Transcript Slide 1
Post-main sequence evolution of debris discs Amy Bonsor Supervised by Dr. M. C. Wyatt Institute of Astronomy University of Cambridge Outline Observations of dusty discs Evolution of debris discs from main sequence to the white dwarf phase Population of debris discs around white dwarfs and how this relates to the observations Hot, dusty discs around white dwarfs e.g Farihi et al 2009, von Hippel et al 2007, etc..... Radii ~ Rsolar Tidally disrupted asteroids or comets Connected with metal rich DAZ white dwarfs Wavelength (m) Farihi et al 2009 Discs around white dwarfs Helix Nebula : a young white dwarf, surrounded by a planetary nebula. Su et al 2007 Spitzer 24, 70um, upper limit at 160um Radius 30-130AU Further observation in talks by Chu and Bilikova Connection with main sequence planetary systems? >400 exoplanet detections >33% of MS A stars have excess emission associated with a debris disc Debris discs are belts of dust and rocks analogous to the solar system's asteroid or Kuiper belt. Discs on the Main Sequence There are lots of observations.... Population of debris discs Fraction of subsample Fraction of subsample on the main sequence 24um Age (Myr) 70um Age (Myr) Constrain population of debris discs around main sequence A stars using the steady state collisional models of Wyatt et al 2007 and Spitzer observations at 24um and 70um. Population of debris discs around main sequence A stars Wyatt et al 2007 Bonsor & Wyatt 2010 http://arxiv.org/abs/1007.4517 Stellar evolution Hurley et al 2000 Population of debris discs around evolved stars Provide a theoretical framework that investigates all of the processes affecting a disc during its evolution What do our models include? Changes to the stellar luminosity, mass and effective temperature Bonsor & Wyatt 2010 Collisions Radiation pressure http://arxiv.org/abs/1007.4517 Poynting-Robertson drag Stellar wind pressure Stellar wind drag Sublimation (of silicates) White dwarf discs have large radii Adiabatic mass loss on timescales long compared to the orbital timescales for bodies in a disc cause material to spiral outwards. White dwarf White dwarf discs have low mass Collisions reduce the mass in the disc Number of particles Diameter Largest object present 5 km White dwarfs have faint discs. Disc temperature follows stellar luminosity Initial radius: 100AU Initial mass 30Mearth Star: 2.9 Msolar Distance: 10pc Spitzer at 70um Time (Myr) White dwarfs have faint discs. Only detect discs around young nearby white dwarfs. Dust around young white dwarfs Distance within which there is one WD with t< tcool Spitzer at 70um Herschel PACs at 70um Alma at 450um Max. distance at which WD disc is detectable 0.5 Msolar WD with a disc mass of 10-2 Mearth, observed with Spitzer at 70um Optimum age for detection ~ 1Myr @ ~200pc Helix Nebula Discs around white dwarfs White dwarfs have a population of cold, large radii, low mass, hard to detect Kuiper belt like discs. Observations find hot, dusty discs within tidal radius Can we link the two populations? Maybe the Kuiper-belts provide the reservoir of material required to replenish the hot discs? We just need a mechanism to move the material inwards ….. Stellar wind drag During high mass loss rates on AGB small bodies leave the disc due to stellar wind drag. Material left between the inner edge of the belt and the star at the end of the AGB Planet truncates belt The inner edge of a MS debris disc can be sculpted by a planet.star planet clears chaotic zone material scattered as star looses mass As the star looses mass the width of the chaotic zone increases and more material is scattered. Conclusions Bonsor & Wyatt 2010 Evolved known population of main sequence debris discs to the white dwarf phase, investigating the effects of stellar evolution on the disc. Implications for white dwarfs: •A population of cold Kuiper-like discs, only detectable around very young white dwarfs. •Provide reservoir of material to explain hot dusty discs