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Register a new userIntracluster Age Gradients In Numerous Young Stellar Clusters
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Konstantin V. Getman
Publication date
2018
fields
Physics
and research's language is
English
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The pace and pattern of star formation leading to rich young stellar clusters is quite uncertain. In this context, we analyze the spatial distribution of ages within 19 young (median t<3 Myr on the Siess et al. (2000) timescale), morphologically simple, isolated, and relatively rich stellar clusters. Our analysis is based on young stellar object samples from the MYStIX and SFiNCs surveys, and a new estimator of pre-main sequence (PMS) stellar ages, AgeJX, derived from X-ray and near-infrared photometric data. Median cluster ages are computed within four annular subregions of the clusters. We confirm and extend the earlier result of Getman et al. (2014): 80% percent of the clusters show age trends where stars in cluster cores are younger than in outer regions. Our cluster stacking analyses establish the existence of an age gradient to high statistical significance in several ways. Time scales vary with the choice of PMS evolutionary model; the inferred median age gradient across the studied clusters ranges from 0.75 Myr/pc to 1.5 Myr/pc. The empirical finding reported in the present study -- late or continuing formation of stars in the cores of star clusters with older stars dispersed in the outer regions -- has a strong foundation with other observational studies and with the astrophysical models like the global hierarchical collapse model of Vazquez-Semadeni et al. (2017).
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Photometric detections of dust circumstellar disks around pre-main sequence (PMS) stars, coupled with estimates of stellar ages, provide constraints on the time available for planet formation. Most previous studies on disk longevity, starting with Haisch, Lada & Lada (2001), use star samples from PMS clusters but do not consider datasets with homogeneous photometric sensitivities and/or ages placed on a uniform timescale. Here we conduct the largest study to date of the longevity of inner dust disks using X-ray and 1--8 micrometre infrared photometry from the MYStIX and SFiNCs projects for 69 young clusters in 32 nearby star-forming regions with ages t<=5 Myr. Cluster ages are derived by combining the empirical AgeJX method with PMS evolutionary models, which treat dynamo-generated magnetic fields in different ways. Leveraging X-ray data to identify disk-free objects, we impose similar stellar mass sensitivity limits for disk-bearing and disk-free YSOs while extending the analysis to stellar masses as low as M~0.1 Mo. We find that the disk longevity estimates are strongly affected by the choice of PMS evolutionary model. Assuming a disk fraction of 100% at zero age, the inferred disk half-life changes significantly, from t1/2 ~ 1.3--2 Myr to t1/2 ~ 3.5 Myr when switching from non-magnetic to magnetic PMS models. In addition, we find no statistically significant evidence that disk fraction varies with stellar mass within the first few Myr of life for stars with masses <2 Mo, but our samples may not be complete for more massive stars. The effects of initial disk fraction and star-forming environment are also explored.
A high number of embedded clusters is found in the Galaxy. Depending on the formation scenario, most of them can evolve to unbounded groups that are dissolved within a few tens of Myr. A systematic study of young stellar clusters showing distinct characteristics provide interesting information on the evolutionary phases during the pre-main sequence. In order to identify and to understand these phases we performed a comparative study of 21 young stellar clusters. Near-infrared data from 2MASS were used to determine the structural and fundamental parameters based on surface stellar density maps, radial density profile, and colour-magnitude diagrams. We obtained age and mass distributions by comparing pre-main sequence models with the position of cluster members in the colour-magnitude diagram. The mean age of our sample is ~ 5 Myr, where 57% of the objects is found in the 4 - 10 Myr range of age, while 43% is < 4 Myr old. Their low E(B-V) indicate that the members are not suffering high extinction Av < 1 mag), which means they are more likely young stellar groups than embedded clusters. Relations between structural and fundamental parameters were used to verify differences and similarities that could be found among the clusters. The parameters of most of the objects have the same trends or correlations. Comparisons with other young clusters show similar relations among mass, radius and density. Our sample tends to have larger radius and lower volumetric density, when compared to embedded clusters. These differences are compatible with the mean age of our sample, which we consider intermediary between the embedded and the exposed phases of the stellar clusters evolution.
Recent discoveries have put the picture of stellar clusters being simple stellar populations into question. In particular, the color-magnitude diagrams of intermediate age (1-2 Gyr) massive clusters in the Large Magellanic Cloud (LMC) show features that could be interpreted as age spreads of 100-500 Myr. If multiple generations of stars are present in these clusters then, as a consequence, young (<1 Gyr) clusters with similar properties should have age spreads of the same order. In this paper we use archival Hubble Space Telescope (HST) data of eight young massive LMC clusters (NGC 1831, NGC 1847, NGC 1850, NGC 2004, NGC 2100, NGC 2136, NGC 2157 and NGC 2249) to test this hypothesis. We analyzed the color-magnitude diagrams of these clusters and fitted their star formation history to derive upper limits of potential age spreads. We find that none of the clusters analyzed in this work shows evidence for an extended star formation history that would be consistent with the age spreads proposed for intermediate age LMC clusters. Tests with artificial single age clusters show that the fitted age dispersion of the youngest clusters is consistent with spreads that are purely induced by photometric errors. As an additional result we determined a new age of NGC 1850 of ~100 Myr, significantly higher than the commonly used value of about 30 Myr, although consistent with early HST estimates.
I review progress towards understanding the time-scales of star and cluster formation and of the absolute ages of young stars. I focus in particular on the areas in which Francesco Palla made highly significant contributions - interpretation of the Hertzsprung-Russell diagrams of young clusters and the role of photospheric lithium as an age diagnostic.
Extended main sequence turn-off (eMSTO) regions are a common feature in color-magnitude diagrams of young and intermediate-age star clusters in the Magellanic Clouds. The nature of eMSTOs remains debated in the literature. The currently most popular scenarios are extended star formation activity and ranges of stellar rotation rates. Here we study details of differences in MSTO morphology expected from spreads in age versus spreads in rotation rates, using Monte Carlo simulations with the Geneva SYCLIST isochrone models that include the effects of stellar rotation. We confirm a recent finding of Niederhofer et al. that a distribution of stellar rotation velocities yields an MSTO extent that is proportional to the cluster age, as observed. However, we find that stellar rotation yields MSTO crosscut widths that are generally smaller than observed ones at a given age. We compare the simulations with high-quality Hubble Space Telescope data of NGC 1987 and NGC 2249, which are the two only relatively massive star clusters with an age of ~1 Gyr for which such data is available. We find that the distribution of stars across the eMSTOs of these clusters cannot be explained solely by a distribution of stellar rotation velocities, unless the orientations of rapidly rotating stars are heavily biased towards an equator-on configuration. Under the assumption of random viewing angles, stellar rotation can account for ~60% and ~40% of the observed FWHM widths of the eMSTOs of NGC 1987 and NGC 2249, respectively. In contrast, a combination of distributions of stellar rotation velocities and stellar ages fits the observed eMSTO morphologies very well.
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