Updated High-Temperature Opacities for the Dartmouth Stellar Evolution Program and their Effect on the Jao Gap Location

The Jao Gap, a 17 percent decrease in stellar density at $M_G \sim10$ identified in both Gaia DR2 and EDR3 data, presents a new method to probe the interior structure of stars near the fully convective transition mass. The Gap is believed to originate from convective kissing instability wherein asymmetric production of $^{3}$He causes the core convective zone of a star to periodically expand and contract and consequently the stars' luminosity to vary. Modeling of the Gap has revealed a sensitivity in its magnitude to a population's metallicity primarily through opacity. Thus far, models of the Jao Gap have relied on OPAL high-temperature radiative opacities. Here we present updated synthetic population models tracing the Gap location modeled with the Dartmouth stellar evolution code using the OPLIB high-temperature radiative opacities. Use of these updated opacities changes the predicted location of the Jao Gap by $\sim0.05$ mag as compared to models which use the OPAL opacities. This difference is likely too small to be detectable in empirical data.Comment: 15 pages, 13 figures; Accepted for publication in The Astrophysical Journa

The impact of pollution on stellar evolution models

An approach is introduced for incorporating the concept of stellar pollution into stellar evolution models. The approach involves enhancing the metal content of the surface layers of stellar models. In addition, the surface layers of stars in the mass range of 0.5-2.0 Solar masses are mixed to an artificial depth motivated by observations of lithium abundance. The behavior of polluted stellar evolution models is explored assuming the pollution occurs after the star has left the fully convective pre main sequence phase. Stellar models polluted with a few Earth masses of iron are significantly hotter than stars of the same mass with an equivalent bulk metallicity. Polluted stellar evolution models can successfully reproduce the metal-rich, parent star tau Bootis and suggest a slightly lower mass than standard evolution models. Finally, the possibility that stars in the Hyades open cluster have accreted an average of 0.5 Earth masses of iron is explored. The results indicate that it is not possible to rule out stellar pollution on this scale from the scatter of Hyades stars on a color-magnitude diagram. The small amount of scatter in the observational data set does rule out pollution on the order of 1.5 Earth masses of iron. Pollution effects at the low level of 0.5 Earth masses of iron do not produce substantial changes in a star's evolution.Comment: 14 pages, 3 figures, AASTeX, to appear in the 10/10/03 issue of Ap

From Canonical to Enhanced Extra Mixing in Low-Mass Red Giants: Tidally Locked Binaries

Stellar models which incorporate simple diffusion or shear induced mixing are used to describe canonical extra mixing in low mass red giants of low and solar metallicity. These models are able to simultaneously explain the observed Li and CN abundance changes along upper red giant branch (RGB) in field low-metallicity stars and match photometry, rotation and carbon isotopic ratios for stars in the old open cluster M67. The shear mixing model requires that main sequence (MS) progenitors of upper RGB stars possessed rapidly rotating radiative cores and that specific angular momentum was conserved in each of their mass shells during their evolution. We surmise that solar-type stars will not experience canonical extra mixing on the RGB because their more efficient MS spin-down resulted in solid-body rotation, as revealed by helioseismological data for the Sun. Thus, RGB stars in the old, high metallicity cluster NGC 6791 should show no evidence for mixing in their carbon isotopic ratios. We develop the idea that canonical extra mixing in a giant component of a binary system may be switched to its enhanced mode with much faster and somewhat deeper mixing as a result of the giant's tidal spin-up. This scenario can explain photometric and composition peculiarities of RS CVn binaries. The tidally enforced enhanced extra mixing might contribute to the star-to-star abundance variations of O, Na and Al in globular clusters. This idea may be tested with observations of carbon isotopic ratios and CN abundances in RS CVn binaries.Comment: 47 pages, 19 figures, accepted for publication in Ap

