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Register a new userThe Complex Distribution of Recently Formed Stars. Bimodal Stellar Clustering in the Star-Forming Region NGC 346
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Added by
Dimitrios Gouliermis
Publication date
2014
fields
Physics
and research's language is
English
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We present a detailed stellar clustering analysis with the application of the two-point correlation function on distinct young stellar ensembles. Our aim is to understand how stellar systems are assembled at the earliest stages of their formation. Our object of interest is the star-forming region NGC 346 in the Small Magellanic Cloud. It is a young stellar system well-revealed from its natal environment, comprising complete samples of pre--main-sequence and upper main-sequence stars, very close to their formation. We apply a comprehensive characterization of the autocorrelation function for both centrally condensed stellar clusters and self-similar stellar distributions through numerical simulations of stellar ensembles. We interpret the observed autocorrelation function of NGC 346 on the basis of these simulations. We find that it can be best explained as the combination of two distinct stellar clustering designs, a centrally concentrated, dominant at the central part of the star-forming region, and an extended self-similar distribution of stars across the complete observed field. The cluster component, similar to non-truncated young star clusters, is determined to have a core radius of ~2.5 pc and a density profile index of ~2.3. The extended fractal component is found with our simulations to have a fractal dimension of ~2.3, identical to that found for the interstellar medium, in agreement to hierarchy induced by turbulence. This suggests that the stellar clustering at a time very near to birth behaves in a complex manner. It is the combined result of the star formation process regulated by turbulence and the early dynamical evolution induced by the gravitational potential of condensed stellar clusters.
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Deep F555W and F814W Hubble Space Telescope ACS images are the basis for a study of the present day mass function (PDMF) of NGC346, the largest active star forming region in the Small Magellanic Cloud (SMC). We find a PDMF slope of Gamma=-1.43+/-0.18 in the mass range 0.8-60 Mo, in excellent agreement with the Salpeter Initial Mass Function (IMF) in the solar neighborhood. Caveats on the conversion of the PDMF to the IMF are discussed. The PDMF slope changes, as a function of the radial distance from the center of the NGC 346 star cluster, indicating a segregation of the most massive stars. This segregation is likely primordial considering the young age (~3 Myr) of NGC346, and its clumpy structure which suggests that the cluster has likely not had sufficient time to relax. Comparing our results for NGC346 with those derived for other star clusters in the SMC and the Milky Way (MW), we conclude that, while the star formation process might depend on the local cloud conditions, the IMF does not seem to be affected by general environmental effects such as galaxy type, metallicity, and dust content.
The full stellar population of NGC 6334, one of the most spectacular regions of massive star formation in the nearby Galaxy, have not been well-sampled in past studies. We analyze here a mosaic of two Chandra X-ray Observatory images of the region using sensitive data analysis methods, giving a list of 1607 faint X-ray sources with arcsecond positions and approximate line-of-sight absorption. About 95 percent of these are expected to be cluster members, most lower mass pre-main sequence stars. Extrapolating to low X-ray levels, the total stellar population is estimated to be 20-30,000 pre-main sequence stars. The X-ray sources show a complicated spatial pattern with about 10 distinct star clusters. The heavily-obscured clusters are mostly associated with previously known far-infrared sources and radio HII regions. The lightly-obscured clusters are mostly newly identified in the X-ray images. Dozens of likely OB stars are found, both in clusters and dispersed throughout the region, suggesting that star formation in the complex has proceeded over millions of years. A number of extraordinarily heavily absorbed X-ray sources are associated with the active regions of star formation.
We present observations of twelve rotational transitions of H2O-16, H2O-18, and H2O-17 toward the massive star-forming region NGC 6334 I, carried out with Herschel/HIFI as part of the guaranteed time key program Chemical HErschel Surveys of Star forming regions (CHESS). We analyze these observations to obtain insights into physical processes in this region. We identify three main gas components (hot core, cold foreground, and outflow) in NGC 6334 I and derive the physical conditions in these components. The hot core, identified by the emission in highly excited lines, shows a high excitation temperature of 200 K, whereas water in the foreground component is predominantly in the ortho- and para- ground states. The abundance of water varies between 4 10^-5 (outflow) and 10^-8 (cold foreground gas). This variation is most likely due to the freeze-out of water molecules onto dust grains. The H2O-18/H2O-17 abundance ratio is 3.2, which is consistent with the O-18/O-17 ratio determined from CO isotopologues. The ortho/para ratio in water appears to be relatively low 1.6(1) in the cold, quiescent gas, but close to the equilibrium value of three in the warmer outflow material (2.5(0.8)).
We present a study of the spatial distribution of the stellar cluster populations in the star forming galaxy NGC 628. Using Hubble Space Telescope broad band WFC3/UVIS UV and optical images from the Treasury Program LEGUS (Legacy ExtraGalactic UV Survey), we have identified 1392 potential young (<100 Myr) stellar clusters within the galaxy, identified from a combination of visual inspection and automatic selection. We investigate the clustering of these young stellar clusters and quantify the strength and change of clustering strength with scale using the two-point correlation function. We also investigate how image boundary conditions and dust lanes affect the observed clustering. The distribution of the clusters is well fit by a broken power law with negative exponent $alpha$. We recover a weighted mean index of $alpha$ ~ -0.8 for all spatial scales below the break at 3.3 (158 pc at a distance of 9.9 Mpc) and an index of $alpha$ ~ -0.18 above 158 pc for the accumulation of all cluster types. The strength of the clustering increases with decreasing age and clusters older than 40 Myr lose their clustered structure very rapidly and tend to be randomly distributed in this galaxy whereas the mass of the star cluster has little effect on the clustering strength. This is consistent with results from other studies that the morphological hierarchy in stellar clustering resembles the same hierarchy as the turbulent interstellar medium.
We discuss the star formation history of the SMC region NGC 346 based on Hubble Space Telescope images. The region contains both field stars and cluster members. Using a classical synthetic CMD procedure applied to the field around NGC 346 we find that there the star formation pace has been rising from a quite low rate 13 Gyr ago to approx 1.4 times 10^{-8} Mo yr^{-1}pc^{-2} in the last 100 Myr. This value is significantly higher than in other star forming regions of the SMC. For NGC 346 itself, we compare theoretical and observed Color-Magnitude Diagrams (CMDs) of several stellar sub-clusters identified in the region, and we derive their basic evolution parameters. We find that NGC 346 experienced different star formation regimes, including a dominant and focused high density mode, with the sub-clusters hosting both pre-main sequence (PMS) and upper main sequence (UMS) stars, and a diffuse low density mode, as indicated by the presence of low-mass PMS sub-clusters. Quantitatively, the star formation in the oldest sub-clusters started about 6 Myr ago with remarkable synchronization, it continued at high rate (up to 2 times 10^{-5} Mo yr^{-1} pc^{-2}) for about 3 Myr and is now progressing at a lower rate. Interestingly, sub-clusters mainly composed by low mass PMS stars seem to experience now the first episode of star formation, following multi-seeded spatial patterns instead of resulting from a coherent trigger. Two speculative scenarios are put forth to explain the deficiency of UMS stars: the first invokes under-threshold conditions of the parent gas; the second speculates that the initial mass function (IMF) is a function of time, with the youngest sub-clusters not having had sufficient time to form more massive stars.
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