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Star Formation in our Galaxy

Star Formation in our Galaxy. Dr Andrew Walsh (James Cook University, Australia). Lecture 3 – High Mass Star Formation and Masers Is high mass star formation different? Competitive Accretion vs Coalescence vs Monolithic Collapse Disks around high mass stars

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Star Formation in our Galaxy

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  1. Star Formation in our Galaxy Dr Andrew Walsh (James Cook University, Australia) • Lecture 3 – High Mass Star Formation • and Masers • Is high mass star formation different? • Competitive Accretion vs Coalescence vs Monolithic Collapse • Disks around high mass stars • Masers (H2O, OH, SiO, NH3 and CH3OH)

  2. pro⋅to⋅star A quick note on nomenclature Massive star formation High mass star formation Young stellar object High Mass Protostellar Object

  3. What is different about High Mass Star Formation? • Happens much faster • Ionising radiation and HII regions • Only happens in clusters • Is the formation mechanism • fundamentally different?

  4. What is different about High Mass Star Formation? Happens much faster • No pre-main sequence stage for high mass stars. • (tKH << tff) • Still accreting while on the main sequence • Rare (form quickly and low probability from the IMF) • Very high accretion rates (10-3 or even 10-2 Mʘ /yr) • Always embedded in natal molecular cloud

  5. What is different about High Mass Star Formation? Ionising Radiation and HII Regions • Stars M > 8Mʘ produce significant UV photons capable of • ionising hydrogen •  HII regions Panagia 1973

  6. What is different about High Mass Star Formation? Ionising Radiation and HII Regions • Stars M > 8Mʘ produce significant UV photons capable of • ionising hydrogen •  HII regions Kurtz 2005

  7. G305.2+0.2 Contours: 4.8GHz radio continuum (ATCA)

  8. What is different about High Mass Star Formation? Ionising Radiation and HII Regions CORNISH images of UCHII region G43.890-0.784, courtesy Cormac (Mopra Boy) Purcell

  9. What is different about High Mass Star Formation? Only Happens in Clusters Isolated low mass star formation

  10. What is different about High Mass Star Formation? Only Happens in Clusters

  11. What is different about High Mass Star Formation? Only Happens in Clusters

  12. What is different about High Mass Star Formation? • Is the formation mechanism fundamentally different? • Problems with scaling low mass star formation up: • Radiation pressure is too great (spherically symmetric) •  high mass star on the main sequence while still trying to accrete matter •  Incredibly high accretion rates required • No clear cases of isolated high mass star formation • Fragmentation results in prestellar cores that are too small

  13. What is different about High Mass Star Formation? Competitive Accretion, Coalescence and Monolithic Collapse Competitive accretion Introduced to provide both an alternate mechanism to forming high mass stars and to explain the Initial Mass Function (IMF) Prestellar cores start off with similar masses. Their location within the molecular cloud determines the final mass of the star. High mass stars are formed in the cluster centres where the gas reservoir is denser.

  14. What is different about High Mass Star Formation? Competitive Accretion, Coalescence and Monolithic Collapse Competitive accretion One possible interpretation used ballistic motions of cores with respect to the gas to give the final stellar mass.  Walsh et al. 2006 compared motions of high density cores (traced by N2H+) with the low density surrounding gas (traced by 13CO and C18O), but found no clear signs for relative motion.

  15. What is different about High Mass Star Formation? Competitive Accretion, Coalescence and Monolithic Collapse Coalescence High mass stars form in the centres of clusters Intermediate mass prestellar cores (or the stars themselves) merge to form high mass core/star Will work if stellar densities are ~106 pc-3, but highest stellar densities seen (eg. Orion) are only ~104 pc-3 Difficult to sustain a disk and an outflow during coalescence  Good evidence for outflows  Not so good evidence for disks

  16. Matthews et al. 2007

  17. Chini et al. 2007 M17 What is different about High Mass Star Formation? Finding Disks Around High Mass Stars Too big (40 000 AU)!

