400 research outputs found
Imaging the circumstellar envelopes of AGB stars
We report the results of an exploratory program to image the extended
circumstellar envelopes of asymptotic giant branch (AGB) stars in
dust-scattered galactic light. The goal is to characterize the morphology of
the envelopes as a probe of the mass-loss process. The observations consist of
short exposures with the VLT and longer exposures with 1-2m telescopes,
augmented with archival images from the Hubble Space Telescope. We observed 12
AGB stars and detected the circumstellar envelopes in 7. The detected envelopes
have mass loss rates more than about 5 10E-6 solar mass per year, and they can
be seen out to distances of about 1 kpc. The observations provide information
on the mass loss history on time scales up to about 10,000 years. For the five
AGB envelopes in which the circumstellar geometry is well determined by
scattered light observations, all except one (OH348.2-19.7) show deviations
from spherical symmetry. Two (IRC+10216 and IRC+10011) show roughly spherical
envelopes at large radii but asymmetry or bipolarity close to the star; one
(AFGL 2514) shows an extended, elliptical envelope, and one (AFGL 3068) shows a
spiral pattern. The non-spherical structures are all consistent with the
effects of binary interactions. Our observations are in accord with a scenario
in which binary companions play a role in shaping planetary nebulae, and show
that the circumstellar gas is already partly shaped on the AGB, before
evolution to the proto-planetary nebula phase.Comment: Accepted by AA 21 Feb 2006; 18 pages, 14 figs; for high resolution
images, contact mauron at graal.univ-montp2.f
On the Formation of Multiple-Shells Around Asymptotic Giant Branch Stars
Two types of models for the formation of semi-periodic concentric multiple
shells (M-shells) around asymptotic giant branch (AGB) stars and in planetary
nebulae are compared against observations. Models that attribute the M-shells
to processes in an extended wind acceleration zone around AGB stars result in
an optically thick acceleration zone, which reduces the acceleration efficiency
in outer parts of the extended acceleration zone. This makes such models an
unlikely explanation for the formation of M-shells. Models which attribute the
M-shell to semi-periodic variation in one or more stellar properties are most
compatible with observations. The only stellar variation models on time scales
of 50-1500 years that have been suggested are based on an assumed solar-like
magnetic cycle. Although ad-hoc, the magnetic cycle assumption fits naturally
into the increasingly popular view that magnetic activity plays a role in
shaping the wind from upper AGB stars.Comment: 8 pages, Submitted to Ap
Deep optical imaging of AGB circumstellar envelopes
We report results of a program to image the extended circumstellar envelopes
of asymptotic giant branch (AGB) stars in dust-scattered Galactic light. The
goal is to characterize the shapes of the envelopes to probe the mass-loss
geometry and the presence of hidden binary companions. The observations consist
of deep optical imaging of 22 AGB stars with high mass loss rates: 16 with the
ESO 3.5 m NTT telescope, and the remainder with other telescopes. The
circumstellar envelopes are detected in 15 objects, with mass loss rates > 2E-6
Msun/year. The surface brightness of the envelopes shows a strong decrease with
Galactic radius, which indicates a steep radial gradient in the interstellar
radiation field. The envelopes range from circular to elliptical in shape, and
we characterize them by the ellipticity (E = major/minor axis) of iso-intensity
contours. We find that about 50 percent of the envelopes are close to circular
with E
1.2. We interpret the shapes in terms of populations of single stars and
binaries whose envelopes are flattened by a companion. The distribution of E is
qualitatively consistent with expectations based on population synthesis models
of binary AGB stars. We also find that about 50 percent of the sample exhibit
small-scale, elongated features in the central regions. We interpret these as
the escape of light from the central star through polar holes, which are also
likely produced by companions. Our observations of envelope flattening and
polar holes point to a hidden population of companions within the circumstellar
envelopes of AGB stars. These companions are expected to play an important role
in the transition to post-AGB stars and the formation of planetary nebulae.Comment: 19 pages, 13 figures, color pictures in Appendix, accepted by A&
Gas phase atomic metals in the circumstellar envelope of IRC+10216
We report the results of a search for gas phase atomic metals in the
circumstellar envelope of the AGB carbon star IRC+10216. The search was made
using high resolution (R=50000) optical absorption spectroscopy of a backgound
star that probes the envelope on a line of sight 35" from the center. The metal
species that we detect in the envelope include NaI, KI, CaI, CaII, CrI, and
FeI, with upper limits for AlI, MnI, TiI, TiII, and SrII. The observations are
used to determine the metal abundances in the gas phase and the condensation
onto grains. The metal depletions range from a factor of 5 for Na to 300 for
Ca, with some similarity to the depletion pattern in interstellar clouds. Our
results directly constrain the condensation efficiency of metals in a
carbon-rich circumstellar envelope and the mix of solid and gas phase metals
returned by the star to the ISM. The abundances of the uncondensed metal atoms
that we observe are typically larger than the abundances of the metal-bearing
molecules detected in the envelope. The metal atoms are therefore the major
metal species in the gas phase and likely play a key role in the metal
chemistry.Comment: 11 pages, 8 Figures. Accepted by Astronomy and Astrophysic
The Shapes of AGB Envelopes as Probes of Binary Companions
We describe how the large scale geometry of the circumstellar envelopes of
asymptotic giant branch stars can be used to probe the presence of unseen
