Physical conditions in Orion's Veil. III. UV iron absorption lines toward Θ¹B Orionis

We have used archival Space Telescope Imaging Spectrograph (STIS) data in the range 2100-2500 toward the Trapezium star Θ¹ B Orionis (HD 37021) to estimate the Fe column density and other parameters in the neutral Veil of Orion. The Veil lies 1-3 pc in front of the Orion H⁺ region, it has an estimated volume density of 10^2.5-3.5cm^-3, and its H° column density has been previously measured via the STIS-observed Lyα line. We find N(Fe)/N(H°) = 4.4 ± 1.3 × 10 ^-7, implying a log depletion of -1.81 ± 0.13 relative to solar. Our estimate of N(Fe) comes from direct integration of the moderate optical depth Fe II λ2249 and λ2260 lines. From this analysis, we are also able to estimate empirically new values for the oscillator strengths of the low optical depth Fe II λ2234 and λ2367 lines. N(Fe)/N(H) in the Veil is consistent with the most recent estimates of this ratio for the main Orion H⁺ region. Therefore, there is no evidence that significant Fe has been released from grains into the gaseous state in the H⁺ region via grain destruction processes. The Fe/H ratio in the Veil is also comparable to other Fe/H measurements made in the diffuse interstellar medium for lower density gas. Therefore, the present result suggests that depletion is not strongly correlated with local gas density at least up to n ≈ 10 ³cm^-3. Finally, we have identified several outlying velocity components in some of the Fe II line profiles, components that have been previously identified in optical Ca II and Na I line profiles. These outlying velocity components are probably small concentrations of cold neutral material lying tens of pc or more in front of the Veil and unrelated to it.