arXiv subjects

Brian Chaboyer

Publications and source records attributed to Brian Chaboyer.

84 records · Page 5Linked to original sources

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.

astro-ph↗

arXiv1995

Globular Cluster Ages and the Formation of the Galactic Halo

Main sequence turnoff magnitudes from the recent set of Yale isochrones (Chaboyer \ea 1995) have been combined with a variety of relations for the absolute magnitude of RR Lyr stars (\mvrr) to calibrate age as a function of the difference in magnitude between the main sequence turn-off and the horizontal branch (\dv). A best estimate for the calibration of \mvrr is derived from a survey of the current literature: $\mvrr = 0.20 \feh + 0.98$. This estimate, together with other calibrations (with slopes ranging from 0.15 to 0.30) has been used to derive \dv ages for 43 Galactic globular clusters. Independent of the choice of \mvrr, there is no strong evidence for an age-Galacto\-centric distance relationship among the 43 globular clusters. However, an age-metallicity relation exists, with the metal-poor clusters being the oldest. A study of the age distribution reveals that an age range of 5 Gyr exists among the bulk of the globular clusters. In addition, about 10\% of the sample are substantially younger, and including them in the analysis increases the age range to 9 Gyr. Once again, these statements are independent of the \mvrr ~relation. Evidence for age being the second parameter governing horizontal branch morphology is found by comparing the average \dv ~age of the second parameter clusters to the normal clusters. The second parameter clusters are found to be on average 2 - 3 Gyr younger than the other clusters, which is consistent with age being the second parameter. These results suggest that globular clusters were formed over an extended period of time, with progressively more metal-rich globular clusters ($\feh \ga -1.7$) being formed at later times.

astro-ph↗

arXiv1995

Halo Star Evolution

In this review, I will discuss a few problems which point to the need for improved stellar evolution models of halo stars. Current stellar evolution models do not match the observed $^7$Li abundance patterns, suggesting that the input physics and/or the assumptions used in constructing the models are in need of revision. It appears that all halo stars have suffered some $^7$Li depletion, implying that the primordial $^7$Li abundance is higher than that presently observed in hot halo stars. Observations of abundances of various elements in globular cluster giant branch stars have suggested for some time now that some form of deep mixing, which is not present in theoretical models, occurs in halo stars. The driving mechanism for this mixing, and its incorporation into stellar models remain one of the key problems in stellar modeling. Current theoretical isochrones are able to provide a good match to observed colour-magnitude diagrams. However, there is some evidence that the theoretical luminosity functions are in disagreement with observations. This is an area which requires further study, as it suggests that the relative main sequence/giant branch lifetimes predicted by the models are incorrect. A discussion of some of the uncertainties involved in determining the ages of globular clusters is presented. The absolute ages of globular clusters provide a lower bound to the age of the universe, and so are of great interest to cosmologists. Unfortunately, present uncertainties in stellar models lead to a rather large range in the inferred ages of globular clusters of 11 -- 18 Gyr.

astro-ph↗

arXiv1995

The OPAL Equation of State and Low Metallicity Isochrones

The Yale stellar evolution code has been modified to use the OPAL equation of state tables (Rogers 1994). Stellar models and isochrones were constructed for low metallicity systems ($-2.8 \le [Fe/H] \le -0.6$). Above $M\sim 0.7\,\msun$, the isochrones are very similar to those which are constructed using an equation of state which includes the analytical Debye-Huckel correction at high temperatures. The absolute magnitude of the main sequence turn-off (\mvto) with the OPAL or Debye-Huckel isochrones is about 0.06 magnitudes fainter, at a given age, than \mvto derived from isochrones which do not include the Debye-Huckel correction. As a consequence, globular clusters ages derived using \mvto are reduced by 6 -- 7\% as compared to the ages determined from the standard isochrones. Below $M\sim 0.7\,\msun$, the OPAL isochrones are systematically hotter (by approximately 0.04 in B-V) at a given magnitude as compared to the standard, or Debye-Huckel isochrones. However, the lower mass models fall out of the OPAL table range, and this could be the cause of the differences in the location of the lower main-sequences.

