All Figures

$\begin{figure} \par\includegraphics[width=8.2cm,clip]{9176fi01.eps}\end{figure}$ Figure 1: System transmission curves of IRAC channels 1-4 (Spitzer). The four major bands of PAH emission are marked by dotted lines. IRAC channel 4 (centred at 8.0 $\mu$ m) contains two strong bands while images in channel 2 have very smooth backgrounds as they are essentially free from PAH emission.
Open with DEXTER

In the text

$\begin{figure} \par\includegraphics[width=8.2cm,clip]{9176fi02.eps}\end{figure}$ Figure 2: SIRCA L' band image of central L1641N. The field shown has a size of 73 $\hbox {$^{\prime \prime }$ }$ $\times$ 47 $\hbox {$^{\prime \prime }$ }$ .
Open with DEXTER

In the text

$\begin{figure} \par\includegraphics[width=16.7cm,clip]{9176fi03.eps}\end{figure}$ Figure 3: Spitzer composite of 3.6 $\mu$ m (blue), 4.5 $\mu$ m (green) and 8.0 $\mu$ m (red). All our sources are marked by circles. Sources for which we have spectroscopy have thick, dashed circles. The surveyed region has a size of 8 $.\mkern -4mu^\prime$ 45 $\times$ 7 $.\mkern -4mu^\prime$ 38 (1.11 $\times$ 0.97 pc at a distance of 450 pc), although the field size in this figure has been made larger to give a better overview of the region. The green square outlines our J band observations.
Open with DEXTER

In the text

$\begin{figure} \par\includegraphics[width=8.2cm,clip]{9176fi04.eps}\end{figure}$ Figure 4: Comparison of ISO and Spitzer photometry. The ISOCAM 6.7 $\mu$ m channel is compared to the mean flux of the Spitzer (IRAC) 5.8 and 8.0 $\mu$ m channels. The upper panel shows our ISOCam photometry and the lower panel that of Ali et al. (2004).
Open with DEXTER

In the text

$\begin{figure} \par\includegraphics[width=16.7cm,clip]{9176fi05.eps}\end{figure}$ Figure 5: The central region of L1641N in a composite using I (blue colours), 2.12 $\mu$ m H₂ S(1) (green) and $K_{\rm S}$ (red) filters. The big ellipse marks IRAS beam centred on IRAS 05338-0624, circles denote our sources and plus signs are corresponding sources of Chen et al. (1993). The square is centred on the 2 mm dust peak (see text). The field shown has a size of 83 $.\!\!^{\prime\prime}$ 0 $\times$ 80 $.\!\!^{\prime\prime}$ 5 for both images.
Open with DEXTER

In the text

$\begin{figure} \par\includegraphics[width=16.8cm,clip]{9176fi06.eps}\end{figure}$ Figure 6: Stellar models (BaSeL v2.2) for log g = 4.5 and $T_{{\rm eff}}$ = 2000-10 000 K. Overplotted are the ground based filter transmission curves (I, J and $K_{\rm S}$ ) used in our observations.
Open with DEXTER

In the text

$\begin{figure} \par\includegraphics[width=16.7cm,clip]{9176fi07.eps}\end{figure}$ Figure 7: Stellar models (BaSeL v2.2) for log g = 4.5 and $T_{{\rm eff}}$ = 2000-10 000 K. Overplotted are the IRAC filter transmission curves.
Open with DEXTER

In the text

$\begin{figure} \par\includegraphics[width=8.2cm,clip]{9176fi08.eps}\end{figure}$ Figure 8: Colour-magnitude diagram using only Spitzer photometry (4.5 and 8.0 $\mu$ m). Sources marked with an asterisk show clear IR excess at 8 $\mu$ m compared to photospheres reddened by extinction alone. Sources that show no or very little excess are marked with plus signs. Double sources are indicated by an AB suffix. The solid line separates non-excess and excess sources while some sources below the dashed line could possibly be an extragalactic contribution. Source No. 24 appears to be an extended background source in our $K_{\rm S}$ image and is located in the part of the diagram expected for a galaxy. The arrow indicates the expected interstellar extinction vector (A_K = 5 corresponds to A_V = 47.4). YSO candidates without excess are marked with a red square.
Open with DEXTER

