The X-factor in astrophysics, often labeled X
CO, is an empirically determined proportionality constant which converts
carbon monoxide
Carbon monoxide ( chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...
(CO) emission line brightness to molecular
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
(H
2) mass.
Origin
The term X-factor was coined in a 1983 paper titled "Gamma-rays from atomic and molecular gas in the first galactic quadrant" and published in ''
The Astrophysical Journal
''The Astrophysical Journal'', often abbreviated ''ApJ'' (pronounced "ap jay") in references and speech, is a peer-reviewed scientific journal of astrophysics and astronomy, established in 1895 by American astronomers George Ellery Hale and Ja ...
''.
Calibration
Calibrating X
CO requires an independent method of determining the amount of molecular hydrogen in a given astrophysical region. While direct emission from molecular hydrogen is difficult to observe, there are other ways of inferring molecular hydrogen mass, outlined below.
* The mass of an individual
molecular cloud
A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit absorption nebulae, the formation of molecules (most commonly molecular hydroge ...
can be inferred from its velocity dispersion. Assuming that the cloud is self gravitating and
virialized, and that nearly all of the gas mass is molecular hydrogen, one obtains an estimate of the cloud's H
2 mass.
* H
2 mass can be estimated through gamma ray observations, where gamma ray emission arises from the interaction of
cosmic rays
Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our ow ...
with H
2 molecules.
* Using optical and infrared observations, one can estimate the amount of dust in a region and convert to H
2 mass through an (empirically determined) dust-to-gas ratio.
References
External links
Review: The CO-to-H2 Conversion Factor
Astrophysics
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