stellar companions. A nearby companion modifies the mass loss by
gravitationally focusing the wind towards the orbital plane, and thereby
determines the shape of the envelope at large distances from the star. Using
available simulations, we develop a prescription for the observed shapes of
envelopes in terms of the binary parameters, envelope orientation, and type of
observation. The prescription provides a tool for the analysis of envelope
images at optical, infrared, and millimetre wavelengths, which can be used to
constrain the presence of companions in well observed cases. We illustrate this
approach by examining the possible role of binary companions in triggering the
onset of axi-symmetry in planetary nebula formation. If interaction with the
primary leads to axi-symmetry, the spherical halos widely seen around newly
formed nebulae set limits on the companion mass. Only low mass objects may
orbit close to the primary without observable shaping effects: they remain
invisible until the interaction causes a sudden change in the mass loss
geometry.Comment: 11 pages, 7 figures, to appear in MNRA
Envelope density pattern around wide binary AGB stars: a dynamical model
The goal is to build up a simple dynamical model for the out-flowing
circumstellar envelope around AGB stars in a wide binary system to explore the
parameter dependence of the geometrical characteristics of column density
patterns. An AGB star in a wide binary system is considered as a 3-D piston
model that can induce a 3-D quasi-spherical density structure in the
circumstellar envelope by orbital motion of the AGB star. The column density
pattern only depends on two parameters: eccentricity of the orbit e and the
terminal outflow velocity to mean orbital velocity ratio gamma. When viewed
perpendicular to the orbital plane, spiral, broken spiral, and incomplete
concentric shell patterns can be seen, while when viewed along the orbital
plane, alternative concentric half-shell, egg-shell, and half-shell half-gap
patterns will develop. Non-zero eccentricity causes asymmetry, while larger
gamma makes a weaker pattern and helps bring out asymmetry. A spiral pattern
may becomes broken when e > 0.4. The spiral center is always less than 12% of
spiral pitch away from the orbit center. One should have more chances (~ 80%)
seeing spiral-like patterns than seeing concentric shells (~ 20%) in the
circumstellar envelope of wide binary AGB stars.Comment: 5 pages, 5 figures, accepted to be published in A&A in 200
Imaging Polarimetric Observations of a New Circumstellar Disk System
Few circumstellar disks have been directly observed. Here we use sensitive
differential polarimetric techniques to overcome atmospheric speckle noise in
order to image the circumstellar material around HD 169142. The detected
envelope or disk is considerably smaller than expectations based on the
measured strength of the far-IR excess from this system
A Detailed Analysis of the Dust Formation Zone of IRC+10216 Derived from Mid-IR Bands of C2H2 and HCN
A spectral survey of IRC+10216 has been carried out in the range 11 to 14 um
with a spectral resolution of about 4 km s^-1. We have identified a forest of
lines in six bands of C2H2 involving the vibrational states from the ground to
3nu5 and in two bands of HCN, involving the vibrational states from the ground
up to 2nu2. Some of these transitions are observed also in H13CCH and H13CN. We
have estimated the kinetic, vibrational, and rotational temperatures, and the
abundances and column densities of C2H2 and HCN between 1 and 300 R* (1.5E16
cm) by fitting about 300 of these ro-vibrational lines. The envelope can be
divided into three regions with approximate boundaries at 0.019 arcsec (the
stellar photosphere), 0.1 arcsec (the inner dust formation zone), and 0.4
arcsec (outer dust formation zone). Most of the lines might require a large
microturbulence broadening. The derived abundances of C2H2 and HCN increase by
factors of 10 and 4, respectively, from the innermost envelope outwards. The
derived column densities for both C2H2 and HCN are 1.6E19 cm^-2. Vibrational
states up to 3000 K above ground are populated, suggesting pumping by
near-infrared radiation from the star and innermost envelope. Low rotational
levels can be considered under LTE while those with J>20-30 are not
thermalized. A few lines require special analysis to deal with effects like
overlap with lines of other molecules.Comment: 8 pages, 16 figures, 2 machine-readable tables, accepted in the
Astrophysical Journa
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
New, nearby bright southern ultracool dwarfs
We report the discovery of twenty-one hitherto unknown bright southern
ultracool dwarfs with spectral types in the range M7 to L5.5, together with new
observations of a further three late M dwarfs previously confirmed. Three more
objects are already identified in the literature as high proper motion stars;we
derive their spectral types for the first time. All objects were selected from
the 2MASS All Sky and SuperCOSMOS point source databases on the basis of their
optical/near-infrared colours, -band magnitudes and proper motions. Low
resolution (R 1000) spectroscopy with the ESO/NTT SOFI spectrograph
has confirmed the ultracool nature of 24 targets, out of a total of 25
candidates observed. Spectral types are derived by direct comparison with
template objects and compared to results from HO and FeH indices. We also
report the discovery of one binary, as revealed by SOFI acquisition imaging;
spectra were taken for both components. The spectral types of the two
components are L2 and L4 and the distance 19 pc. Spectroscopic distances
and transverse velocities are derived for the sample. Two L5 objects lie
only 10 pc distant. Such nearby objects are excellent targets for
further study to derive their parallaxes and to search for fainter, later
companions with AO and/or methane imaging.Comment: 11 pages, 10 figures, accepted to MNRA
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