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Register a new userNGC 1866: a milestone for understanding the chemical evolution of stellar populations in the LMC
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Alessio Mucciarelli
Publication date
2010
fields
Physics
and research's language is
English
Authors
A. Mucciarelli
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We present new FLAMES@VLT spectroscopic observations of 30 stars in the field of the LMC stellar cluster NGC 1866. NGC 1866 is one of the few young and massive globular cluster that is close enough so that its stars can be individually studied in detail. Radial velocities have been used to separate stars belonging to the cluster and to the LMC field and the same spectra have been used to derive chemical abundances for a variety of elements, from [Fe/H] to the light (i.e. Na, O, Mg...) to the heavy ones. The average iron abundance of NGC 1866 turns out to be [Fe/H]= -0.43+-0.01 dex (with a dispersion of 0.04 dex), from the analysis of 14 cluster-member stars. Within our uncertainties, the cluster stars are homogeneous, as far as chemical composition is concerned, independent of the evolutionary status. The observed cluster stars do not show any sign of the light elements anti-correlation present in all the Galactic globular clusters so far studied, and also found in the old LMC stellar clusters. A similar lack of anti-correlations has been detected in the massive intermediate-age LMC clusters, indicating a different formation/evolution scenario for the LMC massive clusters younger than ~3 Gyr with respect to the old ones. Also opposite to the Galactic globulars, the chemical composition of the older RGB field stars and of the young post-MS cluster stars show robust homogeneity suggesting a quite similar process of chemical evolution. The field and cluster abundances are in agreement with recent chemical analysis of LMC stars, which show a distinctive chemical pattern for this galaxy with respect to the Milky Way. We discuss these findings in light of the theoretical scenario of chemical evolution of the LMC.
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More complete knowledge of galaxy evolution requires understanding the process of star formation and interaction between the interstellar radiation field and the interstellar medium in galactic environments traversing a wide range of physical parameter space. Here we focus on the impact of massive star formation on the surrounding low metallicity ISM in 30 Doradus in the Large Magellanic Cloud. A low metal abundance, as is the case of some galaxies of the early universe, results in less ultra-violet shielding for the formation of the molecular gas necessary for star formation to proceed. The half-solar metallicity gas in this region is strongly irradiated by the super star cluster R136, making it an ideal laboratory to study the structure of the ISM in an extreme environment. Our spatially resolved study investigates the gas heating and cooling mechanisms, particularly in the photo-dissociation regions where the chemistry and thermal balance are regulated by far-ultraviolet photons (6 eV< h u <13.6 eV). We present Herschel observations of far-infrared fine-structure lines obtained with PACS and SPIRE/FTS. We have combined atomic fine-structure lines from Herschel and Spitzer observations with ground-based CO data to provide diagnostics on the properties and the structure of the gas by modeling it with the Meudon PDR code. We derive the spatial distribution of the radiation field, the pressure, the size, and the filling factor of the photodissociated gas and molecular clouds. We find a range of pressure of ~ 10^5 - 1.7x10^6 cm^{-3} K and a range of incident radiation field G_UV ~ 10^2 - 2.5x10^4 through PDR modeling. Assuming a plane-parallel geometry and a uniform medium, we find a total extinction of 1-3 mag , which correspond to a PDR cloud size of 0.2 to 3pc, with small CO depth scale of 0.06 to 0.5pc. We also determine the three dimensional structure of the gas. (Abridged)
Context: Cepheids are excellent tracers of young stellar populations. They play a crucial role in astrophysics as standard candles. The chemistry of classical Cepheids in the Milky Way is now quite well-known. Despite a much larger sample, the chemical composition of Magellanic Cepheids has been only scarcely investigated. Aims: For the first time, we study the chemical composition of several Cepheids located in the same populous cluster: NGC 1866, in the Large Magellanic Cloud (LMC). To also investigate the chemical composition of Cepheids at lower metallicity, four targets are located in the Small Magellanic Cloud (SMC). Our sample allows us to increase the number of Cepheids with known metallicities in the LMC/SMC by 20%/25% and the number of Cepheids with detailed chemical composition in the LMC/SMC by 46%/50%. Methods: We use canonical spectroscopic analysis to determine the chemical composition of Cepheids and provide abundances for a good number of $alpha$, iron-peak and neutron-capture elements. Results: We find that six Cepheids in the LMC cluster NGC 1866 have a very homogeneous chemical composition, also consistent with red giant branch (RGB) stars in the cluster. Period--age relations that include no or average rotation indicate that all the Cepheids in NGC 1866 have a similar age and therefore belong to the same stellar population. Our results are in good agreement with theoretical models accounting for luminosity and radial velocity variations. Using distances based on period-luminosity relations in the near- or mid-infrared, we investigate for the first time the metallicity distribution of the young population in the SMC in the depth direction. Preliminary results show no metallicity gradient along the SMC main body, but our sample is small and does not contain Cepheids in the inner few degrees of the SMC.
