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Lecture 3 – High Mass Star Formation

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1 Lecture 3 – High Mass Star Formation
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 A quick note on nomenclature
pro⋅to⋅star   A quick note on nomenclature Massive star formation High mass star formation Young stellar object High Mass Protostellar Object pro⋅to⋅star   [proh-toh-stahr] an early stage in the evolution of a star, after the beginning of the collapse of the gas cloud from which it is formed, but before sufficient contraction has occurred to permit initiation of nuclear reactions at its core.

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. tff = 7  106 yr tKH = 3  107 yr tKH << tff Rare (form quickly and low probability from the IMF) Very high accretion rates (10-3 or even 10-2 Mʘ /yr) Still accreting while on the main sequence Always embedded in natal molecular cloud -1/2 M R 3/2 105 Mʘ 25 pc 2 M R -1 L -1 1 Mʘ 1 Rʘ 1 Lʘ

5 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

6 Ionising Radiation and HII Regions
What is different about High Mass Star Formation? Ionising Radiation and HII Regions Spectral Type Mass (Mʘ) Luminosity (Lʘ) Ionising flux (s-1) O4 60 1.3  106 8.5  1049 O5 50 6.8  105 4.2  1049 O5.5 42 4.0  105 2.3  1049 O6 37 2.5  105 1.2  1049 O6.5 32 1.5  105 6.6  1048 O7 28 1.0  105 4.2  1048 O7.5 27 8.3  104 3.2  1048 O8 25 6.5  104 2.2  1048 O8.5 23 5.4  104 1.6  1048 O9 22 4.6  104 1.2  1048 O9.5 21 3.8  104 6.9  1047 B0 19 2.5  104 2.3  1047 B0.5 15 1.1  104 1.7  1046 B1 12 5.2  103 1.9  1045 B2 10 2.8  103 4.5  1044 B3 8 1.0  103 4.9  1043 Stars M > 8Mʘ produce significant UV photons capable of ionising hydrogen  HII regions Panagia 1973

7 Ionising Radiation and HII Regions
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 Class of Region Size (pc) Density (cm-3) Emission Measure (pc cm-6) Ionised Mass (Mʘ) Hypercompact 0.03 106 1010 ~10-3 Ultracompact 0.1 104 107 ~10-2 Compact 0.5 5103 ~1 Classical ~10 ~100 ~102 ~105 Giant ~30 ~5105 Supergiant >100 Kurtz 2005

8 G Contours: 4.8GHz radio continuum (ATCA)

9 Ionising Radiation and HII Regions
What is different about High Mass Star Formation? Ionising Radiation and HII Regions CORNISH images of UCHII region G , courtesy Cormac (Mopra Boy) Purcell

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

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

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

13 Is the formation mechanism fundamentally different?
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

14 Competitive Accretion, Coalescence and Monolithic Collapse
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.

15 Competitive Accretion, Coalescence and Monolithic Collapse
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 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.

16 Competitive Accretion, Coalescence and Monolithic Collapse
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

17 Matthews et al. 2007

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

19 The Disk candidate in IRAS 18089-1732
Color: NH3(4,4) Contours: continuum VLA 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 ATCA

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

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

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

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

24 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.

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

26 Masers in Star Forming Regions
H2O, OH, SiO, NH3 and CH3OH Known maser lines: Species Frequency (GHz) OH 1.612, 1.665, 1.667, 1.720, 6.035 H2O 22.235, 187, 233, 658 NH3 20.719, , , , SiO 42.820, , , CH3OH (many) Other maser species: H, CH, H2CO, HCCCN, HCN, SiS?

27 Masers in Star Forming Regions
H2O, OH, SiO, NH3 and CH3OH Known maser lines: Species Frequency (GHz) OH 1.612, 1.665, 1.667, 1.720, 6.035 H2O 22.235 NH3 20.719, , , , SiO 42.820, , , CH3OH 6.669, , Other maser species: H, CH, H2CO, HCCCN, HCN, SiS?

28 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

29 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

30 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

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 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.

33 Matthews et al. 2007 Masers in Star Forming Regions SiO Masers
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.

34 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.

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

36 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.

37 CH3OH (methanol) Masers
Muller et al. (2007) H2O, OH, SiO, NH3 and CH3OH Frequency (GHz) Class 6.669 II 12.178 19.967 24.928 I 24.933 24.934 24.959 36.169 44.069 84.521 95.169 CH3OH (methanol) Masers

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 Class II CH3OH masers traditionally associated with UCHII regions

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.

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

43 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
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

45 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

46 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

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

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

49 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)

50 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)

51 HOPS daily monitoring of Orion

52 Class II CH3OH maser monitoring
Periodic flaring of the 6.7GHz CH3OH maser in G9.62 van der Walt et al. (2009)

53 Lecture 3 – High Mass Star Formation
Summary 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)

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