for accretion shock based on
high density
• Additional diagnostics needed to test accretionshock 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
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Classical T Tauri star (accreting)
i=7o (pole-on)
M = 0.8 MSun
R = 0.7 RSun
Distance 57 pc
10 million yr old
Romanova et al. 2004
Poised to make planets
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
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. 2006
Smith 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
Light
curve
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 =
2GM*
(1 – R*/rt )1/2
R*
~ 510 km/s
Te = 3.4 MK
●
Macc = f A* ρpre vff
(Konigl 1991;
Cranmer 2008)
Accretion shock cools radiatively
Vff =
2GM*
(1 – R*/rt )1/2
R*
~ 510 km/s
Te = 3.4 MK
●
Macc = f A* ρpre vff
(Konigl 1991;
Cranmer 2008)
“Settling”
The Splash:
A New Accretion-Fed
Post-Shock Structure
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
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!
Definitely not “settling”
Soft X-ray Excess (OVII) Ubiquitous
Gudel & Telleschi 2007
Accretion Variation: Te, NH, Ne from Ne IX
Te from
1.9 to 3.1 MK
• 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.
NH from
0.9 to 3.2 1021 cm-2
• Observed
● diagnostics constrain
model Macc, B, f, rin and rout
Brickhouse et al. 2012
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. 2008
BP Tau