The X-Ray Life
of Stars:
Low-Mass and
Pre-Main Sequence
Manuel Güdel
University of Vienna
Outline
• On the main sequence: from the Sun back to ZAMS
• Younger still: Protoplanetary disks and accretion
• More massive: Herbig stars
• More embedded: Jets and outflows
• Eruptive variables
• Summary
The Sun Among Stars
YEAR
α Cen AB: solar-like behavior (Ayres+ 2009)
• cycles
• rotational modulation,
• slow changes in coronal T
The Sun behaves like a normal
X-ray G2V star and therefore
as an example and anchor for
stellar X-ray astronomy.
Line Shifts: Contact Binary
(composite, 15 lines in MEG)
VV Cep
Rapidly Rotating Giant
(NeX 12.14A centroids, HEG)
FK Com
at rest
(Drake+ 2008)
• no eclipse
• near-polar
• on primary
• height 0.06-0.2 R*
(Huenemoerder+ 2006)
line shift,
broadening,
Doppler im.,
near-polar
region, ≤ 1R*
(Drake+ 2008)
redshift
60-140 km/s
(Flaring) Coronal Structure from Fluorescence
Monte-Carlo modeling of fluorescent efficiency
for different source heights (depending on
flux > 7.11 keV):
h < 0.3R*
(Testa+ 2008; see also Osten et al. 2007: alternatively electron impact but inefficient)
Beyond the Limit of Stars: Brown Dwarfs
LX/Lbol
log LX
stars
----------------BDs---------------
(Preibisch+ 2005)
log L(Hα)
* Young BDs (M6-M9): like stars (M6-M9):
coronal activity depends on Teff, not mass!
* Old BDs: X-ray faint – but not radio faint!
Magnetic activity persists, but coronal heating
declines.
(Berger+ 2010)
log LR
Toward Forming Stars:
Accretion and the
„High-Energy“ Environment
Shocks in accretion streams:
T = 3mHv2 / 16k
v  vff =
vff
(Günther+
2008)
f
ne = 1012 cm-3
r
(2GM/R)1/2
(Günther+ 2006)
i
 T = a few MK
f
dM/dt = 4R2fvffnemp  ne  1012-1014 cm-3
Dense, cool plasma in accretion shocks?
(Kastner+ 2002, Stelzer & Schmitt 2004, Schmitt+ 2005, Günther+ 2006, Argiroffi+ 2007, Robrade &
Schmitt 2006/07, Huenemoerder+ 2007, etc)
hot
OVIII 3-4 MK
OVII 2 MK cool
10-30 MK
non-accreting
accreting
1-2 MK
X-Ray Soft Excess
L(OVII)
(Güdel 2006; Telleschi+ 2007; G&T 2007)
WTTS
Related to
accretion AND coronal activity
L(OVIII)
But Shocks are Complex...
0.3 dex
2.5 dex
(Curran+ 2011)
.
.
.
• MX << Mopt: need right conditions:
• MX nearly constant!
too fast: chromospheric absorption
Not correl. with Mopt.
too slow: T too low, no X-rays;
(Curran+ 2011): optical-depth effects
mixture of structured flows
increasing with accretion rate?
(Sacco+2010)
Post-shock
Problems with Cooling....
• higher densities
• higher absorption
than Ne IX or Mg XII
Observations: (TW Hya, Brickhouse+ 2010)
• OVII lower density
• OVII lower absorption:
OVII
NeIX
NH = 4.1x1020 cm-2
NH = 1.8x1021 cm-2
NeIX
OVII
shock
Post-shock cools with distance.
O VII should show
MgXI
(Günther+ 2007)
(Brickhouse+ 2010)
Shock-heated gas channeled back to
the corona? „Accretion-fed corona“
(Brickhouse+ 2010)
(2D simulations by Orlando+ 2010; By=1 G, plasma-β >> 1)
Or fibril-structured accretion streams,
dense core developing shock deeper in
chromosphere: NeIX from deep, dense
layers, OVII only from outer, low-dens layer
(Sacco+ 2010)
chromosphere
shock
accretion flow
denser
OVII, NeIX
NeIX
more NH
Disk Ionization by Stellar X-Rays: Fluorescence
(flaring protostar in COUP)
Fe K
6.4 keV
Fluorescence of cool
disk material
(COUP, Tsujimoto+ 2005)
Fe XXV 6.7 keV
(30 – 100 MK)
Orion YSO (COUP):
6.4 keV
6.4 keV line during impulsive phase
like HXR or radio: EW = 1.4 keV!!
theoretical disks: <150 eV
(George & Fabian 1991, Drake+ 2008)
source height
EWcalc
(Drake+ 2008)
SXR flare
K shell electron ejection by
nonthermal electrons?
