593 research outputs found
The nearby AGB star L2 Puppis: the birth of a planetary nebula ?
Adaptive optics observations in the infrared (VLT/NACO, Kervella et al. 2014)
and visible (VLT/SPHERE, Kervella et al. 2015) domains revealed that the nearby
AGB star L2 Pup (d=64 pc) is surrounded by a dust disk seen almost edge-on.
Thermal emission from a large dust "loop" is detected at 4 microns up to more
than 10 AU from the star. We also detect a secondary source at a separation of
32 mas, whose nature is uncertain. L2 Pup is currently a relatively "young" AGB
star, so we may witness the formation of a planetary nebula. The mechanism that
breaks the spherical symmetry of mass loss is currently uncertain, but we
propose that the dust disk and companion are key elements in the shaping of the
bipolar structure. L2 Pup emerges as an important system to test this
hypothesis.Comment: 6 pages, 3 figures, Proceedings of the Physics of Evolved Stars
conference, 8-12 June 2015, Nice, Franc
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
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
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
VISIR-VLT high resolution study of the extended emission of four obscured post-AGB candidates
The onset of the asymmetry of planetary nebulae (PNe) is expected to occur
during the late Asymptotic Giant Branch (AGB) and early post-AGB phases of low-
and intermediate-mass stars. Among all post-AGB objects, the most heavily
obscured ones might have escaped the selection criteria of previous studies
detecting extreme axysimmetric structures in young PNe. Since the most heavily
obscured post-AGB sources can be expected to descend from the most massive PN
progenitors, these should exhibit clear asymmetric morphologies. We have
obtained VISIR-VLT mid-IR images of four heavily obscured post-AGB objects
barely resolved in previous Spitzer IRAC observations to analyze their
morphology and physical conditions across the mid-IR. The VISIR-VLT images have
been deconvolved, flux calibrated, and used to construct RGB composite pictures
as well as color and optical depth maps that allow us to study the morphology
and physical properties of the extended emission of these sources. We have
detected extended emission from the four objects in our sample and resolved it
into several structural components that are greatly enhanced in the temperature
and optical depth maps. They reveal the presence of asymmetry in three young
PNe (IRAS 15534-5422, IRAS 17009-4154, and IRAS 18454+0001), where the
asymmetries can be associated with dusty torii and slightly bipolar outflows.
The fourth source (IRAS 18229-1127), a possible post-AGB star, is better
described as a rhomboidal detached shell. The heavily obscured sources in our
sample do not show extreme axisymmetric morphologies. This is at odds with the
expectation of highly asymmetrical morphologies in post-AGB sources descending
from massive PN progenitors. The sources presented in this paper may be
sampling critical early phases in the evolution of massive PN progenitors,
before extreme asymmetries develop.Comment: 9 pages, 4 figure
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&
Carbon-rich dust production in metal-poor galaxies in the Local Group
We have observed a sample of 19 carbon stars in the Sculptor, Carina, Fornax,
and Leo I dwarf spheroidal galaxies with the Infrared Spectrograph on the
Spitzer Space Telescope. The spectra show significant quantities of dust around
the carbon stars in Sculptor, Fornax, and Leo I, but little in Carina. Previous
comparisons of carbon stars with similar pulsation properties in the Galaxy and
the Magellanic Clouds revealed no evidence that metallicity affected the
production of dust by carbon stars. However, the more metal-poor stars in the
current sample appear to be generating less dust. These data extend two known
trends to lower metallicities. In more metal-poor samples, the SiC dust
emission weakens, while the acetylene absorption strengthens. The bolometric
magnitudes and infrared spectral properties of the carbon stars in Fornax are
consistent with metallicities more similar to carbon stars in the Magellanic
Clouds than in the other dwarf spheroidals in our sample. A study of the carbon
budget in these stars reinforces previous considerations that the dredge-up of
sufficient quantities of carbon from the stellar cores may trigger the final
superwind phase, ending a star's lifetime on the asymptotic giant branch.Comment: ApJ, in press, 21 pages, 12 figures. Replaced Fig 12, corrected two
reference
Study of the inner dust envelope and stellar photosphere of the AGB star R Doradus using SPHERE/ZIMPOL
We use high-angular-resolution images obtained with SPHERE/ZIMPOL to study
the photosphere, the warm molecular layer, and the inner wind of the close-by
oxygen-rich AGB star R Doradus. We present observations in filters V,
cntH, and cnt820 and investigate the surface brightness distribution of
the star and of the polarised light produced in the inner envelope. Thanks to
second-epoch observations in cntH, we are able to see variability on
the stellar photosphere. We find that in the first epoch the surface brightness
of R Dor is asymmetric in V and cntH, the filters where molecular
opacity is stronger, while in cnt820 the surface brightness is closer to being
axisymmetric. The second-epoch observations in cntH show that the
morphology of R Dor changes completely in a timespan of 48 days to a more
axisymmetric and compact configuration. The polarised intensity is asymmetric
in all epochs and varies by between a factor of 2.3 and 3.7 with azimuth for
the different images. We fit the radial profile of the polarised intensity
using a spherically symmetric model and a parametric description of the dust
density profile, . On average, we find exponents of
that correspond to a much steeper density profile than that of
a wind expanding at constant velocity. The dust densities we derive imply an
upper limit for the dust-to-gas ratio of at 5.0
. Given the uncertainties in observations and models, this value is
consistent with the minimum values required by wind-driving models for the
onset of a wind, of . However, if the steep density
profile we find extends to larger distances from the star, the dust-to-gas
ratio will quickly become too small for the wind of R Dor to be driven by the
grains that produce the scattered light.Comment: 10 pages, 8 figures, 4 table
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