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SPIRou : investigating magnetised star/planet formation

Par Jean-Francois Donati - 4/07/2008


SPIRou : investigating magnetised star/planet formation

Whereas the understanding of most phases of stellar evolution made considerable progress throughout the whole of the twentieth century, stellar formation remained rather enigmatic and poorly constrained by observations until about three decades ago. One major discovery obtained at this time is that protostellar accretion discs are often associated with are often associated with bipolar flows (eg Snell et al 1980, ApJ 239, L17), now known to be powerful, highly-collimated jets escaping the disc along its rotation axis. These jets (and in particular their collimation) have been attributed to the presence of magnetic fields and to the so-called magneto-centrifugal processes (eg Pudritz & Norman, 1983, ApJ 274, 677). Another important discovery is that low-mass magnetic protostars are rotating significantly slower than predicted by a non magnetized collapse (eg Bertout 1989, ARA&A 27, 351); this is likely due to the large-scale magnetic field coupling the protostar to its accretion disc (eg Königl, 1991, ApJ 370, L39). Both results demonstrate that magnetic fields play a central role throughout stellar formation.

For exploring magnetic fields (through Zeeman polarisation of spectral lines) in star forming regions, and in particular in low-mass protostars and protostellar accretion discs, a nIR spectropolarimeter is optimally suited (with respect to an optical instrument), Zeeman splitting in magnetically sensitive spectral lines being much stronger in the nIR. Moreover, low-mass protostars are often quite cool, the youngest ones being often surrounded by a dust envelope (mostly opaque at visible wavelengths); studying them at nIR wavelengths is by far the optimal solution. By detecting Zeeman signatures in young stars/discs and by monitoring them as the stars/discs rotate, we will be able to model their large-scale magnetic topologies (eg Donati et al 2007, MNRAS 380, 1297) and derive a wealth of brand new constraints on how magnetic fields impact the birth of stars and their planetary systems, how they participate in launching jets and how they control their angular momentum history.

For this program, we need high spectral resolution (>50,000) to provide as much details as possible on the shape of Zeeman signatures; we also need the largest possible spectral domain accessible in a single exposure (ie 0.98-2.5µm, the YJHK bands) to ensure that (i) we maximise the number of spectral lines from which Zeeman signatures are extracted and improve the accuracy to which magnetic fields are detected and modeled (ii) we collect as many different spectral proxies as possible (eg lines formed at the footpoints of accretion funnels, or at the inner rim of the accretion disc) to monitor the magnetospheric accretion processes simultaneously with (and further constrain) the magnetic topology. Magnetically sensitive Ti lines @ 2.22µm, as well as CO lines @ 2.31µm have often been used in this aim (eg Johns Krull 2007, ApJ 664, 975); many other atomic and molecular lines are also suitable for this purpose throughout the whole YJHK bands. With good RV stability (10m/s), we can also investigate whether close-in giant planets are already present around forming protostars (ie before their disc is fully dissipated).

With an efficient high-resolution nIR spectrograph like SPIRou, we should be able to access more that 200 young objects with mK<11 for such studies. This program would typically require as much as 50 nights/yr over a period of 5yr to investigate a large enough sample of protostars and protostellar discs and study how their large-scale magnetic topologies correlate with fundamental parameters (such as mass, age, rotation rate, outflow properties, disc density distribution) and vary on a timescale of a few yr (to look for activity or accretion cycles, or at planet migration within the inner regions of protostellar accretion discs like FUOrs).



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