... Observatory

The CSO is operated by the California Institute of Technology under funding from the National Science Foundation, GrantNo. AST-9980846.

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... SHARC

Submillimeter High Angular Resolution Camera.

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... place

This is supported, for example, by observations of high abundances of gas-phase atomic oxygen in molecular clouds with density $\sim$ $10^2 {-} 10^3~{\rm cm}^{-3}$ (Lis et al. 2001), suggesting that much of the oxygen which is not locked in CO could well be present in the gas in its atomic form, rather than depleted on dust grains.

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... Phase I

Note that Phase I includes, but is not limited to, the hot-corecomponent of the cloud (see sketch in Fig. 4).

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... sufficient

In fact, the column density of o- ${\rm H}_2^{18}{\rm O}$ is of the same order of magnitude as that of HDO, but the Einstein B coefficient of the o- ${\rm H}_2^{18}{\rm O}$( 1_1,0-1_0,1) transition is about a factor of 2 smaller than that of the HDO( 1_1,1-0_0,0) transition, resulting in a shallower absorption line.

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... determined

We can reasonably assume that the gas-phase water abundance at this stage be determined mostly by evaporation from dust grains or freeze-out onto dust grains, and that these processes affect both ${\rm H}_2{\rm O}$ and HDO equally. In other words, we assume the [HDO]/[ ${\rm H}_2{\rm O}$] ratio to be constant throughout Phases I and II. However, even considering that fractionation is likely to take place in the cold portions of the gas, the [HDO]/[ ${\rm H}_2{\rm O}$] ratio in the Galactic Center is expected to assume a maximum value of $\sim$10^-3 (E. Bergin, priv. comm.; also cf. Roberts & Millar 2000), not very far from our estimate and therefore not affecting our conclusions. To make our point even stronger, fractionation will be most active in the coldest, outermost regions of the envelope, which contribute less than 1% to the total gas column density.

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... Center

The ratio of the abundance of a deuterated species with respect to its non-deuterated counterpart, [XD]/[XH], is expected to scale with the [D]/[H] ratio in the gas (Gerin & Roueff 1999; E. Bergin, priv. comm.).

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