All Figures

$\begin{figure} {\includegraphics[angle=0,width=9cm]{6105fg1.ps} }\end{figure}$ Figure 1: Spectra of IRAS 05345+3157 in N₂H⁺ (1-0) and (3-2) lines ( left panel) and N₂D⁺ (2-1) and (3-2) lines ( right panel). The lines under the N₂H⁺ spectra indicate the position of the hyperfine components.
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In the text

$\begin{figure} {\includegraphics[angle=-90,width=8cm]{6105fg2.ps} } \end{figure}$ Figure 2: Deuterium fractionation ( $D_{\rm frac}$ ) versus integrated CO depletion factor ( $f_{\rm D}$ ) for our sources (filled circles) and the low-mass pre-stellar cores of Crapsi et al. (2005) (open squares). The arrows indicate the upper limits on $D_{\rm frac}$ (see Table 9) or the lower limit on $f_{\rm D}$ (see Table 11).
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In the text

$\begin{figure} {\includegraphics[angle=0,width=8cm]{6105fg3.ps} }\end{figure}$ Figure 3: Top left: deuterium fractionation versus kinetic temperaturederived from NH3 observations. The arrows indicate the deuterium fractionation upper limits. The curves represent the prediction of the theoretical model described in Sect. 4.2. The solid curve is model prediction when using the rate coefficients measured by Gerlich et al. (2002) for the proton-deuteron exchange reactions. The dotted curve is from the standard model, with (a factor of about 3) larger rate coefficients. Dashed curve is from the standard model, when 70% of CO molecules are trapped in H₂O ice. Bottom: same as top for the integrated CO depletion factor. Top right: deuterium fractionation versus depletion factor for the same models and data in the previous two panels. Note that the presence of trapped CO does not significantly affect the deuterium fractionation.
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In the text

$\begin{figure}\par {\includegraphics[angle=0,width=14cm]{6105fgA1.ps} } \end{figure}$ Figure A.1: Spectra of the N₂H⁺ (1-0) line ( left panels) and the N₂D⁺ (2-1) line ( right panels) for IRAS 05345+3157, IRAS 05373+2349, IRAS 18144+3157, IRAS 18511+0146 and IRAS 18517+0437 ( from top to bottom). All spectra have been taken at the position of the N₂H⁺ (1-0) line emission peak.

In the text

$\begin{figure}{\includegraphics[angle=0,width=14cm]{6105fgA2.ps} } \end{figure}$ Figure A.2: Same as Fig. A.1 for IRAS 19092+0841, IRAS 20126+4104, IRAS 20216+4107, IRAS 20343+4129 and IRAS 22172+5549. For IRAS 20343+4129, we show the spectra obtained towards position ^a (see Table 5).

In the text

$\begin{figure}{\includegraphics[angle=-90,width=16cm]{6105fgA3.ps} } \end{figure}$ Figure A.3: Left panel: plot of the N₂H⁺ (1-0) ( top panel) and (3-2) ( bottom panel) integrated emission, superimposed on the 850 $\mu$ m map (grey-scale), obtained with SCUBA towards IRAS 05345+3157. Right panel: same as left panel for the C¹⁷O (2-1) line ( top panel) and DCO⁺ (2-1) line ( bottom panel). The levels are: from 20 $\%$ to 95 $\%$ of the peak (step 15 $\%$ ) for the SCUBA map; from 30 $\%$ to 90 $\%$ of the peak (step 10 $\%$ ) for the N₂H⁺ lines; from 40 $\%$ to 90 $\%$ (step 10 $\%$ ) for the other lines. The contours at half of the maximum are indicated by thick lines for each tracer.

In the text

$\begin{figure}{\includegraphics[angle=-90,width=11cm]{6105fgA4.ps} } \end{figure}$ Figure A.4: Same as Fig. A.3 for IRAS 05373+2349, which is undetected in C¹⁷O.

In the text

$\begin{figure}{\includegraphics[angle=0,width=7.5cm]{6105fgA5.ps} } \end{figure}$ Figure A.5: Same as Fig. A.3 for IRAS 18144+3157. We do not show the C¹⁷O map because it is too noisy, nor the DCO⁺ map because the source is not detected in this tracer.

In the text

$\begin{figure}{\includegraphics[angle=-90,width=11cm]{6105fgA6.ps} } \end{figure}$ Figure A.6: Same as Fig. A.3 for IRAS 18511+0146.

In the text

$\begin{figure}{\includegraphics[angle=-90,width=11cm]{6105fgA7.ps} } \end{figure}$ Figure A.7: Left panel: integrated N₂H⁺ (1-0) ( top panel) and (3-2) ( bottom panel) line emission in IRAS 18517+0437. Right panel: integrated emission of the C¹⁷O (2-1) line. In each panel, the filled triangles at map center indicate the position of the 1.2 mm emission peak (Beuther et al. 2002).

In the text

$\begin{figure}{\includegraphics[angle=-90,width=11cm]{6105fgA8.ps} } \end{figure}$ Figure A.8: Same as Fig. A.3 for IRAS 19092+0841.

In the text

$\begin{figure}{\includegraphics[angle=-90,width=11cm]{6105fgA9.ps} } \end{figure}$ Figure A.9: Same as Fig. A.7 for IRAS 20126+4104. The triangle corresponds to the position of the 850 $\mu$ m emission peak (Williams et al. 2004).

In the text

$\begin{figure}{\includegraphics[angle=-90,width=11cm]{6105fA10.ps} } \end{figure}$ Figure A.10: Same as Fig. A.9 for IRAS 20216+4107.

In the text

$\begin{figure}{\includegraphics[angle=-90,width=11cm]{6105fA11.ps} } \end{figure}$ Figure A.11: Same as Fig. A.9 for IRAS 20343+4129.

In the text

$\begin{figure}{\includegraphics[angle=-90,width=11cm]{6105fA12.ps} } \end{figure}$ Figure A.12: Same as Fig. A.3 for IRAS 22172+5549.

In the text

$\begin{figure} {\includegraphics[angle=0,width=9cm]{6105fg1.ps} }\end{figure}$	Figure 1: Spectra of IRAS 05345+3157 in N₂H⁺ (1-0) and (3-2) lines ( left panel) and N₂D⁺ (2-1) and (3-2) lines ( right panel). The lines under the N₂H⁺ spectra indicate the position of the hyperfine components.
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$\begin{figure} {\includegraphics[angle=-90,width=8cm]{6105fg2.ps} } \end{figure}$	Figure 2: Deuterium fractionation ( $D_{\rm frac}$ ) versus integrated CO depletion factor ( $f_{\rm D}$ ) for our sources (filled circles) and the low-mass pre-stellar cores of Crapsi et al. (2005) (open squares). The arrows indicate the upper limits on $D_{\rm frac}$ (see Table 9) or the lower limit on $f_{\rm D}$ (see Table 11).
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