Magnetospheric Emissions from the Planet Orbiting tau Boo: A Multi-Epoch Search

Abstract

All of the solar system gas giants produce electron cyclotron masers, driven by the solar wind impinging on their magnetospheres. Extrapolating to the planet orbiting tau Boo, various authors have predicted that it may be within the detection limits of the 4-meter wavelength (74 MHz) system on the Very Large Array. This paper reports three epochs of observations of tau Boo. In no epoch do we detect the planet; various means of determining the upper limit to the emission yield single-epoch limits ranging from 135 to 300 mJy. We develop a likelihood method for multi-epoch observations and use it to constrain various radiation properties of the planet. Assuming that the planet does radiate at our observation wavelength, its typical luminosity must be less than about 10^{16} W, unless its radiation is highly beamed into a solid angle Omega << 1 sr. While within the range of luminosities predicted by various authors for this planet, this value is lower than recent estimates which attempt to take into account the stellar wind of tau Boo using the known properties of the star itself. Electron cyclotron maser emission from solar systems planets is beamed, but with characteristic solid angles of approximately 1 sr illuminated. Future long-wavelength instruments (e.g., the Long Wavelength Array and the Low Frequency Array) must be able to make typical flux density measurements on short time scales (~ 15 min.) of approximately 25 mJy in order to improve these constraints significantly.