C. M. Walmsley

Observations of SiO towards photon dominated regions

We report observations with the IRAM 30-m telescope of SiO(2–1), (3–2), and (5–4) towards the Orion Bar, S 140, NCC 2023 and NGC 7023. We detect SiO towards the Orion Bar and the S 140 photon dominated region (PDR) with an abundance of order 10−11 relative to H2. Towards the Orion Bar, SiO is extended and is not confined to a layer close to the ionization front. We put limits of 2–4 × 10−11 on [SiO]/[H2] towards several positions in NGC 2023 and NGC 7023. In order to interpret these data, we present model calculations of the SiO abundance in PDRs as a function of the incident radiation field and consider also the expected Si 35 μm fine structure line intensity. Our data suggest that silicon is very highly depleted onto dust grains within the molecular cloud but that a fraction of the order of ten percent of the silicon comes back into the gas phase in a layer close to the ionization front. We attempted to estimate whether the observed silicon fine structure line emission could originate in fully ionized gas and conclude that in the case of the Orion Bar, it is quite likely. Finally, a by–product of our observations was the detection of the 86773.43 MHz J = 1/2→ 1/2 F = 1→ 1 component of the 101− 000 line of HCO in all four sources confirming that this species is relatively abundant in PDRs.

Detection of F[CLC]e[/CLC]O toward Sagittarius B2

We have observed the J=5-4 ground state transition of FeO at a frequency of 153 GHz towards a selection of galactic sources. Towards the galactic center source SgrB2, we see weak absorption at approximately the velocity of other features towards this source (62 km s

Detection of FeO toward Sagittarius B2

^{-1}

Detection of FeO towards SgrB2

LSR). Towards other sources, the results were negative as they were also for MgOH(3-2) and FeC(6-5). We tentatively conclude that the absorption seen toward SgrB2 is due to FeO in the hot (

A recalibration of the interstellar ammonia thermometer

\sim

A study of HCN, HNC and their isotopomers in OMC-1. I : Abundances and chemistry

500 K) relatively low density absorbing gas known to be present in this line of sight. This is the first (albeit tentative) detection of FeO or any iron--containing molecule in the interstellar gas. Assuming the observed absorption to be due to FeO, we estimate [FeO]/[SiO] to be of order or less than 0.002 and [FeO]/[H

Observations of deuterated cyanoacetylene in dark clouds

_{2}

SILICON CHEMISTRY IN PDRS

] of order

Observations of Hot Molecular Cores

3 10^{-11}

Hot shocked ammonia towards SGR B2

. This is compatible with our negative results in other sources. Our results suggest that the iron liberated from grains in the shocks associated with SgrB2 remains atomic and is not processed into molecular form.