Masers and Massive Star Formation Claire Chandler

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Masers and Massive Star Formation Claire 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?

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Masers and Massive Star Formation Claire 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?. - PowerPoint PPT Presentation

Transcript of Masers and Massive Star Formation Claire Chandler

Page 1: Masers and Massive Star Formation Claire Chandler

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?

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Current picture of low-mass star formation

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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 = L/4r2c

– Same dependence on r 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 (1/Lacc) to the Kelvin-Helmholtz timescale (1/Lint) is ~10 M

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So, how do stars with M*>10M form?

• Accretion:– Need to reduce 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?

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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

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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 ~ 1067 cm3, T ~ 100 K

– CH3OH: n ~ 1067 cm3, T ~ 100 K

– SiO (v=0): n ~ 106 cm3, T ~ few 100 K (very rare)

– H2O: n ~ 101012 cm3, T ~ few 100 K

– SiO (v=1, v=2): n ~ 101012 cm3, T ~ 10002500 K (rare)

– Other OH transitions, HCN, NH3, HCO2H, etc…

• Zeeman effect for OH, H2O gives B-field

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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)

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Argon, Reid & Menten (2003)

Moscadelli et al. (1999)

Bloemhof et al. (1992)

• H2O maser proper motions outflow from TW object• CH3OH masers roughly coincident with OH masers• OH maser proper motions expansion at ~ few km/s• B-field from OH Zeeman ~10 mG (Baudry & Diamond 1998)

W3(OH):

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W3(Main):

• H2O maser proper motions trace several outflows from the IRS5 region

• Zeeman effect in H2O masers close to “c” give B ~ 1540 mG (Sarma et al. 2001)

Tieftrunk et al. (1997)

Imai et al. (2000)

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Cepheus AMost dense molecular core in the molecular cloud complex

associated with Cep OB3 association, d~725pc, Lbol~2.5104L

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VLA observations originally interpreted as a disk around HW2

H2O masers in the vicinity of Cep A HW2

HW2

Torrelles et al. (2001)

Garay et al. (1996)

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H2O maser proper motions of R13

VLBA and MERLIN observations identify multiple sources for the masers:– R1, R2, R3 shocks outlining walls of outflow cavity

Torrelles et al. (2001)

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H2O maser proper motions of R4

– R4 possibly a disk around a ~3 M star

Gallimore et al. (2003)

Torrelles et al. (2001)

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H2O maser proper motions of R5

– R5 edge of an expanding bubble caused by spherical mass ejection from an embedded protostar

Torrelles et al. (2001)

Curiel et al. (2002)

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Orion BN/KL

Shocked H2 emission traces an explosive outflow event centred close to radio Sources “I” and “n”

Lbol ~ 5-8104 L

Menten & Reid (1995)

Schultz et al. (1999)

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OH, H2O and SiO masers in the vicinity of Source I

Johnston et al. (1989)H2O masers: Gaume et al. (1998), Greenhill et al. (1998)

H2O masers trace a ~20km/s flow, and SiO v=1 masers trace an “X” centred on Source I

SiO

Greenhill et al. (1998); Doeleman et al. (1999)

OHH2OH2O

Model

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Monthly monitoring of SiO masers in Source I with the VLBA

Greenhill, 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=10 SiO masers (~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 ~60GB/epoch just for the images

• Sneak preview of results from 4 epochs…

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Single epoch SiO radial velocities and VLA 7mm continuum

North Arm

East Arm

South Arm

West Arm

Bridge

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Source I: the movie preview

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Models for the Source I disk/outflow system

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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 M*~1015M; 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