Until recently, X-rays from low-mass young stars (105–106 yr) were thought to be a universal proxy for magnetic activity, enhanced by 3-4 orders of magnitude with respect to the Sun, but otherwise similar in nature to all low-mass, late-type convective stars (including the Sun itself). However, there is now evidence that other X-ray emission mechanisms are at work in young stars. The most frequently invoked mechanism is accretion shocks along magnetic field lines (“magnetic accretion”). In the case of the more massive A- and B-type stars, and their progenitors the Herbig AeBe stars, other, possibly more exotic mechanisms can operate: star-disk magnetic reconnection, magnetically channeled shocked winds, etc. In any case, magnetic fields, both on small scale (surface activity) and on large scale (dipolar magnetospheres), play a distinctive role in the emission of X-rays by young stars, probably throughout the IMF.

Accreting T Tauri stars are observed to be less luminous in X-rays than non-accretors, an effect that has been detected in various star forming regions. To explain this we have combined, for the first time, a radiative transfer code with an accretion model that considers magnetic fields extrapolated from surface magnetograms obtained from Zeeman-Doppler imaging. Such fields consist of compact magnetic regions close to the stellar surface, with extended field lines interacting with the disk. We study the propagation of coronal X-rays through the magnetosphere and demonstrate that they are strongly absorbed by the dense gas in accretion columns.

In this contribution we first briefly review our current knowledge on the physics of accretion discs driving self-confined jets. It will be shown that a large scale magnetic field is expected to thread the innermost disc regions, giving rise to a transition from an outer standard accretion disc to an inner jet emitting disc. We then report new progresses on the theory of star-disc interaction, allowing to explain the formation of accretion funnel flows with stellar dipole fields consistent with observational constraints. Such a connection is now not only probed by modern observations but it is also requested for spinning down protostars, which are known to be both actively accreting and contracting. This spin down most probably relies on the angular momentum removal by ejection. Two such scenarios will be addressed here, namely “accretion-powered stellar winds” (Matt & Pudritz 2005) and “Reconnection X-winds” (Ferreira, Pelletier & Appl 2000). The latter can slow down a protostar on time scales shorter or comparable to the embedded phase. It will be shown that these two scenarios are not incompatible and that transitions from one to another may even occur as they mainly depend on the stellar dynamo.

Emission line profiles from pre-main-sequence objects accreting via magnetically-controlled funnel flows encode information on the geometry and kinematics of the material on stellar radius scales. In order to extract this information it is necessary to perform radiative-transfer modelling of the gas to produce synthetic line profiles. In this review I discuss the physics that needs to be included in such models, and the numerical methods and assumptions that are used to render the problem tractable. I review the progress made in the field over the last decade, and summarize the main successes and failures of the modelling work.

Our present understanding of the coronal structure of T Tauri stars is fragmentary and observations in different wavelength regimes often appear to give contradictory results. X-ray data suggest the presence of magnetic loops on a variety of scales, from compact loops of size less than a stellar radius, up to very large loops of up to 10 stellar radii which may connect to the disk. While some stars show a clear rotational modulation in X-rays, implying distinct bright and dark regions, many do not. This picture is complicated by the accretion process itself, which also contributes to the X-ray emission. The location of the inner edge of the accretion disk and the nature of the magnetic field there are still hotly-contested issues. Accretion indicators often suggest the presence of discrete accretion funnels. This has implications for the structure of the corona, as does the presence of an outflowing wind. All of these factors are linked to the structure of the magnetic field, which we are now beginning to unravel through Zeeman-Doppler imaging. In this review I will describe the present state of our understanding of the magnetic structure of T Tauri coronae and the impact this has during such an early evolutionary stage.

We apply results from FUV and X-ray spectroscopy to evaluate the role of photoevaporation in dispersing the disk around TW Hya. Accretion produces bright EUV emission that may be smothered by the accretion column. Solar-like magnetic activity produces fewer ionizing photons, which may be absorbed by an accretion-powered neutral wind. We estimate a photoevaporation rate of ∼ 5 × 10−11M⊙ yr−1 for the disk around TW Hya. These models can be tested by detecting gas in the ionized disk surface, including emission in the [Ne II] 12.8μm line. Photoevaporation is likely a minor process in disk dispersal during the accretion phase, but could remove ∼ 1 MJ of remnant gas around a solar-mass star after accretion ceases.

The presence of close (≲ 0.1 AU) stellar companions must greatly alter the circumstellar environment of classical T Tauri stars, including severe truncation if not elimination of circumstellar disks. It is thus remarkable how little impact the presence of a close companion has on our observable diagnostics for accretion and outflow. Emission line shapes, degrees of continuum veiling, and spectral energy distributions are all indistinguishable between single classical T Tauri stars and classical T Tauri close binaries. Some of the most classical T Tauri stars that laid the foundation for our single-star accretion-disk paradigm have turned out to have close companions. Periodicities in spectral signatures are suggestive of the presence of accretion flows from circumbinary disks to the circumstellar regions; the subsequent flow of material through the circumstellar region to the stellar surface in the presence of a stellar magnetosphere is unstudied. Observations of stellar rotation distributions in close binaries suggest that inner disk regions may act to regulate stellar angular momentum.

