attachment:chandra.jpg © Chandra / E Feigelson

Magnetised collapse, accretion disc, jets & protoplanets

Using Doppler tomography on time-resolved spectropolarimetric observations of protostellar accretion discs, we can map the temperature, density and magnetic field in the core regions of the disc (eg Donati et al 2005, Nature 438, 466), and compare these distributions with those expected from numerical simulations of molecular cloud collapse in the presence of magnetic fields (eg Banerjee & Pudritz 2006, ApJ 641, 949). This comparison should indicate whether the magnetic field is indeed triggering the disc instabilities causing the enhanced accretion (eg Balbus & Hawley 2003, LNP 614, 329) and tell how efficiently the field can inhibit the formation of protostellar/protoplanetary clumps in the disc (eg Fromang 2005, A&A 441, 1); moreover, it should clarify the origin of the disc field and indicate whether it is a fossil remnant or a dynamo output.

Using the disc and magnetic field structure derived from observations, we can test and update existing theoretical models of collimated jet and wind formation from the young stars and/or their accretion discs (eg Ferreira et al 2006, A&A 453, 785) and try to work out the role of magnetic fields in controlling the disc ability to fire jets and winds (eg Ménard & Duchène 2004, A&A 425, 973). We should be able to conclude how much mass and angular momentum is extracted from the disc (and thus made unavailable to the forming star) and/or from the star through such processes.

We can also look at density gaps in accretion discs potentially indicating the presence of close-in giant planets around newly born stars. By looking at how gaps spatially correlate with magnetic topologies, we can check whether magnetic fields are capable of stopping giant protoplanets in their inward spiral migration (eg Terquem 2003, MNRAS 341, 1157). Migrating planets may also survive by entering the low-density magnetospheric gaps that magnetised protostars forming dig at the centre of accretion discs; simulations can help testing this option more quantitatively (Romanova & Lovelace 2006, ApJ 645, L73).

Magnetospheric accretion and stellar structure

Using Zeeman-Doppler imaging on sets of spectropolarimetric observations of classical T Tauri stars (cTTSs), we can map magnetic structures at the surface of newly born stars (eg Donati et al 2007, MNRAS 380, 1297; Donati et al 2008, MNRAS 386, 1234) and investigate how the magnetic field connects the star to the accretion disc using the derived disc models. We can then look at how disc material is transferred to the star through magnetospheric accretion processes (eg Jardine et al 2006, MNRAS 367, 917). Global 3D simulations of disc accretion to rotating magnetised stars successfully captured many aspects of magnetospheric flows, using simple field configurations (tilted dipole, Romanova et al 2003, ApJ 595, 1009; tilted dipole+quadrupole, Long, Romanova & Lovelace, 2007, ApJ MNRAS 374, 436). Future simulations will use more realistic magnetospheric topologies derived from observed surface magnetic maps of cTTSs and will produce realistic spectra of emission lines formed in accretion funnels, to be compared with observations (eg Kurosawa, Romanova, Harries 2008, MNRAS 385, 1931).

X-ray observations also reveal clues on the magnetic topologies of low-mass protostars; they tell, eg, that coronal emission from low-mass protostars is lower that those of active stars of similar spectral type and rotation rate on the main sequence, accreting protostars showing even smaller X-rays than non-accreting protostars (eg Preibisch et al 2005, ApJS 160, 401). Recording simultaneous X-ray observations and spectropolarimetric monitoring on a statistically significant sample of protostars and producing models reproducing consistently the trends revealed by the data (eg Jardine et al 2006, MNRAS 367, 917, Gregory et al 2007, MNRAS 379, L35) will ultimately provide a better description of the 3D magnetospheric geometry of newly-born low-mass stars.

These studies give us means to explore how the accretion process and the magnetic field impacts the structure of the forming star (eg by inhibiting convection and mixing where the field is strong, Mullan & Mac Donald, 2001, ApJ 559, 353; Gallardo, Chabrier & Baraffe, 2007, A&A 472, L17) and how it affects the angular momentum history (eg Long, Romanova & Lovelace, 2005, ApJ 634, 1214).

Post accretion phase and angular momentum evolution

By investigating the large-scale magnetic topologies of disc-less post T Tauri stars, we can explore the efficiency with which young stars lose most of their angular momentum. This angular momentum loss indeed critically depends on how the immediate circumstellar environment (wind, prominences) magnetically couple to the central star (eg Bouvier Forestini Allain 1997, A&A 326, 1023). Looking at the magnetic properties of pre-main-sequence and zero-age-main-sequence stars with different ages, masses and rotation rates and looking at how often they eject prominences (eg Donati et al 2000, MNRAS 316, 699) can help us constrain models of the angular momentum evolution of young low-mass stars (eg Feigelson et al 2004, ApJ 611, 1107).