All Figures

$\begin{figure} \par\includegraphics[width=8cm,clip]{0324fig1.eps} \end{figure}$ Figure 1: Spectra of N₂D⁺ 1-0, N₂D⁺ 2-1, and N₂D⁺ 3-2 toward HH 211, one of the youngest sources in our sample.
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In the text

$\begin{figure} \par\includegraphics[width=13cm,clip]{0324fig2.eps} \end{figure}$ Figure 2: Spectra of N₂H⁺ 1-0, N₂D⁺ 2-1, C¹⁸O 1-0, and HCO⁺ 3-2 ( from left to right) of all 20 protostars of our sample. To be able to plot all spectra in one plot, we divided the intensities of HCO⁺ by three.
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In the text

$\begin{figure} \par\includegraphics[width=7.8cm,clip]{0324fig3.eps} \end{figure}$ Figure 3: Deuterium fractionation (N₂D⁺/N₂H⁺ column density ratio) versus dust temperature. The highest N₂D⁺/N₂H⁺ ratios are seen at the lowest temperatures. As $T_{\rm Dust}$ increases, the deuterium fractionation declines. The objects marked with triangles are located in the Perseus cloud.
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In the text

$\begin{figure} \par\includegraphics[width=7.8cm,clip]{0324fig4.eps} \end{figure}$ Figure 4: N₂D⁺/N₂H⁺ column density ratio versus CO depletion factor for prestellar cores ( upper panel, Crapsi et al. 2005), Class 0 protostars ( middle panel, this work) and massive prestellar cores ( lower panel, Fontani et al. 2006). The protostellar objects marked with triangles are located in the Perseus cloud. Among the Perseus objects, $f\rm _D$ (CO) and the deuterium fraction appear correlated. The solid curve in the middle panel is the result of fitting the sub-sample of Perseus cores with a formula adopted from Caselli et al. (1998, see text).
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In the text

$\begin{figure} \par\includegraphics[width=7.8cm,clip]{0324fig5.eps} \end{figure}$ Figure 5: N₂D⁺/N₂H⁺ versus $L_{\rm BOL}/ F_{1.3}^{D_0}$ . All objects with high deuterium fractionation have low $L_{\rm BOL}/ F_{1.3}^{D_0}$ ratios and are likely to be young objects. L 1527, which has one of the lowest $L_{\rm BOL}/ F_{1.3}^{D_0}$ ratios, also has a quite upper limit on the N₂D⁺/N₂H⁺ ratio.
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In the text

$\begin{figure} \par\includegraphics[width=7.8cm,clip]{0324fig6.eps} \end{figure}$ Figure 6: FWHM of N₂D⁺ 2-1 versus N₂D⁺/N₂H⁺. Triangles are Perseus cores.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{0324fig7.eps} \end{figure}$ Figure 7: Asymmetry parameter $\delta v$ versus N₂D⁺/N₂H⁺ ratio. Only the sources with an unambiguously determined asymmetry parameter are shown. All sources with enhanced deuterium fractionation are located at moderate $\delta v$ of about -0.4, and is increasing with decreasing deuterium fractionation.
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In the text

$\begin{figure} \par\includegraphics[width=7.8cm,clip]{0324fig8.eps} \end{figure}$ Figure 8: Deuterium fractionation of NH₃ (red triangles) and N₂H⁺ (black circles) versus dust temperature.
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In the text

$\begin{figure} \par\includegraphics[width=8.5cm,clip]{0324fig9.eps} \end{figure}$ Figure 9: Density (dashed line) and temperature (solid lines) profiles of the protostellar core models. The four temperature profiles represent models that yield values for $T_{\rm Dust}$ of 20 K, 27 K, 35 K, and 45 K.
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In the text

$\begin{figure} \par\includegraphics[width=7.8cm,clip]{0324fi10.eps} \end{figure}$ Figure 10: Upper panel: N₂D⁺/N₂H⁺ ratio versus dust temperature. Lower panel: N₂D⁺/N₂H⁺ ratio and the CO depletion factor $F\rm_D$ (CO). The open circles are the observed values for the subsample of cores in the Perseus cloud; the crosses are the model results. The N₂D⁺/N₂H⁺ ratio of the models was determined using the CTEX method.
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In the text

$\begin{figure} \par\includegraphics[width=7.8cm,clip]{0324fi11.eps} \end{figure}$ Figure 11: The relative abundance of CO ( upper panel), N₂H⁺ ( middle panel), and the N₂D⁺/N₂H⁺ ratio are shown as a function of radius. It can be clearly seen that the radius at which CO starts to deplete shifts outwards with increasing temperature, as does the peak of the deuterium fractionation.
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In the text

