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THE BIRTH OF STARS AND PLANETARY SYSTEMS Stephen E. Strom National Optical Astronomy Observatory 07 January, 2003 Overview of Presentation • Theoretical overview • Confrontation with theory: – what we know and how we know it • Current key questions • Answering key questions Theory Stellar Conception • A star’s life begins in darkness, in an optically opaque molecular cloud • Shielded by dust and gas from galactic starlight and cosmic rays, the cloud cools • In the densest clumps of molecular gas, gravity overcomes internal pressure: clumps contract A Collapsing Molecular Clump Gravity ~ M/R2 Stellar Gestation • Clumps are initially spinning as well – a result of tidal encounters among clumps • Spinning, collapsing clumps produce: – a flattened envelope from which material flows toward a …. – circumstellar disk, through which material flows toward a…. – central, prestellar core (a “stellar seed”) Spinning Protostellar Core Forming the Star-Disk System Accretion Disk Infalling envelope Stellar seed Building a Full-Term Star • Gas and dust transported: envelope accretion disk stellar seed • Stellar mass builds up over time (~ 1 Myr) • Accreting material arises from regions that rotate – absent a way of slowing down the star, the star will rotate so rapidly that material is flung off the equator – a star cannot reach ‘full-term’ absent spin regulation • Stellar winds and jets act as ‘rotation regulators’ Building a Full-term Star Rotating accretion disk Accreting material Wind/Jet removes angular momentum Forming star A Star in Formation: Artist Conception Forming Planets • Planets form in circumstellar disks • Two processes may be operative: – disk instabilities leading to rapid agglomeration of gas into giant (Jupiter mass) planets during disk accretion phase – agglomeration of dust into km-size planetesimals • buildup of earth mass solid cores via planetesimal collisions • buildup of gas giants if enough disk gas is available Formation via Disk Instability Forming Jupiter Formation via Agglomeration; Collisions Growth of larger bodies via collisions Planetesimal swarm formed via collisions among small dust grains Mature planets Star and Planet Formation Summary Molecular Cloud Rotating Clump Forming Star + disk Confrontation with theory: What we know and how we know it Stellar Conception • Radio maps of molecular clouds reveal rotating pre-stellar clumps – diagnosed via tracers of dense, cold gas: CO, CS • Observations of multiple molecules provide – – – – temperature density clump mass kinematics: internal gas motions; rotation • Clump self-gravity exceeds internal pressure Star-Forming Molecular Cloud 30 Light Years Ophiuchus Molecular Cloud (d ~ 500 light years) Opaque Molecular Clump 0.2 light years Stellar Gestation • Doppler analysis (mm-wave) of gas motions shows – clumps are collapsing – clumps are rotating • Hubble Space Telescope observations reveal – flattened envelopes – opaque disks embedded within envelopes – central star • Doppler analysis (infrared) of gas motions shows – gas accreting onto the central star Disks and Envelopes Around Young Stars Building a Mature Star • Hubble space telescope observations reveal – disks of solar system dimension around young stars • Infrared observations show – spectral signatures expected for accretion disks • Radio observations: disk masses ~ solar system • Doppler analysis (infrared) of gas motions shows – gas accreting onto the central star – winds emanating from star or inner disk • Optical and infrared images reveal – jets emanating from star-disk systems HST Observes Protoplanetary Disks HST Observes Edge-on Disk Diagnosing Disks in the Infrared Accretion Disks and Stellar Jets Implications for Planet Building • In combination, these observations suggest: – accretion disks surround all forming stars – disk masses and sizes are similar to our solar system • As a consequence of the processes that give birth to stars, raw material for planet-building is in place Evidence for Planetesimal Building • Earth-like planets believed built via planetesimal collisions – produce larger bodies – produce small dust grains as a by-product of collisions • Planetesimals not observed directly • In solar system, evidence of collisions comes from – cratering history (moon; other bodies) – inclination of planet rotation axes • Outside solar system, evidence of collisions come from – light scattered earthward by small dust grains – thermal emission from heated grains • Dust grain