2007MNRAS.376..895M

Recent observations and theoretical investigations of neutron stars indicate that their atmospheres consist not of hydrogen or iron but possibly other elements such as helium. We calculate the ionization and dissociation equilibrium of helium in the conditions found in the atmospheres of magnetized neutron stars. For the first time, this investigation includes the internal degrees of freedom of the helium molecule. We found that at the temperatures and densities of neutron star atmospheres, the rotovibrational excitations of helium molecules are populated. Including these excitations increases the expected abundance of molecules by up to two orders of magnitude relative to calculations that ignore the internal states of the molecule; therefore, if the atmospheres of neutron stars indeed consist of helium, helium molecules and possibly polymers will make the bulk of the atmosphere and leave signatures on the observed spectra from neutron stars. We applied our calculation to nearby radio-quiet neutron stars (RQNS) with B_dipole∼ 10¹³-10¹⁴ G. If helium comprises their atmospheres, our study indicates that isolated neutron stars with T_BB∼ 10⁶ K such as RX J0720.4-3125 and RX J1605.3+3249 will have He⁺ ions predominantly, while isolated neutron stars with lower temperature (T_BB∼ 5x10⁵ K) such as RX J1856.5-3754 and RX J0420.0-5022 will have some fraction of helium molecules. We found that ionization, dissociation and electric excitation energies of helium molecules are larger than 100 eV at B ≳ 10¹³ G. On the other hand, rotovibrational excitation energies are in the range of 10-100 eV at B = 10¹²-10¹⁴ G. If helium molecules are abundant, their spectroscopic signatures may be detected in the optical, ultraviolet and X-ray band.