Activity, rotation and weather in Ultracool Dwarfs

First NAHUAL meeting, La Gomera

Eduardo L. Martín,

IAC

Outline

• 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 Henry 1998 in BDExp

The 2004 census

Martin et al. 2005, RevMex AA Martin et al. 2005, RevMex AA

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.

Martin et al. 1997,1998,1999; Kirkpatrick et al. 1999; 2000; Burgasser et al. 2000,2001; Leggett et al. 2001; Geballe et al. 2002; Cushing et al. 2002

Martin et al. 1997,1998,1999; Kirkpatrick et al. 1999; 2000; Burgasser et al. 2000,2001; Leggett et al. 2001; Geballe et al. 2002; Cushing et al. 2002

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.

Kumar 1963; D’Antona & Mazzitelli 1995; Saumon et al. 1996; Chabrier & Baraffe 2000Kumar 1963; D’Antona & Mazzitelli 1995; Saumon et al. 1996; Chabrier & Baraffe 2000

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.

Kippenhahn 1970; Martin et al. 1997; Basri et al. 2000; Reid et al. 2002. Kippenhahn 1970; Martin et al. 1997; Basri et al. 2000; Reid et al. 2002.

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.

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.

Joergens et al. 2003Joergens et al. 2003

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.

Gizis et al. 2000; Zapatero Osorio et al. 2002Gizis et al. 2000; Zapatero Osorio et al. 2002

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?

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

Liebert et al. 1999; Martin 1999; Reid et al. 2001; Martin & Ardila 2001 Liebert et al. 1999; Martin 1999; Reid et al. 2001; Martin & Ardila 2001

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.

Mohanty & Basri 2002; Meyer & Meyer-Hofmeister 1999Mohanty & Basri 2002; Meyer & Meyer-Hofmeister 1999

Fleming et al. 1993, 2000; Mokler & Stelzer 2002; Martin & Bouy 2002Fleming et al. 1993, 2000; Mokler & Stelzer 2002; Martin & Bouy 2002

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%

Berger et al. 2001, NatureBerger et al. 2001, Nature

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.

Guedel & Benz 1993 ApJ Guedel & Benz 1993 ApJ

Yet Another Surprise!

• BRI0021-0214 is another inactive fast rotating dM9.5 with persistent and flaring radio emission.

• It violates the Guedel-Benz relation by a factor of >1700

Berger 2002, ApJ Berger 2002, ApJ

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

Weather observations

• VLT/ISAAC and IRTF/Spex time series observations of late L and T dwarfs.

• No variability detected larger than 5%

Goldman et al. 2004Goldman et al. 2004

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.