Download Wasp-17b: An Ultra-Low Density Planet in a Probable Retrograde

Wasp-17b: An Ultra-Low Density Planet in a
Probable Retrograde Orbit
Anderson, D. R. et al., ApJ 709, 159-167
Thomas Kupfer
March 29, 2011
Astrophysical Background
Formation of planetary systems
SuperWASP
Observations
Photometric observations
Spectroscopic observations
Results
Discussion
Outlook
Formation of planetary systems
◮
Gravitational collapse of a
molecular cloud
◮
Conservation of the angular
momentum of the cloud
⇒ A disk is formed with
the new star in the center
◮
In the disk planets can be
formed
http://astrobiology.nasa.gov/nai/
seminars/detail/117
Formation of planetary systems
◮
Gas giants supposed to be formed beyond the frost line
◮
Angular momentum of star and planets derive from parent
molecular cloud
⇒ Close alignment between the stellar spin and the
planetary orbit axis is expected
SuperWASP (Wide Angle Search for Planets)
◮
Consists of two robotic
observatories (SuperWASP
North and South)
◮
Each observatory consists
of eight 200mm lenses
◮
Aim is to observe the
entire sky up to 15
Magnitude and look for
transiting planets
◮
26 proved planet detections
http://wasp.astro.keele.ac.uk/
Photometric Observations
◮
WASP-17 was observed by WASP-South over two years (2006
– 2008)
⇒ Resulting in 15 509 usable photometric measurements
◮
They found a transit at 3.7 day periodicity
◮
Follow up observations with the EulerCAM over 6 hours
Spectroscopic Observations
http://www.isdc.unige.ch/result.cgi?061127 novaX
◮
41 spectra were obtained
using the CORALIE
spectrograph mounted at
the Euler Swiss Telescope
◮
3 additional spectra with
HARPS spectrograph
Rossiter–McLaughlin effect
http://www.naoj.org/Pressrelease/2010/12/20/index.html
◮
When the planet obscures a part of the star blue- or
red-shifted light is ”obscured”
⇒ Leading to an ”annomalous” radial velocity
◮
The shape of the effect is sensitive to the path of the planet
relative to the stars spin axis
Rossiter–McLaughlin effect in WASP-17
◮
Three RV measurements
during the transits suggest
large spin-orbit
misalignment
◮
first and third in
transit-point are discrepant
by 6σ and 3.8σ for a
prograde orbit
⇒ First planet found to be in a probable retrograde orbit
Stellar and System Parameter
◮
From spectroscopy WASP-17
is a F6 star
◮
System Parameter were
determined in a Markov-Chain
Monte Carlo analysis
◮
Planet is in a 3.735 days
orbit with a separation of
∼ 0.05 AU
Parameters
Extra constraints
eccentricity
λ (deg)
MP (MJup )
RP (RJup )
ρP (ρJup )
Case 1
...
0.129
−147
0.490
1.74
0.092
Case 2
MS Prior
0.237
−148.7
0.496
1.51
0.144
Case 3
e=0
0 (fixed)
−149.3
0.498
1.97
0.0648
Discussion
◮
WASP-17b the least dense planet known with a density of
0.06 – 0.14 ρJupiter
◮
Radius of 1.5 – 2 RJupiter larger than predicted by standard
evolution models
Possible Explanations:
◮
Enhanced atmospheric opacity can delay radius shrinkage
BUT 10 times solar insufficient to account for planets size
◮
Additional energy source is necessary
◮
Because of tidal dissipation orbital energy is deposited within
the planet
⇒ Leading to an inflated planet which could account for the
radius of WASP-17b
Discussion
◮
Planet is in a close and retrograde orbit
◮
Supposed to be formed outside the ice boundary (∼ 3 AU)
and in a prograde orbit
Possible Explanations:
◮
Migration of the planet due to tidal interaction with the gas
disk
BUT Not able to produce a retrograde orbit
◮
Additional planet – planet or star – planet scattering is
necessary to produce highly eccentric and retrograde orbit
⇒ A combination of tidal dissipation and scattering are
necessary to produce such a system
Outlook
◮
Further RV measurements and imaging are necessary to
search for stellar and planetary companions
◮
Due to low surface gravity the planet has the largest known
atmospheric scale hight for a planet (1100 – 2100 km)
⇒ good prospect for transmission spectroscopy
⇒ WASP-17b is an example that the formation or planetary
systems could be quite complicated