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About a quarter of all post-asymptotic giant branch (AGB) stars are hydrogen-deficient. Stellar evolutionary models explain the carbon-dominated H-deficient stars by a (very) late thermal pulse scenario where the hydrogen-rich envelope is mixed with the helium-rich intershell layer. Depending on the particular time at which the final flash occurs, the entire hydrogen envelope may be burned. In contrast, helium-dominated post-AGB stars and their evolution are yet not understood. A small group of very hot, helium-dominated stars is formed by O(He)-type stars. We performed a detailed spectral analysis of ultraviolet and optical spectra of four O(He) stars by means of state-of-the-art non-LTE model-atmosphere techniques. We determined effective temperatures, surface gravities, and the abundances of H, He, C, N, O, F, Ne, Si, P, S, Ar, and Fe. By deriving upper limits for the mass-loss rates of the O(He) stars, we found that they do not exhibit enhanced mass-loss. The comparison with evolutionary models shows that the status of the O(He) stars remains uncertain. Their abundances match predictions of a double helium white dwarf merger scenario, suggesting that they might be the progeny of the compact and of the luminous helium-rich sdO-type stars. The existence of planetary nebulae that do not show helium enrichment around every other O(He) star, precludes a merger origin for these stars. These stars must have formed in a different way, for instance via enhanced mass-loss during their post-AGB evolution or a merger within a common-envelope (CE) of a CO-WD and a red giant or AGB star. A helium-dominated stellar evolutionary sequence exists, that may be fed by different types of mergers or CE scenarios. It appears likely, that all these pass through the O(He) phase just before they become white dwarfs., Comment: 29 pages, 27 figures, accepted for publication in A&A

For the spectral analysis of high-resolution and high-signal-to-noise spectra of hot stars, state-of-the-art non-local thermodynamic equilibrium (NLTE) model-atmospheres are mandatory. These are strongly dependent on the reliability of the atomic data that is used for their calculation. In a recent analysis of the ultraviolet (UV) spectrum of the DA-type white dwarf G191-B2B, 21 Zn IV lines were newly identified. Because of the lack of Zn IV data, transition probabilities of the isoelectronic Ge VI were adapted for a first, coarse determination of the photospheric Zn abundance. We performed new calculations of Zn IV and Zn V oscillator strengths to consider their radiative and collisional bound-bound transitions in detail in our NLTE stellar-atmosphere models for the analysis of the Zn IV - V spectrum exhibited in high-resolution and high-S/N UV observations of G191-B2B and RE0503-289. In the UV spectrum of G191-B2B, we identify 31 Zn IV and 16 Zn V lines. Most of these are identified for the first time in any star. We can reproduce well almost all of them at log Zn = -5.52 +/- 0.2 (mass fraction, about 1.7 times solar). In particular, the Zn IV / Zn V ionization equilibrium, which is a very sensitive indicator for the effective temperature, is well reproduced with the previously determined Teff = 60000 +/- 2000 and log g = 7.60 +/- 0.05. In the spectrum of RE0503-289, we identified 128 Zn V lines for the first time and determined log Zn = -3.57 +/- 0.2 (155 times solar). Reliable measurements and calculations of atomic data are a pre-requisite for stellar-atmosphere modeling. Observed Zn IV and Zn V line profiles in two white dwarf (G191-B2B and RE0503-289) ultraviolet spectra were well reproduced with our newly calculated oscillator strengths. This allowed us to determine the photospheric Zn abundance of these two stars precisely., Comment: 53 pages, 8 figures

