6 A comparison with previous observations

Table 11 lists the [HDCO]/[H₂CO] and [DCN]/[HCN] ratios which have been observed towards other sources.

   

Table 11: Previous observations of formaldehyde and hydrogen cyanide towards a selection of interstellar sources.

Source	[H2CO]	$\frac{\rm [HDCO]}{\rm [H_2CO]}$	[HCN]	$\frac{\rm [DCN]}{\rm [HCN]}$
IRAS16293	2.0 (-9)1	0.141	1.9 (-9)2	0.012
L134N	2.0 (-8)3	0.0684	4.0 (-9)3	0.054
TMC-1	7.0 (-8)5	0.0594	1.0 (-8)6	0.0114
OCRa	3.7 (-8)7	0.148	2.0 (-8)7	0.049
OHCb	2.6 (-8)7	--	3.0 (-7)7	0.0039

NOTES: a(-b) implies $a \times 10 ^{-b}$.
           ^a Orion Compact Ridge; ^b Orion Hot Core.
REFS: 1. Loinard et al. (2000); 2. vanDishoeck et al. (1995);
3. Ohishi et al. (1992); 4. Turner (2001); 5. Ohishi (1998);
6. Willacy & Millar (1998); 7. Charnley et al. (1992); 8. Turner (1990);
9. Schilke et al. (1992).

A major motivation for this study was to see if the high [HDCO]/[H₂CO] ratios and low [DCN]/[HCN] ratios seen towards IRAS16293 are typical of low-mass star formation. Instead, we found [HDCO]/[H₂CO] ratios less than half of the value observed towards IRAS16293 and the Orion Compact Ridge (OCR), and slightly lower than those seen in the cold cloud TMC-1. [DCN]/[HCN] ratios, on the other hand, appear to be significantly higher than are seen towards both TMC-1 and IRAS16293, but similar to those seen towards the OCR. The [DCN]/[HCN] ratios we have observed are also similar to those measured in L134N (Turner 2001), a source where [DCO⁺]/[HCO⁺], [NH₂D]/[NH₃] and [NHD₂]/[NH₃] ratios are also enhanced over cold cloud values (Tiné et al. 2000; Roueff et al. 2000).

6.1 Low-mass star-forming cores

Shah & Wootten (2001) have observed [NH₂D]/[NH₃] and [DCO⁺]/[HCO⁺] ratios towards a wide range of low-mass, protostellar cores. Their source list overlaps, to some extent, with ours, the relevant results being included in Table 12.

   

Table 12: Observations of other molecular D/H ratios towards the sources in our survey (for the data from Butner et al. (1995), ratios have been recalculated for a ¹²C/¹³C ratio of 60).

Source	[DCO+]/[HCO+]	[NH2D]/[NH3]
B5IRS1	0.0391
L1448mms	0.013 ($\pm$0.004)2	0.029 ($\pm$0.011)2
L1448NW		0.09 ($\pm$0.02)2
IRAS03282		0.007 ($\pm$0.003)2
L1527	0.019 ($\pm$0.005)2
L1551IRS5	0.0353/0.0841	0.087 ($\pm$0.013)2

REFS: 1. Butner et al. (1995); 2. Shah & Wootten (2001); 3. Williams et al. (1998).

The [NH₂D]/[NH₃] ratio towards IRAS03282 is 0.007, slightly lower than that predicted by the gas-phase chemical model at steady state ($\sim$0.015 at 10 K), however higher ratios were seen towards other sources. [NH₂D]/[NH₃] ratios are also expected to rise as accretion occurs, see Fig. 6, and after $5\times10^4$ yrs the predicted ratio has risen to 0.03, the value observed towards L1448mms, but it takes almost $2 \times 10 ^5$ yrs of accretion to increase the [NH₂D]/[NH₃] ratio to $\sim$0.09, as was observed towards L1551IRS5 and L1448NW.

