We present recent coronagraphic observations of β Pictoris obtained with the Space Telescope Imaging Spectrograph (STIS) aboard the Hubble Space Telescope. The superb, high-resolution images show that the inner part of the disk is inclined by about 5° with respect to the main disk. Long-slit coronagraphic spectrograms oriented along the inner disk indicate that the reflectance of the inner disk is neutral over the spectral region, 3000-5600 Å.

The new Wide Field Imager attached to the MPG/ESO 2.2 m telescope on La Silla, Chile, has been used to monitor ∽2000 stars in the Orion Nebula Cluster (ONC) at 815 nm on 45 nights between 25 Dec 1998 and 28 Feb 1999. Over 400 periodic variables have been found, most or all of which are rotating, spotted T Tauri stars (TTS), more than doubling the number of known rotation periods for cluster members. Masses and ages are available for 335 of these from the literature. We confirm the existence of a bimodal period distribution for stars with M > 0.25 M⊙. A surprising new result is that stars of lower mass tend to rotate faster than higher mass stars, perhaps indicating that their disks have dissipated more rapidly in the harsh cluster environment. In the mass range 0.1 − 1 M⊙ between 40% and 80% of the stars have the variability characteristics of weak-line TTS (WTTS), suggesting that the half-life for accretion disks is ∽1 Myr in this cluster and probably even smaller for the lower mass stars.

The frequently made assumption that gas and dust temperatures are equal cannot be applied to the low density disks (104 cm−3 < ntot < 108 cm−3) around young A stars (“Vega-type” stars), because the collisional coupling is too weak. It turns out that OI fine structure lines and CO rotational lines are two very important cooling terms in the heating/cooling balance for the gas. Their level population numbers are far from LTE and depend strongly on the background radiation field, given by the thermal dust emission of the disk and the cosmic microwave background.

Interstellar dust particles, which are supposedly aggregates, penetrate dust disks around stars because of the star's motion relative to the surrounding interstellar medium (ISM). We discuss the interrelation of the physical properties of local interstellar dust, the relative motion of the star and the surrounding ISM, and the evolution of the circumstellar disk.

We propose the global baroclinic instability as a source for vigorous turbulence leading to angular momentum transport in Keplerian accretion disks. We know from analytical considerations and three-dimensional radiation hydro simulations that, in particular, protoplanetary disks have a negative radial entropy gradient, which makes them baroclinic. Two-dimensional numerical simulations show that this baroclinic flow is unstable and produces turbulence. These findings were tested for numerical effects by performing barotropic simulations which show that imposed turbulence rapidly decays. The turbulence in baroclinic disks draws energy from the background shear, transports angular momentum outward and creates a radially inward bound accretion of matter, thus forming a self consistent process. Gravitational energy is transformed into turbulent kinetic energy, which is then dissipated, as in the classical accretion paradigm. We measure accretion rates in 2D and 3D simulations of Ṁ = −;10−9 to −10−7 M⊙ yr−1 and viscosity parameters of α = 10−4–10−2, which fit perfectly together and agree reasonably with observations. The turbulence creates pressure waves, Rossby waves, and vortices in the (R – ø) plane of the disk. We demonstrate in a global simulation that these vortices tend to form out of little background noise and to be long-lasting features, which have already been suggested to lead to the formation of planets.

We have numerically investigated the dynamics of how a close stellar fly-by encounter of a symmetrical circumstellar planetesimal disc can give rise to the many kinds of asymmetries and substructures attributed to the dusty disc of β Pic. We find three recognizable groupings of test particles that can be related to the morphology of the β Pic disc. These are: highly eccentric and inclined orbit particles that reach apocentre in the southwest, moderately eccentric and inclined orbit particles that reach apocentre in the northeast, and a relatively unperturbed region inside ∽200 AU radius.

We present Infrared Space Observatory (ISO) spectra of 14 isolated Herbig Ae/Be stars. The IR spectra were combined with photometric data from the UV to the sub-mm region. We defined two key groups, based upon the spectral shape of the IR region. The results can be summarized as follows (see also Meeus et al. 2001): (1) the continuum of the IR to sub-mm region can be reconstructed by the sum of a power-law and a cool component, which can be represented by a black body. Possible locations for these components are an optically thick, geometrically thin disc (power-law component) and an optically thin flared region (black body); (2) remarkably, some sources lack the silicate bands; (3) PAH bands are present in at least 50% of our sample; with one exception, PAHs are not present in sources which only show a power-law continuum in the IR; (4) the dust in HAEBE stars shows strong evidence for coagulation; this dust processing is unrelated to any of the central star properties.

In this paper we present results on the effect of the vertical stratification of magnetic diffusivity, expected in current models of protoplanetary discs, on the development of the magnetorotational instability. Specifically, on the basis of a quasi-global, linear analysis we study the operation of the magnetorotational instability across the so-called dead zone of protoplanetary discs. Our results indicate that the predicted strong vertical diffusivity gradients can damp the instability in such regions. This suggests the necessity of a revision of current models for the structure and evolution of protoplanetary discs.

Water maser emission from star forming regions has been monitored for several decades using the Puschino radio telescope, showing radial velocity variations consistent with material in Keplerian orbit around protostars. MERLIN and the EVN are now being used to image the 22 GHz emission on au scales and measure proper motions. This will distinguish discs from outflows, and provide an estimate of the central mass and possibly orbiting condensations.