For the spectral analysis of high-resolution and high-signal-to-noise (S/N) spectra of hot stars, state-of-the-art non-local thermodynamic equilibrium (NLTE) model atmospheres are mandatory. These are strongly dependent on the reliability of the atomic data that is used for their calculation. Reliable Ba V - VII oscillator strengths are used to identify Ba lines in the spectra of the DA-type white dwarf G191-B2B and the DO-type white dwarf RE0503-289 and to determine their photospheric Ba abundances. We newly calculated Ba V - VII oscillator strengths to consider their radiative and collisional bound-bound transitions in detail in our NLTE stellar-atmosphere models for the analysis of Ba lines exhibited in high-resolution and high-S/N UV observations of G191-B2B and RE0503-289. For the first time, we identified highly ionized Ba in the spectra of hot white dwarfs. We detected Ba VI and Ba VII lines in the Far Ultraviolet Spectroscopic Explorer (FUSE) spectrum of RE0503-289. The Ba VI / Ba VII ionization equilibrium is well reproduced with the previously determined effective temperature of 70000 K and surface gravity of $\log g = 7.5$. The Ba abundance is $3.5 \pm 0.5 \times 10^{-4}$ (mass fraction, about 23000 times the solar value). In the FUSE spectrum of G191-B2B, we identified the strongest Ba VII line (at 993.41 \AA) only, and determined a Ba abundance of $4.0 \pm 0.5 \times 10^{-6}$ (about 265 times solar). Reliable measurements and calculations of atomic data are a pre-requisite for stellar-atmosphere modeling. Observed Ba VI - VII line profiles in two white dwarfs' (G191-B2B and RE0503-289) far-ultraviolet spectra were well reproduced with our newly calculated oscillator strengths. This allowed to determine the photospheric Ba abundance of these two stars precisely., Comment: 36 pages, 8 figures

In the framework of the Virtual Observatory (VO), the German Astrophysical Virtual Observatory (GAVO) developed the registered service TheoSSA (Theoretical Stellar Spectra Access). It provides easy access to stellar spectral energy distributions (SEDs) and is intended to ingest SEDs calculated by any model-atmosphere code, generally for all effective temperature, surface gravities, and elemental compositions. We will establish a database of SEDs of flux standards that are easily accessible via TheoSSA's web interface. The OB-type subdwarf Feige 110 is a standard star for flux calibration. State-of-the-art non-local thermodynamic equilibrium (NLTE) stellar-atmosphere models that consider opacities of species up to trans-iron elements will be used to provide a reliable synthetic spectrum to compare with observations. In case of Feige 110, we demonstrate that the model reproduces not only its overall continuum shape from the far-ultraviolet (FUV) to the optical wavelength range but also the numerous metal lines exhibited in its FUV spectrum. We present a state-of-the-art spectral analysis of Feige 110. We determined $T_\mathrm{eff} = 47\,250 \pm 2000\,\mathrm{K}$, $\log g = 6.00 \pm 0.20$ and the abundances of He, N, P, S, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, and Ge. Ti, V, Mn, Co, Zn, and Ge were identified for the first time in this star. Upper abundance limits were derived for C, O, Si, Ca, and Sc. The TheoSSA database of theoretical SEDs of stellar flux standards guarantees that the flux calibration of astronomical data and cross-calibration between different instruments can be based on models and SEDs calculated with state-of-the-art model-atmosphere codes., Comment: 19 pages, 7 figures

