arXiv subjects

Pierre Demarque

Publications and source records attributed to Pierre Demarque.

At least 37 records · Page 2Linked to original sources

arXiv2006

The hot stars in orbit around the M31 central supermassive black hole: are they young or old?

AIMS: The cluster of hot stars observed in orbit around the central black hole of M31 has been interpreted as a 200 Myr starburst. The formation of a population of young stars in close proximity to a massive black hole presents a difficult challenge to star formation theory. We point out that in a high stellar density environment, the course of stellar evolution is modified by frequent collisions and mergers. METHODS: Blue stragglers, which are the results of mergers in globular clusters, occupy the same position in the color-magnitude diagram as the observed hot stars in M31. For confirmation, the integrated spectrum of P3 is shown to be compatible with the spectral energy distribution of a blue horizontal branch field star. RESULTS: We suggest an old stellar population of evolved blue horizontal-branch stars and of merger products cannot be ruled out on the basis of the available data. Observations are suggested that would help distinguish between a ``young'' and ``old'' stellar population interpretation of the observations.

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arXiv2006

Asteroseismic Diagnostics of Stellar Convective Cores

We present a detailed study of the small frequency separations as diagnostics of the mass of the convective core and evolutionary stage of solar-type stars. We demonstrate how the small separations can be combined to provide sensitive tests for the presence of convective overshoot at the edge of the core. These studies are focused on low degree oscillation modes, the only modes expected to be detected in distant stars. Using simulated data with realistic errors, we find that the mass of the convective core can be estimated to within 5% if the total stellar mass is known. Systematic errors arising due to uncertainty in the mass could be up to 20%. The evolutionary stage of the star, determined in terms of the central hydrogen abundance using our proposed technique, however, is much less sensitive to the mass estimate.

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arXiv2006

On Dissipation inside Turbulent Convection Zones from 3D Simulations of Solar Convection

The development of 2D and 3D simulations of solar convection has lead to a picture of convection quite unlike the usually assumed Kolmogorov spectrum turbulent flow. We investigate the impact of this changed structure on the dissipation properties of the convection zone, parametrized by an effective viscosity coefficient. We use an expansion treatment developed by Goodman & Oh 1997, applied to a numerical model of solar convection (Robinson et al. 2003) to calculate an effective viscosity as a function of frequency and compare this to currently existing prescriptions based on the assumption of Kolmogorov turbulence (Zahn 1966, Goldreich & Keeley 1977). The results match quite closely a linear scaling with period, even though this same formalism applied to a Kolmogorov spectrum of eddies gives a scaling with power-law index of 5/3.

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arXiv2005

NGC 6791: an exotic open cluster or the nucleus of a tidally disrupted galaxy?

We report on high resolution Echelle spectroscopy of 20 giant stars in the Galactic old open clusters NGC 6791 obtained with Hydra at the WIYN telescope. High precision radial velocity allow us to isolate 15 {\it bona fide} cluster members. From 10 of them we derive a global [M/H]=+0.39$\pm$0.05. We therefore confirm that NGC 6791 is extremely metal rich, exhibits a few marginally sub-solar abundance ratios, and within the resolution of our spectra does not show evidences of spread in metal abundance. With these new data we re-derive the cluster fundamental parameters suggesting that it is about 8 Gyr old and 4.3 kpc far from the Sun. The combination of its chemical properties, age, position, and Galactic orbit hardly makes NGC 6791 a genuine Population I open cluster. We discuss possible interpretations of the cluster peculiarities suggesting that the cluster might be what remains of a much larger system, whose initial potential well could have been sufficient to produce high metallicity stars, and which has been depopulated by the tidal field of the Galaxy. Alternatively, its current properties may be explained by the perturbation of the Galactic bar on an object originated well inside the solar ring, where the metal enrichment had been very fast.

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arXiv2005

The Non-Homologous Nature of Solar Diameter Variations

We show in this paper that the changes of the solar diameter in response to variations of large scale magnetic fields and turbulence are not homologous. For the best current model, the variation at the photospheric level is over 1000 times larger than the variation at a depth of 5 Mm, which is about the level at which f-mode solar oscillations determine diameter variations. This model is supported by observations that indicate larger diameter changes for high degree f-modes than for low degree f-modes, since energy of the former are concentrated at shallower layers than the latter.

