Infrared molecular spectroscopy is a key tool for the observation of gas in the innermost region of disks around T Tauri stars. In this contribution, we examine how infrared spectroscopy of CO can be used to study the inner truncation region of disks around T Tauri stars. The inferred inner gas radii for T Tauri star disks are compared to the inner dust radii of disks, to the expectations of models for disk truncation, and to the orbital distribution of short-period extra-solar planets.

Hydrogen emission lines observed from T Tauri stars (TTS) are associated with the accretion/outflow of gas in these young star forming systems. Magnetospheric accretion models have been moderately successful at reproducing the shapes of several Hi emission line profiles, suggesting that the emission arises in the accretion funnels. Despite considerable effort to model and observe these emission features, the physical conditions of the gas confined to the funnel flows remain poorly constrained by observation. We conducted a mutli-epoch near-infrared spectroscopic survey of 16 actively accreting classical TTS in the Taurus-Auriga star forming region. We present an analysis of these simultaneously acquired line flux ratios of many Paschen and Brackett series emission lines, in which we compare the observed ratios to those predicted by the Case B approximation of hydrogen recombination line theory. We find that the line flux ratios for the Paschen and Brackett decrements as well as a comparison between Brγ and Paschen transitions agree well with the Case B models with T < 5000 K and ne ≈ 1010 cm−3.

I review, from an observational perspective, the interactions of accretion discs with magnetic fields in cataclysmic variable stars. I start with systems where the accretion flows via a stream, and discuss the circumstances in which the stream forms into an accretion disc, pointing to stars which are close to this transition. I then turn to disc-fed systems and discuss what we know about how material threads on to field lines, as deduced from the pattern of accretion footprints on the white dwarf. I discuss whether distortions of the field lines are related to accretion torques and the changing spin periods of the white dwarfs. I also review the effect on the disc–magnetosphere interaction of disc-instability outbursts. Lastly, I discuss the temporary, dynamo-driven magnetospheres thought to occur in dwarf-nova outbursts, and whether slow-moving waves are excited at the inner edges of the disc.

I review the results obtained by long-baseline interferometry at infrared wavelengths on the innermost regions around young stars. These observations directly probe the location of the dust and gas in the disks. The characteristic sizes of these regions are found larger than previously thought. These results have motivated in part a new class of models of the inner disk structure. However the precise understanding of the origin of these low visibilities is still in debate. Mid-infrared observations have probed disk emission over a larger range of scales revealing mineralogy gradients in the disk. Recent spectrally resolved observations allow the dust and gas to be studied separately. The few results show that the Brackett gamma emission can find its origin either in a wind or in a magnetosphere but there are no definitive answers yet. In a number of cases, the very high spatial resolution seems to reveal very close companions. In any case, these results provide crucial information on the structure and physical properties of disks surrounding young stars especially as initial conditions for planet formation.

This symposium was characterized by an intense exchange of new information and by lively discussions on many aspects of the formation and early evolution of low-mass stars. The observational data presented at this meeting, obtained in spectral regions ranging from X-rays to submm waves, were found to be remarkably consistent with the current magnetospheric disc-accretion paradigm for young stellar objects. But there remain open questions, and a full understanding of the star-formation process will require much additional work.

We review the theory of the formation and gravitational collapse of magnetized molecular cloud cores, leading to the birth of T Tauri stars surrounded by quasi-Keplerian disks whose accretion is driven by the magnetorotational instability (MRI). Some loss of magnetic flux during the collapse results typically in a dimensionless mass-to-flux ratio for the star plus disk of λ0 ≈ 4. Most of the mass ends up in the star, while almost all of the flux and the angular momentum ends up in the disk; therefore, a known mass for the central star implies a computable flux in the surrounding disk. A self-contained theory of the MRI that drives the viscous/resistive spreading in such circumstances then yields the disk radius needed to contain the flux trapped in the disk as a function of the age t. This theory yields analytic predictions of the distributions with distance ϖ from the central star of the surface density Σ(ϖ), the vertical magnetic field Bz(ϖ), and the (sub-Keplerian) angular rotation rate Ω (ϖ). We discuss the implications of this picture for disk-winds, X-winds, and funnel flows, and we summarize the global situation by giving the energy and angular-momentum budget for the overall problem.

