Jeremy R. King

The Problem of Hipparcos Distances to Open Clusters. I. Constraints from Multicolor Main-Sequence Fitting

Parallax data from the Hipparcos mission allow the direct distance to open clusters to be compared with the distance inferred from main-sequence (MS) fitting. There are surprising differences between the two distance measurements, indicating either the need for changes in the cluster compositions or reddening, underlying problems with the technique of MS fitting, or systematic errors in the Hipparcos parallaxes at the 1 mas level. We examine the different possibilities, focusing on MS fitting in both metallicity-sensitive B-V and metallicity-insensitive V-I for five well-studied systems (the Hyades, Pleiades, α Per, Praesepe, and Coma Ber). The Hipparcos distances to the Hyades and α Per are within 1 σ of the MS-fitting distance in B-V and V-I, while the Hipparcos distances to Coma Ber and the Pleiades are in disagreement with the MS-fitting distance at more than the 3 σ level. There are two Hipparcos measurements of the distance to Praesepe; one is in good agreement with the MS-fitting distance and the other disagrees at the 2 σ level. The distance estimates from the different colors are in conflict with one another for Coma but in agreement for the Pleiades. Changes in the relative cluster metal abundances, age related effects, helium, and reddening are shown to be unlikely to explain the puzzling behavior of the Pleiades. We present evidence for spatially dependent systematic errors at the 1 mas level in the parallaxes of Pleiades stars. The implications of this result are discussed.

STELLAR KINEMATIC GROUPS. II. A REEXAMINATION OF THE MEMBERSHIP, ACTIVITY, AND AGE OF THE URSA MAJOR GROUP

Utilizing Hipparcos parallaxes, original radial velocities and recent literature values, new Ca ii H and K emission measurements, literature-based abundance estimates, and updated photometry (including recent resolved measurements of close doubles), we revisit the Ursa Major moving group membership status of some 220 stars to produce a final clean list of nearly 60 assured members, based on kinematic and photometric criteria. Scatter in the velocity dispersions and H-R diagram is correlated with trial activity-based membership assignments, indicating the usefulness of criteria based on photometric and chromospheric emission to examine membership. Closer inspection, however, shows that activity is considerably more robust at excluding membership, failing to do so only for � 15% of objects, perhaps considerably less. Our UMa members demonstrate nonzero vertex deviation in the Bottlinger diagram, behavior seen in older and recent studies of nearby young disk stars and perhaps related to Galactic spiral structure. Comparison of isochrones and our final UMa group members indicates an age of 500 � 100 Myr, some 200 Myr older than the canonically quoted UMa age. Our UMa kinematic=photometric members’ mean chromospheric emission levels, rotational velocities, and scatter therein are indistinguishable from values in the Hyades and smaller than those evinced by members of the younger Pleiades and M34 clusters, suggesting these characteristics decline rapidly with age over 200–500 Myr. None of our UMa members demonstrate inordinately low absolute values of chromospheric emission, but several may show residual fluxes a factor of � 2 below a Hyades-defined lower envelope. If one defines a Maunder-like minimum in a relative sense, then the UMa results may suggest that solar-type stars spend 10% of their entire main-sequence lives in periods of precipitously low activity, which is consistent with estimates from older field stars. As related asides, we note six evolved stars (among our UMa nonmembers) with distinctive kinematics that lie along a 2 Gyr isochrone and appear to be late-type counterparts to disk F stars defining intermediate-age star streams in previous studies, identify a small number of potentially very young but isolated field stars, note that active stars (whether UMa members or not) in our sample lie very close to the solar composition zero-age main sequence, unlike Hipparcos-based positions in the H-R diagram of Pleiades dwarfs, and argue that some extant transformations of activity indices are not adequate for cool dwarfs, for which Ca ii infrared triplet emission seems to be a better proxy than H� -based values for Ca ii H and K indices.

Beryllium Abundances in Halo Stars from Keck/HIRES Observations

We have determined the abundance of Be in stars with an array of metal abundances in order to enhance our understanding of the chemical evolution of the Galaxy, cosmic-ray theory, and cosmology. Observations of the Be II resonance lines at j3130 and j3131 were made at the Keck telescope with the HIRES spectrometer at a resolution of 46,000 and signal-to-noise ratios of 60¨110 (per pixel) typically. Our sample includes 22 halo dwarfs and —ve disk stars (including the Sun). We have taken special care in determining the stellar parameters for these stars in a consistent manner. The Be abundances were found (1) from the measured equivalent width of the relatively unblended Be II line at 3131.065 with Ae an analysis that included 11 weak atomic and molecular lines near that wavelength and (2) from spec- trum synthesis that included newly derived enhanced O (relative to Fe) in the synthesis calculations. The two methods are in excellent agreement. We —nd straight-line —ts between Be and Fe: log N(Be/H) \ 0.96(^0.04)(Fe/H) ( 10.59(^0.03) ; and between Be and O: log N(Be/H) \ 1.45(^0.04)(O/H) ( 10.69(^0.04) . It seems that Be and Fe increase at the same rate during the course of the evolution of the Galaxy. But as O increases by a factor of 100, Be increases more rapidly, by a factor of 800. Traditional models in which energetic cosmic rays interact with ambient CNO nuclei in the interstellar medium to produce Be are consistent with this —nding, as long as certain chemical evolution eUects (such as mass out—ow from the halo) are taken into account. However, models predicting a linear relationship between Be and O, such as those producing Be in the vicinity of Type II supernovae, are less consistent with our result. There is some evidence for an intrinsic spread in Be at a given (Fe/H) or (O/H). There is currently no evidence of a primordial plateau level of Be down to log N(Be/H) \( 13.5.

