Forbidden Velocity Gas Detected in Ultraviolet Interstellar Absorption toward Four Distant Inner Galaxy Stars

Details Forbidden Velocity Gas Detected in Ultraviolet Interstellar Absorption toward Four Distant Inner Galaxy Stars Forbidden Velocity Gas Detected in Ultraviolet Interstellar Absorption toward Four Distant Inner Galaxy Stars

Oct 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993apj...415..652t&link_type=abstract

Astrophysical Journal v.415, p.652

Astronomy and Astrophysics

Astronomy

15

Galaxy: Kinematics And Dynamics, Ism: Abundances, Ultraviolet: Interstellar

Scientific paper

We analyze International Ultraviolet Explorer high-dispersion observations of interstellar absorption toward HD 167402, HD 168941, HD 172140, and HD 173502, four distant, inner Galaxy, lower halo/thick disk stars (2°.3 < 1 < 5°.8, 6°.3 < |b| < 12°.3). On the basis of ultraviolet stellar photospheric and wind lines, we classify HD 167402, HD 168941, HD 172140, and HD 173502 as B0 II/B0.5 Ib, 09.5-9.7 II-III, B0.5 III, and B0.5 III, respectively. These classifications result in line-of-sight distance estimates ranging from 6.1 to 6.8 kpc ±25%, and therefore the stars are 0.69 to 1.5 kpc below the Galactic plane. Low and high ion interstellar absorption profiles (including high resolution and high S/N optical Ca II profiles) observed toward three of the four stars show significant amounts of absorption at negative velocities. Negative velocities are not allowed by normal Galactic rotation in the directions sampled. If the thick disk/lower halo gas is corotating with the thin disk and has a radial velocity dispersion of ˜30 km s-1, then we would expect the absorption profiles observed with the lUE to extend from -40 to +100 km s-1 in these directions. However, the observed profiles extend from roughly -100 to +100 km s-1. We discuss some viable interpretations of the negative (i.e., forbidden) velocity gas. The most plausible interpretation is that the absorption at negative velocities occurs in outflowing gas, a few hundred parsecs from the plane, associated with the 3 kpc arm. This negative velocity gas is consistent with the recent numerical studies by Martos and Cox of gas flows in the vicinity of spiral arms. The HD 167402 and HD 168941 observations, based on multiple lUE spectra, reveal strong absorption by N V, C IV, and Si IV. The clear presence of N V absorption implies the existence of hot (T 2 × 105 K) collisionally ionized gas along these inner Galaxy sight lines. The strong Si IV absorption appears to require an additional source of photoionization. The self-ionizing radiation produced by hot cooling gas is a likely possibility.

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