activity, rotation and weather in ultracool dwarfs
DESCRIPTION
Activity, rotation and weather in Ultracool Dwarfs. First NAHUAL meeting, La Gomera Eduardo L. Mart í n, IAC. Outline. Introduction on brown and ultracool dwarfs Rotation observations H observations X-ray and radio observations Weather observations Final Remarks. - PowerPoint PPT PresentationTRANSCRIPT
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Activity, rotation and weather in Ultracool Dwarfs
First NAHUAL meeting, La Gomera
Eduardo L. Martín,
IAC
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Outline
• Introduction on brown and ultracool dwarfs
• Rotation observations
• H observations
• X-ray and radio observations
• Weather observations
• Final Remarks
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The 1997 census of the solar neighborhood
Henry 1998 in BDExp Henry 1998 in BDExp
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The 2004 census
Martin et al. 2005, RevMex AA Martin et al. 2005, RevMex AA
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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
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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
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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.
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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.
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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
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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
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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?
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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
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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
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Fleming et al. 1993, 2000; Mokler & Stelzer 2002; Martin & Bouy 2002Fleming et al. 1993, 2000; Mokler & Stelzer 2002; Martin & Bouy 2002
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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
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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
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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
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Radio Emission in an L dwarf
• 2MASS 0036+18, L3.5
• Unusual flare profile and variable persistent emission
• No evidence of H emission
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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
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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
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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.