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

Pres

sure

~ T

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

Infalling envelope

Forming the Star-Disk System

Stellar seedAccretion Disk

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

Wind/JetRotating accretion disk

Accreting material Forming star

Infalling gas/d

ustremoves angular momentum

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

Planetesimal swarm formed via collisions among small dust grains

Growth of larger bodies via collisions

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 Clump0.2 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

= 7”

Stellar density ~ 100x Orion Nebula Cluster

Galactic Center Superclusters: d = 10 kpc

Probing the IMF: Measurements

R 136

20”

Stellar density ~ 10x Orion Nebula Cluster

LMC Massive Cluster: d = 200 kpc

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