1. THE BIRTH OF STARS AND PLANETARY SYSTEMS Stephen E. Strom National Optical Astronomy Observatory 07 January, 2003

2. Overview of Presentation Theoretical overview Confrontation with theory: –what we know and how we know it Current key questions Answering key questions

3. Theory

4. 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

5. A Collapsing Molecular Clump Pressure ~ T Gravity ~ M/R 2

6. 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”)

7. Spinning Protostellar Core

8. Infalling envelope Forming the Star-Disk System Stellar seedAccretion Disk

9. 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’

10. Building a Full-term Star Wind/Jet Rotating accretion disk Accreting material Forming star Infalling gas/dust removes angular momentum

11. A Star in Formation: Artist Conception

12. 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

13. Formation via Disk Instability Forming Jupiter

14. Formation via Agglomeration; Collisions Planetesimal swarm formed via collisions among small dust grains Growth of larger bodies via collisions Mature planets

15. Star and Planet Formation Summary Molecular Cloud Rotating Clump Forming Star + disk

16. Confrontation with theory: What we know and how we know it

17. 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

18. Star-Forming Molecular Cloud 30 Light Years Ophiuchus Molecular Cloud (d ~ 500 light years)

19. Opaque Molecular Clump 0.2 light years

20. 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

21. Disks and Envelopes Around Young Stars

22. 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

23. HST Observes Protoplanetary Disks

24. HST Observes Edge-on Disk

25. Diagnosing Disks in the Infrared

26. Accretion Disks and Stellar Jets

27. 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

28. 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

29. A Post-Planet-Building Disk HST Observtions of an IRAS-discovered disk

30. Disk Warping: Evidence of Planets?

31. 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

32. Detecting Extrasolar Planets

33. Extrasolar Planetary Systems

34. Extrasolar Planet Transit

35. Key Questions & Paths to Answers

36. 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 ?

37. 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

38. Diagnosing Planet Formation: GSMT AURA-NIO Point Design 30-m ground-based telescope Emission from tidal gaps

39. Diagnosing Planet Formation: ALMA Star at 10pc

40. SIRTF SIRTF: Artist Conception

41. Locating Candidate Planetary Systems with SIRTF Inflections in spectra can diagnose gaps in dust disks Dust excess can diagnose planetesimal collision rates

42. Dust Emission from Planet-Forming Disks: Resolving Candidate Mature Systems Gemini observation of Dust Ring Artist conception of system

43. 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)

44. Finding Planets: Precise Position Measurements

45. 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

46. 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)

47. Diagnosing Mature Planets Spectra diagnose structure and chemistry of planetary atmospheres

48. Terrestrial Planet Finder TPF will have the ability to image and take spectra of earth-like planets surrounding nearby stars

49. 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?

50. 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

51. Probing the IMF: Measurements = 7” Stellar density ~ 100x Orion Nebula Cluster Galactic Center Superclusters: d = 10 kpc

52. Probing the IMF: Measurements R 136 20” Stellar density ~ 10x Orion Nebula Cluster LMC Massive Cluster: d = 200 kpc

53. Probing the IMF: Measurements M82 Superclusters: d = 4 Mpc

54. 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)

55. JWST will observe first generation stars

56. GSMT will enable analyis of distant star-forming regions HST GSMT