Eric L. Sandquist

Age and helium content of the open cluster NGC 6791 from multiple eclipsing binary members - II. Age dependencies and new insights

Context. Models of stellar structure and evolution can be constrained by measuring accurate parameters of detached eclipsing binaries in open clusters. Multiple binary stars provide the means to determine helium abundances in these old stellar systems, and in turn, to improve estimates of their age. Aims. In the first paper of this series, we demonstrated how measurements of multiple eclipsing binaries in the old open cluster NGC6791 sets tighter constraints on the properties of stellar models than has previously been possible, thereby potentially improving both the accuracy and precision of the cluster age. Here we add additional constraints and perform an extensive model comparison to determine the best estimates of the cluster age and helium content, employing as many observational constraints as possible. Methods. We improve our photometry and correct empirically for differential reddening effects. We then perform an extensive comparison of the new colour-magnitude diagrams (CMDs) and eclipsing binary measurements to Victoria and DSEP isochrones in order to estimate cluster parameters. We also reanalyse a spectrum of the star 2‐17 to improve [Fe/H] constraints. Results. Wefind abest estimateof theage of ∼8.3Gyr for NGC6791 whiledemonstrating that remaining age uncertainty is dominated by uncertainties in the CNO abundances. The helium mass fraction is well constrained at Y = 0.30 ± 0.01 resulting in ΔY/ΔZ ∼1.4 assuming that such a relation exists. During the analysis we firmly identify blue straggler stars, including the star 2‐17, and find indications for the presence of their evolved counterparts. Our analysis supports the RGB mass-loss found from asteroseismology and we determine precisely the absolute mass of stars on the lower RGB, MRGB = 1.15 ± 0.02 M� . This will be an important consistency check for the detailed asteroseismology of cluster stars. Conclusions. Using multiple, detached eclipsing binaries for determining stellar cluster ages, it is now possible to constrain parameters of stellar models, notably the helium content, which were previously out of reach. By observing a suitable number of detached eclipsing binaries in several open clusters, it will be possible to calibrate the age-scale and the helium enrichment parameter ΔY/ΔZ, and provide firm constraints that stellar models must reproduce.

Ccd photometry of the globular cluster m5. I. the color-magnitude diagram and luminosity functions

We present new BV I photometry for the halo globular cluster M5 (NGC 5904 = C1516+022), and examine the B- and I-band luminosity functions (LFs), based on over 20,000 stars — one of the largest samples ever gathered for a cluster luminosity function. Extensive artificial star tests have been conducted to quantify incompleteness as a function of magnitude and cluster radius. We do not see evidence in the LF of a “subgiant excess” or of a discrepancy in the relative numbers of stars on the red-giant branch and main sequence, both of which have been claimed in more metal-poor clusters. Enhancements of �-element have been taken into account in our analysis. This improves the agreement between the observed and predicted positions of the “red-giant bump”. Depending on the average �-element enhancement among globular clusters and the distance calibration, the observed discrepancy between the theoretical and observed position for a large number of clusters (Fusi Pecci et al. 1990) can be almost completely removed. The helium abundance of M5, as determined by the population ratio R, is found to be Y = 0.19 ± 0.02. However, there is no other indication that the helium abundance is different from other clusters of similar metallicity, and values calculated for other helium indicators are consistent with Y ≈ 0.23. The relative ages of M5, Palomar 5, M4, NGC 288, NGC 362, NGC 1261, NGC 1851 and � , , !

A Critical Examination of Li Pollution and Giant-Planet Consumption by a Host Star

Given the high metal contents observed for many stars with planets (SWPs), we examine the overall likelihood that the consumption of a giant planet could pollute its host star. First, we discuss 6Li and 7Li as indicators of pollution, verifying that 6Li is a strong indicator of pollution 30 Myr after star formation and showing that it strongly constrains the amount of heavy-element pollution incorporated into the star. Detection of 6Li in SWPs could also be used to distinguish between giant planet formation theories and can be used to detect the consumption of giant planets independent of planet mass. Second, we examine the probability that giant planets between 1 and 3 MJ could be destroyed in the outer convection zone of stars slightly more massive than the Sun (for which detection of a chemical signature of pollution would be easiest). We find that heated giant planets would be efficiently destroyed near the surface of the star, while the cores of cold giant planets might be able to survive a plunge through the base of the stars convection zone. Third, we examine whether dynamical processes could bring a giant planet close enough to the star to destroy it and whether the destruction of a planet would necessarily affect other planets in the system. While tidal interaction between protoplanets and their nascent disks may have led them to the proximity of their host stars, postformation star-planet interaction can lead to tidal disruption of the planet and accretion of its material or orbital decay followed by hydrodynamical interaction. Throughout, we consider the case of HD 82943, a star with a preliminary detection of 6Li that is known to have two planets. Using stellar models including diffusion, we estimate the mass of HD 82943 to be ~1.2 M☉ and its age to be ~0.5-1.5 Gyr. The observed 7Li abundance for HD 82943 is consistent with stars of similar Teff and age in the open cluster NGC 752. We describe a possible dynamical history for a hypothetical planet in the presence of the two resonant planets currently known. We present stable orbital configurations in which the hypothetical planet has low eccentricity and semimajor axis near 0.02 AU, so that it is dynamically decoupled from the resonant planets. Tidal interactions with the slowly rotating star can subsequently drag the planet into the stellar surface within the age of the star.

