30 research outputs found
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
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
Do dusty A stars exhibit accretion signatures in their photospheres?
We determined abundances of O, Ca, Fe, Ba and Y for a sample of dusty and
dust-free A stars, taken from the list of Cheng et al. (1992). Five of the
stars have an infrared-excess due to circumstellar dust. Ongoing accretion from
their circumstellar surroundings might have modified the abundances in the
photospheres of these stars, but our results clearly show, that there is no
difference in the photospheric composition of the dusty and dust-free stars.
Instead all of them show the typical diffusion pattern which diminishes towards
larger rotational velocities.Comment: 8 pages, 3 figures, accepted for publication in A&
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
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
Oxygen in the Very Early Galaxy
Oxygen abundances in a sample of ultra-metal-poor subdwarfs have been derived
from measurements of the oxygen triplet at 7771--5 A and OH lines in the near
UV performed in high-resolution and high signal-to-noise spectra obtained with
WHT/UES, KeckI/HIRES, and VLT/UVES. Our Fe abundances were derived in LTE and
then corrected for NLTE effects following Thevenin and Idiart (1999). The new
oxygen abundances confirm previous findings for a progressive linear rise in
the oxygen-to-iron ratio with a slope -0.33+-0.02 from solar metallicity to
[Fe/H] -3. A slightly higher slope would be obtained if the Fe NLTE corrections
were not considered. Below [Fe/H]= -2.5 our stars show [O/Fe] ratios as high as
~ 1.17 (G64-12), which can be interpreted as evidence for oxygen overproduction
in the very early epoch of the formation of the halo, possibly associated with
supernova events with very massive progenitor stars. We show that the arguments
against this linear trend given by Fulbright and Kraft (1999), based on the LTE
Fe analysis of two metal-poor stars cannot be sustained when an NLTE analysis
is performed. Using 1-D models our analysis of three oxygen indicators
available for BD +23 3130 gives consistent abundances within 0.16 dex and
average [O/Fe] ratio of 0.91.Comment: 45 pages, 11 figures, accepted for publication in The Astrophysical
Journa