code: def mass_2d(self, r, rho0, Ra, Rs):
          
          Ra, Rs = self._sort_ra_rs(Ra, Rs)
          sigma0 = self.rho2sigma(rho0, Ra, Rs)
          m_2d = 2 * np.pi * sigma0 * Ra * Rs / (Rs - Ra) * (np.sqrt(Ra ** 2 + r ** 2) - Ra - np.sqrt(Rs ** 2 + r ** 2) + Rs)
          return m_2d
docs: mass enclosed projected 2d sphere of radius r
          :param r:
          :param rho0:
          :param Ra:
          :param Rs:
          :return: