Constantine P. Deliyannis

The Evolution of Galactic Boron and the Production Site of the Light Elements

The Goddard High Resolution Spectrograph (GHRS) of the Hubble Space Telescope (HST) has been used to obtain spectra of the 2500 A region in eight stars with metallicities ranging from [Fe/H] = -0.4 to -3.0, including the most metal-poor star ever observed for boron. Spectrum synthesis utilizing latest Kurucz model atmospheres has been used to determine the B abundance for each star, with particular attention paid to the errors of each point, to permit judgment of the quality of the fit of models of Galactic chemical evolution. Previous observations were combined with new ones, bringing the number of stars analyzed to 11. A straight line of slope ≈ 1 gives an excellent fit to a plot of log (BLTE) versus [Fe/H], and if NLTE B abundances are used, the slope is ≈ 0.7. Plotting B versus [O/H] rather than [Fe/H] increases the slope of either plot by about 0.2. The observed relation suggests that the production of light elements such as B and Be is directly related to the production of heavier elements. Our data do not show a change in slope between halo and disk metallicities, but the number of stars near the disk-halo transition is small, and a modest change is not precluded. The NLTE B/Be ratio is typically ≈ 15 throughout the lifetime of the Galaxy, a ratio naturally produced by cosmic-ray (CR) spallation. Our data support a model in which most light-element production comes from low-energy CR spallation of C and O nuclei onto protons and α-particles, probably in the vicinity of massive supernovae in star-forming regions. Until recently, most models have emphasized light-element production in the general ISM from the spallation of high-energy protons and α-particles onto CNO nuclei. Especially during the Galaxys early history, when the metallicity of the ISM was low, the spallation of protons and α-particles onto CNO nuclei cannot account for as much B as we observe, unless the CR flux was sufficiently high for compensation. The observed relation also constrains any direct production of B by the ν-process in supernovae to be at most a small part of total B production. It is possible that the gamma rays recently detected from the Orion Nebula region are the signature of spallation by energetic C and O nuclei. Nevertheless, B, Be, and Fe data alone give the strongest evidence of the importance of spallation by C and O for producing light elements.

Primordial Lithium: Keck Observations in M92 Turnoff Stars

We present new Keck I/HIRES observations at R = 45,000 (=3 pixels) of seven stars near the turnoff of the old, metal-poor globular cluster M92. In three of these stars, we have signal-to-noise ratios (S/Ns) of 40 pixel-1, and in the other four, the S/N is near 20. The Li abundance in star 18 is high compared with the halo field-star plateau and is similar to that in the remarkable Li-rich halo field star BD +23°3912. In addition to the high Li abundance in star 18, there is a dispersion in Li abundance in our seven stars covering the full range of a factor of 3. We have attempted to determine whether the excess Li in star 18 is due to less than average Li depletion in this star from an even higher initial abundance, as predicted by the Yale rotational models, or whether it is due to the extraordinary action of Li production mechanisms in the material that formed this star. We have found no convincing evidence that favors Li production: (1) Stars 18, 21, and 46 have identical Ba abundances, which argues against Li production carrying an s-process signature. (2) These three stars have indistinguishable Ca, Cr, Fe, and Ti, which argues against supernova Li production. (3) We discuss ν-process production of Li and find no convincing observational evidence for this from the strengths of the Mg, Ca, and Fe lines. (4) The similarity in age of these cluster stars argues against cosmic-ray Li production that requires age differences of gigayears. The most likely explanation for the Li dispersion is differential Li depletion from a (possibly significantly) higher primordial Li abundance due to differences in the initial angular momentum in each star followed by spin-down; the most rapid rotators destroy the most Li, whereas the initially slower rotators preserve more Li.

Extremely Metal-Poor Stars. III. The Lithium-depleted Main-Sequence Turnoff Dwarfs*

We present abundances of 14 elements for the metal-poor, near-main-sequence turnoff stars G66-30, G139-8, and G186-26, which are well known to possess less than 1/5-1/10 the value of Li/H observed in field halo Spite Plateau dwarfs and are thought by many to have been produced in the currently accepted standard big bang cosmology. The stars have [Fe/H] = -1.53, -2.24, and -2.68, respectively, and for these values their collective abundance patterns are not abnormal. That is to say, there is no common abundance abnormality that one might associate with their Li deficiencies. In G186-26, we find an overabundance of the heavy neutron-capture elements that increases with atomic mass. We measure [Ba/Fe] = +0.35, whereas most stars of this metallicity have [Ba/Fe] ~ -0.5, together with smaller enhancements for Sr and possibly Y. Such supersolar, neutron-capture, element abundances, however, are exhibited by ~25% of stars having [Fe/H] ~ -2.7. No such enhancement is found in G66-30 or G139-8, and the simplest explanation for the data of the three stars is that there is no connection between Li depletion and the abundance patterns of the heavy neutron-capture elements. G66-30 and G186-26 appear to be multiple systems, and we discuss the possible role of binarity in producing the observed Li depletion. We concur with J. A. Thorburn that this group of objects is not the progeny of blue stragglers, and we discuss the alternative that some of the Li-depleted stars may be the Population II counterparts of the disk population subgiant CH, barium dwarf, and F str λ4077 stars. As suggested by others, the most likely explanation for the abundance anomalies in these objects, which includes universal Li deficiency, involves mass transfer across a binary during the asymptotic giant branch evolutionary phase of the erstwhile primary of the system. We note that while such mass transfer might be expected to produce Li depletion, variations in C, N, and the heavy neutron-capture elements are possible but not all necessary. Neither G66-30 nor G139-8 shows enhancement of C or the neutron-capture elements, and no information is available for N. Further work is necessary to settle the issue. If mass transfer is responsible for Li depletions, such stars justifiably could be excluded from investigations of the primordial Li abundance. In the absence of evidence for such mass transfer, however, the possibility remains that some of these objects, if not all, are extreme examples of a process that has affected all Plateau stars.

Hubble Space Telescope Beryllium Abundances in the α Centauri System

High signal-to-noise ratio Hubble Space Telescope Goddard High-Resolution Spectrograph spectra of α Centauri A (spectral type G2 V) and α Centauri B (spectral type K1 V) have been analyzed in the Be II λ3130 spectral region. Both stars offer an excellent opportunity for testing predictions of 9Be destruction since they are nearby, have a well-determined orbit and parallax, and thus have very well known physical parameters. A detailed spectrum synthesis has been made using a line list that has been carefully tested by comparison with the Sun and with metal-poor stars of different temperature and metallicity. Our analysis gives [Be/H] = +0.06 ± 0.09 dex for α Cen A and -0.54 ± 0.28 dex for α Cen B, using a model atmosphere with a metallicity [M/H] = +0.10. The implications of the new beryllium abundances and the previous lithium studies for models of stellar light-element depletion are then studied. Both the Sun and α Cen A are more highly depleted in lithium than younger stars; this is not consistent with standard stellar models. The Sun, α Cen A, and α Cen B have photospheric 9Be abundances lower than the current accepted solar meteoritic value. Because the initial 9Be abundance for the α Cen system is not known, and the depletion is much smaller than that for lithium, it is difficult to determine precise beryllium depletion factors for this system. However, because neither star is expected to deplete beryllium in standard models, the lower relative abundance of α Cen B might be evidence for main-sequence beryllium depletion. The theoretical implications of beryllium depletion are briefly discussed.

Astrometry with Hubble Space Telescope Fine Guidance Sensor 3: The Parallax of the Cataclysmic Variable RW Triangulum

RW Triangulum (RW Tri) is a 13th magnitude nova-like cataclysmic variable star with an orbital period of 0.2319 days (5.56 hr). Infrared observations of RW Tri indicate that its secondary is most likely a late-K dwarf (Dhillon). Past analyses predicted a distance of 270 pc, derived from a blackbody fit to the spectrum of the central part of the disk (Rutten, van Paradijs, & Tinbergen). Recently completed Hubble Space Telescope Fine Guidance Sensor interferometric observations allow us to determine the first trigonometric parallax to RW Tri. This determination puts the distance of RW Tri at 341, one of the most distant objects with a direct parallax measurement. We compare our result with methods previously employed to estimate distances to cataclysmic variables.

The Distance to the Hyades Cluster Based on Hubble Space Telescope Fine Guidance Sensor Parallaxes

Trigonometric parallax observations made with the Hubble Space Telescope (HST) Fine Guidance Sensor (FGS) 3 of seven Hyades members in six fields of view have been analyzed along with their proper motions to determine the distance to the cluster. Knowledge of the convergent point and mean proper motion of the Hyades is critical to the derivation of the distance to the center of the cluster. Depending on the choice of the proper-motion system, the derived cluster center distance varies by 9%. Adopting a reference distance of 46.1 pc or m - M = 3.32, which is derived from the ground-based parallaxes in the General Catalogue of Trigonometric Stellar Parallaxes (1995 edition), the FK5/PPM proper-motion system yields a distance 4% larger, while the Hanson system yields a distance 2% smaller. The HST FGS parallaxes reported here yield either a 14% or 5% larger distance, depending on the choice of the proper-motion system. Orbital parallaxes (Torres et al.) yield an average distance 4% larger than the reference distance. The variation in the distance derived from the HST data illustrates the importance of the proper-motion system and the individual proper motions to the derivation of the distance to the Hyades center; therefore, a full utilization of the HST FGS parallaxes awaits the establishment of an accurate and consistent proper-motion system.

Keck Observation of Lithium in a Cool Halo Dwarf Common Proper Motion Pair

We exploit the uniform age and initial chemical composition of the halo common proper motion pair, HD 134439 and HD 134440, to eliminate metallicity- and age-dependent effects in an investigation of Li processing in cool, metal-poor dwarfs. We have obtained a very high S/N (480) Keck observation of Li in HD 134440 and derive a 3 σ upper limit on its lithium abundance of A(Li) ≤ -0.59. This contrasts with an abundance over 1 dex higher, A(Li) = 0.45 ± 0.05, in HD 134439, which is hotter by only 200 K. This difference verifies that Li processing in cool, metal-poor dwarfs is very sensitively dependent on effective temperature. Additionally, the stark contrast between Li in HD 134440 and in the tidally locked binary star BD -0°4234, reinforces the significance of the detection in the latter, which may lend support to the suggestion that rotationally induced mixing is a significant mechanism for Li depletion.

Lithium in the Old Open Cluster M 67: Constraints for the Cause of the Boesgaard Li Gap

Wide field multi-object high resolution spectroscopy can address the cause of the decade-old stellar evolutionary problem known as the Boesgaard Li gap, a severe main sequence Li depletion in F stars within a few hundred K in Teff.This Li gap points to a fundamental failure of the standard stellar evolution theory. Additional mechanisms, such as mass loss, microscopic diffusion, and rotationally-induced mixing have all been proposed to explain the gap. Subgiants of the old open cluster M 67 have evolved out of the gap, and their deepening convection zones can potentially reveal the signature(s) of the dominant mechanism(s). We present observations of 54 stars near the turnoff of M 67, obtained simultaneously in January 1996 with the 3.5-m WIYN telescope and Hydra multi-object fiber fed spectrograph. The Li decline in our M 67 subgiants argues strongly against diffusion, and weakly against mass loss (but field star Li and Be data argue strongly against mass loss). Our data support models which include rotationally-induced mixing. The importance of short-period binaries is also discussed, and implications for cosmology are mentioned.

Evolutionary Models of Rotating Stars

In this paper we review the principal elements of the Yale code for evolving rotating stars (YREC). The various mechanisms to distribute angular momentum in the solar interior used in the code are discussed, both in regard to their properties and their consequences in the stellar models. In particular, two general types of mechanisms exist, one which depends on the velocity of rotation, and the other which depends on the radial gradient of this quantity. The regions where these mechanisms operate effectively, and the times when this occurs, and possibly the efficiency of these transfer mechanisms to cause material mixing, depend on which of the two types they belong.

Ages of Globular Clusters

A brief historical introduction is given. Recent advances in precision photometry of faint stars in globular clusters and in the modeling of turnoffs and synthetic horizontal branches, indicate that there is a spread in age of at least 4 Gyr among Galactic globular clusters. Absolute ages are more difficult to determine and the sources of uncertainty are discussed. On the observational side, chemical abundances, [Fe/H] and [O/Fe], are the major sources of uncertainty. Two effects hitherto ignored in evolutionary calculations and currently under study, rotation and diffusion, are discussed. Internal rotation affects the ages only in a minor way. The diffusion of helium near the main sequence could be important in reducing the derived ages of the oldest clusters; its efficiency, which is poorly known, can however be constrained and evaluated by lithium abundance observations. Finally, an approximate chronology of the Galaxy is presented on the basis of our current best understanding of cluster dating. The lowest age derived for the cluster M92 is 14 Gyr, which puts a strong constraint on the cosmological timescale.