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