Invited and contributed abstracts

Invited talks

The accretion disk paradigm for young stars

Claude Bertout, Institut d’Astrophysique de Paris

Accretion plays a major role in several of the many models put forward to explain the properties of T Tauri stars since their discovery by Alfred Joy in the 1940s. Early investigators already recognized in the 1950s that a source of energy external to the star was needed to account for the emission properties of these stars in the optical range.

The opening of new spectral windows from the infrared to the ultraviolet showed in the 1970s and 1980s that the excess emission of T Tauri stars and related objects extends into all wavelength domains, while evidence of outflow and/or infall in their circumstellar medium accumulated.

Although the disk hypothesis had been put forward by Merle Walker as early as 1972 to explain properties of YY Orionis stars and although Lynden-Bell and Pringle worked out the accretion disk model and applied it specifically to T Tauri stars in 1974, the prevailing model for young stellar objects until the mid-1980s assumed that they experienced extreme solar-type activity. It then took until the late 1980s before the indirect evidence of disks presented by several teams of researchers became so compelling that a paradigm shift occurred, leading to the current consensual picture.

I will briefly review the various models proposed for explaining the properties of young stellar objects, from their discovery to the direct observations of circumstellar disks that have so elegantly confirmed the nature of young stars. I will go on to discuss more modern issues concerning their accretion properties and conclude with some results obtained in a recent attempt to better understand the evolution of Taurus-Auriga young stellar objects.

From Boundary Layers to Magnetospheric Accretion

Gibor Basri, UC Berkeley

In recognition of the critical contributions of Claude Bertout and his students and collaborators to the understanding of T Tauri stars, I review some aspects of how our understanding of T Tauri stars and the process of star formation has undergone a full revolution in the past 25 years. In the early 1980s, studies of these stars concentrated on what their mass loss might be like (with some hints of inflows as well), how much of their emission activity could be accounted for by chromospheric activity (and the first studies of UV and X-ray emission), beginning studies of their rotational properties, and attempts to understand their strange line profiles. A few compendia of T Tauri spectra had appeared (eg. the Cohen and Kuhi collection). The advent of the IRAS satellite (which measured infrared excesses) and the "re-discovery" of a seminal paper by Lynden-Bell and Pringle on accretion disks provoked the paradigm shift which has become standard today. I trace the history of our conceptions of the basic phenomena associated with the star-disk interaction, and remind the reader of how we arrived where we are today. Of particular interest is the increasing realization of the role stellar magnetic fields play in this, and the development of methods for measuring accretion rates on T Tauri stars. Our picture of the relevant geometry near the star went from spherical to purely disk-like to rather complex. Synoptic spectral observations along with proper interpretation of line profiles were helpful in supporting the basic scenarios which prevail today.

What can X-rays tell us about magnetic fields, accretion and mass loss in young stars ?

Thierry Montmerle, Laboratoire d’Astrophysique de Grenoble

Until recently, X-rays from young stellar objects (YSOs) were thought to be a 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). However, there is now increasing evidence that specific X-ray emission mechanisms are at work in the early stages when the YSOs are still accreting from their circumstellar disk. This evidence comes not only from the frequent existence of a soft X-ray component, but also from EUV observations from FUSE. The most frequently invoked mechanism is accretion shocks along magnetic field lines ("magnetic accretion"), but, especially in the case of the more massive Herbig AeBe stars, other, possibly more exotic mechanisms can operate: star-disk magnetic reconnection, magnetically channeled shocked winds, etc. Some of these mechanisms are also now seen in massive stars. In any case, magnetic fields, both on small scale (surface activity) and on large scale (dipolar magnetospheres) play a central role in the emission of X-rays by young stars, probably throughout the IMF.

