All Figures

$\begin{figure} \par\includegraphics[width=8.4cm,clip]{3925fig1.ps} \end{figure}$ Figure 1: Comparison of line flux values obtained in this work to those found in the literature. For clarity, only the initial section of the distributions is shown (0 to $60 \times 10^{-19}$ W cm^-2), but the linear regression fits refer to the entire data sets (up to $400 \times 10^{-19}$ W cm^-2). The dotted line is of unit slope and meant to guide the eye only. Next to this line is the fit for [O I] 63 $\mu$ m, followed by that for [O I] 145 $\mu$ m and [C II] 157 $\mu$ m, respectively.
Open with DEXTER

In the text

$\begin{figure} \par\includegraphics[width=8.5cm,clip]{3925fig2.ps} \end{figure}$ Figure 2: Line ratio plot for the fine structure lines of O I and C II. Dots with error bars designate ratios based on differential line fluxes (ON-OFF measurements), whereas the arrows indicate lower limits at the $3 \sigma$ level. In the majority of observations, an OFF-source measurement was not made and in several cases, the OFFs refer to the average flux in the associated flows or, for non-mapped objects, the symbols refer merely to ON source measurements (see the text).
Open with DEXTER

In the text

$\begin{figure} \par\includegraphics[width=8.4cm,clip]{3925fig3.ps} \end{figure}$ Figure 3: Similar to Fig. 2 but only ON-source measurements are shown, i.e. no subtraction of OFF data have been made. Limit symbols and error bars have been omitted to avoid crowding (see Fig. 2). The horizontal dashed line indicates the optically thin regime for the [O I] 63 $\mu$ m/[O I] 145 $\mu$ m line ratio above about ten. The two open symbols with vertical error bars refer to the average values of data lying above and below the dividing line, respectively.
Open with DEXTER

In the text

$\begin{figure} \par\includegraphics[width=7.1cm,clip]{3925fig4.ps} \end{figure}$ Figure 4: [O I] 63 $\mu$ m/[O I] 145 $\mu$ m line intensity ratios for collisions with neutral hydrogen, H I, as the function of the temperature are shown by the dotted lines. The logarithms of the densities (in cm^-3) are indicated above each curve. Optically thin ratios are given for the volume densities $n({\rm H})= 10^3$ , 10⁴ and 10⁵ cm^-3. The broken line outlines the ratio of optically thick lines of equal velocity width, each of which is approximated by the integrated Planck function at the given temperature, i.e. $\int_{\Delta \nu}\!I_{\nu}~{\rm d}\nu = \Delta \nu B_{\nu}(T)$ . The optically thin intensity ratios for the inverted levels, due to collisions with H₂, are shown by the solid lines, with the logarithmic densities in cm^-3 below. Observed line ratios fall roughly into the area inside the thin horizontal lines (compare with Fig. 3).
Open with DEXTER

In the text

$\begin{figure} \par\includegraphics[width=7.6cm,clip]{3925fig5.ps} \end{figure}$ Figure 5: Hydrogen column densities $N({\rm H})$ (in cm^-2) versus volume densities $n({\rm H})$ (in cm^-3) for unit line center optical depth in the [O I] 145 $\mu$ m line, $\tau _0=1$ . Collisional excitation with atomic hydrogen is assumed, in addition to a cosmic oxygen abundance and a Gaussian line width of 1 km s^-1. At temperatures below 100 K, column densities for optically thick [O I] 145 $\mu$ m emission become prohibitively large.
Open with DEXTER

In the text

$\begin{figure} \par\includegraphics[width=8.2cm,clip]{3925fig6.ps} \end{figure}$ Figure 6: The level inversion parameter g₀/g₁-n₀/n₁ for O I is shown for collisional excitation with atomic hydrogen (dashed lines) and molecular hydrogen (solid lines), respectively. For collisions with H₂, this parameter is negative at temperatures above 50 K and densities below 10⁵ cm^-3, implying inversion of the upper level population. At low temperatures, this parameter becomes independent of the density and the two curves become indistiguishable, approaching the ratio of the statistical weights ( $g_0/g_1-n_0/n_1\rightarrow g_0/g_1 = 1/3$ , see the curves for 10 K).
Open with DEXTER

In the text

$\begin{figure} \par\includegraphics[width=7.4cm,clip]{3925fig7.ps} \end{figure}$ Figure 7: The hydrogen column density, $N({\rm H})$ , of a parcel of molecular cloud gas along the line of sight to attain an optical depth of $\tau _0=-1$ in a thermally broadened 145 $\mu$ m maser line. Logarithmic densities in cm^-3 are given next to each curve.
Open with DEXTER

In the text

$\begin{figure} \par\includegraphics[width=7.7cm,clip]{3925fig8.ps} \end{figure}$ Figure 8: Numerical results for oxygen line ratios at characteristic interstellar cloud temperatures. Displayed are the column densities of atomic oxygen $N({\rm O})$ , for $\delta v=1$ km s^-1, needed to produce the line ratios for an H₂ density of $3\times 10^4$ cm^-3. For these parameters and at 20 K, line center optical depths in this plot range from $\min \tau_0 = 0.05$ to $\max \tau_0 = 5 \times 10^5$ in the 145 $\mu$ m and in the 63 $\mu$ m lines, respectively. At 100 K, these values are 26 and in excess of $4 \times 10^5$ .
Open with DEXTER

In the text

$\begin{figure} \par\includegraphics[width=7.4cm,clip]{3925fig7.ps} \end{figure}$	Figure 7: The hydrogen column density, $N({\rm H})$ , of a parcel of molecular cloud gas along the line of sight to attain an optical depth of $\tau _0=-1$ in a thermally broadened 145 $\mu$ m maser line. Logarithmic densities in cm^-3 are given next to each curve.
Open with DEXTER