2009A&A...497..379M

We present Spitzer infrared observations to constrain disc and dust evolution in young T Tauri stars in MBM 12, a star-forming cloud at high latitude with an age of 2Myr and a distance of 275pc. The region contains 12 T Tauri systems, with primary spectral types between K3 and M6; 5 are weak-line and the rest classical T Tauri stars. We first use MIPS and literature photometry to compile spectral energy distributions for each of the 12 members in MBM 12, and derive their IR excesses. Of the 8 stars that are detected with MIPS (spectral types between K3 and M5), only 1 lacks an IR excess - the other 7 all have an IR excess that can be attributed to a disc. This means that in MBM 12, for the detected spectral types K3-M5, we have a very high disc fraction rate, about 90%. Furthermore, 3 of those 7 excess sources are candidate transitional discs. The four lowest-mass systems in the cloud, with spectral types of M5-M6, were undetected by Spitzer. Their upper limits indicate that they either have a transitional disc, or no disc at all. The IRS spectra are analysed with the newly developed two-layer temperature distribution (TLTD) spectral decomposition method. For the 7 T Tauri stars with a detected IR excess, we analyse their solid-state features to derive dust properties such as mass-averaged grain size, composition and crystallinity. The mass-averaged grain size we determine from the 10-micron feature has a wide range, between 0.4 and 6µm. This grain size is much smaller in the longer-wavelength region: between 0.1 and 1.5µm. We find that later-type objects have larger grain sizes, as was already shown by earlier studies. Furthermore, we find a wide range in mass fraction of the crystalline grains, between 3 and (at least) 30%, with no relation to the spectral type nor grain size. We do find a spatial gradient in the forsterite to enstatite range, with more enstatite present in the warmer regions. The fact that we see a radial dependence of the dust properties indicates that radial mixing is not very efficient in the discs of these young T Tauri stars. The sources that have the least amount of disc flaring have the largest grain sizes, pointing to dust settling. A comparison between the objects with companions closer than 400 AU (``binaries'') and those with wider or no companions (``singles''), shows that disc evolution already starts to differentiate at an age of 2Myr: the excess at 30µm is a factor of 3 greater for the ``single'' group. The SED analysis shows that the discs in MBM 12, in general, undergo rapid inner disc clearing, while the binary sources have faster disc evolution. The dust grains seem to evolve independently of the stellar properties, but are mildly related to disc properties such as flaring and accretion rates.