Investigating the Consistency of Stellar Evolution Models with Globular Cluster Observations via the Red Giant Branch Bump

Synthetic RGBB magnitudes are generated with the most recent theoretical stellar evolution models computed with the Dartmouth Stellar Evolution Program (DSEP) code. They are compared to the observational work of Nataf et al., who present RGBB magnitudes for 72 globular clusters. A DSEP model using a chemical composition with enhanced $\alpha$ capture [$\alpha$/Fe] $=+0.4$ and an age of 13 Gyr shows agreement with observations over metallicities ranging from [Fe/H] = $0$ to [Fe/H] $\approx-1.5$, with discrepancy emerging at lower metallicities.Comment: 11 pages, 12 figure

The Pulsation Properties of Procyon A

A grid of stellar evolution models for Procyon A has been calculated. These models include the best physics available to us (including the latest opacities and equation of state) and are based on the revised astrometric mass of Girard et al (1996). Models were calculated with helium diffusion and with the combined effects of helium and heavy element diffusion. Oscillation frequencies for l=0,1,2 and 3 p-modes and the characteristic period spacing for the g-modes were calculated for these models. We find that g-modes are sensitive to model parameters which effect the structure of the core, such as convective core overshoot, the heavy element abundance and the evolutionary state (main sequence or shell hydrogen burning) of Procyon A. The p-modes are relatively insensitive to the details of the physics used to model Procyon A, and only depend on the evolutionary state of Procyon A. Hence, observations of p-mode frequencies on Procyon A will serve as a robust test of stellar evolution models.Comment: 4 pages, to appear in ApJ

A New Color-Magnitude Diagram for Palomar 11

We present new photometry for the Galactic thick disk globular cluster Palomar 11 extending well past the main sequence turn-off in the V and I bands. This photometry shows noticeable red giant and subgiant branches. The difference in magnitude between the red horizontal branch (red clump) and the subgiant branch is used to determine that Palomar 11 has an age of 10.4+/-0.5 Gyr. The red clump is used to derive a distance d_\sun=14.3+/-0.4 kpc, and a mean cluster reddening of E(V-I)=0.40+/-0.03. There is differential reddening across the cluster, of order \delta E(V-I)~0.07. The colour magnitude diagram of Palomar 11 is virtually identically to that of the thick disk globular cluster NGC 5927, implying that these two clusters have a similar age and metallicity. Palomar 11 has a slightly redder red giant branch than 47 Tuc, implying that Palomar 11 is 0.15 dex more metal-rich, or 1 Gyr older than 47 Tuc. Ca II triplet observations (Rutledge et al. 1997) favour the hypothesis that Palomar 11 is the same age as 47 Tuc, but slightly more metal-rich.Comment: to appear in AJ (19 pages, 4 B&W figures, 1 colour figure

The Age of the Inner Halo Globular Cluster NGC 6652

HST (V,I) photometry has been obtained for the inner halo globular cluster NGC 6652. The photometry reaches approximately 4 mag below the turn-off and includes a well populated horizontal branch. This cluster is located close to the Galactic center at a galactocentric distance of approximately 2.0 kpc with a reddening of E(V-I) = 0.15 +/- 0.02 and has a metallicity of [Fe/H] approximately -0.85. Based upon Delta(V) between the point on the sub-giant branch which is 0.05 mag redder than the turn-off and the horizontal branch, NGC 6652 is 11.7 +/- 1.6 Gyr old. Using this same Delta(V), precise differential ages for 47 Tuc (a thick disk globular), M107 and NGC 1851 (both halo clusters) were obtained. NGC 6652 appears to be the same age as 47 Tuc and NGC 1851 (within +/- 1.2 Gyr), while there is a slight suggestion that M107 is older than NGC 6652 by 2.3 +/- 1.5 Gyr. As this is a less than 2-sigma result, this issue needs to be investigated further before a definitive statement regarding the relative age of M107 and NGC 6652 may be made.Comment: accepted for publication in the Astronomical Journal, December 2000 issu

Are Stars with Planets Polluted?

We compare the metallicities of stars with radial velocity planets to the metallicity of a sample of field dwarfs. We confirm recent work indicating that the stars-with-planet sample as a whole is iron rich. However, the lowest mass stars tend to be iron poor, with several having [Fe/H]<-0.2, demonstrating that high metallicity is not required for the formation of short period Jupiter-mass planets. We show that the average [Fe/H] increases with increasing stellar mass (for masses below 1.25 solar masses) in both samples, but that the increase is much more rapid in the stars-with-planet sample. The variation of metallicity with stellar age also differs between the two samples. We examine possible selection effects related to variations in the sensitivity of radial velocity surveys with stellar mass and metallicity, and identify a color cutoff (B-V>0.48) that contributes to but does not explain the mass-metallicity trend in the stars-with-planets sample. We use Monte Carlo models to show that adding an average of 6.5 Earth masses of iron to each star can explain both the mass-metallicity and the age-metallicity relations of the stars-with-planets sample. However, for at least one star, HD 38529, there is good evidence that the bulk metallicity is high. We conclude that the observed metallicities and metallicity trends are the result of the interaction of three effects; accretion of about 6 Earth masses of iron rich material, selection effects, and in some cases, high intrinsic metallicity.Comment: 19 pages 11 figure

On Using the Color-Magnitude Diagram Morphology of M67 to Test Solar Abundances

The open cluster M67 has solar metallicity and an age of about 4 Gyr. The turnoff (TO) mass is close to the minimum mass for which solar metallicity stars develop a convective core during main sequence evolution as a result of the development of hydrogen burning through the CNO cycle. The morphology of the color-magnitude diagram (CMD) of M67 around the TO shows a clear hook-like feature, a direct sign that stars close to the TO have convective cores. VandenBerg et al. investigated the possibility of using the morphology of the M67 TO to put constraints on the solar metallicity, particularly CNO elements, for which solar abundances have been revised downward by more than 30% over the last few years. Here, we extend their work, filling the gaps in their analysis. To this aim, we compute isochrones appropriate for M67 using new (low metallicity) and old (high metallicity) solar abundances and study whether the characteristic TO in the CMD of M67 can be reproduced or not. We also study the importance of other constitutive physics on determining the presence of such a hook, particularly element diffusion, overshooting and nuclear reaction rates. We find that using the new solar abundance determinations, with low CNO abundances, makes it more difficult to reproduce the characteristic CMD of M67. This result is in agreement with results by VandenBerg et al. However, changes in the constitutive physics of the models, particularly overshooting, can influence and alter this result to the extent that isochrones constructed with models using low CNO solar abundances can also reproduce the TO morphology in M67. We conclude that only if all factors affecting the TO morphology are completely under control (and this is not the case), M67 could be used to put constraints on solar abundances

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 ^8B neutrinos; this is approximately half-way between the standard solar model without diffusion, and the standard solar model with diffusion alone