63 research outputs found
Spectral analysis of the barium central star of the planetary nebula Hen 2-39
Barium stars are peculiar red giants characterized by an overabundance of
s-process elements along with an enrichment in carbon. These stars are
discovered in binaries with white dwarf companions. The more recently formed of
these stars are still surrounded by a planetary nebula. Precise abundance
determinations of the various s-process elements, especially, of the lightest,
short-lived radionuclide technetium will establish constraints for the
formation of s-process elements in asymptotic giant branch stars as well as
mass transfer through, for example, stellar wind, Roche-lobe overflow, and
common-envelope evolution. We performed a detailed spectral analysis of the
K-type subgiant central star of the planetary nebula Hen 2-39 based on
high-resolution optical spectra obtained with the Ultraviolet and Visual
Echelle Spectrograph at the Very Large Telescope using LTE model atmospheres.
We confirm the effective temperature of K for
the central star of the planetary nebula Hen 2-39. It has a photospheric carbon
enrichment of and a barium overabundance of
. We find a deficiency for most of the
iron-group elements (calcium to iron) and establish an upper abundance limit
for technetium (). The quality of the
available optical spectra is not sufficient to measure abundances of all
s-process elements accurately. Despite large uncertainties on the abundances as
well as on the model yields, the derived abundances are most consistent with a
progenitor mass in the range 1.75-3.00 and a metallicity of
. This result leads to the conclusion that the
formation of such systems requires a relatively large mass transfer that is
most easily obtained via wind-Roche lobe overflow.Comment: 26 pages, 18 figure
High-precision Atomic Physics Laboratories in Space: White Dwarfs and Subdwarfs
The 21st European Workshop on White Dwarfs was held in Austin, TX from July 23rd to 27th of 2018Stellar atmospheres are prime laboratories to determine
atomic properties of highly ionized species.
Reliable opacities are crucial ingredients for the
calculation of stellar atmospheres of white dwarfs
and subdwarfs. A detailed investigation on the
precision of many iron-group oscillator strengths
is still outstanding. To make progress, we used
the Hubble Space Telescope Imaging Spectrograph
to measure high-resolution spectra of three hot
subdwarfs that exhibit extremely high iron-group
abundances. The predicted relative strengths of
the identified lines are compared with the observations
to judge the quality of Kurucz’s line data
and to determine correction factors for abundance
determinations of the respective elements.Astronom
Exploring the Structural Complexity of Compartmentalized Nanostructures of Triblock Terpolymers
Morphology of the first zoeal stage of the commensal southwestern Atlantic crab Austinixa aidae (Righi 1967) (Brachyura: Pinnotheridae), hatched in the laboratory
6 páginas, 2 figuras, 1 tabla.The first zoeal stage of the endemic southern Atlantic pinnotherid crab Austinixa aidae is described and illustrated based on laboratory-hatched material from ovigerous females collected from the upper burrows of the thalassinidean shrimp Callichirus major at Ubatuba, São Paulo, Brazil. The zoeae of Austinixa species can be distinguished from other pinnotherids and especially from zoeae of the closely related species of Pinnixa by the telson structure.FLM is grateful to CNPq for a research fellowship (Proc. 301359/2007-210 5).Peer reviewe
An extremely hot white dwarf with a rapidly rotating K-type subgiant companion: UCAC2 46706450
UCAC2 46706450 is a late-type star with an ultraviolet (UV) excess. It was
considered a candidate to establish a sample of FGK stars with white dwarf (WD)
companions that can be used to test binary evolution models. To verify the WD
nature of the companion, UV spectroscopy was performed by Parsons et al.
(2016). By a detailed model-atmosphere analysis we show that the UV source is
an extremely hot WD with effective temperature =
K, mass , radius , and luminosity , i.e.,
the compact object is just about to enter the WD cooling sequence.