  18. The Disk candidate in IRAS 18089-1732 Color: NH3(4,4) Contours: continuum VLA ATCA Beuther & Walsh 2008 • New high-excitation ammonia • NH3(4,4)/(5,5) data • Clear east-west velocity gradient. • Non-Keplerian motions. • T > 100K in rotating structure. • Dv(NH3(5,5)) ~ 4.7 km/s • Beuther & Walsh 2008

  19. G305.21+0.21 NH3 (4,4) 0th moment NH3 (4,4) 1st moment NH3 (5,5) 0th moment NH3 (5,5) 1st moment

  20. G327.3-0.60 NH3 (4,4) 0th moment NH3 (4,4) 1st moment NH3 (5,5) 0th moment NH3 (5,5) 1st moment

  21. G328.81+0.63 Continuum 24.142GHz NH3 (4,4) 0th moment NH3 (5,5) 0th moment Continuum 24.536GHz

  22. G351.77-0.54 NH3 (4,4) 0th moment NH3 (4,4) 1st moment NH3 (5,5) 0th moment NH3 (5,5) 1st moment

  23. Summary – disks around high mass stars • Few good examples available • Enough evidence to say they almost certainly exist • Promising results from high-J NH3 observations • but so far no flattened structures.

  24. Masers in Star Forming Regions H2O, OH, SiO, NH3 and CH3OH

  25. Masers in Star Forming Regions H2O, OH, SiO, NH3 and CH3OH Known maser lines: Other maser species: H, CH, H2CO, HCCCN, HCN, SiS?

  26. Masers in Star Forming Regions H2O, OH, SiO, NH3 and CH3OH Known maser lines: Other maser species: H, CH, H2CO, HCCCN, HCN, SiS?

  27. Masers in Star Forming Regions H2O, OH, SiO, NH3 and CH3OH Water Masers First discovered by Knowles et al. (1969) Pumped by collisions with H2 molecules Requires high densities ~109 cm-3 Usually associated with shocks/outflows

  28. Masers in Star Forming Regions H2O, OH, SiO, NH3 and CH3OH Water Masers AFGL 5142 high mass star forming region Goddi et al. 2007 Water masers Methanol masers

  29. Masers in Star Forming Regions H2O, OH, SiO, NH3 and CH3OH W3(OH) high mass star forming region Distance to W3(OH) is 2.04 ±0.07 kpc Hachisuka et al. 2006

  30. Masers in Star Forming Regions H2O, OH, SiO, NH3 and CH3OH OH (hydroxyl) Masers First discovered by Wilson and Darrett (1968) Radiatively Pumped Found close to high mass YSOs Forster & Caswell (1989) found OH masers tend to be close to larger (older) HII regions, than H2O masers.

  31. Masers in Star Forming Regions H2O, OH, SiO, NH3 and CH3OH OH (hydroxyl) Masers First discovered by Wilson and Darrett (1968) Radiatively Pumped Found close to high mass YSOs Forster & Caswell (1989) found OH masers tend to be close to larger (older) HII regions, than H2O masers.

  32. Matthews et al. 2007 Masers in Star Forming Regions H2O, OH, SiO, NH3 and CH3OH SiO Masers Vibrationally excited SiO transitions are masers whilst non-vibrationally excited transitions exhibit thermal emission. Thermal SiO is a good outflow tracer in high mass star forming regions. SiO masers common in evolved stars, but only three known in star forming regions: Orion, SGR B2 and W51.

  33. Masers in Star Forming Regions H2O, OH, SiO, NH3 and CH3OH NH3 (ammonia) Masers Some NH3 inversion transitions exhibit maser emission: (3,3), (5,5), (6,6), (9,6), (6,3), (5,4), (7,5), (9,8), (6,5), (10,8), (8,6) and (11,9). Limited examples show emission closely associated with high mass YSOs.

