Abundances in Stars from the Red Giant Branch Tip to the Near Main Sequence in M71: II. Iron Abundance

We present [Ffe/H] abundance results that involve a sample of stars with a wide range in luminosity from luminous giants to stars near the turnoff in a globular cluster. Our sample of 25 stars in M71 includes 10 giant stars more luminous than the RHB, 3 horizontal branch stars, 9 giant stars less luminous than the RHB, and 3 stars near the turnoff. We analyzed both Fe I and Fe II lines in high dispersion spectra observed with HIRES at the W. M. Keck Observatory. We find that the [Fe/H] abundances from both Fe I and Fe II lines agree with each other and with earlier determinations. Also the [Fe/H] obtained from Fe I and Fe II lines is constant within the rather small uncertainties for this group of stars over the full range in Teff and luminosity, suggesting that NLTE effects are negligible in our iron abundance determination. In this globular cluster, there is no difference among the mean [Fe/H] of giant stars located at or above the RHB, RHB stars, giant stars located below the RHB and stars near the turnoff.Comment: Minor changes to conform to version accepted for publication, with several new figures (Paper 2 of a pair

Stellar Iron Abundances: non-LTE Effects

We report new statistical equilibrium calculations for Fe I and Fe II in the atmosphere of Late-Type stars. We used atomic models for Fe I and Fe II having respectively 256 and 190 levels, as well as 2117 and 3443 radiative transitions. Photoionization cross-sections are from the Iron Project. These atomic models were used to investigate non-LTE effects in iron abundances of Late-Type stars with different atmospheric parameters. We found that most Fe I lines in metal-poor stars are formed in conditions far from LTE. We derived metallicity corrections of about 0.3 dex with respect to LTE values, for the case of stars with [Fe/H] ~ -3.0. Fe II is found not to be affected by significant non-LTE effects. The main non-LTE effect invoked in the case of Fe I is overionization by ultraviolet radiation, thus classical ionization equilibrium is far to be satisfied. An important consequence is that surface gravities derived by LTE analysis are in error and should be corrected before final abundances corrections. This apparently solves the observed discrepancy between spectroscopic surface gravities derived by LTE analyses and those derived from Hipparcos parallaxes. A table of non-LTE [Fe/H] and log g values for a sample of metal-poor late-type stars is given.Comment: 22 pages, 9 figures, 1 table, ApJ style, accepte

VINCI / VLTI observations of Main Sequence stars

Main Sequence (MS) stars are by far the most numerous class in the Universe. They are often somewhat neglected as they are relatively quiet objects (but exceptions exist), though they bear testimony of the past and future of our Sun. An important characteristic of the MS stars, particularly the solar-type ones, is that they host the large majority of the known extrasolar planets. Moreover, at the bottom of the MS, the red M dwarfs pave the way to understanding the physics of brown dwarfs and giant planets. We have measured very precise angular diameters from recent VINCI/VLTI interferometric observations of a number of MS stars in the K band, with spectral types between A1V and M5.5V. They already cover a wide range of effective temperatures and radii. Combined with precise Hipparcos parallaxes, photometry, spectroscopy as well as the asteroseismic information available for some of these stars, the angular diameters put strong constraints on the detailed models of these stars, and therefore on the physical processes at play.Comment: 5 pages, 3 figures. To appear in the Proceedings of IAU Symposium 219, "Stars as Suns", Editors A. Benz & A. Dupree, Astronomical Society of the Pacifi

Abundances in Stars from the Red Giant Branch Tip to Near the Main Sequence Turn Off in M71: III. Abundance Ratios

We present abundance ratios for 23 elements with respect to Fe in a sample of stars with a wide range in luminosity, from luminous giants to stars near the turnoff, in the globular cluster M71. The analyzed spectra, obtained with HIRES at the Keck Observatory, are of high dispersion (R=35,000). We find that the neutron capture, the iron peak and the alpha-element abundance ratios show no trend with Teff, and low scatter around the mean between the top of the RGB and near the main sequence turnoff. The alpha-elements Mg, Ca, Si and Ti are overabundant relative to Fe. The anti-correlation between O and Na abundances, observed in other metal poor globular clusters, is detected in our sample and extends to the main sequence. A statistically significant correlation between Al and Na abundances is observed among the M71 stars in our sample, extending to Mv = +1.8, fainter than the luminosity of the RGB bump in M5. Lithium is varying, as expected, and Zr may be varying from star to star as well. M71 appears to have abundance ratios very similar to M5 whose bright giants were studied by Ivans et al. (2001), but seems to have a smaller amplitude of star-to-star variations at a given luminosity, as might be expected from its higher metallicity. The results of our abundance analysis of 25 stars in M71 provide sufficient evidence of abundance variations at unexpectedly low luminosities to rule out the mixing scenario. Either alone or, even more powerfully, combined with other recent studies of C and N abundances in M71 stars, the existence of such abundance variations cannot be reproduced within the context of our current understanding of stellar evolution.Comment: AJ, in press (June 2002), 18 figure

Oxygen Abundances in Two Metal-Poor Subgiants from the Analysis of the 6300 A Forbidden O I Line

