X-rays in cool stars From present challenges to future
observations Marc Audard ISDC & Observatoire de Genve Marc
Audard ISDC & Observatoire de Genve X-ray Universe 2008 Slide 2
Slide 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) Slide 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 Hnoux 1995,
1998) Schematic representation (Feldman 1992) The solar First
Ionization Potential (FIP) effect Slide 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. Slide 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)
Slide 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) Slide 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. Gdel et al. (1999, 2002) Slide 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) Gdel et al. (2002)
Proxima Centauri Slide 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) Slide 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 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; Gnther et al. 2006; Argiroffi et
al. 2007), but not all (Telleschi et al. 2007; Gdel et al. 2007)
Very limited sample, with poor signal-to- noise ratio in grating
spectra Gnther et al. (2007) Slide 12 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; Gdel &
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) Gdel & Telleschi (2007) Slide 13 Audard et
al. (2005, 2008) V1647 Ori Kastner et al. (2006) Similar T50 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) Slide 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) Slide 15 See Pravdo
et al. 2001; Favata et al. 2002; Bally et al. 2003; Kastner et al.
2005; Grosso et al. 2006; Gdel et al. 2005; 2007; 2008 DG Tau A
Cool Hot Jets Gdel et al. (2005,2008) Gdel et al. (2008) Slide 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 (
