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X-ray shining stars Advances foreseen with ASTRO-H. Marc Audard (University of Geneva). ASTRO-H Special Session 8, EWASS 2012, Roma, 3 July 2012. Introduction. Why should we care about stellar science with ASTRO-H?

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x ray shining stars advances foreseen with astro h

X-ray shining starsAdvances foreseen with ASTRO-H

Marc Audard

(University of Geneva)

ASTRO-H Special Session 8, EWASS 2012, Roma, 3 July 2012


Why should we care about stellar science with ASTRO-H?

Stars are nearby astrophysical cosmic plasmas ideal to study MHD physics, the importance of magnetic fields, stellar winds, and X-ray photons on the surrounding environment (chemical enrichment, energy input; habitability of planets; irradiation of accretion disks)

Stars are rich line-emitters!

xmm newton chandra

The high-resolution grating spectra on-board XMM-Newton and Chandra have allowed excellent, new science to be done on stars:

  • Abundance studies (FIP and inverse FIP effect)
  • Average density & opacity measurements
  • Eclipse & Doppler mapping of corona (limited)
  • Density measurements in a handful of young stars (excitement! Accretion may produce sufficient X-rays)
  • Detection of Fe Ka at 6.4 keV: information on source size, height, mechanism (but very limited!)
  • Density variations during flares (rare! Low S/N)
  • Line profiles in stellar winds
  • Etc…
science with astro h
Science with ASTRO-H

The effective area and spectral resolution above 1 keV of ASTRO-H will help:

  • Dynamical MHD processes at the ≤kilosecond time scale
  • Probe densities in accreting stars
  • Detect Fe Ka emission and constrain geometries

These are selected topics!

a dynamical picture
A dynamical picture

Audard et al. (2003)

  • Chromospheric evaporation can lead to mass motions into the corona with speeds of a few 100 km/s
  • Non-equilibrium conditions in the fast rise phase

Rise time < 100s

Flares of a few ks


Proxima Centauri

F ~ ne2V  V ~ F/ne2 M ~ F/ne

Güdel et al. (2002)


Same application for accretion disks

Testa et al. (2008). See also Osten et al. (2007), Giardino et al. (2007), Drake et al. (2008)


Czesla & Schmitt (2007)

V1486 Ori (Czesla & Schmitt 2007):

Evidence of very hot plasma > 10 keV

Strong fluorescent Fe Ka line (EW≈1400 eV) in the initial phase of the flare rise

Fluorescence models underpredict the observed Fe Ka flux (Czesla & Schmitt 2007)

Other examples (e.g., Tsujimoto et al. 2005, Giardino et al. 2007; Stelzer et al. 2011)

Fe Ka line also detected in quiescence

Origin of line not definitive (fluorescence, electron impact ionization)

ASTRO-H will allow to do reverbation mapping of the star-disk system


Detailed flare studies

Model: 1/6 of

AB Dor flare (2-T; 20+80 MK; Maggio et al. 2000), i.e., about 10x quiescence

Bai et al. (1979) model for Fe Ka

SXS: 31.5 c/s

Fe Ka


Caveat: moving

flare plasma dominates the emission (but not so unreasonable for large flares)

Caveat2: thermal broadening

Caveat3: line of sight!

100 km/s shift

Even better at 7keV

Other lines of hot plasma at longer wavelengths useful

hard x ray non thermal flare emission
Hard X-ray non-thermal flare emission

Hard X-ray emission detected in a stellar flare

Preferred explanation as non-thermal thick-target bremsstrahlung emission (also supported by Fe Ka emission)

Superflare on II Peg; Osten et al. 2007. Previously inconclusive with BeppoSAX (Pallavicini et al. 2000)

Osten et al. 2007

Note that non-thermal X-rays must exist: evidence in the Sun and in non-thermal radio emission of stars in quiescence


ne ≈ 1011-12 cm-3

ne ≈ 109-10 cm-3






Fe blend




AB Dor in quiescence: 2.4 c/s (SXS)

ne ≈ 1011.5-12.5 cm-3

SXS will not deblend the many Fe lines around the Ne IX triplet. Accurate modeling and atomic data will be required to derive the density.

