Technische Universität Braunschweig, Institut für Theoretische Physik, Germany

1. ionosperic cutoff Discrimination of exoplanetary and stellar radio flux J.-M. Grießmeier, U. Motschmann, G. Mann thermal nonthermal Technische Universität Braunschweig, Institut für Theoretische Physik,Germany Email: j-m.griessmeier@tu-bs.de Astrophysikalisches Institut Potsdam, Germany 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? • 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 (cutoff-frequency of the radio emission) Stellar radio flux Assuming a solar twin for the extrasolar host star, the following components have to be discussed: 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. Problem: Only the total flux (star plus planet) can be measured. How can stellar and planetary emission be distinguished? 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. 1: Thermal (black-body) and nonthermal emission of Jupiter (normalized to a dis-tance of 1AU). Due to the Earth’s ionosphere, frequen-cies below 10MHz cannot be detected on Earth. [adapted from Bastian, Dulk, Leblanc, ApJ, 545, 1058, (2000)] 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 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: Typically, the radio emission is generated close to the electron gyrofrequency. The emission mechanism probably is the Cyclotron Maser Instability (CMI), a wave-particle-interaction in-volving electrons gyrating in a magnetic field. Instability, i.e. positive growth rates require the distribution function to fulfill the condition • The emitted power scales with the received stellar wind power [Farrell et al., JGR, 104, 14025, (1999)]: • The received solar wind power depends on the cross-section of the magnetosphere: • The size of the magnetosphere depends on the planetary magnetic moment: • For the magnetic moment, different scalings are in use [e.g. Grießmeier et al., A&A, submitted (2003)]: Fig. 4: Model of the Cyclotron Maser operating in a denstiy cavity [Ergun et al., ApJ, 538, 456, (2000)]. 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). Fig. 3: Schematic view of the conversion from solar wind power incident on the magnetosphere to reemitted radio power. This is true for loss-cone and horseshoe distributions. 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). 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