2,379 research outputs found
A photon-counting photodiode array detector for far ultraviolet (FUV) astronomy
A compact, stable, single-stage intensified photodiode array detector designed for photon-counting, far ultraviolet astronomy applications employs a saturable, 'C'-type MCP (Galileo S. MCP 25-25) to produce high gain pulses with a narrowly peaked pulse height distribution. The P-20 output phosphor exhibits a very short decay time, due to the high current density of the electron pulses. This intensifier is being coupled to a self-scanning linear photodiode array which has a fiber optic input window which allows direct, rigid mechanical coupling with minimal light loss. The array was scanned at a 250 KHz pixel rate. The detector exhibits more than adequate signal-to-noise ratio for pulse counting and event location
Evidence for deuterium astration in the planetary nebula Sh2-216?
We present FUSE observations of the line of sight to WD0439+466 (LS V +46
21), the central star of the old planetary nebula Sh2-216. The FUSE data shows
absorption by many interstellar and stellar lines, in particular D I, H2 (J = 0
- 9), HD (J = 0 - 1), and CO. Many other stellar and ISM lines are detected in
the STIS E140M HST spectra of this sightline, which we use to determine N(HI).
We derive, for the neutral gas, D/H=(0.76 +0.12 -0.11)E-5, O/H = (0.89 +0.15
-0.11)E-4 and N/H = (3.24 +0.61-0.55)E-5. We argue that most of the gas along
this sightline is associated with the planetary nebula. The low D/H ratio is
likely the result of this gas being processed through the star (astrated) but
not mixed with the ISM. This would be the first time that the D/H ratio has
been measured in predominantly astrated gas. The O/H and N/H ratios derived
here are lower than typical values measured in other planetary nebulae likely
due to unaccounted for ionization corrections.Comment: Accepted for publication is ApJ
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Evidence for neutral beam injected oxygen impurities in 2XIIB
A series of experiments indicates that the principal source of impurities in the 2XIIB mirror confinement plasma experiment at Lawrence Livermore Laboratory is oxygen in the neutral beams. The dependence of 0 II 539 A emissions on neutral beam current, spatial scans of oxygen emissions, impurity injection experiments, spectral scans of the 0 VI 1032 A line, and other experiments all support this conclusion
The deuterium-to-oxygen ratio in the interstellar medium
Because the ionization balances for HI, OI, and DI are locked together by
charge exchange, D/O is an important tracer for the value of the D/H ratio and
for potential spatial variations in the ratio. As the DI and OI column
densities are of similar orders of magnitude for a given sight line,
comparisons of the two values will generally be less subject to systematic
errors than comparisons of DI and HI, which differ by about five orders of
magnitude. Moreover, D/O is additionally sensitive to astration, because as
stars destroy deuterium, they should produce oxygen. We report here the results
of a survey of D/O in the interstellar medium performed with FUSE. We also
compare these results with those for D/N. Together with a few results from
previous missions, the sample totals 24 lines of sight. The distances range
from a few pc to ~2000 pc and log N(DI) from ~13 to ~16 (cm-2). The D/O ratio
is constant in the local interstellar medium out to distances of ~150 pc and
N(DI) ~ 1x10^15 cm-2, i.e. within the Local Bubble. In this region of the
interstellar space, we find D/O = (3.84+/-0.16)x10^-2 (1 sigma in the mean).
The homogeneity of the local D/O measurements shows that the spatial variations
in the local D/H and O/H must be extremely small, if any. A comparison of the
Local Bubble mean value with the few D/O measurements available for low
metallicity quasar sight lines shows that the D/O ratio decreases with cosmic
evolution, as expected. Beyond the Local Bubble we detected significant spatial
variations in the value of D/O. This likely implies a variation in D/H, as O/H
is known to not vary significantly over the distances covered in this study.
Our dataset suggests a present-epoch deuterium abundance below 1x10^-5, i.e.
lower than the value usually assumed, around 1.5x10^-5.Comment: 17 pages, 9 figures, 4 tables, accepted for publication in the
Astrophysical Journa
The D/H Ratio in the Interstellar Medium toward the White Dwarf PG0038+199
We determine the D/H ratio in the interstellar medium toward the DO white
dwarf PG0038+199 using spectra from the Far Ultraviolet Spectroscopic Explorer
(FUSE), with ground-based support from Keck HIRES. We employ curve of growth,
apparent optical depth and profile fitting techniques to measure column
densities and limits of many other species (H2, NaI, CI, CII, CIII, NI, NII,
OI, SiII, PII, SIII, ArI and FeII) which allow us to determine related ratios
such as D/O, D/N and the H2 fraction. Our efforts are concentrated on measuring
gas-phase D/H, which is key to understanding Galactic chemical evolution and
comparing it to predictions from Big Bang nucleosynthesis. We find column
densities log N(HI) = 20.41+-0.08, log N(DI)=15.75+-0.08 and log N(H2) =
19.33+-0.04, yielding a molecular hydrogen fraction of 0.14+-0.02 (2 sigma
errors), with an excitation temperature of 143+-5K. The high HI column density
implies that PG0038+199 lies outside of the Local Bubble; we estimate its
distance to be 297 (+164,-104)pc (1 sigma). D/[HI+2H2] toward PG0038+199 is
1.91(+0.52,-0.42) e-5 (2 sigma). There is no evidence of component structure on
the scale of Delta v > 8 km/s based on NaI, but there is marginal evidence for
structure on smaller scales. The D/H value is high compared to the majority of
recent D/H measurements, but consistent with the values for two other
measurements at similar distances. D/O is in agreement with other distant
measurements. The scatter in D/H values beyond ~100pc remains a challenge for
Galactic chemical evolution.Comment: 59 pages, 7 tables, 18 figures (1 standalone), accepted by ApJ v2
minor typos correcte
Oxygen Gas Phase Abundance Revisited
We present new measurements of the interstellar gas-phase oxygen abundance
along the sight lines towards 19 early-type galactic stars at an average
distance of 2.6 kpc. We derive O {\small I} column densities from {\it
HST}/STIS observations of the weak 1355 \AA intersystem transition. We derive
total hydrogen column densities [N(H {\small I})+2N(H)] using {\it
HST}/STIS observations of \lya and {\it FUSE} observations of molecular
hydrogen. The molecular hydrogen content of these sight lines ranges from
f(H) = 2N(H)/[N(H {\small I})+2N(H)] = 0.03 to 0.47. The average
of 6.3 cm mag with a standard
deviation of 15% is consistent with previous surveys. The mean oxygen abundance
along these sight lines, which probe a wide range of galactic environments in
the distant ISM, is 10 \oh = (1 in the mean). %(1 ). We see no evidence for decreasing
gas-phase oxygen abundance with increasing molecular hydrogen fraction and the
relative constancy of \oh suggests that the component of dust containing the
oxygen is not readily destroyed. We estimate that, if 60% of the dust grains
are resilient against destruction by shocks, the distant interstellar total
oxygen abundance can be reconciliated with the solar value derived from the
most recent measurements %by Holweger and by Allende Prieto, Lambert & Asplund:
of 10 \oh = 517 58 (1 ). We note that the smaller
oxygen abundances derived for the interstellar gas within 500 pc %by Meyer,
Cardelli & Jura or from nearby B star surveys are consistent with a local
elemental deficit.Comment: 9 figures, 37 page
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