The spin density matrix element $\rho_{00}$ of prompt D$^*$ mesons as a function of $p_{\rm T}$ in the rapidity interval $0.3 < |y| < 0.8$ compared to that of inclusive J$/\psi$ mesons in the rapidity interval $2.5< y< 4$ measured in Pb$-$Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV in the 30$-$50% centrality class. The bottom panel shows the deviation of the measurements from the null hypothesis ($\rho_{00} = 1/3$) in units of the total uncertainties $\sigma_{\rho_{00}}$.