Bibcode
Berger, E.; Rutledge, R. E.; Phan-Bao, N.; Basri, G.; Giampapa, M. S.; Gizis, J. E.; Liebert, J.; Martín, E.; Fleming, T. A.
Bibliographical reference
The Astrophysical Journal, Volume 695, Issue 1, pp. 310-316 (2009).
Advertised on:
4
2009
Journal
Citations
113
Refereed citations
103
Description
We present an 8.5 hr 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
nonvariable 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 misaligned 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 (1) the field is dominated by a
quadrupole configuration, which can naturally explain the 1/4 phase lag;
or (2) 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 (v sin i ≈ 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 sun. This is the first measurement of the radius of
an L dwarf, and along with a mass of 0.085 ± 0.010 M
sun it provides a constraint on the mass-radius relation
below 0.1 M sun. 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 misalignment between the
rotational and orbital axes.