Mohanty, Subhanjoy

Contact Email: subu@astron.berkeley.edu
Institute: UC Berkeley
First Coauthor: Gibor Basri
Institute: UC Berkeley
Second Coauthor: Frank Shu
Institute: UC Berkeley
Presenting Thesis: Yes
Subject Area: Brown Dwarfs & Extrasolar Planets
Waveband: Multi-wavelength
Technique: Spectroscopy
Other: and MHD simulations
Presentation: Poster Display
Title: Activity in Very Cool Stars: Magnetic Dissipation in Late- M and L Dwarf Atmospheres
Abstract: We present rotational velocities and H-alpha activity for 65 mid-M to L dwarfs. We observe a distinct weakening in the rotation-activity connection in these objects. While the rotational velocity continues to increase from mid-M to L, the H-alpha activity only weakly follows rotation from mid- to late-M, and drops precipitously thereafter. For spectral type later than M9, we find a strong correlation instead between activity and spectral type. We interpret our results in terms of a predominantly neutral atmosphere that is rapidly decoupled, as the effective temperature is lowered, from any magnetic fields that may be present. This gives rise to the spectral- type / activity connection. We test this hypothesis by investigating the behaviour of the magnetic diffusion equation in these cool atmospheres. We find that the combination of very low ionization fraction and high density in these atmospheres results in very high field-diffusivities. Therefore the atmosphere is effectively decoupled from the field, and atmospheric motions cannot lead to equilibrium field configurations very different from potential ones. We confirm this through simulations of the equilibrium solution of the magnetic diffusion equation in the presence of atmospheric flows, using a simplified axisymmetric model. The rapid decay of currents also implies that any field-twisting produced in the atmosphere by fluid motions in interior layers (where the coupling between field and matter is good) is likely to dissipate before appreciable currents are built up in the atmosphere. All of this implies that the magnetic free energy available for the support of a corona, chromosphere and activity is very small, accounting for the low activity levels observed in late-M and L-dwarfs. Moreover, the field diffusivity due to neutral - charged particle collisions is inversely proportional to the fractional ionization, which is a strong function of spectral type in these cool objects. Thus, the marked trend of decreasing activity with later type observed in late-M and L dwarfs follows naturally from our analysis.