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

The 2004 census 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 CH 4. Teff<1400K. A 35M Jupiter 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

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=0.075M Sun =79M Jupiter. The lower limit of a brown dwarf mass is at the DBMM=0.012M Sun =13M Jupiter. Kumar 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

NIR high-resolution spectroscopy of a T dwarf. Eps Ind B is the nearest T dwarf known (d=3.6 pc), Scholtz et al Smith et al 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 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. 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. 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

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 1999

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

Radio Observations Very Large Array observations at 8.5 GHz of LP Quiescent and flaring emission B~5G from synchrotron theory. Duty cycle ~ 2.5% Berger et al. 2001, Nature

Violation of the Guedel-Benz Relation Coronal activity in G,K,M stars L R ~L X / Hz -1 Measured radio flux is at least 4 orders of magnitude higher than predicted. Guedel & Benz 1993 ApJ

Yet Another Surprise! BRI 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

Radio Emission in an L dwarf 2MASS , 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. 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.