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

Star cluster disruption in the starburst galaxy Messier 82

Using high-resolution, multiple-passband Hubble Space Telescope images spanning the entire optical/near-infrared wavelength range, we obtained a statistically complete sample, $U$-band selected sample of 846 extended star clusters across the disk of the nearby starburst galaxy M82. Based on careful analysis of their spectral energy distributions, we determined their galaxy-wide age and mass distributions. The M82 clusters exhibit three clear peaks in their age distribution, thus defining a relatively young, log(t/yr) < 7.5, an intermediate-age, log(t/yr) $\in$ [7.5, 8.5], and an old sample, log(t/yr) > 8.5. Comparison of the completeness-corrected mass distributions offers a firm handle on the galaxy's star cluster disruption history. The most massive star clusters in the young and old samples are (almost) all concentrated in the most densely populated central region, while the intermediate-age sample's most massive clusters are more spatially dispersed, which may reflect the distribution of the highest-density gas throughout the galaxy's evolutionary history, combined with the solid-body nature of the galaxy's central region.Comment: 10 pages, 6 figures, 2 online-only data tables; ApJS, in pres

Small Molecule Hsp90 Modulator and Neuregulin-induced Peripheral Demyelination

Abstract Modulating molecular chaperones is emerging as an attractive approach to treat neurodegenerative diseases associated with protein aggregation, diabetic peripheral neuropathy (DPN) and possibly, demyelinating neuropathies. KU-32 is a small molecule inhibitor of heat shock protein 90 (Hsp90) and reverses sensory deficits associated with myelinated fiber dysfunction in DPN. Additionally, KU-32 prevented the loss of myelinated internodes induced by treating myelinated Schwann cell-dorsal root ganglia sensory neuron co-cultures with neuregulin-1 Type 1 (NRG1). Since KU-32 decreased NRG1-induced demyelination in an Hsp70-dependent manner, the goal of the current study was to clarify how Hsp70 may be mechanistically linked to preventing demyelination. The activation of p42/p44 MAPK and induction of the transcription factor c-jun function as negative regulators of myelination. NRG1 activated MAPK, induced c-jun expression and promoted a loss of myelin segments in DRG explants isolated from both wild type and Hsp70 KO mice. Although KU-32 did not block the activation of MAPK, it blocked c-jun induction and protected against a loss of myelinated segments in wildtype (WT) mice. KU-32 did not prevent the NRG1-dependent induction of c-jun and loss of myelin segments in explants from Hsp70 KO mice. Over-expression of Hsp70 in myelinated DRG explants prepared from WT or Hsp70 KO mice was sufficient to block the induction of c-jun and the loss of myelin segments induced by NRG1. Lastly, inhibiting the proteasome prevented KU-32 from decreasing c-jun levels. Collectively, these data support that Hsp70 induction is sufficient to prevent NRG1-induced demyelination by enhancing the proteasomal degradation of c-jun

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

Discovery of Extended Main Sequence Turn-offs in Four Young Massive Clusters in the Magellanic Clouds

An increasing number of young massive clusters (YMCs) in the Magellanic Clouds have been found to exhibit bimodal or extended main sequences (MSs) in their color--magnitude diagrams (CMDs). These features are usually interpreted in terms of a coeval stellar population with different stellar rotational rates, where the blue and red MS stars are populated by non- (or slowly) and rapidly rotating stellar populations, respectively. However, some studies have shown that an age spread of several million years is required to reproduce the observed wide turn-off regions in some YMCs. Here we present the ultraviolet--visual CMDs of four Large and Small Magellanic Cloud YMCs, NGC 330, NGC 1805, NGC 1818, and NGC 2164, based on high-precision Hubble Space Telescope photometry. We show that they all exhibit extended main-sequence turn-offs (MSTOs). The importance of age spreads and stellar rotation in reproducing the observations is investigated. The observed extended MSTOs cannot be explained by stellar rotation alone. Adopting an age spread of 35--50 Myr can alleviate this difficulty. We conclude that stars in these clusters are characterized by ranges in both their ages and rotation properties, but the origin of the age spread in these clusters remains unknown.Comment: 14 pages, 9 figures, ApJ accepte