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