Copyright by Athena Ranice Stacy 2011 - The University of Texas at ...

Figure 3.7: Projected ionization structure of gas at 1700, 2300, and 4200 yr
after initial sink formation. White lines in left two panels depict the density
contours of the disk at densities of 10 8.5 , 10 9 , 10 9.5 , and 10 10 cm −3 . White lines
in the right panel depict contours at densities of 10 8 , 10 8.25 , and 10 8.75 cm −3 .
Box length is 10,000 AU. Note the pronounced hour-glass morphology of the
developing ultra-compact H ii region, roughly perpendicular to the disk. This
morphology becomes more spherical as the I-front expands.
collapse on roughly the orbital timescale, is harder for these sinks to satisfy.
When the disks are undergoing fragmentation during the first few hundred
yr, the sinks form within ∼ 100 AU of each other and orbit at approximately
5-10 km s −1 , yielding an orbital timescale on the order of 100 yr. This is not
significantly longer than the free-fall timescale of the sinks, which is < ∼ 100
yr. This leads to sink merging before an entire orbit is complete, particularly
during early times in the ‘no-feedback’ case. The one merger that occurs for
the main sink in the ‘with-feedback’ case occurs after multiple orbits, however,
and is a result of migration.
3.3.3 Ionization Front Evolution
The I-front initially appears around the main sink ∼ 500 yr after the
sink first forms, once it has grown to 15 M⊙ (see panel d of Fig. 3.1). The
morphology of the growing I-front is shown in Figure 3.7, where we can see
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