From Pre-Stellar Cores to Protostars: The Initial Conditions of Star Formation

1. From Pre-Stellar Cores to Protostars: The Initial Conditions of Star Formation Andre P., Ward-Thompson D., Barsony M., 2000, in Protostars and Planets IV, ed. V. Mannings, A. P. Boss, S. S. Russell (Tucson: Univ. Ari- zona Press), 59 Gao Yang, THCA GaoYang, THCA

2. Content • Introduction • Pre-Stellar Cores • The Youngest Protostars • Conclusions and Implications GaoYang, THCA

3. Introduction: Stages of low-mass star formation • The fragmentation of a molecular cloud into a number of gravitationally-bound cores; Govern the origin of the stellar initial mass function (IMF).But the mechanism is still poorly understand! Ambipolar diffusion? Turbulence dissipation? Outside impulse? • Pre-stellar cores; • Protostars; • Pre-main sequence star (PMS).Planet system formation. GaoYang, THCA

4. James Clerk Maxwell Telescope (JCMT) , submm Infra Red Astronomical Satellite (IRAS), IR Introduction: Telescopes GaoYang, THCA

5. Pre-stellar cores: Definition • A gravitationally bound core has formed in a molecular cloud, and evolves toward higher degrees of central condensation. • No central hydrostatic protostellar object exists yet within the core. GaoYang, THCA

6. Pre-stellar cores: Identification • NH_3 transitions survey of isolated dense cores in dark clouds (Myers et al. 1983). • IRAS detection of embedded source to classify starless cores and cores with stars (Beichman et al. 1986). • JCMT observation (Ward-Thompson et al. 1994) and other observations: • Larger size than, but comparable mass to the envelope of Class 0 sours. • Flat inner radial density profiles; contract more slowly than Class 0 protostars. GaoYang, THCA

7. Pre-stellar cores: Spectral Energy Distributions (SED) and Temperatures • Images of L1544 at different wavelength • ISO (IR), • SCUBA on JCMT (submm) • IRAM (mm) GaoYang, THCA

8. Pre-stellar cores: Spectral Energy Distributions (SED) and Temperatures • Grey-body spectra curve: black body+ dust opacity+ source solid angle • Optical depth: • Dust opacity per unit (dust+ gas) mass column density: • For L1544, no warm dust, no embedded protostellar objects GaoYang, THCA

9. Pre-stellar cores: Spectral Energy Distributions (SED) and Temperatures • SED for IRAS16293 and L1544 • fitted by grey-body curve GaoYang, THCA

10. Pre-stellar cores: Mass and Density Structure • Dust emission is generally optically thin at (sub) millimeter wavelength: • The total (gas+ dust) mass contained within a radius R derived from submillimeter flux density (proportion to): • Radial density profile: by integral the submm emission in circular or elliptical annmli GaoYang, THCA

11. Radial intensity profile of L1689B at 1.3mm Inner flattening Disk like core Spherical core Sharp out edge: steeper than Pre-stellar cores: Mass and Density Structure GaoYang, THCA

12. Pre-stellar cores: Lifetimes • Calculation (Beichman et al. 1986): • N_ starless cores/N_ cores with stars • The lifetime of core with stars • 10^6 yr • An anti-correlation between lifetime and density is shown for samples from different star-forming clusters GaoYang, THCA

13. Pre-stellar cores: Pre-stellar Condensations in Star Forming Clusters • 59 starless cores and 41 YSOs • Multiple star formation regions • Denser • More compact GaoYang, THCA

14. These pre-stellar mass spectra resemble the shape of initial mass function (IMF) The IMF of clusters is primarily determined at the pre-stellar stage of star formation Pre-stellar cores: Pre-stellar Condensations in Star Forming Clusters GaoYang, THCA

15. Infrared YSO classes: Class I: Class II (PMS): Class III (PMS): Evolution stages: Class 0 (T<70K), Class I (T: 70-650K), Class II (T: 650-2880K), Class III(T:>2880K). T: the bolometric temperature The Youngest Protostars: Class 0 Protostars and Other YSO Stages GaoYang, THCA

16. Class 0 protostars: A central YSO A spheroidal circumstellar dust envelope High ratio of submm to bolometric luminosity as an evolutionary indicator The Youngest Protostars: Class 0 Protostars and Other YSO Stages GaoYang, THCA

17. The Youngest Protostars: Class 0 Protostars and Other YSO Stages • Evolutionary diagrams for YSOs: • Early main accretion phase (Class 0) • Late accretion phase (Class I) • PMS stars with protoplanetary disks (Class II) • PMS stars with debris disks (Class III) GaoYang, THCA

18. The Youngest Protostars: Class 0 Protostars and Other YSO Stages • Class 0 protostars survey: • Submm continuum mapping; • HIRES processing of the IRAS data; • CO mapping; • Large-scale near-IR/optical imaging of shocked H_2 and [SII] emission. • Typical lifetime of class 0 protostars (derived from the number ratio of class 0 to class I): , short-lived. GaoYang, THCA

19. The Youngest Protostars: Density Structure of the Protostellar Environment • Envelope: no central flattening, consistent with the standard • Sometimes shallower; • In star-forming clusters: more compact and steeper. • Disks of Class 0 objects are a factor of >=10 less massive than their envelopes. • Multiplicity: proto binaries sharing a common envelopes and sometimes a common disk GaoYang, THCA

20. The Youngest Protostars: Direct Evidence for Infall • Spectroscopic signatures indicating infall: self-absorbed profiles skewed to the blue. • Often complicated by the simultaneous presence of rotation and/or outflow. • Infall is more prominent in Class 0 objects than in Class I objects, consistent with a decline of infall/accretion rate with evolutionary stages. GaoYang, THCA

21. The Youngest Protostars: Decline of Outflow and Inflow with Time • Declination of outflow momentum flux versus during evolutionary stages (circle for Class 0; filed circle for Class I). GaoYang, THCA

22. The Youngest Protostars: Decline of Outflow and Inflow with Time • Model predicts that accretion rate is in proportion to ejection rate –accretion rate decline. • The decline of accretion rate is linked with the collapse initial conditions. • If the density profile at the beginning of collapse varies from , the accretion rate varies with time. GaoYang, THCA

23. Conclusions and Implications • Density profile varies from predicted: dynamical, not quasi-static process. • Implications on the origin of IMF from the observation of star-forming clusters. • With the advent of new facilities at far-IR and submm, the next decades will be rich in observation. • Thanks For Your Attention! GaoYang, THCA