Mass of helium atom11/11/2022 ![]() In this paper, we focus on He atoms and their radiative transitions in magnetic NS atmospheres. Mid- Z element atmospheres for B∼ 10 12–10 13 G were recently studied by Mori & Ho (2007). These improvements have been incorporated into partially ionized, magnetic NS atmosphere models ( Ho et al. Recently, a thermodynamically consistent equation of state and opacities for magnetized ( B= 10 12–10 15 G), partially ionized H plasma have been obtained ( Potekhin & Chabrier 2003, 2004), and the effect of bound atoms on the dielectric tensor of the plasma has also been studied ( Potekhin et al. Rajagopal, Romani & Miller 1997) relied on oversimplified treatments of atomic physics and plasma thermodynamics (ionization equilibrium, equation of state and non-ideal plasma effects). Early considerations of partially ionized and strongly magnetized atmospheres (e.g. Because a strong magnetic field greatly increases the binding energies of atoms, molecules and other bound species (for a review, see Lai 2001), these bound states may have appreciable abundances in the NS atmosphere, as guessed by Cohen, Lodenquai & Ruderman (1970) and confirmed by calculations of Lai & Salpeter (1997) and Potekhin, Chabrier & Shibanov (1999). 2001), including the effect of vacuum polarization (see Lai & Ho 2002, 2003 Ho & Lai 2003 van Adelsberg & Lai 2006). Fully ionized atmosphere models in various magnetic field regimes have been extensively studied (e.g. The atmosphere composition may also be affected by (slow) diffusive nuclear burning in the outer NS envelope ( Chang, Arras & Bildsten 2004), as well as by the bombardment on the surface by fast particles from NS magnetospheres (e.g. Fe) atmosphere may be possible if no fallback/accretion occurs. A pure He atmosphere results if H is completely burnt out, and a heavy-element (e.g. Because of the efficient gravitational separation of light and heavy elements, a pure H atmosphere is expected even if a small amount of fallback or accretion occurs after NS formation. The atmosphere composition of the NS is unknown a priori. ![]() 2005 Medin & Lai 2006, 2007), detailed modelling of radiative transfer in magnetized NS atmospheres is important. Since the thermal radiation from a NS is mediated by its atmosphere (if T is sufficiently high so that the surface does not condense into a solid see e.g. Multiple lines also have the potential of constraining the mass–radius relation of NSs (through the measurement of gravitational redshift). Clearly, understanding these absorption lines is very important as it would lead to direct measurement of the NS surface magnetic fields and compositions, shedding light on the nature of these objects. The identifications of these features, however, remain uncertain, with suggestions ranging from proton cyclotron lines to atomic transitions of H, He or mid- Z atoms in a strong magnetic field (see Sanwal et al. 2006) and possibly RBS 1774 (∼0.7 keV Zane et al. 2007), single- or multiple-absorption features at E≃ 0.2–1 keV have been detected from several sources (see van Kerkwijk & Kaplan 2007): e.g. 2003) or by emission from a condensed surface covered by a thin atmosphere ( Ho et al. RX J1856.5−3754) have featureless X-ray spectrum remarkably well described by blackbody (e.g. The true nature of these sources, however, is unclear at present: they could be young cooling NSs, or NSs kept hot by accretion from the interstellar medium, or magnetar descendants. Haberl 2006): they share the common property that their spectra appear to be entirely thermal, indicating that the emission arises directly from the NS surfaces, uncontaminated by magnetospheric emission. Of great interest are the radio-quiet, thermally emitting NSs (e.g. equation of state at super-nuclear densities, cooling history, surface magnetic field and composition). Such studies can potentially provide invaluable information on the physical properties and evolution of NSs (e.g. ![]() This was made possible by X-ray telescopes such as Chandra and XMM–Newton. Harding & Lai 2006 Kaspi, Roberts & Harding 2006). Atomic processes, magnetic fields, stars: atmospheres, stars: neutron 1 INTRODUCTIONĪn important advance in neutron star (NS) astrophysics in the last few years has been the detection and detailed studies of surface emission from a large number of isolated NSs, including radio pulsars, magnetars and radio-quiet NSs (e.g. ![]()
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