Angular Momentum Losses and the Orbital Period Distribution of Cataclysmic Variables below the Period Gap: Effects of Circumbinary Disks

The population synthesis of cataclysmic variable binary systems at short orbital periods (<2.75 hr) is investigated. A grid of detailed binary evolutionary sequences has been calculated and included in the simulations to take account of additional angular momentum losses beyond that associated with gravitational radiation and mass loss, due to nova outbursts, from the system. As a specific example, we consider the effect of a circumbinary disk to gain insight into the ingredients necessary to reproduce the observed orbital period distribution. The resulting distributions show that the period minimum lies at about 80 minutes, with the number of systems monotonically increasing with increasing orbital period to a maximum near 90 minutes. There is no evidence for an accumulation of systems at the period minimum, which is a common feature of simulations in which only gravitational radiation losses are considered. The shift of the peak to about 90 minutes is a direct result of the inclusion of systems formed within the period gap. The period distribution is found to be fairly flat for orbital periods ranging from about 85 to 120 minutes. The steepness of the lower edge of the period gap can be reproduced, for example, by an input of systems at periods near 2.25 hr due to a flow of cataclysmic variable binary systems from orbital periods longer than 2.75 hr. The good agreement with the cumulated distribution function of observed systems within the framework of our model indicates that the angular momentum loss by a circumbinary disk or a mechanism that mimics its features, coupled with a weighting factor to account for selection effects in the discovery of such systems and a flow of systems from above the period gap to below the period gap, is an important ingredient for understanding the overall period distribution of cataclysmic variable binary systems.

A LONG-PERIOD TOTALLY ECLIPSING BINARY STAR AT THE TURNOFF OF THE OPEN CLUSTER NGC 6819 DISCOVERED WITH KEPLER *

We present the discovery of the totally eclipsing long-period (P = 771.8 d) binary system WOCS 23009 in the old open cluster NGC 6819 that contains both an evolved star near central hydrogen exhaustion and a low-mass (0.45M⊙) star. This system was previously known to be a single-lined spectroscopic binary, but the discovery of an eclipse near apastron using data from the Kepler space telescope makes it clear that the system has an inclination that is very close to 90 ◦ . Although the secondary star has not been identified in spectra, the mass of the primary star can be constrained using other eclipsing binaries in the cluster. The combination of total eclipses and a mass constraint for the primary star allows us

The Blue Straggler RS Canum Venaticorum Star S1082 in M67: A Detailed Light Curve and the Possibility of a Triple* **

Using both photometric and spectroscopic data, we present a picture of the very unusual blue straggler S1082 in the old open cluster M67 whose light is the sum of a close binary (P = 1.0677978 days) and another cluster member. The primary of the close binary and the third star are both blue stragglers in their own rights. Using relative photometry with millimagnitude accuracy, we provide a complete V-band light curve for the system and show a number of unusual features: brightness variations at the 0.01?0.03 mag level from month to month at all phases, a narrow primary and broad secondary eclipse, brightness differences between phases 0.25 and 0.75, and short duration (~1 hr) drops in brightness. Much of the light-curve variation appears to be because of spot activity on the cooler, fainter, nearly synchronized component of the close binary. We use spectra from several sources to constrain the temperatures of the three known components, the relative flux contributions, rotational velocities, and radial velocities. The data clearly show that the brightest star (narrow-line component) seen in the spectrum is on an orbit with period P = 1189 ? 7 days and eccentricity e = 0.57 ? 0.08, although we cannot prove that there is a dynamical link between it and the close binary. The systematic velocities of all of the stars indicate that they are members of the cluster. Our models of the system indicate that the cooler component of the close binary lies on the main sequence near the cluster turnoff, while the hotter component lies near an extension of the main sequence blueward of the turnoff, and thus qualifies as a blue straggler by itself. The reduction of the masses of the stars in the close binary compared with previous models indicates that it is possible that the more massive component of the close binary formed from a merger of just two turnoff-mass stars.