SAO244567, the exciting star of the Stingray nebula, is rapidly evolving. Previous analyses suggested that it has heated up from an effective temperature of about 21kK in 1971 to over 50kK in the 1990s. Canonical post-asymptotic giant branch evolution suggests a relatively high mass while previous analyses indicate a low-mass star. Fitting line profiles from static and expanding non-LTE model atmospheres to the observed UV and optical spectra, taken during 1988-2013, allowed us to study the temporal change of effective temperature, surface gravity, mass-loss rate, and terminal wind velocity. In addition, we determined the chemical composition of the atmosphere. We find that the central star has steadily increased its effective temperature from 38kK in 1988 to a peak value of 60kK in 2002. During the same time, the star was contracting, as concluded from an increase in surface gravity from log g = 4.8 to 6.0 and a drop in luminosity. Simultaneously, the mass-loss rate declined from log (dM/dt/Msun/yr)=-9.0 to -11.6 and the terminal wind velocity increased from 1800km/s to 2800km/s. Since around 2002, the star stopped heating and has cooled down again to 55kK by 2006. It has a largely solar surface composition with the exception of slightly subsolar carbon, phosphorus, and sulfur. By comparison with stellar-evolution calculations, we confirm that SAO244567 must be a low-mass star (M < 0.55 Msun). However, the slow evolution of the respective stellar evolutionary models is in strong contrast to the observed fast evolution and the young planetary nebula with a kinematical age of only about 1000 years. We speculate that the star could be a late He-shell flash object. Alternatively, it could be the outcome of close-binary evolution. Then SAO244567 would be a low-mass (0.354 Msun) helium prewhite dwarf after the common-envelope phase, during which the planetary nebula was ejected., Comment: 16 pages, 13 figures, accepted for publication in A&A

H-rich, DA-type white dwarfs are particularly suited as primary standard stars for flux calibration. State-of-the-art NLTE models consider opacities of species up to trans-iron elements and provide reliable synthetic stellar-atmosphere spectra to compare with observation. We establish a database of theoretical spectra of stellar flux standards that are easily accessible via a web interface. In the framework of the Virtual Observatory, the German Astrophysical Virtual Observatory developed the registered service TheoSSA. It provides easy access to stellar spectral energy distributions (SEDs) and is intended to ingest SEDs calculated by any model-atmosphere code. In case of the DA white dwarf G 191-B2B, we demonstrate that the model reproduces not only its overall continuum shape but also the numerous metal lines exhibited in its ultraviolet spectrum. TheoSSA is in operation and contains presently a variety of SEDs for DA white dwarfs. It will be extended in the near future and can host SEDs of all primary and secondary flux standards. The spectral analysis of G 191-B2B has shown that our hydrostatic models reproduce the observations best at an effective temperature of 60000 +/- 2000K and a surface gravity of log g = 7.60 +/- 0.05. We newly identified Fe VI, Ni VI, and Zn IV lines. For the first time, we determined the photospheric zinc abundance with a logarithmic mass fraction of -4.89 (7.5 times solar). The abundances of He (upper limit), C, N, O, Al, Si, O, P, S, Fe, Ni, Ge, and Sn were precisely determined. Upper abundance limits of 10% solar were derived for Ti, Cr, Mn, and Co. The TheoSSA database of theoretical SEDs of stellar flux standards guarantees that the flux calibration of all astronomical data and cross-calibration between different instruments can be based on the same models and SEDs calculated with different model-atmosphere codes and are easy to compare., Comment: 42 pages, 27 figures

Large sets of objects with spectroscopic redshift measurements will be needed for imaging dark energy experiments to achieve their full potential, serving two goals:_training_, i.e., the use of objects with known redshift to develop and optimize photometric redshift algorithms; and_calibration_, i.e., the characterization of moments of redshift (or photo-z error) distributions. Better training makes cosmological constraints from a given experiment stronger, while highly-accurate calibration is needed for photo-z systematics not to dominate errors. In this white paper, we investigate the required scope of spectroscopic datasets which can serve both these purposes for ongoing and next-generation dark energy experiments, as well as the time required to obtain such data with instruments available in the next decade. Large time allocations on kilo-object spectrographs will be necessary, ideally augmented by infrared spectroscopy from space. Alternatively, precision calibrations could be obtained by measuring cross-correlation statistics using samples of bright objects from a large baryon acoustic oscillation experiment such as DESI. We also summarize the additional work on photometric redshift methods needed to prepare for ongoing and future dark energy experiments., Comment: White paper for the "Dark Energy and CMB" working group for the American Physical Society's Division of Particles and Fields long-term planning exercise ("Snowmass")

