2004.02.09

Structure, star formation and magnetic fields
in the OMC 1 region
Coppin et al., A&A, 356, 1031 (2000)
UMEKAWA Michihisa (ASAFAS)
Plasma seminar
Feb. 2
• Observation of OMC1 using 450 and 850 micro m
• clump mass function
• polarimetry (direction of magnetic fields)
Orion star forming region
•~４５０ｐｃ
•5h20m-6h00m
•+5°- -10°
OMC1
James
Clerk Maxwell Telescope (JCMT)
SCUBA (Submillimetre Common-User Bolometer Array)
Dec. 1997
Jan. 1999 for polarimetry data, with a rotating half-wave plate
and a fixed etched grid in front of the SCUBA
15m, the largest
submillimetre telescope in the world.
SCUBA
450 micro m: hexagonal array of 91 bolometers
850 micro m: 37 bolometers
field of view: 2’.3
Calibration sources: Uranus, Mars, OH231.8, CRL618......
Nine over lapping fields of view over two nights
Bright bar region could be observed only second night
north-east bar
KL
Clump identification
1.
2.
3.
4.
part of a clump being truncated by the map edge
having a negative flux
having a higher mean signal in larger areas
having a peak flux above the local background
lower than the noise cut-off
reject criteria
The fluxes of the peaks above the local backgroud for clumps
•39 clumps at 850 micro m, 28 clumps at 450 micro m
55 source are finally listed
•3 clumps are on common regions 850 and 450 micro m
covered, but not seen on 450 micro m map. All have
some emission on 450 micro m map although not a distinct
centroid.
•Clumps are generally circle, however at 850 micro m 7
clumps and at 450 micro m 4 clumps are elongated.
•～20% of the flux within their beam size region
•<7% of the total flux of the whole jiggle-map region.
•S_ν∝ν^(2+β) (black body). The mean βis 0.5.
This is small than 1.75 to 2.5 of the previous work at
350 and 1100 micro m. For clouds This is 1.8 to 2.3.
Expecting the growth of grains?
Clump mass function
•The number of clumps dN in mass bin M to M+dM is
dN/dM ∝ M^(-α)
•S_clumps directly traces mass of the clumps.
β and temperature of dust are approximately constant.
log(dN/dS_clump) vs log S_clump is plotted.
α= 1.51 at 850 micro m and 1.54 at 450 micro m.
Polarimetry data
Conclusions
•4 clouds larger than 0.1 pc contain most of the mass and flux
in the region, while the 55 clumps identified on ~0.02pc
scales have <7% of the total flux.
•magnetic field may have influenced the formation of the
brightest cores, Orion-KL and Orion-S, but overall the HII
region expansion and fragmentation on the Jeans length
appear to be more important dynamically.
•a single power-law function of slope -1.5 provides a good fit
to the clump masses over a very wide range from 0.1 to 100
Msun.
• One more NEXT
Cloud mass function in Motte et al. (1998)
Motte et al., A&A 336, 150 (1998)
ρOphiuchi dark cloud : 160pc from the sun.
IRAM 30-m telescope located near Granada Spain
1.3mm dust continuum emission from the ρOphiuchi main cloud
MPIfR 19-channel bolometer
run: 1995 Mar. 17 to 21, 1996 Mar. 13 to 17, 1996 Apr. 3 to 4,
and 1997 Feb. 17 to 23
Results
• dN/dM ∝ M^-α
• α~2.5 for 1 – 10 Msun
• α~1.5 for less than 1Msun
• The mass distribution for clumps mimics
the behavior of the stellar initial mass
function.
Comments in Coppin et al.
• Motte et al. found two slopes of cloud mass
function in ρOphiuchi. In contrust their result has
only -1.5.
• The changes in mas function slope are potentially
a difference between regions of high-mass and
low-mass star formation.
Statistical problem: only afew high mass objects
identified in each cloud.
Umekawa’s simulations
(a)
(b)
Umekawa’s simulations
• (a) Mc~3.5x10 Msun
• (b) Mc~1.1x10^-1Msun
• 微妙である

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