1,721,397 research outputs found
The intermediate-age open cluster NGC 2158
Full text linkWe report on UBVRI CCD photometry of two overlapping fields in the region of the intermediate-age open cluster NGC 2158 down to V =21. By analysing colour-colour (CC) and colour-magnitude diagrams (CMDs) we infer a reddening E (B -V )=0.55+/-0.10, a distance of 3600+/-400pc, and an age of about 2Gyr. Synthetic CMDs constructed with these parameters (but fixing E (B -V )=0.60 and [Fe/H]=-0.60), and including binaries, field contamination and photometric errors, yield a good description of the observed CMD. The elongated shape of the clump of red giants in the CMD is interpreted as resulting from a differential reddening of about ΔE (B -V )=0.06 across the cluster, in the direction perpendicular to the Galactic plane. NGC 2158 turns out to be an intermediate-age open cluster with an anomalously low metal content. The combination of these parameters, together with the analysis of the cluster orbit, suggests that the cluster belongs to the old thin disc population
Population Synthesis Models with AGB Stars
No full textWe present new synthetic stellar models for the thermally-pulsing asymptotic giant branch (TP-AGB) phase of low- and intermediate-mass stars. These models are particularly suitable for population synthesis studies, as they i) cover the whole mass range of interest (0.8-5.0 M_{sun}), ii) span a wide range in metallicity (0.0001 ≤ Z ≤ 0.03), iii) include several important improvements in the adopted input physics (e.g.variable molecular opacities), and iv) are calibrated on basic observables so as to provide a realistic description of the contribution from AGB stars to both resolved and integrated properties of galaxies
Towards Simulating the Photometry, Chemistry, Mass Loss and Pulsational Properties of AGB Star Populations in Resolved Galaxies
No full textExtended and updated grids of TP-AGB tracks have been implemented in the trilegal population-synthesis code, which generates mock stellar catalogues for a galaxy given its mass, distance, star formation history and age-metallicity relation, including also the Milky Way foreground population. Among the stellar parameters that are simulated, we now include the surface chemistry, mass-loss rates, pulsation modes and periods of LPVs. This allows us to perform a series of consistency checks between AGB model predictions and observations, which we are just starting to explore. We present a few examples of model--data comparisons, mostly regarding the near-infrared and variability data for AGB stars in the Magellanic Clouds
The carbon star luminosity functions in the Magellanic Clouds
No full textWe address the question of reproducing the observed luminosity functions of carbons stars in both the LMC and SMC, characterized by quite different features (i.e. faint end and peak location). To this aim, we use recent synthetic TP-AGB evolutionary calculations for a dense grid of stellar masses in the range 0.8 Mo ~< M ~< 5 Mo and metallicities Z = 0.008 and Z = 0.004. These models rely on updated inputs, treating envelope burning and dregde-up in detail by means of envelope integrations (Marigo et al. 1996, 1998ab). The observed carbon star luminosity functions turn out to be strong calibrators of the dredge-up parameters, namely: the minimum temperature at the base of the convective envelope, Tbdred, and the efficiency, lambda. Both distributions are reproduced remarkably well. We also investigate the sensitiveness of the predictions to the star formation history (SFR). The peak location results essentially invariant with respect to the SFR, whereas the shape of the bright wing may give hints on the recent epochs
Constraining the third dredge-up via carbon stars in the Magellanic Clouds
No full textWe use the available data for Magellanic Cloud carbon stars to constrain the efficiency of the third dredge-up process in TP-AGB models. We show that star counts in LMC clusters provide quite stringent limits to the lifetime of the C-star phase, with a duration between 2 and 3 Myr for stars in the mass range from 1.5 to 2.8 Msun. Together with the luminosity functions of field C stars, this information allows us to re-calibrate the third dredge-up parameters log T_b_dred and lambda in TP-AGB models that include variable molecular opacities (Marigo 2002). Preliminary results are presented here
The third dredge-up and the carbon star luminosity functions in the Magellanic Clouds
Full text linkWe investigate the formation of carbon stars as a function of the stellar mass and parent metallicity. Theoretical modelling is based on an improved scheme for treating the third dredge-up in synthetic calculations of thermally pulsing asymptotic giant branch (TP-AGB) stars. In this approach, the usual criterion (based on a constant minimum core mass for the occurrence of dredge-up, M_c() min) is replaced by one on the minimum temperature at the base of the convective envelope, T_b() dred, at the stage of the post-flash luminosity maximum. Envelope integrations then allow determination of M_c() min as a function of stellar mass, metallicity, and pulse strength (see Wood 1981), thus inferring if and when dredge-up first occurs. Moreover, the final possible shut down of the process is predicted. Extensive grids of TP-AGB models were computed by Marigo (1998a,b) using this scheme. In this paper, we present and discuss the calibration of the two dredge-up parameters (i.e. efficiency lambda and T_b() dred) aimed at reproducing the carbon star luminosity function (CSLF) in the LMC, using TP-AGB models with original metallicity Z=0.008. In addition to this, the effects of different input quantities on the theoretical CSLF are analyzed. It turns out that the faint tail is almost insensitive to the history of star formation rate (SFR) in the parent galaxy, in contrast to the bright wing which may be more affected by the details of the recent history. Actually, we find that the faint end of the CSLF is essentially determined by the temperature parameter T_b() dred. Once the faint end is reproduced, the peak location is a stringent calibrator of the efficiency parameter lambda . The best fit to the observed CSLF in the LMC is obtained with lambda =0.50, log T_b() dred = 6.4, and a constant SFR up to an age of about 5 x 10(8) yr. This recent drop of the SFR is invoked to remove a slight excess of bright carbon stars otherwise predicted. A good fit to the observed CSLF in the SMC is then easily derived from the Z=0.004 models, with a single choice of parameters lambda =0.65, log T_b() dred = 6.4, and a constant SFR over the entire significant age interval. The result for lambda is consistent with the theoretical expectation that the third dredge-up is more efficient at lower metallicities
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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