A Spitzer IRAC Search for Substellar Companions of the Debris Disk Star ɛ Eridani

Abstract

We have used the Infrared Array Camera (IRAC) on board the Spitzer Space Telescope to search for low-mass companions of the nearby debris disk star ɛ Eri. The star was observed in two epochs 39 days apart, with different focal plane rotation to allow the subtraction of the instrumental point-spread function, achieving a maximum sensitivity of 0.01 MJy sr^-1 at 3.6 and 4.5 μm, and 0.05 MJy sr^-1 at 5.8 and 8.0 μm. This sensitivity is not sufficient to directly detect scattered or thermal radiation from the ɛ Eri debris disk. It is, however, sufficient to allow the detection of Jovian planets with mass as low as 1M_J in the IRAC 4.5 μm band. In this band, we detected over 460 sources within the 5.70 arcmin field of view of our images. To test if any of these sources could be a low-mass companion to ɛ Eri, we have compared their colors and magnitudes with models and photometry of low-mass objects. Of the sources detected in at least two IRAC bands, none fall into the range of mid-IR color and luminosity expected for cool, 1 Gyr substellar and planetary mass companions of ɛ Eri, as determined by both models and observations of field M, L, and T dwarfs. We identify three new sources that have detections at 4.5 μm only, the lower limit placed on their [3.6]-[4.5] color consistent with models of planetary mass objects. Their nature cannot be established with the currently available data, and a new observation at a later epoch will be needed to measure their proper motion in order to determine if they are physically associated to ɛ Eri.