NGC 346 in the Small Magellanic Cloud. III. Recent Star Formation and Stellar Clustering Properties in the Bright HiiRegion N66

Abstract

In the third part of our photometric study of the star-forming region NGC 346/ N66 and its surrounding field in the Small Magellanic Cloud (SMC), we focus on the large number of low-mass pre-main-sequence (PMS) stars revealed by the Hubble Space Telescope observations with the Advanced Camera for Surveys. We investigate the origin of the observed broadening of the PMS population in the V À I; V color-magnitude diagram. The most likely explanations are either the presence of differential reddening or an age spread among the young stars. Assuming the latter, simulations indicate that we cannot exclude the possibility that stars in NGC 346 might have formed in two distinct events occurring about 10 and 5 Myr ago, respectively. We find that the PMS stars are not homogeneously distributed across NGC 346, but instead are grouped in at least five different clusters. On spatial scales from 0.8 00 to 8 00 (0.24Y2.4 pc at the distance of the SMC) the clustering of the PMS stars as computed by a two-point angular correlation function is self-similar with a power-law slope % À0:3. The clustering properties are quite similar to Milky Way star-forming regions like Orion OB or Oph. Thus molecular cloud fragmentation in the SMC seems to proceed on the same spatial scales as in the Milky Way. This is remarkable given the differences in metallicity and hence dust content between SMC and Milky Way star-forming regions.

A slitless spectroscopic survey for Hαemission-line objects in SMC clusters

“…Their large number may have introduced biases. Hennekemper et al (2008) report numerous premain-sequence T Tauri stars, which are unfortunately too faint (see Fig. 4) to be detected by the present study.…”

“…However, Nota et al (2006) found several populations with different ages: 4.5 Gyr for stars in the field, a young population with ages ranging from 3 to 5 Myr, in which stars with a mass less than 3 M are still pre-main sequence stars, and a population with an intermediate age of 150 Myr. In this open cluster and in its vicinity, we found 55 emissionline stars. Dereddening with a global value of 0.008 mag for massive stars from Hennekemper et al (2008) suggests that most of them are on the main sequence. The spectral classification obtained differs on average by 1 spectral subtype for the 12 stars shared with spectral classifications by other authors.…”

“…Owing to the large spread in age of the stars in this cluster, some of them could be pre-mainsequence stars. Where possible, cross-references to the studies by Hennekemper et al (2008), Wisniewski & Bjorkman (2006), Wisniewski et al (2007b), andHunter et al (2008) and to the Simbad database are, therefore, also included.…”

“…This figure shows that the slitless spectroscopy allows to find circumstellar (CS) emissionline stars while slit-spectroscopy cannot disentangle CS and nebular emission lines in diffuse emission nebulae. According to Hennekemper et al (2008), there are many pre main sequence T Tauri stars in the central regions of the cluster. But they are too faint to be detected here.…”

Hubble Tarantula Treasury Project. V. The Star Cluster Hodge 301: The Old Face of 30 Doradus*

“…They found that the ages based on PMS isochrones are 1.5-2.0 times shorter than the "nuclear" MSTO ages. However, our PMS age is mostly based on the TOn luminosity, and not on the PMS stars, whose color is affected by several uncertainties (see e.g., Hennekemper et al 2008). Another possible source of error is the use of different sets of models to study different phases: we do not have this kind of problem because in this work we used the PARSEC models which follow the entire evolution from the PMS to the post-MS. Figure 10 shows the synthetic LF (blue line) corresponding to the best SFH compared with the observed counterpart (red line).…”