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Session 51 - Interstellar Medium II.
Display session, Thursday, January 08
Exhibit Hall,
On a flight of the ORFEUS-SPAS mission in November-December 1996, the Interstellar Medium Absorption Profile Spectrograph (IMAPS) recorded high-resolution spectra of a number of hot stars over a wavelength interval that included the resonance absorption features of O VI at 1032 and 1038ÅO VI is an important tracer of interstellar gases at a temperature T\approx 3\times 10^5K. Two noteworthy targets that were observed were 15 Mon, a star behind the Monogem Ring, and HD64760, a star within or behind the Gum Nebula. Distinct absorption features from interstellar O VI appear on top of the slowly varying stellar continua in the spectra of both stars: 15 Mon shows a shallow and very broad feature centered at a heliocentric radial velocity of about +35 km s^-1 and a stronger, asymmetric one with a peak absorption at v=+105 km s^-1, while HD64760 exhibits an extraordinarily strong feature at v=+35 km s^-1 (central depth = 45% at 1032ÅFWHM = 80 km s^-1) that is situated between distinct, weaker ones centered at -195 and +155 km s^-1. In the future, the O VI features may be compared with much weaker absorptions by species at lower ionization levels so that we can improve our understanding of the origin and dynamics of the O VI-bearing material. Already, there is evidence from an HST archive spectrum of 15 Mon that the main, low-velocity interstellar Mg II absorption for this star is accompanied by narrow, weak features at v=+57 and +86 km s^-1.