In the text

$\begin{figure} \par\includegraphics[width=18cm,clip]{9176fi09.eps}\end{figure}$ Figure 9: Colour-colour diagrams using ground based and Spitzer photometry. The solid black curve represents model stellar atmospheres ( $\log~$ g = 4.5, $T_{{\rm eff}} = 2500{-}10~000$ K) and the brown lines the reddening band due to interstellar extinction. The slopes of the reddening vectors were found by fitting the ``blue'' sources. Symbols are the same as in Fig. 8 with the addition of green circles that denote sources not detected at 8.0 $\mu$ m and thus having unknown red/blue status in the [4.5]-[8.0] index. Error bars are shown for selected sources located on the red side of the reddening band (all such green circles with error bars are added to our list of YSO candidates) (A_J = 5 corresponds to A_V = 19.4).
Open with DEXTER

In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{9176fi10.eps}\end{figure}$ Figure 10: Colour-magnitude diagram showing that most of the sources detected in L1641N by the ISO satellite in fact have excess at 14.3 $\mu$ m. Symbols are the same as in Fig. 8. The most luminous and red source in this plot, L1641N-172, is an outflow source and the brightest source at mid-IR wavelengths in the entire L1641N. It is marked with a circle since it could not be shown to have excess in the Spitzer data because of saturation.
Open with DEXTER

In the text

$\begin{figure} \par\includegraphics[width=8.8cm]{9176fi11.eps}\end{figure}$ Figure 11: The completeness of the survey as determined by source absolute magnitude, distance and extinction in the I and J band observations. The age of the stars are shown in parenthesis following the spectral type. The horizontal axis represents distance and J band extinction. Limiting magnitudes are marked by red dashed lines (A_J = 5 corresponds to A_V = 19.4).
Open with DEXTER

In the text

$\begin{figure} \par\includegraphics[width=8.8cm]{9176fi12.eps}\end{figure}$ Figure 12: Interstellar extinction curve from the optical (0.67 $\mu$ m) through IR. The red plus signs represents measurements from our observations (I, J, $K_{\rm S}$ , [3.6], [4.5] and [8.0]). The dotted line is a power law fit to the observed E(I-J)/ $E(J-K_{\rm S})$ and the solid line is a theoretical R=A_V/E(B-V)=5.5 extinction curve from Draine et al. (2003).
Open with DEXTER

In the text

$\begin{figure} \par\includegraphics[width=9cm,clip]{9176fi13.eps}\end{figure}$ Figure 13: Polynomial fits (solid curves) of bolometric correction BC(J) and effective temperature $T_{{\rm eff}}$ to intrinsic colour index (I-J)₀. Model atmospheres (BaSeL v2.2) with a surface gravity of $\log~g = 4.5$ have been used for calculations in the temperature range 2000-10 000 K with a step size of 100 K (circles). The blue curves are fits made using all effective temperatures, while the red curves are more accurate fits in the 2500-4000 K region (which we use in our photometric temperature determinations).
Open with DEXTER

In the text

$\begin{figure} \par\includegraphics[width=9cm,clip]{9176fi14.eps}\end{figure}$ Figure 14: Comparison of spectroscopic and photometrically derived effective temperatures. The solid line indicates where $T_{{\rm eff}}$ (spec) = $T_{{\rm eff}}$ (phot). As can be seen in the lower panel, the residuals are fairly well centred on the solid line with a standard deviation of a few hundred Kelvins. The red asterisks represent two sources, L1641N-120 and 121, having larger residuals than the other stars. This is probably caused by a small amount of intrinsic excess emission in the $K_{\rm S}$ band (enough to keep them within the reddening band in Fig. 9 but yielding higher photometric temperatures).
Open with DEXTER

