Reading a text about gas nuclei I encountered the following formula:

$r = \alpha + \beta \left( \frac{T}{P} \right)^\frac{1}{3} + \kappa \left( \frac{T}{P} \right)^\frac{2}{3}$

$r$ is the critical radius of the nucleus, while $T$ and $P$ are the current temperature and pressure respectively. However I have no idea what $\alpha$, $\beta$ or $\kappa$ are. Also, I don't even known the name of such formula and the principle behind it.

Any clues?