Accretion Processes in X-rays: From
White Dwarfs to Quasars
Boston, MA
14 July 2010
A New View of Accretion
Shock Structure
Nancy S. Brickhouse
Harvard-Smithsonian Center for Astrophysics
Collaborators: Steve Cranmer,
Andrea Dupree, Juan Luna, and Scott Wolk
Diverse X-ray Spectra from Young
Stars Observed with Chandra HETG
Accretion shock models → Te and
Ne for given mass accretion rate
• Kastner et al. (2002) find high Ne in TW Hya
• Chandra Large Observing Program to definitively
establish accretion as the source of the emission,
and, if confirmed, bring new diagnostics to bear,
using 500 ksec High Energy Transmission Grating
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
X-rays from the accretion shock
(Kastner et al. 2002)
• X-ray plasma has high Neon abundance
(Kastner et al. 2002; Drake, Testa, & Hartmann 2005)
Accretion and a Corona
Emission Measure
vs Te
Light
curve
Emission measure distribution and variability
allow us to isolate the accretion shock.
Brickhouse et al. 2010, ApJ, 710, 1835
He-like Line Ratio Diagnostics
He-like Energy Levels
Ne and Te Diagnostic Ratios
(Smith et al. 2009)
Accurate Atomic Theory:
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!
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
Complex absorption
• O VII: NH = 4.1 x 1020 cm-2
• Ne IX: NH = 1.8 x 1021 cm-2
Series lines rule out
resonance scattering:
Tau ~ g f λ, for a given ion
Testing the Accretion Shock Model
Vff =
2GM*
(1 – R*/rt )1/2
R*
~ 510 km/s
Te = 3.4 MK
●
Macc = f A* ρpre vff
(Konigl 1991; Calvet &
Gullbring 1998; Gunther et
al. 2007; Cranmer 2008)
Testing the Accretion Shock Model
Vff =
2GM*
(1 – R*/rt )1/2
R*
~ 510 km/s
Te = 3.4 MK
●
Macc = f A* ρpre vff
(Konigl 1991; Calvet &
Gullbring 1998; Gunther et
al. 2007; Cranmer 2008)
“Settling”
Te and Ne from Ne IX agree
with the shock model.
Model predicts Ne at O VII 7
times larger than observed.
•Consider a new 2-Region model:
Region 1 = the shock front
Region 2 = the post-shock region
•Each region: Ne , Te, NH, V, and M
•Predict r, i, and f for He-like ions
•V2 = 300 x V1 => M2 = 30 x M1
Te and Ne from Ne IX agree
with the shock model.
Model predicts Ne at O VII 7
times larger than observed.
•Consider a new 2-Region model:
Region 1 = the shock front
Region 2 = the post-shock region
•Each region: Ne , Te, NH, V, and M
•Predict r, i, and f for He-like ions
•V2 = 300 x V1 => M2 = 30 x M1
Definitely not “settling”
Soft X-ray Excess (OVII) Ubiquitous
Gudel & Telleschi 2007
also see Robrade & Schmitt 2007
Courtesy A. Szentgyorgyi
Accretion Variability
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Ne IX diagnostics for 3
observation segments ~150 ksec
each give different Te and NH
(Ne does not vary).
Variable Te implies changing rin,
and thus Mdot
Observed diagnostics constrain
model Mdot , B, and rout
Te varies from 3.1 to 1.9 K
Mdot varies a factor of ~5
Filling factor varies a factor of ~7
rin varies from 1.75 R* to 3.52 R*
B varies from 800 to 500 G
Brickhouse et al. 2010, in progress
Conclusions
• High S/N HETG spectrum derives from 3 regions:
a hot 10 MK corona, an accretion shock, and a cool
post-shock region.
• 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 work.
• Te and NH vary (Ne does not), implying variability in
Mdot, B, and rtrunc
• Our values for Mdot are in good agreement with
optical and UV methods.