1,720,982 research outputs found
Microscopic diffusion and the calibration of globular cluster ages
No full textWe discuss current uncertainties in the predicted magnitudes of Turn-Off (TO) and Zero Age Horizontal Branch (ZAHB) stars, as produced by uncertainties in the efficiency of element sedimentation in low mass, metal poor stars. One finds that for ages of the order of 10 Gyr the uncertainties in sedimentation affect the calibration of the TO luminosity in terms of cluster ages less than 1 Gyr, this uncertainty becoming however larger for larger cluster ages. The difference in magnitude between TO and ZAHB stars, Delta V(TO-HB), appears less affected by sedimentation, since the variation in the TO luminosity is partially balanced by a similar variation of ZAHB luminosities. However additional uncertainties in the theoretical ZAHB luminosity are discussed, showing that a conservative estimate of uncertainties in the ZAHB V magnitude gives an error of +0.106/ - 0.047 and an uncertainty in the calibration of Delta V(TO-HB) in terms of the cluster age of the same order of the uncertainty in the calibration of TO magnitudes
Theoretical stellar models for old galactic clusters
No full textWe present new evolutionary stellar models suitable for old Population I clusters, discussing both the consequences of the most recent improvements in the input physics and the effect of element diffusion within the stellar structures. Theoretical cluster isochrones are presented, covering the range of ages from 1 to 9 Gyr for the four selected choices of the metallicity Z = 0.007, 0.010, 0.015 and 0.020. Theoretical uncertainties on the efficiency of superadiabatic convection are discussed in some detail. Isochrone fitting to the CM diagrams of the two well-observed galactic clusters NGC 2420 and M67 indicates that a mixing-length parameter alpha = 1.9 appears adequate for reproducing the observed colour of cool giant stars. The problems in matching theoretical predictions to the observed slope of MS stars are discussed
Theoretical Zero Age Main Sequences revisited
No full textZero Age Main Sequence (ZAMS) models with updated physical inputs are presented for selected assumptions about the chemical composition, covering the ranges 0.6 < M < 1.2 M., 0.0001 < Z < 0.04, 0.23 < Y < 0.34. The HR diagram location of the ZAMS as a function of Y and Z is discussed both in the theoretical (logL, logT(eff)) and in the observational (M(upsilon), B-V) diagrams, showing that the V magnitude presents an increased dependence on Z to be taken into account when discussing observational evidences. Analytical relations quantifying both these dependences are derived. Implications for the galactic helium to heavier elements enrichment Delta Y/Delta Z are finally discussed
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