Astronomy and Astrophysics – Astrophysics
Scientific paper
Dec 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005phdt........23c&link_type=abstract
PhD Thesis, Proquest Dissertations And Theses 2005. Section 0035, Part 0606 149 pages; [Ph.D. dissertation].United States -- Cal
Astronomy and Astrophysics
Astrophysics
1
Neutron Stars, Atomic Lines, Spectra
Scientific paper
The surface composition of neutron stars determines their luminosity, spectral shape and radiation. We examine their effects in young neutron stars and type I X-ray burst sources. We study the mechanisms by which surface composition can change on a neutron star and the radiative transfer through a thin heavy element scattering layer on a neutron star surface during a type I X-ray burst.
We focus our attention on a mechanism of nuclear processing on the surfaces of young neutron stars known as diffusive nuclear burning. Hydrogen diffuses down to deeper layers in the neutron star where higher temperatures and the presence of proton-capturing elements consume it. We find the rate of this process by calculating in detail the compositional structure of a neutron star envelope. We then include the effects of nuclear burning in two different regimes: nuclear-limited diffusive nuclear burning and diffusion-limited diffusive nuclear burning. We also study the effects of magnetic fields on this mechanism. For B < 10 12 --10 13 G, we find that increasing B fields decreases the rate of burning by up to two orders of magnitude. However, for larger B fields, the rate of nuclear burning is enhanced. Applying this to young neutron stars, we find that for their observed effective temperatures this mechanism can consume all the hydrogen on an astrophysically short timescale. We also show that primordial hydrogen must have been consumed by this process early in its cooling history when it was much hotter. We also find for magnetars ( B > 10 14 G) that no hydrogen can exist on its surface because their high effective temperatures and large B -fields conspired to quickly capture all the hydrogen even onto heavy elements like Fe.
We also study the effects of trace heavy metals on the surfaces of neutron stars during a type I X-ray burst. Temperatures are in excess of 10 7 K during a type I X-ray burst, thus ionizing all abundant material in cosmic composition except for Fe. This signature of this material was found during an observation of EXO 0748-676 by Cottam, Paerels and Mendez. Noting that the diffusion time on the neutron star surface is extremely rapid, we study the origin of the Fe observed. We speculate on a mode of accretion known as the accretion + spallation scenario where the Fe is deposited above the continuum photosphere by the accretion beam and destroyed by nuclear spallation reactions due to incoming protons. We study the line produced by hydrogenic Fe under these conditions. We show that the primary broadening mechanism is Stark broadening, that the line is a resonant line and that the material is in NLTE. We include these effects and self consistently calculate the line profile. We find that the Fe column required to produce this line is [approximate] 3--10 times that of a solar metallicity photosphere. With the intrinsic line profile in hand, we then calculate the rotationally broadened line profiles to fit the data and find that the parameters of EXO 0748-676 is ν s sin i ( R /10km) = [Special characters omitted.] Hz, lg 10 ( N Fe,n=2 /cm -2 ) = [Special characters omitted.] and 1 + z = [Special characters omitted.] with 95% confidence.
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