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arXiv2005

Space and Ground Based Pulsation Data of Eta Bootis Explained with Stellar Models Including Turbulence

The space telescope MOST is now providing us with extremely accurate low frequency p-mode oscillation data for the star Eta Boo. We demonstrate in this paper that these data, when combined with ground based measurements of the high frequency p-mode spectrum, can be reproduced with stellar models that include the effects of turbulence in their outer layers. Without turbulence, the l=0 modes of our models deviate from either the ground based or the space data by about 1.5-4.0 micro Hz. This discrepancy can be completely removed by including turbulence in the models and we can exactly match 12 out of 13 MOST frequencies that we identified as l=0 modes in addition to 13 out of 21 ground based frequencies within their observational 2 sigma tolerances. The better agreement between model frequencies and observed ones depends for the most part on the turbulent kinetic energy which was taken from a 3D convection simulation for the Sun.

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arXiv2005

Core Overshoot: An Improved Treatment and Constraints from Seismic Data

We present a comprehensive set of stellar evolution models for Procyon A in an effort to guide future measurements of both traditional stellar parameters and seismic frequencies towards constraining the amount of core overshoot in Procyon A and possibly other stars. Current observational measurements of Procyon A when combined with traditional stellar modeling only place a large upper limit on overshoot of alphaOV < 1.1. By carrying out a detailed pulsation analysis, we further demonstrate, how p- and g-mode averaged spacings can be used to gain better estimates of the core size. For both p- and g-modes, the frequency spacings for models without overshoot are clearly separated from the models with overshoot. In addition, measurements of the l=0 averaged small p-mode spacings could be used to establish Procyon A's evolutionary stage. For a fixed implementation of overshoot and under favorable circumstances, the g-mode spacings can be used to determine the overshoot extent to an accuracy of +-0.05 Hp. However, we stress that considerable confusion is added due to the unknown treatment of the overshoot region. This ambiguity might be removed by analyzing many different stars. A simple non-local convection theory developed by Kuhfuss is implemented in our stellar evolution code and contrasted with the traditional approaches. We show that this theory supports a moderate increase of the amount of convective overshoot with stellar mass of Delta(alphaOV) = +0.10 between 1.5 Msun and 15 Msun. This theory places an upper limit on Procyon A's core overshoot extent of ~0.4 Hp which matches the limit imposed by Roxburgh's integral criterion.

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arXiv2005

Super Helium-Rich Population and the Origin of Extreme Horizontal-Branch Stars in Globular Clusters

Recent observations for the color-magnitude diagrams (CMDs) of the massive globular cluster Omega Centauri have shown that it has a striking double main sequence (MS), with a minority population of bluer and fainter MS well separated from a majority population of MS stars. Here we confirm, with the most up-to-date Y2 isochrones, that this special feature can only be reproduced by assuming a large variation (Delta Y = 0.15) of primordial helium abundance among several distinct populations in this cluster. We further show that the same helium enhancement required for this special feature on the MS can by itself reproduce the extreme horizontal-branch (HB) stars observed in Omega Cen, which are hotter than normal HB stars. Similarly, the complex features on the HBs of other globular clusters, such as NGC 2808, are explained by large internal variations of helium abundance. Supporting evidence for the helium-rich population is also provided by the far-UV (FUV) observations of extreme HB stars in these clusters, where the enhancement of helium can naturally explain the observed fainter FUV luminosity for these stars. The presence of super helium-rich populations in some globular clusters suggests that the third parameter, other than metallicity and age, also influences CMD morphology of these clusters.

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arXiv2004

Y2 Isochrones with an Improved Core Overshoot

Convective core overshoot affects stellar evolution rates and the dating of stellar populations. In this paper, we provide a patch to the $Y^2$ isochrones with an improved treatment of convective core overshoot. The new tracks cover the transition mass range from no convective core to a fully developed convective core. We compare the improved isochrones to CMDs of a few well observed open star clusters in the Galaxy and the Large Magellanic Cloud. Finally we discuss future prospects for improving the treatment of core overshoot with the help of asteroseismology.

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arXiv2004

Asteroseismic determination of helium abundance in stellar envelopes

Intermediate degree modes of the solar oscillations have previously been used to determine the solar helium abundance to a high degree of precision. However, we cannot expect to observe such modes in other stars. In this work we investigate whether low degree modes that should be available from space-based asteroseismology missions can be used to determine the helium abundance, Y, in stellar envelopes with sufficient precision. We find that the oscillatory signal in the frequencies caused by the depression in Γ_1 in the second helium ionisation zone can be used to determine the envelope helium abundance of low mass main sequence stars. For frequency errors of 1 part in 10^4, we expect errors σ_Y in the estimated helium abundance to range from 0.03 for 0.8M_sun stars to 0.01 for 1.2M_sun stars. The task is more complicated in evolved stars, such as subgiants, but is still feasible if the relative errors in the frequencies are less than 10^{-4}.

