HD 22049  HR 1084   Gliese144   HIP 16537
RA (2000) = 03 32 55.8442     Dec (2000) = -09 27 29.744
SpT = K2V    V = 3.73 mag    d = 3.22pc
Proper Motion (mas/yr) = -975.17 +19.49

This image of epsilon Eri essentially traces the thermal emission of dust. Notice that the white star in the center represents the location of the star. The hotter the dust, the yellower the color. The disk appears as a 60 AU radius ring that is lumpy and close to face-on to our line of sight. Note that the ring must be thinner than what is shown because the 13.8" resolution of the instrumentation broadens the intrinsic signal.

Dent et al. (2000) present a thorough analysis of this image. They find the dust is located between 50 and 80 AU radius, has characteristic size 30 microns, characteristic temperature 35 K, and total mass 0.07 lunar mass. They estimate that the lifetime of these grains before collisions destroy them is 10 Myr. If the age of epsilon Eridani is 700 Myr, then the 30 micron grains observed with SCUBA have been replenished 70 times over. The source of replenishment is thought to be collisions of larger dust grains or planetesimals. These objects are a parent population of primordial bodies that can survive up to the present age of the star. Dent et al. estimate that objects larger than 0.5 mm in size replenish the observed 30 micron grains.

There are three things that distinguish epsilon Eri from beta Pic, Fomalhaut, and Vega.  First, it is the closest star and therefore we are resolving structure on a much smaller scale than is possible for the other stars. Second, it is a K star, meaning that this is the oldest system with a resolved dust disk. Epsilon Eri is thought to be between 500-1000 million years old, whereas beta Pic, Vega, and Fomalhaut are 10-300 million years old. Third, it is the least massive, least luminous star of the lot.  This means that the observed 35 AU radius hole in the middle of the disk is unlikely to result from an ice sublimation boundary, which for our Sun is at roughly 5 AU.

The central hole seems to be good evidence for planetary sweeping of material.  In other words, as far as we know, the most likely mechanism to generate and maintain the hole is if an unseen planet is responsible for sweeping the region clear of dust. In our solar system, the giant planets were responsible for ejecting small bodies. These small bodies are now found in the Kuiper Belt (35-50 AU radius) and the Oort cloud (10,000-100,000 AU radius).   Thus, what we see around epsilon Eri is a ring of dust analogous to our Kuiper Belt.

Below we illustrate this relationship:

The image of our solar system represent the locations of known Kuiper Belt objects, as shown in the Kuiper Belt Home Page. Though most of the irregularity in the ring is due to observational incompleteness, the break in the ring is expected due to dynamical interactions with Neptune. This is demonstrated in the dynamical model published by Liou and Zook (1999, Astron. J., vol 118, pg. 580). Here a disk of particles orbiting the Sun experiences various forces such as Poynting-Robertson radiation drag and dynamical interactions with planets, shown by small white dots. Gravitational scattering by planets creates both a central hole and irregularities along the ring, such as density enhancements and a gap around the position of Neptune.

Basic facts about Epsilon Eridani:

1) age is estimated to be between 500 Myr and 1 Gyr

2) structure is a ring with inner and outer radii of 50 and 80 AU, respectively.

3) characteristic grain size is 30 microns, with characteristic temperature 35 K.

4) total dust mass is 0.07 lunar mass (5.1x10^24 g or 2.6x10^-9 M_sun)

5) morphology is significantly asymmetric

6) ring is seen near face-on from our point of view.