A magnetic field evolution scenario for brown dwarfs and giant planets

Abstract

Very little is known about magnetic fields of extrasolar planets and brown dwarfs. We use the energy flux scaling law presented by Christensen et al. to calculate the evolution of average magnetic fields in extrasolar planets and brown dwarfs under the assumption of fast rotation, which is probably the case for most of them. We find that massive brown dwarfs of about 70 M Jup can have fields of a few kilo-Gauss during the first few hundred Million years. These fields can grow by a factor of two before they weaken after deuterium burning has stopped. Brown dwarfs with weak deuterium burning and extrasolar giant planets start with magnetic fields between ∼100 G and ∼1 kG at the age of a few Myr, depending on their mass. Their magnetic field weakens steadily until after 10 Gyr it has shrunk by about a factor of 10. We use observed X-ray luminosities to estimate the age of the known extrasolar giant planets that are more massive than 0.3 M Jup and closer than 20 pc. Taking into account the age estimate, and assuming sun-like wind-properties and radio emission processes similar to those at Jupiter, we calculate their radio flux and its frequency. The highest radio flux we predict comes out as 700 mJy at a frequency around 150 MHz for τ Boo b, but the flux is below 60 mJy for the rest. Most planets are expected to emit radiation between a few Mhz and up to 100 MHz, well above the ionospheric cutoff frequency.

Linear polarization of rapidly rotating ultracool dwarfs

“…-Strong magnetic fields can also produce linear polarization by Zeeman splitting of atomic and molecular lines or by synchrotron emission. X-ray and radio observations show that ultracool dwarfs can host magnetic fields in the range of 0.1-3 kG (Neuhauser & Comeron 1998;Berger et al 2005;Berger 2006;Berger et al 2008Berger et al , 2010, which agrees with predictions from numerical simulations (Reiners & Christensen 2010). Leroy (1995) linear polarization amplitude is a function of the intensity of the magnetic field and the number of atomic lines in the stellar spectra.…”

Star-planet magnetic interaction and activity in late-type stars with close-in planets

“…Here, we adopt a field as strong as possible, i.e., B p = 100 G, which is about seven times the Jupiter's field, because P rec and P hel scale as B 2/3 p0 and we aim at maximizing the energy dissipation rate. Such extreme fields cannot be excluded in the framework of the dynamo models proposed by Christensen et al (2009) and Reiners & Christensen (2010) provided that the internal heat flux of the planet, which controls the strength of the field, is sufficiently high. Since the dissipated power scales as B 2/3 p0 , adopting a field of 15 G, close to the value of Jupiter, will reduce the estimated powers by a factor of ∼3.7.…”

“…(45) and (8), respectively, with B 0 = 10 G in all the cases with the exception of HD 189733 for which B 0 = 40 G. The planetary field strength B p0 = 100 G in all the cases to maximize the energy dissipation rate as in the case of linear force-free fields. Assuming B p0 = 15 G -a much more realistic value in view of the models of Reiners & Christensen (2010) for planets with ages of at least 1−2 Gyr, and the field observed in Jupiter -the given powers are reduced by a factor of ∼3.7. The variation of αB φ across the planetary magnetosphere does not exceed 5 percent in all the cases, thus our assumption of a constant coronal field over the volume of the planetary magnetosphere is well justified.…”

Magnetic Fields from Low-Mass Stars to Brown Dwarfs

“…Additionally, Christensen et al (2009) showed that the scaling law between surface magnetic field strength and convective energy density initially derived by Christensen and Aubert (2006) from geodynamo simulations, can account for the observed field strengths of a number of objects including the Earth, Jupiter as well as rapidly rotating main sequence M dwarfs and young T Tauri stars. The concept of a dynamo continuum from planets to low-mass stars is already at the root of several studies (e.g., Reiners and Christensen 2010;Morin et al 2011;Schrinner et al 2012) and will likely result in new advances in the forthcoming years.…”