Jupiters radiation belts and their effects on spacecraft

The effects of electron and proton radiation on spacecraft which will operate in the trapped radiation belts of the planet Jupiter are described, and the techniques and results of the testing and simulation used in the radiation effects program are discussed. Available data from the Pioneer 10 encounter of Jupiter are compared with pre-encounter models of the Jupiter radiation belts. The implications that the measured Jovian radiation belts have for future missions are considered

UV Spectral Synthesis of Vega

We show that the UV spectrum (1280-3200 A) of the "superficially normal" A-star Vega, as observed by the IUE satellite at a resolution comparable to the star's rotational broadening width, can be fit remarkably well by a single-temperature synthetic spectrum based on LTE atmosphere models and a newly constructed UV line list. If Vega were a normal, equator-on, slow-rotating star, then its spectrum and our analysis would indicate a temperature of Teff ~ 9550 K, surface gravity of log g ~ 3.7, general surface metallicity of [m/H] ~ -0.5, and a microturbulence velocity of v(turb) ~ 2.0 km/s. Given its rapid rotation and nearly pole-on orientation, however, these parameters must be regarded as representing averages across the observed hemisphere. Modeling the complex UV line spectrum has allowed us to determine the specific surface abundances for 17 different chemical elements, including CNO, the light metals, and the iron group elements. The resultant abundance pattern agrees in general with previous results, although there is considerable scatter in the literature. Despite its peculiarities, Vega has turned out to provide a powerful test of the extent of our abilities to model the atmospheric properties of the early A-stars, particularly the detailed UV line spectrum. The value of the measurements from this pilot study will increase as this analysis is extended to more objects in the rich high-dispersion IUE data archive, including both normal and peculiar objects.Comment: To appear in the Astrophysical Journa

First Stellar Abundances in the Dwarf Irregular Galaxy Sextans A

We present the abundance analyses of three isolated A-type supergiant stars in the dwarf irregular galaxy Sextans A from high-resolution spectra the UVES spectrograph at the VLT. Detailed model atmosphere analyses have been used to determine the stellar atmospheric parameters and the elemental abundances of the stars. The mean iron group abundance was determined from these three stars to be [(FeII,CrII)/H]=-0.99+/-0.04+/-0.06. This is the first determination of the present-day iron group abundances in Sextans A. These three stars now represent the most metal-poor massive stars for which detailed abundance analyses have been carried out. The mean stellar alpha element abundance was determined from the alpha element magnesium as [alpha(MgI)/H]=-1.09+/-0.02+/-0.19. This is in excellent agreement with the nebular alpha element abundances as determined from oxygen in the H II regions. These results are consistent from star-to-star with no significant spatial variations over a length of 0.8 kpc in Sextans A. This supports the nebular abundance studies of dwarf irregular galaxies, where homogeneous oxygen abundances are found throughout, and argues against in situ enrichment. The alpha/Fe abundance ratio is [alpha(MgI)/FeII,CrII]=-0.11+/-0.02+/-0.10, which is consistent with the solar ratio. This is consistent with the results from A-supergiant analyses in other Local Group dwarf irregular galaxies but in stark contrast with the high [alpha/Fe] results from metal-poor stars in the Galaxy, and is most clearly seen from these three stars in Sextans A because of their lower metallicities. The low [alpha/Fe] ratios are consistent with the slow chemical evolution expected for dwarf galaxies from analyses of their stellar populations.Comment: 40 pages, 8 figures, accepted for publication in A

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

The HgMn Binary Star Phi Herculis: Detection and Properties of the Secondary and Revision of the Elemental Abundances of the Primary

Observations of the Mercury-Manganese star Phi Herculis with the Navy Prototype Optical Interferometer (NPOI) conclusively reveal the previously unseen companion in this single-lined binary system. The NPOI data were used to predict a spectral type of A8V for the secondary star Phi Her B. This prediction was subsequently confirmed by spectroscopic observations obtained at the Dominion Astrophysical Observatory. Phi Her B is rotating at 50 +/-3 km/sec, in contrast to the 8 km/sec lines of Phi Her A. Recognizing the lines from the secondary permits one to separate them from those of the primary. The abundance analysis of Phi Her A shows an abundance pattern similar to those of other HgMn stars with Al being very underabundant and Sc, Cr, Mn, Zn, Ga, Sr, Y, Zr, Ba, Ce, and Hg being very overabundant.Comment: Accepted to ApJ, 45 pages, 11 figure

Analysis of Four A-F Supergiants in M31 from Keck HIRES Spectroscopy

The first stellar abundances in M31 are presented, based on Keck I HIRES spectroscopy and model atmospheres analyses of three A-F supergiants, 41-2368, 41-3712, and A-207. We also present the preliminary analysis of a fourth star, 41-3654. We find that the stellar oxygen abundances are in good agreement with those determined from nebular studies, even though the stars do {\it not} show a clear radial gradient in oxygen. The uncertainties in the stellar abundances are smaller than the range in the nebular results, making these stars ideal objects for further studies of the distribution of oxygen in M31. We show that the stars can be used to study the abundance distributions of other elements as well, including iron-group and heavier elements. The A-F supergiants also provide direct information on the metallicity and reddening of nearby Cepheid stars. We have examined the metallicity and reddening assumptions used for Cepheids within 1' of our targets and noted the differences from values used in the literature.Comment: Accepted for publication in the ApJ, October 2000 (23 pages, 4 tables, 11 figures