The Astro2010 Decadal Survey recommended a Wide Field Infrared Survey Telescope (WFIRST) as its top priority for a new large space mission. The report of the WFIRST-AFTA Science Definition Team (SDT) presents a Design Reference Mission for WFIRST that employs one of the 2.4-m, Hubble-quality mirror assemblies recently made available to NASA. The 2.4-m primary mirror enables a mission with greater sensitivity and higher angular resolution than the smaller aperture designs previously considered for WFIRST, increasing both the science return of the primary surveys and the capabilities of WFIRST as a Guest Observer facility. The option of adding an on-axis, coronagraphic instrument would enable imaging and spectroscopic studies of planets around nearby stars. This short article, produced as a companion to the SDT report, summarizes the key points of the WFIRST-2.4 DRM. It highlights the remarkable opportunity that the 2.4-m telescope affords for advances in many fields of astrophysics and cosmology, including dark energy, the demographics and characterization of exoplanets, the evolution of galaxies and quasars, and the stellar populations of the Milky Way and its neighbors., Comment: 15 pages, 9 figures, 1 table, Companion article to the SDT report, arXiv:1305.5422, added pointer to WFIRST-AFTA SDT report and corrected line color description in Figure 2 caption

The Astro2010 Decadal Survey recommended a Wide Field Infrared Survey Telescope (WFIRST) as its top priority for a new large space mission. As conceived by the decadal survey, WFIRST would carry out a dark energy science program, a microlensing program to determine the demographics of exoplanets, and a general observing program utilizing its ultra wide field. In October 2012, NASA chartered a Science Definition Team (SDT) to produce, in collaboration with the WFIRST Project Office at GSFC and the Program Office at JPL, a Design Reference Mission (DRM) for an implementation of WFIRST using one of the 2.4-m, Hubble-quality mirror assemblies recently made available to NASA. This DRM builds on the work of the earlier WFIRST SDT, reported by Green et al. (2012). The 2.4-m primary mirror enables a mission with greater sensitivity and higher angular resolution than the 1.3-m and 1.1-m designs considered previously, increasing both the science return of the primary surveys and the capabilities of WFIRST as a Guest Observer facility. The option of adding an on-axis, coronagraphic instrument would enable imaging and spectroscopic studies of planets around nearby stars. This document presents the final report of the SDT., Comment: 190 pages, 118 figures, 15 tables, For a short summary of the report highlights, see arXiv:1305.5425, added pointer to the summary of this report and corrected line labels in the caption of Figure 2-2

River management and restoration measures are of increasing importance for integrated water resources management (IWRM) as well as for ecosystem services. However, often river management mainly considers engineering and construction aspects only and the hydrogeological settings as the properties and functions of ancient fluvial systems are neglected which often do not lead to the desired outcome. Knowledge of the distribution of sediment units could contribute to a more efficient restoration. In this study, we present two noninvasive approaches for delineation of fluvial sediment architecture that can form a basis for the restoration, particularly in areas where site disturbance is not permitted. We investigate the floodplain of a heavily modified low-mountain river in Switzerland using different hydrogeophysical methods. In the first approach, we use data from electromagnetic induction (EMI) with four different integral depths (0.75–6 m) and gamma-spectrometry as well as the elevation data as input for a K-means cluster algorithm. The generated cluster map of the surface combines the main characteristics from multilayered input data and delineates areas of varying soil properties. The resulting map provides an indication of areas with different sedimentary units. In the second approach, we develop a new iterative method for the generation of a geological structure model (GSM) by means of various EMI forward models. We vary the geological input parameters based on the measured data until the predicted EMI maps match the measured EMI values. Subsequently, we use the best matched input data for the GSM generation. The derived GSM provides a 3D delineation of possible ancient stream courses. A comparison with an independent ground penetrating radar (GPR) profile confirmed the delineations on the cluster map as well as the vertical changes of the GSM qualitatively. Thus, each of the approaches had the capacity for detecting sedimentary units with distinct hydra

