Beryllium Abundances in Stars of One-Solar-Mass

We have determined Be abundances in 50 F and G dwarfs in the mass range of 0.9 $\leq$ M$_\odot$ $\leq$ 1.1 as determined by Lambert & Reddy. The effective temperatures are 5600 to 6400 K and metallicities from $-$0.65 to +0.11. The spectra were taken primarily with Keck I + HIRES. The Be abundances were found via spectral synthesis of Be II lines near 3130 \AA. The Be abundances were investigated as a function of age, temperature, metallicity and Li abundance in this narrow mass range. Even though our stars are similar in mass, they show a range in Be abundances of a factor of $>$40. We find that [Be/Fe] has no dependence on temperature, but does show a spread of a factor of 6 at a given temperature. The reality of the spread is shown by two identical stars which differ from each other by a factor of two only in their abundances of Li and Be. Our thin-disk-star sample fits the trend between Be abundance and [Fe/H] found for halo and thick disk stars, extending it to about 4 orders of magnitude in the two logarithmic quantities. Both Fe and Be appear to increase similarly over time in the Galaxy. One-third of our sample may be classified as subgiants; these more-evolved stars have lower Be abundances than the dwarfs. They have undergone Be depletion by slow mixing on the main sequence and Be dilution during their trip toward the red giant base. There are both Li and Be detections in 60 field stars in the "Li-plateau" of 5900 - 6300 K now and the abundances of the two light elements are correlated with a slope of 0.34 $\pm$0.05, with greater Li depletion than Be depletion.Comment: 38 pages, 14 figures, 5 tables Astrophysical Journal, Accepted Sep. 19, 200

Beryllium and Alpha-Element Abundances in a Large Sample of Metal-Poor Stars

The light elements, Li, Be, and B, provide tracers for many aspects of astronomy including stellar structure, Galactic evolution, and cosmology. We have taken spectra of Be in 117 metal-poor stars ranging in metallicity from [Fe/H] = -0.5 to -3.5 with Keck I + HIRES at a resolution of 42,000 and signal-to-noise ratios of near 100. We have determined the stellar parameters spectroscopically from lines of Fe I, Fe II, Ti I and Ti II. The abundances of Be and O were derived by spectrum synthesis techniques, while abundances of Fe, Ti, and Mg were found from many spectral line measurements. There is a linear relationship between [Fe/H] and A(Be) with a slope of +0.88 +-0.03 over three orders of magnitude in [Fe/H]. We fit the relationship between A(Be) and [O/H] with both a single slope and with two slopes. The relationship between [Fe/H] and [O/H] seems robustly linear and we conclude that the slope change in Be vs. O is due to the Be abundance. Although Be is a by-product of CNO, we have used Ti and Mg abundances as alpha-element surrogates for O in part because O abundances are rather sensitive to both stellar temperature and surface gravity. We find that A(Be) tracks [Ti/H] very well with a slope of 1.00 +-0.04. It also tracks [Mg/H] very well with a slope of 0.88 +-0.03. We find that there are distinct differences in the relationships of A(Be) and [Fe/H] and of A(Be) and [O/H] for our dissipative stars and our accretive stars. We suggest that the Be in the dissipative stars was primarily formed by GCR spallation and Be in the accretive stars was formed in the vicinity of SN II.Comment: Accepted for Ap.J. Nov. 10, 2011, v. 741 70 pages, 27 figures, 5 table

Rotational modulation of the chromospheric activity in the young solar-type star, X-1 Orionis

The IUE satellite was used to observe one of the youngest G stars (GO V) for which Duncan (1981) derives an age of 6 x 10 to the 8th power years from the Li abundance. Rotational modulation was looked for in the emission flux in the chromospheric and transition region lines of this star. Variations in the Ca 11 K-lines profile were studied with the CHF telescope at Mauna Kea. Results show that the same modulation of the emission flux of Ca 11 due to stellar rotation is present in the transition region feature of C IV and probably of He II. For other UV lines the modulation is not apparent, due to a more complex surface distribution of the active areas or supergranulation network, or a shorter lifetime of the conditions which give rise to these features, or to the uncertainities in the measured line strengths. The Mg II emission flux is constant to within + or - 3.4% implying a rather uniform distribution of Mg II emission areas. The Ca II emission not only shows a measurable variation in intensity but also variations in detailed line profile shape when observed at high resolution

IUE observations of the chromospheric activity-age relation in young solar-type stars

