48 research outputs found
V605 Aql: The Older Twin of Sakurai's Object
New optical spectra have been obtained with VLT/FORS2 of the final helium
shell flash (FF) star, V605 Aql, which peaked in brightness in 1919. New models
suggest that this star is experiencing a very late thermal pulse. The evolution
to a cool luminous giant and then back to a compact hot star takes place in
only a few years. V605 Aql, the central star of the Planetary Nebula (PN), A58,
has evolved from T5000 K in 1921 to 95,000 K today. There are
indications that the new FF star, Sakurai's Object (V4334 Sgr), which appeared
in 1996, is evolving along a similar path. The abundances of Sakurai's Object
today and V605 Aql 80 years ago mimic the hydrogen deficient R Coronae Borealis
(RCB) stars with 98% He and 1% C. The new spectra show that V605 Aql has
stellar abundances similar to those seen in Wolf-Rayet [WC] central stars of
PNe with ~55% He, and ~40% C. The stellar spectrum of V605 Aql can be seen even
though the star is not directly detected. Therefore, we may be seeing the
spectrum in light scattered around the edge of a thick torus of dust seen
edge-on. In the present state of evolution of V605 Aql, we may be seeing the
not too distant future of Sakurai's Object.Comment: 12 pages, 1 figure, ApJ Letters in pres
Freshly ionized matter around the final Helium shell flash object V4334 Sgr (Sakurai's object)
We report on the discovery of recently ionized hydrogen-deficient gas in the
immediate circumstellar environment of the final helium shell flash star V4334
Sgr (Sakurai's object). On spectra obtained with FORS2 multi-object
spectroscopy we have found spatially extended (about 2") emission from [N II],
[O I], [O II] and very faint Halpha and [S II]. In the [N II] (ll6548,83) lines
we have identified two components located at velocities -350 +/-50 and +200
+/-50 km/s, relative to V4334 Sgr itself. The full width of the [N II] l6583
feature at zero intensity corresponds to a velocity spread of about 1500 km/s.
Based on the available data it is not possible to conclusively determine the
mechanism of ionization. Both photo-ionization, from a rapidly evolving central
star, and shock excitation, as the result of the collision of the fast ouflows
with slower circumstellar matter, could account for the observed lines. The
central star is still hidden behind strong dust absorption, since only a faint
highly reddened continuum is apparent in the spectra. Theory states that it
will become hotter and will retrace its post-asymptotic giant branch evolution
towards the planetary nebula domain. Our detection of the ionized ejecta from
the very late helium shell flash marks the beginning of a new phase in this
star's amazingly rapid evolution.Comment: 11 pages, 2 figures. Accepted by ApJ
The Anomalous Infrared Emission of Abell 58
We present a new model to explain the excess in mid and near infrared
emission of the central, hydrogen poor dust knot in the planetary nebula (PN)
Abell 58. Current models disagree with ISO measurement because they apply an
average grain size and equilibrium conditions only. We investigate grain size
distributions and temperature fluctuations affecting infrared emission using a
new radiative transfer code and discuss in detail the conditions requiring an
extension of the classical description. The peculiar infrared emission of V605
Aql, the central dust knot in Abell 58, has been modeled with our code. V605
Aql is of special interest as it is one of only three stars ever observed to
move from the evolutionary track of a central PN star back to the post-AGB
state.Comment: 17 pages, 4 figures; accepted and to be published in Ap
V605 Aquilae: a born again star, a nova or both?