UBVI CCD Photometry of the Old Open Cluster Berkeley 17

Photometric UBVI CCD photometry is presented for NGC 188 and Berkeley 17. Color-magnitude diagrams (CMDs) are constructed and reach well past the main-sequence turn-off for both clusters. Cluster ages are determined by means of isochrone fitting to the cluster CMDs. These fits are constrained to agree with spectroscopic metallicity and reddening estimates. Cluster ages are determined to be 7.0+/-0.5 Gyr for NGC 188, and 10.0+/- 1.0 Gyr for Berkeley 17, where the errors refer to uncertainties in the relative age determinations. These ages are compared to the ages of relatively metal-rich inner halo/thick disk globular clusters and other old open clusters. Berkeley 17 and NGC 6791 are the oldest open clusters with an age of 10 Gyr. They are 2 Gyr younger than the thick disk globular clusters. These results confirm the status of Berkeley 17 as one of the oldest known open cluster in the Milky Way, and its age provides a lower limit to the age of the Galactic disk.Comment: to appear in AJ; 28 pages, 9 figure

The RR Lyrae variables and Horizontal Branch of NGC6656 (M22)

The first calibrated broadband UBVI time-series photometry is presented for the RR Lyrae variable stars in NGC6656 (M22), with observations spanning a range of twenty-two years. We have also redetermined the variability types and periods for the RR Lyrae stars identified previously by photographic observations, revising the number of fundamental-mode RR Lyrae variables (RR0) to 10 and the number of first-overtone variables (RR1) to 16. The mean periods of the RR0 and RR1 variables are _{RR0}=0.66 \pm 0.02 d and _{RR1}=0.33 \pm 0.01 d, respectively, supporting an Oosterhoff II classification for the cluster. The number ratio of RR1- to all RR-type variables is N_1/N_{RR}=0.61, also consistent with an Oosterhoff II designation. Both the RR Lyrae stars' minimum light colors and the blue edge of the RR Lyrae instability strip suggest E(B-V)=0.36 \pm 0.02 mag toward M22. Regarding the HB morphology of M22, we find (B-R)/(B+V+R)=+0.97 \pm 0.1 and at least one "gap" located in an unusual part of the blue HB, in the middle of the so-called hot HB stars.Comment: accepted to A

STELLAR POPULATION MODELS AND INDIVIDUAL ELEMENT ABUNDANCES. II. STELLAR SPECTRA AND INTEGRATED LIGHT MODELS

ABSTRACT The first paper in this series explored the effects of altering the chemical mixture of the stellar population on an element-by-element basis on stellar evolutionary tracks and isochrones to the end of the red giant branch. This paper extends the discussion by incorporating the fully consistent synthetic stellar spectra with those isochrone models in predicting integrated colors, Lick indices, and synthetic spectra. Older populations display element ratio effects in their spectra at higher amplitude than younger populations. In addition, spectral effects in the photospheres of stars tend to dominate over effects from isochrone temperatures and lifetimes, but, further, the isochrone-based effects that are present tend to fall along the age-metallicity degeneracy vector, while the direct stellar spectral effects usually show considerable orthogonality

The ACS Survey of Galactic Globular Clusters. I. Overview and Clusters Without Previous HST Photometry

We present the first results of a large ACS Survey of Galactic globular clusters. This Hubble Space Telescope (HST) Treasury project is designed to obtain photometry with S/N > ~10 for main sequence stars with masses > ~0.2Msun in a sample of globulars using the Advanced Camera for Surveys (ACS) Wide Field Channel. Here we focus on clusters without previous HST imaging data. These include NGC 5466, 6779, 5053, 6144, Palomar 2, E 3, Lynga 7, Palomar 1, and NGC 6366. Our CMDs extend reliably from the horizontal branch to as much as seven magnitudes fainter than the main sequence turnoff and represent the deepest CMDs published to-date for these clusters. Using fiducial sequences for three standard clusters (M92, NGC 6752, and 47 Tuc) with well-known metallicities and distances, we perform main sequence fitting on the target clusters in order to obtain estimates of their distances and reddenings. These comparisons along with fitting the cluster main sequences to theoretical isochrones yield ages for the target clusters. We find that the majority of the clusters have ages that are consistent with the standard clusters at their metallicities. The exceptions are E 3 which appears ~2 Gyr younger than 47 Tuc, and Pal 1, which could be as much as 8 Gyr younger than 47 Tuc.Comment: 22 pages, 29 figures, accepted for publication in The Astronomical Journal, April 2007. High resolution version available at http://www.astro.ufl.edu/~ata/GC_Treasury/Web_Page/publications.htm