Problem less important for OVII

“Hotter” plasma will also be probed (Mg XI, Si XIII, etc) but critical densities may be too high (except during flares)





high densities in accreting stars
High densities in accreting stars
  • High i/f ratio in He-like triplets of TW Hya indicate ne≈1013 cm-3 (Kastner et al. 2002; Stelzer & Schmitt 2004). Also Fe XVII (Ness & Schmitt 2005)
  • Plasma T≈3 MK consistent with adiabatic shocks from gas in free fall (v≈150-300 km s-1)
  • High densities in accreting young stars (Schmitt et al. 2005; Robrade & Schmitt 2006; Günther et al. 2006; Argiroffi et al. 2007), but not all (Telleschi et al. 2007; Güdel et al. 2007, Argiroffi et al. 2011; etc)
  • Very limited sample (only 8 CTTS), with generally poor signal-to-noise ratio in grating spectra

Günther et al. (2007)

accretion in young stars
Accretion in young stars
  • Very small sample of accreting stars with gratings
  • SXS will allow us to measure densities in the critical range of 1011-12 cm-3, i.e., with the Ne IX and Mg XI triplets
  • Measurement at lower densities (O VII) will be difficult due to NH (1020-1022 cm-2) and to the limited spectral resolution at 22 Å



130ks, 0.03 c/s

NH=6x1020 cm-2




Schmitt et al. (2005)


Potential of ASTRO-H to observe several “nearby” (≤140pc) star forming regions also with SXS to obtain densities

  • Importance to determine the contribution of accreting plasma
  • Access to Ne IX (but blends with Fe) and Mg XI, crucial for accreting plasmas. Access to OVII will depend on column density (OK for NH<5x1021 cm-2->AV=3m at 500pc) and spectral resolution.

Fx ≈ 10-15-10-14 erg s-1 cm-2

Hartmann (1997)

Audard et al. (2010)

During outbursts in young stars, due to the increase in accretion rate, the accretion disk closes in and may have disrupted the magnetic loops, modifying the magnetospheric configuration (Kastner et al. 2004; 2006; Grosso et al. 2005; Audard et al. 2005; 2010; Hamaguchi et al. 2012)

x ray emission from interacting wind massive binaries what astro h can tell us
X-ray emission from interacting wind massive binaries: what Astro-H can tell us
  • The interactions of stellar winds in massive binary systems produce X-ray emission due to heating of the winds in the interaction zone (e.g. Stevens et al. 1992, ApJ, 386, 265).

Courtesy G. Rauw


The morphology of the wind interaction zone depends on the ratio between the wind momenta:

  • X-ray emission lines formed in the interaction region are expected to display a changing morphology depending on the shock properties and the orbital phase (e.g. Henley et al. 2003, MNRAS 346, 773). This provides important diagnostics on the wind interaction.

Courtesy G. Rauw


Fe K is probably one of the best diagnostic lines for this purpose as it traces only the hottest plasma produced in the wind – wind interaction zone.

  • The spectral resolution of SXS (assumed value 5 eV equivalent to 225 km s-1) is just about right to resolve lines that are a few 1000 km s-1 wide.
  • Observations at different orbital phases of a sample of such binaries will allow to probe the physics of wind interactions.

Courtesy G. Rauw


Hydrodynamic shocks in colliding wind systems are known to accelerate relativistic particles (seen through synchrotron radio emission in a subsample of massive binaries, e.g. De Becker 2007, A&ARv 14, 171).

  • The copious photospheric UV radiation is expected to produce hard X-ray emission through inverse Compton scattering.
  • This has been observed (so far) for η Carinae (Leyder et al. 2008, A&A 477, L29) and WR140 (Sugawara et al. 2011, BSRSL 80, 724).
  • Astro-H offers the ideal combination of instrumental capabilities to detect such a hard tail and simultaneously characterize the thermal emission of a colliding wind system (see also posters by De Becker et al. and Khangulyan et al. at this meeting).

Courtesy G. Rauw

  • Dynamical processes (flare physics) will be studied with good S/N
  • Stochastic processes, however, require some integration time (20-50 ks) to capture flares with sufficient energy and signal
  • ASTRO-H will also probe high densities in young accreting stars via the Ne IX and Mg XI triplets
  • ASTRO-H will help constrain geometries via the Fe Ka line
  • The SXI will also provide additional coverage in the field of stellar clusters, in particular star forming regions
  • ASTRO-H SXS will also be useful for colliding wind binaries: it will resolve lines in shocks and trace the wind-wind shock zone
  • The broad-band coverage of ASTRO-H will also help detect the non-thermal emission in stellar coronae and colliding wind binaries
  • Count rates should not be too problematic except during very strong flares, and optical loading should be minimal but can be alleviated thanks to optical blocking filters