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arXiv2003

A Preliminary Seismic Analysis of 51 Peg: Large and Small Spacings from Standard Models

We present a preliminary theoretical seismic study of the astronomically famous star 51 Peg. This is done by first performing a detailed analysis within the Hertzsprung-Russell diagram (HRD). Using the Yale stellar evolution code (YREC), a grid of stellar evolutionary tracks has been constructed for the masses 1.00 M_sun, 1.05 M_sun and 1.10 M_sun, in the metallicity range Z=0.024-0.044, and for values of the Galactic helium enrichment ratio DY/DZ in the range 0-2.5. Along these evolutionary tracks, we select 75 stellar model candidates that fall within the 51 Peg observational error box in the HRD (all turn out to have masses of 1.05 M_sun and 1.10 M_sun. The corresponding allowable age range for these models, which depends sensitively on the parameters of the model, is relatively large and is ~2.5 - 5.5 Gyr. For each of the 75 models, a non-radial pulsation analysis is carried out, and the large and small frequency spacings are calculated. The results show that just measuring the large and small frequency spacings will greatly reduce the present uncertainties in the derived physical parameters and in the age of 51 Peg. Finally we discuss briefly refinements in the physics of the models and in the method of analysis which will have to be included in future models to make the best of the precise frequency determinations expected from space observations.

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arXiv2002

The Y^2 Stellar Evolutionary Tracks

We present a database of the latest stellar models of the $Y^2$ (Yonsei-Yale) collaboration. This database contains the stellar evolutionary tracks from the pre-main-sequence birthline to the helium core flash that were used to construct the $Y^2$ isochrones. We also provide a simple interpolation routine that generates stellar tracks for given sets of parameters (metallicity, mass, and $α$-enhancement).

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arXiv2002

The YY Isochrones for alpha-element Enhanced Mixtures

We present a new set of isochrones in which the effect of the alpha-element enhancement is fully incorporated. These isochrones are an extension of the already published set of YY Isochrones (Yi et al. 2001: Paper 1), constructed for the scaled-solar mixture. As in Paper 1, helium diffusion and convective core overshoot have been taken into account.The range of chemical compositions covered is 0.00001 < Z < 0.08. The models were evolved from the pre-main-sequence stellar birthline to the onset of helium burning in the core. The age range of the full isochrone set is 0.1 -- 20 Gyr, while younger isochrones of age 1 -- 80 Myr are also presented up to the main-sequence turn-off. Combining this set with that of Paper 1 for scaled-solar mixture isochrones, we provide a consistent set of isochrones which can be used to investigate populations of any value of alpha-enhancement. We confirm the earlier results of Paper 1 that inclusion of alpha-enhancement effects further reduces the age estimates of globular clusters by approximately 8 percent if [alpha/Fe]=+0.3. It is important to note the metallicity dependence of the change in age estimates (larger age reductions in lower metallicities). This reduces the age gap between the oldest metal-rich and metal-poor Galactic stellar populations and between the halo and the disk populations. The isochrone tables, together with interpolation routines have been made available via internet; http://www.astro.yale.edu/demarque/yyiso.html http://www-astro.physics.ox.ac.uk/~yi/yyiso.html http://csaweb.yonsei.ac.kr/~kim/yyiso.html

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arXiv2002

Seismic Test of Solar Models, Solar Neutrinos and Implications for Metal-Rich Accretion

The Sun is believed to have been the recipient of a substantial amount of metal-rich material over the course of its evolution, particularly in the early stages of the Solar System. With a long diffusion timescale, the majority of this accreted matter should still exist in the solar convection zone, enhancing its observed surface abundance, and implying a lower-abundance core. While helioseismology rules out solar models with near-zero metallicity cores, some solar models with enhanced metallicity in the convection zone might be viable, as small perturbations to the standard model. Because of the reduced interior opacity and core temperature, the neutrino flux predicted for such models is lower than that predicted by the standard solar model. This paper examines how compatible inhomogeneous solar models of this kind are with the observed low and intermediate degree p-mode oscillation data, and with the solar neutrino data from the SNO Collaboration. We set an upper limit on how much metal-rich accretion took place during the early evolution of the Sun at about 2 Earth masses of iron (or about 40 Earth masses of meteoric material).