In this paper we present a study and comparison of the star formation rates (SFR) in the fields around NGC 1898 and NGC 2154, two intermediate-age star clusters located in very different regions of the Large Magellanic Cloud. We also present a photometric study of NGC 1898, and of seven minor clusters which happen to fall in the field of NGC 1898, for which basic parameters were so far unknown. We do not focus on NGC 2154, because this cluster was already investigated in Baume et al. 2007, using the same theoretical tools. The ages of the clusters were derived by means of the isochrone fitting method on their $clean$ color-magnitude diagrams. Two distinct populations of clusters were found: one cluster (NGC 2154) has a mean age of 1.7 Gyr, with indication of extended star formation over roughly a 1 Gyr period, while all the others have ages between 100 and 200 Myr. The SFRs of the adjacent fields were inferred using the downhill-simplex algorithm. Both SFRs show enhancements at 200, 400, 800 Myr, and at 1, 6, and 8 Gyr. These bursts in the SFR are probably the result of dynamical interactions between the Magellanic Clouds (MCs), and of the MCs with the Milky Way.
Galactic open and globular clusters (OCs, GCs) appear to inhabit separate regions of the age-mass plane. However, the transition between them is not easily defined because there is some overlap between high-mass, old OCs and low-mass, young GCs. We are exploring the possibility of a clear-cut separation between OCs and GCs using an abundance feature that has been found so far only in GCs: (anti)correlations between light elements. Among the coupled abundance trends, the Na-O anticorrelation is the most widely studied. These anticorrelations are the signature of self-enrichment, i.e., of a formation mechanism that implies multiple generations of stars. Here we concentrate on the old, massive, metal-rich OC NGC 6791. We analyzed archival Keck/HIRES spectra of 15 NGC 6791 main sequence turn-off and evolved stars, concentrating on the derivation of C, N, O, and Na abundances. We also used WIYN/Hydra spectra of 21 evolved stars (one is in common). Given the spectral complexity of the very metal-rich NGC 6791 stars, we employed spectrum synthesis to measure most of the abundances. We confirmed the cluster super-solar metallicity and abundances of Ca and Ni that have been derived in past studies. More importantly, we did not detect any significant star-to-star abundance dispersion in C, N, O and Na. Based on the absence of a clear Na-O anticorrelation, NGC 6791 can still be considered a true OC, hosting a single generation of stars, and not a low-mass GC.
Near infrared (IR) studies of Cepheid variables in the LMC take advantage of the reduced light curve amplitude and metallicity dependence at these wavelengths. This work presents such photometry for two young clusters known to contain sizeable Cepheid populations: NGC 1866 and NGC 2031. Our goal is to determine light curves and period-luminosity (PL) relations in the near-IR, to assess the similarity between cluster and field pulsators, and to examine the predictive capability of current pulsation models. The light curves are obtained from multiwavelength broadband J,H,Ks photometry of Cepheids in both clusters, with periods previously established from optical photometry. Mean magnitudes for the Cepheids are used to construct PL relations in the near-IR. The properties in the PL planes are compared with the behavior of field Cepheids in the LMC and with the predictions of recent pulsational models, both canonical and overluminous. Cluster and field Cepheids are homogeneous and the inclusion of the cluster Cepheids in the field sample extends nicely the PL relation. The slope of the PL relation is constant over the whole period range and does not show -- at least in the adopted IR bands -- the break in slope at P ~ 10 d reported by some authors. A comparison with the predictions of pulsation models allows an estimate for the distance moduli of NGC 1866 and NGC 2031. The two clusters are found to lie at essentially the same distance. Fitting of theoretical models to the data gives, for the K filter, (m-M)_0 = 18.62+-0.10 if canonical models are used and (m-M)_0 = 18.42+-0.10 if overluminous models are used. On the basis of this result, some considerations on the relationship between the clusters and the internal structure of the LMC are presented.
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