Stellar winds may be important for angular momentum transport from accreting T Tauri stars, but the nature of these winds is still not well-constrained. We present some simulation results for hypothetical, hot (∼ 106 K) coronal winds from T Tauri stars, and we calculate the expected emission properties. For the high mass loss rates required to solve the angular momentum problem, we find that the radiative losses will be much greater than can be powered by the accretion process. We place an upper limit to the mass loss rate from accretion-powered coronal winds of ∼ 10−11M yr−1. We conclude that accretion powered stellar winds are still a promising scenario for solving the stellar angular momentum problem, but the winds must be cool (e.g., 104 K) and thus are not driven by thermal pressure.

Submillimetre imaging polarimetry is one of the most powerful tools at present for studying magnetic fields in star-forming regions, and the only way to gain significant information on the structure of these fields. We present analysis of the largest sample (to date) of both high- and low-mass star-forming regions observed using this technique. A variety of magnetic field morphologies are observed, with no single field morphology favoured. Both the continuum emission morphologies and the field morphologies are generally more complex for the high-mass sample than the low-mass sample. The large scale magnetic field (observed with the JCMT; 14″ resolution) of NGC1333 IRAS2 is interpreted to be weak (compared to the energetic contributions due to turbulence) from the random field pattern observed. On smaller scales (observed with the BIMA array; 3″ resolution) the field is observed to be almost radial, consistent with the polarisation nulls in the JCMT data – suggesting that on smaller scales, the field may be more important to the star formation process. An analysis of the magnetic field direction and the jet/outflow axis is also discussed. Cumulative distribution functions of the difference between the mean position angle of the magnetic field vectors and the jet/outflow axis reveal no correlation. However, visual inspection of the maps reveal alignment of the magnetic field and jet/outflow axis in 7 out of 15 high-mass regions and 3 out of 8 low-mass regions.

I present and discuss a unified scheme for jet launching that is based on stochastic dissipation of the accretion disk kinetic energy, mainly via shock waves. In this scheme, termed thermally-launched jet model, the kinetic energy of the accreted mass is transferred to internal energy, e.g., heat or magnetic energy. The internal energy accelerates a small fraction of the accreted mass to high speeds and form jets. For example, thermal energy forms a pressure gradient that accelerates the gas. A second acceleration stage is possible wherein the primary outflow stretches magnetic field lines. The field lines then reconnect and accelerate small amount of mass to very high speeds. This double-stage acceleration process might form highly relativistic jets from black holes and neutron stars. The model predicts that detail analysis of accreting brown dwarfs that launch jets will show the mass accretion rate to be ṀBD ≳ 10−9 − 10−8M⊙ yr−1, which is higher than present claims in the literature.

The role of the star-disk interaction region in launching the high velocity component of accretion-driven outflows is examined. Spectroscopic indicators of high velocity inner winds have been recognized in T Tauri stars for decades, but identifying the wind launch site and the accompanying mass loss rates has remained elusive. A promising new diagnostic is He I λ10830, whose metastable lower level results in a powerful probe of the geometry of the outflowing gas in the interaction region. This, together with other atomic and molecular spectral diagnostics covering a wide range of excitation and ionization states, suggests that more than one launch site of the innermost wind is operational in most accreting stars.

We discuss a number of topics relevant to disk-magnetosphere interaction and how numerical simulations illuminate them. The topics include: (1) disk-magnetosphere interaction and the problem of disk-locking; (2) the wind problem; (3) structure of the magnetospheric flow, hot spots at the star's surface, and the inner disk region; (4) modeling of spectra from 3D funnel streams; (5) accretion to a star with a complex magnetic field; (6) accretion through 3D instabilities; (7) magnetospheric gap and survival of protoplanets. Results of both 2D and 3D simulations are discussed.