The evolution of angular momentum is a key to our understanding of star formation and stellar evolution. The rotational evolution of solar-mass stars is mostly controlled by magnetic interaction with the circumstellar disc and angular momentum loss through stellar winds. Major differences in the internal structure of very low-mass stars and brown dwarfs – they are believed to be fully convective throughout their lives, and thus should not operate a solar-type dynamo – may lead to major differences in the rotation and activity of these objects. Here, we report on observational studies to understand the rotational evolution of the very low-mass stars and brown dwarfs.

We calculated profiles of CIV, 1550Å, Si IV 1400Å, NV 1240Å and OVI 1035Å doublet lines using results of 3D MHD simulations of disc accretion onto young stars with a dipole magnetic field. It appeared that our calculations cannot reproduce the profiles of these lines observed (HST/GHRS-STIS and FUSE) in CTTSs spectra. We also found that the theory predicts much larger CIV 1550Å line flux than observed (up to two orders of magnitude in some cases) and argue that the main portion of accretion energy in CTTSs is liberated outside the accretion shock. We conclude that the reason of disagreement between the theory and observation is the strongly non-dipolar character of CTTS magnetic field near its surface.

The birth of a young star is accompanied not only by accretion but by the expulsion of matter as well in the form of a collimated outflow. These outflows are seen at various wavelengths from X-rays to the radio band, but ultimately the driving mechanism appears to be a highly collimated supersonic jet that contains not only atomic but molecular components as well. These jets may also play a key role in the star formation process itself since they could be one of the primary mechanism for removing angular momentum from the accretion disk thereby allowing accretion to occur. Whereas much is known about their propagation on large-scales (i.e., hundreds of AU to several parsecs) from both observations and simulations, we must explore the “central engine” in order to understand how they are generated. While this is particularly challenging, high spatial resolution studies are beginning to reveal interesting data from which we can confront the various models. In this review, I will summarise what these studies suggest and note how they already favour certain models over others. I will also describe some of the results from spectro-astrometry and interferometry that are revealing details of outflows on milliarcsecond scales from the source.

Studies of stellar magnetism at the pre-main sequence phase can provide important new insights into the detailed physics of the late stages of star formation, and into the observed properties of main sequence stars. This is especially true at intermediate stellar masses, where magnetic fields are strong and globally organised, and therefore most amenable to direct study. This talk reviews recent high-precision ESPaDOnS observations of pre-main sequence Herbig Ae-Be stars, which are yielding qualitatively new information about intermediate-mass stars: the origin and evolution of their magnetic fields, the role of magnetic fields in generating their spectroscopic activity and in mediating accretion in their late formative stages, and the factors influencing their rotational angular momentum.

While numerous studies have been aimed at understanding the properties of young brown dwarfs relatively little exploration of their potential as drivers of outflows has occurred. Forbidden emission lines are important probes of outflows from young stellar objects, as they trace the shocks which form as an outflow interacts with the ambient medium of its driving source. While forbidden emission was identified in the spectra of young brown dwarfs, indicating the presence of outflows, these lines were weak and confined to the brown dwarf continuum position. Hence their origin in an outflow could not be confirmed. Our approach to this problem, is to analyse the forbidden line regions of brown dwarfs using spectro-astrometry. Spectro-astrometry is a novel technique which allows the user to recover spatial information from a spectrum beyond the limitations of the seeing of the observation. Using this technique we have found two brown dwarf outflows to date. In this chapter we outline this technique, describe our results for the brown dwarfs ρ-Oph 102 and 2MASS1207-3932 and discuss our future plans.

Followup infrared spectroscopy is reported for V1647 Ori, a young star whose recent eruption illuminated McNeil's Nebula. Lines of HI, H2, and CO are compared to previous observations. We find that the accretion rate fell two orders of magnitude and the CO bandheads disappeared at the end of the outburst. We also report a striking metamorphosis of the fundamental CO spectrum from centrally peaked profiles to emission lines with superimposed blue-shifted absorption lines and back again one year later. This remarkable change in spectral appearance indicates that the system did not return to equilibrium immediately following the outburst. In this paper we propose a mechanism to explain the emergence of a transient post-outburst outflow.

Magnetospheric accretion models are the current consensus to explain the main observed characteristics of classical T Tauri stars. In recent years the concept of a static magnetosphere has been challenged by synoptic studies of classical T Tauri stars that show strong evidence for the accretion process to be dynamic on several timescales and governed by changes in the magnetic field configuration. At the same time numerical simulation results predict evolving funnel flows due to the interaction between the stellar magnetosphere and the inner disk region. In this contribution we will focus on the main recent observational evidences for time variable funnel flows and compare them with model predictions.

