20 research outputs found
Formation of stars and planets: the role of magnetic fields
Star formation is thought to be triggered by gravitational collapse of the
dense cores of molecular clouds. Angular momentum conservation during the
collapse results in the progressive increase of the centrifugal force, which
eventually halts the inflow of material and leads to the development of a
central mass surrounded by a disc. In the presence of an angular momentum
transport mechanism, mass accretion onto the central object proceeds through
this disc, and it is believed that this is how stars typically gain most of
their mass. However, the mechanisms responsible for this transport of angular
momentum are not well understood. Although the gravitational field of a
companion star or even gravitational instabilities (particularly in massive
discs) may play a role, the most general mechanisms are turbulence viscosity
driven by the magnetorotational instability (MRI), and outflows accelerated
centrifugally from the surfaces of the disc. Both processes are powered by the
action of magnetic fields and are, in turn, likely to strongly affect the
structure, dynamics, evolutionary path and planet-forming capabilities of their
host discs. The weak ionisation of protostellar discs, however, may prevent the
magnetic field from effectively coupling to the gas and shear and driving these
processes. Here I examine the viability and properties of these
magnetically-driven processes in protostellar discs. The results indicate that,
despite the weak ionisation, the magnetic field is able to couple to the gas
and shear for fluid conditions thought to be satisfied over a wide range of
radii in these discs.Comment: Invited Review. 11 figures and 1 table. Accepted for publication in
Astrophysics & Space Scienc
Outgassing History and Escape of the Martian Atmosphere and Water Inventory
The evolution and escape of the martian atmosphere and the planet’s water inventory can be separated into an early and late evolutionary epoch. The first epoch started from the planet’s origin and lasted ∼500 Myr. Because of the high EUV flux of the young Sun and Mars’ low gravity it was accompanied by hydrodynamic blow-off of hydrogen and strong thermal escape rates of dragged heavier species such as O and C atoms. After the main part of the protoatmosphere was lost, impact-related volatiles and mantle outgassing may have resulted in accumulation of a secondary CO2 atmosphere of a few tens to a few hundred mbar around ∼4–4.3 Gyr ago. The evolution of the atmospheric surface pressure and water inventory of such a secondary atmosphere during the second epoch which lasted from the end of the Noachian until today was most likely determined by a complex interplay of various nonthermal atmospheric escape processes, impacts, carbonate precipitation, and serpentinization during the Hesperian and Amazonian epochs which led to the present day surface pressure