Post on 06-Feb-2016
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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?
Current picture of low-mass star formation
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
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?
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
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
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)
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):
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)
Cepheus AMost dense molecular core in the molecular cloud complex
associated with Cep OB3 association, d~725pc, Lbol~2.5104L
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)
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)
H2O maser proper motions of R4
– R4 possibly a disk around a ~3 M star
Gallimore et al. (2003)
Torrelles et al. (2001)
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)
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)
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
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…
Single epoch SiO radial velocities and VLA 7mm continuum
North Arm
East Arm
South Arm
West Arm
Bridge
Source I: the movie preview
Models for the Source I disk/outflow system
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