Far-Infrared Observations of [S\I.] in the Orion Wind Shock

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Session 44 -- Interstellar Medium
Display presentation, Thursday, January 13, 9:30-6:45, Salons I/II Room (Crystal Gateway)

[44.03] Far-Infrared Observations of [S\I.] in the Orion Wind Shock

M.R.Haas, D.J.Hollenbach, E.F.Erickson (NASA/Ames Research Center)

We have used the facility Cryogenic Grating Spectrometer (CGS) on the Kuiper Airborne Observatory to map the newly discovered [S\I.] 25.2 \mm. line (Haas et al. , 1991, ApJ 374, 555) in the vicinity of the Orion molecular shock. A cut across the source through peaks 1 and 2 of the molecular hydrogen $v = 1 - 0$ S(1) distribution shows that the [S\I.] emission is correlated with the H$_2$ emission and is confined to the shocked region. There is no significant [S\I.] emission from the vicinity of the ionization bar or the ionizing star $\Theta^1$C. This confirms that the observed emission arises entirely from shocks and is uncontaminated by contributions from either the overlying H\II. region or the photodissociation region (PDR), where the sulfur is expected to be ionized. Since all the other far-infrared fine-structure lines and radio recombination lines observed to date have significant H\II. and/or PDR contributions, [S\I.] is a uniquely valuable diagnostic of shock activity. The shocked [S\I.] distribution peaks near H$_2$ peak~1 and the observed intensity is consistent with recent models of fast, dissociative J-shocks if H$_2$ reforms on grains in the postshock gas and if the postshock density is $\sim 10^5$ cm$^{-3}$. These same J-shock models can also produce the observed intensities of [O\I.] and [Si\II.]. Although C-shocks might also produce a sufficient column density of S$^0$, they do not readily explain the observed [S\I.]/[O\I.] ratio and cannot produce the observed [Si\II.] intensity because of their low fractional ionization.

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