Not-So-Simple Stellar Populations in the Intermediate-age Large Magellanic Cloud Star Clusters NGC 1831 and NGC 1868

Using a combination of high-resolution Hubble Space Telescope/WFPC2 observations, we explore the physical properties of the stellar populations in two intermediate-age star clusters in the Large Magellanic Cloud, NGC 1831 and NGC 1868, based on their color-magnitude diagrams. We show that both clusters exhibit extended main-sequence turn-offs. To explain the observations, we consider variations in helium abundance, binarity, age dispersions, and fast rotation of the clusters' member stars. The observed narrow main sequence excludes significant variations in helium abundance in both clusters. We first establish the clusters' main-sequence binary fractions using the bulk of the clusters' main-sequence stellar populations >1 mag below their turn-offs. The extent of the turn-off regions in color--magnitude space, corrected for the effects of binarity, implies that age spreads of order 300 Myr may be inferred for both clusters if the stellar distributions in color--magnitude space were entirely due to the presence of multiple populations characterized by an age range. Invoking rapid rotation of the population of cluster members characterized by a single age also allows us to match the observed data in detail. However, when taking into account the extent of the red clump in color--magnitude space, we encounter an apparent conflict for NGC 1831 between the age dispersion derived from that based on the extent of the main-sequence turn-off and that implied by the compact red clump. We therefore conclude that, for this cluster, variations in stellar rotation rate are preferred over an age dispersion. For NGC 1868, both models perform equally well.Comment: 34 pages, 19 figures, accepted in Ap

Discovery of Super-Li Rich Red Giants in Dwarf Spheroidal Galaxies

Stars destroy lithium (Li) in their normal evolution. The convective envelopes of evolved red giants reach temperatures of millions of K, hot enough for the 7Li(p,alpha)4He reaction to burn Li efficiently. Only about 1% of first-ascent red giants more luminous than the luminosity function bump in the red giant branch exhibit A(Li) > 1.5. Nonetheless, Li-rich red giants do exist. We present 15 Li-rich red giants--14 of which are new discoveries--among a sample of 2054 red giants in Milky Way dwarf satellite galaxies. Our sample more than doubles the number of low-mass, metal-poor ([Fe/H] <~ -0.7) Li-rich red giants, and it includes the most-metal poor Li-enhanced star known ([Fe/H] = -2.82, A(Li)_NLTE = 3.15). Because most of these stars have Li abundances larger than the universe's primordial value, the Li in these stars must have been created rather than saved from destruction. These Li-rich stars appear like other stars in the same galaxies in every measurable regard other than Li abundance. We consider the possibility that Li enrichment is a universal phase of evolution that affects all stars, and it seems rare only because it is brief.Comment: 6 pages, 3 figures, accepted to ApJ Letters, version 3 includes additional references and minor typographical change

Data characterization using artificial-star tests: performance evaluation

Traditional artificial-star tests are widely applied to photometry in crowded stellar fields. However, to obtain reliable binary fractions (and their uncertainties) of remote, dense, and rich star clusters, one needs to recover huge numbers of artificial stars. Hence, this will consume much computation time for data reduction of the images to which the artificial stars must be added. In this paper, we present a new method applicable to data sets characterized by stable, well-defined point-spread functions, in which we add artificial stars to the retrieved-data catalog instead of the raw images. Taking the young Large Magellanic Cloud cluster NGC 1818 as an example, we compare results from both methods and show that they are equivalent, while our new method saves significant computational time.Comment: 23 pages, 10 figures, accepted by PAS

First Observational Signature of Rotational Deceleration in a Massive, Intermediate-age Star Cluster in the Magellanic Clouds

While the extended main-sequence turn-offs (eMSTOs) found in almost all 1--2 Gyr-old star clusters in the Magellanic Clouds are often explained by postulating extended star-formation histories, the tight subgiant branches (SGBs) seen in some clusters challenge this popular scenario. Puzzlingly, the SGB of the eMSTO cluster NGC 419 is significantly broader at bluer than at redder colors. We carefully assess and confirm the reality of this observational trend. If we would assume that the widths of the features in color--magnitude space were entirely owing to a range in stellar ages, the star-formation histories of the eMSTO stars and the blue SGB region would be significantly more prolonged than that of the red part of the SGB. This cannot be explained by assuming an internal age spread. We show that rotational deceleration of a population of rapidly rotating stars, a currently hotly debated alternative scenario, naturally explains the observed trend along the SGB. Our analysis shows that a `converging' SGB could be produced if the cluster is mostly composed of rapidly rotating stars that slow down over time owing to the conservation of angular momentum during their evolutionary expansion from main-sequence turn-off stars to red giants.Comment: 11 pages, preprint format (uses aastex6.cls); ApJ Letters, in pres

Simple stellar population models including blue stragglers

Observations show that nearly all star clusters and stellar populations contain blue straggler stars (BSs). BSs in a cluster can significantly enhance the integrated spectrum of the host population, preferentially at short wavelengths, and render it much bluer in photometric colours. Current theoretical simple stellar population (SSP) models constructed within the traditional framework of single and binary stellar evolution cannot fully account for the impact of these objects on the integrated spectral properties of stellar populations. Using conventional SSP models without taking into account BS contributions may significantly underestimate a cluster's age and/or metallicity, simply because one has to balance the observed bluer colours (or a bluer spectrum) with a younger age and/or a lower metallicity. Therefore, inclusion of BS contributions in SSP models is an important and necessary improvement for population synthesis and its applications. Here, we present a new set of SSP models, which include BS contributions based on our analysis of individual star clusters. The models cover the wavelength range from 91~{\AA} to 160~$\mu$m, ages from 0.1 to 20 Gyr and metallicities $Z=0.0004, 0.004, 0.008, 0.02$ (solar) and 0.05. We use the observed integrated spectra of several Magellanic Cloud star clusters to cross-check and validate our models. The results show that the age predictions from our models are closer to those from isochrone fitting in the clusters' colour-magnitude diagrams compared to age predictions based on standard SSP models.Comment: 16 pages, 15 figures, 4 tables, accepted for publication in MNRA