Transcript Document
ASTEROSEISMOLOGY CoRoT session, January 13, 2007 Jadwiga Daszyńska-Daszkiewicz Instytut Astronomiczny, Uniwersytet Wrocławski European Helio- and Asteroseismology Network Participants HELAS Activities: Global Helioseismology Local Helioseismology Asteroseismology Public Outreach CoRoT Mission Sir Arthur Eddington (1882 – 1944) „At first sight it would seem that the deep interior of the sun and stars is less accessible to scientific investigation than any other region of the universe.” Asteroseismology Investigation of stellar interiors by means of the oscillation frequencies aster – from Greek means star seismos – Gr. quake, tremor logos – Gr. word, reason Helioseismology helios – Gr. Sun Pulsating star - star in which variability is due to pulsations, i.e. acoustic and/or gravity waves propagating in its envelope and interior. Changes of the brightness and/or the radial velocity are the observed evidences of pulsations. WHY STARS PULSATE ? 1. self-excitation 2. excitation by an external force Ad. 1. there are regions in a star which work like a heat engine, e.g. pulsation of classical Cepheids Ad. 2. stochastic excitation by turbulent convection in the nearsurface regions, e.g. solar-like oscillations When a Cepheid envelope begins to shrink (red arrows), it is almost transparent for the outgoing radiation (brown arrows). This phase corresponds to the onset of the compression stroke in an internal combustion engine. In the phase of maximum compression the envelope absorbs outgoing radiation and begins to expand. This phase corresponds to the ignition at the beginning of the combustion stroke. The driving zone has to be located at an optimal geometrical depth in the stellar envelope. The driving region located too shallow the amount of the energy absorbed by thin matter will be insufficient to maintain pulsations The driving region located too deep the amplitude of the temperature variations is very small and the layer will absorb too small amount of energy to be efficient In a star cooler than Teff~5500K convection prevents the accumulation of heat and pressure. log (L/L) A star hotter than Teff~7500K has regions of partial ionization too close to the surface. Blue edge of the classical instability strip Red edge of the log Teffclassical instability strip Various types of pulsating stars in the HR diagram J. Christensen-Dalsgaard The sound waves are generated by a stochastic velocity field in the near-surface convection, where turbulent motions have speeds close to the speed of sound. These waves propagate into the interior and produce the standing waves. Solar oscillations are damped oscillations excited stochasticaly by near-surface convection. The main effect of excitation takes place in a thin subphotospheric layer, where the speeds are close to the sound speed, cs. The Sun as pulsating star 5-minute oscillations of the Sun were discovered in 1962. amplitudes of the brightness variations: ~2 mag amplitudes of the radial velocity variations: ~20 cm/s oscillations periods: 3-25 min lifetimes of modes: of the order of days, weeks number of modes: ~ 107 HOW STARS PULSATE ? 1-dimensional oscillations Fundamental First overtone Second overtone nodes D. Kurtz 2-dimensional radial oscillations Fundamental First overtone Second overtone 3-dimensional radial pulsations with n = 2 2-dimensional non-radial oscillations dipole =1 quadrupole =2 3-dimensional non-radial oscillations = 3 W. Zima = 1, m=0 = 1, m=1 T. Bedding = 2, m=1 = 2, m=2 = 3, m=0 = 3, m=2 = 3, m=1 = 3, m=3 = 4, m=1 = 4, m=2 = 4, m=4 = 5, m=0 = 5, m=2 = 5, m=3 = 8, m=1 = 8, m=2 = 8, m=3 CAN WE HEAR STELLAR PULSATIONS ? NO ! BUT WE CAN OBSERVE THE EFFECTS OF PULSATIONS Mira ( Cet ) – the first pulsating star discovered in 1596 by David Fabricius. Visual magnitude: from 3.5 to 9, period equal to 332 days Doppler shift can be used to derive radial velocity Line profile variations Amplitude Asteroseismology Pulsation frequency [c/d] =2 = 20 = 25 = 75 http://astro.phys.au.dk/helio_outreach SEISMIC MODEL OF THE STAR theoretical frequencies = observed frequencies Which constraints can be obtained from asteroseismology ? Mass Age Chemical abundance Efficiency of convection Test of atomic data (opacities) Internal rotation Helioseismology Oscillation frequencies can be used to yield information on the structure and dynamics inside the Sun. Periodogram from the radial velocity measurements on the Sun (BiSON experiment) What have we learnt from helioseismology ? Age of the Sun Depth of convection zone Test of opacities, equation of state Helium abundance Internal rotation rate of the Sun Inferred rotation rate of the Sun as a function of radius for indicated heliographic latitudes; from MDI data. J. Christensen-Dalsgaard Rotation of the Sun J. Christensen-Dalsgaard Local helioseismology L. Gizon ASTEROSEISMOLOGY: THE MUSIC OF THE SPHERES The audible range from 20 to 20.000 Hz 1 cycle per second = 1 Hz 5 min 0.003 Hz „SOUNDS” OF PULSATIONS The Sun Centauri Hydrae Zoltan Kollath