astro-ph↗

arXiv1995

ROTATION, DIFFUSION, AND OVERSHOOT IN THE SUN: EFFECTS ON THE OSCILLATION FREQUENCIES AND THE NEUTRINO FLUX

We have studied the importance of the combined effects of rotation, diffusion, and convective overshoot on the p-mode oscillation spectrum and the neutrino flux of the standard solar model. To isolate the various physical affects included in the new rotation plus diffusion models we also constructed solar models to test the significance of diffusion and of overshoot by themselves. In previous studies, models that include helium diffusion during solar evolution were found to improve the predicted p-mode frequencies for some modes and worsen the agreement for others (Guenther \ea 1993). Here we verify this result for both the Bahcall and Loeb (1990) formulation of diffusion and the Proffitt and Michaud (1991) formulation of diffusion. We find that the effects of rotation on the Sun's structure in the outer layers perturbs the $p$-mode frequencies only slightly when compared to the more substantial effects due to diffusion. In the thin overshoot layer (taken here to be $0.1\, H_p$), we have compared the effect of overmixing in a radiative layer versus convective (adiabatic) penetration. Neither radiative overmixing nor adiabatic penetration has any significant effect on the $p$-modes, probably in part because the overshoot layer is constrained to be thin. The predicted neutrino flux in our diffusion plus rotation model is 7.12 SNU for Cl detectors, 127 SNU for Ga detectors and $5.00\times 10^6\,{\rm erg \, cm^{-2} }$ for the $^8$B neutrinos; this is approximately half-way between the standard solar model without diffusion, and the standard solar model with diffusion alone.

astro-ph↗

arXiv1994

A Supergiant Supernova-Blown Bubble in the Spiral Galaxy NGC 1620

We present UBR and H$α$ imaging of NGC 1620, a highly inclined spiral galaxy that contains a large scale, arc-like feature of radius 3 kpc in its outer disk at a distance of $\sim$ 11 kpc from the center. What is unusual about this arc-like feature is its stellar nature and the presence of a luminous star cluster at its center. The arc is fragmented into HII region complexes and OB star clusters and shows two kinks in optical continuum light. It spans an angle of 220$^{\circ}$ on our U image and a full, though fragmented, circle on an unsharp masked R image. It is centered on a young star cluster that is the most luminous clump in blue optical continuum light besides the nucleus of the galaxy. This central star cluster has UBR colors and a surface brightness similar to those of other HII regions, but is a relatively weak H$α$ emitter. It consists of at least three unresolved condensations in optical continuum light. Its location at the center of the arc and its prominence within the galaxy suggests that it has been the site of several generations of supernova explosions that swept up the surrounding gas into a supershell. When it attained a radius of $0.5-1$ kpc, this shell became gravitationally unstable and formed the stars which now delineate the arc. The constraints imposed by the survival of the expanding arc against random stellar motions and the age of the stars in the arc yield a required energy input by a minimum of 400 and a maximum of 6500 supernovae. In this scenario the asymmetry in surface brightness of the arc reflects the radial gradient of the gas density in the disk of NGC 1620, while the kinks reflect inhomogeneities in the original gas distribution with respect to the central star cluster. The supernova superbubble formed at least $5 \times 10^7$ yr ago so that, unless

astro-ph↗

arXiv1994

Absolute Ages of Globular Clusters and the Age of the Universe

The main sequence turnoff luminosity is the best stellar `clock' which can be used to determine the absolute ages of globular clusters. This is due to the fact that it is generally assumed that the luminosity and lifetimes of main sequence globular cluster stars are independent of the properties of stellar convection and atmospheres, two areas of stellar evolution which are poorly understood. Several possible sources of error in this stellar clock are discussed, and isochrones are constructed using a variety of different physical assumptions. The mean age of the oldest globular clusters are determined from these isochrones and it is found that the uncertainties in the input physics can lead to changes in the derived age of $\pm 15\%$. Surprisingly the largest source of error is the mixing length theory of convection. It is well known that uncertainties in the distance scale and chemical composition of globular cluster stars lead to changes of order $\sim 22\%$ in the determination of absolute ages. Combining the various sources of error, the absolute age of the oldest globular clusters are found to lie in the range 11 --- 21 Gyr. This is meant to be a total theoretical range. For the standard inflationary model ($Ω= 1, Λ= 0$), a minimum age of the universe of 11 Gyr requires $H_o \la 60~\kmsmpc$.

astro-ph↗

arXiv1994

Ages of Galactic Globular Clusters from the New Yale Isochrones

A new grid of theoretical isochrones based on the Yale stellar evolution code using the OPAL and Kurucz opacities has been constructed. The grid of isochrones spans a wide range of metallicities, helium abundances and masses. The construction of the isochrones is described and the isochrones are compared to galactic globular cluster observations. A solar calibrated mixing length ($α= 1.7$) yields a good fit to globular cluster colour-magnitude diagrams. Ages for 40 globular clusters are determined using the \dv method and the formation of the halo is discussed.

