Surprisingly Little OviEmission Arises in the Local Bubble

Abstract

This paper reports the first study of the O VI resonance line emission (λλ1032, 1038) originating in the Local Bubble (or Local Hot Bubble) surrounding the solar neighborhood. In spite of the fact that O VI absorption within the Local Bubble has been observed, no resonance line emission was detected during our 230 ks Far Ultraviolet Spectroscopic Explorer observation of a "shadowing" filament in the southern Galactic hemisphere. As a result, tight 2 σ upper limits are set on the intensities in the 1032 and 1038 Å emission lines: 500 and 530 photons cm-2 s-1 sr-1, respectively. These values place strict constraints on models and simulations. They suggest that the O VI-bearing plasma and the X-ray emissive plasma reside in distinct regions of the Local Bubble and are not mixed in a single plasma, whether in equilibrium with T ~ 106 K or highly overionized with T ~ 4 to 6 × 106 K. If the line of sight intersects multiple cool clouds within the Local Bubble, then the results also suggest that hot/cool transition zones differ from those in current simulations. With these intensity upper limits, we establish limits on the electron density, thermal pressure, path length, and cooling timescale of the O VI-bearing plasma in the Local Bubble. Furthermore, the intensity of O VI resonance line doublet photons originating in the Galactic thick disk and halo is determined (3500-4300 photons cm-2 s-1 sr-1), and the electron density, thermal pressure, path length, and cooling timescale of its O VI-bearing plasma are calculated. The pressure in the Galactic halo's O VI-bearing plasma (3100-3800 K cm-3) agrees with model predictions for the total pressure in the thick disk/lower halo. We also report the results of searches for the emission signatures of interstellar C I, C II, C III, N I, N II, N III, Mg II, Si II, S II, S III, S IV, S VI, Fe II, and Fe III.