All Figures

$\begin{figure} \par\includegraphics[angle=-90,width=8.4cm,clip]{7583fig1a.ps}\includegraphics[angle=-90,width=8.4cm,clip]{7583fig1b.ps}\end{figure}$ Figure 1: ( Left) JCMT-SCUBA 450 $\mu$ m emission from IRAS 18511 region. The contours levels are from 2.0 to 13.4 Jy/beam (peak flux density $\sim$ 13.4 Jy/beam) in steps of 1 Jy/beam, where the beam size is 10 $^{\prime \prime }$ . ( Right) JCMT-SCUBA 850 $\mu$ m emission from IRAS 18511 region. The contours levels are from 0.18 to 2.28 Jy/beam (peak flux density $\sim$ 2.28 Jy/beam) in steps of 0.2 Jy/beam, beam is 15 $.\!\!^{\prime\prime}$ 5.
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In the text

$\begin{figure} \par\includegraphics[angle=-90,width=8.4cm,clip]{7583fig2a.ps}\in... ...3fig2c.ps}\includegraphics[angle=-90,width=8.4cm,clip]{7583fig2d.ps}\end{figure}$ Figure 2: ( Top left) MIPS 70 $\mu$ m image of IRAS 18511 region. The plus symbol and the ellipse represent the IRAS position and the corresponding 1- $\sigma$ position error ellipse. ( Top right) MIPS 24 $\mu$ m image of IRAS 18511 region with the sources A, B and C marked. ( Bottom left) IRAC 8.0 $\mu$ m image of IRAS 18511 region. The white cross marks the position of radio emission at 8.5 and 15 GHz. The solid triangle marks the position of the methanol maser. ( Bottom right) IRAC 3.6 $\mu$ m image with sources G1-G5 marked.
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In the text

$\begin{figure} \par\includegraphics[angle=-90,width=8.4cm,clip]{7583fig3.ps}\end{figure}$ Figure 3: Grayscale image of IRAS 18511 at GLIMPSE-IRAC 8.0 $\mu$ m showing the dark filaments against diffuse emission. Overlaid as contours is the JCMT-SCUBA sub-millimetre emission at 850 $\mu$ m. The region covered by the SCUBA map is also shown.
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In the text

$\begin{figure} \par\includegraphics[width=8.4cm,clip]{7583fig4.ps}\end{figure}$ Figure 4: Spitzer colour-colour diagram of the observed sources and the models used in the simulations. The filled circles represent the sources detected in all four IRAC bands. The empty (red) circles represent the colours of the unreddened class I and class II models used in the simulations (see text for details). The solid line square approximately delineates the region occupied by class II sources whereas the dotted-line square covers the region occupied by the class I models shown in the models of Allen et al. (2004). The arrow represents the extinction vector for A_V=20 mag.
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In the text

$\begin{figure} \par\includegraphics[width=8.4cm,clip]{7583fig5.ps}\end{figure}$ Figure 5: $K_{\rm s}$ band Palomar image of the region around IRAS 18511. The axes are in J2000 coordinates. The sources marked by open (red) circles are the YSO candidates selected from the various colour-colour diagrams (see Table 2). The (blue) solid triangles are the sources G1-G5.
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In the text

$\begin{figure} \par\includegraphics[width=7.6cm,clip]{7583fig6a.ps}\hspace*{4mm} \includegraphics[width=7.6cm,clip]{7583fig6b.ps}\end{figure}$ Figure 6: Colour-colour diagrams (CCDs) of sources detected in Palomar and Spitzer-IRAC images within 10% contour level of the sub-millimetre 850 $\mu$ m peak at IRAS 18511 A (see text for details). ( Left) J-H vs. $H-K_{\rm s}$ CCD where the locii of the main sequence and giant branches are shown by the solid and dotted lines respectively. The short-dash line represents the locus of T-Tauri stars. The dash-dotted straight line follows the reddening vectors of main sequence stars (or dwarfs). The long dashed line represents the locus of Herbig Ae/Be stars. The position of IRAS 18511 A is marked as "A'' in the CCD. ( Right) J-[3.6] vs. $K_{\rm s}-[$ 4.5] CCD where the solid line represents the line separating the normal stars and the young stellar objects while the dotted line represents the YSO locus of Wang & Looney (2007). The arrow represents the extinction vector A_V=5 mag.
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In the text

$\begin{figure} \par\mbox{\includegraphics[width=7.6cm,clip]{7583fig7a.ps}\hspace*{4mm} \includegraphics[width=7.6cm,clip]{7583fig7b.ps} } \end{figure}$ Figure 7: ( Left) The solid line represents the spectral energy distribution (SED) of IRAS 18511 constructed using the IRAS and MSX fluxes. The dotted line depicts the lower limit to the spectral energy distribution of A constructed using Palomar, Spitzer-IRAC, Spitzer-MIPS and JCMT-SCUBA. While the filled circles represent the fluxes of IRAS 18511 A, the solid triangles represent the lower limits. ( Right) The SEDs of young stellar objects, G1-G5, detected using IRAC colour-colour diagram.
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In the text

