All Figures

$\begin{figure} \par\includegraphics[width=13cm,clip]{fig_spectra_h2co_1.eps} \end{figure}$ Figure 1: Spectra of the eigth H₂CO transitions in Table 2 observed towards NGC 1333-IRAS 4A, NGC 1333-IRAS 4B, NGC 1333-IRAS 2, and L1448-MM respectively.
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In the text

$\begin{figure} \par\includegraphics[width=12.75cm,clip]{fig_spectra_h2co_2.eps} \end{figure}$ Figure 2: As Fig. 1 for the sources L1448-N, L1157-MM, L1527 and VLA1623.
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In the text

$\begin{figure} \par\includegraphics[angle=270,width=17.05cm,clip]{fig_spectra_h213co.eps} \end{figure}$ Figure 3: Spectra of the observed H₂¹³CO lines.
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In the text

$\begin{figure} \par\includegraphics[width=16.4cm,clip]{fig_popdiag.eps} \end{figure}$ Figure 4: H₂CO rotational diagrams derived for the observed sources. Circles and squares mark the ortho and para H₂CO transitions respectively. Fluxes of the para transitions have been multiplied by the ortho to para ratio, obtained by minimizing the $\chi ^2$ between the observations and the predictions of the rotational diagram (see text). Solid lines show the best fit curves.
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In the text

$\begin{figure} \par\includegraphics[width=16.4cm,clip]{fig_chi2_dens_temp.eps} \end{figure}$ Figure 5: $\chi ^2$ contours as a function of the density and temperature of the emitting gas. Contours indicates the 1, 2 and 3 $\sigma$ confidence levels respectively.
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In the text

$\begin{figure} \par\includegraphics[width=7.9cm,clip]{fig_denstemp_iras2.eps} \end{figure}$ Figure 6: Density (solid line), gas (dashed line) and dust (dotted line) temperature across the envelope of NGC 1333-IRAS 2.
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In the text

$\begin{figure} \par\includegraphics[width=16.6cm,clip]{fig_chi2_xin_xout.eps} \end{figure}$ Figure 7: Protostellar envelope model $\chi _{\rm red}^2$ contours as a function of the outer and inner H₂CO abundances. The contours levels show the 1, 2, and 3 $\sigma$ confidence levels respectively.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{fig_modelobs.eps} \end{figure}$ Figure 8: Ratio of the model predictions over the observed fluxes of H₂CO lines as a function of the upper energy of the line, for NGC 1333-IRAS 2. In the left panel a jump in the abundance (Table 6) is adopted, while the in the right panel a constant abundance across the envelope is assumed. The circles and the squares represent H₂¹²CO and H₂¹³CO lines respectively. The arrows represent lower limits.
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In the text

$\begin{figure} \par\includegraphics[width=8.6cm,clip]{fig_chi2_diffvel.eps} \end{figure}$ Figure 9: $\chi _{\rm red}^2$ diagrams of L1448-MM for three different velocity profiles. The upper left and upper right panels show the $\chi _{\rm red}^2$ contours derived for a free falling envelope with a central mass of 0.3 and 0.8 $M_{\hbox {$\odot $ }}$ respectively. The lower panel shows the static envelope case, with a 1 km s^-1 turbulent velocity. Contours indicate the 1, 2 and 3 $\sigma$ confidence levels respectively.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{fig_chi2_diffop.eps} \end{figure}$ Figure 10: $\chi _{\rm red}^2$ diagrams of NGC 1333-IRAS 4 for three different ortho to para ratios. The upper left, upper right and lower panels shows the $\chi _{\rm red}^2$ contours derived for an ortho to para ratio of 1, 2 and 3 respectively. Contours indicates the 1, 2 and 3 $\sigma$ confidence levelsrespectively.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{fig_chi2_difftemp.eps} \end{figure}$ Figure 11: $\chi _{\rm red}^2$ diagrams of NGC 1333-IRAS 2 for an evaporation temperature of 100 K ( left panel) and 50 K ( right panel). Contours indicates the 1, 2 and $3 \sigma$ confidence levels respectively.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{fig_correl_xin_1.eps} \end{figure}$ Figure 12: Derived H₂CO $X_{\rm in}$ abundances as a function of the density ( left panel) and the radius ( right panel) where $T_{\rm dust} = 100$ K. The star represent IRAS 16293-2422.
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In the text

$\begin{figure} \par\includegraphics[width=6.8cm,clip]{fig_correl_xin_2.eps} \end{figure}$ Figure 13: Derived H₂CO $X_{\rm in}$ abundances as a function of $L_{\rm smm}/L_{\rm bol}$ . The star represent IRAS 16293-2422.
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In the text

