Periodic Radio and Hα emission from the L dwarf binary 2MASSW J0746425+200032: exploring the magnetic field topology and radius of an L dwarf
Rutledge, R. E.
Giampapa, M. S.
Gizis, J. E.
Fleming, T. A.
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CitationBerger, E., R. E. Rutledge, N. Phan-Bao, G. Basri, M. S. Giampapa, J. E. Gizis, J. Liebert, E. Martín, and T. A. Fleming. 2009. Periodic Radio and Hα emission from the L dwarf binary 2MASSW J0746425+200032: exploring the magnetic field topology and radius of an L dwarf. The Astrophysical Journal 695, no. 1: 310–316. doi:10.1088/0004-637x/695/1/310.
AbstractWe present an 8.5-hour simultaneous radio, X-ray, UV, and optical observation of the L dwarf binary 2MASSW J0746425+200032. We detect strong radio emission, dominated by short-duration periodic pulses at 4.86 GHz with P = 124.32±0.11 min. The stability of the pulse profiles and arrival times demonstrates that they are due to the rotational modulation of a B ≈ 1.7 kG magnetic field. A quiescent non-variable component is also detected, likely due to emission from a uniform large-scale field. The Hα emission exhibits identical periodicity, but unlike the radio pulses it varies sinusoidally and is offset by exactly 1/4 of a phase. The sinusoidal variations require chromospheric emission from a large-scale field structure, with the radio pulses likely emanating from the magnetic poles. While both light curves can be explained by a rotating mis-aligned magnetic field, the 1/4 phase lag rules out a symmetric dipole topology since it would result in a phase lag of 1/2 (poloidal field) or zero (toroidal field). We therefore conclude that either (i) the field is dominated by a quadrupole configuration, which can naturally explain the 1/4 phase lag; or (ii) the Hα and/or radio emission regions are not trivially aligned with the field. Regardless of the field topology, we use the measured period along with the known rotation velocity (vsini ≈ 27 km s−1), and the binary orbital inclination (i ≈ 142◦), to derive a radius for the primary star of 0.078 ± 0.010 R⊙. This is the first measurement of the radius of an L dwarf, and along with a mass of 0.085±0.010 M⊙ it provides a constraint on the mass-radius relation below 0.1 M⊙. We find that the radius is about 30% smaller than expected from theoretical models, even for an age of a few Gyr. The origin of this discrepancy is either a breakdown of the models at the bottom of the main sequence, or a significant mis-alignment between the rotational and orbital axes.
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