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virial parameter pârâmun-e viriyâl Fr.: paramètre du viriel A dimensionless parameter that measures the ratio of thermal plus kinetic energies to
gravitational energy of a physical system, such as a molecular cloud. The virial
parameter is expressed as:
R and M are the radius and mass of the
cloud respectively, σ is the one-dimensional
→ velocity dispersion inside the cloud, and G
the → gravitational constant. It indicates whether a
cloud could be bound or not. For molecular clouds that are
confined by their surface pressure and for which self-gravity is unimportant,
α is much larger than unity, whereas _{vir}α
is ~ 1 when the gravitational energy of a clump becomes comparable to its kinetic energy.
See, e.g., Bertoldi & McKee, 1992 (ApJ 395, 140).
See also → _{vir}virial theorem. |