Measurements of Magnetic Fields in Young Stars

Christopher Johns-Krull, Rice University

Stellar magnetic fields including a strong dipole component are believedto play a critical role in the early evolution of newly formed stars andtheir circumstellar accretion disks. It is currently believed that the stellar magnetic field truncates the accretion disk several stellar radiiabove the star. This action forces accreting material to flow along the field lines and accrete onto the star preferentially at high stellar latitudes. It is also thought that the stellar rotation rate becomeslocked to the Keplerian velocity at the radius where the disk is truncated. I will review recent efforts to measure the magnetic field properties of T Tauri stars, focussing on how the observations compare with the theoreticalexpectations. A picture is emerging indicating that quite strong fields do indeed cover the majority of the surface on young stars; however, the dipole component of the field appears to be alarmingly small. However, I willalso show that at least one accretion model which takes into account thenon-dipole nature of the magnetic field provides predicitons relating various stellar and accretion parameters which are present in the currentdata.

Magnetic field at the inner disk edge

Moira Jardine, SUPA, School of Physics and Astronomy

The nature of the magnetic field that links young stars to their disks is a crucial factor that influences the accretion of disk material onto the star and the exchange of torques between the disk and the star. It is only recently however that it has been possible to infer the structure of this magnetic field from observations. In this review I will describe recent advances that have been made in determining the nature of the field that links the star to its disk. I will show how these observations are being used to guide current progress in modeling magnetospheric accretion and discuss implications for stellar angular momentum evolution.

Time variable funnel flows

Silvia Alencar, UFMG, Brazil

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 simulations results predict evolving funnel flows due to theinteraction between the stellar magnetosphere and the inner disk region.In this contribution we will focus on the main recent observationalevidences for time variable funnel flows and compare them with modelpredictions.

Radiative transfer in funnel flows

Tim Harries, University of Exeter

The intensity and morphology of line profiles produced in magnetospheric accretion streams encode information on both the geometry and physical conditions of the accreting material. However extracting this information, and testing models for the structure of the magnetosphere, requires radiative-transfer modelling of moving material in a non-spherical geometry - a formidable numerical problem. Despite these difficulties significant progress has been made in this area, and I will review these developments by exploring both the numerical techniques and their application to observations. I will highlight some successes of the models, and also critically explore their weaknesses.

Observations of Accretion Shocks in young stars

David Ardila, Spitzer Science Center / Caltech

I will discuss the observational evidence suggesting that accretion flows in pre main-sequence stars end in a radiative shock. The existence of this shock is one of the basic elements of the the magnetospheric accretion paradigm, as the UV and optical veiling continua are hard to understand without it. It seems clear that the shock also contributes to line emission at UV and X-ray wavelengths. For example, we observe strong "transition region" lines in the UV spectra of CTTS, while they are absent, for the most part, from the spectra of WTTS. For the handful of stars for which OVII or NeIX triplets are observed, their origin is roughly consistent with cool, dense regions, with characteristics similar to those expected in an accretion shock. However, until very recently detailed models have been lacking. I will explore the basic observational constraints imposed by recent observations on the shock structure as well as the role of the shock physics in the interpretation of these observations.

Gas in the Inner Disk Region

John Carr, Naval Research Laboratory

I will review the observation of gas in the innermost region of disks around T Tauri stars. In particular, I will examine what can be learned about inner disk truncation from infrared spectroscopy of disk gas. The inferred inner gas radii for T Tauri star disks will be compared to the expectations of models for disk truncation, to the inner dust radii of disks, and to the radial distribution of short-period extra-solar planets.

Inner disk regions revealed by infrared interferometry

Fabien Malbet, Laboratoire d’Astrophysique de Grenoble

I will 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 hottest dust. The characteristic sizes of these regions found are larger than previously thought and correlate with the luminosity of the central young stars. This relation has motivated in part a new class of models of the inner disk structure. However the precise understanding of their origin is still in debate. Mid-infrared observations have probed disk emission over a larger range of scales revealing mineralogy gradients in the disk providing crucial information on the structure and physical properties of young circumstellar disks as initial conditions for planet formation.

Signatures of the Inner Wind

Suzan Edwards, Smith College

The role of the star-disk interaction region in launching the high velocity component of accretion-driven outflows will be 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 mass loss rates has remained elusive. A promising new diagnostic is He I 10830, 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.

