Structure,%20star%20formation%20and%20magnetic%20fields%20in%20the%20OMC%201%20region

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Structure, star formation and magnetic fields in
the OMC 1 region
Coppin et al., AA, 356, 1031 (2000)

• UMEKAWA Michihisa (ASAFAS)

Plasma seminar Feb. 2
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• Observation of OMC1 using 450 and 850 micro m
• clump mass function
• polarimetry (direction of magnetic fields)

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Orion star forming region

• 450pc
• 5h20m-6h00m
• 5- -10

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OMC1
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• 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 tele- scope in
the world.
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• 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

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north-east bar KL
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Clump identification

• 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
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• 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
• lt7 of the total flux of the whole jiggle-map
  region.
• S_???(2ß) (black body). The mean ßis 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?

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Clump mass function

• The number of clumps dN in mass bin M to MdM is
• dN/dM ? M(-a)
• S_clumps directly traces mass of the clumps.
• ß and temperature of dust are approximately
  constant.
• log(dN/dS_clump) vs log S_clump is
  plotted.

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a 1.51 at 850 micro m and 1.54 at 450 micro m.
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Polarimetry data
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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 lt7 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

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Cloud mass function in Motte et al. (1998)
Motte et al., AA 336, 150 (1998) ?Ophiuchi dark
cloud 160pc from the sun. IRAM 30-m telescope
located near Granada Spain 1.3mm dust continuum
emission from the ?Ophiuchi 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
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Results
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• dN/dM ? M-a
• a2.5 for 1 10 Msun
• a1.5 for less than 1Msun
• The mass distribution for clumps mimics the
  behavior of the stellar initial mass function.

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Comments in Coppin et al.

• Motte et al. found two slopes of cloud mass
  function in ?Ophiuchi. 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.

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Umekawas simulations
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(a) (b)
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Umekawas simulations

• (a) Mc3.5x10 Msun
• (b) Mc1.1x10-1Msun
• ?????