Varied Mechanisms for Star-Formation in Bright-Rimmed Clouds

1. Larry Morgan (St Mary’s University) James Urquhart (ATNF) Mark Thompson (University of Hertfordshire) Charles Figura (Wartburg College) Kristen Thomas (University of Kentucky) Varied Mechanisms for Star-Formation in Bright-Rimmed Clouds

2. Introduction • Star-Formation Theory Refresher • Background to Triggered Star-Formation Theory • Observational Sample • Discoveries! • Conclusions

3. Star-Formation Theory

4. Star-Formation Theory • Wildly generalising, star-formation is mostly dependent upon the balance, or lack of it, of forces acting on a lump of stuff • ISRF, Self-Gravity, Winds, Turbulence (Huge range of scales) act inward • Temperature, Turbulence, Ambipolar diffusion may act outward • Resolved by radiation transport

5. Star-Formation Theory Fragmentation & Collapse Phase (Prestellar) t ∼1 Myr

6. Star-Formation Theory Accretion & Ejection Phase (Protostellar) t ∼ 0.1 Myr

7. Star-Formation Theory Gravitational Contraction Phase (PMS) t ∼1 - 10 Myr

8. The Problem with Bright-Rimmed Clouds • Isolated star-formation is isolated • Triggered star-formation may be triggered

9. Observations Vs. Theory • Despite large numbers of inconclusive observations theory of triggered star-formation is reasonably solid • However! They supply few diagnostics that can unambiguously separate triggered and isolated star-formation • Several proposed basic mechanisms for triggered star-formation. • Two major competing theories

10. The All-Important Ionisation Layer

11. Triggered Star-Formation • “A shock front can compress pre-existing dense clumps forcing them to contract and form new stars. The swept-up shell itself may become gravitationally unstable and fragment into new protostars.” • Kirsanova et al. (2008)

12. Radiatively Driven Implosion

13. Radiatively Driven Implosion • Dense core ‘excavated’ from medium by photoionisation evaporation • Collapse of core initiated by shocks driven into cloud by Ionised Boundary Layer (IBL) • Sequential star-formation directly influenced through interaction of radiation field

14. Radiatively Driven Implosion • RDI process controlled by balance between internal pressure of molecular material and the pressure in the IBL Cloud Pressure IBL Pressure

15. Radiatively Driven Implosion

16. Radiatively Driven Implosion Rim Types Observed A - 66% B - 29% C - 5% Modelled (old) A - 5% B - 5% C - 90% Modelled (new) A - 63% B + C - 37% Models exclusive of internal star-formation!

17. Radiatively Driven Implosion

18. Bright Rimmed Clouds:Observations

19. Observational Sample • Catalogue of Sugitani et al (1991) • IRAS sources within optically bright rims • Confirmation of status depends upon detection at multiple wavelengths

20. Observational Sample

21. Observational Sample Archival Data • Radio Observations • - NVSS fluxes over IBL region give electron densities and pressures of ionised gas (no temperature information)

22. Observational Sample Archival Data • IR Observations • - MSX images trace PDRs at interface of ionisation features and molecular material through emission of PAH features at ∼8 μm

23. Observational Sample Collected Data • Line Observations • - CO, 13CO, C18O pointed observations taken with the JCMT in J=2-1 transitions. Also NH3 in production, observed in pointed mode with at least one map with the GBT at 23 GHz.

24. Observational Sample • Line widths of CO are indicative of dynamical activity in ∼90% of the sample self-absorbed profiles showing infall occurring. NH3 data supportive of these conclusions but still undergoing analysis

25. Observational Sample • Submillimetre Continuum Observations • - Observations taken at 450 & 850 μm using the SCUBA on the JCMT, fully sample jiggle maps of submm condensations at head of BRCs

26. Observational Sample • Morgan (2008) reported the presence of YSOs at the head of many of the Sugitani BRCs with 1 < Lbol < 7000 L⊙ (median 63 L⊙) • Spitzer data, SED fitting and molecular observations show intermediate to massive star-formation in early to mid-stages occuring out of the Galactic Plane

27. Results • 44 observed BRCs yielded a total of 47 dense cores. 5 BRCs showed no submm emission. • 34 of these cores have data sufficient to determine mass, luminosity, evolutionary class, etc. • Mean mass - 6.5 M⊙, Mean T - 23 K

28. Analysis • Star-formation found in large number of BRCs • Support for interaction of multiple layers given by multi-wavelength observations • The morphology of BRCs supportive of RDI scenario

29. Extracting Potential Triggered Sources • Combining Radio/IR/Submm/Molecular data

30. Analysis

31. Analysis

32. A Rims Vs. B Rims • A rims hosts to star-formation apparently unrelated to exterior radiation field • B rims contain star-formation with a direct correlation to ionising radiation

33. Observational Sample

34. A Rims Vs. B Rims • No correlation between rim type and YSO properties, if RDI evolution were true, we should see older YSOs in B rims Text • BUT! Classification system ropey at best and we are dealing with small numbers

35. Observations Compared with Modelling ATCA

36. Radiatively Driven Implosion

37. Observations Compared with Modelling

38. 3D Molecular Maps

39. Conclusions • Star-formation is rife amongst BRCs • Different mechanisms responsible for the formation of sources • Situation not fully described by current models • First real linking of triggering mechanism to properties of embedded YSOs • Need larger sample base!