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arXiv2001

Empirical Constraints on Convective Core Overshoot

In stellar evolution calculations, the local pressure scale height is often used to empirically constrain the amount of convective core overshoot. However, this method brings unsatisfactory results for low-mass stars (< 1.1 -1.2 Mo for Z= \Zs) which have very small cores or no convective core at all. Following Roxburgh's integral constraint,we implemented an upper limit of overshoot within the conventional method of alpha parameterization in order to remove an overly large overshoot effect on low-mass stars. The erroneously large effect of core overshoot due to the failure of alpha parameterization can be effectively corrected by limiting the amount of overshoot to < 15 % of the core radius. 15 % of the core radius would be a proper limit of overshoot which can be implemented in a stellar evolution code for intermediate to low mass stars. The temperature structure of the overshoot region does not play a crucial role in stellar evolution since this transition region is very thin.

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arXiv1999

The Metallicity Dependence of RR Lyrae Absolute Magnitudes from Synthetic Horizontal-Branch Models

A grid of synthetic horizontal-branch (SHB) models based on HB evolutionary tracks with improved physics has been constructed to reconsider the theoretical calibration of the dependence of M_v(RR) on metallicity in globular clusters, and the slope of the mean -[Fe/H] relation. The SHB models confirm Lee's earlier finding (Lee 1991) that the slope of the -[Fe/H] relation is itself a function of the metallicity range considered, and that in addition, for a given [Fe/H], RR Lyrae luminosities depend on HB morphology. This is due to the fact that HB stars pass through the RR Lyrae instability strip at different evolutionary stages, depending on their original position on the HB. At [Fe/H]=-1.9, and for HB type 0, the models yield M_v(RR)=0.47 \pm 0.10. The mean slope for the zero-age HB models is 0.204. Since there is no simple universal relation between M_v(RR) and metallicity that is applicable to all globular clusters, the HB morphology of each individual cluster must be taken into account, in addition to [Fe/H], in deriving the appropriate M_v(RR). Taking HB morphology into account, we find that the slope of the mean -[Fe/H] relation varies between 0.36 for the clusters with galactocentric distances R_gc less than 6 kpc and 0.22 for clusters with 6<R_gc<20 kpc. Implications for interpreting observations of field RR Lyrae variables and for absolute globular cluster ages and galactic chronology are briefly discussed.

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arXiv1997

The Relative Ages of Galactic Globular Clusters

We present a review of the present state of knowledge regarding the relative ages of Galactic globular clusters. First, we discuss the relevant galaxy formation models and describe the detailed predictions they make with respect to the formation timescale and chemical evolution of the globular clusters. Next, the techniques used to estimate globular cluster ages are described and evaluated with particular emphasis on the advantages and disadvantages of each method. With these techniques as a foundation, we present arguments in favor of the following assertions: 1) The age of a globular cluster is the likeliest candidate to be the global second parameter, which along with metal abundance, controls the morphology of the horizonal branch. 2) A total age range of as much as $\sim$5 Gyr exists among the bulk of the Galactic globulars. 3) There is a significant relation between age and metallicity among the Galactic globular clusters if the slope of the \mvrr-\feh relation is less than $\sim$0.23. These conclusions along with other supporting evidence favor a formation scenario in which the inner regions of the Galactic halo collapsed in a monotonic fashion over a short time period much less than 1 Gyr. In contrast, the outer regions of the halo fragmented and collapsed in a chaotic manner over several Gyrs.

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arXiv1997

On the Origin of the UV Upturn in Elliptical Galaxies. II. Test of the HB Hypothesis

Whether or not metal-rich HB stars are the dominant UV source in giant elliptical galaxies (gEs) is an important question in current astronomical research. We follow up our previous evolutionary population synthesis study with quantitative tests to answer this question affirmatively under the following three conditions: (1) Reimers' empirical mass loss formula is proper, (2) the mass loss efficiency parameter in metal-rich stars is somewhat larger than the value estimated from the metal-poor star studies, and (3) the true value of the helium enrichment parameter is positive. All three important empirical characteristics of the UV upturn (i.e., the fact that strong UV upturns are restricted to gEs, the positive UV upturn-metallicity correlation, and the narrow range of the temperature of the UV sources) are closely reproduced for reasonable ranges of input parameters. We discuss the major sources of uncertainties in the models, such as the production and role of hot horizontal-branch stars in gEs, and the importance of galactic nucleosynthesis.

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