1 Introduction

Observations of deuterated molecules have become an important tool in the study of interstellar chemistry. Although the underlying (or cosmic) D/H ratio is low ($\sim$10^-5), the formation of deuterated molecules is preferred at low temperatures ($\leq$80 K) and leads to a high degree of fractionation in cold, dark clouds. For example, in the quiescent dark cloud, TMC-1, molecular D/H ratios, including [HDCO]/[H₂CO] and [DCN]/[HCN] are observed to be >10^-2.

Enhanced molecular D/H ratios are also observed in hot molecular cores (HMC's), clumps of hot, dense gas, usually associated with high mass star formation. The temperatures of these cores (typically 70-150 K) should be high enough to preclude the enhancement of molecular D/H ratios through gas-phase reactions. However, the ratios which have been measured are generally $\sim$10^-3 (e.g. Hatchell et al. 1998, 1999), lower than TMC-1, yet still enhanced over the cosmic value. It is now generally accepted that these ratios have been preserved from an earlier, colder phase of the cloud's history in the ice-mantles of dust grains. Once some heating event, such as the formation of a star or the passage of a shock, heats the grains sufficiently to evaporate their mantles, the D/H ratios can survive for $\sim$10⁴ yrs in the hot gas (Rodgers & Millar 1996).

In cold cores which are forming low mass stars, we might expect a situation intermediate between hot cores and dark clouds. To date, the only survey of deuterated molecules in a low-mass star forming region has been that of IRAS16293-2422 (hereafter, IRAS16293), a class 0, proto-binary system in $\rho$Oph, by vanDishoeck et al. (1995). However, their survey revealed discrepancies in the levels of fractionation of different molecules, with over 10% deuteration seen in species such as HDCO and HDS, yet only a few percent in species such as DCN. While the [DCN]/[HCN] ratio is similar to that observed in TMC-1, the [HDCO]/[H₂CO] ratio is at least twice as high. Neither the very large [HDCO]/[H₂CO] or [HDS]/[H₂S] ratios can be explained by a standard gas-phase chemistry.

We wished to confirm whether these high D/H ratios were a general feature of low-mass star formation, or particular to IRAS16293. Therefore we have carried out a survey to measure both the [HDCO]/[H₂CO] and [DCN]/[HCN] ratios in the dense gas associated with young protostars (`protostellar cores') in three different star forming regions. Our sources are listed in Table 1.

 

 

Table 1: Sources observed, along with velocities, bolometric temperatures and protostellar class taken from the literature.

Region	Source	$\alpha_{1950}$	$\delta_{1950}$	$v_{\rm lsr}$	$T_{\rm bol}$	Class
		[$^{\rm h}$ $^{\rm m}$ $^{\rm s}$]	[$^{\circ}$ ' '']	(kms-1)	(K)
Perseus	B5IRS1	03:44:31.7	+32:42:29	10.2	85	I
	L1448mms	03:22:34.3	+30:33:35	5.6	56	0
	L1448NW	03:22:31.1	+30:35:3.8	5.0	24	0
	HH211	03:40:48.7	+31:51:24	9.2	30	0
	IRAS03282	03:28:15.2	+30:35:14	7.0	26	0
Taurus	L1527	04:36:49.3	+25:57:16	5.6	59	0
	L1551IRS5	04:28:40.2	+18:01:42	6.4	97	I
Orion	RNO43	05:29:30.6	+12:47:25	9.6	33	0
	HH111	05:49:9.3	+02:47:48	8.5	38	0

Sections 2 and 3 describe the observations and data reduction techniques, presenting the resulting column densities and molecular D/H ratios, Sect. 4 summarises and discusses these results. In Sect. 5 we describe the chemical models and compare model predictions with the observations. Section 6 compares these results with those from previous observations of high-mass star forming regions. Throughout, we adopt the conventions; "N(ABC)'' for the column density of molecule ABC, "[ABC]'' for N(ABC)/N(H₂), i.e. the fractional abundance of molecule ABC, and "fractionation of XD'' for [XD]/[XH].