Detecting a rotation in the ε Eridani debris disc

Abstract

The evidence for a rotation of the ε Eridani debris disc is examined. Data at 850-μm wavelength were previously obtained using the Submillimetre Common User Bolometer Array (SCUBA) over periods of 1997-1998 and 2000-2002. By χ2 fitting after shift and rotation operations, images from these two epochs were compared to recover proper motion and orbital motion of the disc. The same procedures were then performed on simulated images to estimate the accuracy of the results. Minima in the χ2 plots indicate a motion of the disc of approximately 0.6 arcsec per year in the direction of the star's proper motion. This underestimates the true value of 1 arcsec per year, implying that some of the structure in the disc region is not associated with ε Eridani, originating instead from background galaxies. From the χ2 fitting for orbital motion, a counterclockwise rotation rate of ∼2°.75 per year is deduced. Comparisons with simulated data in which the disc is not rotating show that noise and background galaxies result in approximately Gaussian fluctuations with a standard deviation of ±1°.5 per year. Thus, counterclockwise rotation of disc features is supported at approximately a 2σ level, after a 4-yr time difference. This rate is faster than the Keplerian rate of 0°.65 per year for features at ≈65 au from the star, suggesting their motion is tracking a planet inside the dust ring. Future observations with SCUBA-2 can rule out no rotation of the ε Eridani dust clumps with ∼4σ confidence. Assuming a rate of about 2°.75 per year, the rotation of the features after a 10-yr period could be shown to be ≥ 1° per year at the 3σ level. © 2006 RAS.