Observational Clues to the Origin of Jets

Tom Ray, Dublin Institute for Advanced Studies

Jets and outflows accompany the birth not only of stars but sub-stellar objects as well such as brown dwarfs. The phenomenon may occur during the formation of giant planets. While their generation is not entirely understood, it is almost certainly due to MHD processes involving an accretion disk. Even for the nearest star formation region, high spatial resolution is required to determine the precise mechanism. I will briefly review a number of clues as to what that mechanism might be from observations close to the source. These include rotation in the ionized, neutral and molecular components, jet opening angles, poloidal velocities, and basic gas parameters.

The accretion-ejection connexion and jet launching mechanism(s) in young stars

Silvie Cabrit, Observatoire de Paris

Winds and jets from young accreting stars possibly include contributions from several components: (1) an inner coronal stellar wind, (2) an episodic flow from reconnexion sites in the star-disk magnetopause, (3) an outer centrifugally-driven MHD disk wind. In order to investigate which of these mechanisms is the dominant contributor to stellar jets, I will review theoretical predictions for steady pressure-driven and centrifugally-driven MHD winds, and compare them with observational measurements of jet properties (poloidal speeds, rotation velocities, ejection/accretion ratio, jet widths...). It will be shown that centrifugally-driven disk winds are favored over pressure-driven winds on energetic grounds, although the range of launch radii remains uncertain. Possible clues to this issue, including comparison of jet properties across various evolutionary stages, will be briefly discussed.

Stellar angular momentum evolution : observations

Jerome Bouvier, Laboratoire d’Astrophysique de Grenoble

I will review recent results on the rotational evolution of young stars. Large scale photometric surveys of young clusters have yielded precise rotation period measurements for hundreds of stars at various ages in the range 1-200 Myr. As a result, the evolution of the rotation period distributions can now be closely followed from the early PMS up to the ZAMS for stars in the mass range 0.1-1.0 Mo. The results provide clear evidence for PMS spin up for a fraction of stars, PMS braking on a timescale of a few Myr for another fraction, and a mass-dependent spin down on the ZAMS. Significant differences are also observed between the rotational evolution of low mass stars (0.3-1.0Mo) and of very low mass ones (0.1-0.3Mo), in the sense that the latter do not seem to experience significant PMS braking. I will further address the origin of PMS braking and discuss whether or not observations support the idea that the rotation of low mass young stars is regulated by an accretion-related process (e.g., X-winds, disk-locking, accretion-driven stellar winds).

Magnetization, Accretion, and Outflows in YSOs

Frank Shu, University of California, San Diego

We compute the likely level of magnetization of the accretion disks in young stars from fields brought in by the process of star formation. We model the global problems of the viscous transport of angular momentum and the resistive transport of matter across the embedded poloidal field in the disk, and we show that the ratio of resistivity to viscosity must equal a specific ratio in steady state. The angle from the vertical at which mean field lines emerge from the surface of the disk is always greater than 30 degrees in realistic circumstances, but the amount by which disks rotate below the Keplerian rate make it difficult for T Tauri and FU Orioinis stars to drive cold disk winds by the magnetocentrifugal mechanism. A model of the process of magnetorotational instability in such disks allows their rotation rates, surface densities, and strengths of the poloidal magnetic field to be calculated as a function of disk radius, which compare well with the existing data. The implied levels of magnetization are enough to affect the predictions of X-wind theory and may explain better the bowl-like shape of the cavities of embedded objects evacuated by wide-angle winds.

Numerical simulations of the star-disk magnetosphere

Christian Fendt, Max Planck Institute for Astronomy

I will discuss the large-scale evolution of the stellar magnetosphere interaction with the accretion disk. In particular the generation of outflows from disk and star will be investigated. Recent MHD simulations suggest that outflows launched from a very concentrated region close to the inner disk radius tend to be uncollimated.