We calculate the SEDs of young stars accompanied by protoplanetary disks, taking into account the effect of the settling of dust particles. Observed disk candidates contain significant number of binary or multiple systems. We compare our results with observed data and obtain many excellent fittings. From the results, it is found that in close binary systems the disk masses are relatively low and dust particles is hard to settle as compared with the cases of disks in wide binaries or around isolated stars. This is attributable to gravitational perturbation from the companion stars, which will disturb formation of planetary systems in close binary systems.

Spectroscopic observation between 3-14 μm of 3 stars in the TW Hya Association were obtained using the Aerospace Corporation Broadband Array Spectrograph System (BASS) on the NASA IRTF telescope. The targets observed were TW Hya, HR 4796A, and HD 98800. Both of the late-type stars, TW Hya and HD 98800, exhibit a strong 10 μm silicate emission band. The strong emission indicates the presence of small dust grains close to the star, at angular distances not currently accessible to coronagraphic techniques. The spectral structure due to crystalline material that is present in some young early-type stars (HD 163296, HD 31648, etc.) is not apparent in these two objects. For HR 4796A, the dust emission is weak.

We have selected several stars with far-IR (IRAS) fluxes indicating the presence of circumstellar dust shells and disks. In this paper we present an analysis of the spectra of these stars. In several of these stars we find evidence for gaseous envelopes in addition to dusty disks. In some of these stars we find complex and variable Ha profiles indicating the variations in the circumstellar envelopes and disks.

In optically thin gas disks around young Vega-type stars, dust grains are exposed to the stellar radiation pressure and gas drag force. The combination of these forces pushes the grains away from the central star. Typically, 10–100 μm grains migrate outward to become concentrated at the outer edge of the gas disk. A dust ring naturally forms without the help of clearing bodies, such as planets or brown dwarfs.

We present the results of a study of the evolution of the gas and dust in disks around T Tauri, Herbig Ae and Vega-like stars. We observed the two lowest rotational lines of H2 with the ISO-SWS as well as 12CO 3–2 and 13CO 3–2 with the JCMT, and CO 6–5 with the CSO. The H2 lines trace the warm (∽ 100 K) inner region whereas the CO lines probe the colder outer disks. Substantial amounts of H2 have been detected toward T Tauri and Herbig Ae stars, and, surprisingly, also around three Vega-like objects (49 Cet, HD 135344 and β Pictoris). In contrast with previous conclusions derived from CO data, a significant mass of warm gas is found to persist up to ages of several tens of Myrs, suggesting that slow formation of gas-rich giant planets is possible.

An analysis of solid-state features in infrared spectra of 46 Herbig Ae/Be stars is presented. The presence of solid-state emission bands is compared to other indicators of circumstellar material, such as Hα emission, optical variability and sub-mm continuum fluxes. The correlation between these different indicators is weak, if present at all, in our sample. However, a strong dependence on spectral type of the central star seems to be present: stars with spectral type earlier than B9 show either amorphous silicate in absorption or infrared spectra dominated by PAH emission, whereas more than 70% of the stars of later spectral type show silicate emission. We conclude that the infrared spectrum of Herbig Be stars is in general dominated by emission from the circumstellar envelope, whereas the lower-mass Herbig Ae stars show a spectrum that is dominated by a disk that is passively heated by the central star.

Observations of submillimeter lines of CO, HCO+, HCN and their isotopes from circumstellar disks around low-mass pre-main sequence stars can be used to set constraints on the temperature and density distributions in these disks. The lines considered here originate from levels with higher excitation temperatures and critical densities than studied before (CO 6–5, HCO+ and HCN 4–3), and are combined with interferometer data on lower excitation lines. We discuss the results for two disks, i.e., those around LkCa 15 and TW Hya. We find that the TW Hya disk has a warm surface layer and agrees well with a flaring disk geometry, while the LkCa 15 disk is cooler and can be described by either dust-settling in a flared disk or a flatter disk overall. The densities are well described by disk models in the literature.

The photometer (ISOPHOT) on the ESA Infrared Space Observatory (ISO) satellite was used to study the four Vega-like prototypes (Vega, β Pic, α PsA, ɛ Eri) and a set of candidate stars believed to have similar dust/debris discs. Most of the candidate stars were main sequence stars, but probably younger than Vega and ɛ Eri since they showed residual emission in the cores of their stellar spectral lines. Low resolution spectra were combined with long wavelength multi-filter photometry (between 60/μm and 200/μm) to give the spectral energy distributions for the dust/debris discs. Models suggested that the masses of the discs were between 10−7 and 10−4 M⊙ (2–2000 times the mass of the moon). These were more massive than the discs around the four prototypes, but less massive than the discs around young stars such as T Tau stars.

We present spectroscopic data of 3 isolated Vega-type systems which exhibit the 3.3 μm‘PAH’ feature ascribed to stretching of the C-H bonds in aromatic hydrocarbons. These are part of a sample of mainly hot stars. We discuss the implications of Vega-type stars having an organic component to their surroundings and compare them with the unique star, HD135344 (SAO 206462).

We present the results of short baseline interferometry observations at submillimetre wavelengths, using a two-element interferometer comprising the JCMT/CSO, of circumstellar discs around young YSOs. We model data for the Class 0 protostar IRAS03282 and the Class I protostar L1709B with discs and compare them with previously published results. We find evidence for the ratio of the disc to envelope mass to increase as the objects evolve.

Multicolour imaging polarimetry of the main sequence star BD+31°643 shows that the Kalas & Jewitt (1997) disc extends to at least 13000AU. A disc of this radius must be dynamically very young compared to the central star, unlike the situation found in the β Pictoris system. A striking linear filament of 850μm emission detected 50″ south, that parallels the dust disk, is probably not physically associated.