Transcript planets

The High-Energy Environment of
Extrasolar Planets
J. Schmitt
Hamburger Sternwarte
Email: [email protected]
Internet: http://www.hs.uni-hamburg.de
X-ray Universe 2011
Outline:
Motivation: The Sun as an X-ray
jjjjsource
 X-ray properties of planet-bearing
kkstars
 Star-planet interactions (SPI)
 Conclusions

Subject of X-ray emission and extrasolar planets is further
persued by:
Session A.1 Monday 15:20
Scott Walk: X-ray Observations of Hot Jupiters
Poster A13:
K. Poppenhaeger: Star-Planet Interactions in X-rays mimicked by selection effects ?
What would the Sun/solar system look like
to an extrasolar astronomer (equipped
with our instrumentation) ?
The SOHO Sun
Robrade et al. (2009)
k
RV-signal dominated by Jupiter !
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(Hypothetical) Extraterrestrial astronomers know
that
Sun is a (weak) X-ray source
Sun shows cyclic activity with a period of 11 years
Sun possesses a cold Jupiter with a period of about 11/12
years
„Types“ of extrasolar planets:
1. Radial velocity detections (blue, nearby))
2. Transit detections (green, further away)
3. Microlensing detections (brown, very distant)
Spectral type distribution of extrasolar planet host stars
Poppenhaeger et al. (2010)
F
G
K
M
Log FX
Mean X-ray surface flux
Volume-limited sample of F,G,K,M dwarfs: FX vs. MV
Solar
coronal
hole
Schmitt & Liefke (2004)
MV
Saturation limit
Solar level
Hot stars
Pizzolato et al. (2003)
Rossby number
Oxygen VII + VIII
XMM-Newton RGS:
α Centauri A+B (inactive star)
(Liefke & Schmitt 2006)
Chandra LETGS:
Accretion/Jet sources
Courtesy: J. Robrade
An analogy from close binaries ?
What are we talking about ?
Courtesy K. Poppenhaeger
Why do we care about X-rays ?
Star-Planet interaction:
(a) Star influences planet (trivial at first sight)
(b) Planet influences star
Planet might affect star through
tidal interaction (Earth-Moon system !)
Half period
magnetic interaction (joint magnetospheres)
full period
Jupiter-Io-like interaction
full period
Clarke et al. (2002)
X-ray Universe 2011
Key elements of Jupiter-Io interaction:
1. Strong magnetic field of Jupiter
2. Evaporation due to volcanism and formation of plasma
torus (high density environment)
3. Corotation of Jupiter‘s magnetosphere beyond Io
4. Magnetospheric rotation is super-Keplerian at Io‘s
distance
All required ingredients present in late-type stars
albeit not necessarily in any given star !
X-ray Universe 2011

