Modification to the Jeans criterion by external tides: Anisotropic fragmentation and formation of filaments

Abstract: The Jeans criterion sets the foundation of our understanding of gravitational collapse. Jog studied the fragmentation of gas under external tides and derived a dispersion relation $$ l' = l_{\rm Jeans} \frac{1} {(1 + \lambda_0' / 4 \pi G \rho_0)^{1/2}} \;. $$ She further concludes that the Jeans mass is $m_{\rm incorrect}'=m_{\rm Jeans} ( 1/(1 + \lambda_0' / 4 \pi G \rho_0)^{3/2})$. We clarify that due to the inhomogeneous nature of tides, this characteristic mass is incorrect. Under weak tides, the mass is $m \approx \rho\, l_1 l_2 l_3$, where the modifications to Jeans lengths along all three dimensions need to be considered; when the tide is strong enough, collapse can only occur once 1 or 2 dimensions. In the latter case, tides can stretch the gas, leading to the formation of filaments.