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Session 120 - The Local Interstellar Medium.
Oral session, Thursday, January 18
Corte Real, Hilton

[120.04] Observations of O \small VI Emission from the Galactic Corona with the Hopkins Ultraviolet Telescope

W. V. Dixon, A. F. Davidsen (JHU), H. C. Ferguson (STScI)

We have searched for far-UV emission from coronal gas in the Galactic halo along lines of sight with galactic latitudes between 42\arcdeg\ and 88\arcdeg\ using spectra obtained by the Hopkins Ultraviolet Telescope (HUT) during the Astro-2 space shuttle mission in 1995 March. Of four spectra taken through a \SLITfive\ aperture, two show Øsixwave\ emission at a significance of 4 \sigma. Two out of six spectra taken through a \SLITsix\ aperture also show Øsix\ emission, though at only 2 \sigma levels of significance. Three of the detections lie near regions of enhanced soft X-ray emission associated with Radio Loop I. The fourth, at l = 218\arcdeg, b = 56\arcdeg, may represent a more typical region of the halo. In its spectrum, we find I(Øsix) = (3.59 \pm 0.96) \times 10^-7 \ERGS. This is the first detection of Øsix\ emission from the Galactic halo. None of the spectra exhibit significant emission from \Cfourwave, though our upper limits are greater than the reported intensities along other lines of sight. We set a limit on I(Øsix) / I(\Cfour) \geq 3.4, consistent with the predictions of self-photoionizing galactic fountain models, but higher than those of models based on turbulent mixing layers. Combining our measured Øsix\ intensity with estimates of N(Øsix) through the halo, we find that, for 5.3 \leq \log T \leq 5.8, the data are consistent with n_e \approx 0.06 cm^-3 and 22000 \leq P/k \leq 67000 cm^-3 K, substantially greater than the values derived from \Cfour\ observations, suggesting that the \Cfour\ and Øsix\ emission arise from physically distinct clouds and/or that a substantial portion of the \Cfour\ absorption arises from nonemitting photoionized gas. This result is consistent with galactic halo models incorporating self-photoionization of the cooling gas.

Program listing for Thursday