Gregg Hallinan NRAO & UC Berkeley E mail: gregg@astro.berkeley.edu Image credit: Hubble January 2012 > Caltech Astronomy with Adaptive Optics on Moderate sized Telescopes IUCAA, August 22 25, 2011
Mass-Luminosity relationship Mass-Luminosity-Age relationship Adapted from Reiners (2007)
Evolutionary Models for Brown Dwarfs Evolutionary models attempt to predict the physical characteristics of brown dwarfs. Stellar Substellar Such models attempt to incorporate effect of parameters such as the presence of magnetic fields and the onset of dust formation. Empirical data required to constrain such models. Chabrier et al. 2000, Burrows et al. 1997 We need to weigh brown dwarfs...
Weighing Up Matters: The binary L dwarf 2M0746+20 - L0+L1.5 dwarf - Separation: 0.1-0.25 arcseconds - Orbital Period: 12.71 years - Total Mass = 0.146 +0.016-0.006 M sun Bouy et al. A&A 2004
From NGS to LGS AO LGS AO allows more accurate measurement of existing sample and dramatically expands number of possible targets. NGS > 2 binary systems LGS > >25 binary systems Spectral range M7 T5.5 See Konopacky et al. 2010 ApJ, 711, 1087 and Dupuy et al. 2011 arxiv:1103.5747 Keck Laser Guide Star Adaptive Optics (LGS-AO)
Results from LGS AO N E 0.25 Konopacky et al. 2010 ApJ, 711, 1087 2M0746+20: Improved determination of total mass = 0.151 ± 0.003 M sun However, all such measurements determine relative orbital parameters. Individual masses poorly constrained crucial for constraining evolutionary models.
Results from LGS AO Dupuy, Liu & Ireland 2011 Improved mass measurements suggest discrepancy with models. However these measurements are still model dependent.
Results from LGS AO Konopacky et al. 2010 ApJ, 711, 1087 Use radial velocity measurements to constrain individual masses. Results are still very poorly constrained: 0.12 +0.01 0.09 M sun and 0.03 +0.09 0.01 M sun for the primary and secondary respectively. Need to be improved by a factor of 10 need absolute orbit determination.
Brown Dwarfs are Radio Stars Hallinan et al. 2007ApJ...663L..25H Two such radio active brown dwarfs are in tight binary systems...
EVLA observations of 2M0746+20 12.5 hours of data with 2 GHz bandwidth. RMS noise <1.5 μjy.
Dynamical Mass Measurement Using VLBI The fortuitous presence of an in beam calibrator at a distance of 10 arcseconds allows astrometric accuracy of ~10 15 microarcseconds 3 year multi observation campaign required to measure dynamical mass of 2M0746+20. Will measure the masses of the individual components of a binary L dwarf system for the first time.
Global VLBI Requires a very large allocation of resources. 50 hours of global VLBI over 3 years.
What about Robo AO on the Palomar 60? The proposed astrometric accuracy of Robo AO on the P60 is on par with LGS AO on Keck. Sample can be observed within 2 hour period every few months. Drawbacks: Observations will need to be conducted in J, H, K or possibly i. mk = 9 15. Lower resolution may rule out some systems. Possible additional science goal The isoplanatic angle of Robo AO on P60 is such that a source with Mv < 17 will probably be within the FOV. Can this source can be used as an astrometric reference source to determine the absolute orbit of tight binary brown dwarf systems?
Relative vs Absolute Orbit External ref source
Conclusions Dynamical mass measurements of brown dwarf binary systems are ongoing using LGS AO on 8 10m class telescopes. Such measurements determine the total mass of the system only. Additional radial velocity measurements are required for individual mass determination. Not yet possible for L and T dwarfs. VLBI observations are ongoing to try and determine the absolute orbit of the components of a binary system relative to an external field source. Very resource expensive. Robo AO allows much cheaper astrometric monitoring of tight binary systems. Very competitive against larger LGS AO systems. However, may be limited to the wider binary systems. May even allow absolute orbital determination relative to field sources.