THE IONIZATION STRUCTURE OF SHARPLESS 2-264: MULTIWAVELENGTH OBSERVATIONS OF THE λ Ori H II REGION

Abstract

ABSTRACT We present velocity-resolved maps taken with the Wisconsin H-Alpha Mapper (WHAM) in Hα, [S ii] λ 6716 , and [N ii] λ 6583 around the well-known O8 III star λ Ori A (HD 36861) ( ℓ = 185 ° to 205 ° , b = − 24 ° to − 1 ° ). The integrated intensity ( v LSR = − 80 to +80 km s−1), I H α , within WHAM’s one-degree beams varies from ∼190 R near the center to ∼10 R on the periphery of the nebula where it becomes comparable to foreground and/or background emission in this complex region. Intensity ratios for [N ii]/Hα and [S ii]/Hα average 0.28 and 0.35, respectively. In both ratios, higher values are found preferentially at larger radii from λ Ori, although the behavior of [N ii]/Hα is complicated near the edges of the nebula. The [S ii]/[N ii] intensity ratio ranges from ∼0.5 to ∼1.0, with the value increasing toward larger radii (and lower Hα intensities). Variations of the [S ii]/Hα, [N ii]/Hα, and [S ii]/[N ii] line ratios in this diffuse region show some similar trends to those seen in the warm ionized medium (WIM) but with generally lower metal-line ratios. As with the WIM, the trends are driven by changes in the underlying physical parameters, most notably the ionization states and gas temperature. To investigate which cause might be dominant in this region, we use these extremely high signal-to-noise observations to construct a map of temperature and non-thermal velocity throughout the nebula. Using the line widths of Hα and [S ii], we separate thermal and non-thermal components and find spatial trends of these parameters within the nebula. Ion temperatures range between 4000 and 8000 K throughout the nebula. The non-thermal velocity map reveals a decrease in velocity from about 10 to 5 km s−1 from the center to the edge of the lower half of the H ii region. In addition to using the widths as a measure of temperature, we also use the variation in [N ii]/Hα to estimate electron temperature. The results obtained from this diffuse H ii region around λ Ori can be compared to studies of the WIM to provide important insight into the nature of the diffuse ionized gas throughout the disk and halo of the Galaxy.