From Pre-stellar Cores to Proto-stars: The Initial Conditions of Star Formation PHILIPPE ANDRE DEREK WARD-THOMPSON MARY BARSONY Reported by Fang Xiong, PMO

Authors Mary Barsony Research Projects: Spitzer IRS (Infrared Spectrometer) imaging of the outflow driven by the nearest, well- isolated protostar. Derek Ward-Thompson Research Interests: The very earliest stages of star formation, including pre- stellar cores. Philippe Andre Research Projects: Early Stage of star formation. 2

Authors 3

OUTLINE I. Introduction I I. Pre-Stellar Cores I II. The Youngest Proto-stars I V. Summary 4

I. INTRODUCTION The formation of low-mass stars is believed to involve a series of different stages. The first stage: Fragmentation of a molecular cloud into a number of gravitationally-bound cores. Thermal Pressure Gravity = Magnetic Pressure Turbulent Pressure 5

I. INTRODUCTION The second stage: The pre-stellar fragment becomes gravitationally unstable and collapses, the released gravitational energy radiates away and the fragment stays roughly isothermal. Then, an opaque, hydrostatic proto-stellar object forms in the center. The third stage: The central object builds up its mass from a surrounding infalling envelope and accretion disk, while progressively warming up. 6

I. INTRODUCTION Standard collapsing theory: Larson: Runaway Model, “outside first, then inside”. Shu: Inside-out Model, “inside first, then outside”. Both Larson and Shu’s model reqiue isothermal condition. Observations have shown that the accretion phase is always accompanied by a powerful bipolar outflows. 7

CONTENT I. Introduction I I. Pre-Stellar Cores I II. The Youngest Proto-stars I V. Summary 8

II. PRE-STELLAR CORES A. Definition and Identification B. Spectral Energy Distributions and Temperatures C. Mass and Density Structure D. Lifetimes 9

II. PRE-STELLAR CORES A. Definition and Identification In 1980s, Myers and co-workers catalogued about 90 cores by transitions of NH 3. These cores were separated into starless cores and cores with stars according to whether an embedded source was detected by IRAS or not. The starless NH 3 were the potential sites of future isolated low-mass star formation. 10

II. PRE-STELLAR CORES A. Definition and Identification In 1994, Ward-Thompson observed the 800μm dust continuum emission of about 20 starless NH 3 cores from the Myers’s list. They found these starless cores have larger FWHM sizes than, but comparable masses to the envelopes of the youngest proto-stars. They also demonstrated that pre-stellar cores have flat inner radial density profiles. 11

II. PRE-STELLAR CORES A. Definition and Identification B. Spectral Energy Distributions and Temperatures C. Mass and Density Structure D. Lifetimes 12

II. PRE-STELLAR CORES B. Spectral Energy Distributions and Temperatures Detected at 90 & 200 μm (from ISO) 13

II. PRE-STELLAR CORES B. Spectral Energy Distributions and Temperatures Detected at 850 μm (from SCUBA) Detected at 1.3 mm (from IRAM) 14

II. PRE-STELLAR CORES B. Spectral Energy Distributions and Temperatures The spectral energy distribution of L1544 in the far-infrared and sub-millimeter wavelength 15

II. PRE-STELLAR CORES A. Definition and Identification B. Spectral Energy Distributions and Temperatures C. Mass and Density Structure D. Lifetimes 16

II. PRE-STELLAR CORES C. Mass and Density Structure 17

II. PRE-STELLAR CORES C. Mass and Density Structure 18

II. PRE-STELLAR CORES A. Definition and Identification B. Spectral Energy Distributions and Temperatures C. Mass and Density Structure D. Lifetimes 19

II. PRE-STELLAR CORES D. Lifetimes Generally, we use the ratio of numbers of starless cores to numbers of cores with embedded IRAS sources to estimate their relative timescales. This figure shows the estimated lifetime of starless cores for each of six dark cloud. 20