In the text

$\begin{figure} \par\includegraphics[width=16.7cm,clip]{9176fi15.eps} \end{figure}$ Figure 15: $K_{\rm S}$ band images of reflection nebulosity ( left panel) and double sources ( right panel) with and without the main source (illuminating source and primary, respectively) removed using a normalized PSF from an isolated star. Each image has a field size of 34 $.\!\!^{\prime\prime}$ 7 $\times$ 34 $.\!\!^{\prime\prime}$ 7.
Open with DEXTER

In the text

$\begin{figure} \par\includegraphics[clip]{9176fi16.eps}\end{figure}$ Figure 16: H-R diagram of the full (spectroscopic and photometric) sample for which we could obtain effective temperatures and extinctions. The evolution track models (blue curves with masses indicated) were obtained from the Dartmouth Stellar Evolution Database (Dotter et al. 2007). Isochrones are drawn with red dashed lines and their ages shown on the right side of the grid. Observations are marked using green asterisks, with parentheses for the suggested four early foreground stars and source numbers given for double sources. The orange curves and numbers illustrate the completeness of the survey across the grid in terms of the A_J extinction.
Open with DEXTER

In the text

$\begin{figure} \par\includegraphics[width=16cm,clip]{9176fi17.eps}\end{figure}$ Figure 17: Resulting temperature, luminosity, age and mass functions for the YSO sample shown in Fig. 16. The first four panels include all sources in the H-R diagram while the last two panels show the MF for the completeness limits given above the respective panels (the limiting mass is also illustrated by the red dotted lines). The red, hashed region in the age plot illustrates stars with intrinsic IR excess (disk stars). It is interesting to note that even though the excess YSOs are present at all ages, seven out of the eight youngest YSOs in the sample have IR excess.
Open with DEXTER

In the text

$\begin{figure} \par\includegraphics[width=9cm,clip]{9176fi18.eps}\end{figure}$ Figure 18: Cumulative star formation history plot for the sample where ages could be determined. The blue (dashed) curves are simple theoretical models assuming a constant star formation rate, an unbound cluster and velocity dispersions of 1 and 3 km s^-1, respectively.
Open with DEXTER

In the text

$\begin{figure} \par\includegraphics[width=9cm,clip]{9176fi19.eps} \end{figure}$ Figure 19: Spatial distribution of stellar ages. The two major bi-polar outflows are illustrated by arrows, with the corresponding outflow sources (and two additional deeply embedded sources, probably very young) marked by red asterisks.
Open with DEXTER

In the text

$\begin{figure} \par\includegraphics[width=9cm,clip]{9176fi20.eps}\end{figure}$ Figure 20: Cumulative radial distribution of YSOs. The relative number of sources versus distance from the 2 mm continuum dust peak (Chen et al. 1995) at the centre of L1641N. The younger group (0-1.5 Myr) is clearly more concentrated towards the centre while the older group (>1.5 Myr) is more spread out (similar to a uniform distribution, dashed curve). This supports the age determinations and shows that the velocity dispersion of the cluster members is large enough that the older YSOs have had time to move away significantly from their birth sites.
Open with DEXTER

In the text

$\begin{figure} \par\includegraphics[width=16cm,clip]{9176fi21.eps}\end{figure}$ Figure 21: Deep $K_{\rm S}$ and 2.12 $\mu$ m H₂ colour composite of the central and south east region of L1641N. All visible sources in our sample are marked by a circle. The field shown has a size of 4 $.\mkern -4mu^\prime$ 05 $\times$ 4 $.\mkern -4mu^\prime$ 35.
Open with DEXTER

In the text

$\begin{figure} \par\includegraphics[width=16cm,clip]{9176fi22.eps}\end{figure}$ Figure 22: Deep $K_{\rm S}$ and 2.12 $\mu$ m H₂ colour composite of the central and north west region of L1641N. All visible sources in our sample are marked by a circle. The field shown has a size of 4 $.\mkern -4mu^\prime$ 40 $\times$ 4 $.\mkern -4mu^\prime$ 37.
Open with DEXTER