Ultraviolet data obtained with the IUE spacecraft are presented for a dozen solar-type stars in the field. The stars are of spectral type F6 V - G1 V; on the basis of their high Li content, they range in age from 0.1 to 2.8 Gyr. The evolution of transition regions and chromospheric emission with stellar age is studied along with the surface distribution of magnetically active regions as revealed by rotational modulation of UV emission line fluxes

Gyroscopic pumping of large-scale flows in stellar interiors, and application to Lithium Dip stars

The maintenance of large-scale differential rotation in stellar convective regions by rotationally influenced convective stresses also drives large-scale meridional flows by angular--momentum conservation. This process is an example of ``gyroscopic pumping'', and has recently been studied in detail in the solar context. An important question concerns the extent to which these gyroscopically pumped meridional flows penetrate into nearby stably stratified (radiative) regions, since they could potentially be an important source of non-local mixing. Here we present an extensive study of the gyroscopic pumping mechanism, using a combination of analytical calculations and numerical simulations both in Cartesian geometry and in spherical geometry. The various methods, when compared with one another, provide physical insight into the process itself, as well as increasingly sophisticated means of estimating the gyroscopic pumping rate. As an example of application, we investigate the effects of this large-scale mixing process on the surface abundances of the light elements Li and Be for stars in the mass range 1.3-1.5 solar masses (so-called ``Li-dip stars''). We find that gyroscopic pumping is a very efficient mechanism for circulating material between the surface and the deep interior, so much in fact that it over-estimates Li and Be depletion by orders of magnitude for stars on the hot side of the dip.However, when the diffusion of chemical species back into the surface convection zone is taken into account, a good fit with observed surface abundances of Li and Be as a function of stellar mass in the Hyades cluster can be found for reasonable choices of model parameters.Comment: Submitted to Ap

New Keck Observations of Lithium in Very Metal-poor Stars

Lithium abundances have been determined in more than 100 metal-poor halo stars both in the field and in clusters. From these data we find trends of Li with both temperature and metallicity and a real dispersion in Li abundances in the Spite Li plateau. We attribute this dispersion primarily to Li depletion (presumably due to extra mixing induced by stellar rotation) and to Galactic chemical evolution. We derive a primordial Li of 2.44 $\pm$0.18 for A(Li)$_p$ = log N(Li/H) + 12.00. This agrees with the Li abundances predicted by the $WMAP$ results. For stars cooler than the Li plateau we have evidence that Li depletion sets in at hotter temperatures for the higher metallicity stars than for the low-metal stars. This is the opposite sense of predictions from stellar models. The smooth transition of the Li content from the Li plateau stars to the cool stars adds weight to the inference of Li depletion in the plateau stars.Comment: Invited talk for IAU Symposium 228 "From Lithium to Uranium..." held in Paris in May, 2005. 6 pages, 6 figure

Tracing mixing in stars: new beryllium observations of the open clusters NGC 2516, Hyades, and M67

Determinations of beryllium abundance in stars, together with lithium, provide a key tool to investigate the so far poorly understood extra-mixing processes at work in stellar interiors. We measured Be in three open clusters,complementing existing Be surveys, and aiming at gathering a more complete empirical scenario of the evolution of Be as a function of stellar age and temperature. Specifically, we analyzed VLT/UVES spectra of members of NGC 2516, the Hyades, and M 67 to determine their Be and Li abundances. In the first two clusters we focused on stars cooler than 5400 K, while the M 67 sample includes stars warmer than 6150 K, as well as two subgiants and two blue stragglers. We also computed the evolution of Be for a 0.9 Mo star based on standard evolutionary models. We find different emprical behaviours for stars in different temperature bins and ages. Stars warmer than 6150 K show Be depletion and follow a Be vs. Li correlation while Be is undepleted in stars in the ~6150-5600 K range. NGC 2516 members cooler than 5400 K have not depleted any Be, but older Hyades of similar temperature do show some depletion. Be is severely depleted in the subgiants and blue stragglers. The results for warm stars are in agreement with previous studies, supporting the hypothesis that mixing in this temperature regime is driven by rotation. The same holds for the two subgiants that have evolved from the "Li gap". This mechanism is instead not the dominant one for solar-type stars. We show that Be depletion of cool Hyades cannot simply be explained by the effect of increasing depth of the convective zone. Finally, the different Be content of the two blue stragglers suggests that they have formed by two different processes (i.e., collisions vs. binary merging).Comment: 19 pages, 10 figures, accepted for publication in Astronomy & Astrophysic