Transcript The Birth and Evolution of Brown Dwarfs

Activity, rotation and weather in
Ultracool Dwarfs
First NAHUAL meeting, La Gomera
Eduardo L. Martín,
IAC
Outline
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Introduction on brown and ultracool dwarfs
Rotation observations
H observations
X-ray and radio observations
Weather observations
Final Remarks
The 1997 census of the solar
neighborhood
Henry 1998 in BDExp
Martin et al. 2005, RevMex AA
The 2004 census
Martin et al. 1997,1998,1999; Kirkpatrick et al. 1999; 2000; Burgasser et al. 2000,2001; Leggett e
al. 2001; Geballe et al. 2002; Cushing et al. 2002
Ultracool dwarfs (L, T)
• Two new spectral classes
have been defined for
ultracool dwarfs.
• The L class is
characterized by weak or
absent TiO, strong FeH,
and huge alkali lines.
Teff~2200K-1400K.
• The T class is
characterized by CH4.
Teff<1400K.
• A 35MJupiter evolves from
M-type at 10 Myr to T
type at 1 Gyr.
Kumar 1963; D’Antona & Mazzitelli 1995; Saumon et al. 1996; Chabrier & Baraffe 2000
Brown dwarfs
• A brown dwarf is defined primarily by its mass,
irrespective of how it forms.
• The low-mass limit of a star corresponds to the
minimum mass for stable Hydrogen burning.
• The HBMM depends on chemical composition
and rotation. For solar abundances and no rotation
the HBMM=0.075MSun=79MJupiter.
• The lower limit of a brown dwarf mass is at the
DBMM=0.012MSun=13MJupiter.
Kippenhahn 1970; Martin et al. 1997; Basri et al. 2000; Reid et al. 2002.
Rotation
• Projected rotational
velocities (vsini) have
been measured in 40 field
dwarfs M9-L6, using the
rotational broadening of
atomic and molecular
lines with Keck/Hires.
• Average vsini=21km/s,
corresponding to Prot~6hr
• Rotation makes the star
more degenerate, and
increases the HBMM.
NIR high-resolution spectroscopy
of a T dwarf.
• Eps Ind B is the nearest T
dwarf known (d=3.6 pc),
Scholtz et al. 2003.
• Smith et al. 2003 have
obtained R=50,000
spectroscopy with Phoenix
at Gemini South.
• Many spectral features for
accurate radial velocity
and rotational broadening
determination.
Joergens et al. 2003
Evolution of rotational periods
• Acceleration of the
rotation of brown dwarfs
due to contraction during
the first 50-100 Myr.
• Magnetic braking due to
interaction with a disk
may play a role.
• Lack of efficient braking
during most of dwarf’s
evolution.
Gizis et al. 2000; Zapatero Osorio et al. 2002
H activity
• H is a diagnostic of hot
plasma. It can be caused
by a chromosphere or by
an accretion boundary
layer (CTTS activity).
• The average H emission
level in young BDs is
higher than in the older
counterparts of the solar
vicinity. Accretion rates
are very low.
Field M,L,T Dwarfs
• The general trend is
that H activity level
declines with
decreasing
temperature
• A few very low-mass
dwarfs have
extraordinary
persistent H emission
• Interacting binaries?
Liebert et al. 1999; Martin 1999; Reid et al. 2001; Martin & Ardila 2001
H flares
• Duty cycle 1-3%
• Sometimes HeI, KI,
NaI, OI and CaII
emission, and blue
veiling
• Energy release can be
a few percent of
bolometric luminosity
Mohanty & Basri 2002; Meyer & Meyer-Hofmeister 1999
H-rotation connection breaks
down
• For SpT>M7 there is
no connection between
rotation and activity.
• In the neutral
atmospheres of L
dwarfs the magnetic
fields may be
decoupled from
convective motions.
Fleming et al. 1993, 2000; Mokler & Stelzer 2002; Martin & Bouy 2002
Berger et al. 2001, Nature
Radio Observations
• Very Large Array
observations at 8.5
GHz of LP944-20
• Quiescent and flaring
emission
• B~5G from
synchrotron theory.
• Duty cycle ~ 2.5%
Guedel & Benz 1993 ApJ
Violation of the Guedel-Benz
Relation
• Coronal activity in G,K,M stars
LR~LX/1015.5 Hz-1
• Measured radio flux is at least 4 orders of
magnitude higher than predicted.
Berger 2002, ApJ
Yet Another Surprise!
• BRI0021-0214 is
another inactive fast
rotating dM9.5 with
persistent and flaring
radio emission.
• It violates the GuedelBenz relation by a
factor of >1700
Radio Emission in an L dwarf
• 2MASS 0036+18,
L3.5
• Unusual flare profile
and variable persistent
emission
• No evidence of H
emission
Radio Activity does not Decline
in Very Low-Mass Dwarfs
• Contrary to H activity,
there is not a clear decline
of radio emission for
spectral types cooler than
M8. The decline may be
shifted to cooler
temperatures.
• Radio emission requires
magnetic fields B~5-20 G,
similar to Jupiter’s
Goldman et al. 2004
Weather observations
• VLT/ISAAC and
IRTF/Spex time series
observations of late L
and T dwarfs.
• No variability detected
larger than 5%
Final Remarks
• Coronal activity in brown dwarfs is scarce.
Possibly less RV jitter. Weather may also not be a
problem.
• H activity dies off quickly for SpT>M8, with a
few exceptions (interacting binaries?). NAHUAL
can test this possibility.
• Ultracool dwarfs tend to be fast rotators. Could
this limit the RV accuracy?
• Is activity switching from “stellar” to “planetary”
mode in the ultracool dwarfs? NAHUAL could be
used to measure zeeman splitting.