7,841 research outputs found
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
Metal abundances in hot white dwarfs with signatures of a superionized wind
About a dozen hot white dwarfs with effective temperatures Teff =
65,000-120,000 K exhibit unusual absorption features in their optical spectra.
These objects were tentatively identified as Rydberg lines of ultra-high
excited metals in ionization stages V-X, indicating line formation in a dense
environment with temperatures near one million Kelvin. Since some features show
blueward extensions, it was argued that they stem from a superionized wind. A
unique assignment of the lines to particular elements is not possible, although
they probably stem from C, N, O, and Ne. To further investigate this
phenomenon, we analyzed the ultraviolet spectra available from only three stars
of this group; that is, two helium-rich white dwarfs, HE 0504-2408 and HS
0713+3958 with spectral type DO, and a hydrogen-rich white dwarf, HS 2115+1148
with spectral type DAO. We identified light metals (C, N, O, Si, P, and S) with
generally subsolar abundances and heavy elements from the iron group (Cr, Mn,
Fe, Co, Ni) with solar or oversolar abundance. The abundance patterns are not
unusual for hot WDs and can be interpreted as the result of gravitational
settling and radiative levitation of elements. As to the origin of the
ultra-high ionized metals lines, we discuss the possible presence of a
multicomponent radiatively driven wind that is frictionally heated.Comment: Accepted for publication in A&
The hot white dwarf in the peculiar binary nucleus of the planetary nebula EGB6
EGB6 is an extended, faint old planetary nebula (PN) with an enigmatic
nucleus. The central star (PG0950+139) is a hot DAOZ-type white dwarf (WD). An
unresolved, compact emission knot was discovered to be located 0.166" away from
the WD and it was shown to be centered around a dust-enshrouded low-luminosity
star. It was argued that the dust disk and evaporated gas (photoionized by the
hot WD) around the companion are remnants of a disk formed by wind material
captured from the WD progenitor when it was an asymptotic giant branch (AGB)
star. In this paper, we assess the hot WD to determine its atmospheric and
stellar parameters. We performed a model-atmosphere analysis of ultraviolet
(UV) and optical spectra. We found Teff = 105,000 +/- 5000 K, log g = 7.4 +/-
0.4, and a solar helium abundance (He = 0.25 +/- 0.1, mass fraction). We
measured the abundances of ten more species (C, N, O, F, Si, P, S, Ar, Fe, Ni)
and found essentially solar abundance values, indicating that radiation-driven
wind mass-loss, with a theoretical rate of log(dot-M/M_sun/yr) = -11.0
(+1.1)(-0.8) prevents the gravitational separation of elements in the
photosphere. The WD has a mass of M/M_sun = 0.58 (+0.12)(-0.04) and its
post-AGB age (log(t_evol/yr) = 3.60 (+1.26)(-0.09)) is compatible with the PN
kinematical age of log(t_PN}/yr) = 4.2. In addition, we examined the UV
spectrum of the hot nucleus of a similar object with a compact emission region,
TOL26 (PN G298.0+34.8), and found that it is a slightly cooler DAOZ WD (Teff
about 85,000 K), but this WD shows signatures of gravitational settling of
heavy elements.Comment: A&A accepte
The far-ultraviolet spectra of two hot PG1159 stars
PG1159 stars are hot, hydrogen-deficient (pre-) white dwarfs with atmospheres
mainly composed of helium, carbon, and oxygen. The unusual surface chemistry is
the result of a late helium-shell flash. Observed element abundances enable us
to test stellar evolution models quantitatively with respect to their
nucleosynthesis products formed near the helium-burning shell of the progenitor
asymptotic giant branch stars. Because of the high effective temperatures
(Teff), abundance determinations require ultraviolet spectroscopy and non-local
thermodynamic equilibrium model atmosphere analyses. Up to now, we have
presented results for the prototype of this spectral class and two cooler
members (Teff in the range 85,000-140,000 K). Here we report on the results for
two even hotter stars (PG1520+525 and PG1144+005, both with Teff = 150,000 K)
which are the only two objects in this temperature-gravity region for which
useful far-ultraviolet spectra are available, and revisit the prototype star.
Previous results on the abundances of some species are confirmed, while results
on others (Si, P, S) are revised. In particular, a solar abundance of sulphur
is measured in contrast to earlier claims of a strong S deficiency that
contradicted stellar evolution models. For the first time, we assess the
abundances of Na, Al, and Cl with newly constructed non-LTE model atoms.
Besides the main constituents (He, C, O), we determine the abundances (or upper
limits) of N, F, Ne, Na, Al, Si, P, S, Cl, Ar, and Fe. Generally, good
agreement with stellar models is found.Comment: Accepted for publication in A&
The virtual observatory service TheoSSA: Establishing a database of synthetic stellar flux standards. I. NLTE spectral analysis of the DA-type white dwarf G 191-B2B
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 figure
Stellar laboratories III. New Ba V, Ba VI, and Ba VII oscillator strengths and the barium abundance in the hot white dwarfs G191-B2B and RE0503-289
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 . The Ba
abundance is (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 (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 figure
First detection of bromine and antimony in hot stars
Bromine (atomic number Z=35) and antimony (Z=51) are extremely difficult to
detect in stars. In very few instances, weak and mostly uncertain
identifications of Br I, Br II, and Sb II in relatively cool, chemically
peculiar stars were successful. Adopted solar abundance values rely on
meteoritic determinations. Here, we announce the first identification of these
species in far-ultraviolet spectra of hot stars (with effective temperatures of
49,500-70,000 K), namely in helium-rich (spectral type DO) white dwarfs. We
identify the Br VI resonance line at 945.96 A. A previous claim of Br detection
based on this line is incorrect because its wavelength position is inaccurate
by about 7 A in atomic databases. Taking advantage of precise laboratory
measurements, we identify this line as well as two other, subordinate Br VI
lines. Antimony is detected by the Sb V resonance doublet at 1104.23/1225.98 A,
as well as two subordinate Sb VI lines. A model-atmosphere analysis reveals
strongly oversolar Br and Sb abundances that are caused by radiative-levitation
dominated atomic diffusion.Comment: Accepted for publication in A&
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