Download Discrimination of exoplanetary and stellar radio flux

Technische Universität Braunschweig,
Institut für Theoretische Physik,
Germany
Email: [email protected]
Astrophysikalisches Institut Potsdam,
Germany
Discrimination of exoplanetary and stellar radio flux
J.-M. Grießmeier, U. Motschmann, G. Mann
Abstract
Magnetized extrasolar giant planets in close (but not tidally locked) orbits are expected to be strong nonthermal radio emitters. The radiation may
be strong enough to be detected on earth with the next generation of instruments. But as most observation techniques radio detection does not
simply yield the pure planetary signal, but a combination of the planetary and stellar emission. We compare the expected stellar and planetary
signal for the low-frequency radio range and discuss methods useful to separate the effect of stellar and planetary emission. The additional noise
due to the galactic background is discussed. We show that for a planet with sufficient radio emission the separation of the planetary signal from
the stellar emission seems feasible.
Why to look for radio flux?
Stellar radio flux
• better intensity ratio of stellar flux to planetary flux than in
the visible range (109) or in the infrared (106): between 10-4
(quiet sun) and 103 (strong radio bursts)
• information on planetary rotation (modulation of the
emission with the rotation frequency)
• information on the planetary magnetic field (cutofffrequency of the radio emission)
Assuming a solar twin for the extrasolar host star, the following components have to be discussed:
Problem: Only the total flux (star plus planet) can be
measured. How can stellar and planetary emission be
distinguished?
Fig. 1: Thermal (blackbody) and nonthermal
emission of Jupiter
(normalized to a distance of 1AU).
thermal
Due to the Earth’s
ionosphere,
frequencies
below
10MHz
cannot be detected on
Earth.
nonthermal
ionosperic
cutoff
[adapted from Bastian,
Dulk, Leblanc, ApJ,
545, 1058, (2000)]
Quiet sun emission
The quiet sun emission is due to thermal emission of ionized plasma close to the (local) electron plasma
frequency. It is randomly polarized.
Slowly varying component
It is due to thermal emission from regions of hot and dense plasma (e.g. over sunspots). It leads to flux
density variations of a factor of two in the decimetric and centimetric wavelength range with 27 days
period (due to the solar rotation). It is frequently circularly polarized.
Noise storms
During solar maximum, noise storms frequently
occur (about 10% of the time). The typical
duration is between a few hours and several
days. The emission consists of a broadband
continuum plus short-lived bursts. The emission
is circularly polarized.
Radio bursts
Radio bursts are generated by high-energy
particles originating from solar flares or shock
fronts. Typically, their frequency drifts. Their flux
densities are much higher than that of the quiet
sun or of noise storms. Different kinds of radio
bursts exist.
Fig. 2: The various components of the solar radio spectrum
[Boischot et al., Adv. in Electronics and Electron Phys., 20,
147, (1964); Nelson et al., in “Solar Radiophysics”, p.113,
(1985)]. The flux density associated with solar radio bursts by
far exceeds the quiet sun conditions.
Planetary radio flux
Typically, the radio emission
is generated close to the
electron gyrofrequency.
All strongly magnetized planets of the solar system are sources of nonthermal radio emission. The source
region was found to be close to the auroral fieldlines at distances of 2 to 4 planetary radii.
For the total emitted power, simple scaling laws can be applied:
• The emitted power scales with the received stellar wind
1.2
Prad  PSW
power [Farrell et al., JGR, 104, 14025, (1999)]:
The emission mechanism
probably is the Cyclotron
Maser Instability (CMI), a
wave-particle-interaction involving electrons gyrating in a
magnetic field.
• The received solar wind power depends on the cross2
2
section of the magnetosphere:
PSW  RM d
Fig. 3: Schematic view of the
conversion from solar wind power
incident on the magnetosphere to
reemitted radio power.
• The size of the magnetosphere depends on the
planetary magnetic moment:
RM  M
• For the magnetic moment, different scalings are in use
[e.g. Grießmeier et al., A&A, submitted (2003)]:
M   r
1/ 3
1/ 2
c
d
1/ 3
 
c
For close-in extrasolar giant planets (i.e. small d), much stronger radio
emission is expected than for Jupiter [Farrell et al., JGR, 104, 14025, (1999),
Zarka et al., ASS, 277, 293, (2001)] as long as they are not tidally locked
(this would lead to smaller magnetic moments).
Radio flux comparison
Quiet sun emission
The quiet sun emission is much weaker than Jupiter’s emission, and has a different
polarisation (i.e. it is randomly polarised).
Slowly varying component
The slowly varying component does not contribute in the frequency range relevant for
Jupiter’s emission, and changes on a relatively slow timescale.
Noise storms
Noise storms could be a problem for more active stars (or weak planetary emissions, but
then detection is much more the problem than discrimination).
Galactic background
An additional measurement (slightly off-target) may become necessary to be able to
subtract the galactic background from the measured signal.
Radio bursts
The flux density is much higher than for Jupiter’s emission, which could be problematic.
However, for the sun, radio bursts are either rare (Type IV bursts: 3 per month at solar
maximum) or of limited duration (Type III bursts: a few seconds).
Instability, i.e. positive growth
rates require the distribution
function to fulfill the condition
Fig. 4: Model of the
Cyclotron Maser operating in
a denstiy cavity [Ergun et al.,
ApJ, 538, 456, (2000)].
 f v   0
This is true for loss-cone and
horseshoe distributions.
Fig. 5: Comparison of
solar
and
average
planetary flux densities
[Boischot et al., Adv. in
Electronics and Electron
Phys., 20, 147, (1964);
Nelson et al., in “Solar
Radiophysics”,
p.113,
(1985), Zarka et al., in
“Neptune and Triton”,
p.341, (1995)]. Jupiter:
normalized to a distance
of 1AU.
Stellar radio bursts vs. planetary radio emission
• use statistical methods to reduce influence of stellar bursts
• observe secondary eclipses of transiting planets (compare spectra of “star plus
planet” with spectra of “star only” type)
• For a system without tidal locking, the planetary emission is modulated with the
rotation rate
• observe close-in giant planet with high flux density