Investigating spectra of the cool star ( = K) we
found that it is a K-type subgiant with , , and , that is rapidly rotating
with km s. Optical light curves reveal a period of two
days and an o-band peak-to-peak amplitude of 0.06 mag. We suggest, that it is
caused by stellar rotation in connection with star spots. With the radius we
infer an extremely high rotational velocity of
km s, thus marking the star as one of
the most rapidly rotating subgiants known. This explains chromospheric activity
observed by H emission and emission-line cores in CaII H and K as well
as NUV flux excess. From equal and constant radial velocities of the WD and the
K subgiant as well as from a fit to the spectral energy distribution we infer
that they form a physical, wide though unresolved binary system. Both
components exhibit similar metal abundances and show iron-group elements with
slightly oversolar (up to 0.6 dex) abundance, meaning that atomic diffusion in
the WD atmosphere is not yet active due to a residual, weak radiation-driven
wind. (abridged)Comment: 13 pages, accepted for publication in A&
First discovery of trans-iron elements in a DAO-type white dwarf (BD)
We have identified 484 lines of the trans-iron elements (TIEs) Zn, Ga, Ge,
Se, Br, Kr, Sr, Zr, Mo, In, Te, I, Xe, and Ba, for the first time in the
ultraviolet spectrum of a DAO-type WD, namely BD, surrounded
by the ionized nebula Abell 35. Our TIE abundance determination shows extremely
high overabundances of up to five dex -- a similar effect is already known from
hot, H-deficient (DO-type) white dwarfs. In contrast to these where a
pulse-driven convection zone has enriched the photosphere with TIEs during a
final thermal pulse and radiative levitation has established the extreme TIE
overabundances, {here the extreme TIE overabundances are exclusively driven by
radiative levitation on the initial stellar metallicity. The very low mass
() of BD implies that a
third dredge-up with enrichment of s-process elements in the photosphere did
not occur in the AGB precursor.Comment: 34 pages, 19 Figure
Trans-iron elements in evolved stars
In dieser Arbeit werden Spektralanalysen verschiedener Sterntypen vorgestellt, deren Fokus auf den Häufigkeiten von Transeisenelementen liegt. Das übergeordnete Ziel ist es, die Nukleosynthese schwerer Elemente durch Neutroneneinfang auf dem asymptotischen Riesenast zu untersuchen. Diese Analysen können helfen, ein besseres Verständnis von Prozessen wie Konvektion, Diffusion, Sternwind und Massenverlust zu erlangen, die für die Entstehung des beobachteten Häufigkeitsmusters verantwortlich sind.This work presents spectral analyses of different types of stars focusing on the abundances of trans-iron elements. The overarching aim is to constrain neutron-capture nucleosynthesis via the slow (s-) process on the asymptotic giant branch as well as processes like mixing, diffusion, stellar wind, and mass loss that are responsible for the formation of the observed abundance pattern
NLTE spectral analysis of the intermediate helium-rich subdwarf B star CPD-20 degrees 1123
Subdwarf B (sdB) stars are core helium-burning stars with stratified
atmospheres. Their atmospheres are dominated by hydrogen (H) while the helium
(He) and metal abundances are shaped by an interplay of gravitational settling
and radiative levitation. However, a small fraction of these show spectra
dominated by He I absorption lines. In between these groups of He-deficient and
extreme He-rich sdBs, some are found to have intermediate surface He
abundances. These objects are proposed to be young "normal" (He-deficient) sdBs
for which the dynamical stratification of the atmosphere is still ongoing. We
present an analysis of the optical spectrum of such an intermediate He-rich
sdB, namely CPD-20{\deg}1123, by means of non-local thermodynamic equilibrium
(NLTE) stellar atmosphere models. It has a He-to-H number ratio of
and its effective temperature of
together with a surface gravity of
places the star close to
the high-temperature edge until which it may be justified to use LTE model
atmospheres. This work states a test of the T\"ubingen NLTE Model Atmosphere
Package for this temperature regime. We present the first application of
revised, elaborated model atoms of low ionization stages of light metals usable
with this atmosphere code.Comment: 16 pages, 13 figure
Sliding along the Eddington Limit—Heavy-Weight Central Stars of Planetary Nebulae
Due to thermal pulses, asymptotic giant branch (AGB) stars experience periods of convective mixing that provide ideal conditions for slow neutron-capture nucleosynthesis. These processes are affected by large uncertainties and are still not fully understood. By the lucky coincidence that about a quarter of all post-AGB stars turn hydrogen-deficient in a final flash of the helium-burning shell, they display nuclear processed material at the surface providing an unique insight to nucleosynthesis and mixing. We present results of non-local thermodynamic equilibrium spectral analyses of the extremely hot, hydrogen-deficient, PG 1159-type central stars of the Skull Nebula NGC 246 and the “Galactic Soccerballs” Abell 43 and NGC 7094
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