83 research outputs found
Rapid evolution of [WC] stars in the Magellanic Clouds
We obtained new spectra of fourteen Magellanic Cloud planetary nebulae with
the South African Large Telescope to determine heating rates of their central
stars and to verify evolutionary models of post-asymptotic giant branch stars.
We compared new spectra with observations made in previous years. Five
planetary nebulae showed an increase in excitation over time. Four of their
central stars exhibit [WC] features in their spectra, including three new
detections. This raises the total number of [WC] central stars of PNe in the
Magellanic Clouds to ten. We compared determined heating rates of the four [WC]
central stars with the He-burning post-asymptotic giant branch evolutionary
tracks and the remaining star with the H-burning tracks. Determined heating
rates are consistent with the evolutionary models for both H and He-burning
post-asymptotic giant branch stars. The central stars of the PNe that show the
fastest increase of excitation are also the most luminous in the sample. This
indicates that [WC] central stars in the Magellanic Clouds evolve faster than
H-burning central stars, and they originate from more massive progenitors.Comment: 11 pages, 2 figure
A multi-wavelength view on the dusty Wolf-Rayet star WR 48a
We present results from the first attempts to derive various physical
characteristics of the dusty Wolf-Rayet star WR 48a based on a multi-wavelength
view of its observational properties. This is done on the basis of new optical
and near-infrared spectral observations and on data from various archives in
the optical, radio and X-rays. The optical spectrum of WR 48a is acceptably
well represented by a sum of two spectra: of a WR star of the WC8 type and of a
WR star of the WN8h type. The strength of the interstellar absorption features
in the optical spectra of WR 48a and the near-by stars D2-3 and D2-7 (both
members of the open cluster Danks 2) indicates that WR 48a is located at a
distance of ~4 kpc from us. WR 48a is very likely a thermal radio source and
for such a case and smooth (no clumps) wind its radio emission suggests a
relatively high mass-loss rate of this dusty WR star (dM/dt = a few x 10^(-4)
solar masses per year). Long timescale (years) variability of WR 48a is
established in the optical, radio and X-rays. Colliding stellar winds likely
play a very important role in the physics of this object. However, some
LBV-like (luminous blue variable) activity could not be excluded as well.Comment: Accepted for publication in MNRAS; 16 pages, 16 figures, 6 table
Nuclear ashes and outflow in the eruptive star Nova Vul 1670
CK Vulpeculae was observed in outburst in 1670-16721, but no counterpart was
seen until 1982, when a bipolar nebula was found at its location. Historically,
CK Vul has been considered to be a nova (Nova Vul 1670), but a similarity to
'red transients', which are more luminous than classical nova and thought to be
the result of stellar collisions, has re-opened the question of CK Vul's
status. Red transients cool to resemble late M-type stars, surrounded by
circumstellar material rich in molecules and dust. No stellar source has been
seen in CK Vul, though a radio continuum source was identified at the expansion
centre of the nebula. Here we report CK Vul is surrounded by chemically rich
molecular gas with peculiar isotopic ratios, as well as dust. The chemical
composition cannot be reconciled with a nova or indeed any other known
explosion. In addition, the mass of the surrounding gas is too high for a nova,
though the conversion from observations of CO to a total mass is uncertain. We
conclude that CK Vul is best explained as the remnant of a merger of two stars.Comment: an older version of an article that appeared in Nature; published in
Nature, online version, 23 March 201
Evolution of the central stars of young planetary nebulae
The evolution of central stars of planetary nebulae was so far documented in
just a few cases. However, spectra collected a few decades ago may provide a
good reference for studying the evolution of central stars using the emission
line fluxes of their nebulae. We investigated evolutionary changes of the
[OIII] 5007 A line flux in the spectra of planetary nebulae. We compared
nebular fluxes collected during a decade or longer. We used literature data and
newly obtained spectra. A grid of Cloudy models was computed using existing
evolutionary models, and the models were compared with the observations. An
increase of the [OIII] 5007 A line flux is frequently observed in young
planetary nebulae hosting H-rich central stars. The increasing nebular
excitation is the response to the increasing temperature and hardening
radiation of the central stars. We did not observe any changes in the nebular