MHD Simulations of Disk-Star Interaction

Marina Romanova, Cornell University

I will discuss a number of topics relevant to disk-magnetosphere interaction and how numerical simulations illuminate them. The topics include: (1) disk-magnetosphere coupling and the problem of spin-down; (2) structure of the magnetospheric flow and the inner regions of the disk, and properties of the hot spots on the surface of the star; (3) evolution of the magnetic field in the vicinity of the star, and outflows. Results of both 2D and 3D simulations will be presented.

The Impact of Close Stellar Companions on Inner Disks

Robert Mathieu, University of Wisconsin - Madison

Three disks may be present in a close binary system: a circumprimary disk, a circumsecondary disk, and a circumbinary disk. Typically star-disk magnetospheric interactions will involve the circumprimary and circumsecondary disks. Yet from a purely theoretical point of view the processes by which such disks are created and replenished remain unclear. I will discuss the observational evidence for circumprimary and circumsecondary disks, and for possible flows of material between the circumbinary disk and these inner disks. I will also present new evidence suggesting that at 150 Myr the rotation periods of stars in close binaries are shorter the rotation periods of single stars or stars in wide binaries. This new result may reflect on the degree of magnetic disk locking in close binaries.

Insights on Funnel Accretion and the Disc-Magnetosphere Interaction from Magnetic Cataclysmic Variables

Coel Hellier, University of Keele

In the Cataclysmic variables (CVs) mass transfer occurs via Roche lobe overflow from a donor star mediated by an accretion disc onto a magnetic white dwarf. If the white dwarf has a sufficiently strong magnetic field a disc does not form (the Polar subclass), or is truncated (the Intermediate Polar sub class), and the material is funnelled directly along field lines via accretion funnels onto the magnetic poles of the white dwarf. Given the short rotation periods of the magnetic white dwarfs in these systems (tens of minutes to hours), they provide an ideal opportunity for a detailed study of the dynamics of the disc - magnetosphere interaction and of the radiation properties of accretion funnels. We review our current state of knowledge in these area drawing parallels with accretion flows in Young Stellar Objects as appropriate.

Star-disk interaction in young intermediate-mass stars

Antonella Natta, INAF-Osservatorio di Arcetri

I will review our current knowledge of the accretion properties of stars of intermediate mass. More specifically,I will discuss methods to determine mass accretion rate and the mostrecent results, evidence of stellar winds and measurements of stellar magnetic fields. Finally, I will comment on the evolution of disks around intermediate-mass stars.

Halting planetary migration at the star-disk interface

Caroline Terquem, Institut d’Astrophysique de Paris

The distribution of semi-major axes of extrasolar planets suggests that there is a pile-up at a rather well-defined separation (about 0.4 AU) or period (about 3 days). The different mechanisms that have been put forward to account for this pile-up are: i) existence of a magnetospheric cavity, ii) tidal interaction with a rotating star, iii) Roche lobe overflow, iv) presence of a toroidal magnetic field with a strong gradient. I will review these mechanisms.

The orbital properties of inner exoplanets

Frederic Pont, Observatoire de Geneve

Doppler surveys have shown that close-in extra-solar planets are relatively common, and that they tend to pile up near a lower limit of a 0.05 AU (or P 3 days). These hot planets are thought to have migrating inwards after their formation, when the protoplanetary disc was still present. One of the leading explanation for the pile-up at P 3 days is that migration just stops at the inner end of the disk, at the edge of the magnetic cavity. Close-in planets are being discovered at a steady rate now, most remarquably by surveys for transiting planets. Most than 50 planets with periods shorter than 10 days are known, covering all the mass range from a few Earth masses to heavier than Jupiters. In this presentation we shall revisit the inner disk edge hypothesis in light of the newest observational results.

Star-Disk Interaction in Young Brown Dwarfs

Subhanjoy Mohanty, Harvard-Smithsonian CfA

I will review the current state of knowledge regarding accretion in brown dwarfs (BDs), and the interaction of the disk with the central object. In particular, I will discuss (1) observations of accretion/outflow phenomena in BDs; (2) techniques for measuring the accretion rates (Mdot); (3) the dependence of Mdot on the central mass from stars to BDs; (4) temporal evolution of Mdot; (5) observations of variability in the accretion line profiles and implications for Mdot; (6) evidence for disk-locking; and finally (7) role of the central BD in disk evolution, e.g., creation of large inner holes through planetesimal formation and/or photoevaporation. I will conclude with a summary of the implications of the above for the formation mechanism of BDs versus stars.

