X-Ray Spectroscopy of Cool Stars
From Coronal Heating to Accretion
Manuel Güdel
Paul Scherrer Institut, Switzerland
Max-Planck-Institute for Astronomy, Heidelberg, Germany
Cambridge, July 11, 2007
ESA
Coronal statics: Structure and extent of magnetic fields
Radio VLBI (0.8 mas)
(UV Cet, Benz et al. 1998)
X-ray eclipse map
(0.015 mas)
( CrB, Guedel et al. 2003)
...but marginal or exceptional and always challenging
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Coronal structure
Cambridge, July 11, 2007

coronal heating and dynamics
First step toward coronal structure: densities and EM
(Audard et al. 2001, Ayres et al. 2001, Güdel et al.
2001, Huenemoerder et al. 2001, Mewe et al. 2001,
Ness et al. 2001, Phillips et al. 2001, etc;
Surveys: Nes et al. 2004, Testa et al. 2004):
(Testa et al. 2004)
• Coronal densities typically ≈ 1010 cm-3
• In active stars up to 1011 cm-3
Cambridge, July 11, 2007
Combine
- density at T (homogenous assumption) and EM at T
- reasonable scale height at T (e.g., loop scaling laws)
 surface filling factor for structures at T
(Testa et al. 2004)
(Ness et al. 2004)
MgXI
7 MK
solar active
regions
NeIX
3-4 MK
“activity”
cool: fill up to 10%
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then: add hot plasma
add cool plasma

interactions between

active regions: flares
more heating, higher T,
more pasma, higher ne
Are flares heating active stellar coronae?
(e.g.,Güdel et al. 1997, Drake et al. 2000, Ness et al. 2004)
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Composition of stellar coronae: Indicator of mass transport?
active stars
enhanced high-FIP :
IFIP
Solar
analogs
inverse FIP effect
activity
Brinkman et al. 2001,
Güdel et al. 2001)
(1 Ori, Telleschi et al. 2005)
Sun and inactive
stars (+Sun)
enhanced low-FIP:
FIP effect
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FIP
What determines IFIPness among most active stars?
Fe/Ne
weaker IFIP
IFIPness
determined by
the stellar Teff:
Ionisation structure in
chromosphere?
stronger IFIP
(XEST + published
values; after
Telleschi et al. 2007:
EPIC: Scelsi et al. 2007)
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Teff
Abundances as accretion indicators?
1. Metals like Fe, Mg, Si, C, O, may condense into grains and be retained
in the disk (planets). Not so Ne and N (TW Hya, Herczeg et al. 2002 for Si/UV;
Stelzer & Schmitt 2004 for Ne, N, C, Fe/X-rays)
 Accretion streams Fe-depleted / Ne- and N rich
2. But: similar in other active stars
“old” TW Hya: Ne/O high;
“young” BP Tau: Ne/O normal
Grain growth toward planets
retains metals only in old TW
Hya disk. In younger CTTS, dust
accretes as well (Drake et al. 2005).
3. MP Mus: “old”, but low Ne!
(Argiroffi et al. 2007)
Cambridge, July 11, 2007
ESA
Proxima Centauri,
quiescent
inactive star
...also
Proxima Centauri:
average flare
active star
...not
Proxima Centauri:
YY Gem, quiescent
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similar
active star
Anything left for "quiescence"?
(Audard et al. 2003)
Flare distributions in light curves: Favor dominance of small flares:
All coronal heating may be due to the sum of all flares.
(Audard et al. 1999, Kashyap et al. 2002, Guedel et al. 2003, Arzner & Guedel 2004,
Stelzer et al. 2007)
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(Guedel et al. 2003)
OVII
ne 5x109
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4x1011
2x1010
4x1011
2x1010
average flare
log ne = 10.50 +/- 0.28
quiescent YY Gem log ne = 10.35 +0.13
-0.45
DEM steep on low-T side:
DEM T4
(static loops: DEM T1.0-1.5)
(Laming & Drake 1999)
T, EM, ne
superposed flaring (heating - cooling)
DEM T3-5
from hydrodynamic decay
(Guedel et al. 2003)
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active star:
IFIP
• Flares bring
new, chromospheric
material into corona
(cromospheric evaporation)
• Flares not directly responsible
for IFIP in active stars
“activated”
(flaring) star:
• IFIP composition builds up
gradually
relative FIP
(Nordon & Behar 2006)
inactive star:
FIP
FIP
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11, 2007
flare
How does accretion interact with the „high-energy“ environment?
Shocks in accretion streams:
T = 3mHv2 / 16k
vff
f
v  vff = (2GM/R)1/2
 T = a few MK (<< 10 MK)
dM/dt = 4R2fvffnemp  ne  1012-1014 cm-3
Can test these predictions using high-res X-ray spectroscopy
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High-resolution X-ray spectroscopy
of classical T Tauri stars
TW Hya
BP Tau
(Kastner et al. 02)
(Schmitt et al. 05)
very soft spectrum
hard
very high densities
(1013 cm-3, NeIX)
intermed. dens.
(3x1011 cm-3)
NeIX
OVII
Hypothesis: Shock-induced soft X-rays
Cambridge, July 11, 2007
Dense, cool plasma in accretion shocks?
Possible for TW Hya, BP Tau, V4046 Sgr,
MP Mus (Kastner et al. 2002, Stelzer & Schmitt
r
i
T Tau
BP
f
2004, Schmitt et al. 2005, Günther et al. 2006,
Argiroffi et al. 2007)
But: Not measured in XEST targets
• AB Aur
• T Tau
Density
AB Aur
<
few x 1010 cm-3 << shock ne
So, is accretion really important?
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(Telleschi et al. 2007,
Güdel et al. 2007)
OVIII
3-4 MK
OVII
2 MK
10-30 MK
hot
WTTS:
non-accreting
CTTS:
accreting
1-2 MK
"SOFT EXCESS"
Cambridge, July 11, 2007
(Telleschi et al. 2007, Güdel et al. 2007)
CTTS
Soft Excess
≈ 2-3x
WTTS
hotter
(Güdel &
Telleschi 2007)
Cambridge, July 11, 2007
“Accretion adds cool material in CTTS”
New insight into coronal statics and dynamics from high-res spectroscopy:
- Active coronae may be driven by magnetic explosive energy release:
density, temperatures, EM distributions
Open questions:
what drives abundance anomalies?
how are dynamic coronal systems structured?
- Coronal magnetic structures modified by accretion:
density, temperatures, abundances(?),
soft excess
Open questions:
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how is soft excess achieved?
what exactly do abundances reflect?
end
Cambridge, July 11, 2007