II. PRE-STELLAR CORES D. Lifetimes 21

CONTENT I. Introduction I I. Pre-Stellar Cores I II. The Youngest Proto-stars I V. Summary 22

III. THE YOUNGEST PROTOSTARS A. Class 0 Proto-stars and Other YSO Stages B. Density Structure of the Proto-stellar Environment C. Direct Evidence for Infall D. Decline of Outflow and Inflow with Time 23

III. THE YOUNGEST PROTOSTARS A. Class 0 Proto-stars and Other YSO Stages 24

III. THE YOUNGEST PROTOSTARS A. Class 0 Proto-stars and Other YSO Stages (ii) Centrally peaked but extended sub- millimeter continuum emission tracing the presence of a spheroidal circumstellar dust envelope. (iii) High ratio of sub-millimeter to bolometric luminosity suggesting the envelope mass exceeds the central stellar mass: L smm /L bol >0.5%. 25

III. THE YOUNGEST PROTOSTARS A. Class 0 Proto-stars and Other YSO Stages Combining infrared and submillimeter data, it is therefore to define a complete, empirical evolutionary sequence for low-mass YSOs: Class 0→Class I → Class II → Class III This sequence can also be parameterized by the “bolometric temperature”, T bol. 26

III. THE YOUNGEST PROTOSTARS A. Class 0 Proto-stars and Other YSO Stages 27

III. THE YOUNGEST PROTOSTARS A. Class 0 Proto-stars and Other YSO Stages B. Density Structure of the Proto-stellar Environment C. Direct Evidence for Infall D. Decline of Outflow and Inflow with Time 28

III. THE YOUNGEST PROTOSTARS B. Density Structure of the Proto-stellar Environment 29

III. THE YOUNGEST PROTOSTARS B. Density Structure of the Proto-stellar Environment Many Class 0 protostars are in fact multiple systems, when viewed at sub-arcsecond resolution, sharing a common envelope and sometimes a circumbinary disk. These protobinaries are probably formed by dynamical fragmentation during the isothermal collapse phase. 30

III. THE YOUNGEST PROTOSTARS A. Class 0 Proto-stars and Other YSO Stages B. Density Structure of the Proto-stellar Environment C. Direct Evidence for Infall D. Decline of Outflow and Inflow with Time 31

III. THE YOUNGEST PROTOSTARS C. Direct Evidence for Infall A comprehensive survey of a sample of 47 embedded YSOs in H 2 CO and CS suggests that infall is more prominent in Class 0 than in Class I sources. In these transitions, infall asymmetries are detected toward 40–50 % of Class 0 objects but less than 10 % of Class I sources. This is consistent with a decline of rate with evolutionary stage. 32

III. THE YOUNGEST PROTOSTARS A. Class 0 Proto-stars and Other YSO Stages B. Density Structure of the Proto-stellar Environment C. Direct Evidence for Infall D. Decline of Outflow and Inflow with Time 33

III. THE YOUNGEST PROTOSTARS D. Decline of Outflow and Inflow with Time Most Class 0 protostars drive powerful CO outflows, some outflow activity even exists throughout the accretion phase. Luminosity of these outflows are often of the same order as the bolometric luminosity of the central sources. For Class I objects, CO outflows are much less powerful and less collimated than Class 0 objects. 34

III. THE YOUNGEST PROTOSTARS D. Decline of Outflow and Inflow with Time The decline of outflow power with evolutionary stage reflects a corresponding decrease in the mass-accretion/infall rate. Decline of mass-accretion with time does not imply a higher accretion luminosity for Class 0 compared to Class I because the central stellar mass is smaller at the Class 0 stage and the stellar radius is likely to be larger. 35

CONTENT I. Introduction I I. Pre-Stellar Cores I II. The Youngest Proto-stars I V. Summary 36

IV. Summary 37

IV. Summary 38

IV. Summary 39

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