Contributed talks

Accretion and Outflow-Related X-Rays in T Tauri Stars

Manuel Guedel, Paul Scherrer Institut and Institute of Astronomy

We report on new observations of accretion- and outflow-related X-rays in T Tauri stars, obtained as part of the "XMM-Newton Extended Survey of the Taurus Molecular Cloud." X-rays potentially form in accretion shocks of magnetically guided accretion streams near the stellar surface, although we hypothesize that direct interactions between the streams and magnetic coronae occur as well. We report on the discovery of a "soft excess" in accreting T Tau stars supporting this scenario. We further discuss a new type of X-ray source in jet-driving T Tauri stars. It shows a strongly absorbed coronal X-ray 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.

The Rotation of Very-Low Mass Stars and Brown Dwarfs

Jochen Eisloffel, Thüringer Landessternwarte

In recent years a large number of new observations on the rotation and angular momentum evolution of very low-mass stars and brown dwarfs have become available. Periods at a 1% accuracy level have been measured for several hundreds such objects in various young clusters. It turns out, that very-low mass objects show some remarkable differences in their rotational evolution as compared to solar-mass stars. In this talk, I will present possible consequences from the differences in the very-low mass objects’ interaction with their discs and in their magnetic activity as opposed to higher mass stars.

Observational constraints for models of disk photoevaporation

Gregory Herczeg, Caltech

Photoevaporation by the central star may play a fundamental role in disk dispersal. We analyze the strength and origin of FUV and X-ray emission to describe the radiation field incident upon the circumstellar disk. We then discuss observations of H2 and [Ne II] emission, which provide evidence for a warm disk surface layer that may be heated by emission from the central star. We discuss challenges to disk photoevaporation models in the context of these results.

On the origin of continuum and line emission in CTTSs

Sergey Lamzin, Sternberg Astronomical Institute, Moscow State University

We calculated profiles of CIV 1550, SiIV 1400, NV 1240 and OVI 1035 doublet lines using results of 3D MHD simulations of disc accretion onto magnetized young stars. It follows from the comparison of theoretical and observed (HST/GHRS-STIS and FUSE) profiles that in the case of CTTS accreted matter has large horizontal velocity component near stellar surface. We also found that the theory predicts much larger CIV 1550 line flux than observed (up to two orders of magnitude in some cases) and concluded that the main portion of accretion energy in CTTSs is liberated outside accretion shock. We then discuss: 1) the origin of emission continuum and lines of different ions; 2) the problem of "wide" and "narrow" components of emission lines; 3) some results of magnetic field measurements in CTTS.

Measuring the Physical Conditions of Accreting Gas in T Tauri Systems

Jeffrey Bary, University of Virginia

I present one aspect of the results from a low-resolution near-infrared spectroscopic survey of actively accreting T Tauri stars (TTS) in the Taurus-Auriga star forming region. Using CorMASS, a cross-dispersed spectrograph with excellent wavelength coverage (0.8-2.5 $mu$m), we simultaneously observed several Paschen and Brackett series HI recombination lines in 106 spectra of 16 TTS. An analysis of the Paschen and Brackett decrements in the context of recombination line theory has provided us with an accurate determination of the temperature and density of the emitting hydrogen gas. Since these emission lines have long been associated with material confined to the accretion funnels linking the star to the disk, we assume this temperature and density to be that of the accreting gas.

Why are accreting T Tauri stars less luminous in X-rays than non-accretors?

Scott Gregory, University of St Andrews

Accreting T Tauri stars show lower levels of X-ray activity than non-accretors. 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 cannot penetrate the dense material in accretion columns. The reduction in L$_X$ depends strongly on the field geometry, and may explain why CTTs show a larger scatter in L$_X$ values compared to WTTs.

