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Register a new userOn the determination of the He abundance distribution in globular clusters from the width of the main sequence
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Santi Cassisi
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One crucial piece of information to study the origin of multiple stellar populations in globular clusters, is the range of initial helium abundances $Delta{Y}$ amongst the sub-populations hosted by each cluster. These estimates are commonly obtained by measuring the width in colour of the unevolved main sequence in an optical colour-magnitude-diagram. The measured colour spread is then compared with predictions from theoretical stellar isochrones with varying initial He abundances, to determine $Delta{Y}$. The availability of UV/optical magnitudes thanks to the {sl HST UV Legacy Survey of Galactic GCs} project, will allow the homogeneous determination of $Delta{Y}$ for a large Galactic globular cluster sample. From a theoretical point of view, accurate UV CMDs can efficiently disentangle the various sub-populations, and main sequence colour differences in the ACS $F606W-(F606W-F814W)$ diagram allow an estimate of $Delta{Y}$. We demonstrate that from a theoretical perspective the ($F606W-F814W$) colour is an extremely reliable He-abundance indicator. The derivative d$Y$/d($F606W-F814W$), computed at a fixed luminosity along the unevolved main sequence, is largely insensitive to the physical assumptions made in stellar model computations, being more sensitive to the choice of the bolometric correction scale, and is only slightly dependent on the adopted set of stellar models. From a theoretical point of view the ($F606W-F814W$) colour width of the cluster main sequence is therefore a robust diagnostic of the $Delta{Y}$ range.
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Extended main sequence turn-offs (eMSTOs) are a common feature in color-magnitude diagrams (CMDs) of young and intermediate-age star clusters in the Magellanic Clouds. The nature of eMSTOs is still debated. The most popular scenarios are extended star formation and ranges of stellar rotation rates. Here we study implications of a kink feature in the main sequence (MS) of young star clusters in the Large Magellanic Cloud (LMC). This kink shows up very clearly in new emph{Hubble Space Telescope} observations of the 700-Myr-old cluster NGC 1831, and is located below the region in the CMD where multiple or wide MSes, which are known to occur in young clusters and thought to be due to varying rotation rates, merge together into a single MS. The kink occurs at an initial stellar mass of $1.45 pm 0.02;M_{odot}$; we posit that it represents a lower limit to the mass below which the effects of rotation on the energy output of stars are rendered negligible at the metallicity of these clusters. Evaluating the positions of stars with this initial mass in CMDs of massive LMC star clusters with ages of $sim,$1.7 Gyr that feature wide eMSTOs, we find that such stars are located in a region where the eMSTO is already significantly wider than the MS below it. This strongly suggests that stellar rotation emph{cannot} fully explain the wide extent of eMSTOs in massive intermediate-age clusters in the Magellanic Clouds. A distribution of stellar ages still seems necessary to explain the eMSTO phenomenon.
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The radial metallicity distribution in the Galactic thin disc represents a crucial constraint for modelling disc formation and evolution. Open clusters allow us to derive both the radial metallicity distribution and its evolution over time. In this paper we perform the first investigation of the present-day radial metallicity distribution based on [Fe/H] determinations in late type members of pre-main-sequence clusters. Because of their youth, these clusters are therefore essential for tracing the current inter-stellar medium metallicity. We used the products of the Gaia-ESO Survey analysis of 12 young regions (age<100 Myr), covering Galactocentric distances from 6.67 to 8.70 kpc. For the first time, we derived the metal content of star forming regions farther than 500 pc from the Sun. Median metallicities were determined through samples of reliable cluster members. For ten clusters the membership analysis is discussed in the present paper, while for other two clusters (Chamaeleon I and Gamma Velorum) we adopted the members identified in our previous works. All the pre-main-sequence clusters considered in this paper have close-to-solar or slightly sub-solar metallicities. The radial metallicity distribution traced by these clusters is almost flat, with the innermost star forming regions having [Fe/H] values that are 0.10-0.15 dex lower than the majority of the older clusters located at similar Galactocentric radii. This homogeneous study of the present-day radial metallicity distribution in the Galactic thin disc favours models that predict a flattening of the radial gradient over time. On the other hand, the decrease of the average [Fe/H] at young ages is not easily explained by the models. Our results reveal a complex interplay of several processes (e.g. star formation activity, initial mass function, supernova yields, gas flows) that controlled the recent evolution of the Milky Way.
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