The Universe in High-resolution X-ray Spectra
Chandra Workshop 2015
Cambridge, MA
19 August 2015
X-ray Absorption as a Powerful
New Probe of the Accretion
Physics in Young Stars
Nancy S. Brickhouse
Harvard-Smithsonian Center for Astrophysics
Collaborators: Steve Cranmer,
Andrea Dupree, Juan Luna, Moritz Guenther, and Scott Wolk
Accretion shock models
●
→ Te and Ne for given Macc
• Kastner et al. (2002) find high Ne at low Te (~ 3 MK)
on TW Hya, using Chandra High Energy
Transmission Grating (HETG) for 50 ksec
• Chandra HETG Large Observing Program for
500 ksec (Brickhouse et al. 2010, ApJ, 710, 1835)
Testing the Accretion Shock Model
Vff = [ 2GM* (1 – R*/rt )]1/2
R*
●
Macc ~ f A* ρpre vff
~ 510 km/s for rt = 4.5 R*
→Te = 3.4 MK
(Konigl 1991; Calvet & Gullbring 1998; Guenther et
al. 2007; Cranmer 2008)
Brickhouse et al. 2010
TW Hya
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Classical T Tauri Star
M = 0.8 MSun
R = 0.7 RSun
Distance 57 pc
10 million yr old
Making planets?
High Neon abundance
(Kastner et al. 2002; Drake, Testa, & Hartmann 2005)
Romanova et al. 2004
Diverse X-ray Spectra from Young
Stars Observed with Chandra HETG
TW Hya Is Viewed Face-on
CO (3-2) indicates i=7o (Qi et al. 2004)
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.
Resonance Line Scattering Is Ruled Out
Optical Depth Tau of Strongest Line
Series lines scale as Tau ~ g f λ
Photoelectric absorption
Assume a neutral/near-neutral absorber to derive column density NH
O VII: NH = 4.1 x 1020 cm-2
Ne IX: NH = 1.8 x 1021 cm-2
Accurate Atomic Theory
Benchmarked by Experiment
Ne IX G-ratio (Te) Diagnostic
Chen et al. 2006
Smith et al. 2009
He-like Line Ratio Diagnostics
O VII
Ne IX
He-like Energy Levels
Ne and Te Diagnostic Ratios
(Smith et al. 2009)
Mg XI
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
As predicted by accretion shock models!
The Shock Structure
• Te and Ne from Ne IX (the shock front) agree with the standard
shock model.
• But standard model predicts Ne at O VII 7 times larger than
observed.
• And EM analysis indicates that the post-shock region (O VII)
has 30 x more mass than the shock (?!).
Soft X-ray Excess (OVII) Ubiquitous
Among Accreting Stars
Gudel & Telleschi 2007
also see Robrade & Schmitt 2007
OVII behaves strangely
Too much of it? Gives the wrong accretion rate
MAYBE an absorption effect? (Low NH)
Ne IX behaves like we expect
Reasonable Te and Ne
●
Macc =
reasonable accretion rate:
5 x 10-9 MSun/yr (Batalha et al. 2002)
to 4 x 10-10 MSun/yr (Muzerolle et al. 2000)
Accretion and Corona
Emission Measure
vs Te
Light
curve
Emission measure distribution and variability allow us
to isolate the accretion shock.
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 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
A TW Hya accretion event
…
X-Ray accretion lines:
N VII, O VIII, Ne IX,
Fe XVII, Mg XI
10 hours
H-alpha asymmetry change 9 minutes later
increased inflow for 1.5 hours
Dupree et al. 2012
H-beta shows similar behavior as H-alpha
Helium D3 exhibits abrupt
red side enhancement
Accretion
X-rays
Delay after
X-ray event
H-alpha
9 minutes
H-beta
9+ minutes
He D3
Veiling
30 minutes
Broad component
~2 hours
Coronal enhancements follow
increase in veiling
Conclusions
• High S/N HETG spectrum derives from 3 regions:
a hot 10 MK corona, an accretion shock, and a cool
post-shock region.
• Ne IX diagnostics show excellent agreement with
simple models of the shock itself.
• Standard 1D models of the post-shock cooling plasma
don’t fit the O VII observations.
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• Te and NH vary: observations give Macc, B, f, and rin
and rout . X-ray absorption is key.
• The shock impacts the stellar atmosphere as
observed in optical diagnostics.