Activity in Very Cool Stars: Magnetic Dissipation in Late M and L Dwarf Atmospheres

By S. Mohanty, G. Basri, F. Shu, F. Allard, G. Chabrier. 2002

204 citations recorded by the source corpus.

Recent observations show that chromospheric Hα activity in late M and L dwarfs is much lower than in the earlier M types. This is particularly surprising given that the late M and L dwarfs are comparatively very rapid rotators: in the early M dwarfs, rapid rotation is associated with high activity levels. One possibility is that the drop-off in activity in the late M and L dwarfs is a result of very high electrical resistivities in their dense, cool, and predominantly neutral atmospheres.We calculate the magnetic field diffusivity in the atmospheres of objects with Teff in the range 3000-1500 K (mid M to late L) using the atmospheric structure models of Allard and Hauschildt. We find that the combination of very low ionization fraction and high density in these atmospheres results in very large resistivities and thus efficient field diffusion. While both ambipolar diffusion and Ohmic decay of currents due to ion-electron collisions occur, the primary diffusion effects are due to current decay through collisions of charged particles with neutrals. Moreover, the latter resistivity is a strong function of both effective temperature and optical depth, increasing rapidly as either Teff or optical depth decreases. This has two implications: (1) Any magnetic field present is increasingly decoupled from atmospheric fluid motions as one moves from mid M to L. In the late M and L dwarfs, atmospheric motions cannot lead to equilibrium field configurations very different from potential ones. That is, the magnitude of magnetic stresses generated by atmospheric motions is very small in these objects. We quantify this effect by a simple Reynolds number calculation. (2) Even if magnetic stresses are easily produced by fluid motions in the hot interior (where the coupling between field and matter is good), their propagation up through the atmosphere will be increasingly hampered by the growing atmospheric resistivity as one moves from mid M to late L. Thus both the generation and propagation of magnetic stresses are increasingly damped with decreasing Teff in these cool dwarfs. As a result, the magnetic free energy available for the support of a chromosphere, and activity becomes smaller and smaller with later type. This can account for the observed drop in Hα activity from mid M to L, assuming that activity in these dwarfs is magnetically driven. To check the latter assumption, we estimate the emergent acoustic fluxes in these objects through a Lighthill-Proudman calculation. While the acoustic fluxes also decrease with decreasing Teff, they appear inadequate to explain the observed Hα fluxes in mid M to L dwarfs. In the absence of acoustic heating, magnetic heating indeed seems the most viable way of generating activity. We emphasize that our calculations are equilibrium ones and do not address time-dependent phenomena. We also do not examine the highest atmospheric layers, where correction is not expected, but which we show are rarefied enough to permit charged particles to remain coupled to the field. Finally, while our calculations do not address flares in late M and L dwarfs, we speculate that the latter could be created by buoyant flux tubes that are generated in the interior and rise rapidly through the atmosphere, dissipating their associated currents in the upper atmospheric layers.

