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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. H2012 ADAS Workshop Cadarache, France 24-25 Sept 2012. Outline.

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Diagnosing the Shock from Accretion onto a Young StarNancy S. BrickhouseHarvard-Smithsonian Center for AstrophysicsCollaborators: Steve Cranmer, Moritz GuentherAndrea Dupree, Juan Luna, and Scott Wolk

H2012 ADAS Workshop

Cadarache, France

24-25 Sept 2012

outline
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
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 n e 2 v of stars
Emission Measure (Ne2 V) of Stars

(Kastner et al. 2002)

log Ne2V

(cm-3)

log Te (K)

Stellar coronae, but accretion shock in TW Hydrae? (Kastner et al. 2002)

accretion or corona
Accretion or Corona?
  • Original argument for accretion shock based on high density
  • Additional diagnostics needed to test accretion-shock model

ChandraLarge Observing Program

TW Hya

500 ks with High Energy Transmission Grating

(Brickhouse et al. 2010)

tw hya
TW Hya
  • Classical T Tauri star (accreting)
  • i=7o (pole-on)
  • M = 0.8 MSun
  • R = 0.7 RSun
  • Distance 57 pc
  • 10 million yr old
  • Poised to make planets

Romanova et al. 2004

slide9

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 Line Ratio Diagnostics

He-like Energy Levels

(Smith et al. 2009)

x ray line ratio diagnostics for density and temperature
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
Accretionand aCorona

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
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
Accretion shock cools radiatively

2GM*

R*

Vff = (1 – R*/rt )1/2

~ 510 km/s

Te = 3.4 MK

Macc = f A*ρpre vff

(Konigl 1991;

Cranmer 2008)

accretion shock cools radiatively1
Accretion shock cools radiatively

2GM*

R*

Vff = (1 – R*/rt )1/2

~ 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
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 structure1
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”

accretion variation t e n h n e from ne ix
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.
  • Observed diagnostics constrain model Macc, B, f, rin and rout

NH from

0.9 to 3.2 1021 cm-2

Brickhouse et al. 2012

accretion model variations
Accretion Model Variations

Brickhouse et al. 2012

conclusions
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
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