Are magnetically-powered phenomena on brown dwarfs similar to or very different from M dwarfs?

Jeffrey L. Linsky

JILA/University of Colorado and NIST

The EVLA Vision:

Stars on and off the Main Sequence

Socorro NM

26-28 May 2009

Outline for this “tale of thermonuclear failure and its many consequences”

• Low mass “objects” do not have stable thermonuclear reactions to heat their cores and halt the gravitational contraction → rapid rotation and degenerate convective cores → secular cooling of the atmosphere.

• Cool atmospheres have very low ionization → decoupling of turbulent flows and magnetic fields until very deep in the atmosphere → photospheric magnetic fields with very little free energy.

• Very small Rossby numbers predict saturated activity if brown dwarfs are like M dwarfs, but LX is weak. Why?

• Weak coronal heating (due to small convective speeds and near potential magnetic fields) → low density coronae → gyrosynchrotron radio emission (LR~nrelB2E2) but weak X-ray emission (Lx~ne

2f(T)).

Spectra (not mass) determine star types (Burgasser in Physics Today June 2008)

Evolutionary tracks (age and mass)(Burrows et al. ApJ 491,856 (1997))

• Theoretical BD models from Burrows et al. (Rev. Mod. Phys. 73, 719 (2001)).

• With time: Tc and ρc increase.

• Blue: H burning stars (0.075-0.2 Msun).

• Green: BDs that burned D and now have electron degenerate cores and cool.

• Red: BDs that did not burn D (0.3-13 MJ).

• Dots mark ages when 50% of D and Li burned in core.

• Cores are convective metallic H/He mixtures

The interior structure of a star changes with decreasing mass

• Central temperatures (Tc) decrease with lower mass

• Central densities (ρc) increase then decrease

• Cores degenerate when (ψ=kT/kTF <0.1)

• Core fully convective for M<0.35Msun (M3 V)

• Uncertainties: EOS, convection in molecular atmosphere, opacities.

• Jupiter: M=0.001Msun

Chabrier

Solid lines (t=5Gyr), dashed lines (t=108 yr)(Chabrier & Baraffe ARAA 38, 337 (2000)

From M dwarfs to brown dwarfs:TC as function of mass and age

• Dashed lines indicate temperatures for burning H, Li, & D.

• Minimum mass for burning H is 0.075Msun (about M8 but depends on age)

Chabrier & Baraffe (2000)

Brown dwarf photosphere models

• There are nonLTE radiative/convective equilibrium models for M, L, T dwarfs and gas giant planets by Allard, Hauschildt, Tsuji, etc.

• Major issues include: completeness of molecular opacities, convection (ML or 3D hydro), dust formation and opacity, initial conditions for young (t<few Myr) BDs.

• Photospheres are neutral and the depth where ionization becomes important increases to later spectral type.

• With decreasing Teff , the magnetic field and convective motions are uncoupled deeper into the star. This will be important for MHD coronal heating and structure. Very different from the Sun and M dwarfs.

• H2 dissociation produces small (dT/dh)ad and low vconv

Saturation at small Rossby numbers(Reiners et al. ApJ 692, 538 (2009))

• Rossby number = R0=Prot/τconv

• For M dwarfs and hotter stars, saturation of Bf, Lx/Lbol, and LHα/Lbol at R0<0.1

• For R0>0.1, activity indicators depend on rotation (and age).

• All seven M3.5-M6 rapid rotators (vsini>5) are in saturation regime.

• Saturation behavior for both fully convective stars and stars with radiative cores.

All L dwarfs are rapid rotators and in the saturation regime

(Reiners & Basri ApJ 684, 1390 (2008))

Rotational evolution of M and BDs: observations and theory (Reiners & Basri (2008))

• Blue = young stars; red = old stars• Solid lines: rotation models for stars of mass 0.06 – 0.10 Msun• Dashed lines: isochrones for 2, 5, 10 Myr (upper left to lower right)• Theory: gravitation contraction and a magnetic-wind breaking law

depending on mass and Teff (lower convective speed → decreased coronal heating and lower mass loss and angular momentum loss).