  34. Masers in Star Forming Regions H2O, OH, SiO, NH3 and CH3OH NH3 (ammonia) Masers Walsh et al. (2007)

  35. Masers in Star Forming Regions H2O, OH, SiO, NH3 and CH3OH CH3OH (methanol) Masers Two types of CH3OH masers, divided by pumping mechanism Class I masers are pumped by collisions with H2 molecules, are usually offset from high mass YSOs and are traditionally associated with outflows Class II masers are pumped by infrared photons and are usually found coincident with high mass YSOs.

  36. Muller et al. (2007) H2O, OH, SiO, NH3 and CH3OH CH3OH (methanol) Masers

  37. Masers in Star Forming Regions H2O, OH, SiO, NH3 and CH3OH CH3OH (methanol) Masers Class II CH3OH masers traditionally associated with UCHII regions

  38. Masers in Star Forming Regions H2O, OH, SiO, NH3 and CH3OH CH3OH (methanol) Masers Class II CH3OH masers traditionally associated with UCHII regions

  39. Masers in Star Forming Regions H2O, OH, SiO, NH3 and CH3OH CH3OH (methanol) Masers Walsh et al. (1998) observed 364 CH3OH maser sites at high spatial resolution to determine where, in relation to the UCHII region, the masers were located.

  40. Masers in Star Forming Regions H2O, OH, SiO, NH3 and CH3OH CH3OH (methanol) Masers Walsh et al. (1998) observed 364 CH3OH maser sites at high spatial resolution to determine where, in relation to the UCHII region, the masers were located.

  41. Masers in Star Forming Regions H2O, OH, SiO, NH3 and CH3OH CH3OH (methanol) Masers Walsh et al. (1998) observed 364 CH3OH maser sites at high spatial resolution to determine where, in relation to the UCHII region, the masers were located. 75% of CH3OH masers are NOT associated with UCHII regions

  42. Masers in Star Forming Regions H2O, OH, SiO, NH3 and CH3OH CH3OH (methanol) Masers Walsh et al. (1998) observed 364 CH3OH maser sites at high spatial resolution to determine where, in relation to the UCHII region, the masers were located. 75% of CH3OH masers are NOT associated with UCHII regions Walsh et al. (1998) also found ~33% of maser sites in lines or arcs

  43. Norris et al. (1993) Masers in Star Forming Regions H2O, OH, SiO, NH3 and CH3OH CH3OH (methanol) Masers Walsh et al. (1998) observed 364 CH3OH maser sites at high spatial resolution to determine where, in relation to the UCHII region, the masers were located. 75% of CH3OH masers are NOT associated with UCHII regions Walsh et al. (1998) also found ~33% of maser sites in lines or arcs

  44. Masers in Star Forming Regions H2O, OH, SiO, NH3 and CH3OH CH3OH (methanol) Masers Walsh et al. (1998) observed 364 CH3OH maser sites at high spatial resolution to determine where, in relation to the UCHII region, the masers were located. 75% of CH3OH masers are NOT associated with UCHII regions Walsh et al. (1998) also found ~33% of maser sites in lines or arcs

  45. Masers in Star Forming Regions H2O, OH, SiO, NH3 and CH3OH CH3OH (methanol) Masers Followup observations (Walsh et al. 1997; 2001; 2003) showed All CH3OH masers coincident with dense cores  Class II CH3OH masers are only found with sites of high mass star formation – an easily observable and reliable signpost

  46. Masers in Star Forming Regions H2O, OH, SiO, NH3 and CH3OH CH3OH (methanol) Masers Voronkov et al. (2005) 6.7GHz Class II CH3OH masers in Orion

  47. Masers in Star Forming Regions H2O, OH, SiO, NH3 and CH3OH CH3OH (methanol) Masers Voronkov et al. (2005)

  48. Masers in Star Forming Regions H2O, OH, SiO, NH3 and CH3OH Maser Variability H2O masers are highly variable (~weeks) OH and CH3OH masers slowly variable (~years)

  49. H2O maser monitoring in high mass star forming regions H2O masers are highly variable (~weeks) OH and CH3OH masers slowly variable (~years) Felli et al. (2007)

  50. HOPS daily monitoring of Orion

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