C. Johns-Krull: The Magnetic Fields on T Tauri Stars

T Tauri stars are low mass, pre-main sequence stars, many of which are still surrounded by active accretion disks where it is believed planet formation is currently under way. Stellar magnetic fields including a strong dipole component on these newly formed stars are believed to play a critical role in the early evolution of the young star plus disk system. It is currently believed that the stellar magnetic field truncates the accretion disk several stellar radii above the star. This action forces accreting material to flow along the field lines and accrete onto the star at high stellar latitudes. It is also thought that the stellar rotation rate becomes locked 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 theoretical expectations. 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. I will also briefly discuss recent work on the origins of magnetic fields in fully convective stars such as T Tauri stars.

R. Pudritz:  Accretion powered winds in PMS stars

The slow spin of accreting young stellar objects presents a fascinating problem for the physics of cool stars.  A large literature has already explored the proposal that disk locking can explain slow PMS rotation. Recently, Matt and Pudritz (2005, 2006) have shown that the coupling of stellar fields to disks is minimal and may be unequal to the task under fiducial conditions.  Instead, we argue that a magnetized stellar wind, powered by the accretion flow onto the star, can remove the angular momentum accreted by the star and leave it spinning slowly.  In this talk, I will report on some recent results that further explore this idea that will be complementary to Sean Matt's review talk of the theory of stellar winds.  I will also show some recent calculations of the formation of protostars in magnetized disks in the context of the origin of magnetic fields in young stars.

J. Bouvier: The magnetic star-disk connection: insights from CTTS variability

Synoptic studies of the variability of accreting T Tauri stars offer
clues to the structure of the magnetospheric region extending from the inner disk edge to the stellar surface. I will present recent results
which provide clear evidence for the physical association between
inner disk warps, funnel flows and accretion shocks. These new results globally support the magnetically-controlled accretion/ejection
paradigm. However, they also point to a highly variable large-scale (r
>> R_star) magnetic structure on timescales ranging from days to
years, which casts doubt on the validity of static magnetospheric
accretion models. The origin of this variability will be shortly
discussed. On short timescales (days-weeks), it probably results from
differential rotation shearing the field lines which connect the star
to the inner disk, while on longer timescales (years) it might reflect
dynamo evolution.

Fred Walter: Highly Time-variable Accretion in the Classical T Tauri Star S CrA

We will summarize the results of 4 years of intensive spectroscopic and photometric monitoring of the S CrA system with SMARTS. S CrA is a 1.3" visual pair of highly veiled classical T Tauri stars. Each star varies in brightness by up to 2 magnitudes on timescales of days. Spatially-unresolved low dispersion spectroscopy reveals the coming and going of inverse P-Cygni absorption components in the upper Balmer lines on similar timescales.  Spatially-resolved high dispersion VLT/UVES spectra reveal the true complexity of the system. The brightness changes must be due to accretion, and will trace the total accretion onto each star in the system. The spectroscopic inverse P-Cygni absorption components trace absorption on the line of sight only. This may provide a clue into the azimuthal distribution of the accretion regions. We will discuss our attempts to understand the timescales of the accretion in this system.

Jan Forbrich: Simultaneous X-ray, radio, near-infrared, and optical monitoring of Young Stellar Objects in the Coronet cluster

Multi-wavelength monitoring can provide important information about physical processes in young stellar objects such as the relation between accretion processes and X-ray emission. While coronal processes should mainly cause variations in the X-ray and radio bands, accretion processes may be traced by time-correlated variability in the X-ray and optical/infrared bands. We present the first simultaneous X-ray, radio, near-infrared, and optical multi-wavelength monitoring campaign succeeding in detecting X-ray to radio variability in extremely young objects like class I and class 0 protostars. Our target is the compact Coronet cluster in the Corona Australis star-forming region, harbouring at least one class 0 protostar, several class I objects, numerous T Tauri stars, and a few Herbig AeBe stars. A core sample of seven objects  was detected simultaneously in the X-ray, radio, and optical/infrared bands. While most of these sources exhibit clear variability in the X-ray regime! and several display also optical/infrared variability, none of them shows significant radio variability on the timescales probed. We also do not find any case of clearly time-correlated optical/infrared and X-ray variability. This suggests that there is no direct link between the X-ray and optical/infrared emission, supporting the notion that accretion is not an important source for the X-ray emission of these YSOs. Combining our Chandra data with previous observations results in one of the most sensitive X-ray observations yet of a star-forming region allowing a virtually complete census of YSOs in the Coronet region.

Ettore Flacommio: X-ray observations and coronal structures in the PMS: COUP and beyond

I will summarize some of the results obtained by the Chandra Orion Ultradeep Project (COUP), relevant to the study of the coronal structures of Pre Main Sequence (PMS) stars. COUP consisted of a ~850 ksec long Chandra observation of the Orion Nebula Cluster performed in January 2003 and whose results were published roughly one year ago (special issue of ApJS, n. 160). I will focus on thee studies that, taking advantage of the exceptional length of the observation and of the richness of the observed stellar sample, have given a significant contribution to the characterization of the X-ray emitting regions of PMS stars both in the temporal and the spatial domains: a statistical study of flare variability in a subset of young solar analogs (Wolk et al. 2005), a detailed physical modeling of flaring magnetic loops (Favata et al. 2005), and a study of X-ray rotational modulation (Flaccomio et al. 2005). New developments in these fields will be mentioned.


J. Schmitt: Accretion-related X-ray emission in cTTS

The observed X-ray emission of cTTS is usually attributed to some scaled-up version of solar activity.  This ``standard'' picture of cTTS activity has recently been challenged on the basis of high-resolution spectroscopic X-ray observations. Spectra obtained with XMM-Newton and Chandra have provided strong evidence for accretion-induced X-ray emission in cTTS. The evidence consists of unsual forbidden-intercombination line ratios in OVII and NeIX triplets, which indicate plasma densities not encountered in coronal sources, and unusually strong emission in OVII, indicating the presence of unusually large soft emission emasures.  I will discuss the observational evidence for the cTTS TW Hya, BP Tau, CR~Cha, V4046 Sgr and T Tau and argue that the X-ray data are consistent with a magnetically funnelled accretion column scenario.


Hans-Moritz Guenther: Separating  X-rays from CTTS in accretion and corona


The X-ray spectra of classical T Tauri stars (CTTS) usually show small f/i rations in the He-like triplet. This  component can be modelled in terms of an optically thin accretion shock, but in addition these stars also posses an active corona. Using the accretion model the observed grating X-ray spectra of TW Hya (XMM-Newton) and V4046 Sgr (Chandra) are decomposed in an accretion and a low-density coronal component, which dominates the flux above 1 kev. The resulting coronae are not fundamentally different from those observed on "normal" stars.