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.

HD 98800: A unique stellar system of post-T tauri stars

HD 98800 is a system of four stars, and it has a large infrared excess that is thought to be due to a dust disk within the system. In this paper we present new astrometric observations made with Hipparcos, as well as photometry from Hubble Space Telescope WFPC2 images. Combining these observations and reanalyzing previous work allow us to estimate the age and masses of the stars in the system. Uncertainty in these ages and masses results from uncertainty in the temperatures of the stars and any reddening they may have. We find that HD 98800 is most probably about 10 Myr old, although it may be as young as 5 Myr or as old as 20 Myr old. The stars in HD 98800 appear to have metallicities that are about solar. An age of 10 Myr means that HD 98800 is a member of the post T Tauri class of objects, and we argue that the stars in HD 98800 can help us understand why post T Tauris have been so elusive, HD 98800 may have formed in the Centaurus star-forming region, but it is extraordinary in being so young and yet so far from where it was born.

Beryllium in the Hyades F and G Dwarfs from Keck HIRES Spectra

Although there are extensive observations of Li in field stars of all types and in both open and (recently) globular cluster stars, there are relatively few observations of Be. Because Be is not destroyed as easily as Li, the abundances of Li and Be together can tell us more about the internal physical processes in stars than either element can alone. We have obtained high-resolution (45,000) and high signal-to-noise ratio (typically 90 per pixel) spectra of the Be II resonance lines in 34 Hyades F and G dwarfs with the Keck I telescope and HIRES. In addition we took a spectrum of the daytime sky to use as a surrogate for the solar spectrum so we could determine the value for Be in the Sun, analyzed in the same manner as that for the stars. We have adopted stellar temperatures and some Li abundances for these stars from the literature. For most of the F dwarfs we have rederived Li abundances. The Be abundances have been derived with the spectrum synthesis method. We find that Be is depleted, but detected, in the Li gap in the F stars reaching down to values of A(Be) = 0.60 dex, or a factor of nearly 7 below the meteoritic Be abundance (a factor of 3.5 below the solar value of Chmielewski et al.). There is little or no depletion of Be in stars cooler than 6000 K, in spite of the large depletions (0.5-2.5 dex) in Li. The mean value of A(Be) for the 10 coolest stars is 1.33 ± 0.06, not far from the meteoritic value of 1.42. The pattern in the Be abundances—a Be dip and undepleted Be in the cool stars—is well matched by the predictions of slow mixing due to stellar rotation. We have interpolated the calculations of Deliyannis and Pinsonneault for Be depletion due to rotational mixing to the age of the Hyades; we find excellent agreement of the predictions with the observed Be abundances but less good agreement with the observed Li abundances. Some of our Hyades stars have photometrically determined rotation periods, but there is no relation between Be and rotation period. (Generally, the lower mass stars have less Li and longer periods, which may indicate greater spin-down and thus more Li depletion relative to Be.) The Li and Be abundances are correlated for stars in the temperature range of 5850-6680 K, similar to results from earlier work on Li and Be in F and G field stars. This indicates that the depletions are not just correlated—the only claim that can be made for the field stars—but are probably occurring together during main-sequence evolution. The Hyades G dwarfs have more Be than the Sun; their initial Be may have been larger or they may not be old enough to have depleted much Be. For those Hyades stars that seem to have little or no depletion of Li or Be, the Li/Be ratio is found to be 75 ± 30; the meteoritic ratio Li/Be is 78. The Hyades ratio is a representative value for the initial ratio in the material out of which the Hyades cluster was formed.

Galactic evolution of beryllium

The abundance of Be in the lowest-metallicity stars is a probe of Big Bang Nucleosynthesis, and its abundance in halo and disk stars is a probe of galactic evolution and stellar structure. We present observations of the Be II resonance lines in 14 halo stars and 27 (mostly old) disk stars with [Fe/H] from −2.7 to +0.13. The spectra were obtained at the Canada-France-Hawaii 3.6 m telescope and have a measured resolution of 0.13 A and a median signal-to-noise ratio of ∼50. For 18 of the 41 stars we have also made observations of the O I triplet at the Palomar 5 m telescope, the UH 2.2 m telescope, and the CFH telescope. Stellar parameters of T eff , log g, and [Fe/H] were carefully determined from several independent estimates

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.

Keck HIRES Spectroscopy of M92 Subgiants: Surprising Abundances near the Turnoff

Using high-resolution, moderate signal-to-noise ratio spectroscopy obtained with the 10 m Keck I Telescope and efficient HIRES echelle spectrograph, we derive abundances of several elements in sub- giants near the M92 turno†. As a consistency check, we also analyze the metal-poor -eld star HD 140283 and -nd an Fe abundance in -ne agreement with many previous determinations. However, our M92 value ((Fe/H) \( 2.52) is a factor of 2 lower than the abundance derived from its red giant members. Di†erences in model atmospheres, gf-values, and instrumental e†ects might account for this di†erence, but whether they in fact do so is unclear. We note possible evidence for (Fe/H) di†erences within M92. Our spectroscopic analysis suggests that the M92 reddening, E(B(V ), may be 0.04E0.05 mag greater than canonical values, but various uncertainties mean that this conclusion is not de-nitive; the signi-cant di†erence in interstellar Na I line strengths in the M92 and HD 140283 spectra may be consistent with an increased reddening. Regardless, the conclusion that either the (Fe/H) of M92 has been signi-cantly overestimated from red giants or current reddening/photometry estimates are too small/red is not easily escaped. If the reddening/photometry were in error by this amount, turno† colorE based ages for M92 could be reduced by D4 Gyr. The adjustment to the M92 distance modulus required for a similarly reduced turno† age that is luminosity-based can be accommodated by increases in extinction and alterations to the metal-poor -eld star distance scale recently inferred from Hipparcos Cepheid and subdwarf data. Our M92 subgiants demonstrate (Cr/Fe), (Ca/Fe), and (Ti/Fe) ratios that are unremarkable and essentially identical to the values for HD 140283. (Ba/Fe) is 0.45 dex larger for the M92 subgiants than for HD 140283. Surprisingly, we -nd (Mg/Fe) to be 0.55 dex lower in our M92 subgiants than in HD 140283, and (Na/Fe) to be 0.76 dex larger in our M92 subgiants than in HD 140283. These di†erences (and indeed nearly all our abundance ratios) seem immune to various data, analysis, and parameter errors. If real, this striking abundance pattern is suggestive of material in our M92 starsI photospheres that has undergone Ne ) Na and Mg ) Al cycling like that inferred for red giants in M92 and other clusters. While this is generally believed to be an in situ process in cluster giants, the presence of abun- dant Li in our M92 objects suggests a polluting source acting either primordially or via accretion after cluster star formation. This may be consistent with CN and Na variations on the 47 Tucanae main sequence, recently reported Ba and Eu variations in M15 red giants, possible cluster-to-cluster n-capture abundance di†erences, and very low (O/Fe) ratios observed near the base of the M13 giant branch. We thus suggest that a polluting source of light-element alteration, in addition to the in situ source for more evolved stars, may be required for M92. Comparison of our M92 subgiant abundance ratios with those of M92 red giants may indicate that pollution occurred after the present generation of cluster stars formed, but until the cause or causes of the subgiant versus giant Fe abundance discrepancy are de-ni- tively identi-ed, this conclusion is uncertain. A polluting source of our Na and Mg anomalies produced via processing in a previous stellar generation also has complications; namely, how the Mg and Na anomalies arise without apparently any net inNuence on our subgiantsI Li abundances and on the C abundances of other M92 subgiants. A similar quandary may exist in some 47 Tuc turno† stars. An understanding of cluster abundance variations (by whatever mechanisms) and their behavior with evolu- tionary state may be needed for a complete understanding of absolute and relative globular clusters ages, and for derivation of the primordial Li abundance.

Abundances of Stars with Planets: Trends with Condensation Temperature

Precise abundances of 18 elements have been derived for 10 stars known to host giant planets from high signal-to-noise ratio, high-resolution echelle spectroscopy. Internal uncertainties in the derived abundances are typically 0.05?dex. The stars in our sample have all been previously shown to have abundances that correlate with the condensation temperature (T c) of the elements in the sense of increasing abundances with increasing T c; these trends have been interpreted as evidence that the stars may have accreted H-depleted planetary material. Our newly derived abundances also correlate positively with T c, although slopes of linear least-square fits to the [m/H]-T c relations for all but two stars are smaller here than in previous studies. When considering the refractory elements (T c >900 K) only, which may be more sensitive to planet formation processes, the sample can be separated into a group with positive slopes (four stars) and a group with flat or negative slopes (six stars). The four stars with positive slopes have very close-in giant planets (three at 0.05 AU) and slopes that fall above the general Galactic chemical evolution trend. We suggest that these stars have accreted refractory-rich planet material but not to the extent that would increase significantly the overall stellar metallicity. The flat or negative slopes of the remaining six stars are consistent with recent suggestions of a planet formation signature, although we show that the trends may be the result of Galactic chemical evolution.