In the text

$\begin{figure}\par\includegraphics[angle=180,width=17cm]{9176f23a.ps} \end{figure}$ Figure 23: Optical spectra - Exposure times and I band magnitudes are given for each source.
Open with DEXTER

In the text

$\begin{figure}\par\includegraphics[angle=180,width=17cm]{9176f23b.ps} \end{figure}$ Figure 23: continued.
Open with DEXTER

In the text

$\begin{figure}\par\includegraphics[angle=180,width=17cm]{9176f23c.ps} \end{figure}$ Figure 23: continued.
Open with DEXTER

In the text

$\begin{figure}\par\includegraphics[angle=180,width=17cm]{9176f23d.ps} \end{figure}$ Figure 23: continued.
Open with DEXTER

In the text

$\begin{figure}\par\includegraphics[angle=180,width=17cm]{9176f23e.ps} \end{figure}$ Figure 23: continued.
Open with DEXTER

In the text

$\begin{figure}\par\includegraphics[angle=180,width=17cm]{9176f23f.ps} \end{figure}$ Figure 23: continued.
Open with DEXTER

In the text

$\begin{figure}\par\includegraphics[angle=180,width=17cm]{9176f24a.ps} \end{figure}$ Figure 24: Evidence of youth - H $\alpha$ in emission and Li I $\lambda 6707$ in absorption. The shifts in wavelength for each source is an artefact caused by each star being differently positioned within the wide slit (1 $.\!\!^{\prime\prime}$ 2).
Open with DEXTER

In the text

$\begin{figure}\par\includegraphics[angle=180,width=11cm]{9176f24b.ps} \end{figure}$ Figure 24: continued.
Open with DEXTER

In the text

$\begin{figure} \par\includegraphics[width=8.2cm,clip]{9176fi01.eps}\end{figure}$	Figure 1: System transmission curves of IRAC channels 1-4 (Spitzer). The four major bands of PAH emission are marked by dotted lines. IRAC channel 4 (centred at 8.0 $\mu$ m) contains two strong bands while images in channel 2 have very smooth backgrounds as they are essentially free from PAH emission.
Open with DEXTER

$\begin{figure} \par\includegraphics[width=8.2cm,clip]{9176fi02.eps}\end{figure}$	Figure 2: SIRCA L' band image of central L1641N. The field shown has a size of 73 $\hbox {$^{\prime \prime }$ }$ $\times$ 47 $\hbox {$^{\prime \prime }$ }$ .
Open with DEXTER

$\begin{figure} \par\includegraphics[width=8.2cm,clip]{9176fi04.eps}\end{figure}$	Figure 4: Comparison of ISO and Spitzer photometry. The ISOCAM 6.7 $\mu$ m channel is compared to the mean flux of the Spitzer (IRAC) 5.8 and 8.0 $\mu$ m channels. The upper panel shows our ISOCam photometry and the lower panel that of Ali et al. (2004).
Open with DEXTER

$\begin{figure} \par\includegraphics[width=16.8cm,clip]{9176fi06.eps}\end{figure}$	Figure 6: Stellar models (BaSeL v2.2) for log g = 4.5 and $T_{{\rm eff}}$ = 2000-10 000 K. Overplotted are the ground based filter transmission curves (I, J and $K_{\rm S}$ ) used in our observations.
Open with DEXTER

$\begin{figure} \par\includegraphics[width=16.7cm,clip]{9176fi07.eps}\end{figure}$	Figure 7: Stellar models (BaSeL v2.2) for log g = 4.5 and $T_{{\rm eff}}$ = 2000-10 000 K. Overplotted are the IRAC filter transmission curves.
Open with DEXTER

