1,928 research outputs found
BD+30 3639: The Infrared Spectrum During Post-AGB Stellar Evolution
We present a radiative-transfer calculation which reproduces the infrared
spectrum of the planetary nebula BD~+303639. We calculate the
transfer process through absorption and scattering in a spherical-symmetric
multi-grain dust shell. The emission of transiently heated particles is taken
into account, as well as polycyclic aromatic hydrocarbons. We obtain an
acceptable fit to most of the spectrum, including the PAH infrared bands. At
submillimetre wavelengths the observed emission is larger than the model
predicts, indicating that large dust conglomerates (``f{}luffy grains'') may be
needed as an additional constituent. The fit favours a distance of kpc, which implies that BD~+303639 has evolved from a massive
progenitor of several solar masses. A low dust-to-gas mass ratio is found in
the ionised region. The calculations yield an original mass-loss rate of
2\times10^{-4} \msolar \peryr on the Asymptotic Giant Branch. Using this
mass-loss rate, we calculate how the infrared spectrum has evolved during the
post-AGB evolution. We show in particular the evolution of the IRAS colours
during the preceding post-AGB evolution.Comment: accepted for publication in MNRAS. LaTeX, 15 pages, hardcopy and 8
figures available from [email protected] or [email protected]
Structure and shaping processes within the extended atmospheres of AGB stars
We present recent studies using the near-infrared instrument AMBER of the VLT
Interferometer (VLTI) to investigate the structure and shaping processes within
the extended atmosphere of AGB stars. Spectrally resolved near-infrared AMBER
observations of the Mira variable S Ori have revealed wavelength-dependent
apparent angular sizes. These data were successfully compared to dynamic model
atmospheres, which predict wavelength-dependent radii because of geometrically
extended molecular layers. Most recently, AMBER closure phase measurements of
several AGB stars have also revealed wavelength-dependent deviations from 0/180
deg., indicating deviations from point symmetry. The variation of closure phase
with wavelength indicates a complex non-spherical stratification of the
extended atmosphere, and may reveal whether observed asymmetries are located
near the photosphere or in the outer molecular layers. Concurrent observations
of SiO masers located within the extended molecular layers provide us with
additional information on the morphology, conditions, and kinematics of this
shell. These observations promise to provide us with new important insights
into the shaping processes at work during the AGB phase. With improved imaging
capabilities at the VLTI, we expect to extend the successful story of imaging
studies of planetary nebulae to the photosphere and extended outer atmosphere
of AGB stars.Comment: 6 pages, Proc. of "Asymmetric Planetary Nebulae V", A.A. Zijlstra, F.
Lykou, I. McDonald, and E. Lagadec (eds.), Jodrell Bank Centre for
Astrophysics, Manchester, UK, 201
PAH Formation in O-rich Planetary Nebulae
Polycyclic aromatic hydrocarbons (PAHs) have been observed in O-rich
planetary nebulae towards the Galactic Bulge. This combination of oxygen-rich
and carbon-rich material, known as dual-dust or mixed chemistry, is not
expected to be seen around such objects. We recently proposed that PAHs could
be formed from the photodissociation of CO in dense tori. In this work, using
VISIR/VLT, we spatially resolved the emission of the PAH bands and ionised
emission from the [SIV] line, confirming the presence of dense central tori in
all the observed O-rich objects. Furthermore, we show that for most of the
objects, PAHs are located at the outer edge of these dense/compact tori, while
the ionised material is mostly present in the inner parts of these tori,
consistent with our hypothesis for the formation of PAHs in these systems. The
presence of a dense torus has been strongly associated with the action of a
central binary star and, as such, the rich chemistry seen in these regions may
also be related to the formation of exoplanets in post-common-envelope binary
systems.Comment: 14, accepted for publication in the MNRAS Journa
Investigating the nature of the Fried Egg nebula: CO mm-line and optical spectroscopy of IRAS 17163-3907
Through CO mm-line and optical spectroscopy, we investigate the properties of
the Fried Egg nebula IRAS 17163-3907, which has recently been proposed to be
one of the rare members of the yellow hypergiant class. The CO J=2-1 and J=3-2
emission arises from a region within 20" of the star and is clearly associated
with the circumstellar material. The CO lines show a multi-component
asymmetrical profile, and an unexpected velocity gradient is resolved in the
east-west direction, suggesting a bipolar outflow. This is in contrast with the
apparent symmetry of the dust envelope as observed in the infrared. The optical
spectrum of IRAS 17163-3907 between 5100 and 9000 {\AA} was compared with that
of the archetypal yellow hypergiant IRC+10420 and was found to be very similar.
