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Session 7 - Gas and Dust in the ISM.
Display session, Monday, June 10
Great Hall,
Over 50 absorption features from the Lyman and Werner bands of interstellar H_2 were observed in the uv spectrum of \zeta Ori A by the Interstellar Medium Absorption Profile Spectrograph (IMAPS) on the ORFEUS-SPAS I orbital mission in September 1993. Hydrogen molecules are found mostly in two prominent velocity components: one has \log N(H_2)=14.5, an average rotational temperature (for J=0-5) T_rot=950K and a heliocentric radial velocity v_ødot\approx 0 km s^-1, while the other has \log N(H_2)=15.9, T_rot=320K and v_ødot=25 km s^-1. The component near 0 km s^-1 has the unusual property of showing a steady increase in the width of the velocity profile as J increases from 0 to 5, coupled with a small shift (\sim 2 km s^-1) in the profile's center toward negative velocities. This behavior is not consistent with the conventional picture that the excitation is caused by optical pumping by uv starlight photons. We suggest that the H_2 in this component is forming in a gas region that is undergoing recombination, cooling and compression behind a bow shock created by the collision of a high (negative) velocity gas flow and a stationary cloud.
