MagIcS
stellar formation with magnetic fields
stellar formation with magnetic fields
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© 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 & Duchčne 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 the accretion discs that could indicate the presence of planets
in very close orbits around the newly born star. By looking for correlations between these gaps and
the disc field distribution, we can check whether magnetic fields are capable of stopping 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 the magnetised forming stars succeed at
digging in the central regions of accretion discs; simulations can help testing this option by
estimating how empty this gap really is, depending on, eg the stellar magnetospheric topology
(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) 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 2007, MNRAS in press).
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 &
McDonald, 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).
Related threads, funded research and associated Large Program
This research shares obvious links with that on dynamos of low-mass stars,
as magnetic fields of most youngs low-mass stars are presumably generated through dynamo processes.
It also has links with that on magnetic fields of planet-hosting stars and
should provide it with information on how, where and when close-in giant planets form and migrate.
Links with the research thread on magnetic fields of massive stars
are likely to build up in the near future, when observations clearly demonstrate that magnetospheric
accretion processes play a significant role in the formation of Herbig Ae/Be stars.
The two first issues of this research thread (accretion discs and magnetospheric accretion) are the
main focus of an ANR-funded research program
MAPP (MAgnetic Protostars and Planets). The corresponding
Large Program with ESPaDOnS at CFHT was awarded time in 2008a; additional data will
simultaneously collected with NARVAL at TBL.
Core team and collaborators
Thread coordinators: JF Donati, J Bouvier, F Ménard, R Pudritz
Agency/Country Core team Collaborators F Bouvier, Chabrier, Donati, Dougados, Ferreira, Fromang, Henebelle, Ménard, Rincon, Terquem
Baraffe, Casse, Catala, Montmerle, Moutou O Cameron, Gregory, Harries, Hussain, Jardine, Mohanty, Unruh
Küker C Banerjee, Duffin, Houde, Ouyed, Pudritz
Wade, V Petit T Trung
US Johns-Krull, Kurosawa, Long, Romanova, Valenti
Feigelson, Kulkarni, Lovelace
| Agency/Country | Core team | Collaborators |
| F | Bouvier, Chabrier, Donati, Dougados, Ferreira, Fromang, Henebelle, Ménard, Rincon, Terquem | Baraffe, Casse, Catala, Montmerle, Moutou |
| O | Cameron, Gregory, Harries, Hussain, Jardine, Mohanty, Unruh | Küker |
| C | Banerjee, Duffin, Houde, Ouyed, Pudritz | Wade, V Petit |
| T | Trung | |
| US | Johns-Krull, Kurosawa, Long, Romanova, Valenti | Feigelson, Kulkarni, Lovelace |