fluxes in the planetary nebulae hosting late-type Wolf-Rayet (WR) central
stars. This may indicate a slower temperature evolution (which may stem from a
different evolutionary status) of late-[WR] stars. In young planetary nebulae
with H-rich central stars, the evolution can be followed using optical spectra
collected during a decade or longer. The observed evolution of H-rich central
stars is consistent with the predictions of the evolutionary models provided in
the literature. Late-[WR] stars possibly follow a different evolutionary path.Comment: Accepted for publication in A&
Lithium in coalesced non-compact stars
Context. Galactic red novae are thought to be produced in stellar mergers
between non-compact stars, such as main-sequence stars and cool giants. They
are hoped to help in explaining physical processes involved in common envelope
evolution and stellar binary collisions. Aims. We investigate the presence of
lithium in three best-observed Galactic red nova remnants. Explaining the
origin of lithium may point to mixing mechanism present before, during, or
after the merger. Methods. The lithium line at 6707.81 A was compared to a
feature of [Ca I at 6572.78 A to derive relative abundances in circumstellar
gas. Absolute abundances were next calculated assuming the Solar calcium to
lithium abundance ratio. Results. Lithium abundances were measured in the
merger remnants of V838 Mon with A(Li)=2.3, CK Vul with A(Li)=2.5, and V1309
Sco with A(Li)=1.8. Conclusions. Lithium is overabundant in red novae,
suggesting that at least some merger products activate the Cameron-Fowler
mechanism whereby convective mixing can reach the deep stellar interior.
Whether deep convection and associated diffusion alone or some other processes
(e.g. spin down) can be responsible for driving the Cameron-Fowler mechanism in
the remnants requires further studies. Early observations of lithium in V838
Mon hint that these mechanisms can activate early, perhaps already in the
common envelope phase. These observations should be taken into account in
modelling these complex systems.Comment: submitted to A&A; comments welcom
The second release of the Toru\'n catalogue of Galactic post-AGB objects: new classification scheme
The investigation of post-AGB objects (proto-planetary nebulae) is very
important from the standpoint of physical and chemical changes occurring during
the late stages of stellar evolution. The Toru\'n catalogue of Galactic
post-AGB and related objects is an evolutive catalogue containing astrometric,
photometric and spectroscopic data as well as HST images for all known post-AGB
objects and candidates in our Galaxy. This free-access catalogue can serve as
an ideal tool to study different groups of post-AGB objects, especially due to
the fact that all information is gathered in one place. The second release of
our catalogue introduces a simple classification scheme of post-AGB objects and
includes a significant number of new objects, photometric data, spectra and
images. Here, using objects from the catalogue we consider the problem of the
termination of the AGB phase.Comment: 1 figure, poster during IAUS 283, Planetary Nebulae: an Eye to the
Futur
Validating post-AGB candidates in the LMC and SMC using SALT spectra
We selected a sample of post-AGB candidates in the Magellanic Clouds on the
basis of their near- and mid-infrared colour characteristics. Fifteen of the
most optically bright post-AGB candidates were observed with the South African
Large Telescope in order to determine their stellar parameters and thus to
validate or discriminate their nature as post-AGB objects in the Magellanic
Clouds. The spectral types of absorption-line objects were estimated according
to the MK classification, and effective temperatures were obtained by means of
stellar atmosphere modelling. Emission-line objects were classified on the
basis of the fluxes of the emission lines and the presence of the continuum.
Out of 15 observed objects, only 4 appear to be genuine post-AGB stars (27\%).
In the SMC, 1 out of 4 is post-AGB, and in the LMC, 3 out 11 are post-AGB
objects. Thus, we can conclude that the selected region in the colour-colour
diagram, while selecting the genuine post-AGB objects, overlaps severely with
other types of objects, in particular young stellar objects and planetary
nebulae. Additional classification criteria are required to distinguish between
post-AGB stars and other types of objects. In particular, photometry at far-IR
wavelengths would greatly assist in distinguishing young stellar objects from
evolved ones. On the other hand, we showed that the low-resolution optical
spectra appear to be sufficient to determine whether the candidates are
post-AGB objects.Comment: 19 pages, 19 figures, A&A in pres
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