Los ascomicetos mitospóricos entomopatógenos en el control microbiano de la

Focusing on Pismis 24, a first diagnostic diagram to investigate the stellar popu-lation in this cluster and in the associated Hii region G353.2+0.9 is the (J − H) vs. (H − Ks) colour-colour plot. Figure 5.15 shows this diagram for the

northern-5.3. Past and current star formation in NGC6357 190

Figure 5.14: Left: Contours of the surface density of IRAC sources. Only sources detected in at least the two lowest IRAC bands, with photometric errors < 0.3 mag, and up to [3.6]= 12.25 are considered. The contours range in steps of 3σ from the estimated average surface density of field stars plus 3σ (14+ 11 = 25 stars arcmin⁻²). Underlying (greyscale), the image at 3.6 µm. Also labelled, the tentatively identified subclusters (A stands for AH03J1725-34.4). Right: Same as left, but showing the positions of identified Class II sources (full blue squares) and Class I sources (open red triangles). Other YSO concentrations are enclosed in dashed-line circles and a rectangular box.

and southern parts of the SofI field (separated at DEC δ= −34^◦11⁰14⁰⁰, and char-acterised by the molecular gas and Pismis 24, respectively), including all sources with a photometric uncertainty < 0.3 mag and inside the completeness limit in the Ksband.

The black solid line shows the main sequence locus, while the dashed lines delineate the corresponding reddening band, following the Rieke & Lebofsky (1985) extinction law. The crosses on the dashed lines indicate intervals of AV = 10 mag. A significant fraction of the sources are found below the reddening band, indicating the presence of a large population of YSOs in the region. There are, however, a large number of sources above the reddening band as well. This may point to a steeper extinction law; an anomalous reddening law for NGC 6357 is suggested by several authors (e.g., Chini & Kr¨ugel 1983; Bohigas et al. 2004;

Russeil et al. 2012). Knowing the slope of the extinction law is fundamental to identify sources with an IR-excess (and hence a circumstellar disk; cf. Sect. 1.3.1), and thus their fraction with respect to the total stellar population, which can be used to constrain the age of the cluster. Checking the positions of the sources above the reddening band, we find that they are anti-correlated with the molecular gas and

5.3. Past and current star formation in NGC6357 191

Figure 5.15: a) SofI J − H vs. H − Ks diagram for the northern field. b) SofI J − Hvs. H − Ksdiagram for the southern field. Only sources with Ks< 16 mag and photometric errors < 0.3 mag have been selected. The grey triangles mark the sources whose colours have been taken from 2MASS. Those labelled G1, G2, and G3 are candidate giant stars based on their brightness and extinction. The thick solid line is the main sequence locus (using the colours from Koornneef 1983). The dashed lines are reddening paths with crosses every AV = 10 mag, following Rieke & Lebofsky (1985). Also shown as grey full lines, reddening vectors according to the extinction law derived by Straiˇzys & Laugalys (2008).

with the cluster core, but are uniformly distributed elsewhere. This is consistent with the distribution expected for shielded background objects. We conclude that these are likely background giants, whose unreddened locus is found above that of the main sequence in a diagram like Fig. 5.15. We thus make the (conservative) choice of a Rieke & Lebofsky (1985) extinction law.

Figure 5.16 shows the colour-magnitude diagrams for the northern and southern fields. In agreement with Massey et al. (2001) and Fang et al. (2012), we find a typical extinction of AV ≈ 5.7 − 7.6 mag for the stars in Pismis 24. The extinction has the effect of moving the points along the reddening vector indicated in the figure, away from the ZAMS- or PMS tracks. Therefore, in principle, we can

5.3. Past and current star formation in NGC6357 192

Figure 5.16: a) SofI Ksvs. H − Ksdiagram for the northern field. Also shown as a grey full line, the 1 Myr isochrone for PMS stars from the evolutionary tracks of Palla & Stahler (1999), for AV = 20 mag, and masses in the 0.1 − 6 Mrange.

b) SofI Ksvs. H − Ksdiagram for the southern field. In both panels, only sources with Ks< 16 mag and photometric errors < 0.3 mag have been included. The grey full triangles indicate the sources whose colours have been taken from 2MASS.

Some of them, belonging to Pismis 24, are labelled. The thick solid vertical line marks the main sequence locus (using absolute magnitudes from Allen 1973 and colours from Koornneef 1983) for a distance of 1.7 kpc. The arrow indicates a reddening of AV = 20 mag according to Rieke & Lebofsky (1985). Spectral types are labelled next to the ZAMS. The dashed grey line marks the completeness limit.

obtain the masses for each of the objects, tracing them back to the appropriate track, as described in Massi et al. (2006). This procedure does not take into account the IR-excess, thus leading to an overestimate in stellar mass. In the figure, the photometric depth of our image can be fully appreciated: converting the completeness limits into masses using the PMS tracks (1 Myr old objects) of Palla & Stahler (1999), we find that we are complete down to M ∼ 0.2 Mwith AV ∼ 10 mag for the southern field and M ∼ 0.4 − 2 Mwith AV ∼ 20 − 40 mag for the northern field.

5.3. Past and current star formation in NGC6357 193

The surface density of sources in the Ks field was computed in squares of 29⁰⁰ × 29⁰⁰ (100 × 100 pixel²) spaced by half a cell, and is shown in contours in Fig. 5.17, above a level of 240 stars/arcmin² (180+ 60 stars/arcmin², mean field density plus 2σ). Pismis 24 appears substructured also in the central regions observed in the near-IR; all of the most massive members are found in the same subcluster, while the others host only low- and intermediate-mass stars. Parker

& Meyer (2012) performed N-body simulations of clusters with 1000 members, and show that substructured distributions with radius of 1 pc collapse to central condensations after 1 Myr, if they are initially subvirial. On the other hand, if the cluster is initially supervirial, as expected in case they experience an early phase of fast gas removal, it still appears substructured after 5 Myr. Differential reddening may simulate the subclustering, but our data indicate that this is not the case. With the SofI near-IR data we are able to find more members than with IRAC data, due to the better angular resolution and mass completeness. However, the core appears smaller because of the small field covered by the Ksframe, implying that the field count estimate is contaminated by the halo of cluster members visible in X-ray data. With a larger frame we would be able to have a better determination of the field counts, and the number of cluster members would be even higher.

5.3.4 The K luminosity function and the initial mass function