The chemical abundance analysis of normal early A- and late B-type stars

Modern spectroscopy of early-type stars often aims at studying complex physical phenomena. Comparatively less attention is paid to identifying and studying the "normal" A- and B-type stars and testing how the basic atomic parameters and standard spectral analysis allow one to fit the observations. We wish to stablish whether the chemical composition of the solar photosphere can be regarded as a reference for early A- and late B-type stars. We have obtained optical high-resolution, high signal-to-noise ratio spectra of three slowly rotating early-type stars (HD 145788, 21 Peg and pi Cet) that show no obvious sign of chemical peculiarity, and performed a very accurate LTE abundance analysis of up to 38 ions of 26 elements (for 21 Peg), using a vast amount of spectral lines visible in the spectral region covered by our spectra. We provide an exhaustive description of the abundance characteristics of the three analysed stars with a critical review of the line parameters used to derive the abundances. We compiled a table of atomic data for more than 1100 measured lines that may be used in the future as a reference. The abundances we obtained for He, C, Al, S, V, Cr, Mn, Fe, Ni, Sr, Y, and Zr are compatible with the solar ones derived with recent 3D radiative-hydrodynamical simulations of the solar photosphere. The abundances of the remaining studied elements show some degree of discrepancy compared to the solar photosphere. Those of N, Na, Mg, Si, Ca, Ti, and Nd may well be ascribed to non-LTE effects; for P, Cl, Sc and Co, non-LTE effects are totally unknown; O, Ne, Ar, and Ba show discrepancies that cannot be ascribed to non-LTE effects. The discrepancies obtained for O (in two stars) and Ne agree with very recent non-LTE abundance analysis of early B-type stars in the solar neighbourhood.Comment: Accepted for publication on Astronomy and Astrophysic

The Chemical Composition of Two Supergiants in the Dwarf Irregular Galaxy WLM

[Abridged] The chemical composition of two stars in WLM has been determined from high quality UVES data obtained at the VLT UT2 (program 65.N-0375). The model atmospheres analysis shows that they have the same metallicity, [Fe/H] = -0.38 +/-0.20, and [Mg/Fe] = -0.24 +/-0.16. This result suggests that the [alpha(Mg)/Fe] ratio in WLM may be suppressed relative to solar abundances (also supported by differential abundances relative to similar stars in NGC6822 and the SMC). The absolute Mg abundance, [Mg/H] = -0.62 is high relative to what is expected from the nebulae though, where two independent spectroscopic analyses of the HII regions in WLM yield [O/H] = -0.89. Intriguingly, the oxygen abundance determined from the OI 6158 feature in one WLM star is [O/H] = -0.21 +/-0.10, corresponding to five times higher than the nebular oxygen abundance. This is the first time that a significant difference between young stellar and nebular oxygen abundances has been found, and presently, there is no simple explanation for this difference. If the stellar abundances reflect the true composition of WLM, then this galaxy lies well above the metallicity-luminosity relationship for dwarf irregular galaxies. It also suggests that WLM is more chemically evolved than currently interpreted from its color-magnitude diagram.Comment: 27 pages, 7 tables, 10 figures. accepted for publication in the Astronomical Journa

Abundance analysis of prime B-type targets for asteroseismology II. B6--B9.5 stars in the field of view of the CoRoT

The CoRoT satellite is collecting precise time-resolved photometry for tens of asteroseismology targets. To ensure a correct interpretation of the CoRoT data, the atmospheric parameters, chemical compositions, and rotational velocities of the stars must be determined. The main goal of the ground-based seismology support program for the CoRoT mission was to obtain photometric and spectroscopic data for stars in the fields monitored by the satellite. These ground-based observations were collected in the GAUDI archive. High-resolution spectra of more than 200 B-type stars are available in this database, and about 45% of them is analysed here. To derive the effective temperature of the stars, we used photometric indices. Surface gravities were obtained by comparing observed and theoretical Balmer line profiles. To determine the chemical abundances and rotational velocities, we used a spectrum synthesis method, which consisted of comparing the observed spectrum with theoretical ones based on the assumption of LTE. Atmospheric parameters, chemical abundances, and rotational velocities were determined for 89 late-B stars. The dominant species in their spectra are iron-peak elements. The average Fe abundance is 7.24+/-0.45 dex. The average rotational velocity is 126 km/sec, but there are 13 and 20 stars with low and moderate Vsin i values, respectively. The analysis of this sample of 89 late B-type stars reveals many chemically peculiar (CP) stars. Some of them were previously known, but at least 9 new CP candidates, among which at least two HgMn stars, are identified in our study. These CP stars as a group exhibit Vsin i values lower than the stars with normal surface chemical composition.Comment: 21 pages, 13 figures, accepted to Astronomy and Astrophysic

The effect of rotation on the spectrum of Vega

The discovery that Vega is a rapidly rotating pole-on star has raised a number of questions about this fundamental standard, including such issues as its composition, and in turn its mass and age. We report here a reanalysis of Vega's composition. A full spectral synthesis based on the Roche model derived earlier from NPOI interferometry is used. We find the line shapes in Vega's spectrum to be more complex than just flat-bottomed, which have been previously reported; profiles range from slightly self-reversed to simple ``V'' shapes. A high SNR spectrum, obtained by stacking spectra from the ELODIE archive, shows excellent agreement with the calculations, provided we add about 10 km/s of macroturbulence to the predicted spectra. From the abundance analysis, we find that Vega shows the peculiar abundance pattern of a \W Bootis star as previously suggested. We investigate the effects of rotation on the deduced abundances and show that the dominant ionization states are only slightly affected compared to analyses using non-rotating models. We argue that the rapid rotation requires the star be fully mixed. The composition leads to masses and particularly ages that are quite different compared to what are usually assumed.Comment: 16 pages, 4 figures, accepted by Ap