Measurements and Analysis of Helium-Like Triplet Ratios in the X-Ray Spectra of O-Type Stars Page: 4 of 52
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( 2.2), and of spatial distribution of the X-ray emitting plasma (@ 2.3), while incorporating
the line-ratio modeling into a self-consistent line-profile model (@ 2.4). In 3 we discuss
the reduction and analysis of archival O star X-ray spectra. In @ 4 we give the results of
this analysis, fitting high signal-to-noise complexes with the self-consistent line-profile model
described in 2.4 and fitting the lower signal-to-noise complexes with multiple Gaussians
and interpreting these results according to the spatially distributed picture described in 2.3.
In 5 we discuss the implications of these results, and in 6 we give our conclusions.
2.1. Radial dependence of the f/i ratio
The physics of helium-like ions in coronal plasmas has been investigated in numerous
papers (Gabriel & Jordan 1969; Blumenthal et al. 1972; Gabriel & Jordan 1973; Mewe &
Schrijver 1975, 1978a,b,c; Pradhan & Shull 1981; Pradhan 1982; Porquet et al. 2001). The
principal diagnostic is the ratio of the strengths of the forbidden to intercombination lines,
R = f/i. We will use the calligraphic R to refer to this ratio, and the italic R to refer to
distances comparable to the stellar radius.
The upper level of the forbidden line (2 331) is metastable and relatively long-lived.
When the excitation rate from 2 331 to the upper levels of the intercombination line (2 3P,2)
becomes comparable to the decay rate of the forbidden transition, the line ratio is altered.'
The excitations may be due to electron impacts in a high density plasma, or due to an
external UV radiation source.
Gabriel & Jordan (1969) (hereafter GJ) and Blumenthal, Drake, & Tucker (1972) (here-
after BDT) derive the expression
R = Ro 1 (1)
1 + #/#c + ne /nc
where 0 is the photoexcitation rate from 23S to 23P, and 0c is the critical rate at which R
is reduced to Ro/2. Similarly, ne is the electron density, and n, is the critical density.
In Table 1 we give our adopted values for the atomic parameters necessary for calculation
of He-like triplet ratios. We adopt the BDT values for 0, because they have calculated it for
'The 2 31 state may also be excited to the 2 3Po state, but this state does not decay to ground, so we
omit it from our discussion. However, in Gabriel & Jordan (1969) and Blumenthal, Drake, & Tucker (1972),
the formal treatment involves all states.
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Leutenegger, Maurice A.; Paerels, Frits B.S.; /Columbia U., Astron. Astrophys.; Kahn, Steven M.; /SLAC; Cohen, David H. et al. Measurements and Analysis of Helium-Like Triplet Ratios in the X-Ray Spectra of O-Type Stars, article, June 19, 2006; [Menlo Park, California]. (digital.library.unt.edu/ark:/67531/metadc892235/m1/4/: accessed December 15, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.