These results build on previous evidence that IRAS 17163-3907 is a yellow
hypergiant.Comment: 14 pages including appendix, accepted for publication in A&
Disk evaporation in a planetary nebula
We study the Galactic bulge planetary nebula M 2-29 (for which a 3-year
eclipse event of the central star has been attributed to a dust disk) using HST
imaging and VLT spectroscopy, both long-slit and integral field. The central
cavity of M 2-29 is filled with a decreasing, slow wind. An inner high density
core is detected, with radius less than 250 AU, interpreted as a rotating
gas/dust disk with a bipolar disk wind. The evaporating disk is argued to be
the source of the slow wind. The central star is a source of a very fast wind
(1000 km/s). An outer, partial ring is seen in the equatorial plane, expanding
at 12 km/s. The azimuthal asymmetry is attributed to mass-loss modulation by an
eccentric binary. M 2-29 presents a crucial point in disk evolution, where
ionization causes the gas to be lost, leaving a low-mass dust disk behind.Comment: 11 pages, accepted for publication in "Astronomy and Astrophysics
Submillimeter polarisation and magnetic field properties in the envelopes of proto-planetary nebulae CRL 618 and OH 231.8+4.2
We have carried out continuum and line polarisation observations of two
Proto-planetary nebulae (PPNe), CRL 618 and OH 231.8+4.2, using the
Submillimeter Array (SMA) in its compact configuration. The frequency range of
observations, 330-345 GHz, includes the CO(J=3-2) line emission. CRL 618 and OH
231.8+4.2 show quadrupolar and bipolar optical lobes, respectively, surrounded
by a dusty envelope reminiscent of their AGB phase. We report a detection of
dust continuum polarised emission in both PPNe above 4 sigma but no molecular
line polarisation detection above a 3 sigma limit. OH 231.8+4.2 is slightly
more polarised on average than CRL 618 with a mean fractional polarisation of
4.3 and 0.3 per cent, respectively. This agrees with the previous finding that
silicate dust shows higher polarisation than carbonaceous dust. In both
objects, an anti-correlation between the fractional polarisation and the
intensity is observed. Neither PPNe show a well defined toroidal equatorial
field, rather the field is generally well aligned and organised along the polar
direction. This is clearly seen in CRL 618 while in the case of OH 231.8+4.2,
the geometry indicates an X-shaped structure coinciding overall with a
dipole/polar configuration. However in the later case, the presence of a
fragmented and weak toroidal field should not be discarded. Finally, in both
PPNe, we observed that the well organised magnetic field is parallel with the
major axis of the 12CO outflow. This alignment could indicate the presence of a
magnetic outflow launching mechanism. Based on our new high resolution data we
propose two scenarios to explain the evolution of the magnetic field in evolved
stars.Comment: 11 pages, 8 figures and 1 table. Accepted for publication in the
Monthly Notices of the Royal Astronomical Societ
The metallicity extremes of the Sagittarius dSph using SALT spectroscopy of PNe
In this work we present the first spectroscopic results obtained with the
Southern African Large Telescope (SALT) telescope during its
perfomance-verification phase. We find that the Sagittarius dwarf spheroidal
galaxy (Sgr) Sgr contains a youngest stellar population with [O/H] -0.2 and age
t>1 Gyr, and an oldest population with [O/H]=-2.0. The values are based on
spectra of two planetary nebulae (PNe), using empirical abundance
determinations. We calculated abundances for O, N, Ne, Ar, S, Cl, Fe, C and He.
We confirm the high abundances of PN StWr2-21 with 12+log(O/H) = 8.57+/-0.02
dex. The other PN studied, BoBn1, is an extraordinary object in that the neon
abundance exceeds that of oxygen. The abundances of S, Ar and Cl in BoBn1 yield
the original stellar metallicity, corresponding to 12+log(O/H) = 6.72+/-0.16
dex which is 1/110 of the solar value. The actual [O/H] is much higher: third
dredge-up enriched the material by a factor of ~12 in oxygen, ~240 in nitrogen
and ~70 in neon. Neon as well as nitrogen and oxygen content may have been
produced in the intershell of low-mass AGB stars. Well defined broad WR lines
are present in the spectrum of StWr2-21 and absent in the spectrum of BoBn1.
This puts the fraction of [WR]-type central PNe stars to 67% for dSph galaxies.Comment: 14 pages, 4 figures, accepted to MNRA
The low wind expansion velocity of metal-poor carbon stars in the Halo and the Sagittarius stream
We report the detection, from observations using the James Clerk Maxwell
Telescope, of CO J 3 2 transition lines in six carbon stars, selected
as members of the Galactic Halo and having similar infrared colors. Just one
Halo star had been detected in CO before this work. Infrared observations show
that these stars are red (J-K 3), due to the presence of large dusty
circumstellar envelopes. Radiative transfer models indicates that these stars
are losing mass with rather large dust mass-loss rates in the range 1--3.3
Myr, similar to what can be observed in the
Galactic disc. We show that two of these stars are effectively in the Halo, one
is likely linked to the stream of the Sagittarius Dwarf Spheroidal galaxy (Sgr
dSph), and the other three stars certainly belong to the thick disc. The wind
expansion velocities of the observed stars are low compared to carbon stars in
the thin disc and are lower for the stars in the Halo and the Sgr dSph stream
than in the thick disc. We discuss the possibility that the low expansion
velocities result from the low metallicity of the Halo carbon stars. This
implies that metal-poor carbon stars lose mass at a rate similar to metal-rich
carbon stars, but with lower expansion velocities, as predicted by recent
theoretical models. This result implies that the current estimates of mass-loss
rates from carbon stars in Local Group galaxies will have to be reconsidered.Comment: 10 pages, 7 figures, accepted for publication in MNRA
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