All Figures

$\begin{figure} \par\includegraphics[width=8.3cm,clip]{3860f1.ps} \end{figure}$ Figure 1: Spatial distribution of all good 2MASS sources in the NGC 6530 region. Indicated are the WFI and ACIS FOVs, as well as the selected Check Field.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{3860f2.ps} \end{figure}$ Figure 2: Plot of the reddening-free index Q_JHHK (see text) vs. H-K, for objects in the ACIS FOV. $\times$ 's are X-ray detected sources, while small dots are sources not detected in X-rays. The reddening and excess vectors are shown. ZAMS stellar colors are shown for $A_{\rm V}$ equal to the minimum, average, and maximum values for NGC 6530 (from SCB) and $A_{\rm V}=5$ and 10, respectively. A dashed line is the locus of giant stars. Thick $\times$ 's are hard X-ray sources. The horizontal dotted line indicates the lowest Q_JHHK value attained by normal photospheres. Empty circles are sources with significant excess in the Q_JHHK index. Thick circles are candidate protostars (Q_JHHK-excess sources without optical counterpart).
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{3860f3.ps} \end{figure}$ Figure 3: A (V-I,J-K) optical-infrared color-color diagram, for stars in the ACIS FOV. $\times$ 's are X-ray detected stars, while small dots are X-ray undetected ones. The ZAMS locus (solid line) is shown for the nominal cluster extinction and for $A_{\rm V}=5$ and 10, respectively. The reddening vector is also shown. The dotted line indicates a limit for normal photospheric colors. Stars significantly to the right of this limit, plotted with error bars, are Q_VIJK-excess stars.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{3860f4.ps} \end{figure}$ Figure 4: A (V-I,I-J) diagram, for stars in the ACIS FOV. Symbols are as in Fig. 3. Stars plotted with error bars have significant Q_VIIJ excesses.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{3860f5.ps} \end{figure}$ Figure 5: (B-V,J-K) diagram, analogous to Fig. 3, for stars in the ACIS FOV.
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In the text

$\begin{figure} \par\includegraphics[width=8.3cm,clip]{3860f6.ps} \end{figure}$ Figure 6: Map of all X-ray sources without a 2MASS ( $\times$ 's) or WFI object (plus signs) identification, strongly clustered around the cluster center.
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In the text

$\begin{figure} \par\includegraphics[width=8.3cm,clip]{3860f7.ps} \end{figure}$ Figure 7: Map of all stars in the region showing significant Q_VIJK(dots) or Q_JHHK excesses (empty circles). $\times$ 's are X-ray detected Q_VIJK-excess stars. The two labeled big dots are the only two stars in this region listed in the Herbig & Bell (1988) catalog of PMS stars. A density gradient of Q_VIJK-excess stars can be seen across the WFI FOV, decreasing from northwest to southeast. Thick empty circles are optically undetected Q_JHHK-excess objects.
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In the text

$\begin{figure} \par\includegraphics[width=8.3cm,clip]{3860f8.ps} \end{figure}$ Figure 8: Map of all stars in the region showing significant Q_VIIJ excesses (small dots), with an overall similar distribution as Q_VIJK-excess stars in Fig. 7. Big dots are all stars from the Simbad database with V<11.5, with indicated spectral types if available. $\times$ 's are X-ray detected Q_VIIJ-excess stars (much less numerous than X-ray detected Q_VIJK-excess stars in Fig. 7).
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In the text

$\begin{figure} \par\includegraphics[width=8.3cm,clip]{3860f9.ps} \end{figure}$ Figure 9: Spatial map of stars with significant Q_BVJK excess. $\times$ 's are X-ray detected stars with Q_BVJK excess. Big dots are stars with Q_BVJK excess, but showing no excess in any of the Q_VIJK and Q_VIIJ indices. The Q_BVJK excess stars are correlated more with X-ray detected stars and more concentrated towards the ACIS FOV than the Q_VIJK- or Q_VIIJ-excess stars (cf. Figs. 7 and 8).
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In the text

$\begin{figure} \par\includegraphics[width=8.3cm,clip]{3860f10.ps} \end{figure}$ Figure 10: (V,V-I) color-magnitude diagram for stars in the ACIS FOV, showing stars with Q_VIIJ excess (big dots). $\times$ 's are X-ray sources. Isochrones at 0.3, 1, and 2 Myr are shown, as well as the ZAMS, at cluster reddening. A reddening vector is also shown.
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In the text

$\begin{figure} \par\includegraphics[width=8.3cm,clip]{3860f11.ps} \end{figure}$ Figure 11: (J-H,H-K) color-color diagram for the ACIS FOV. The ZAMS locus and reddening vector are shown. The dotted line is the locus of giant stars. $\times$ 's are X-ray detected sources. Big filled dots are Q_VIJK-excess stars, and big empty circles are Q_VIIJ-excess stars.
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In the text

$\begin{figure} \par\includegraphics[width=8.3cm,clip]{3860f12.ps} \end{figure}$ Figure 12: Diagram of indices Q_VIIJ vs. Q_VIJK for the ACIS FOV. The thick solid line is the ZAMS. The dotted curve is the locus of giant stars. $\times$ 's are X-ray detected stars. Filled dots are Q_VIJK-excess stars, while empty circles are Q_VIIJ-excess stars. The arrow is the displacement caused by a 5-mag excess only in the V band. The subset of stars having both excesses shows an apparent correlation.
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In the text

