Critical adiabatic index for dipolar accretion
= Critical adiabatic index for dipolar accretion
{title2=$\gamma_{\rm crit}=7/5$}
For <transonic accretion in a dipolar flux tube>, the <Bernoulli equation> at the <sonic point> contains $c_s^2[1/(\gamma-1)-5/2]$. A positive reservoir energy requires $\gamma<7/5$. Compression in the $r^3$ <dipolar flux-tube area> raises the <specific enthalpy> more rapidly than in the $r^2$ geometry of spherical <Bondi accretion>, whose corresponding upper index is $5/3$.