Comment on “Dynamic transition of supercritical hydrogen: Defining the boundary between interior and atmosphere in gas giants”

Trachenko et al. [Phys. Rev. E 89, 032126 (2014)] have argued for the existence of a “Frenkel line” in fluid hydrogen that separates “rigid” and “nonrigid” regimes in a supercritical region. On that basis, they proposed a criterion for locating the boundary between the interior and the atmosphere for gas giants. This Comment shows that the two methods they use to locate the transition between the rigid and nonrigid states are both questionable, which casts doubt on the claims in the paper.

Figure 1

Location of the minimum of $c_s\left(T\right)$ along the isobars in the $P-T$ plane for supercritical hydrogen (plus symbol) and Ar (cross symbol) taken from the NIST database [4]. Circles show the corresponding critical points. The short-dashed line corresponds to the Frenkel line for supercritical Ar according to the condition for specific heat, $C_V=2k_B$ [8], as shown in the inset.

Figure 2

Pair distribution functions for hydrogen at two pressures and temperature 2500 K, obtained from ab initio molecular-dynamics simulations. The dimensionless parameter $r_s$ corresponds to the densities for which the effective H-H interaction contains an attractive intramolecular well [15].