The Evolution of L and T Dwarfs in Color-Magnitude Diagrams

Abstract

We present new evolution sequences for very low mass stars, brown dwarfs and giant planets and use them to explore a variety of influences on the evolution of these objects. We compare our results with previous work and discuss the causes of the differences and argue for the importance of the surface boundary condition provided by atmosphere models including clouds. The L- to T-type ultracool dwarf transition can be accommodated within the Ackerman & Marley (2001) cloud model by varying the cloud sedimentation parameter. We develop a simple model for the evolution across the L/T transition. By combining the evolution calculation and our atmosphere models, we generate colors and magnitudes of synthetic populations of ultracool dwarfs in the field and in galactic clusters. We focus on near infrared color- magnitude diagrams (CMDs) and on the nature of the ``second parameter'' that is responsible for the scatter of colors along the Teff sequence. Variations in metallicity and cloud parameters, unresolved binaries and possibly a relatively young population all play a role in defining the spread of brown dwarfs along the cooling sequence. We find that the transition from cloudy L dwarfs to cloudless T dwarfs slows down the evolution and causes a pile up of substellar objects in the transition region, in contradiction with previous studies. We apply the same model to the Pleiades brown dwarf sequence. Taken at face value, the Pleiades data suggest that the L/T transition occurs at lower Teff for lower gravity objects. The simulated populations of brown dwarfs also reveal that the phase of deuterium burning produces a distinctive feature in CMDs that should be detectable in ~50-100 Myr old clusters.

Accepted for publication in the ApJ. 52 pages including 20 figures

References in corpus (19)

• Line and Mean Opacities for Ultracool Dwarfs and Extrasolar Planets
• Atomic and Molecular Opacities for Brown Dwarf and Giant Planet Atmospheres
• Discovery of Two T Dwarf Companions with the Spitzer Space Telescope
• Hubble Space Telescope NICMOS Observations of T Dwarfs: Brown Dwarf Multiplicity and New Probes of the L/T Transition
• Ammonia as a tracer of chemical equilibrium in the T7.5 dwarf Gliese 570D
• M dwarfs: effective temperatures, radii and metallicities
• Consistent simulations of substellar atmospheres and non-equilibrium dust-cloud formation
• HD 203030B: an Unusually Cool Young Sub-Stellar Companion near the L/T Transition
• Clouds, Gravity and Metallicity in Blue L dwarfs: The Case of 2MASS J11263991-5003550
• Physical parameters of two very cool T dwarfs
• Keck Laser Guide Star Adaptive Optics Monitoring of 2MASS J1534-2952AB: First Dynamical Mass Determination of a Binary T Dwarf
• Physical and Spectral Characteristics of the T8 and Later-Type Dwarfs
• Evidence for a Turnover in the IMF of Low Mass Stars and Sub-stellar Objects: Analysis from an Ensemble of Young Clusters
• 3.6-7.9 um Photometry of L and T Dwarfs and the Prevalence of Vertical Mixing in their Atmospheres
• Space Velocities of L- and T-type Dwarfs
• Multiepoch Radial Velocity Observations of L Dwarfs
• Dynamical Mass of GJ 802B: a brown dwarf in a triple system
• Proper motion L and T dwarf candidate members of the Pleiades
• Pleiades low-mass brown dwarfs: the cluster L dwarf sequence

Cited by in corpus (3)

• The 0.8-14.5 micron Spectra of Mid-L to Mid-T Dwarfs: Diagnostics of Effective Temperature, Grain Sedimentation, Gas Transport, and Surface Gravity
• Keck Laser Guide Star Adaptive Optics Monitoring of the M8+L7 Binary LHS 2397aAB: First Dynamical Mass Benchmark at the L/T Transition
• An Age Constraint for the Very Low-Mass Stellar/Brown Dwarf Binary 2MASS J03202839-0446358AB