A BLIND SEARCH FOR MAGNETOSPHERIC EMISSIONS FROM PLANETARY COMPANIONS TO NEARBYSOLAR-TYPE STARS

摘要

This paper reports a blind search for magnetospheric emissions from planets around nearby stars. Young starsare likely to have much stronger stellar winds than the Sun, and because planetary magnetospheric emissions arepowered by stellar winds, stronger stellar winds may enhance the radio luminosity of any orbiting planets. Usingvarious stellar catalogs, we selected nearby stars (< 30 pc) with relatively young age estimates ( < 3 Gyr). Weconstructed different samples from the stellar catalogs, finding between 100 and several hundred stars. We stackedimages from the 74 MHz (4 m wavelength) VLA Low-frequency Sky Survey, obtaining 3a limits on planetaryemission in the stacked images of between 10 and 33 mJy. These flux density limits correspond to average planetaryluminosities less than 5-10 x 10~(23)erg s~(-1).Using recent models for the scaling of stellar wind velocity, density, andmagnetic field with stellar age, we estimate scaling factors for the strength of stellar winds, relative to the Sun, in oursamples. The typical kinetic energy carried by the stellar winds in our samples is 15-50 times larger than that of theSun, and the typical magnetic energy is 5-10 times larger. If we assume that every star is orbited by a Jupiter-likeplanet with a luminosity larger than that of the Jovian decametric radiation by the above factors, our limits onplanetary luminosities from the stacking analysis are likely to be a factor of 10-100 above what would be requiredto detect the planets in a statistical sense. Similar statistical analyses with observations by future instruments, such asthe Low Frequency Array and the Long Wavelength Array, offer the promise of improvements by factors of 10-100.