$\begin{figure} \par\includegraphics[width=7.3cm,clip]{fig_correl_xout.eps} \end{figure}$ Figure 14: Derived outer formaldehyde abundance $X_{\rm out}$ as a function the CO abundance of the outer enveloppe. Circles represent Class 0 sources, The star represents IRAS 16293-2422, the square represents VLA1623, and the triangles represent the pre-stellar cores of Bacmann et al. (2002,2003) sample.
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In the text

$\begin{figure} \par\includegraphics[width=16.3cm,clip]{fig_model_l1448n.eps} \end{figure}$ Figure A.1: Observations and result of the modelling of L1448N. The upper left panel shows the SCUBA observations at 450 and 850 $\mu$ m. The upper right panel shows the SED (diamonds) and the fitted model (solid line). The lower left panel shows the brightness profile and the fitted model. The dashed line represents the beam profile. The lower right panel shows the derived temperature dust (solid line) and density profile (dashed line).

In the text

$\begin{figure} \par\includegraphics[width=13cm,clip]{fig_spectra_h2co_1.eps} \end{figure}$	Figure 1: Spectra of the eigth H₂CO transitions in Table 2 observed towards NGC 1333-IRAS 4A, NGC 1333-IRAS 4B, NGC 1333-IRAS 2, and L1448-MM respectively.
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$\begin{figure} \par\includegraphics[width=12.75cm,clip]{fig_spectra_h2co_2.eps} \end{figure}$	Figure 2: As Fig. 1 for the sources L1448-N, L1157-MM, L1527 and VLA1623.
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$\begin{figure} \par\includegraphics[angle=270,width=17.05cm,clip]{fig_spectra_h213co.eps} \end{figure}$	Figure 3: Spectra of the observed H₂¹³CO lines.
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$\begin{figure} \par\includegraphics[width=16.4cm,clip]{fig_popdiag.eps} \end{figure}$	Figure 4: H₂CO rotational diagrams derived for the observed sources. Circles and squares mark the ortho and para H₂CO transitions respectively. Fluxes of the para transitions have been multiplied by the ortho to para ratio, obtained by minimizing the $\chi ^2$ between the observations and the predictions of the rotational diagram (see text). Solid lines show the best fit curves.
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$\begin{figure} \par\includegraphics[width=16.4cm,clip]{fig_chi2_dens_temp.eps} \end{figure}$	Figure 5: $\chi ^2$ contours as a function of the density and temperature of the emitting gas. Contours indicates the 1, 2 and 3 $\sigma$ confidence levels respectively.
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$\begin{figure} \par\includegraphics[width=7.9cm,clip]{fig_denstemp_iras2.eps} \end{figure}$	Figure 6: Density (solid line), gas (dashed line) and dust (dotted line) temperature across the envelope of NGC 1333-IRAS 2.
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$\begin{figure} \par\includegraphics[width=16.6cm,clip]{fig_chi2_xin_xout.eps} \end{figure}$	Figure 7: Protostellar envelope model $\chi _{\rm red}^2$ contours as a function of the outer and inner H₂CO abundances. The contours levels show the 1, 2, and 3 $\sigma$ confidence levels respectively.
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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{fig_chi2_diffop.eps} \end{figure}$	Figure 10: $\chi _{\rm red}^2$ diagrams of NGC 1333-IRAS 4 for three different ortho to para ratios. The upper left, upper right and lower panels shows the $\chi _{\rm red}^2$ contours derived for an ortho to para ratio of 1, 2 and 3 respectively. Contours indicates the 1, 2 and 3 $\sigma$ confidence levelsrespectively.
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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{fig_chi2_difftemp.eps} \end{figure}$	Figure 11: $\chi _{\rm red}^2$ diagrams of NGC 1333-IRAS 2 for an evaporation temperature of 100 K ( left panel) and 50 K ( right panel). Contours indicates the 1, 2 and $3 \sigma$ confidence levels respectively.
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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{fig_correl_xin_1.eps} \end{figure}$	Figure 12: Derived H₂CO $X_{\rm in}$ abundances as a function of the density ( left panel) and the radius ( right panel) where $T_{\rm dust} = 100$ K. The star represent IRAS 16293-2422.
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$\begin{figure} \par\includegraphics[width=6.8cm,clip]{fig_correl_xin_2.eps} \end{figure}$	Figure 13: Derived H₂CO $X_{\rm in}$ abundances as a function of $L_{\rm smm}/L_{\rm bol}$ . The star represent IRAS 16293-2422.
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$\begin{figure} \par\includegraphics[width=7.3cm,clip]{fig_correl_xout.eps} \end{figure}$	Figure 14: Derived outer formaldehyde abundance $X_{\rm out}$ as a function the CO abundance of the outer enveloppe. Circles represent Class 0 sources, The star represents IRAS 16293-2422, the square represents VLA1623, and the triangles represent the pre-stellar cores of Bacmann et al. (2002,2003) sample.
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