105 research outputs found
WFIRST Observatory Performance
The WFIRST observatory will be a powerful and flexible wide-field near-infrared facility. The planned surveys will provide data applicable to an enormous variety of astrophysical science. This presentation will provide a description of the observatory and its performance characteristics. This will include a discussion of the point spread function, signal-to-noise budgets for representative observing scenarios and the corresponding limiting sensitivity. Emphasis will be given to providing prospective Guest Observers with information needed to begin thinking about new observing programs
The WFIRST Interim Design Reference Mission: Capabilities, Constraints, and Open Questions
The Project Office and Science Definition Team for the Wide-Field Infrared Survey Telescope (WFIRST) are in the midst of a pre-Phase A study to establish a Design Reference Mission (DRM). An Interim report was released in June 2011, with a final report due later in 2012. The predicted performance of the Interim DRM Observatory will be described, including optical quality, observing efficiency, and sensitivity for representative observing scenarios. Observing constraints and other limitations on performance will also be presented, with an emphasis on potential Guest Observer programs. Finally, a brief status update will be provided on open trade studies of interest to the scientific community. The final DRM may differ from the Interim DRM presented here. However, the underlying requirements of the scientific programs are not expected to change, hence the capabilities of the IDRM are likely to be maintained even if the implementation changes in significant ways
First Detection of Krypton and Xenon in a White Dwarf
We report on the first detection of the noble gases krypton (Z = 36) and xenon (54) in a white dwarf. About 20 KrVI-VII and Xe VI-VII lines were discovered in the ultraviolet spectrum of the hot DO-type white dwarf RE 0503-289. The observations, performed with the Far Ultraviolet Spectroscopic Explorer, also reveal highly ionized photospheric lines from other trans-iron group elements, namely Ga (31), Ge (32), As (33), Se (34), Mo (42), Sn (50), Te (52), and I (53), from which gallium and molybdenum are new discoveries in white dwarfs, too. For Kr and Xe, we performed an NLTE analysis and derived mass fractions of log Kr = -4.3 plus or minus 0.5 and log Xe = -4.2 plus or minus 0.6, corresponding to an enrichment by factors of 450 and 3800, respectively, relative to the Sun. The origin of the large overabundances is unclear. We discuss the roles of neutron-capture nucleosynthesis in the-precursor star and radiation-driven diffusion. It is possible that diffusion is insignificant and thaI the observed metal abundances constrain the evolutionary history of the star. Its hydrogen deficiency may be the consequence of a late helium-shell nash or a binary white dwarf merger
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
Non-LTE Spectral Analysis of Extremely Hot Post-AGB Stars: Constraints for Evolutionary Theory
Spectral analysis by means of Non-LTE model-atmosphere techniques has arrived
at a high level of sophistication: fully line-blanketed model atmospheres which
consider opacities of all elements from H to Ni allow the reliable
determination of photospheric parameters of hot, compact stars. Such models
provide a crucial test of stellar evolutionary theory: recent abundance
determinations of trace elements like, e.g., F, Ne, Mg, P, S, Ar, Fe, and Ni
are suited to investigate on AGB nucleosynthesis. E.g., the strong Fe depletion
found in hydrogen-deficient post-AGB stars is a clear indication of an
efficient s-process on the AGB where Fe is transformed into Ni or even heavier
trans iron-group elements. We present results of recent spectral analyses based
on high-resolution UV observations of hot stars.Comment: 6 pages, 2 figure
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