7,543 research outputs found
Stellar laboratories. V. The Xe VI ultraviolet spectrum and the xenon abundance in the hot DO-type white dwarf RE0503-289
For the spectral analysis of spectra of hot stars with a high resolution and
high signal-to-noise ratio (S/N), advanced non-local thermodynamic equilibrium
(NLTE) model atmospheres are mandatory. These are strongly dependent on the
reliability of the atomic data that are used for their calculation.
Reliable Xe VI oscillator strengths are used to identify Xe lines in the
ultraviolet spectrum of the DO-type white dwarf RE0503-289 and to determine its
photospheric Xe abundance.
We publish newly calculated oscillator strengths that are based on a recently
measured Xe VI laboratory line spectrum. These strengths were used to consider
their radiative and collisional bound-bound transitions in detail in our NLTE
stellar-atmosphere models to analyze Xe VI lines exhibited in high-resolution
and high S/N UV observations of RE0503-289.
We identify three hitherto unknown Xe VI lines in the ultraviolet spectrum of
RE0503-289 and confirm the previously measured photospheric Xe abundance of
this white dwarf (log Xe = -4.2 +/- 0.6).
Reliable measurements and calculations of atomic data are prerequisite for
stellar-atmosphere modeling. Observed Xe VI line profiles in the ultraviolet
spectrum of the white dwarf RE0503-289 were well reproduced with the newly
calculated Xe VI oscillator strengths.Comment: 3 pages, 4 figure
EC 11481-2303 - A Peculiar Subdwarf OB Star Revisited
EC 11481-2303 is a peculiar, hot, high-gravity pre-white dwarf. Previous
optical spectroscopy revealed that it is a sdOB star with an effective
temperature (Teff) of 41790 K, a surface gravity log(g)= 5.84, and He/H = 0.014
by number. We present an on-going spectral analysis by means of non-LTE
model-atmosphere techniques based on high-resolution, high-S/N optical
(VLT-UVES) and ultraviolet (FUSE, IUE) observations. We are able to reproduce
the optical and UV observations simultaneously with a chemically homogeneous
NLTE model atmosphere with a significantly higher effective temperature and
lower He abundance (Teff = 55000 K, log (g) = 5.8, and He / H = 0.0025 by
number). While C, N, and O appear less than 0.15 times solar, the iron-group
abundance is strongly enhanced by at least a factor of ten.Comment: 8 pages, 11 figure
Complete spectral energy distribution of the hot, helium-rich white dwarf RX J0503.9-2854
In the line-of-sight toward the DO-type white dwarf RX J0503.9-2854, the
density of the interstellar medium (ISM) is very low, and thus the
contamination of the stellar spectrum almost negligible. This allows us to
identify many metal lines in a wide wavelength range from the extreme
ultraviolet to the near infrared. In previous spectral analyses, many metal
lines in the ultraviolet spectrum of RX J0503.9-2854 have been identified. A
complete line list of observed and identified lines is presented here. We
compared synthetic spectra that had been calculated from model atmospheres in
non-local thermodynamical equilibrium, with observations. In total, we
identified 1272 lines (279 of them were newly assigned) in the wavelength range
from the extreme ultraviolet to the near infrared. 287 lines remain
unidentified. A close inspection of the EUV shows that still no good fit to the
observed shape of the stellar continuum flux can be achieved although He, C, N,
O, Al, Si, P, S, Ca, Sc, Ti, V, Cr, Mn, Fe, Cr, Ni Zn, Ga, Ge, As, Kr, Zr, Mo,
Sn, Xe, and Ba are included in the stellar atmosphere models. There are two
possible reasons for the deviation between observed and synthetic flux in the
EUV. Opacities from hitherto unconsidered elements in the model-atmosphere
calculation may be missing and/or the effective temperature is slightly lower
than previously determined.Comment: 92 pages, 45 figure
Temperature and Kinematics of CIV Absorption Systems
We use Keck HIRES spectra of three intermediate redshift QSOs to study the
physical state and kinematics of the individual components of CIV selected
heavy element absorption systems. Fewer than 8 % of all CIV lines with column
densities greater than 10^{12.5} cm^{-2} have Doppler parameters b < 6 km/s. A
formal decomposition into thermal and non-thermal motion using the simultaneous
presence of SiIV gives a mean thermal Doppler parameter b_{therm}(CIV) = 7.2
km/s, corresponding to a temperature of 38,000 K although temperatures possibly
in excess of 300,000 K occur occasionally. We also find tentative evidence for
a mild increase of temperature with HI column density. Non-thermal motions
within components are typically small (< 10 km/s) for most systems, indicative
of a quiescent environment. The two-point correlation function (TPCF) of CIV
systems on scales up to 500 km/s suggests that there is more than one source of
velocity dispersion. The shape of the TPCF can be understood if the CIV systems
are caused by ensembles of objects with the kinematics of dwarf galaxies on a
small scale, while following the Hubble flow on a larger scale. Individual high
redshift CIV components may be the building blocks of future normal galaxies in
a hierarchical structure formation scenario.Comment: submitted to the ApJ Letters, March 16, 1996 (in press); (13 Latex
pages, 4 Postscript figures, and psfig.sty included
A New Method for Controlling Tree Size?