population decreases with age – similar to solar system record A Post-Planet-Building Disk HST Observtions of an IRAS-discovered disk Disk Warping: Evidence of Planets? Evidence for Extrasolar Planets • Reflex Doppler motions in parent stars – periodic signals indicative of orbital motions – velocity amplitudes + periods yield mass estimates • More than 50 systems now known – many contain multiple planets – unexpected distribution of orbital distances • unfavorable for survival of terrestrial planets • Direct evidence of giant planet planet via eclipse – gas envelope inferred from light curve shape Detecting Extrasolar Planets Extrasolar Planetary Systems Extrasolar Planet Transit Key Questions & Paths to Answers Current Key Questions: Planets • When do planets form? – disk accretion phase? – later, following accretion of disk gas? • How diverse are planetary system architectures? – are close-in (r < 1 AU) Jupiter-mass planets favored? – are planets in habitable zones common or rare? • Can we observe extra-solar planets directly? – can we determine atmospheric structure and chemistry ? – can we detect signatures of life ? When do Planets Form? • Key observations: – probing accretion disks surrounding young stars and searching for tidal gaps diagnostic of forming planets – searching for gaps in beta-Pic-like disks around mature stars – determining accurate ages for star-disk systems • Key facilities – ALMA – next generation O/IR telescopes – SIRTF + current generation telescopes Diagnosing Planet Formation: GSMT AURA-NIO Point Design 30-m ground-based telescope Emission from tidal gaps Diagnosing Planet Formation: ALMA Star at 10pc SIRTF SIRTF: Artist Conception Locating Candidate Planetary Systems with SIRTF Inflections in spectra can diagnose gaps in dust disks Dust excess can diagnose planetesimal collision rates Dust Emission from Planet-Forming Disks: Resolving Candidate Mature Systems Gemini observation of Dust Ring Artist conception of system How Diverse are Planetary System Architectures? • Key observations – Statistical studies of dust distributions – Precise measurements of reflex motions: • continuation of current radial velocity programs • precise proper motion measurements • Key facilities – SIRTF – SIM (Space Interferometry Mission) Finding Planets: Precise Position Measurements Space Interferometry Mission SIM can (1) detect earth-like planets around nearby stars (2) determine distribution of planetary architectures from statistical studies of large samples of stars Observing Planets Directly • Key observations – imaging and spectroscopy • Key theoretical work – develop understanding of how to diagnose life from spectroscopic signatures • Key facilities – Devices designed to enable high contrast imaging; spectroscopy • coronagraphs that block out light from central star – use on current (Gemini; Keck) and future (GSMT) ground-based telescopes • infrared interferometers (ground: e.g. Keck; Large Binocular Telescope) • Terrestrial Planet Finder/Darwin (space) Diagnosing Mature Planets Spectra diagnose structure and chemistry of planetary atmospheres Terrestrial Planet Finder TPF will have the ability to image and take spectra of earth-like planets surrounding nearby stars Current Key Questions: Stars • How does the distribution of stellar masses depend on initial conditions? – chemical abundance? – collisions among molecular clouds? • How has star formation activity changed over the lifetime of the universe? How Stars of Different Mass Form • Key observations – physical conditions and kinematics in molecular clouds – observations of stellar mass distributions in these clouds • Key facilities – ALMA • high spatial resolution maps of molecular clouds – large ground-based telescopes (Gemini; Keck; GSMT) • photometry and spectroscopy of emerging stellar populations Probing the IMF: Measurements Galactic Center Superclusters: d = 10 kpc = 7” Stellar density ~ 100x Orion Nebula Cluster Probing the IMF: Measurements LMC Massive Cluster: d = 200 kpc 20” R 136 Stellar density ~ 10x Orion Nebula Cluster Probing the IMF: Measurements M82 Superclusters: d = 4 Mpc Star Formation: From the First Stars to the Current Epoch • Key observations – trace star formation rate to earliest epochs – study starburst systems • star formation rates • distribution of stellar masses • Key facilities – NGST (multi-wavelength photometry) – large ground-based telescopes (spectroscopy) JWST will observe first generation stars GSMT will enable analyis of distant star-forming regions HST GSMT