conversion factor کروند ِ هاگرد karvand-e hâgard Fr.: facteur de conversion A numerical factor that, by multiplication or division, translates one
unit or value into another.
In → molecular cloud studies, a factor used to convert
the → carbon monoxide (CO) line intensity to
→ molecular hydrogen (H2)
→ column density; usually denoted
XCO = I(CO) / N(H2). This useful factor relates
the observed CO intensity to the cloud mass. A general method to derive XCO
is to compare the → virial mass
and the 12CO (J = 1-0) luminosity of a cloud.
The basic assumptions are that the CO and H2 clouds are co-extensive, and
molecular clouds obey the → virial theorem.
However, if the molecular cloud is subject to ultraviolet radiation,
selective → photodissociation may take place, which will change
the situation. Moreover, molecular clouds may not be in
→ virial equilibrium.
To be in virial equilibrium molecular clouds must have enough mass, greater than
about 105 solar masses.
The way → metallicity affects
XCO is a matter of debate, and there is no clear correlation
between XCO and metallicity. Although lower metallicity brings about
higher ultraviolet fields than in the solar vicinity, other factors appear to be
as important as metallicity for the determination of XCO.
In the case of the → Magellanic Clouds,
XCO(SMC) =
14 ± 3 × 1020 cm-2 (K km s-1)-1, which is
larger than XCO (LMC) = 7 ± 2 × 1020 cm-2
(K km s-1)-1.
An independent method to derive XCO is to make use of the
gamma ray emission from a cloud. The flow of → cosmic ray
protons interacts with interstellar low-energy hydrogen nuclei in clouds creating neutral
→ pions. These pions quickly decay into two gamma rays. It is therefore
possible to estimate the number of hydrogen nuclei and hence the cloud mass
from the gamma ray counts. Such a gamma-ray based conversion factor is estimated to be
2.0 × 1020 cm-2 (K km s-1)-1
for Galactic clouds, in good agreement with the result obtained from the
virial method. However, the gamma ray flux is not well known in general, so this
method is uncertain as well. See, e.g., Fukui & Kawamura, 2010 (ARAA 48, 547).
See also: → conversion; → factor. |