Recent LTE analyses (Israelian et al. 1998 and Bosegaard et al. 1999) of the OH bands in the optical-ultraviolet spectra of nearby metal-poor subdwarfs indicate that oxygen abundances are generally higher than those previously determined. The difference increases with decreasing metallicity and reaches delta([O/Fe]) ~ +0.6 dex as [Fe/H] approaches -3.0. Employing high resolution (R = 50000), high S/N (~ 250) echelle spectra of the two stars found by Israelian et al. (1998) to have the highest [O/Fe]-ratios, viz, BD +23 3130 and BD +37 1458, we conducted abundance analyses based on about 60 Fe I and 7-9 Fe II lines. We determined from Kurucz LTE models the values of the stellar parameters, as well as abundances of Na, Ni, and the traditional alpha-elements, independent of the calibration of color vs $T_{eff}$ scales. We determined oxygen abundances from spectral synthesis of the stronger line (6300 A) of the [O I] doublet. The syntheses of the [O I] line lead to smaller values of [O/Fe], consistent with those found earlier among halo field and globular cluster giants. We obtain [O/Fe] = +0.35 +/- 0.2 for BD +23 3130 and +0.50 +/- 0.2 for BD +37 1458. In the former, the [O I] line is very weak (~ 1 mA), so that the quoted [O/Fe] value may in reality be an upper limit. Therefore in these two stars a discrepancy exists between the [O/Fe]- ratios derived from [O I] and the OH feature, and the origin of this difference remains unclear. Until the matter is clarified, we suggest it is premature to conclude that the ab initio oxygen abundances of old, metal-poor stars need to be revised drastically upward.Comment: 38 pages, 5 tables, 14 figures To appear in July 1999 AJ Updated April 16, 1999. Fixed typo

A toolkit of mechanism and context independent widgets

Most human-computer interfaces are designed to run on a static platform (e.g. a workstation with a monitor) in a static environment (e.g. an office). However, with mobile devices becoming ubiquitous and capable of running applications similar to those found on static devices, it is no longer valid to design static interfaces. This paper describes a user-interface architecture which allows interactors to be flexible about the way they are presented. This flexibility is defined by the different input and output mechanisms used. An interactor may use different mechanisms depending upon their suitability in the current context, user preference and the resources available for presentation using that mechanism

Perceptions of Leadership from Hispanic, Latino, and Spanish Employees in the Department of Defense

The purpose of this study is to identify the satisfaction levels within the Department of Defense (DOD) of Hispanic, Latino, or Spanish (HLS) employees. The Office of Personnel Management\u27s (OPM) 2020 Federal Employee Viewpoint Survey (FEVS) is used to identify the perspectives and attitudes of HLS employees as they pertain to organizational and leadership satisfaction. These perspectives are conceptualized using the Leader-Member Exchange (LMX) theory, which points to the relationships among these workplace actors as a principal characteristic of workplace satisfaction and intentions to leave an organization. The sample size acquired from the 2020 FEVS, administered from September 16th, 2020, to October 28th, 2020, includes 12,854 permanent, active, and non-seasonal HLS employees employed as of August 31st, 2019. The dataset acquired from OPM was examined using SPSS 29, which included data analysis using a quantitative methodology and a non-experimental correlational research design. Data analysis shows a positive correlation between leadership, organizational satisfaction, and intent to leave. The implications, future research, and discussion are addressed in the context of the public administration field

Structure and Evolution of Nearby Stars with Planets. I. Short-Period Systems

Using the Yale stellar evolution code, we have calculated theoretical models for nearby stars with planetary-mass companions in short-period nearly circular orbits: 51 Pegasi, Tau Bootis, Upsilon Andromedae, Rho Cancri, and Rho Coronae Borealis. We present tables listing key stellar parameters such as mass, radius, age, and size of the convective envelope as a function of the observable parameters (luminosity, effective temperature, and metallicity), as well as the unknown helium fraction. For each star we construct best models based on recently published spectroscopic data and the present understanding of galactic chemical evolution. We discuss our results in the context of planet formation theory, and, in particular, tidal dissipation effects and stellar metallicity enhancements.Comment: 48 pages including 13 tables and 5 figures, to appear in Ap

s-Process Nucleosynthesis in Carbon Stars

We present the first detailed and homogeneous analysis of the s-element content in Galactic carbon stars of N-type. Abundances of Sr,Y, Zr (low-mass s-elements, or ls) and of Ba, La, Nd, Sm and Ce (high-mass s-elements, hs) are derived using the spectral synthesis technique from high-resolution spectra. The N-stars analyzed are of nearly solar metallicity and show moderate s-element enhancements, similar to those found in S stars, but smaller than those found in the only previous similar study (Utsumi 1985), and also smaller than those found in supergiant post-AGB stars. This is in agreement with the present understanding of the envelope s-element enrichment in giant stars, which is increasing along the spectral sequence M-->MS-->S-->SC-->C during the AGB phase. We compare the observational data with recent $s$-process nucleosynthesis models for different metallicities and stellar masses. Good agreement is obtained between low mass AGB star models (M < 3 M_o) and s-elements observations. In low mass AGB stars, the 13C(alpha, n)16O reaction is the main source of neutrons for the s-process; a moderate spread, however, must exist in the abundance of 13C that is burnt in different stars. By combining information deriving from the detection of Tc, the infrared colours and the theoretical relations between stellar mass, metallicity and the final C/O ratio, we conclude that most (or maybe all) of the N-stars studied in this work are intrinsic, thermally-pulsing AGB stars; their abundances are the consequence of the operation of third dredge-up and are not to be ascribed to mass transfer in binary systems.Comment: 31 pages, 10 figures, 6 tables. Accepted in Ap