Number Densities and Rate Coefficients in Astrochemistry

Basic ideas about astrochemistry began to be explored theoretically in the middle of the 20th century when it was conventional to use centimetre-gram-seconds (cgs) units rather than SI. This convention became embedded in the astrochemical literature (a few examples of which are appended to each chapter of this book) and research papers in astronomy are conventionally written using a mixture of cgs and the units of astronomy, as appropriate. Therefore, we have retained this convention in this book, so that our readers may consult the astrochemical literature more easily.

Number densities are given per cm³ rather than per m³, and rate coefficients of binary reactions are in units of cm³ s⁻¹ rather than m³ s⁻¹. Thus, a binary rate coefficient of (say) 10⁻¹⁷ m³ s⁻¹ is replaced by one of 10⁻¹¹ cm³ s⁻¹. We like to think that this convention has some small but particular value in astronomy, because the mean number density of hydrogen atoms in the interstellar medium of the Milky Way galaxy is about one per cm³ (or 10⁶ m⁻³), and so all other number densities (say, of molecular clouds with a hydrogen number density of 10⁴ cm⁻³) are automatically compared to this mean.