THEORY AND OBSERVATION OF KINEMATICALLY OLD, LOW-METALLICITY OBJECTS
Ecole Normale Superieur de Lyon
We first examine the characteristic properties of metal-depleted low-mass stars and brown dwarfs, compared to their solar-metallicity counterparts: central and effective temperature, luminosity, radius. We present the metallicity dependence of the mass/effective-temperature relationship and of the hydrogen-burning limit. We give the characteristic magnitudes of the brown dwarf limit for subsolar metal abundances in various optical and infrared filters. We examine the expected photometric signatures of substellar objects, which stem from their very physical properties, in these filters.
Theory is then compared with observation in different (optical and infrared) color-magnitude diagrams (CMD) for various globular clusters with different metallicities observed with the Hubble Space Telescope (HST), including the most recent observations with the Near-Infrared Camera and Multi-Object Spectrometer (NICMOS). This yields the derivation of the characteristic bolometric luminosity functions for these clusters and of their present-day and initial mass functions. These latter are well represented by slowly rising power-law functions down to the bottom of the main sequence. We present the predicted luminosity functions for these clusters in the brown dwarf domain, which should be in reach with ongoing infrared projects. This yields the determination of the stellar and brown dwarf densities in various globular clusters.
We then turn to field high-velocity low-mass stars, characteristic of the spheroid (stellar halo) and dark halo populations. We first determine the metallicity dispersion of these objects from their photometric signature in CMD and the luminosity functions obtained from ground-based observations and from HST standard and deep-field observations. We transform these luminosity functions into mass functions. This yields the determination of the halo stellar density and halo/disk normalization down to the brown dwarf limit. Extrapolation into the substellar regime yields the maximum contribution of brown dwarfs to the spheroid and dark halo populations and to the galactic dynamical mass.
We examine the prospects for the search of stellar and substellar kinematically old objects with ongoing or near-future projects like the European Southern Observatory's (ESO) Imaging Survey (EIS), and the “Son of ISAAC” (SOFI) 1- to 2.5μm imager and spectrometer for ESO's New Technology Telescope (NTT).