Spectral analyses of hot, compact stars with NLTE (non-local thermodynamical equilibrium) model-atmosphere techniques allow the precise determination of photospheric parameters. The derived photospheric metal abundances are crucial constraints for stellar evolutionary theory. Previous spectral analyses of the exciting star of the nebula A 35, BD-22 3467, were based on He+C+N+O+Si+Fe models only. For our analysis, we use state-of-the-art fully metal-line blanketed NLTE model atmospheres that consider opacities of 23 elements from hydrogen to nickel. For the analysis of high-resolution and high-S/N (signal-to-noise) FUV (far ultraviolet, FUSE) and UV (HST/STIS) observations, we combined stellar-atmosphere models and interstellar line-absorption models to fully reproduce the entire observed UV spectrum. The best agreement with the UV observation of BD-22 3467 is achieved at Teff = 80 +/- 10 kK and log g =7.2 +/- 0.3. While Teff of previous analyses is verified, log g is significantly lower. We re-analyzed lines of silicon and iron (1/100 and about solar abundances, respectively) and for the first time in this star identified argon, chromium, manganese, cobalt, and nickel and determined abundances of 12, 70, 35, 150, and 5 times solar, respectively. Our results partially agree with predictions of diffusion models for DA-type white dwarfs. A combination of photospheric and interstellar line-absorption models reproduces more than 90 % of the observed absorption features. The stellar mass is M ~ 0.48 Msun. BD-22 3467 may not have been massive enough to ascend the asymptotic giant branch and may have evolved directly from the extended horizontal branch to the white dwarf state. This would explain why it is not surrounded by a planetary nebula. However, the star, ionizes the ambient interstellar matter, mimicking a planetary nebula., Comment: 13 pages, 17 figures

The relatively high abundance of carbon in the hot DO white dwarf RE0503-289 indicates that it is a descendant of a PG1159 star. This is corroborated by the recent detection of the extremely high abundances of trans-Fe elements which stem from s-process nucleosynthesis in the precursor AGB star, dredged up by a late He-shell flash and possibly amplified by radiative levitation. On the other hand, the hottest known DO white dwarf, KPD0005+5106, cannot have evolved from a PG1159 star but represents a distinct He-rich evolutionary sequence that possibly originates from a binary white dwarf merger., Comment: Proceedings, 18th European White Dwarf Workshop, Krakow, Poland, 13-17 Aug. 2012

State-of-the-art spectral analysis of hot stars by means of non-LTE model-atmosphere techniques has arrived at a high level of sophistication. The analysis of high-resolution and high-S/N spectra, however, is strongly restricted by the lack of reliable atomic data for highly ionized species from intermediate-mass metals to trans-iron elements. Especially data for the latter has only been sparsely calculated. Many of their lines are identified in spectra of extremely hot, hydrogen-deficient post-AGB stars. A reliable determination of their abundances establishes crucial constraints for AGB nucleosynthesis simulations and, thus, for stellar evolutionary theory. In a previous analysis of the UV spectrum of RE 0503-289, spectral lines of highly ionized Ga, Ge, As, Se, Kr, Mo, Sn, Te, I, and Xe were identified. Individual abundance determinations are hampered by the lack of reliable oscillator strengths. Most of these identified lines stem from Ge V. In addition, we identified Ge VI lines for the first time. We calculated Ge V and Ge VI oscillator strengths to consider their radiative and collisional bound-bound transitions in detail in our non-LTE stellar-atmosphere models for the analysis of the Ge IV - VI spectrum exhibited in high-resolution and high-S/N UV spectra of RE 0503-289. We identify four Ge IV, 37 Ge V, and seven Ge VI lines. Most of these are identified for the first time in any star. We reproduce almost all Ge IV, Ge VI, and Ge VI lines in the observed spectrum of RE 0503-289 (Teff = 70 kK, log g = 7.5) at log Ge = -3.8 +/- 0.3 (mass fraction, about 650 times solar). Reliable measurements and calculations of atomic data are a prerequisite for stellar-atmosphere modeling. Our oscillator-strength calculations have allowed, for the first time, Ge V and Ge VI lines to be successfully reproduced in a white dwarf's spectrum and to determine its photospheric Ge abundance., Comment: 54 pages, 8 figures

