code: def sigma_s2(self, r, R, r_ani, a, gamma, rho0_r0_gamma):
          
          beta = self._beta_ani(r, r_ani)
          return (1 - beta * R**2/r**2) * self.sigma_r2(r, a, gamma, rho0_r0_gamma, r_ani)
docs: projected velocity dispersion
          :param r:
          :param R:
          :param r_ani:
          :param a:
          :param gamma:
          :param phi_E:
          :return: