Download Brown Dwarfs and M Dwarfs

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~ne2f(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.
Solid lines (t=5Gyr), dashed lines (t=108 yr)
C
• Jupiter: M=0.001Msun
(Chabrier & Baraffe ARAA 38, 337 (2000)
h
a
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
Sp. Type
Log(Lx/Lbol)
Flares?
Log(LHα/Lbol)
UV(2600Å)/photo
Log(νLR/LX)
vsin i (km/s)
Mass/Sun
H burning core?
Early M
M0-M6
Up to -3
yes
Up to -3.5
Large
-15.5
3-5
0.1-0.3
Yes
VB 10
M8
-4.1to -5
yes
-4.4
10xphot
-13.2
6.5
0.08?
Yes?
LSR1835
M8.5
<-5.7
?
-4.5 to -5
~photo
>-11.3
~50
0.06?
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)