Turbulence, orbits, evolution MERLIN observations of H
2O masers
in outer Galactic SFR S128A
Anita Richards, AVO/JBO
Evgueni Lekht, Eduardo Mendoza-Torres, INAOEP
Jim Cohen, Matt Crocker, JBO
Vladimir Samodourov, LPI
S128A
● 8-9 kpc (16 from Galactic centre)
● Haschick & Ho (1985)
● Colliding CO
● 2 HII regions
● 2 H2O masers
● 2+ IR sources
Monitoring ● Puschino 22-GHz
since 1955
● Relatively stable
● Three main features
● MERLIN 1998, 1999
● <10 mas resolution● FoV ~ primary
beam!● ~50-mas absolute
astrometry
Spatialcomplexity
● Colour - separate region (MERLIN)
● Log scale
● 1998-1999
● Single-dish blending not serious (at these epochs) in most regions...
Line drifts
● SD peak blueshifts ~-0.1 km/s/yr
● Resolved - similar● All regions● <dVlsr> -0.5 km/s/yr
B1
B2
B3
B5
B4
Variability
● Unsaturated maser DV-2 ~ ln F0
● Brightening - steep narrowing - pump rate increases?
● Dims again - shallower re-broadening - near saturation as pump rate falls?
● Why?
New maser region● C - several arcsec from
suggested CO front
Maser motions B
● Brightest r ~3 au, no systematic shift
● Centroid shifts 3 mas to ESE - 100 km/s???
Maser structure A
● Features seen in 1998, 1999 in A2
● Individual clouds have linear structure
● No systematic velocity gradient
● ~15 km/s range of V
LSR
Colliding clouds
• CO clouds collide in 3 dimensional space..• Cloud nearer us must be pushing back on background
cloud• Hence blue-shifted acceleration of masers?• About half of 1998 maser features have 1999 matches• No systematic proper motions• Mostly undetectable (1 mas/yr ~ 50 km/s)• Apparent 100 km/s shift probably pattern speed e.g. compression wave going past?
• Heating/shock compression - masers seen if at favourable angle?
• Any evidence for turbulence?
Compressible &/or chaotic● Line shifts and time-variability give Strouhal No <<1
(Lekht et al. 1999) for 1 au clouds● Consistent with compressible or chaotic turbulence on
scales up to 300 au● MERLIN shows cloud radii 3 - 150 au ● Strelnitski et al. 2002 - fractal analysis
● Number density non-integer function of volume ● Approximate by = dM/dV ~ rd-d0
● d0 = 2 for 2D projection, d is fractal dimension● d = d(log )/d(log r ) + d0
● Measure average surface density for increments of spot separation r
Strelniski examples
S128A
● Line shifts and time-variability give Strouhal No <<1
● Mean slope d(log )/d(log r ) = -1.77 (0.04) = d - d0
● d0 = 2● d = 0.23 (0.04)
Bohigas et al. 2003
Star formation in S128A
Masers and hot cores?● Previously, IR nebulosity N0 close to masers A● N2 close to maser region B● Now maser region C found close to N3!● Any direct association of masers with YSO?
● Need to investigate potential outflows● Any masers in discs?
● look for large linear velocity gradient● One candidate● M* ~ (dVlsr/dR)2 x R3/888 Msun (Vlsr km/s, R au)● dVlsr/dR = 0.437(0.044) km/s/au, R=47(12) au● M* = 22(3) Msun if edge-on, Keplerian● More likely just a Gaussian cloud????
Disc candidates
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