1. X-rays in cool stars From present challenges to future observations Marc Audard ISDC & Observatoire de Genève Marc Audard ISDC & Observatoire de Genève X-ray Universe 2008

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

4. 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) Schematic representation (Feldman 1992) The solar First Ionization Potential (FIP) effect

5. 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.

6. The FIP and inverse FIP effects 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) 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). Solar Sanz-Forcada et al. (2003)

7. 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)

8. 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. Güdel et al. (1999, 2002)

9. 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) Güdel et al. (2002) Proxima Centauri

10. Coronal densities Densities in active stars are log n e ≈ 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 n e 2 V). 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)

11. High densities in accreting stars High i/f ratio in He-like triplets of TW Hya indicate n e ≈10 13 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) Very limited sample, with poor signal-to- noise ratio in grating spectra Günther et al. (2007)

12. Accreting stars show a soft X-ray excess (T≈2.5-3 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) Robrade & Schmitt (2007) Güdel & Telleschi (2007)

13. Audard et al. (2005, 2008) V1647 Ori Kastner et al. (2006) Similar T≈50 MK 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) V1118 Ori Hartmann (1997)

14. Low plasma temperature and low density (10 10 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) Montmerle (2000) Telleschi et al. (2007b)

15. 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 DG Tau A Cool Hot Jets Güdel et al. (2005,2008) Güdel et al. (2008)

16. From present challenges to 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 (<50 ks, e.g., Taurus, Ophiuchus, Chamaeleon, Orion, etc) XMM-Newton RGS 131ks Schmitt et al. (2005) XEUS, TES 10ks

17. Orion distance (500pc) XEUS TES (50 ks)

18. Fe K  geometry 100 km/s shift Detailed flare studies NFI TES HXI CdTe

19. 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

21. Conclusion Current X-ray observatories have tremendously improved our understanding of coronal stars 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 … 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 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

23. Electron densities He-like where R 0 = low-density limit (ratio of A radiative decay coefficients) n c = critical density (i.e., R(n=n c )=R 0 /2)  c = radiative excitation