V605 Aquilae is today widely assumed to have been the result of a final
helium shell flash occurring on a single post-asymptotic giant branch star. The
fact that the outbursting star is in the middle of an old planetary nebula and
that the ejecta associated with the outburst is hydrogen deficient supports
this diagnosis. However, the material ejected during that outburst is also
extremely neon rich, suggesting that it derives from an oxygen-neon-magnesium
star, as is the case in the so-called neon novae. We have therefore attempted
to construct a scenario that explains all the observations of the nebula and
its central star, including the ejecta abundances. We find two scenarios that
have the potential to explain the observations, although neither is a perfect
match. The first scenario invokes the merger of a main sequence star and a
massive oxygen-neon-magnesium white dwarf. The second invokes an
oxygen-neon-magnesium classical nova that takes place shortly after a final
helium shell flash. The main drawback of the first scenario is the inability to
determine whether the ejecta would have the observed composition and whether a
merger could result in the observed hydrogen-deficient stellar abundances
observed in the star today. The second scenario is based on better understood
physics, but, through a population synthesis technique, we determine that its
frequency of occurrence should be very low and possibly lower than what is
implied by the number of observed systems. While we could not envisage a
scenario that naturally explains this object, this is the second final flash
star which, upon closer scrutiny, is found to have hydrogen-deficient ejecta
with abnormally high neon abundances. These findings are in stark contrast with
the predictions of the final helium shell flash and beg for an alternative
explanation.Comment: 8 pages, 1 figures, 2 tables, accepted for MNRAS. Better title and
minor corrections compared to previous versio
The Element Abundances in Bare Planetary Nebula Central Stars and the Shell Burning in AGB Stars
We review the observed properties of extremely hot hydrogen-deficient
post-AGB stars of spectral type [WC] and PG1159. Their H-deficiency is probably
caused by a (very) late helium-shell flash or a AGB final thermal pulse, laying
bare interior stellar regions which are usually kept hidden below the hydrogen
envelope. Thus, the photospheric element abundances of these stars allow to
draw conclusions about details of nuclear burning and mixing processes in the
precursor AGB stars. We summarize the state-of-the-art of stellar evolution
models which simulate AGB evolution and the occurrence of a late He-shell
flash. We compare predicted element abundances to those determined by
quantitative spectral analyses performed with advanced non-LTE model
atmospheres. A good qualitative and quantitative agreement is found. Future
work can contribute to an even more complete picture of the nuclear processes
in AGB stars.Comment: Review, accepted for publication in PASP, Febr. 06 issue. For high
resolution versions of Figures 1 and 6 see preprint on
http://astro.uni-tuebingen.de/publications/paper_05_05.shtm
Soziale Netzwerkbildung und Online-Lernen.
Die Dissertation mit dem Titel âSoziale Netzwerkbildung und Online-Lernenâ geht der Fragestellung nach, wie sich Lernprozess und soziales Umfeld, insbesondere in Lernformen, in denen die Gestaltung des Lernprozesses in erheblichem MaĂ auf die Teilnehmenden verlagert wird, gegenseitig beeinflussen. Sie leistet dies am Beispiel des Online-Lernens, dabei steht kontrastierend zur allgemeinen Forschung im Bereich E-Learning mit den Schwerpunkten Lerntheorie, Didaktik und technische Umsetzung, das soziale Umfeld im Zentrum der Untersuchung. Anhand von 18 Interviews wurden eine qualitative, themenbasierte Inhaltsanalyse sowie eine qualitative Netzwerkanalyse vorgenommen. Darauf aufbauend wurden fĂŒnf biographieorientierte Typen identifiziert und deren spezifische Strategien des Aufbaus, der Gestaltung und der Funktionalisierung der relevanten Teilnetzwerke zur UnterstĂŒtzung des Online-Lernens herausgearbeitet. Theoretische Bezugspunkte sind dabei die Erkenntnisse zum Distance Learning, die soziale Netzwerktheorie und das theoretische Konstrukt der Konstitutionsleistungen. Hierzu zĂ€hlen die Aspekte Organisation der Rahmenbedingungen, biographische Passung und soziale Akzeptanz. Es wird gezeigt, dass Online-Lernen ein hoch voraussetzungsreicher Prozess ist, der erheblich durch die den Lernenden umgebenden sozialen Netzwerke beeinflusst wird