Young stellar clusters in the star forming region NGC 6357

Observations have proven that a large fraction of stars form in clusters or clustered environments. In the galactic disk, some of these structures dissolve rather quickly after their birth, but others survive and emerge from their parental gas as open clusters. In addition, high-mass stars are usually found in clusters of low-mass stars and it is not clear yet whether they can also form in isolation. So the topics of stellar cluster and high-mass star formation are deeply related with each other and the physical processes involved are not fully understood yet. In this context, observations of star forming regions and embedded clusters are therefore fundamental to clarify the early evolutionary stages of these entities.

Young open clusters of few Myr of age are characterised by a population of high-mass main sequence stars and low-mass pre-main sequence stars, HII regions, and remnants of their parental gas. The intense UV radiation from the massive cluster members strongly interacts with the gas and can either disperse it or trigger new star formation. In particular, NGC 6357 is a complex of giant molecular clouds and HII regions located at a distance of 1.7 kpc, relatively close for this kind of structure. Interestingly, it houses at least three large young open clusters, of which Pismis 24 is the most prominent one, and exhibits signposts of star formation in progress (Fig. 1). Photometry and spectroscopy of both massive and low-mass stars point to an age of the whole stellar population of 1-3 Myr. It appears as if star formation has been going on in different sites simultaneously, which is a landmark of turbulence in the interstellar medium.

Figure 1: Three-colour image (3.6μm blue, 4.5μm green, 8.0μm  red) of NGC 6357, obtained from Spitzer/IRAC observations. The large circles mark the locations of the most prominent HII regions. The clusters Pismis 24 and AH03J1725-34.4 are also labelled. The diffuse red emission is due to polycyclic aromatic hydrocarbons (PAHs), excited by the intense UV emission from the young high-mass stars in the region, and outlines the large-scale distribution of the molecular gas. The strongest diffuse emission inside the circles is produced by hot dust mixed with the ionised gas in HII regions. The imaged region spans about 7 pc in width.

Astronomers of the Arcetri Observatory have been studying NGC 6357 for many years, mostly focusing on the ionised and molecular gas. A detailed study of the young stellar population in NGC6357, and in Pismis 24 in particular, is presented in the article "Young open clusters in the galactic star forming region NGC 6357" (Astronomy & Astrophysics, in press) led by Fabrizio Massi (INAF-Observatory of Arcetri) and including Andrea Giannetti (INAF-IRA Bologna), Maria Teresa Beltran (INAF-Observatory of Arcetri) and collaborators in Bologna, Teramo and Chile.

Identifying this population is not an easy task, since the related sky area lies in the galactic plane, less than 10 degrees from the galactic centre, and the target stars are mixed with lots of background stars. The authors of the paper performed Spitzer/IRAC infrared photometry (3.6 through 8.0μm) to retrieve the young stars based on their colours and study their spatial distribution (see Fig. 1).

To study in detail the open cluster Pismis 24 in NGC 6357, the Spitzer/IRAC data were complemented with near-infrared (JHKs) photometry, taken with Sofi at the NTT (ESO) telescope, high spatial resolution X-ray observations with the Chandra satellite from the literature, and optical (VI) photometry with WFPC2/HST. By combining all the information at these different wavelengths, the cluster stellar population was cleaned from the background contaminants and analysed. The measured fraction of cluster members with a circumstellar disk (about 30%) and their position in an optical color-magnitude diagram are consistent with an age of 1-3 Myr.  The projected spatial distribution of the cluster members exhibits some sub-clustering which is consistent with the merging of smaller star groups formed in different cores.

The authors find indications of triggered star formation in the gas around Pismis 24. They propose a tentative scenario for the formation of the cluster, in which a few dense cores (> 10⁵ cm^-3, 0.1 pc in size) embedded in a larger (about 1 pc in size), less dense cloud gave birth to different groups of stars. As soon as the massive stars entered the zero age main sequence their intense UV emission and strong stellar winds would have quickly dispersed the parental gas and would be now affecting near-by, pre-existing clouds, triggering the formation of a new generation of stars. The paper is the first devoted to studying the stellar populations in NGC 6357 and the authors are currently working on new near-infrared data towards the two other large clusters in NGC 6357.

Figure 2: The stellar population of the open cluster Pismis 24 was extracted by combining together photometry at different wavelengths. The three panels show optical and infrared magnitude/colour-colour diagrams for stars retrieved in the HST images and matched to near-infrared and X-ray sources. In particular, those associated with X-ray emission are indicated as small blue triangles (pre-main sequence stars are known to be intense X-ray emitters). (a) V vs. V - I for stars towards Pismis 24 from HST/WFPC2 images. The full line marks the Zero Age Main Sequence (ZAMS), the dashed lines are isochrones for (from right to left) 1, 3, and 10 Myr old Pre-Main Sequence (PMS) stars. These have been reddened by AV = 5.5 mag and scaled to 1.7 kpc. The loci of 1 and 0.4 solar masses are labelled near the 1 Myr isochrone. Large open triangles are stars without a NIR counterpart. (b) Ks vs. H - Ks for th NIR counterparts of the optical stars (red open squares: stars on the left of the optical ZAMS; full squares: stars on the right of the optical ZAMS undetected in X-ray). The full lines are the isochrones for (from right to left) 1 and 3 Myr old PMS stars, same reddening and distance as above. The loci of 1 and 0.4 solar masses are also labelled and the dashed lines are their reddening paths (for 1 Myr old stars, up to AV = 10 mag). (c) J - H vs. H - Ks for the NIR counterparts of the optical stars (same symbols as above, but stars on the left of the optical ZAMS are omitted). The full line is the unreddened Main Sequence, the dashed lines are reddening paths with crosses every AV = 10 mag.  Also shown as full lines, the reddening paths according to a different extinction law. The dot-dashed line marks the locus of Classical T-Tauri Stars. Clearly, most of the pre-main sequence stars in the V vs. V - I diagram (i. e., those between the 1 and 10 Myr isochrones) are X-ray emitters and spread parallel to the classical T-Tauri locus in the J - H vs. H - Ks. A comparison of panels (a) and (b) shows that these objects tend to exhibit larger masses in the near-infrared, which can be interpreted as a larger near-infrared emission due to the presence of circumstellar disks.

Edited by Anna Gallazzi and Fabrizio Massi, 29/12/2014