These flux density levels have to be evaluated in view of the at present possible radio telescope sensitivities. Investigations in 1999 and again in 2002 by means of the Very Large Array (VLA, 27 radio antennas in Y-shaped configuration, located close to Socorro, New Mexico, USA), have been performed surveying the region near Tau Bootis with the sofar unsurpassed sensitivity. (Farrell, 2003) As displayesd in Fig. 1, the search at 74 MHz with a sensitivity of < 0.12 Jy would have been within the margins of the estimates, but the negative results may place upper limits on stellar winds, exo-interplanetary magnetic fields. Even the world-largest telescope UTR-2 (located near Kharkov, Ukraine) and connected to state-of-the-art acousto-optic (AOS) and digital receivers (DSP) (Rucker, 2001; Lechacheux, 1998), is at present unable to cope with those necessary low sensitivities.
Taking into account the UTR-2 effective area (∼5*104 m2), the spectrometer bandwidth (1 kHz) and an integration time of 10s, the UTR-2 sensitivity is about 25 Jy, with an integration time of 100s down at ∼8 Jy. As will be shortly addressed below, the development of a new generation of receivers with increased bandwidth and integration time will yield sensitivities below the 1 Jy level. Considering the dependence of flux density S with wavelength λ the probability of detecting exoplanetary radio emission may increase at lower frequencies, i.e. at comparable sensitivties like the VLA 2002 measurements, but considerably below 74 MHz.
Summarizing the many aspects of exoplanetary radio astronomy, nonthermal radio emission from stellar orbiting companions at solar/stellar distances of ≤20 parsec may be within the reach of near future technology, under favorable conditions like strong stellar wind and/or strong exo-interplanetary magnetic field interactions with exoplanetary magnetic fields.
Within the frame of the French-Austrian-Ukrainian cooperation in the field of decameter radio astronomy a new generation of radio receiver is under development to be installed at UTR-2. An interference robust digital radio wave analyzer with pure digital techniques, 14 MHz bandwidth, center frequency between 8 - 80 Mhz, 2 channels for polarization measurements and real time processing, the so-called Robin 2, will provide sensitivities at the 100 mJy level. An extremely ambitious project entitled LOFAR (= Low Frequency Array, http://www.lofar.org/) proposes a multi-element, interferometric imaging telescope operating in the frequency range of 10 - 240 MHz, with a collecting area of 106 m2 at 15 MHz. Sensitivities of 1 mJy (at 15 MHz) or even 0.3 mJy (at 150 MHz) may be possible.
Reaching these new frontiers of low frequency radio astronomy, new insights will be obtained in the physics of extremely remote worlds, e.g. on the magnetic properties of exoplanets and probably on their rotation periods. Beyond that, these investigations touch the question of our uniqueness, the possible existance of magnetic planets within the habitable zones. It is reasonable that strong planetary magnetic fields can also be maintained by planets with solid surfaces (Paschke, priv. comm.), thus providing the necessary shielding effect against cosmic rays. In this sense, planetary radio astronomy significantly contributes in the search for extrasolar planets.
Full article available at ADSABS: http://adsabs.harvard.edu/abs/2002ESASP.518..421R