High-resolution soft X-ray spectroscopic observations of single hot white dwarfs are scarce. With the Chandra Low-Energy Transmission Grating, we have observed two white dwarfs, one is of spectral type DA (LB 1919) and the other is a non-DA of spectral type PG1159 (PG 1520+525). The spectra of both stars are analyzed, together with an archival Chandra spectrum of another DA white dwarf (GD 246). The soft X-ray spectra of the two DA white dwarfs are investigated in order to study the effect of gravitational settling and radiative levitation of metals in their photospheres. LB 1919 is of interest because it has a significantly lower metallicity than DAs with otherwise similar atmospheric parameters. GD 246 is the only white dwarf known that shows identifiable individual iron lines in the soft X-ray range. For the PG1159 star, a precise effective temperature determination is performed in order to confine the position of the blue edge of the GW Vir instability region in the HRD. (abridged), Comment: A&A, in press

In December 2010, NASA created a Science Definition Team (SDT) for WFIRST, the Wide Field Infra-Red Survey Telescope, recommended by the Astro 2010 Decadal Survey as the highest priority for a large space mission. The SDT was chartered to work with the WFIRST Project Office at GSFC and the Program Office at JPL to produce a Design Reference Mission (DRM) for WFIRST. Part of the original charge was to produce an interim design reference mission by mid-2011. That document was delivered to NASA and widely circulated within the astronomical community. In late 2011 the Astrophysics Division augmented its original charge, asking for two design reference missions. The first of these, DRM1, was to be a finalized version of the interim DRM, reducing overall mission costs where possible. The second of these, DRM2, was to identify and eliminate capabilities that overlapped with those of NASA's James Webb Space Telescope (henceforth JWST), ESA's Euclid mission, and the NSF's ground-based Large Synoptic Survey Telescope (henceforth LSST), and again to reduce overall mission cost, while staying faithful to NWNH. This report presents both DRM1 and DRM2., Comment: 102 pages, 57 figures, 17 tables

We describe a mission architecture designed to substantially increase the science capability of the NASA Science Mission Directorate (SMD) Astrophysics Explorer Program for all AO proposers working within the near-UV to far-infrared spectrum. We have demonstrated that augmentation of Falcon 9 Explorer launch services with a 13 kW Solar Electric Propulsion (SEP) stage can deliver a 700 kg science observatory payload to extra-Zodiacal orbit. This new capability enables up to ~13X increased photometric sensitivity and ~160X increased observing speed relative to a Sun-Earth L2, Earth-trailing, or Earth orbit with no increase in telescope aperture. All enabling SEP stage technologies for this launch service augmentation have reached sufficient readiness (TRL-6) for Explorer Program application in conjunction with the Falcon 9. We demonstrate that enabling Astrophysics Explorers to reach extra-zodiacal orbit will allow this small payload program to rival the science performance of much larger long development time systems; thus, providing a means to realize major science objectives while increasing the SMD Astrophysics portfolio diversity and resiliency to external budget pressure. The SEP technology employed in this study has strong applicability to SMD Planetary Science community-proposed missions. SEP is a stated flight demonstration priority for NASA's Office of the Chief Technologist (OCT). This new mission architecture for astrophysics Explorers enables an attractive realization of joint goals for OCT and SMD with wide applicability across SMD science disciplines., Comment: Submitted to proceedings of the SPIE Astronomical Telescopes and Instrumentation conference, Amsterdam, The Netherlands, July 2012