Astronomy & Astrophysics

All Figures

$\begin{figure} \par\includegraphics[width=10.8cm,clip]{H4402F1.EPS} \end{figure}$ Figure 1: Acquisition image of the UCH II region G29.96-0.02 obtained through a narrow-band H₂ 1-0 S(1) filter (central wavelength 2.13 ${\rm ~ \mu m}$ ). Coordinates are relative to the position of the ionizing star: $\alpha(2000)=18^{\rm h}46^{\rm m}4\hbox{$.\!\!^{\rm s}$ }075, \delta(2000)=-2\hbox{$^\circ$ }39\hbox{$^\prime$ }21\hbox{$.\!\!^{\prime\prime}$ }42$ . Contours are 1, 2.5, 5, 10, 25, 50, 75 and 90% the peak flux. The cometary structure of the H II region is clearly revealed. The position of the slit is marked by a dashed line. The ionizing star and the bright arc are indicated. At a distance of 6 kpc, 1 arcsec corresponds to 0.029 pc.
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In the text

$\begin{figure} \par\includegraphics[angle=90,width=12.2cm,clip]{H4402F2.EPS} \end{figure}$ Figure 2: Nebular spectrum of G29.96-0.02 extracted from a $0\hbox{$.\!\!^{\prime\prime}$ }3 \times 0\hbox{$.\!\!^{\prime\prime}$ }882$ region around the peak of ionized emission. The lines observed, their identification and their relative flux with respect to Br $\gamma$ are listed in Table 1. Note the logarithmic scale of the flux scale.
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In the text

$\begin{figure} \par\includegraphics[width=10.8cm,clip]{H4402F3.EPS} \end{figure}$ Figure 3: Spatial variation of a) Br $\gamma$ , b-e) the strongest He I recombination lines and f) H₂ 1-0 S(1) across the symmetry axis of G29.96 (from left to right in Fig. 1). The normalized He I and H₂ intensity distributions are compared to the Br $\gamma$ profile (dashed line). The origin corresponds to the position of the ionizing star. At a distance of 6 kpc, 1 $^{\prime \prime }$ corresponds to 0.029 pc. While the He I and H I distributions trace each other very well, the H₂ line peaks 0 $.\!\!^{\prime\prime}$ 93 ahead of Br $\gamma$ and shows a different structure in the tail. We note that it was not possible to measure line fluxes close to the position of the ionizing star, which explains the absence of points in between -1 $^{\prime \prime }$ and 0 $^{\prime \prime }$ .
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In the text

$\begin{figure} \par\includegraphics[width=12.9cm,clip]{H4402F4.EPS} \end{figure}$ Figure 4: Spatial variation of the velocity centroid (top panel) and line FWHM (bottom panel) of Br $\gamma$ and H₂ along the symmetry axis of G29.96 (from left to right in Fig. 1). Typical error bars are indicated at different positions along the slit. In panels a) and c), the intensity distribution of Br $\gamma$ is plotted (dashed line) for comparison. The dotted lines in panels b) and d) represent the average H₂ centroid velocity and FWHM calculated between positions -3 $^{\prime \prime }$ and +5 $^{\prime \prime }$ , and 0 $^{\prime \prime }$ and +5 $^{\prime \prime }$ , respectively. The zero point along the slit corresponds to the position of the ionizing star. At a distance of 6 kpc, 1 $^{\prime \prime }$ corresponds to 0.029 pc.
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In the text

$\begin{figure} \par\includegraphics[width=14.2cm,clip]{H4402F5.EPS} \end{figure}$ Figure 5: Partial Grotrian energy-level diagram of the He I singlet and triplet transitions showing the infrared lines observed in the K-band spectrum of G29.96 (solid arrows), other important infrared lines (dotted arrows) and the strongest optical lines (dashed arrows). Note that the energy levels are not drawn to scale. The wavelengths of the infrared lines are indicated in ${\rm ~ \mu m}$ ; those of the optical lines in Å. The wavelengths of the lines used in the present analysis are marked by a box.
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In the text

$\begin{figure} \par\includegraphics[width=7.8cm,clip]{H4402F6.EPS} \end{figure}$ Figure 6: Optical depth correction factor, f, for the 2.1128 ${\rm ~ \mu m}$ He I line as a function of the electron temperature for $\tau _{3890}$ between 0 and 100 and an electron density $n_{\rm e}$ = 10⁴ ${\rm cm^{-3}}$ .
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In the text