6.4 keV
(Osten et al. 2007, Czesla & Schmitt 2007)
rather inefficient
(Czesla & Schmitt 2007)
Irradiating hard
source hidden:
suppressed
continuum:
Herbig Ae/Be Stars
CTTS
AB Aur (HAe)
(Telleschi et al. 2007)
• Soft spectrum
• low density
• high Teff (10 kK)
 X-ray source at R > 1.7R*
h
(≈1600Å)
photoexcitation
Magnetically Confined Winds in AB Aur?
Accretion in HD 104237A?
NeIX: 1012 cm-3
(Testa+ 2008)
Jets in HD 163296?
(Günther+ 2009)
And low-mass
companions in
many others?
(Stelzer+ 2009)
Jets, Accretion Flows
L1551 IRS 5: (observations 2001, 2005, 2009)
Star absorbed, but inner jet X-ray strong
• Cooling jet, dominated by expansion
• standing structure at 0.5-1”
(Schneider+ 2011)
Jets, Accretion Flows
TAX spectrum
during 1 week
DG Tau
DG Tau:
observations
2004, 2005/06, 2010
hard/hot:
variable
soft/cool:
hard/hot
constant
(Chandra LP; Güdel+ 2011)
low NH
high NH
>> NH(AV)
time
Hard emission: coronal
excessively absorbed by
dust-depleted accretion flows.
ACIS-S image
similar spectrum:
soft “stellar” component and jet
Deconvolution of SER-treated ACIS data (Güdel+ 2011)
0.17”
1pixel = 0.0615”
(40 AU along jet)
2-8 keV
n = 105 cm-3
0.3-1.5 keV
Offset in 2010 identical to 2005/06 (Schneider+):
standing structure; collimation region?
(Günther+ 2009)
Eruptive Variables
FU Ori stars (FUors): disk dominates optical spectrum; large amplitude; yrs-decades
EX Lup stars (EXors): star dominates optical spectrum; less energetic, shorter
FUOrs not in immediate outburst: (Skinner+ 2009, 2011)
• very hard spectra
• excess absorption: due to accretion streams, winds, puffed-up inner disk?
• possibly X-ray overluminous for known/estimated masses
companion
FU Ori
(Skinner+ 2011)
X-rays
Z CMa:
no X-ray change
(Stelzer+ 2009)
EX Lup:
correlated,
accretion-funnel
absorbed hard
spectrum + accretionrelated soft spectrum
X-rays
(Grosso+ 2010)
V1647 Ori:
strong X-ray increase;
correlated with accretion
rate?
V1118 Ori:
moderate X-ray increase;
disk inner radius 0.4 to 0.2 AU
due to increased accretion
post-outburst: X-rays low
induced magnetic
reconnnection between
star and disk?
disruption of magnetosphere
by narrower disk?
(Kastner+ 2006)
(Audard+ 2010)
Summary
Unexpected diversity of emission mechanisms and heating
processes revealed in cool/pre-main sequence stars:
• Coronal radiation, T = 1-100 MK
• accretion shocks ear photosphere in T Tauri stars
• internal or bow shocks in jets of T Tauris and Herbigs
• standing shocks (?) in jet collimation regions
• magnetically confined winds in Herbig stars
• protoplanetary disk ionisation and fluorescence
• disk-magnetosphere interactions in eruptive variables
Rich field that has helped diagnose basic physical processes
especially around young, pre-main sequence stars.
NGC 2071 IRS1
Extreme Fluorescence?
NGC 2071 IRS 1:
(Skinner+ 2007,10)
XMM:
- EW(6.4 keV) = 2.4 keV
- constant over years
- not accompanied by flares
- no Fe XXV contribution!
Chandra:
(Skinner et al. 2007, 2010)
similar: Hamaguchi et al. (1.5 keV)
Irradiating hard source hidden (behind star, disk, etc):
suppressed continuum, strong fluorescent line