the birth of stars and planetary systems stephen e. strom national optical astronomy observatory 07...
TRANSCRIPT
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THE BIRTH OF STARS AND PLANETARY SYSTEMS
Stephen E. Strom
National Optical Astronomy Observatory
07 January, 2003
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Overview of Presentation
• Theoretical overview
• Confrontation with theory: – what we know and how we know it
• Current key questions
• Answering key questions
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Theory
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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
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A Collapsing Molecular Clump
Pres
sure
~ T
Gravity ~ M/R2
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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”)
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Spinning Protostellar Core
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Infalling envelope
Forming the Star-Disk System
Stellar seedAccretion Disk
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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’
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Building a Full-term Star
Wind/JetRotating accretion disk
Accreting material Forming star
Infalling gas/d
ustremoves angular momentum
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A Star in Formation: Artist Conception
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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
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Formation via Disk Instability
Forming Jupiter
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Formation via Agglomeration; Collisions
Planetesimal swarm formed via collisions among small dust grains
Growth of larger bodies via collisions
Mature planets
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Star and Planet Formation Summary
Molecular Cloud
Rotating Clump
Forming Star + disk
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Confrontation with theory:What we know and how we know it
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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
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Star-Forming Molecular Cloud
30 Light Years
Ophiuchus Molecular Cloud (d ~ 500 light years)
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Opaque Molecular Clump0.2 light years
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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
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Disks and Envelopes Around Young Stars
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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
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HST Observes Protoplanetary Disks
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HST Observes Edge-on Disk
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Diagnosing Disks in the Infrared
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Accretion Disks and Stellar Jets
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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
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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
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A Post-Planet-Building Disk
HST Observtions of an IRAS-discovered disk
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Disk Warping: Evidence of Planets?
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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
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Detecting Extrasolar Planets
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Extrasolar Planetary Systems
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Extrasolar Planet Transit
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Key Questions & Paths to Answers
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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 ?
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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
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Diagnosing Planet Formation: GSMT
AURA-NIO Point Design 30-m ground-based telescope Emission from tidal gaps
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Diagnosing Planet Formation: ALMA
Star at 10pc
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SIRTF
SIRTF: Artist Conception
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Locating Candidate Planetary Systems with SIRTF
Inflections in spectra can diagnose gaps in dust disks
Dust excess can diagnose planetesimal collision rates
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Dust Emission from Planet-Forming Disks: Resolving Candidate Mature Systems
Gemini observation of Dust Ring Artist conception of system
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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)
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Finding Planets: Precise Position Measurements
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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
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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)
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Diagnosing Mature Planets
Spectra diagnose structure and chemistry of planetary atmospheres
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Terrestrial Planet Finder
TPF will have the ability to image and take spectra of earth-like planets surrounding nearby stars
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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?
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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
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Probing the IMF: Measurements
= 7”
Stellar density ~ 100x Orion Nebula Cluster
Galactic Center Superclusters: d = 10 kpc
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Probing the IMF: Measurements
R 136
20”
Stellar density ~ 10x Orion Nebula Cluster
LMC Massive Cluster: d = 200 kpc
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Probing the IMF: Measurements
M82 Superclusters: d = 4 Mpc
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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)
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JWST will observe first generation stars
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GSMT will enable analyis of distant star-forming regions
HST
GSMT