$\begin{figure} \par\includegraphics[width=15.2cm,clip]{H4402F7.EPS} \end{figure}$ Figure 7: a) Plot of the line ratio 2.1615-7³F-4³D/2.1137-4¹S-3¹P against the ratio 2.1649-7^1,3G-4^1,3F/2.1137-4¹S-3¹P. Note that these line ratios should not depend on radiative transfer effects. b) 2.1649-7^1,3G-4^1,3F/2.1128-4³S-3³P against 2.1649-7^1,3G-4^1,3F/2.1138-4¹S-3¹P. In both figures, the solid line represents the Case B predictions for $T_{\rm e}$ = 5, 7.5, 10 and $15 \times 10^3$ K (indicated by solid diamonds) and $n_{\rm e}$ =10⁴ ${\rm cm^{-3}}$ . The data indicate a temperature between 5000 and 7500 K. The non-solid lines in b) represent the calculations by Benjamin et al. (2002) for non-zero optical depth of the 2³S metastable level. Only points located in the interval [-5 $^{\prime \prime }$ , +8 $^{\prime \prime }$ ] are considered. The data points from the tail of G29.96 (>0 $^{\prime \prime }$ ) are plotted as open circles; those from the arc (<0 $^{\prime \prime }$ ) are plotted as solid circles. The open and filled circles have very similar distributions.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{H4402F8.EPS} \end{figure}$ Figure 8: a) Electron temperature structure along the symmetry axis of G29.96 (from left to right in Fig. 1) as derived from the He I line ratio 2.1649/2.1138 ${\rm ~ \mu m}$ . The dashed line represents the average $T_{\rm e}$ of 6400 K. b) He⁺/H⁺ structure. The dashed line represents the average value of 0.087. Only points located in the interval [-5 $^{\prime \prime }$ , +8 $^{\prime \prime }$ ] are considered.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{H4402F9.EPS} \end{figure}$ Figure 9: The line ratio He I 2.1649/Br $\gamma$ is plotted against the ratio He I 2.1649/2.1137. The lines represent the theoretical ratios for 3 different He⁺/H⁺ abundances (0.08, 0.10 and 0.12) and an electron density of 10⁴ ${\rm cm^{-3}}$ . The values corresponding to $T_{\rm e}$ = 5, 7.5 and $10\times 10^3$ K are indicated by solid diamonds. As in previous figures, only points located in the interval [-5 $^{\prime \prime }$ , +8 $^{\prime \prime }$ ] are considered, the data points in the tail of G29.96 are plotted as open circles and those from the arc as solid circles.
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In the text

$\begin{figure} \par\includegraphics[width=8.3cm,clip]{H4402F10.EPS} \end{figure}$ Figure 10: The relative extent of the He⁺ region to that of the H⁺ region, $R_{\rm He^+}/R_{\rm H^+}$ , is plotted against $T_{\rm eff}$ , the effective temperature of the ionizing star. For $T_{\rm eff} \geq 40~000$ K, the He⁺ and H⁺ zones are coincident.
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In the text

$\begin{figure} \par\includegraphics[height=8.7cm,angle=90,clip]{H4402F11.EPS} \end{figure}$ Figure 11: Spectrum of the ionizing star in G29.96. The emission features of C IV at 2.080 ${\rm ~ \mu m}$ and N III at 2.116 ${\rm ~ \mu m}$ are indicated. These features constitute the base of the K-band classification. The lines of H I and He I are contaminated by nebular emission.
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In the text

$\begin{figure} \par\includegraphics[angle=90,width=12.2cm,clip]{H4402F2.EPS} \end{figure}$	Figure 2: Nebular spectrum of G29.96-0.02 extracted from a $0\hbox{$.\!\!^{\prime\prime}$ }3 \times 0\hbox{$.\!\!^{\prime\prime}$ }882$ region around the peak of ionized emission. The lines observed, their identification and their relative flux with respect to Br $\gamma$ are listed in Table 1. Note the logarithmic scale of the flux scale.
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$\begin{figure} \par\includegraphics[width=7.8cm,clip]{H4402F6.EPS} \end{figure}$	Figure 6: Optical depth correction factor, f, for the 2.1128 ${\rm ~ \mu m}$ He I line as a function of the electron temperature for $\tau _{3890}$ between 0 and 100 and an electron density $n_{\rm e}$ = 10⁴ ${\rm cm^{-3}}$ .
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$\begin{figure} \par\includegraphics[width=8.3cm,clip]{H4402F10.EPS} \end{figure}$	Figure 10: The relative extent of the He⁺ region to that of the H⁺ region, $R_{\rm He^+}/R_{\rm H^+}$ , is plotted against $T_{\rm eff}$ , the effective temperature of the ionizing star. For $T_{\rm eff} \geq 40~000$ K, the He⁺ and H⁺ zones are coincident.
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$\begin{figure} \par\includegraphics[height=8.7cm,angle=90,clip]{H4402F11.EPS} \end{figure}$	Figure 11: Spectrum of the ionizing star in G29.96. The emission features of C IV at 2.080 ${\rm ~ \mu m}$ and N III at 2.116 ${\rm ~ \mu m}$ are indicated. These features constitute the base of the K-band classification. The lines of H I and He I are contaminated by nebular emission.
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