$\begin{figure} \includegraphics[width=12cm]{3860f13.ps} \end{figure}$ Figure 13: Color-magnitude and color-color diagrams for PMS stars in Taurus-Auriga (black triangles) and Orion Nebula Cluster (grey squares). In the (V,V-I) and (J,J-K) color-magnitude diagrams, magnitudes are scaled to the distance of NGC 6530 for easier comparison. Compare with Figs. 10, 3, 4, and 11. Isochrones and other reference lines are as in these figures, respectively. In the (V-I,I-J) diagram, the two black triangles at I-J>4and V-I>2) are the Class I sources IRAS 04016+2610 and IRAS 04303+2240 (see text).
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In the text

$\begin{figure} \par\includegraphics[angle=270,width=8.3cm,clip]{3860f14.ps} \end{figure}$ Figure 14: Distribution of "sharpness'' for all WFI optical stars with a 2MASS identification a) and for Q_VIIJ-excess stars b).
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In the text

$\begin{figure} \par\includegraphics[width=8.3cm,clip]{3860f15.ps} \end{figure}$ Figure 15: Diagram of the Johnson Q index vs. our Q_VIJK index. Stars with Q_VIJK excess are indicated by big filled dots, while stars with Q_VIIJ excess are indicated by empty circles. $\times$ 's are X-ray sources, and small dots all remaining stars with U band data. Arrows indicate the observed displacement of data points, with respect to their expected position from the V-I color. The thick solid line indicates ZAMS colors.
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In the text

$\begin{figure} \par\includegraphics[width=12cm]{3860fA1.ps} \end{figure}$ Figure A.1: Features of simulated 2MASS-WFI chance identifications (CI) in the ACIS FOV: a): (V-I,J-K) diagram; big dots are Q_VIJK-excess stars in the simulated data, defined as in the real data. Isochrones as in Fig. 3 (to be compared with this figure). b): (V,V-I) diagram; empty circles are Q_VIIJ-excess stars in simulated data, other symbols as in panel a). c): distribution of spatial offsets between 2MASS and WFI positions, for real Q_VIJK-excess stars in the ACIS FOV (grey histogram), including stars with an offset up to 0.8 arcsec. Narrower bars indicate offsets for simulated CI with Q_VIJK excess, again up to 0.8 arcsec. d): same as panel c) for Q_VIIJ-excess stars (both real and simulated).

In the text

$\begin{figure} \par\includegraphics[width=8.3cm,clip]{3860fC1.ps} \end{figure}$ Figure C.1: Map of optical extinction $A_{\rm V}$ , derived from 2MASS colors.

In the text

$\begin{figure} \par\includegraphics[width=8.3cm,clip]{3860f1.ps} \end{figure}$	Figure 1: Spatial distribution of all good 2MASS sources in the NGC 6530 region. Indicated are the WFI and ACIS FOVs, as well as the selected Check Field.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{3860f4.ps} \end{figure}$	Figure 4: A (V-I,I-J) diagram, for stars in the ACIS FOV. Symbols are as in Fig. 3. Stars plotted with error bars have significant Q_VIIJ excesses.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{3860f5.ps} \end{figure}$	Figure 5: (B-V,J-K) diagram, analogous to Fig. 3, for stars in the ACIS FOV.
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$\begin{figure} \par\includegraphics[width=8.3cm,clip]{3860f6.ps} \end{figure}$	Figure 6: Map of all X-ray sources without a 2MASS ( $\times$ 's) or WFI object (plus signs) identification, strongly clustered around the cluster center.
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$\begin{figure} \par\includegraphics[width=8.3cm,clip]{3860f8.ps} \end{figure}$	Figure 8: Map of all stars in the region showing significant Q_VIIJ excesses (small dots), with an overall similar distribution as Q_VIJK-excess stars in Fig. 7. Big dots are all stars from the Simbad database with V<11.5, with indicated spectral types if available. $\times$ 's are X-ray detected Q_VIIJ-excess stars (much less numerous than X-ray detected Q_VIJK-excess stars in Fig. 7).
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$\begin{figure} \par\includegraphics[width=8.3cm,clip]{3860f9.ps} \end{figure}$	Figure 9: Spatial map of stars with significant Q_BVJK excess. $\times$ 's are X-ray detected stars with Q_BVJK excess. Big dots are stars with Q_BVJK excess, but showing no excess in any of the Q_VIJK and Q_VIIJ indices. The Q_BVJK excess stars are correlated more with X-ray detected stars and more concentrated towards the ACIS FOV than the Q_VIJK- or Q_VIIJ-excess stars (cf. Figs. 7 and 8).
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$\begin{figure} \par\includegraphics[width=8.3cm,clip]{3860f10.ps} \end{figure}$	Figure 10: (V,V-I) color-magnitude diagram for stars in the ACIS FOV, showing stars with Q_VIIJ excess (big dots). $\times$ 's are X-ray sources. Isochrones at 0.3, 1, and 2 Myr are shown, as well as the ZAMS, at cluster reddening. A reddening vector is also shown.
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$\begin{figure} \par\includegraphics[width=8.3cm,clip]{3860f11.ps} \end{figure}$	Figure 11: (J-H,H-K) color-color diagram for the ACIS FOV. The ZAMS locus and reddening vector are shown. The dotted line is the locus of giant stars. $\times$ 's are X-ray detected sources. Big filled dots are Q_VIJK-excess stars, and big empty circles are Q_VIIJ-excess stars.
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$\begin{figure} \par\includegraphics[angle=270,width=8.3cm,clip]{3860f14.ps} \end{figure}$	Figure 14: Distribution of "sharpness'' for all WFI optical stars with a 2MASS identification a) and for Q_VIIJ-excess stars b).
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