X-rays in cool stars X-rays in cool stars

From present challenges to future observationsFrom present challenges to future observations

X-rays in cool stars X-rays in cool stars

From present challenges to future observationsFrom present challenges to future observations

Marc AudardMarc AudardISDC & Observatoire de GenèveISDC & Observatoire de Genève

Marc AudardMarc AudardISDC & Observatoire de GenèveISDC & Observatoire de Genève

X-ray Universe 2008X-ray Universe 2008

The solar-stellar connectionThe solar-stellar connection

• Stars provide a wide range of masses, radii, rotation periods, Stars provide a wide range of masses, radii, rotation periods, ages, abundances, etc. to study magnetic activityages, abundances, etc. to study magnetic activity

• Active stars show enhanced levels of activity compared to the Active stars show enhanced levels of activity compared to the Sun (LSun (LXX ≈ 100-1000x Sun, T ≈ 5-100 MK) ≈ 100-1000x Sun, T ≈ 5-100 MK)

• Coronal mass ejections and X-ray irradiation have strong Coronal mass ejections and X-ray irradiation have strong impact on orbiting planets and on circumstellar matter (e.g., impact on orbiting planets and on circumstellar matter (e.g., proto-planetary disk)proto-planetary disk)

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

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.

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)

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)

• 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)

•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 CentauriProxima Centauri

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)

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)

• Very limited sample, with poor signal-to-noise ratio in grating spectra

Günther et al. (2007)

• 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)

Auda

rd e

t al.

(200

5, 2

008)

V1647 OriKa

stne

r et a

l. (2

006)

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

Har

tman

n (1

997)

• 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)

Montmerle (2000)

Telleschi et al. (2007b)

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 ACool Hot

Jets

Gü

del et

al. (

2005

,2008)

Güdel et al. (2008)

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 (<50 ks, e.g., Taurus, Ophiuchus, Chamaeleon, Orion, etc)

XMM-Newton RGS131ks

Schmitt et al. (2005)

XEUS, TES10ks

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

Fe Kgeometry100 km/s

shift100 km/s

shift

Detailed flare studiesNFI TES

HXI CdTe

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

ConclusionConclusion• Current X-ray observatories have tremendously improved our Current X-ray observatories have tremendously improved our

understanding of coronal starsunderstanding of coronal stars• New insights and challenges on abundances, plasma densities, New insights and challenges on abundances, plasma densities,

impact of accretion in young stars, diffuse X-ray emission in impact of accretion in young stars, diffuse X-ray emission in star forming regions, magnetic activity at the bottom of the star forming regions, magnetic activity at the bottom of the main sequence, flare physics, coronal mapping, effects of X-main sequence, flare physics, coronal mapping, effects of X-rays on proto-planetary disks and exoplanets, time evolution rays on proto-planetary disks and exoplanets, time evolution of magnetic activity …of magnetic activity …

• Any future X-ray observatory should open new windows in Any future X-ray observatory should open new windows in stellar studies thanks to its sensitivity and high spectral stellar studies thanks to its sensitivity and high spectral resolutionresolution

Interested in learning more: go to sessions A.1 and A.2Interested in learning more: go to sessions A.1 and A.2

Electron densities

He-like

€

R =f

i=

R01+ ne nc + φ φc

where

R0 = low-density limit (ratio of A radiative decay coefficients)

nc = critical density (i.e., R(n=nc)=R0/2)

c = radiative excitation