X-ray diagnostics of
magnetospheric accretion in
Thierry
Montmerle
TTS
Laboratoire d'Astrophysique de Grenoble
and Institut d’Astrophysique de Paris, France
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X-rays vs. Accretion shocks
• X-rays only probe of accretion shocks (free-fall
velocities from circumstellar disk: ≈ 200-300 km/s)
• Very hard to detect, because of many other sources of
X-rays in the environment of young stars:
– Magnetic activity ("coronal" X-rays): x1000
– Jet/outflow shocks (ejection and accretion ~ same velocities !):
~ 10 individual cases known
• In other words, accretion is the signal, flares are the
noise !
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Basic magnetic structure of a "classical"
(= accreting) T Tauri star
Stassun 2001
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A complex X-ray emitting
accretion-ejection configuration...
X
X
Hot-base,
Cool
Cooling Disk
Channeled
Wind
X
Hot Coronal
Wind ?
HAeBe ?
X
X
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Pre-shock zone
T ~ 104 K
ne~1011-1012 cm-3
vacc~ 300500 km/s
Magnetospheric accretion and X-rays: some numbers
Shock surface
Post-shock zone
T ~ 106 K
Soft X-rays (E ≲ 0.8 keV)
Example:
M = 1 M
Macc = 10-10 M/yr
RT = 5 R 
chromosphere
T ~ 104 K
(Lamizin 1998, Calvet & Gullbring
1998)
T = 3 MK
LX = 7 x 1031 erg/s ~ 105 x LX,
(© G. Sacco, 2009)CC2YSO (17-19/5/10
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X-ray accretion diagnostics (in a
nutshell)
• First detection: TW Hya (CTTS)
– Chandra HETGS (Kastner et al. 2002)
– Characteristics: soft spectra (TX ~ 2 MK), high plasma
densities (log ne ~ 13: ~ 100-1000x coronal)
– Key diagnostic: He-like triplets (OVII, NeIX…)
• Line ratios: Direct, independent determinations of ne and TX
• Found in no more than ~ 10 individual stars so far
– Requires HRS [high-resolution spectroscopy: E/DE = f(E) ~
300-800] (XMM RGS; Chandra HETGS): lots of photons !
– Some statistical evidence from low-resolution spectroscopy
[E/DE ~ 20-50] (Chandra ACIS, XMM EPIC):
"soft excess" (TX ≲ 1 keV, e.g., XEST survey; flux too weak
for HRS => ne impossible to determine)
– Deepest study (very long exposures): V4046 Sgr
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The OVII and NeIX triplets: signature of
accretion ?
TW Hya
Brickhouse et al. 2010: Chandra
f(forbidden)/i(intercombination) line ratio sensitive to plasma density
f/i ~0.05 => ne ~ 1012 cm-3
(f+i)/r(resonance) line ratio sensitive to temperature TX (MK)
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Key questions…
• Location of accretion shock wrt the stellar photosphere (HD +
transfer problem)
• Interplay between coronal loops and accretion funnels (MHD
problem)
– Magnetic field topology
• Intrinsic variability (bursts…)
• => 2D MHD modeling (Palermo group: Orlando et al. 2008,
Sacco et al. 2010; Brickhouse et al. 2010…)
– Accretion stream falling along magnetic “tube”  photosphere
– => accretion rate, confinement efficiency, instabilities, etc.
– (private show if interested…)
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Case study: XMM “Large Program”
on the “twin binary” V4046 Sgr
(preliminary results)
“VSag consortium”: TM, E. Alecian & J. Bouvier (LAOG), with C.
Argiroffi, F. Damiani, & A. Maggio (Palermo), S. Alencar (Belo
Horizonte), M. Audard (Geneva), J.-F. Donati (Toulouse), S. Gregory
(Exeter), M. Güdel (Vienna),
V. Holzwarth (Freiburg), D.
Huenemoerder (MIT), J. Kastner, R. Montez & G. Sacco (Rochester),
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G. Wade (Kingston)
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V4046 Sgr Identity card
• Two almost identical components, at d ~ 83 pc;
with M ~ 0.9 Ma, R ~ 1.2 Ra; L ~ 0.4 La
• perhaps a 3rd companion (binary ?) at 14000 AU
• Orbital period ~ 2.42 d (close binary => synchronous rotation)
• i = 35°, sep. ~ 4.5 Ra
• CTTS: circumbinary disk
• accretion flow seen in Hb…
• Age ~ few Myr
• => main goal: look for orbital
modulation of accretion
parameters
Stempels & Gahm 2004
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Coordinated space/ground
observations
• XMM “Large Program” (3x100 ksec; 500 ksec total):
– 3 consecutive revolutions (2d = 173 ksec), each partially
sampled
– Looking for orbital/rotational modulation
– 3 instruments: EPIC, RGS, + OM (Optical Monitor)
• Coordinated ground-based observations:
– Spectrophotometry (optical)
• Espadons@CFHT(Hawaii)
– Photometry (optical, NIR)
• REM (La Silla)
• Pico dos Dias (Brazil)
• SARG@TNG (Canary Islands)
✪
✪
✪✪
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ESPaDOnS
REM
Pico dos Dias: —— —— —
A. Maggio 2010
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EPIC (pn + 2xMOS)
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EPIC (pn + 2xMOS)
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(to be continued…)
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Epilogue: IXO (? ~ 2020 ?)
~ 20m
ESA/NASA/JAXA
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High-resolution spectra of T Tauri stars with IXO
XMS matrix: 2.5eV resol.
(E/DE ~ 1000)
IXO/XGS(+XMS) potential:
• good S/N spectra in < 100 ksec for TTS up to 500 pc (Taurus, Cha, Orion, etc)
 ne in CTTS  accretion vs. corona
• time-resolved high-resolution spectroscopy (~ 10 ksec exp. for bright TTS)
 exposure/30 compared to XMM
 ne variations probing accretion structure
 ne variations in stellar flares
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(after B. Stelzer 2010)
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Additional material:
modeling accretion shocks
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MP Mus: Spectral synthesis
Observed spectrum (XMMRGS)
Spectrum synthesized from
the simulations
(taking into account instrumental
response , distance and interstellar
absorption and assuming accretion
flow cross section)
density diagnostic
Temperature diagnostic
(Sacco et al. 2008)
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2D MHD simulations
MP Mus
ne = 1011 cm-3
v = 450 km s-1
B = 1, 10, 50 G
Orlando et al. 2009
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B=1G
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B = 10 G
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B = 50 G
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