Change in properties from M stars to BDs(Berger et al. ApJ 676, 1307 (2008))

Parameter Early M VB 10 LSR1835

Sp. Type M0-M6 M8 M8.5

Log(Lx/Lbol) Up to -3 -4.1to -5 <-5.7

Flares? yes yes ?

Log(LHα/Lbol) Up to -3.5 -4.4 -4.5 to -5

UV(2600Å)/photo Large 10xphot ~photo

Log(νLR/LX) -15.5 -13.2 >-11.3

vsin i (km/s) 3-5 6.5 ~50

Mass/Sun 0.1-0.3 0.08? 0.06?

H burning core? Yes Yes? No?

Late M-BD stars are in saturation regime (log R0<0.1) but LX/Lbol below hotter stars

Violation of the radio vs. X-ray luminosity law of Guedel & Benz (2003) (cf. Berger et al. 2008)

Failure of acoustic heating

BD atmospheres have very low ionization and high resistivity

• Fractional ionization very small except deep in the atmosphere

• High diffusion rate means that the magnetic field has lowest energy (potential) and no twist.

Magnetic Reynolds number and coupling of B to convective motions• Rm=vLv/ηd~BT/B0

(advection/diffusion)• For Teff<1700 K,

photospheric motions completely decoupled from B except deep in photosphere.

• Untwisted fields have no free energy and cannot heat a chromosphere or corona by reconnection.

• With decreasing Teff, dynamos can operate deeper in the star.

A modest proposal for explaining the violation of the radio vs. X-ray luminosity law

• BDs have cool neutral atmospheres → magnetic fields have little free energy in the photosphere → low heating rate for the corona.

• Coronae are cooler with small pressure scale heights and low densities. Coronal magnetic field reconnections likely occur where density very low. (Extremely low β.)

• LX~ne2f(T). More sensitive to density than T.

• LR~nrelB2(ε/m0c2)2 (if gyrosynchrotron emission). Electron energy distribution (power law) is critical.

• What mechanisms could stress coronal magnetic fields? (1) stellar differential rotation, (2) interactions with magnetic fields or winds of a “roaster”, (3) emergence of new fields from below, etc.

Possible scenarios for heating BD chromospheres (Hα, UV) and coronae (X-rays, radio, flares)

• Important papers: Mohanty et al. (ApJ 571, 469 (2002)); Meyer & Meyer-Hofmeister (A&A 341, L23 (1999)).

• Acoustic wave fluxes (Fac~v8conv) fail to explain

Hα emission of L dwarfs by 3-5 orders of magnitude.

• Energy not from photospheric turbulent flows twisting the magnetic fields because little coupling to magnetic fields.

• Differential rotation could be the energy source for winding the field lines in the corona and eventual dissipation.

How are flares possible in BDs?

• Intermittent events are possible.• Mohanty et al. (2002) suggest that some

emerging twisted flux ropes may be thick enough to emerge through the photosphere without being diffused.

• Consider a bootstrap scenario in which emerging twisted flux ropes heat and ionize the surrounding atmosphere and reduce the neutrality. (Perhaps by current dissipation.)

• Flares on BDs may have different properties from M dwarfs because the surrounding gas is low density (e.g., strong nonthermal radio emission with little thermal X-ray emission).

LHα/Lbol as a function of vsini and spectral type(Reiners & Basri (2008))

• Filled circles: near M9; open circles: M9-L1; filled squares: L1.5-L3; open squares: L3.5 and later.

• Late M dwarfs saturated or rotation-activity (at low vsini).• BDs far below M dwarfs at all vsini due to lower heating rates. No

rotation-activity as previously found by Mohanty & Basri (2003).

Evolution of stellar effective temperatures as function of mass

• For M<0.075Msun,Teff decreases with age.

• An ambriguity: at a given Teff, there are young low mass stars and old higher mass stars.

• As Teff decreases the atmospheres become far less ionized and poor electrical conductors.

Chabrier & Baraffe (2000)