Radio Observations of HD 80606 Near Planetary Periastron

This paper reports Very Large Array observations at 325 and 1425 MHz (λ90 cm and λ20 cm) during and near the periastron passage of HD 80606b on HJD 2454424.86 (2007 November 20). We obtain flux density limits (3σ) of 1.7 mJy and 48 μJy at 325 and 1425 MHz, respectively, equivalent to planetary luminosity limits of 2.3 × 10²⁴ erg s^-1 and 2.7 × 10²³ erg s^-1. Unfortunately, these are several orders of magnitude above the nominal Jovian value (at 40 MHz) of 2 × 10¹⁸ erg s^-1. The motivation for these observations was that the planetary magnetospheric emission is driven by a stellar wind-planetary magnetosphere interaction so that the planetary luminosity would be elevated near periastron. We estimate that, near periastron, HD 80606b might be as much as 3000 times more luminous than Jupiter. Recent transit observations of HD 80606b provide reasonably stringent constraints on the planetary mass and radius, and, because of the planet's highly eccentric orbit, its rotation period is likely to be "pseudo-synchronized" to its orbital period, allowing a robust estimate of the former. Consequently, we are able to make relatively robust estimates of the emission frequency of the planetary magnetospheric emission and find it to be around 60-90 MHz. While this is too low for our reported observations, we compare HD 80606b to other high-eccentricity systems and assess the detection possibilities for both near-term and more distant future systems. Of the known high-eccentricity planets, only HD 80606b is likely to be detectable, as the others (HD 20782B and HD 4113) are both lower mass and longer rotational periods, which imply weaker magnetic field strengths. We find that both the forthcoming "EVLA low band" system, which will operate as low as 65 MHz, and the Low Frequency Array may be able to improve upon our planetary luminosity limits for HD 80606b, and do so at a more optimum frequency. If the low-frequency component of the Square Kilometre Array (SKA-lo) and a future lunar radio array are able to approach their thermal noise limits, they should be able to detect an HD 80606b-like planet, unless the amount by which the planet's luminosity increases is substantially less than the factor of 3000 that we estimate; for the SKA-lo, which is to be located in the southern hemisphere, future planetary surveys will have to find southern hemisphere equivalents of HD 80606b.