Discovery of two distinct red clumps in NGC419: a rare snapshot of a cluster at the onset of degeneracy

Colour-magnitude diagrams (CMD) of the SMC star cluster NGC419, derived from HST/ACS data, reveal a well-delineated secondary clump located below the classical compact red clump typical of intermediate-age populations. We demonstrate that this feature belongs to the cluster itself, rather than to the underlying SMC field. Then, we use synthetic CMDs to show that it corresponds very well to the secondary clump predicted to appear as a result of He-ignition in stars just massive enough to avoid electron-degeneracy settling in their H-exhausted cores. The main red clump instead is made of the slightly less massive stars which passed through electron-degeneracy and ignited He at the tip of the RGB. In other words, NGC419 is the rare snapshot of a cluster while undergoing the fast transition from classical to degenerate H-exhausted cores. At this particular moment of a cluster's life, the colour distance between the main sequence turn-off and the red clump(s) depends sensitively on the amount of convective core overshooting, Lambda_c. By coupling measurements of this colour separation with fits to the red clump morphology, we are able to estimate simultaneously the cluster mean age (1.35(-0.04,+0.11) Gyr) and overshooting efficiency (Lambda_c=0.47(-0.04,+0.14)). Therefore, clusters like NGC419 may constitute important marks in the age scale of intermediate-age populations. After eye inspection of other CMDs derived from HST/ACS data, we suggest that the same secondary clump may also be present in the LMC clusters NGC1751, 1783, 1806, 1846, 1852, and 1917.Comment: To appear in MNRAS Letters (www.blackwell-synergy.com). Better printed in colou

A secondary clump of red giant stars: why and where

Based on the results of detailed population synthesis models, it was recently claimed that the clump of red giants in the colour-magnitude diagram (CMD) of composite stellar populations should present an extension to lower luminosities, which goes down to about 0.4 mag below the main clump. This feature is made of stars just massive enough for having ignited helium in non-degenerate conditions. In this paper, we go into more details about the origin and properties of this feature. We first compare the clump theoretical models with data for clusters of different ages and metallicities, basically confirming the predicted behaviours. We then refine the previous models in order to show that: (i) The faint extension is expected to be clearly separated from the main clump in the CMD of metal-rich populations, defining a secondary clump by itself. (ii) It should be present in all galactic fields containing ~1 Gyr old stars and with mean metallicities higher than about Z=0.004. (iii) It should be particularly strong, if compared to the main red clump, in galaxies which have increased their star formation rate in the last Gyr or so of their evolution. In fact, secondary clumps similar to the model predictions are observed in the CMD of nearby stars from Hipparcos data, and in those of some LMC fields observed to date. There are also several reasons why this secondary clump may be missing or hidden in other observed CMDs of galaxy fields (e.g. photometric errors or differential absorption larger than 0.2 mag). Nonetheless, this structure may provide important constraints to the star formation history of Local Group galaxies. We comment also on the intrinsic luminosity variation and dispersion of clump stars, which may limit their use as either absolute or relative distance indicators.Comment: 20 pages with 11 figures, submitted to MNRA

Transformative economics education : using proverbs from around the world in the classroom

This paper discusses an approach to economics education based on transformative learning theory and Roger’s humanistic approach. The aim is to make the study of Economics more related to the students’ lives, enhancing their motivation to engage with Economics material. The author proposes the use of proverbs from around the world as a way of challenging students' meaning perspectives, and describe my experience of using this approach in the classroom

Broad-band photometric evolution of star clusters

I briefly introduce a database of models that describe the evolution of star clusters in several broad-band photometric systems. Models are based on the latest Padova stellar evolutionary tracks - now including the alpha-enhanced case and improved AGB models - and a revised library of synthetic spectra from model atmospheres. As of today, we have revised isochrones in Johnson-Cousins-Glass, HST/WFPC2, HST/NICMOS, Thuan-Gunn, and Washington systems. Several other filter sets are included in a preliminary way, like those used by the EIS and SDSS projects. The database contains also integrated magnitudes of single-burst stellar populations and Monte-Carlo simulations that show the stochastic dispersion of the colours as a function of cluster mass, age, and metallicity. The models are useful for several kinds of studies, including estimates of masses and ages of extragalactic star clusters observed by means of broad-band photometry.Comment: 5 pages, to appear in Extragalactic Star Clusters, IAU Symp 207, eds. E.K. Grebel, D. Geisler, D. Minniti. The isochrone data is in http://pleiadi.pd.astro.it/~lgirardi/isoc_photsys.htm

Fine structure of the red clump in Local Group galaxies

Some fine structures can nowadays be identified in the high-quality colour-magnitude diagrams (CMD) of Local Group galaxies. The clump of red giants, for instance, may present a significant colour spread, and extensions to both brighter and fainter luminosities. Such features are predicted by population synthesis models which consider stars in the complete relevant ranges of ages and metallicities, and are potentially useful for constraining the star formation histories of the parent galaxies over scales of gigayears. We briefly comment the cases of fields in the Magellanic Clouds, M31, and the local CMD from Hipparcos.Comment: To appear in Proc. of the VLT Opening Symposium (Parallel Workshop 2: Star-Way to the Universe

Can rotation explain the multiple main sequence turn-offs of Magellanic Cloud star clusters?

Many intermediate age star clusters in the Magellanic Clouds present multiple main sequence turn-offs (MMSTO), which challenge the classical idea that star formation in such objects took place over short timescales. It has been recently suggested that the presence of fast rotators among main sequence stars could be the cause of such features (Bastian & de Mink 2009), hence relaxing the need for extended periods of star formation. In this letter, we compute evolutionary tracks and isochrones of models with and without rotation. We find that, for the same age and input physics, both kinds of models present turn-offs with an almost identical position in the colour-magnitude diagrams. As a consequence, a dispersion of rotational velocities in coeval ensembles of stars could not explain the presence of MMSTOs. We construct several synthetic colour-magnitude diagrams for the different kinds of tracks and combinations of them. The models that best reproduce the morphology of observed MMSTOs are clearly those assuming a significant spread in the stellar ages - as long as ~400 Myr - added to a moderate amount of convective core overshooting. Only these models produce the detailed "golf club" shape of observed MMSTOs. A spread in rotational velocities alone cannot do anything similar. We also discuss models involving a mixture of stars with and without overshooting, as an additional scenario to producing MMSTOs with coeval populations. We find that they produce turn-offs with a varying extension in the CMD direction perpendicular to the lower main sequence, which are clearly not present in observed MMSTOs.Comment: To appear in MNRAS Letters. Figs. 2 and 3 are in colou

Fiscal spillovers in the Euro area

Copyright @ 2011 Brunel UniversityThis paper analyses the dynamic effects of fiscal imbalances in a given EMU member state on the borrowing costs of other countries in the euro area. The estimation of a multivariate, multi-country time series model (specifically a Global VAR, or GVAR) using quarterly data for the EMU period suggests that euro-denominated government yields are strongly linked with each other. However, financial markets seem to be able to discriminate among different issuers. Consequently, fiscal imbalances in Italy and in other peripheral countries should be closely monitored by their EMU partners and the European institutions