Activity, rotation and weather in Ultracool Dwarfs

1. Activity, rotation and weather in Ultracool Dwarfs First NAHUAL meeting, La Gomera Eduardo L. Martín, IAC

2. Outline • Introduction on brown and ultracool dwarfs • Rotation observations • H observations • X-ray and radio observations • Weather observations • Final Remarks

3. The 1997 census of the solar neighborhood Henry 1998 in BDExp

4. Martin et al. 2005, RevMex AA The 2004 census

5. 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 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.

6. 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.

7. 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.

8. 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.

9. 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.

10. 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.

11. 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?

12. 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

13. 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.

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

15. 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%

16. 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.

17. Berger 2002, 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

18. Radio Emission in an L dwarf • 2MASS 0036+18, L3.5 • Unusual flare profile and variable persistent emission • No evidence of H emission

19. 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

20. 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%

21. 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.