Application to Planet X around a young star:
 R 3
host
Bplanet  Bhost

d

 planet 
Dipole field

PAlfven
 1
1 
Veff , planet  2 d planet

P  P

 host
planet 
Corotating plasma
Pplanet


2
4 1 M A  Phost
MA

2
planet
P
4 2 3

d planet
GM host
Kepler‘s 3 law
 2 
1

P

 planet 
13 / 3
R p2 Bh2 Rh6
GMhost 
2
6
Pplanet RJ2 BkG
RSun
27

1

6
10

 13 / 3 5 / 3
2
Pd M Sun
4 1 M A  Phost
MA
X-ray Universe 2011
5/3
erg /s
Claims for SPI at X-ray wavelengths (1):
Kashyap et al., 2008, ApJ, 687, 1339
„We carry out detailed statistical analysis on a volume-limited sample of mainsequence star systems with detected planets, comparing subsamples of stars
that have close-in planets with stars that have more distant planets. This
analysis reveals strong evidence that stars with close-in giant planets are on
average more X-ray active by a factor of 4 than those with planets that are
more distant.“
close-in planets
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distant planets
Claims for SPI at X-ray wavelengths (2):
Scharf, C., 2010, ApJ, 722, 1547
„We examine the X-ray emission of stars hosting planets and find a positive
correlation between X-ray luminosity and the projected mass of the most
closely orbiting exoplanets ….
Luminosities and upper limits are consistent with the interpretation that there is
a lower floor to stellar X-ray emission dependent on close-in planetary mass.
Under the hypothesis that this is a consequence of planet-star magnetic field
interaction, and energy dissipation, we estimate a possible field strength
increase of a factor of ~8 between planets of 1 and 10 MJ . …
The high-energy photon emission of planet-star systems may therefore
provide unique access to the detailed magnetic, and hence geodynamic,
properties of exoplanets.“
Scharf (2010)
X-ray census of planet bearing host stars
Poppenhaeger et al. (2010):
Known host stars within a volume of 30 pc: 72
20 pc
XMM-Newton
31 detections/4 upper limits
(20d/1 ul)
ROSAT
23 detections/11 upper limits
(20d/3 ul)
Total
54 detections/15 upper limits
(40d/4 ul)
(Uncensored) LX-distribution of nearby host stars is known
Spectral information avaialble for stronger sources
No correlation !
Poppenhaeger et al. (2010)
Poppenhaeger & Schmitt (2011)
Two case studies:
Name
CoRoT 2ab
51 Peg ab
Teff
5600 K
5790 K
P (days)
1.74
4.23
Rplanet
1.465
?
Age
young
old
Then not known as
planet host
Courtesy K. Poppenhaeger
Poppenhäger et al. (2009)
source+background
source
OVII
background
Alonso et al. (2008): CoRoT-2a + b
transits
Spots
Host star
LX ~ 2 1029 erg/s
Companion
LX < 1027 erg/s
Schröter et al. (2011)
Stellar radiation responsible for:
planetary heating (optical and UV)
ionosphere generation (XUV and X-ray)
(all planets with atmospheres in the solar
system have ionospheres !)
A little comparison ……
Earth
Jupiter
51 Peg b
CoRoT 2b
LX,host (cgs)
1027
1027
5 1026
4 1029
a (AU)
1
5.2
0.052
0.028
FX (cgs)
0.35
0.013
65.4
1.8 105
Teff (K)
300
120
1250
1800
Mass loss of (extrasolar ) planets:
1. „Jeans“ escape: atmosphere becomes collisionless
2. Hydrodynamic blowoff:
Parker wind
collisionless
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collisional
Planetary „surface“
escape velocity:
2GM
vescape 
R
J 
Jean‘s flux:

rms speed: 
escape temperature:

kT
N e  (1 )
2 m p
GMm part potential energy


RkTexo
thermal energy
3kT
v rms 
m part
2GMm part
Tescape 
3k R
Escape temperatures of extrasolar planets:
Tescape  15000 K
Exospheric temperatures of extrasolar planets: ???????????

Scaling relation from solar system gas giants:
Texo,1  Teff ,1
Fheating,1g1
v rms 
Texo,2  Teff ,2
Fheating,2 g2
Obtain ridiculous values for CoRoT 2b
Exospheric temperatures ought to exceed
escape temperature !
X-ray Universe 2011
How large is the mass loss ?
Energy limited flux:
Energy limited mass loss:
G M p m part
N energy limited   FX XUV
Rp
3
R
p
Ý
M energy limited   FX XUV
Mp

BUT is the outflow really energy limited ?
there is radiative 
cooling
conduction
expansion ….
eclipse
RX ray  0.4 R*
RX ray  3 R planet


Chandra CoRoT 2
Schröter et al. (2011)
Conclusions:
(Almost) all extrasolar host stars are X-ray
kksources
 Planet-star interactions are elusive
 Expect ionospheres and hydrodynamic
kkblowoff for the close extrasolar planets
 „X-ray radii“ of extrasolar planets should
kkbe much larger than their „visual radii“
