NaI and CaII absorption components observed towards the Orion-Eridanus Superbubble

¹ Experimental Astrophysics Group, Space Sciences Laboratory, UC Berkeley, Berkeley, CA 94720, USA e-mail: bwelsh@ssl.berkeley.edu
² Department of Physics, Univ. of Wisconsin – La Crosse, La Crosse, WI 54601, USA

Received: 13 January 2005
Accepted: 19 May 2005

Abstract

We present medium-resolution spectra (R ~ 7.5 km s^-1) of the interstellar NaI and CaII interstellar absorption lines observed towards 16 early-type stars with distances of 160–1 kpc in the line-of-sight towards the Orion-Eridanus Superbubble (OE-S). These data have been supplemented with measurements of NaI absorption towards a further 13 stars with similar sight-lines taken from the literature. We detect two major absorption components with velocities of  ~ +7.0 and -8.0 km s^-1. The former component, seen in 70% of the sight-lines is associated with the boundary to the Local Bubble cavity located at a distance of 140–150 pc. The other absorption component is only detected towards a limited region of the sky bounded by (190° < l < 215°) and ( < b < ). If gas with this velocity is associated with an outer expansion shell of the OE-S, then we can place its distance at 163–180 pc in agreement with the estimate by Guo et al. (1995, ApJ, 453, 256). Several other negative velocity components at  ~ -20.4, -28.5 and -33.5 km s^-1 have also been detected for sight-line distances > 220 pc within an area coincident with that of the 0.75 keV X-ray enhancement of the OE-S. Column density ratios, N(NaI)/N(CaII), for the most negative velocity components have values < 1.0, suggesting that this gas has been disrupted by a possible shock event. Our data do not support a simple model for the OE-S that involves a single stellar bubble cavity that stretches from the Orion Nebula to high galactic latitudes. Instead, our detection of multiple positive and negative velocity components suggests the presence of several gas shells produced by supernovae and/or stellar wind-driven shocks. We also confirm that the prominent “hook-like” feature of H-α emission that characterizes the OE-S, in in fact composed of two physically separate emission arcs, with the brighter Arc A being at a distance > 500 pc. Finally, we place a similar distance limit for any coherently structured rear shell of neutral gas associated with expansion of the OE-S towards the galactic halo.