DCO⁺ is fractionated directly via deuteron transfer from H₂D⁺, therefore [DCO⁺]/[HCO⁺] ratios are expected to be among the highest molecular D/H ratios. This is illustrated in Fig. 6, the steady-state [DCO⁺]/[HCO⁺] ratio from the gas-phase model, 0.05, rising to $\sim$0.08 after $5\times10^4$ yrs of accretion. The [DCO⁺]/[HCO⁺] ratio observed towards L1551IRS5 by Butner et al. (1995) is similar to this value, but most of the [DCO⁺]/[HCO⁺] ratios observed by Shah & Wootten towards the sources in our survey are lower even than the gas-phase models predict (see Table 12). However, more detailed observations (Shah 2000), indicate that the DCO⁺ emission may be arising from a different region to the HCO⁺, thus, these [DCO⁺]/[HCO⁺] ratios are very sensitive to the precise details of the radiative transfer model used. Alternatively, this relatively low DCO⁺ fractionation may simply be evidence that accretion is not currently occurring in these sources, but occurred during an earlier stage of evolution.

Another source which has been observed to have high [DCN]/[HCN] and [NH₂D]/[NH₃] ratios (0.04 and 0.06, respectively), but relatively low DCO⁺ fractionation (0.002) is the Orion Compact Ridge (OCR).

Despite the fact that both are classed as hot cores, there are marked chemical differences between the OCR and the Orion Hot Core (OHC) which have been noted by previous workers. It appears that both contain species which have been evaporated from grain surfaces, however, the ice mantles in the OHC seem to have been ammonia rich and methanol poor, while the OCR mantles were methanol rich but ammonia poor.

Levels of deuterium fractionation also differ between the OHC and the OCR. Most ratios which have been measured towards the OHC are $\sim$10^-3, similar to other hot cores. However, several of the deuterated species observed towards the OCR, including HDCO and DCN, have levels of fractionation similar to or even larger than is seen towards the cold cloud TMC-1.

If the Hot Core formed earlier in the evolution of the ridge cloud, when the gas was mostly atomic, surface hydrogenation would produce species such as NH₃, CH₄ and H₂O, with deuterium fractionation which reflects the atomic D/H ratio in the accreting gas (Charnley et al. 1992; Hatchell et al. 1999). In the Compact Ridge, forming at a later time when the accreting gas was mostly molecular, accretion over a longer timescale at low temperatures may have increased deuterium fractionation. CO in the gas-phase would then form H₂CO and CH₃OH on the grain surfaces, and deuterium fractionation would be higher in most species.

On the other hand, the differences between the OHC and the OCR could be due to their forming at different temperatures and/or densities (Caselli et al. 1992). Higher temperatures during the collapse phase of the Hot Core would supress deuterium fractionation, and could also mean that CO did not stay on the grain surfaces long enough for significant amounts of H₂CO and CH₃OH to form.

6.2 A comparison with high-mass star formation

Hatchell et al. (1998) conducted a survey of [DCN]/[HCN] ratios towards several HMC's, finding ratios typically (0.9-4.1 $) \times 10 ^{-3}$, later searches for HDS (Hatchell et al. 1999) put limits on [HDS]/[H₂S] ratios of $\leq$10^-3. These values are very similar, and point either to grain surface processing acting to reduce molecular D/H ratios from cold cloud values, or, as deuterium fractionation is so sensitive to temperature variations, to higher temperatures ($\geq$30 K) at the time the molecules froze onto the grain surfaces.

This appears to contrast with levels of deuterium fractionation observed towards low-mass star forming cores. van Dishoeck et al. (1995) measured [HDS]/[H₂S$]\sim 0.1$ towards IRAS16293, we have measured [DCN]/[HCN $] \sim 0.04$ towards other low-mass cores. Both these ratios are higher than those observed towards HMC's or predicted by chemical models.

To form stars, cold, dense clouds, with enhanced D/H ratios, collapse under gravity. Gas phase molecules then freeze onto grain surfaces, possibly undergoing some chemical processing, before a shock or outflow disrupts the grains and evaporates their mantles. The fact that this sequence of events produces such different levels of deuterium fractionation in HMC's and low-mass cores may, as in the case of the OHC and the OCR, point to significant evolutionary differences between these two types of star forming region.

Higher temperatures during the pre-collapse phase of high mass stars has already been suggested as an explanation for the lower molecular D/H ratios in HMC's and for the chemical differences between the Orion Hot Core and Compact Ridge. It might also be the case that the longer collapse timescales associated with low-mass star formation mean that accretion processes have significantly impacted on the chemistry in these regions.