Limits on the Optical Brightness of the Epsilon Eridani Dust Ring

The STIS/CCD camera on the {\em Hubble Space Telescope (HST)} was used to take deep optical images near the K2V main-sequence star $\epsilon$ Eridani in an attempt to find an optical counterpart of the dust ring previously imaged by sub-mm observations. Upper limits for the optical brightness of the dust ring are determined and discussed in the context of the scattered starlight expected from plausible dust models. We find that, even if the dust is smoothly distributed in symmetrical rings, the optical surface brightness of the dust, as measured with the {\em HST}/STIS CCD clear aperture at 55 AU from the star, cannot be brighter than about 25 STMAG/"$^2$. This upper limit excludes some solid grain models for the dust ring that can fit the IR and sub-mm data. Magnitudes and positions for $\approx$59 discrete objects between 12.5" to 58" from $\epsilon$ Eri are reported. Most if not all of these objects are likely to be background stars and galaxies.Comment: Revision corrects author lis

The composition of debris around HD 12039: water from asteroids?

A small number of nearby Sun-like stars possess warm dust belts, made from the debris of collisions within an analogue to the Sun's Asteroid Belt. Our analysis shows that the debris around the 30 Myr-old star HD 12039 could result from a *single* event, the breakup of a planetesimal ~100 km in size. This offers a unique opportunity to study the ejecta, and compare the composition to that of Solar System asteroids - in particular, the ejecta of comet 9P/Tempel 1 observed by Spitzer in the Deep Impact experiment. We wish to determine the derbis composition around HD 12039 and see what it would contribute if added to a growing terrestrial planet - in particular, whether asteroidal water could add to oceans of an `exo-Earth'

Spitzer survey of the Karin Cluster asteroids

The Karin cluster is one of the youngest known families of main-belt asteroids, dating back to a collisional event only 5.8 Myr ago. Using the Spitzer Space Telescope we have sampled the thermal continua of 17 Karin cluster asteroids, down to the smallest members discovered so far, in order to derive accurate sizes and study the physical properties of their surfaces. The albedos of the observed Karins appear to be very similar. The albedos, pv, have a mean of 0.17 and a standard deviation of 0.04, compared to pv = 0.15 ± 0.05 for 832 Karin itself (for H = 11.2 ± 0.3). The derived diameters range from 20 km for 832 Karin to 1.9 km for 93690, with uncertainties of 10%. The Karins data show no evidence of albedo dependence on size, and the small range of albedos is consistent with all program targets being S-type bodies. There is some evidence for higher values of thermal inertia amongst the smaller family members, which may be indicative of coarser regolith. These results are preliminary, pending outstanding Spitzer observations and further analysis. This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA

Evolution from protoplanetary to debris discs: The transition disc around hd 166191

HD 166191 has been identified by several studies as hosting a rare and extremely bright warm debris disc with an additional outer cool disc component. However, an alternative interpretation is that the star hosts a disc that is currently in transition between a full gas disc and a largely gas-free debris disc. With the help of new optical to mid-infrared (IR) spectra and Herschel imaging, we argue that the latter interpretation is supported in several ways: (i) we show that HD 166191 is comoving with the ∼4-Myr-old Herbig Ae star HD 163296, suggesting that the two have the same age; (ii) the disc spectrum of HD 166191 is well matched by a standard radiative transfer model of a gaseous protoplanetary disc with an inner hole and (iii) the HD 166191 mid-IR silicate feature is more consistent with similarly primordial objects. We note some potential issues with the debris disc interpretation that should be considered for such extreme objects, whose lifetime at the current brightness is much shorter than the stellar age, or in the case of the outer component requires a mass comparable to the solid component of the solar nebula. These aspects individually and collectively argue that HD 166191 is a 4-5 Myr old star that hosts a gaseous transition disc. Though it does not argue in favour of either scenario, we find strong evidence for 3-5 μm disc variability. We place HD 166191 in context with discs at different evolutionary stages, showing that it is a potentially important object for understanding the protoplanetary to debris disc transition