Molecular globules in the Veil bubble of Orion. IRAM 30m 12CO, 13CO, and
  C18O 2-1 expanded maps of Orion A
  Strong winds and ultraviolet (UV) radiation from O-type stars disrupt and
ionize their molecular core birthplaces, sweeping up material into parsec-size
shells. Owing to dissociation by starlight, the thinnest shells are expected to
host low molecular abundances and therefore little star formation. Here, we
expand previous maps taken with the IRAM 30m telescope and present
square-degree 12CO and 13CO (J=2-1) maps of the wind-driven "Veil bubble'' that
surrounds the Trapezium cluster and its natal Orion molecular core (OMC).
Although widespread and extended CO emission is largely absent from the Veil,
we show that several CO "globules'' exist and are embedded in the
[CII]158um-bright shell that confines the bubble. This includes the first
detection of quiescent CO at negative LSR velocities in Orion. Given the harsh
UV irradiation conditions in this translucent material, the detection of CO
globules is surprising. These globules are small (R=7,100 AU), not massive
(M=0.3M_Sun), and are moderately dense: n_ H=4x10^4 cm^-3 (median values). They
are confined by the external pressure of the shell, P_ext/k~10^7 cm^-3 K, and
are likely magnetically supported. They are either transient objects formed by
instabilities or have detached from pre-existing molecular structures, sculpted
by the passing shock associated with the expanding shell and by UV radiation
from the Trapezium. Some represent the first stages in the formation of small
pillars, others of isolated small globules. Although their masses do not
suggest they will form stars, one globule matches the position of a known YSO.
The lack of extended CO in the "Veil shell'' demonstrates that feedback from
massive stars expels, agitates, and reprocesses most of the disrupted molecular
cloud gas, thereby limiting the star-formation rate in the region. The presence
of globules is a result of this feedback.
