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Astron. Astrophys. 336, 309-314 (1998)

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3. Results and chemistry

The present paper aims to establish whether there are potentially observable molecules that belong to the CS and NH3 `families' of early and late time formation. Fig. 1 shows the fractional abundances of a number of relevant species as a function of time, the associated changes in the [FORMULA] and [FORMULA] values are shown in Fig. 4 of Paper 1. Fig. 1a details the C+/C/CO conversion that takes place as free carbon is converted into CO; this transition takes place as the gas reaches its final density and as it occurs a number of carbon based molecules achieve peaks in their abundance (most of these peaks being rather more pronounced than CS) - these are the early-time molecules. These peaks occur at [FORMULA]yr. A number of the other molecules achieve their maximum abundance (usually their equilibrium chemical abundance) at least [FORMULA]yr later - these are the late-time molecules. The actual time of the peak of the early-time molecules is not particularly relevant since this is just the free-fall time, these molecules form immediately when the gas is dense and opaque - in this sense they are dynamically led, whereas the late-time species occur at a subsequent time dictated by a chemical timescale.

[FIGURE] Fig. 1a-d. Chemical fractional abundances (relative to H nuclei) as a function of time for a free-fall collapse model halted at density nH=5[FORMULA]104 cm-3. Initially nH=1[FORMULA]103 cm-3 and AV=0.5. Freeze-out parameter FR=0.01

3.1. Early-time molecules

Initially, most of the carbon is in the form of C+, and this can react with H2 by hydrogen abstraction to form ions such as CH[FORMULA] which may recombine to produce molecules such as CH3 and CH4. Since oxygen is predominantly atomic, formaldehyde can be formed via

[EQUATION]

These hydrocarbons can also react with nitrogen atoms,

[EQUATION]

[EQUATION]

Sulphur is in the form S+, so that H2CS can also be formed

[EQUATION]

Further reaction of C+ with CH4 leads to the C2H[FORMULA] and [FORMULA] ions which recombine to give the important acetylene molecule, which is the precursor to a number of carbon-chain molecules,

[EQUATION]

[EQUATION]

[EQUATION]

[EQUATION]

3.2. Late-time molecules

Ammonia does not achieve its maximum fraction until [FORMULA]yr, mostly because the slight endothermicity (equivalent to 85K) of reaction 1 slows the NH3 formation. Oxygen atoms cannot be ionised by UV photons in these regions, and oxygen atom reaction with H2 is endothermic. Thus, when the clump has collapsed to the extent that molecules are not photodissociated, it is still necessary to wait for sufficient time to elapse for oxygen to be ionised by cosmic rays before hydrogenating reactions can take place. [FORMULA] can react with O, but the formation of this molecule also requires cosmic-ray ionisation (of H2). Thus, OH is a late-time molecule;

[EQUATION]

A number of other molecules then follow from OH,

[EQUATION]

[EQUATION]

[EQUATION]

[EQUATION]

[EQUATION]

NO can also be formed from via reaction of O with NH2, but NH2 (which forms during the NH3 production) is itself a late-time molecule. SO is also formed by reaction of the molecules HS and O2, and again both of these molecules are late time (HS being late time because the reaction of S+ with H2 is endothermic and therefore slow at low temperature). HCO+ is an important ion that forms late, mostly through the reaction of [FORMULA] with CO.

Table 1 gives the enhancements in abundance of these late-time molecules over their values at the time of the CS peak. Note that ammonia has one of the smallest enhancements. The larger this factor the greater the chance that the molecule will be seen in long-lived cores.


[TABLE]

Table 1. List of late-time molecules, along with the factor increase in their fractional abundance at their own peak compared with their abundance at the CS family peak time of [FORMULA]yr. For some of these molecules the late time peak occurs at times even later than those shown in Fig. 1


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© European Southern Observatory (ESO) 1998

Online publication: July 7, 1998
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