X-ray Universe 2011

The The High-Energy Environment of High-Energy Environment of Extrasolar PlanetsExtrasolar Planets

J. Schmitt

Hamburger SternwarteEmail: jschmitt@hs.uni-hamburg.de

Internet: http://www.hs.uni-hamburg.de

Outline: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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benötigt.

(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)

Poppenhaeger et al. (2010)

Spectral type distribution of extrasolar planet host stars

Volume-limited sample of F,G,K,M dwarfs: FX vs. MV

Schmitt & Liefke (2004)

F G K M

Solar coronal hole

MV

Log

FX

Mea

n X

-ray

su

rfac

e fl

ux

Pizzolato et al. (2003)

Solar level

Saturation limit

Hot stars

Rossby number

XMM-Newton RGS: α Centauri A+B (inactive star)

(Liefke & Schmitt 2006)

Oxygen VII + VIII

Chandra LETGS:

Courtesy: J. Robrade

Accretion/Jet sources

An analogy from close binaries ?

Courtesy K. Poppenhaeger

What are we talking about ?

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 !)

magnetic interaction (joint magnetospheres)

Jupiter-Io-like interaction

Half period

full period

full period

X-ray Universe 2011

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:

€

Bplanet = Bhost

Rhost

dplanet

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟

3

€

Veff ,planet = 2π dplanet

1

Phost

−1

Pplanet

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟

€

Pplanet2 =

4π 2

GMhost

dplanet3

€

PAlfven =MA

4 1+ MA2

Pplanet

Phost

−1 ⎛

⎝ ⎜

⎞

⎠ ⎟

2π

Pplanet

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟

13 / 3Rp

2Bh2Rh

6

GMhost( )5 / 3

Dipole field Corotating plasma Kepler‘s 3 law

€

=MA

4 1+ MA2

Pplanet

Phost

−1 ⎛

⎝ ⎜

⎞

⎠ ⎟RJ

2BkG2 RSun

6

Pd13 / 3MSun

5 / 3× 6 1027 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 main-sequence 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.“

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close-in planets

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

Poppenhaeger et al. (2010)

No correlation !

Poppenhaeger & Schmitt (2011)

Name Teff P (days) Rplanet Age CoRoT 2ab 5600 K 1.74 1.465 young 51 Peg ab 5790 K 4.23 ? old

Two case studies:

Then not known as

planet hostCourtesy K. Poppenhaeger

Poppenhäger et al. (2009)

source

source+background

background

OVII

Alonso et al. (2008): CoRoT-2a + b

transits

Spots

Schröter et al. (2011)

Host star

LX ~ 2 1029 erg/s

Companion

LX < 1027 erg/s

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 !)

LX,host (cgs) a (AU) FX (cgs) Teff (K) Earth 1027 1 0.35 300 Jupiter 1027 5.2 0.013 120 51 Peg b 5 1026 0.052 65.4 1250 CoRoT 2b 4 1029 0.028 1.8 105 1800

A little comparison ……

Mass loss of (extrasolar ) planets:

1. „Jeans“ escape: atmosphere becomes collisionless

2. Hydrodynamic blowoff: Parker wind

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Planetary „surface“

collisional

collisionless

€

vescape =2GM

Rescape velocity:

Jean‘s flux:

€

ΦJ =kT

2π mp

N e−λ (1+ λ )

€

λ =GMmpart

RkTexo

=potential energy

thermal energy

rms speed:

€

vrms =3kT

mpart

escape temperature:

€

Tescape =2GM mpart

3k R

Escape temperatures of extrasolar planets:

Scaling relation from solar system gas giants:

€

Texo,1 − Teff ,1

Texo,2 − Teff ,2

vrms =Fheating,1g1

Fheating,2g2

€

Tescape ≈15000 K

Exospheric temperatures of extrasolar planets: ???????????

Obtain ridiculous values for CoRoT 2b

Exospheric temperatures ought to exceed

escape temperature !

X-ray Universe 2011

€

G M p mpart

Rp

Nenergy limited = ε FX +XUVEnergy limited flux:

Energy limited mass loss:

€

˙ M energy limited ≈ ε FX +XUV

Rp3

M p

BUT is the outflow really energy limited ?

there is radiative cooling

conduction

expansion ….

How large is the mass loss ?

Schröter et al. (2011)Chandra CoRoT 2

eclipse

€

RX −ray ≤ 0.4 R*

€

RX −ray ≤ 3 Rplanet

Conclusions: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“