Cataclysmic Variable Evolution with Circumbinary Disks

The results of a systematic investigation of the evolution of cataclysmic variable binary systems with angular momentum losses associated with a circumbinary disk are presented. A grid of models characterized by initial donor masses ranging from 0.15 to 1.2 M☉ and white dwarf masses ranging from 0.3 to 1.4 M☉ has been constructed. In contrast to previous studies, the detailed evolution of the binary and that of a geometrically thin, viscous Keplerian circumbinary disk are both fully taken into account in the computations. The numerical results show that the circumbinary disk can promote the mass transfer between the binary components in the system at sufficient rates even for low fractional mass input rates into the circumbinary disk, δ ~ 10-4. The existence of only a few systems in the period gap requires that the majority of systems with initial orbital periods greater than about 3 hr undergo a period bounce. Such evolutionary behavior implies that δ is a function of the evolutionary state of the donor, in the sense that δ must be higher for more evolved systems to bounce above an orbital period of 3 hr. For the orbital periods 2 hr where the majority of dwarf novae exist, small δ are indicated, suggesting that δ is a function of the mean mass transfer rate in the system. In this picture there is no tendency for systems to converge toward a common period as they approach the period minimum, which is consistent with observations indicating that there is not a peak in the period distribution. The implications of these low fractional mass input rates into circumbinary disks on their detectability via their continuum radiation are also discussed.

The Blue Straggler and Main-Sequence Binary Population of the Low-Mass Globular Cluster Palomar 13*

We present high-precision VI photometry of stars from the middle of the giant branch to about 5 mag below the main-sequence turnoff in the globular cluster Palomar 13 based on images obtained with the Keck II 10 m telescope. We tabulate a complete sample of blue stragglers in the cluster out to about 18 core radii. The blue straggler population is significantly more centrally concentrated than the giant-star sample, which is in turn significantly more centrally concentrated than the main-sequence–star sample. Pal 13 has one of the highest specific frequencies of blue stragglers of any known globular cluster, but the specific frequency of blue stragglers in the outskirts of the cluster does not increase, as has been seen in denser clusters. We also identify a group of faint blue stragglers (bluer than the turnoff but having about the same magnitude) that outnumbers the brighter stragglers by more than a factor of 2. The clusters color-magnitude diagram shows a large excess of stars to the red of the main sequence, indicating that the clusters binary fraction is at least 30% ± 4%, which appears to be similar to that of the low-mass cluster E3 but significantly higher than that of the more massive clusters Pal 5 and NGC 288.

Testing Asteroseismic Scaling Relations using Eclipsing Binaries in Star Clusters and the Field

The accuracy of stellar masses and radii determined from asteroseismology is not known! We examine this issue for giant stars by comparing classical measurements of detached eclipsing binary systems (dEBs) with asteroseismic measurements from the Kepler mission. For star clusters, we extrapolate measurements of dEBs in the turn-off region to the red giant branch and the red clump where we investigate the giants as an ensemble. For the field stars, we measure dEBs with an oscillating giant component. These measurements allow a comparison of masses and radii calculated from a classical eclipsing binary analysis to those calculated from asteroseismic scaling relations and/or other asteroseismic methods. Our first results indicate small but significant systematic differences between the classical and asteroseismic measurements. In this contribution we show our latest results and summarize the current status and future plans. We also stress the importance of realizing that for giant stars mass cannot always be translated to age, since an unknown fraction of these evolved through a blue straggler phase with mass transfer in a binary system. Rough estimates of how many such stars to expect are given based on our findings in the open clusters NGC6819 and NGC6791.

BRIGHT VARIABLE STARS IN NGC 6819: AN OPEN CLUSTER IN THE KEPLER FIELD

We describe a variability study of the moderately old open cluster NGC 6819. We have detected four new detached eclipsing binaries near the cluster turnoff (one of which may be in a triple system). Several of these systems should be able to provide mass and radius information, and can therefore constrain the age of the cluster. We have also newly detected one possible detached binary member about 3.5 mag below the turnoff. One EW-type binary (probably not a cluster member) shows unusually strong night-to-night light curve variations in sets of observations separated by eight years. According to the best current information, the three brightest variables we detected (two of them new) are cluster members, making them blue stragglers. The first one is a δ Scu pulsating variable, the second one is a close but detached binary, and the third one contains a detached short-period binary that shows total eclipses. In each case, however, there is evidence hinting that the system may have been produced through the interaction of more than two stars.