X-rays in cool stars From present challenges to future observations

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X-rays in cool stars From present challenges to
future observations

• Marc Audard
• ISDC Observatoire de Genève

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The solar-stellar connection

• Stars provide a wide range of masses, radii,
  rotation periods, ages, abundances, etc. to study
  magnetic activity
• Active stars show enhanced levels of activity
  compared to the Sun (LX 100-1000x Sun, T
  5-100 MK)
• Coronal mass ejections and X-ray irradiation have
  strong impact on orbiting planets and on
  circumstellar matter (e.g., proto-planetary disk)

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The solar First Ionization Potential (FIP) effect
Schematic representation (Feldman 1992)
Low-FIP elements are overabundant, while high-FIP
elements are photospheric The solar chromosphere
has the right temperature (5,000-10,000 K) to
ionize low-FIP elements and keep high-FIP
elements in a neutral state Some fractionation
mechanism in the chromosphere should then
separate selectively elements and bring them into
the solar corona (see Hénoux 1995, 1998)
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Audard et al. (2001)

• A rich spectrum of coronal lines is emitted by
  magnetically active stars, giving us access to
  abundances of C, N, O, Ne, Mg, Al, Si, S, Ar, Ca,
  and Fe.

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The FIP and inverse FIP effects
Previous X-ray observations of stars showed
evidence of a MAD (metal abundance deficiency)
syndrome (Schmitt et al. 1996) in active stars,
and a possible solar-like FIP effect or no FIP
bias in inactive stars (Drake et al. 1995, 1997,
1999).

• Highly active stars show an inverse FIP effect,
  with low-FIP elements depleted relative to the
  high-FIP elements (Brinkman et al. 2001, Audard
  et al. 2003, etc).
• Ne possibly overabundant (e.g., Drake et al.
  2001) or Ne solar abundance too high (Drake
  Testa 2005, Cunha et al. 2006), but some studies
  suggest that the Ne solar abundance is OK (Young
  2005 Schmelz et al. 2005)

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Transition from FIP to IFIP

• Telleschi et al. (2005) suggest a transition from
  inverse FIP effect to FIP effect with decreasing
  activity in solar analogs (see also Audard et al.
  2003 for RS CVn binaries)
• Consistent with earlier findings of solar-like
  FIP effect in inactive stars
• Remaining problem large uncertainties or
  unavailable stellar photospheric abundances
  (e.g., Sanz-Forcada et al. 2004)

Telleschi et al. (2005)
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Güdel et al. (1999, 2002)

• Strong radio gyrosynchrotron emission in
  magnetically active stars. Electron beam could
  separate low-FIP ions from neutral high-FIP
  elements.
• During flares, chromospheric heating brings low-
  and high-FIP elements into the corona, increasing
  the low-FIP element abundances
• Laming (2004) proposed an alternative model in
  which ponderomotive forces due to Alfven waves
  propagating through the chromosphere fractionate
  low- and high-FIP elements. Fine tuning of
  parameters actually can mimic either the solar
  FIP effect of the inverse FIP effect.

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• Additional evidence of chromospheric evaporation
  via Neupert effect (see also Mitra-Kraev et al.
  2005 Smith et al. 2005 Wargelin et al. 08
  Schmitt et al. 2008 short thermal peak in X-rays
  coincident with optical peak)
• In contrast, no Neupert effect nor density
  changes observed in flares in EV Lac (Osten et
  al. 2005)
• Extremely bright flares may produce non-thermal
  hard X-rays (Osten et al. 2007)

Proxima Centauri
Güdel et al. (2002)
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Coronal densities

• Densities in active stars are log ne 9.5-11
  cm-3 (Ness et al. 2004, Testa et al. 2004),
  leading to coronal filling factors of 0.001-0.1
  (EM 0.85 ne2V).
• Possible higher densities at high T (e.g., Testa
  et al. 2004, Osten et al. 2006), but triplets
  suffer from lower spectral resolution. Fe XXI
  lines are consistent with the low-density limit
  in EUV range (Ness et al. 2004).

Testa et al. (2004)
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High densities in accreting stars

• High i/f ratio in He-like triplets of TW Hya
  indicate ne1013 cm-3 (Kastner et al. 2002
  Stelzer Schmitt 2004). Also Fe XVII (Ness
  Schmitt 2005)
• Plasma T3 MK consistent with adiabatic shocks
  from gas in free fall (v150-300 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)
• Very limited sample, with poor signal-to-noise
  ratio in grating spectra

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• Accreting stars show a soft X-ray excess (T2.5-3
  MK) in high-resolution X-ray spectra compared to
  non-accreting and ZAMS stars (Telleschi et al.
  2007c Güdel Telleschi 2007 Robrade Schmitt
  2007)
• The origin of the soft excess is unclear, but if
  the accretion shock mechanism works for some
  stars, it cannot for others (e.g., AB Aur, T Tau)
• Possibly, coronal loops get filled with accreting
  material (cooler and denser, therefore radiative
  cooling is more efficient)

Güdel Telleschi (2007)
Robrade Schmitt (2007)
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V1118 Ori
During outbursts in young stars, due to the
increase in accretion rate in the outburst, 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 2008). Enhanced
X-ray emission was observed during a transit of
an accretion funnel flow in AA Tau (Grosso et al.
2007)
Hartmann (1997)
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• Low plasma temperature and low density (1010
  cm-3) in Herbig A0 star AB Aur (no corona should
  exist Telleschi et al. 2007b)
• X-ray light curve follows similar periodicity as
  rotation period of AB Aur
• The stellar winds from both hemispheres are
  confined by the stellar magnetic field and
  collide at the equator, producing X-rays (Babel
  Montmerle 1997)

Telleschi et al. (2007b)
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Güdel et al. (2005,2008)

• See Pravdo et al. 2001 Favata et al. 2002 Bally
  et al. 2003 Kastner et al. 2005 Grosso et al.
  2006 Güdel et al. 2005 2007 2008

Güdel et al. (2008)
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From present challengesto future observations

• Many grating spectra of magnetically active stars
  (esp. young pre-main sequence stars) suffer from
  low to average signal-to-noise ratios
• It will be possible to obtain densities in many
  sources within 500 pc relatively quickly (lt50 ks,
  e.g., Taurus, Ophiuchus, Chamaeleon, Orion, etc)

XMM-Newton RGS 131ks
XEUS, TES 10ks
Schmitt et al. (2005)
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Orion distance (500pc) XEUS TES (50 ks)
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Detailed flare studies
NFI TES
HXI CdTe
100 km/s shift
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A possible large program the Orion Nebular
Cluster with the XEUS NFI NB the image is
already degraded to FWHM of 2 Hundreds of
sources with potential measurements of densities
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Conclusion

• Current X-ray observatories have tremendously
  improved our understanding of coronal stars
• New insights and challenges on abundances, plasma
  densities, impact of accretion in young stars,
  diffuse X-ray emission in star forming regions,
  magnetic activity at the bottom of the main
  sequence, flare physics, coronal mapping, effects
  of X-rays on proto-planetary disks and
  exoplanets, time evolution of magnetic activity
• Any future X-ray observatory should open new
  windows in stellar studies thanks to its
  sensitivity and high spectral resolution

Interested in learning more go to sessions A.1
and A.2
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Electron densities
He-like

• where
• R0 low-density limit (ratio of A radiative
  decay coefficients)
• nc critical density (i.e., R(nnc)R0/2)
• ???c radiative excitation