$\begin{figure} \par\includegraphics[width=8.8cm]{9176fi11.eps}\end{figure}$	Figure 11: The completeness of the survey as determined by source absolute magnitude, distance and extinction in the I and J band observations. The age of the stars are shown in parenthesis following the spectral type. The horizontal axis represents distance and J band extinction. Limiting magnitudes are marked by red dashed lines (A_J = 5 corresponds to A_V = 19.4).
Open with DEXTER

$\begin{figure} \par\includegraphics[width=8.8cm]{9176fi12.eps}\end{figure}$	Figure 12: Interstellar extinction curve from the optical (0.67 $\mu$ m) through IR. The red plus signs represents measurements from our observations (I, J, $K_{\rm S}$ , [3.6], [4.5] and [8.0]). The dotted line is a power law fit to the observed E(I-J)/ $E(J-K_{\rm S})$ and the solid line is a theoretical R=A_V/E(B-V)=5.5 extinction curve from Draine et al. (2003).
Open with DEXTER

$\begin{figure} \par\includegraphics[width=16.7cm,clip]{9176fi15.eps} \end{figure}$	Figure 15: $K_{\rm S}$ band images of reflection nebulosity ( left panel) and double sources ( right panel) with and without the main source (illuminating source and primary, respectively) removed using a normalized PSF from an isolated star. Each image has a field size of 34 $.\!\!^{\prime\prime}$ 7 $\times$ 34 $.\!\!^{\prime\prime}$ 7.
Open with DEXTER

$\begin{figure} \par\includegraphics[width=9cm,clip]{9176fi18.eps}\end{figure}$	Figure 18: Cumulative star formation history plot for the sample where ages could be determined. The blue (dashed) curves are simple theoretical models assuming a constant star formation rate, an unbound cluster and velocity dispersions of 1 and 3 km s^-1, respectively.
Open with DEXTER

$\begin{figure} \par\includegraphics[width=9cm,clip]{9176fi19.eps} \end{figure}$	Figure 19: Spatial distribution of stellar ages. The two major bi-polar outflows are illustrated by arrows, with the corresponding outflow sources (and two additional deeply embedded sources, probably very young) marked by red asterisks.
Open with DEXTER

$\begin{figure} \par\includegraphics[width=16cm,clip]{9176fi21.eps}\end{figure}$	Figure 21: Deep $K_{\rm S}$ and 2.12 $\mu$ m H₂ colour composite of the central and south east region of L1641N. All visible sources in our sample are marked by a circle. The field shown has a size of 4 $.\mkern -4mu^\prime$ 05 $\times$ 4 $.\mkern -4mu^\prime$ 35.
Open with DEXTER

$\begin{figure} \par\includegraphics[width=16cm,clip]{9176fi22.eps}\end{figure}$	Figure 22: Deep $K_{\rm S}$ and 2.12 $\mu$ m H₂ colour composite of the central and north west region of L1641N. All visible sources in our sample are marked by a circle. The field shown has a size of 4 $.\mkern -4mu^\prime$ 40 $\times$ 4 $.\mkern -4mu^\prime$ 37.
Open with DEXTER

$\begin{figure}\par\includegraphics[angle=180,width=17cm]{9176f23a.ps} \end{figure}$	Figure 23: Optical spectra - Exposure times and I band magnitudes are given for each source.
Open with DEXTER

$\begin{figure}\par\includegraphics[angle=180,width=17cm]{9176f23b.ps} \end{figure}$	Figure 23: continued.
Open with DEXTER

$\begin{figure}\par\includegraphics[angle=180,width=17cm]{9176f24a.ps} \end{figure}$	Figure 24: Evidence of youth - H $\alpha$ in emission and Li I $\lambda 6707$ in absorption. The shifts in wavelength for each source is an artefact caused by each star being differently positioned within the wide slit (1 $.\!\!^{\prime\prime}$ 2).
Open with DEXTER

$\begin{figure}\par\includegraphics[angle=180,width=11cm]{9176f24b.ps} \end{figure}$	Figure 24: continued.
Open with DEXTER