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Session 17 - Molecular Clouds and Star Formation.
Oral session, Monday, June 10
Humanities 2650,
The W3 molecular cloud core has been observed with the BIMA and NRAO 12 m telescopes in the ^13CO (J = 1--0) and (J = 1--0) lines. Two BIMA fields were imaged, centered on the two infrared sources IRS 5 and IRS 4 (80^\prime\prime west of IRS 5).
The total masses of the the molecular cores associated with IRS 4 and IRS 5 are 1700 and 1100 M_ødot, respectively. Seven clumps between 0.11 and 0.33 pc in size are identified with H_2 column densities in the range 1.3 to 2.1 \times 10^23 cm^-2. Derived masses are between 120 and 480 M_ødot; under simple geometrical assumptions, n(H_2) is between 1.8 to 4.5 \times 10^5 cm^-3. For both cores the sum of all the individual clumps was about half of the total mass. The virial masses of the clumps ranged from 80 to 660 M_ødot. The virial masses are near the measured H_2 masses. Comparisons of the ^13CO and C^18O line strengths provide an estimate of the optical depth in ^13CO, assuming the C^18O line has a negligible opacity. For two clumps located at IRS 5 and 20^\prime\primesouth of IRS 4 an opacities derived were 0.3 and 0.7, respectively. At the positions of IRS 4 and IRS 5, clumps are observed; the IRS 4 clump in particular has a n(H_2) about a factor of two larger than the other clumps.
At the position of IRS 5, the data show a systematic velocity structure suggestive of rotation. A Keplarian velocity field was fitted to the data, which yielded a dynamical mass of 100 M_ødot. Integrating the emission from the same clump over the same velocity range gives a H_2 masss of 110 M_ødot. Using the derived H_2 column density toward IRS 5 and a previous HI Zeeman result, we can derive the ratio |B|/N_H (which is proportional to the magnetic flux-to-mass ratio, \Phi/H_H) can be derived. The determined ratio is near the magnetically critical value, which is consistent with the standard theory of high mass star formation.
