Are There Age Spreads in Star Forming Regions?

A luminosity spread at a given effective temperature is ubiquitously seen in the Hertzsprung-Russell (HR) diagrams of young star forming regions and often interpreted in terms of a prolonged period (>=10 Myr) of star formation. I review the evidence that the observed luminosity spreads are genuine and not caused by astrophysical sources of scatter. I then address whether the luminosity spreads necessarily imply large age spreads, by comparing HR diagram ages with ages from independent clocks such as stellar rotation rate, the presence of circumstellar material and lithium depletion. I argue that whilst there probably is a true luminosity dispersion, there is little evidence to support age spreads larger than a few Myr. This paradox could be resolved by brief periods of rapid accretion during the class I pre main-sequence phase.Comment: To appear in the proceedings of JENAM10: Star Clusters in the Era of Large Surveys, 8 page

The stellar and sub-stellar IMF of simple and composite populations

The current knowledge on the stellar IMF is documented. It appears to become top-heavy when the star-formation rate density surpasses about 0.1Msun/(yr pc^3) on a pc scale and it may become increasingly bottom-heavy with increasing metallicity and in increasingly massive early-type galaxies. It declines quite steeply below about 0.07Msun with brown dwarfs (BDs) and very low mass stars having their own IMF. The most massive star of mass mmax formed in an embedded cluster with stellar mass Mecl correlates strongly with Mecl being a result of gravitation-driven but resource-limited growth and fragmentation induced starvation. There is no convincing evidence whatsoever that massive stars do form in isolation. Various methods of discretising a stellar population are introduced: optimal sampling leads to a mass distribution that perfectly represents the exact form of the desired IMF and the mmax-to-Mecl relation, while random sampling results in statistical variations of the shape of the IMF. The observed mmax-to-Mecl correlation and the small spread of IMF power-law indices together suggest that optimally sampling the IMF may be the more realistic description of star formation than random sampling from a universal IMF with a constant upper mass limit. Composite populations on galaxy scales, which are formed from many pc scale star formation events, need to be described by the integrated galactic IMF. This IGIMF varies systematically from top-light to top-heavy in dependence of galaxy type and star formation rate, with dramatic implications for theories of galaxy formation and evolution.Comment: 167 pages, 37 figures, 3 tables, published in Stellar Systems and Galactic Structure, Vol.5, Springer. This revised version is consistent with the published version and includes additional references and minor additions to the text as well as a recomputed Table 1. ISBN 978-90-481-8817-

Multiple populations in globular clusters. Lessons learned from the Milky Way globular clusters

Recent progress in studies of globular clusters has shown that they are not simple stellar populations, being rather made of multiple generations. Evidence stems both from photometry and spectroscopy. A new paradigm is then arising for the formation of massive star clusters, which includes several episodes of star formation. While this provides an explanation for several features of globular clusters, including the second parameter problem, it also opens new perspectives about the relation between globular clusters and the halo of our Galaxy, and by extension of all populations with a high specific frequency of globular clusters, such as, e.g., giant elliptical galaxies. We review progress in this area, focusing on the most recent studies. Several points remain to be properly understood, in particular those concerning the nature of the polluters producing the abundance pattern in the clusters and the typical timescale, the range of cluster masses where this phenomenon is active, and the relation between globular clusters and other satellites of our Galaxy.Comment: In press (The Astronomy and Astrophysics Review

Differential modulation of diet-induced obesity and adipocyte functionality by human apolipoprotein E3 and E4 in mice

OBJECTIVE: Apolipoprotein E (apoE), a key protein in lipid metabolism, is highly expressed in adipose tissues. Studies have shown that human APOE*4 is associated with a lower body mass index but with a greater risk of coronary heart disease compared with other APOE alleles. To define the isoform-specific role of apoE in regulating the expandability and functionality of adipose tissues, we investigated the effects of diet-induced obesity in mice whose endogenous Apoe gene has been replaced by either the human APOE*3 or APOE*4 allele. RESULTS: After 8 weeks on a Western-type high-fat diet, male APOE4 mice displayed impaired tolerance to glucose and fat overload compared with APOE3 mice. Subcutaneous fat tissues in APOE4 and APOE3 mice after high fat feeding were not different. In contrast, although epididymal fat tissues in APOE4 mice gained 30% less weight during the high fat feeding than in APOE3 mice, they showed impaired insulin-stimulated glucose uptake ex vivo. Epididymal APOE4 adipocytes were larger in size than APOE3 adipocytes, and expressed reduced levels of mRNA for peroxisome proliferator-activated receptor γ2 and adiponectin, important markers of adipocyte functionality. Adenoviral expression of apoE3 in apoE-null culture adipocytes induced adiponectin mRNA in a dose-dependent manner, but the induction was significantly blunted in cells overexpressing apoE4. However, in contrast to the apoE3-expressing cells, Glut1, but not Glut4, expression levels were positively correlated with increased apoE4 mRNA, suggesting that apoE4 expression in adipocyte interferes in insulin-sensing pathways. CONCLUSION: Dysfunctional epididymal adipose tissues contribute to the accelerated impairment of glucose tolerance in APOE4 mice fed a Western-type diet. Our results underscore the importance of functionality of individual fat depots rather than total fat mass as a determinant for metabolic disturbance during diet-induced obesity