Two New Extremely Iron-rich Hot DA White Dwarfs and the Nature of the EUV Opacity
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Abstract
We have obtained IUE echelle spectra of two bright extreme ultraviolet (EUV) sources discovered by the ROSAT wide field camera all-sky survey. These stars, RE 2214-492 and RE 0623-377, are previously uncataloged hot DA white dwarfs with respective apparent visual magnitudes of 11.71 and 12.09 and exhibit short-wavelength EUV cutoffs steeper than most other DAs. The IUE echelle spectra are rich in absorption lines due to a large number of Fe V and Fe VI features, in addition to the highly ionized C, N, O ions frequently seen in other hot DA white dwarfs. No He is observed in either star. Comparison of RE 2214-492 and RE 0623-377 with two well-studied hot DA white dwarfs, G191 B2B and Feige 24, indicates that these new stars are both significantly hotter and more metal-rich than G191 B2B and Feige 24. From optical and UV line profiles we find Teff = 63,500 K and 60,300 K for RE 2214-492 and RE 0623-377, respectively. From modeling of Fe V features in the echelle spectra we find corresponding Fe abundances of log [N(Fe)/ n(H)] = -4.25±0.25. The association of steep EUV cutoffs in these stars, and in G191 B2B and Feige 24, with the observed presence of Fe suggests that iron-group elements along with other heavy elements are responsible for much of the EUV opacity observed by ROSA T in the hottest DA stars.
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