Key observational constraints for jet models in T Tauri stars are outlined, including the jet collimation scale, kinematic structure, and ejection/accretion ratio. It is shown that MHD self-collimation is most likely required. The four possible MHD ejection sites (stellar surface, inner disk edge, extended disk region, magnetosphere-disk reconnexion line) are then critically examined against observational constraints, and open issues are discussed.

Herbig Ae/Be stars are pre-main-sequence stars of intermediate mass, which are still accreting material from their environment, probably via a disk composed of gas and dust. Here we present a recent study of the geometry of the inner (AU-scale) circumstellar region around the Herbig Be star MWC 147 using long-baseline interferometry. By combining for the first time near- and mid-infrared spectro-interferometry on a Herbig star, our VLTI/AMBER and VLTI/MIDI data constrain not only the geometry of the brightness distribution, but also the radial temperature distribution in the disk. The emission from MWC 147 is clearly resolved and has a characteristic physical size of ∼1.3 AU and ∼9 AU at 2.2 μm and 11 μm respectively. This increase in apparent size towards longer wavelengths is much steeper than predicted by analytic disk models assuming power-law radial temperature distributions. For a detailed modeling of the interferometric data and the spectral energy distribution of MWC 147, we employ 2-D frequency-dependent radiation transfer simulations. This analysis shows that passive irradiated Keplerian dust disks can easily fit the SED, but predict much lower visibilities than observed, so these models can clearly be ruled out. Models of a Keplerian disk with emission from an optically thick inner gaseous accretion disk (inside the dust sublimation zone), however, yield a good fit of the SED and simultaneously reproduce the observed near- and mid-infrared visibilities. We conclude that the near-infrared continuum emission from MWC 147 is dominated by accretion luminosity emerging from an optically thick inner gaseous disk, while the mid-infrared emission also contains strong contributions from the passive irradiated dust disk.

I review the current state of knowledge regarding disk accretion in young brown dwarfs (BDs), and the interaction of the disk with the central object. In particular, I discuss (1) observations of accretion/outflow phenomena in BDs; (2) techniques for measuring accretion rates (Ṁacc); (3) the dependence of Ṁacc on the central mass from stars to brown dwarfs; (4) the temporal evolution of Ṁacc; and (5) observations of variability in the accretion line profiles. I then examine the implications of these issues for the formation mechanism of BDs, and discuss new observations that can further constrain substellar origins.

In this review the recent development concerning the large-scale evolution of stellar magnetospheres in interaction with the accretion disk is discussed. I put emphasis on the generation of outflows and jets from the disk and/or the star. In fact, tremendous progress has occurred over the last decade in the numerical simulation of the star-disk interaction. The role of numerical simulations is essential in this area because the processes involved are complex, strongly interrelated, and often highly time-dependent. Recent MHD simulations suggest that outflows launched from a very concentrated region tend to be un-collimated. I present preliminary results of simulations of large-scale star-disk magnetospheres loaded with matter from the stellar, resp. the disk surface demonstrating how a disk jet collimates the wind from the star and also how the stellar wind lowers the collimation degree of the disk outflow.

We report on accretion- and outflow-related X-rays from T Tauri stars, based on results from the “XMM-Newton Extended Survey of the Taurus Molecular Cloud.” X-rays potentially form in shocks of accretion streams near the stellar surface, although we hypothesize that direct interactions between the streams and magnetic coronae may occur as well. We report on the discovery of a “soft excess” in accreting T Tauri stars supporting these scenarios. We further discuss a new type of X-ray source in jet-driving T Tauri stars. It shows a strongly absorbed coronal component and a very soft, weakly absorbed component probably related to shocks in microjets. The excessive coronal absorption points to dust-depletion in the accretion streams.

I review our current understanding of accretion shocks in classical T Tauri stars (CTTs), from a UV and X-ray perspective. The region of the accretion shock is a good candidate as a source of UV transition region lines from Li/Na-like ions, which are stronger in CTTs than in naked atmospheres. Disk gas captured by the stellar magnetic field produces a strong radiative shock upon falling on the stellar surface. Radiation from the shock creates a radiative precursor and heats the stellar surface resulting in a hot spot. Stellar and shock models indicate that unless the post-shock column is very large, it will be buried on the stellar photosphere. Models of the continuum emission produced by this configuration can roughly reproduce the observed excess spectra down to 1650 Å. Transition region lines in CTTs are broad, very variable, and present blueshifted, centered, and redshifted centroids. Detailed models of the line emission have so far failed to reproduce the fluxes, line shapes, and line ratios. High resolution X-ray line observations indicate the presence of larger amounts of cool plasma in CTTs with respect to WTTs. Observations of density sensitive line ratios of He-like ions suggest high plasma densities, as expected from lines originating in the accretion shock. For most stars, the interpretation of these ratios in terms of density remains equivocal due to the presence of the strong UV continuum.