Disks Around Low Mass Objects

(Riaz, Gledhill, Hough)

A study of the compositional alteration of dust grains in protoplanetary disks allows us to understand the conditions that initiate the formation of planets. The most dominant dust species in the Solar system and in circumstellar material are oxygen-rich silicates, that are either olivines, ranging from fayalite to forsterite, or pyroxenes, ranging from ferrosillite to enstatite. The spectra of silicate dust have vibrational spectral features near 10µ, due to the Si-O stretching mode, and near 20&mico;, due to the O-Si-O bending mode. A detailed compositional analysis of these features has been immensely valuable in gaining insight into the various grain processing mechanisms. Silicate dust in the interstellar medium (ISM) is dominated by amorphous olivine, whereas in more evolved systems such as the comet Hale-Bopp, the silicate spectrum is dominated by crystalline enstatite and forsterite. We are conducting dust compositional analyses of circumstellar disks around sub-stellar objects (Mstar < 0.1 Msun) in various star-forming regions at ages of ~1-10 Myr. These brown dwarf disks are of a few Jupiter masses, which makes them interesting to study if planet formation signatures, as observed in the Solar system, can also be found in very low mass disk sources. We are using a large inventory of mid-infrared spectra obtained with the Spitzer Space Telescope. Our dust composition model has been developed using laboratory data for various silicate and silica dust species at different grain sizes from sub-micron to millimeter. The aims of this study are:

• To investigate an evolutionary sequence from ISM-like dust to cometary dust in brown dwarf disks.
• To investigate any difference in the crystalline and large-grain mass fractions between disks around brown dwarfs and solar-type stars.
• To study any dependence of the grain processing mechanisms on the age of the system, and on other stellar/disk properties, such as, X-ray emission strength.
• To study any radial dependence of the dust chemical composition, including processes such as low temperature crystallization, turbulent diffusion and large-scale circulation.

The model fits to the 10 and 20&mico; silicate features (1st and 2nd panel, respectively). Colours represent the following: red - observed spectrum; black - model fit, grey - small amorphous olivine; green - small amorphous pyroxene; cyan - silica; blue - forsterite; pink - enstatite; black dashed - large amorphous olivine; orange - large amorphous pyroxene. Thin dashed line represents the continuum. Right panel shows the model SED fit. Grey line represents the stellar photosphere contribution, disk emission is indicated by blue line.

One type of circumstellar disk is a debris disk, which results from the collision of parent bodies in a planetary system surrounding the star. Debris disks indicate the presence of asteroids or icy Kuiper-belt objects in the planetary system. Collisions between these bodies generate tiny rocky particles that absorb a fraction of the visible light from the star and re-emit this at longer wavelengths (in the far-infrared or sub-millimetre region). At Hertfordshire we are leading a novel project to identify debris disks in large area extragalactic surveys such as the Herschel ATLAS. The Herschel ATLAS survey is the largest extragalactic survey being carried out by the Herschel Space Observatory and the large area of this survey means that we can rapidly search over 100,000 stars for the presence of massive debris disks. The main difficulty is that debris disks are incredibly rare compared to the galaxies found in the Herschel ATLAS. In the image below there are ~6000 galaxies and only three candidate debris disks (the Moon is included to set the angular scale). We use sophisticated statistical techniques to match up far-infrared emission with main sequence stars, followed by higher resolution short-wavelength Herschel PACS observations to confirm the association. The inset image shows one of the candidates that we have found, which may be one of the most massive debris disks yet detected.