X-ray diagnostics of magnetospheric accretion

in TTS

X-ray diagnostics of magnetospheric accretion

in TTSThierry MontmerleLaboratoire d'Astrophysique de Grenoble

and Institut d’Astrophysique de Paris, France

Thierry MontmerleLaboratoire 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

XX

ChanneledWind

Hot CoronalWind ?

Hot-base, Cool

Cooling Disk

X

HAeBe ?

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Soft X-rays (E ≲ 0.8 keV)

Post-shock zone T ~ 106 K

Pre-shock zone

T ~ 104 Kne~1011-1012 cm-3

chromosphere T ~ 104 K

Shock surface

vacc

~ 3

00

-5

00

km

/s

(Lamizin 1998, Calvet & Gullbring 1998) Example:

M = 1 M

Macc = 10-10 M/yrRT = 5 R

Magnetospheric accretion and X-rays: some numbers

T = 3 MKLX = 7 x 1031 erg/s ~ 105 x LX,

(© G. Sacco, 2009)CC2YSO (17-19/5/10 5

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/E = f(E) ~

300-800] (XMM RGS; Chandra HETGS): lots of photons ! – Some statistical evidence from low-resolution

spectroscopy [E/E ~ 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 ?

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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TW Hya

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), G. Wade (Kingston) CC2YSO (17-19/5/10

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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 H…• Age ~ few Myr

• => main goal: look for orbitalmodulation of accretionparameters

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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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✪

✪

✪

✪

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Pico dos Dias: —— —— —

REM

A. Maggio 2010

ESPaDOnS

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EPIC (pn + 2xMOS)

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EPIC (pn + 2xMOS)

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(to be continued…)

Epilogue: IXO (? ~ 2020 ?)

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~ 20m

High-resolution spectra of T Tauri stars with IXO

XMS matrix: 2.5eV resol.(E/E ~ 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)

Additional material:modeling accretion shocks

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MP Mus: Spectral synthesis

Observed spectrum (XMM-RGS)

Spectrum synthesized from the simulations (taking into account instrumental response , distance and interstellar absorption and assuming accretion flow cross section)

Temperature diagnostic

density diagnostic

(Sacco et al. 2008)

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ne = 1011 cm-3

v = 450 km s-1

B = 1, 10, 50 G

MP Mus

2D MHD simulations

Orlando et al. 2009

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B = 1 GCC2YSO (17-19/5/10

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B = 10 GCC2YSO (17-19/5/10

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B = 50 GCC2YSO (17-19/5/10

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