astro-ph↗

arXiv1994

Stellar Models with Microscopic Diffusion and Rotational Mixing I: Application to the Sun

The Yale stellar evolution code has been modified to include the combined effects of diffusion and rotational mixing on $^1$H, $^4$He and the trace elements $^3$He, $^6$Li, $^7$Li, and $^9$Be. The interaction between rotational mixing and diffusion is studied by calculating a number of calibrated solar models. The rotational mixing inhibits the diffusion in the outer parts of the models, leading to a decrease in the envelope diffusion by 25 -- 50\%. Conversely, diffusion leads to gradients in mean molecular weight which can inhibit the rotational mixing. The degree to which gradients in mean molecular weight inhibits the rotational mixing is somewhat uncertain. A comparison to the observed solar oblateness suggests that gradients in the mean molecular weight play a smaller role in inhibiting the rotational mixing previously believed. This is reinforced by the fact that the model with the standard value for the inhibiting effect of mean molecular weight on the rotational mixing depletes no Li on the main sequence. This is in clear contrast to the observations. A reduction in the inhibiting effect of mean molecular weight gradients by a factor of ten leads to noticeable main sequence Li depletion.

astro-ph↗

arXiv1994

Stellar Models with Microscopic Diffusion and Rotational Mixing II: Application to Open Clusters

Stellar models with masses ranging from 0.5 to $1.3~M_\odot$ were constructed in order to compare to young cluster observations of Li and of rotation velocities. The amount of Li depletion in cool stars is sensitive to the amount of overshoot at the base of the surface convection zone, and the exact metallicity of the models. Even when this uncertainty is taken into account, the Li observations are a severe constraint for the models and rule out standard models, and pure diffusion models. Stellar models which include diffusion and rotational mixing in the radiative regions of stars are able to simultaneously match the Li abundances observed in the Pleiades, UMaG, Hyades, Praesepe, NGC 752 and M67. They also match the observed rotation periods in the Hyades. However, these models are unable to simultaneously explain the presence of the rapidly rotating late G and K stars in the Pleiades and the absence of rapidly rotating late F and early G stars.

astro-ph↗

arXiv1994

The Primordial Abundance of $^6$Li and $^9$be

Light element ($^6$Li, $^7$Li and $^9$Be) depletion isochrones for halo stars have been calculated with standard stellar evolution models. These models include the latest available opacities and are computed through the sub-giant branch. If $^6$Li is not produced in appreciable amounts by stellar flares, then the detection of $^6$Li in HD 84937 by Smith, Lambert \& Nissen (1993) is compatible with standard stellar evolution and standard big bang nucleosynthesis only if HD 84937 is a sub-giant. The present parallax is inconsistent with HD 84937 being a sub-giant star at the $2.5\, σ$ level. The most metal poor star with a measured $^9$Be abundance is HD 140283, which is a relatively cool sub-giant. Standard stellar evolution predict that $^9$Be will have been depleted in this star by $\sim 0.3$ dex (for ${\rm T_{eff}} = 5640$ K). Revising the abundance upward changes the oxygen-beryllium relation, suggesting incompatible with standard comic ray production models, and hence, standard big bang nucleosynthesis. However, an increase in the derived temperature of HD 140283 to 5740 K would result in no depletion of $^9$Be and agreement with standard big bang nucleosynthesis.

astro-ph↗

arXiv1994

$^7$Li Abundances in Halo Stars: Testing Stellar Evolution Models and the Primordial $^7$Li Abundance

A large number of stellar evolution models with [Fe/H] = -2.3 and -3.3 have been calculated in order to determine the primordial $^7$Li abundance and to test current stellar evolution models by a comparison to the extensive database of Li abundances in extremely metal poor halo stars observed by Thorburn (1994). Standard models do a good job of fitting the observed Li abundances in stars hotter than 5600 K. They predict a primordial $^7$Li abundance of Log N(Li) = 2.24\pm 0.03$. Models which include microscopic diffusion predict a downward curvature in the $^7$Li destruction isochrones at hot temperatures which is not present in the observations. Thus, the observations clearly rule out models which include uninhibited microscopic diffusion of $^7$Li from the surface of the star. The [Fe/H] = -2.3 stellar models which include both diffusion and rotational mixing provide an excellent match to the observations. Both the plateau stars and the heavily depleted cool stars are well fit by these models. The rotational mixing leads to considerable $^7$Li depletion in these models and the primordial $^7$Li abundance inferred from these models is Log N(Li) = $3.08\pm 0.1$.

astro-ph↗