Subfields: Astronomy and Astrophysics

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Citations in the corpus

Simulations of Core Convection in Rotating A-Type Stars: Differential Rotation and Overshooting (2003, background): “More generally, correlations between magnetic fields and rotation have been inferred in main-sequence stars ranging from F5 to M9 (Noyes et al. 1984; Mohanty et al. 2002), with these fields also thought to be a result of dynamo action driven by convection, whether occupying an envelope or the full…”
WEATHER ON OTHER WORLDS. II. SURVEY RESULTS: SPOTS ARE UBIQUITOUS ON L AND T DWARFS (2014, background): “However, in general the neutral atmospheres of L and T dwarfs are considered too electrically resistive to support magnetic starspots (Mohanty et al. 2002; Gelino et al. 2002; Chabrier & Küker 2006).”
Observations of Cool-Star Magnetic Fields (2012, background): “Another effect that probably bears some importance for magnetic field generation in verylow mass stars is growing atmospheric neutrality (Meyer and Meyer-Hofmeister, 1999; Mohanty et al., 2002).”
A VOLUME-LIMITED SAMPLE OF 63 M7–M9.5 DWARFS. II. ACTIVITY, MAGNETISM, AND THE FADE OF THE ROTATION-DOMINATED DYNAMO (2009, background): “This is probably explained by the growing neutrality of the atmosphere which makes the coupling between the magnetic fields and the atmosphere less efficient at lower temperature (Meyer & Meyer-Hofmeister 1999; Mohanty et al. 2002).”
Water loss from terrestrial planets orbiting ultracool dwarfs: implications for the planets of TRAPPIST-1 (2016, background): “Due to UCDs’ atmospheres low degree of ionization (Mohanty et al. 2002) and the high densities, magnetic breaking is inefficient and cannot counteract spin-up due to contraction.”
The First Direct Measurements of Surface Magnetic Fields on Very Low Mass Stars (2006, background): “One explanation for this is the extreme neutrality of the photospheres (Mohanty et al. 2002).”
RADIUS-DEPENDENT ANGULAR MOMENTUM EVOLUTION IN LOW-MASS STARS. I (2011, background): “For the latest type M dwarfs, this link becomes weaker, because the low atmospheric ionization (due to the low Teff) means that the generation of magnetic stresses, and hence activity, becomes inefficient even if the field is strong (Mohanty et al. 2002).”
Confirmation of the Electron Cyclotron Maser Instability as the Dominant Source of Radio Emission from Very Low Mass Stars and Brown Dwarfs (2008, background): “…of the photosphere and atmosphere with later spectral type, such that magnetic fields becomes decoupled from the increasingly cool and neutral atmosphere, reducing the dissipation of magnetic field energy and hence the heating of plasma (Meyer & Meyer-Hofmeister 1999; Mohanty et al. 2002).”
A Unified Near-Infrared Spectral Classification Scheme for T Dwarfs (2005, background): “As the photometric variations have been linked to the surface evolution of condensate dust clouds (Gelino et al. 2002; Mohanty et al. 2002), verifying the spectral stability of the earliest-type T standards, which are most affected by the photospheric condensates, should be a priority for future…”
Simulations of Dynamo Action in Fully Convective Stars (2007, background): “…chromospheric emission (West & Basri 2007, in preparation): these stars are not so cool that low conductivity effects are likely to play a major role (see Mohanty et al. 2002), so these observations raise further questions about how the magnetic fields of such stars are influenced by rotation.”
Chromospheric activity catalogue of 4454 cool stars (2018, background): “Such weak chromospheres could be a product of a neutral stellar atmosphere that could inhibit magnetic activity (Mohanty et al. 2002; Mohanty & Basri 2003).”
Chromospheric Activity, Rotation, and Rotational Braking in M and L Dwarfs (2008, background): “This effect is likely due to the growing neutrality of cool dwarfs’ atmospheres (Mohanty et al. 2002).”
Evolution of low-mass star and brown dwarf eclipsing binaries (2007, background): “The reason invoked for this is the low electrical conductivity and thus the lack of substantial current generation in their atmosphere (Mohanty et al. 2002; Reiners & Basri 2007).”
The Spectra of T Dwarfs. II. Red Optical Data (2003, background): “The detection of H emission in three T dwarfs appears to stand contrary to the reduced ionization fractions predicted in their very cool atmospheres (Mohanty et al. 2002; Gelino et al. 2002).”
Activity and Kinematics of Ultracool Dwarfs, Including an Amazing Flare Observation (2007, background): “Mohanty et al. (2002) find that the decrease is likely due to the high electrical resistivities in the cool, mostly neutral atmospheres of these ultracool objects, but the topic is still open to investigation.”
Spectroscopic Properties of Cool Stars in the Sloan Digital Sky Survey: An Analysis of Magnetic Activity and a Search for Subdwarfs (2004, background): “…et al. 2000b; Gizis et al. 2002), while the decline toward late M types may reflect difficulty in producing and/or maintaining surface magnetic fields due to the physics of turbulent dynamos and/or the increasingly neutral atmosphere (Hawley et al. 2000a; Fleming et al. 2000; Mohanty et al. 2002).”
Radio Observations of a Large Sample of Late M, L, and T Dwarfs: The Distribution of Magnetic Field Strengths (2006, background): “…standpoint, the data indicate that the decrease in X-ray luminosity by 3 orders of magnitude occurs over a narrow range in spectral type at about M7 (or TeA 2500Y2700 K), roughly the same spectral type where a transition to predominantly neutral atmospheres occurs (Mohanty et al. 2002).”
The T Tauri Phase Down to Nearly Planetary Masses: Echelle Spectra of 82 Very Low Mass Stars and Brown Dwarfs (2005, background): “…other hand, Mohanty & Basri (2003) have shown that such activity falls off sharply, at least in field dwarfs, at M9, with little or no H emission at later types; Mohanty et al. (2002) have further argued that this falloff is due to the increasing neutrality of the cool atmospheres with later type.”