$\begin{figure} \par\mbox{\includegraphics[width=7.6cm,clip]{7583fig8a.ps}\hspace*{4mm} \includegraphics[width=7.6cm,clip]{7583fig8b.ps} } \end{figure}$ Figure 8: Normalised spectral energy distribution (with respect to H band) of sources G1, G2, G4 and G5, dereddened by $A_V \sim 7$ mag ( left) and $A_V\sim 22$ mag ( right). The solid lines represent the normalised spectral energy distributions of the class I (red) and class II (blue) sources adopted in the cluster simulations.
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In the text

$\begin{figure} \par\includegraphics[width=7.6cm,clip]{7583fig9a.ps}\hspace*{4mm} \includegraphics[width=7.6cm,clip]{7583fig9b.ps}\end{figure}$ Figure 9: Spectral energy distributions of G4 ( left) and G5 ( right) with some sample models of Robitaille et al. (2007) (RWIW). The observed fluxes are shown by filled circles, the triangles denote upper limits to the observed fluxes and the lines represent the RWIW models. The parameters of these models are listed in Table 4.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{7583fig10.ps}\end{figure}$ Figure 10: Plot of magnitude distribution in the $K_{\rm s}$ band from the simulations and observations (Palomar). The solid line represents the magnitudes of YSOs within the "region of interest'' (Table 2). The dotted line represents the apparent magnitudes from one run of model (general case of constant star formation rate with ages between 0.01 and 1 Myr) simulations. The dashed line denotes the Palomar sensitivity limit.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{7583fig11.ps}\end{figure}$ Figure 11: Plot of the detected number of sources as a function of wavelength for cluster model simulations with observations. The filled circles and solid lines represent the observations and error bars, respectively. The observation at 24 $\mu$ m represents a lower-limit. The solid squares represent median values of the general case of simulations incorporating class I as well as class II objects for the general case of constant star formation rate with ages between 0.01 and 1 Myr and the dot-dashed line represent the quartile values on either side of the median. The cross symbols and dotted line represent the class I model simulations for an age of 0.5 Myr. The open circles and dashed line denote class II model simulations for 1 Myr. The wavelengths for different cases have been slightly shifted for better viewing. Further details can be found in the text.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{7583fig12.ps}\end{figure}$ Figure 12: The clustering indicator (radial profile) centred on the bright NIR source in IRAS 18511, constructed using the Palomar $K_{\rm s}$ (solid circles), IRAC 4.5 $\mu$ m (solid triangles) and IRAC 8.0 $\mu$ m (solid squares) band fluxes of the sources in the region around IRAS 18511. The star densities derived for the IRAC2 and IRAC4 bands have been scaled by factors of 1.7 and 6.4, respectively for suitable comparison with the radial profile in the $K_{\rm s}$ band. The radii for different cases have been slightly shifted for better viewing.
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In the text

$\begin{figure} \par\includegraphics[angle=-90,width=8.4cm,clip]{7583fig3.ps}\end{figure}$	Figure 3: Grayscale image of IRAS 18511 at GLIMPSE-IRAC 8.0 $\mu$ m showing the dark filaments against diffuse emission. Overlaid as contours is the JCMT-SCUBA sub-millimetre emission at 850 $\mu$ m. The region covered by the SCUBA map is also shown.
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$\begin{figure} \par\includegraphics[width=8.4cm,clip]{7583fig5.ps}\end{figure}$	Figure 5: $K_{\rm s}$ band Palomar image of the region around IRAS 18511. The axes are in J2000 coordinates. The sources marked by open (red) circles are the YSO candidates selected from the various colour-colour diagrams (see Table 2). The (blue) solid triangles are the sources G1-G5.
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$\begin{figure} \par\mbox{\includegraphics[width=7.6cm,clip]{7583fig8a.ps}\hspace*{4mm} \includegraphics[width=7.6cm,clip]{7583fig8b.ps} } \end{figure}$	Figure 8: Normalised spectral energy distribution (with respect to H band) of sources G1, G2, G4 and G5, dereddened by $A_V \sim 7$ mag ( left) and $A_V\sim 22$ mag ( right). The solid lines represent the normalised spectral energy distributions of the class I (red) and class II (blue) sources adopted in the cluster simulations.
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$\begin{figure} \par\includegraphics[width=7.6cm,clip]{7583fig9a.ps}\hspace*{4mm} \includegraphics[width=7.6cm,clip]{7583fig9b.ps}\end{figure}$	Figure 9: Spectral energy distributions of G4 ( left) and G5 ( right) with some sample models of Robitaille et al. (2007) (RWIW). The observed fluxes are shown by filled circles, the triangles denote upper limits to the observed fluxes and the lines represent the RWIW models. The parameters of these models are listed in Table 4.
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