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In this article we present the results from mid-infrared spectroscopy of a sample of 14 T?Tauri stars with silicate emission. The qualitative analysis of the spectra reveals a correlation between the strength of the silicate feature and its shape similar to the one which was found recently for the more massive Herbig Ae/Be stars by van Boekel et al. (2003). The comparison with theoretical spectra of amorphous olivine ([Mg,Fe]2SiO4) with different grain sizes suggests that this correlation is indicating grain growth in the disks of T?Tauri stars. Similar mechanisms of grain processing appear to be effective in both groups of young stars.

Aims.We study the inner region (~1.0?AU up to a few 10 AUs) of the circumstellar disk around the ?classical? T?Tauri star RY?Tau. Our aim is to find a physical description satisfying the available interferometric data, obtained with the mid-infrared interferometric instrument at the Very Large Telescope Interferometer, as well as the spectral energy distribution in the visible to millimeter wavelength range. We also compare the findings with the results of similar studies, including those of intermediate-mass Herbig Ae/Be stars. Methods.Our analysis is done within the framework of a passively heated circumstellar disk, which is optionally supplemented by the effects of accretion and an added envelope. To achieve a more consistent and realistic model, we used our continuum transfer code MC3D. In addition, we studied the shape of the 10 ?m silicate emission feature in terms of the underlying dust population, both for single-dish and for interferometric measurements. Results.We show that a modestly flaring disk model with accretion can explain both the observed spectral energy distribution and the mid-infrared visibilities obtained with the mid-infrared infrared instrument. We found an interesting ambiguity: a circumstellar active disk model with an added envelope, and a lower accretion rate than in the active disk model without envelope, could represent the observations equally as well. This type of model with the envelope should be considered a viable alternative in future models of other T?Tauri stars. The approach of a disk with a puffed-up inner rim wall and the influence of a stellar companion is also discussed. We also investigate the influence of various fit parameters on the outcome of the radiative transfer modeling. From the study of the silicate emission feature we see evidence for dust evolution in a T?Tauri star, with a decreasing fraction of small amorphous and an increasing fraction of crystalline particles closer to the star.

We obtained deep H- and K-band images of DENIS-P J104814–395606 using SofI and the speckle camera SHARP-I at the ESO-3.5m-NTT as well as QUIRC at the Mauna Kea 2.2m telescope between December 2000 and June 2001. The target was recently discovered as nearby M9-dwarf among DENIS sources (Delfosse et al. 2001). We detect parallactic motion on our images and determine the distance to be 4.6 ± 0.3 pc, more precise than previously known. From the available colors, the distance, and the spectral type, we conclude from theoretical models that the star has a mass of ~ 0.075 to 0.09 M⊙ and an age of ~ 1 to 2 Gyrs. We also obtained H- and K-band spectra of this star with ISAAC at the VLT. A faint companion candidate is detected 6" NNW of the star, which is 6.4 ± 0.5 mag fainter in H. However, according to another image taken several month later, the companion candidate is not co-moving with the M9-dwarf. Instead, it is a non-moving background object. Limits for undetected companion candidates are such that we can exclude any stellar companions outside of ~ 0.25" (1 AU), any brown dwarf companions (above the deuterium burning mass limit) outside of ~ 2" (9 AU), and also any companion down to ~ 40 Mjup with ≥ 0.15" (0.7 AU) separation, all calculated for an age of 2 Gyrs. Our observations show that direct imaging of sub-stellar companions near the deuterium burning mass limit in orbit around nearby ultra-cool dwarfs is possible, even with separations that are smaller than the semi-major axis of the outermost planet in our solar system, namely a few tens of AU.

We combine Heliosspacecraft photometer brightnesses, in situ plasma densities, and interplanetary scintillation (IPS) velocity data to obtain a global description of solar wind changes. We find an increase of solar mass flux at solar maximum from the photometer brightnesses larger than would be expected from the usual assumption of invariant momentum flux over solar cycle 21 (1975-1986). A portion of this excess mass is related to solar mass ejections, which have an occurrence rate that follows the solar activity cycle.