The Dark Molecular Gas
  The mass of molecular gas in an interstellar cloud is often measured using
line emission from low rotational levels of CO, which are sensitive to the CO
mass, and then scaling to the assumed molecular hydrogen H_2 mass. However, a
significant H_2 mass may lie outside the CO region, in the outer regions of the
molecular cloud where the gas phase carbon resides in C or C+. Here, H_2
self-shields or is shielded by dust from UV photodissociation, where as CO is
photodissociated. This H_2 gas is "dark" in molecular transitions because of
the absence of CO and other trace molecules, and because H_2 emits so weakly at
temperatures 10 K < T < 100 K typical of this molecular component. This
component has been indirectly observed through other tracers of mass such as
gamma rays produced in cosmic ray collisions with the gas and
far-infrared/submillimeter wavelength dust continuum radiation. In this paper
we theoretically model this dark mass and find that the fraction of the
molecular mass in this dark component is remarkably constant (~ 0.3 for average
visual extinction through the cloud with mean A_V ~ 8) and insensitive to the
incident ultraviolet radiation field strength, the internal density
distribution, and the mass of the molecular cloud as long as mean A_V, or
equivalently, the product of the average hydrogen nucleus column and the
metallicity through the cloud, is constant. We also find that the dark mass
fraction increases with decreasing mean A_V, since relatively more molecular
H_2 material lies outside the CO region in this case.