The Role of Stellar Winds in the Star-Disk Interaction

Sean Matt, University of Virginia

Stellar winds may be important for angular momentum transport from accreting young stars, but the nature of these winds is still not well-understood. I will present some simulation results for thermal coronal winds from T Tauri stars, which are powerful enough to solve the stellar angular momentum problem. I will also briefly discuss the emission properties expected from such winds and the implications for the wind temperature and driving mechanism in real systems.

V1647 Ori’s Last Gasp: A Brief Post-Outburst Warm, Molecular Wind

Sean Brittain, Clemson University

Followup infrared spectroscopy is reported for V1647 Ori, a young star whose recent eruption illuminated McNeil’s Nebula. Lines of H1, H$_2$, and CO have been observed and are compared to previous observations. We find that the $JHK$ magnitudes have faded back to their pre-outburst level, the accretion rate has fallen by two orders of magnitude from its peak, and the CO bandheads have disappeared, suggesting that the outburst is complete. However, we also report a striking metamorphosis of the fundamental CO spectrum from centrally peaked profiles to P Cygni profiles and back again, which indicates that the system did not return to equilibrium immediately following the outburst. We use a standard disk-magnetosphere interaction model to interpret the observations. The model predicts a decreasing truncation radius of the disk with increasing accretion rate. When the truncation radius of the disk moves radially inward or outward in response to changes in the accretion rate, the magnetic field must reorganize, leading to an enhanced reconnection rate. Such activity is expected to launch outflows, which have been observed at the onset and completion of the outburst of the system. We show that these trends are consistent with the fact that V1647 Ori produced a fast and hotter H$alpha$ outflow at the onset of the outburst whereas a slower, cooler CO outflow manifested itself as the system approached quiescence. We also discuss how this interpretation of the outburst bears on concurrent X-ray observations of this young stellar object.

The inner gaseous accretion disk around an Herbig Be star revealed by NIR/MIR spectro-interferometry

Stefan Kraus, Max-Planck-Institute for Radioastronomy

The IR excess emission towards Herbig AeBe stars is believed to indicate the presence of substantial amounts of dust and gas in the system, maybe arranged in an accretion disk. To unveil the physical properties and the distribution of this material it is essential to spatially resolve the inner disk regions over a wide wavelength range. I present a recent investigation, in which we study the geometry of the disk around the luminous Herbig Be star MWC 147 on the AU-scale using VLTI/AMBER and MIDI spectro-interferometry. We employ 2-D radiative transfer modeling, finding strong indications that most of the MIR emission originates in an outer dusty disk, while the NIR emission is dominated by an optically thick gaseous disk inside the dust sublimation radius.

MHD Instabilities at the Disk-Magnetosphere Boundary: 3D Simulations

Akshay Kulkarni, Cornell University

We present results of 3D simulations of MHD instabilities at the accretion disk-magnetosphere boundary. The instability is Rayleigh-Taylor, and manifests itself in the form of tall, thin tongues of plasma that reach the star by penetrating through the magnetosphere in the equatorial plane. The tongues rotate around the star in the equatorial plane, and their number changes with time. In contrast with funnel flows, which deposit matter mainly in the polar cap region, the tongues deposit matter much closer to the stellar equator. The occurrence of the instability depends on the mass, rotation rate and magnetic field of the star, the accretion rate, and the angle between the star’s rotation and magnetic axes.

