Masers and Massive Star Formation Claire Chandler

1. Masers and Massive Star FormationClaire Chandler Overview: • Some fundamental questions in massive star formation • Clues from masers • Review of three regions: W3, Cep A, Orion • Preview of a movie of SiO masers associated with Source I in Orion • What have we learned?

2. Current picture of low-mass star formation

3. The problem with extending the picture of low-mass star formation to massive stars is the following: • Radiation pressure acting on dust grains can become large enough to reverse the infall of matter: • Fgrav = GM*m/r2 • Frad = Ls/4pr2c • Same dependence on rÞ happens at all radii • Luminosity prior to onset of nuclear burning comes from • Accretion, Lacc = GM*Macc/R* • Gravitational contraction, Lint • Transition from where the evolution is dominated by the accretion timescale (µ1/Lacc) to the Kelvin-Helmholtz timescale (µ1/Lint) is ~10 M .

4. So, how do stars with M*>10M form? • Accretion: • Need to reduce s, e.g., by making accreting material very optically thick (high Macc) • Reduce the effective luminosity by making the radiation field anisotropic • Form massive stars through collisions of intermediate-mass stars in clusters • May be explained by observed cluster dynamics • Possible problem with cross section for coalescence • Observational consequences of such collisions? .

5. Other differences between low- and high-mass star formation • Physical properties of clouds undergoing low- and high-mass star formation are different: • Massive SF: clouds are warmer, larger, more massive, mainly located in spiral arms; high mass stars form in clusters and associations • Low-mass SF: form in a cooler population of clouds throughout the Galactic disk, as well as GMCs • Energetic phenomena associated with massive SF: UCHII regions, hot molecular cores • Different environments observed has led to the suggestion that different mechanisms (or modes) apply to low- and high-mass SF

6. Clues from high-resolution maser observations • Maser proper motions plus radial velocities give 3-D velocity fields • Masers trace a variety of physical conditions, depending on the molecule and pump mechanism: • OH (1665/7 MHz): n ~ 106-7 cm-3, T ~ 100 K • CH3OH: n ~ 106-7 cm-3, T ~ 100 K • SiO (v=0): n ~ 106 cm-3, T ~ few 100 K (very rare) • H2O: n ~ 1010-12 cm-3, T ~ few 100 K • SiO (v=1, v=2): n ~ 1010-12 cm-3, T ~ 1000-2500 K (rare) • Other OH transitions, HCN, NH3, HCO2H, etc… • Zeeman effect for OH, H2O gives B-field

7. Case studies: W3, Cep A, Orion W3 contains two main sites of massive star formation at ~2kpc: W3(Main), W3(OH): W3(Main) from Tieftrunk et al. (1997) W3(OH) from Reid et al. (1995)

8. W3(OH): Argon, Reid & Menten (2003) Moscadelli et al. (1999) • H2O maser proper motions Þ outflow from TW object • CH3OH masers roughly coincident with OH masers • OH maser proper motions Þ expansion at ~ few km/s • B-field from OH Zeeman Þ ~10 mG • (Baudry & Diamond 1998) Bloemhof et al. (1992)

9. W3(Main): • H2O maser proper motions trace several outflows from the IRS5 region • Zeeman effect in H2O masers close to “c” give B ~ 15-40 mG (Sarma et al. 2001) Tieftrunk et al. (1997) Imai et al. (2000)

10. Cepheus A Most dense molecular core in the molecular cloud complex associated with Cep OB3 association, d~725pc, Lbol~2.5´104L

11. H2O masers in the vicinity of Cep A HW2 VLA observations originally interpreted as a disk around HW2 HW2 Garay et al. (1996) Torrelles et al. (2001)

12. H2O maser proper motions of R1-3 VLBA and MERLIN observations identify multiple sources for the masers: • R1, R2, R3 shocks outlining walls of outflow cavity Torrelles et al. (2001)

13. H2O maser proper motions of R4 • R4 possibly a disk around a ~3 M star Gallimore et al. (2003) Torrelles et al. (2001)

14. H2O maser proper motions of R5 • R5 edge of an expanding bubble caused by spherical mass ejection from an embedded protostar Curiel et al. (2002) Torrelles et al. (2001)

15. Menten & Reid (1995) Orion BN/KL Shocked H2 emission traces an explosive outflow event centred close to radio Sources “I” and “n” Lbol ~ 5-8´104 L Schultz et al. (1999)

16. OH, H2O and SiO masers in the vicinity of Source I H2O masers: Gaume et al. (1998), Greenhill et al. (1998) Johnston et al. (1989) H2O H2O OH Model SiO H2O masers trace a ~20km/s flow, and SiO v=1 masers trace an “X” centred on Source I Greenhill et al. (1998); Doeleman et al. (1999)

17. Monthly monitoring of SiO masers in Source I with the VLBAGreenhill, Chandler, Reid, Moran, Diamond • The velocity field traced by the SiO masers close to the protostar can potentially determine whether the MHD disk wind models currently in vogue for outflows from low-mass protostars will also work for massive protostars. • Monthly monitoring of the v=1 and v=2, J=1-0 SiO masers (n~43GHz) with the VLBA began in June 2000 and is continuing through this summer • Data sets are large: 81922, 512 channels/transition; image ~25% of each cube Þ ~60GB/epoch just for the images • Sneak preview of results from 4 epochs…

18. Single epoch SiO radial velocities and VLA 7mm continuum North Arm West Arm East Arm Bridge South Arm

19. Source I: the movie preview

20. Models for the Source I disk/outflow system

21. Summary: what does it all mean? • VLBA maser proper motion studies provide the highest resolution possible of the dynamics of star formation • Maser spot geometry and kinematics resemble those of low-mass systems, suggesting formation via accretion: such ordered motions are unlikely to result from coalescence • W3 • Masses of outflow sources unknown; probably less than 10M • Cep A • Mass of HW2 probably ~10M; other sources less massive • Source I • If edge-on disk model is correct, rotation ÞM*~10-15M; this may be the first demonstration that accretion models can be scaled to high-mass systems • Future: more proper motion studies needed for M*>10M; B-field measurements needed to constrain MHD wind models