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Massey et al. (2001) and Fang et al. (2012) derive an age < 2.7 Myr for Pismis 24.

With the HST V I photometry we checked the age derivation of these authors. From the V vs. V − I diagram shown in Fig. 5.20a we find that, if taken at face value, the sources seem to have ages between 1 and 10 Myr, with a median of ∼ 3 Myr.

To further investigate this age dispersion we compared the near-IR counterparts of the optical sources with the isochrones in a Ksvs. H − Ksdiagram (cf. Fig. 5.20b).

The stars appear more massive in the IR, and trying to deredden them individually and correct the V vs. V − I magnitudes accordingly moves the stars to the left

5.3. Past and current star formation in NGC6357 195

Figure 5.18: Example of dereddened KLF (see text) for sources in the H − K s colour interval corresponding to a reddening interval 3.2 mag < AV < 7.8 mag (see text), for the southern field. The dotted line indicates the completeness limit.

Figure 5.19: IMF derived from the dereddened KLF following Massi et al. (2006), for sources in the H − K s colour interval corresponding to a reddening interval 3.2 mag < AV < 7.8 mag (see text), for 1 Myr old PMS stars. The dotted line indicates the completeness limit.

of the ZAMS, indicating that the dispersion in the Ksvs. H − Ksdiagram is not only due to differential extinction, but also to an IR-excess which is not evident in a NIR colour-colour diagram, as shown in Fig. 5.20c. PMS stars of Solar and subsolar mass evolve in time contracting with an approximately constant effective temperature, thus decreasing in luminosity. Therefore, if the stars were 10 Myr old they would be fainter in the IR than younger PMS stars and the IR-excess would have to be even more prominent, and still not be evident in the colour-colour diagram, which is very unlikely. The age of the cluster can thus be constrained between 1 − 3 Myr and the dispersion in the V vs. V − I is not real.

With the age constrained in this way, we used the isochrones by Palla & Stahler (1999) (for 1 and 3 Myr) to derive the IMF, following to the method discussed in Massi et al. (2006), and we find that there appears to be a change of slope around

5.3. Past and current star formation in NGC6357 196

M ∼2.5 M(cf. Fig. 5.19). The IMF is defined by:

dN

dlog(M) = kM^Γ. (5.11)

We fitted the logarithm of the IMF with a two segment function, to derive the index Γ (Scalo 1998) in the two regimes. The IMF derived for 3.2 mag < AV < 7.8 mag and that constructed for 3.2 mag < AV < 15 mag are slightly different, and should represent a lower- and an upper limit for the IMF. This is because, limiting the colour, not only do we find fewer members, but we include just those with relatively small IR excesses: therefore the two colour ranges should encompass the real IMF slope. In addition, one has to keep in mind that because the IR excess is not taken into account in PMS tracks, we measure higher masses in the IR, thus leading to a measuredΓ that may be steeper than the real one. We find that in the high-mass regime the IMF has a slope around −1.2 in the former case ∼ −1.9 in the latter, while for M . 2.5 Mthe IMF appears to flatten. The turnover mass is larger than the mass corresponding to the completeness limit for 3.2 mag < AV < 7.8 mag, while it is approximately equal to the mass completeness limit for 3.2 mag < AV < 15 mag. This flattening is consistent with a Scalo (1998)-or Kroupa et al. (1993) IMF.

Using the derived IMF, complemented with a Scalo’s at the lowest masses (down to 0.1 M), to calculate the total number of cluster members, we estimate that Pismis 24 hosts 3600 − 11000 stars (depending on the extinction interval used), consistent with the estimate of Wang et al. (2007) (∼ 5000), for a distance of 1.7 kpc. This implies a stellar mass for the core of (2 − 6) × 10³M.

On the basis of the previous discussion, we can try to infer the properties of the primordial environment out of which Pismis 24 was born.

First of all, we can expect that the properties of the gas are similar to those of massive quiescent clumps, therefore with average temperatures between ∼ 10 − 15 K and possibly with a strong CO depletion.

Then, assuming that the Hii region evolved in a medium of constant density with a radius equal to the distance between the massive clumps and the ionisation front (i.e. ∼ 1 pc), and using the age of the cluster (∼ 1 Myr) as the age of this idealised Hii region, we can derive its Str¨omgren radius (≈ 0.02 pc) from Eq. 12-20 on Spitzer (1978). In turn, from the Str¨omgren radius we can derive the average density of the medium, resulting to be n_H ∼ 10⁶ cm⁻³. A single clump with a radius of ∼ 1 pc and a density of ∼ 10⁶ cm⁻³has a total gas mass of ≈ 10⁵ M. Using the above estimate of the stellar mass in Pismis 24, we find that the star formation efficiency is 2% − 6%. Such a large clump would encompass the whole

5.3. Past and current star formation in NGC6357 197

core of the cluster; however it would be much more massive than the brightest sources in the ATLASGAL survey. In order to reduce this mass, we can consider smaller clumps, still with a density of 10⁶ cm⁻³, but with a radius R ∼ 0.5 pc, more than sufficient to include the single surface density peaks. The sizes of the clumps in the nearby Hii region are similar to this, and several clumps with a similar radius were observed in Chapters 3 and 4. A clump with these properties would have a mass ∼ 10⁴ M; assuming that there were four of them, one for each peak in surface density of sources in the Ksframe, we get a total gas mass comparable to the most massive objects in Chapter 4. On the other hand, it is still much more massive than the clumps studied in Chapter 3, in agreement with our finding that those objects should not form stars more massive than late O stars. That there are very few sources like the one delineated here makes sense, because there are very few clusters like Pismis 24, hosting a large number of OB- and even O3 stars. The diameter size of the superposition of the four smaller clumps would be ∼ 1.5 pc. In this case, the efficiency of star formation would range between 5 − 15%. Given that all the most massive stars in Pismis are in the same subcluster, there is the possibility that only one of the clumps was this massive, thus increasing the efficiency of star formation, or that some material from the low-density layers of nearby objects may have been focused onto the clump near to the bottom of the potential well, as suggested in the competitive accretion scenario. This material would increase the mass available for accretion, and could end up onto the most massive objects, in a situation similar to that shown in Smith et al. (2009).

The estimate of the density of the material is an upper limit, because part of the UV photons are absorbed by dust grains. This would decrease the average density (even by an order of magnitude), and the gas mass of the primordial material.

Consequently the star formation efficiency would increase even more. Because the gas mass cannot be less than the stellar mass, we can interpret this result as proof that the average density of the clumps from which Pismis 24 was born, was very high.

Evolved regions like NGC 6357 show once more that massive stars form together with a significant number of lower mass stars, and that massive clumps do not host just a single massive star. In addition, from the situation observed in G353.2+0.9 we know that massive clumps may show signs of active star formation even if massive stars were not formed yet, if they are among the latest to be formed. In fact, as suggested by the observed substructure in Pismis 24, they may rapidly remove the gas in their immediate surroundings, thus inhibiting further star formation. The presence of low-mass PMS stars along with massive ZAMS objects is no proof against this, if the growth of high-mass stars towards the ZAMS is very

5.3. Past and current star formation in NGC6357 198

fast and takes only a minor fraction of the age of the cluster. Russeil et al. (2012) estimate the duration of the starless- and protostellar phases for the formation of high-mass stars in NGC 6357 to be 10⁴ yr and 10⁵ yr, respectively. This allows massive stars to accrete their mass and reach the MS while their low-mass counterparts are still contracting towards the ZAMS.