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SPIRou : dynamo processes in brown dwarfs

Par Jean-Francois Donati - 20/10/2008


SPIRou : dynamo processes in brown dwarfs

In the last 20yrs, very-low-mass stars and brown dwarfs have triggered an enormous burst of interest. Very little was known about these objects before; despite considerable progress in recent years, their physics, internal structure and atmospheric properties are still poorly understood, with a number of important issues remaining unexplained. For instance, quantities as basic and fundamental as their radius and emergent spectra are still poorly reproduced by existing models; similarly, the magnetic topologies of late M and early L dwarfs are also poorly known, despite having an obvious impact on convection and therefore on the overall structure of these objects (eg Chabrier et al, 2007, A&A 472, L17). Radio observations demonstrate that strong large scale magnetic fields are likely present in brown dwarfs as late as at least L3 (Berger, 2006, ApJ 648, 629); spectropolarimetric data collected with ESPaDOnS (eg Donati et al 2006, Science 311, 633) demonstrate that large-scale magnetic topologies of M dwarfs can be reliably imaged, but late M and early L brown dwarfs are still out of reach due to their intrinsic faintness at optical wavelengths.

Observing at nIR wavelengths should drastically improve the efficiency of spectropolarimetric studies and make them applicable to late M and early L brown dwarfs that radiate most of their photons between 1-2µm. Moreover, Zeeman splitting in magnetically sensitive spectral lines is much stronger in the nIR, where spectra show a large number of both atomic and molecular lines. A nIR spectropolarimeter thus offers a unique opportunity of studying dynamo processes in brown dwarfs and give us the key to understand why these objects rotate significantly faster than the mid-M dwarfs and strongly violate the radio/X-ray flux correlation applying to all cool stars including the Sun; it will also allow us to study how magnetic fields affect convection and impact the overall structure of brown dwarfs. Tomographic techniques applied to molecular lines can also be used to study "weather" patterns on brown dwarfs.

For this program, we need high spectral resolution (>50,000) to provide as much details as possible on the shape of spectral lines and 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 we maximise the number of atomic (eg Ti @ 2.22µm) and molecular (eg FeH @ 1.00µm, CO @ 2.31µm) lines from which Zeeman signatures are extracted and improve the accuracy to which magnetic fields are detected and modeled. With an efficient high-resolution nIR spectropolarimeter, we are able to access more that 200 young objects with mJ<10 & mK<11 for such studies. This program would typically require as much as 25 nights/yr over a period of 5yr to investigate a large enough sample of late M and early L brown dwarfs.



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