Smaller trees reduce labor and equipment costs for fruit growers. The authors describe the effects of a chemical growth retardant on fruit trees in ISU studies
Hydroprocessing and Blending of a Biomass-Based DTG-Gasoline
The number of annually registered internal-combustion vehicles still exceeds electric-driven ones in most regions,
e.g., Germany. Ambitious goals are disclosed with the European Green Deal, which calls for new technical
approaches and greenhouse gas neutral transition technologies. Such bridging technologies are synthetic fuels for
the transportation sector, e.g., using the bioliq® process for a CO2-neutral gasoline supply. Fuels must meet the
applicable national standards to be used in existing engines. Petrochemical parameters can be variably adapted
to their requirements by hydroprocessing. In this work, we considered the upgrading of aromatic-rich DTG
gasoline from the bioliq® process. The heavy gasoline was therefore separated from the light one by rectification.
We investigated how to selectively modify the petrochemical parameters of the heavy gasoline, especially the
boiling characteristics, to make the product suitable as a high-quality blending component. Three commercially
available Pt/zeolite catalysts were tested over a wide range of temperature and space velocity. We achieved high
gasoline yields, while the content of light end compounds up to a boiling temperature of 150°C could be increased
significantly. In contrast to the high naphthenic content of the gasoline, the obtained octane numbers were
satisfactory. Especially the Motor Octane Number turned out unexpectedly high and showed a dependency on
the isomerization of the naphthenic rings. By blending the upgraded heavy gasoline with the previously separated
light gasoline, we could finally show that hydroprocessing is suitable for adjusting petrochemical parameters. The
aromatic concentration was 37.5% lower than that in the original raw gasoline, while the boiling characteristics
improved significantly. Additionally, the final boiling point was 82°C lower, which is beneficial for the emission
behavior
Stellar laboratories. IX. New Se V, Sr IV - VII, Te VI, and I VI oscillator strengths and the Se, Sr, Te, and I abundances in the hot white dwarfs G191-B2B and RE 0503-289
To analyze spectra of hot stars, advanced non-local thermodynamic equilibrium
(NLTE) model-atmosphere techniques are mandatory. Reliable atomic data is for
the calculation of such model atmospheres.
We aim to calculate new Sr IV - VII oscillator strengths to identify for the
first time Sr spectral lines in hot white dwarf (WD) stars and to determine the
photospheric Sr abundances. o measure the abundances of Se, Te, and I in hot
WDs, we aim to compute new Se V, Te VI, and I VI oscillator strengths.
To consider radiative and collisional bound-bound transitions of Se V, Sr IV
- VII, Te VI, and I VI in our NLTE atmosphere models, we calculated oscillator
strengths for these ions.
We newly identified four Se V, 23 Sr V, 1 Te VI, and three I VI lines in the
ultraviolet (UV) spectrum of RE0503-289. We measured a photospheric Sr
abundance of 6.5 +3.8/-2.4 x 10**-4 (mass fraction, 9500 - 23800 times solar).
We determined the abundances of Se (1.6 +0.9/-0.6 x 10**-3, 8000 - 20000), Te
(2.5 +1.5/-0.9 x 10**-4, 11000 - 28000), and I (1.4 +0.8/-0.5 x 10**-5, 2700 -
6700). No Se, Sr, Te, and I line was found in the UV spectra of G191-B2B and we
could determine only upper abundance limits of approximately 100 times solar.
All identified Se V, Sr V, Te VI, and I VI lines in the UV spectrum of
RE0503-289 were simultaneously well reproduced with our newly calculated
oscillator strengths.Comment: 26 pages, 5 figure
On the influence of resonance photon scattering on atom interference
Here, the influence of resonance photon-atom scattering on the atom
interference pattern at the exit of a three-grating Mach-Zehnder interferometer
is studied. It is assumed that the scattering process does not destroy the
atomic wave function describing the state of the atom before the scattering
process takes place, but only induces a certain shift and change of its phase.
We find that the visibility of the interference strongly depends on the
statistical distribution of transferred momenta to the atom during the
photon-atom scattering event. This also explains the experimentally observed
(Chapman et al 1995 Phys. Rev. Lett. 75 2783) dependence of the visibility on
the ratio d_p/\lambda_i = y'_{12} (2\pi/kd\lambda_i), where y'_{12} is distance
between the place where the scattering event occurs and the first grating, k is
the wave number of the atomic center-of-mass motion, is the grating
constant and \lambda_i is the photon wavelength. Furthermore, it is remarkable
that photon-atom scattering events happen experimentally within the Fresnel
region, i.e. the near field region, associated with the first grating, which
should be taken into account when drawing conclusions about the relevance of
"which-way" information for the interference visibility.Comment: 9 pages, 1 figur
The CARMA correlator
The Combined Array for Research in Millimeter-wave Astronomy (CARMA) requires a flexible correlator to process the data from up to 23 telescopes and up to 8GHz of receiver bandwidth. The Caltech Owens Valley Broadband Reconfigurable Array (COBRA) correlator, developed for use at the Owens Valley millimeter-wave array and being used by the Sunyaev-Zeldovich Array (SZA), will be adapted for use by CARMA. The COBRA correlator system, a hybrid analog-digital design, consisting of downconverters, digitizers and correlators will be presented in this paper. The downconverters receive an input IF of 1-9GHz and produce a selectable output bandwidth of 62.5MHz, 125MHz, 250MHz, or 500MHz. The downconverter output is digitized at 1Gsample/s to 2-bits per sample. The digitized data is optionally digitally filtered to produce bands narrower than 62.5MHz (down to 2MHz). The digital correlator system is a lag- or XF-based system implemented using Field-Programmable Gate Arrays (FPGAs). The digital system implements delay lines, calculates the autocorrelations for each antenna, and the cross-correlations for each baseline. The number of lags, and hence spectral channels, produced by the system is a function of the input bandwidth; with the 500MHz band having the coarsest resolution, and the narrowest bandwidths having the finest resolution
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