Resistive MHD Evolution of Protostars

Shu-ichiro Inutsuka, Kyoto University

The evolutions of the magnetic field and angular momentum in collapsing cloud cores and newly born protostars are studied by 3D resistive MHD simulations with nested grids. The calculations that cover a huge dynamic range from molecular cloud core density (n=10$^4$/cc) to stellar density (n=10$^22$/cc) enable us to study realistic evolution of magnetic field and rotation of the protostars and dynamics of outflows and jets. When the central density becomes sufficiently high (10$^12$/cc), Ohmic dissipation largely removes the magnetic field from a collapsing cloud core, and the strongly twisted magnetic field lines are de-collimated. The magnetic field lines are twisted and amplified again for much higher density (10$^16$/cc) where the magnetic field is re-coupled with warm gas. Finally, protostars at their formation epoch (n=10$^21$/cc) have magnetic fields of $sim$0.1—1k Gauss, which is comparable to observations. The angular momentum in a collapsing cloud is removed by magnetic effects such as magnetic braking, outflow and jets. The formed protostars have rotation periods of 0.1 - 2 days at their formation epoch, which is slightly shorter than observations. This indicates that a further removal mechanism for the angular momentum, such as interactions between the protostar and disk, wind gas, or jets, is important in the long-term evolution of protostars.

Jets from Brown Dwarfs

Emma Whelan, Dublin Institute for Advanced Studies

Recent studies have highlighted the numerous ways in which young brown dwarfs bear a striking resemblance to classical T Tauri stars. This comparison not only provides information relevant to theories of BD formation, but also allows the validity of the accretion/ejection models for the formation of solar-mass stars, to be tested in the sub-stellar domain. While much is now understood about accretion in brown dwarfs relatively little is known about their outflow activity. We are presently leading a study to search for jets from young brown dwarfs. In my talk I will give an introduction to the reasoning behind this work, the techniques used and the results achieved to date.

Magnetism, rotation and accretion in Herbig Ae-Be stars

Evelyne Alecian, RMC - LESIA

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, the factors influencing their rotational angular momentum, and the development and evolution of chemical peculiarity in their photospheres.

Large scale magnetic fields in discs: Jets and Reconnexion X-winds

Jonathan Ferreira, Laboratoire d’Astrophysique

In this talk, I will 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 may be expected to thread the innermost disc regions, giving rise to a transition from an outer standard accretion disc to an inner jet emitting disc. I will then address its connection with the central forming star. 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 explicited, namely "reconnection X-winds" (Ferreira, Pelletier & Appl 2000) and "accretion powered" stellar winds (Matt & Pudritz 2005). For the former, it has been shown that it 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 magnetic configuration.

Submillimetre Polarimetric Observations of Magnetic Fields in Star Forming Regions

Rachel Curran, Dublin Institute for Advanced Studies

Submillimetre imaging polarimetry is one of the best ways of studying magnetic fields in star forming regions, and the only way to gain significant information on the structure of the fields. The largest sample (to date) of both high and low mass SFRs observed using this technique is analysed to establish any links between the magnetic field morphology (in the plane-of-the-sky) on these large (JCMT) and intermediate (BIMA) scales with the observed outflow/disc direction. The future importance of this technique with the SMA and ALMA is also discussed.

The structure accompanying young star formation

V.A. Demichev, Space Research Institute

We studied structure of H2O super maser region in Orion with VLBI angular resolution 0.1 mas or 0.05 AU. Bright maser emission (F$sim$8MJy) was determined by highly organized structure: accretion disk, bipolar outflow, torus and surrounding shell. Accretion disk is viewed edge-on. Disk rotates as a rigid body with velocities $V sim Omega R$ and rotation period is $sim$ 180 yrs. The highly collimated bipolar outflow has size 9x0.7 AU, velocity $sim$10 km/s, with a compact ($<$0.05 AU) bright source ejector in the center, surrounded by torus 0.4 AU diameter. The outflow has helix structure, which is determined by precession with period T $sim$ 10 yrs. Comet-like ejections - bullets were observed on the distances up to 70 AU.

The role of thermal pressure in jet launching

N. Soker, Technion

Most of the energy of the accretion process is liberated near the boundary layer. The energy is in the rotating disk material. I will present my view that the most efficient way to use this energy is via shocks, which lead to large pressure gradients. The large pressure gradients can lead to jet launching without any dynamical role of the magnetic fields. I will present calculations of the acceleration of gas by the thermal pressure gradient. A crucial ingredient of the proposed model is that the accreted material is strongly shocked, and then cools down on a long time scale. I will discuss the perturbations that can lead to these shocks.