arXiv subjects

Pierre Demarque

Publications and source records attributed to Pierre Demarque.

43 records · Page 3Linked to original sources

arXiv1997

On the Origin of the UV Upturn in Elliptical Galaxies. I. Sensitivity of UV Population Synthesis to Various Input Parameters

We present models of the late stages of stellar evolution intended to explain the UV upturn phenomenon in elliptical galaxies. Such models are sensitive to values of a number of poorly-constrained physical parameters, including metallicity, age, stellar mass loss, helium enrichment, and the distribution of stars on the zero age horizontal branch (HB). We explore the sensitivity of the results to values of these parameters, and reach the following conclusions. Old, metal rich galaxies, such as giant ellipticals, naturally develop a UV upturn within a reasonable time scale - less than a Hubble time - without the presence of young stars. The most likely stars to dominate the UV flux of such populations are low mass, core helium burning (HB and evolved HB) stars. Metal-poor populations produce a higher ratio of UV-to-V flux, due to opacity effects, but only metal-rich stars develop a UV upturn, in which the flux increases towards shorter UV wavelengths. Model color-magnitude diagrams and corresponding integrated spectra (for various values of age, metallicity, helium enrichment, mass loss efficiency, initial mass function, and the HB mass dispersion factor) are available on S.Y.'s world wide web site http://shemesh.gsfc.nasa.gov/model.html.

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arXiv1997

On the UV Bright Phase of Metal-Rich Horizontal-Branch Stars

We consider the origin of the UV bright phase of metal-rich helium-burning stars, the slow blue phase (SBP), that was predicted by various earlier works. Based on improved physics including OPAL opacities, we confirm the existence of the SBP. In addition to our grid of evolutionary tracks, we provide an analytical understanding of the main characteristics of the SBP phenomenon. The SBP is slow because it is a slow evolving helium-shell-burning phase which is analogous to the early AGB phase. The SBP of a more metal-rich star is slower than a metal-poor counterpart if their Teff's are the same because a more metal-rich helium-burning star has a smaller mass than a metal-poor one and because lifetime increases as mass decreases. Metal-rich helium-burning stars easily become hot because the luminosity from the hydrogen-burning shell is extremely sensitive to the mean molecular weight whereas the luminosity from the helium-burning core is not. We suggest that the SBP phenomenon is a major cause of the UV upturn phenomenon in giant elliptical galaxies as will be shown in subsequent papers. The new HB tracks can be retrieved from S.Y.'s web site http://shemesh.gsfc.nasa.gov/astronomy.html.

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arXiv1997

Evolution of Stellar Collision Products in Globular Clusters -- I. Head-on Collisions

We explore the evolution of collisionally merged stars in the blue straggler region of the HR diagram. The starting models for our stellar evolution calculations are the results of the smoothed particle hydrodynamics (SPH) simulations of parabolic collisions between main sequence stars performed by Lombardi, Rasio and Shapiro (1996). Since SPH and stellar evolution codes employ different and often contradictory approximations, it is necessary to treat the evolution of these products carefully. The mixture and disparity of the relevant timescales (hydrodynamic, thermal relaxation and nuclear burning) and of the important physical assumptions between the codes makes the combined analysis of the problem challenging, especially during the initial thermal relaxation of the star. In particular, the treatment of convection is important, and semiconvection must be modeled in some detail. The products of seven head-on collisions are evolved through their initial thermal relaxation, and then through the main sequence phase to the base of the giant branch. Their evolutionary tracks are presented. In contrast to the assumptions in previous work, these collision products do not develop substantial convective regions during their thermal relaxation, and therefore are not mixed significantly after the collision.

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arXiv1996

A Precision Age Determination Technique for Globular Clusters

Globular cluster age estimates based on the absolute magnitude of the main sequence turn-off (\mvto) are generally considered to be the most reliable from a theoretical viewpoint. However, the difficulty in determining \mvto\ in observed colour-magnitude diagrams leads to a large error in the derived age. In this paper, we advocate the use of the absolute magnitude of the point which is brighter than the turn-off and 0.05 mag redder (\mvb) as a precision age indicator. It is easy to measure this point on observed colour-magnitude diagrams, leading to small observational error bars. Furthermore, an extensive Monte Carlo calculation indicates that the theoretical uncertainty in \mvb\ is similar to \mvto. As a result, ages derived using \mvb\ are at least a factor of 2 more precise than those derived using \mvto. This technique is applied to the globular cluster M68 and an age of $12.8\pm 0.3\,$Gyr is derived (assuming $\mvrr = 0.20\,\feh + 0.98$), indicating that M68 is a `young' globular cluster. A homogeneous set of globular cluster age estimates with this precision would provide unprecedented insight into the formation of the Galactic halo.

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arXiv1995

A Lower Limit on the Age of the Universe

We report the results of a detailed numerical study designed to estimate both the absolute age and the uncertainty in age (with confidence limits) of the oldest globular clusters. Such an estimate is essential if a comparison with the Hubble age of the universe is to be made to determine the consistency, or lack thereof, of various cosmological models. Utilizing estimates of the uncertainty range (and distribution) in the input parameters of stellar evolution codes we produced 1000 Monte Carlo realizations of stellar isochrones, with which we could fit the ages of the 18 oldest globular clusters. Incorporating the observational uncertainties in the measured color-magnitude diagrams for these systems and the predicted isochrones, we derived a probability distribution for the mean age of these systems. The one-sided 95$\%$ C.L.~ lower bound for this distribution occurs at an age of 12.07 Gyr. This puts interesting constraints on cosmology which we discuss. Further details, including a description of the distributions, covariance matrices, dependence upon individual input parameters, etc.\ will appear in a future article.

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arXiv1995

The theoretical calculation of the Rossby number and the `non-local' convective overturn time for pre-main sequence and early post-main sequence stars

This paper provides estimates of convective turnover time scales for Sun-like stars in the pre-main sequence and early post-main sequence phases of evolution, based on up-to-date physical input for the stellar models. In this first study, all models have solar abundances, which is typical of the stars in the Galactic disk where most of the available data have been collected. A new feature of these models is the inclusion of rotation in the evolutionary sequences, thus making it possible to derive theoretically the Rossby number for each star along its evolutionary track, based on its calculated rotation rate and its local convective turnover time near the base of the convection zone. Global turnover times are also calculated for the complete convection zone. This information should make possible a new class of observational tests of stellar theory which were previously impossible with semi-empirical models, particularly in the study of stellar activity and in research related to angular momentum transfer in stellar interiors during the course of stellar evolution.

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arXiv1994

Modelling of Shallow and Inefficient Convection in the Outer Layers of the Sun Using Realistic Physics

In an attempt to understand the properties of convective energy transport in the solar convection zone, a numerical model has been constructed for turbulent flows in a compressible, radiation-coupled, non-magnetic, gravitationally stratified medium using a realistic equation of state and realistic opacities. The time-dependent, three-dimensional hydrodynamic equations are solved with minimal simplifications. The statistical information obtained from the present simulation provides an improved understanding of solar photospheric convection. The characteristics of solar convection in shallow regions is parameterized and compared with the results of Chan and Sofia's simulations of deep and efficient convection (Chan and Sofia 1989). We assess the importance of the zones of partial ionization in the simulation, and confirm that the radiative energy transfer is negligible throughout the region except in the uppermost scale heights of the convection zone, a region of very high super-adiabaticity. When the effects of partial ionization are included, the dynamics of flows are altered significantly. However, we confirm the Chan and Sofia result that kinetic energy flux is non-negligible and can have a negative value in the convection zone.

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