$\begin{figure} \par\includegraphics[width=7.8cm,clip]{0324fi12.eps} \end{figure}$ Figure 12: The abundances of CO, H₃⁺, H₂D⁺, HCO⁺, DCO⁺, N₂H⁺, and N₂D⁺ relative to H₂ are shown as a function of radius. The thick lines show the abundance profile of the protonated species. The thin lines show their singly-deuterated counterparts. These profiles apply to the model with $T_{\rm Dust}=27$ K.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{0324fig1.eps} \end{figure}$	Figure 1: Spectra of N₂D⁺ 1-0, N₂D⁺ 2-1, and N₂D⁺ 3-2 toward HH 211, one of the youngest sources in our sample.
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$\begin{figure} \par\includegraphics[width=13cm,clip]{0324fig2.eps} \end{figure}$	Figure 2: Spectra of N₂H⁺ 1-0, N₂D⁺ 2-1, C¹⁸O 1-0, and HCO⁺ 3-2 ( from left to right) of all 20 protostars of our sample. To be able to plot all spectra in one plot, we divided the intensities of HCO⁺ by three.
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$\begin{figure} \par\includegraphics[width=7.8cm,clip]{0324fig3.eps} \end{figure}$	Figure 3: Deuterium fractionation (N₂D⁺/N₂H⁺ column density ratio) versus dust temperature. The highest N₂D⁺/N₂H⁺ ratios are seen at the lowest temperatures. As $T_{\rm Dust}$ increases, the deuterium fractionation declines. The objects marked with triangles are located in the Perseus cloud.
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$\begin{figure} \par\includegraphics[width=7.8cm,clip]{0324fig5.eps} \end{figure}$	Figure 5: N₂D⁺/N₂H⁺ versus $L_{\rm BOL}/ F_{1.3}^{D_0}$ . All objects with high deuterium fractionation have low $L_{\rm BOL}/ F_{1.3}^{D_0}$ ratios and are likely to be young objects. L 1527, which has one of the lowest $L_{\rm BOL}/ F_{1.3}^{D_0}$ ratios, also has a quite upper limit on the N₂D⁺/N₂H⁺ ratio.
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$\begin{figure} \par\includegraphics[width=7.8cm,clip]{0324fig6.eps} \end{figure}$	Figure 6: FWHM of N₂D⁺ 2-1 versus N₂D⁺/N₂H⁺. Triangles are Perseus cores.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{0324fig7.eps} \end{figure}$	Figure 7: Asymmetry parameter $\delta v$ versus N₂D⁺/N₂H⁺ ratio. Only the sources with an unambiguously determined asymmetry parameter are shown. All sources with enhanced deuterium fractionation are located at moderate $\delta v$ of about -0.4, and is increasing with decreasing deuterium fractionation.
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$\begin{figure} \par\includegraphics[width=7.8cm,clip]{0324fig8.eps} \end{figure}$	Figure 8: Deuterium fractionation of NH₃ (red triangles) and N₂H⁺ (black circles) versus dust temperature.
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$\begin{figure} \par\includegraphics[width=8.5cm,clip]{0324fig9.eps} \end{figure}$	Figure 9: Density (dashed line) and temperature (solid lines) profiles of the protostellar core models. The four temperature profiles represent models that yield values for $T_{\rm Dust}$ of 20 K, 27 K, 35 K, and 45 K.
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$\begin{figure} \par\includegraphics[width=7.8cm,clip]{0324fi10.eps} \end{figure}$	Figure 10: Upper panel: N₂D⁺/N₂H⁺ ratio versus dust temperature. Lower panel: N₂D⁺/N₂H⁺ ratio and the CO depletion factor $F\rm_D$ (CO). The open circles are the observed values for the subsample of cores in the Perseus cloud; the crosses are the model results. The N₂D⁺/N₂H⁺ ratio of the models was determined using the CTEX method.
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$\begin{figure} \par\includegraphics[width=7.8cm,clip]{0324fi11.eps} \end{figure}$	Figure 11: The relative abundance of CO ( upper panel), N₂H⁺ ( middle panel), and the N₂D⁺/N₂H⁺ ratio are shown as a function of radius. It can be clearly seen that the radius at which CO starts to deplete shifts outwards with increasing temperature, as does the peak of the deuterium fractionation.
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$\begin{figure} \par\includegraphics[width=7.8cm,clip]{0324fi12.eps} \end{figure}$	Figure 12: The abundances of CO, H₃⁺, H₂D⁺, HCO⁺, DCO⁺, N₂H⁺, and N₂D⁺ relative to H₂ are shown as a function of radius. The thick lines show the abundance profile of the protonated species. The thin lines show their singly-deuterated counterparts. These profiles apply to the model with $T_{\rm Dust}=27$ K.
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