139 research outputs found
The post-AGB evolution of AGB mass loss variations
We present new numerical hydrodynamical modelling of the evolution of
Asymptotic Giant Branch (AGB) mass loss fluctuations during the
post-AGB/Planetary Nebula phase. These models show that after ionization, the
observable effects of the mass loss fluctuations disappear in a few thousand
years, consistent with the fact that only few PNe have been found to be
surrounded by `rings'. We derive the observational characteristics of these
rings, and compare them to reported observations of the rings around NGC 6543,
finding a good match of emission properties and line shapes. We predict small
variations in the observable electron temperatures.Comment: 12 pages, 13 figures, accepted by A&
Origin of quasi-periodic shells in dust forming AGB winds
We have combined time dependent hydrodynamics with a two-fluid model for dust
driven AGB winds. Our calculations include self-consistent gas chemistry, grain
formation and growth, and a new implementation of the viscous momentum transfer
between grains and gas. This allows us to perform calculations in which no
assumptions about the completeness of momentum coupling are made. We derive new
expressions to treat time dependent and non-equilibrium drift in a hydro code.
Using a stationary state calculation for IRC +10216 as initial model, the time
dependent integration leads to a quasi-periodic mass loss in the case where
dust drift is taken into account. The time scale of the variation is of the
order of a few hundred years, which corresponds to the time scale needed to
explain the shell structure of the envelope of IRC +10216 and other AGB and
post-AGB stars, which has been a puzzle since its discovery. No such
periodicity is observed in comparison models without drift between dust and
gas.Comment: 17 pages, 9 figures, accepted by A&
Bispectrum speckle interferometry of IRC+10216: the dynamic evolution of the innermost circumstellar environment from 1995 to 2001
We present new near-infrared (JHK) bispectrum speckle-interferometry
monitoring of the carbon star IRC+10216 obtained between 1999 and 2001 with the
SAO 6m telescope. The J-, H-, and K-band resolutions are 50mas, 56mas, and
73mas, resp. The total sequence of K-band observations covers now 8 epochs from
1995 to 2001 and shows the dynamic evolution of the inner dust shell. The
present observations show that the appearance of the dust shell has
considerably changed compared to the epochs of 1995 to 1998. Four main
components within a 0.2" radius can be identified in the K-band images. The
apparent separation of the two initially brightest components A and B increased
from ~191mas in 1995 to ~351mas in 2001. Simultaneously, component B has been
fading and almost disappeared in 2000 whereas the initially faint components C
and D became brighter (relative to peak intensity). These changes can be
related to changes of the optical depth caused, e.g., by mass-loss variations
or new dust condensation in the wind. Our 2D radiative transfer model suggests
that the observed relative motion of A and B is not consistent with the known
terminal wind velocity of 15 km/s. The apparent motion with a deprojected
velocity of 19 km/s on average and of recently 27 km/s appears to be caused by
adisplacement of the dust density peak due to dust evaporation in the optically
thicker and hotter environment. Our monitoring, covering more than 3 pulsation
periods, shows that the structural variations are not related to the stellar
pulsation cycle in a simple way. This is consistent with the predictions of
hydrodynamical models that enhanced dust formation takes place on a timescale
of several pulsation periods. The timescale of the fading of component B can
well be explained by the formation of new dust in the circumstellar envelope.Comment: 11 pages including 6 PostScript figures; also available from
http://www.mpifr-bonn.mpg.de/div/ir-interferometry/publications.html;
Astronomy & Astrophysics, in pres
HI and CO in the circumstellar environment of the oxygen-rich AGB star RX Lep
Circumstellar shells around AGB stars are built over long periods of time
that may reach several million years. They may therefore be extended over large
sizes (~1 pc, possibly more), and different complementary tracers are needed to
describe their global properties. In the present work, we combined 21-cm HI and
CO rotational line data obtained on an oxygen-rich semi-regular variable, RX
Lep, to describe the global properties of its circumstellar environment. With
the SEST, we detected the CO(2-1) rotational line from RX Lep. The line profile
is parabolic and implies an expansion velocity of ~4.2 km/s and a mass-loss
rate ~1.7 10^-7 Msun/yr (d = 137 pc). The HI line at 21 cm was detected with
the Nancay Radiotelescope on the star position and at several offset positions.
The linear shell size is relatively small, ~0.1 pc, but we detect a trail
extending southward to ~0.5 pc. The line profiles are approximately Gaussian
with an FWHM ~3.8 km/s and interpreted with a model developed for the detached
shell around the carbon-rich AGB star Y CVn. Our HI spectra are well-reproduced
by assuming a constant outflow (Mloss = 1.65 10^-7 Msun/yr) of ~4 10^4 years
duration, which has been slowed down by the external medium. The spatial offset
of the HI source is consistent with the northward direction of the proper
motion, lending support to the presence of a trail resulting from the motion of
the source through the ISM, as already suggested for Mira, RS Cnc, and other
sources detected in HI. The source was also observed in SiO (3 mm) and OH (18
cm), but not detected. The properties of the external parts of circumstellar
shells around AGB stars should be dominated by the interaction between stellar
outflows and external matter for oxygen-rich, as well as for carbon-rich,
sources, and the 21-cm HI line provides a very useful tracer of these regions.Comment: 15 pages, 9 figures, accepted for publication in A&
A Mid-Infrared Imaging Survey of Proto-Planetary Nebula Candidates
We present the data from a mid-infrared imaging survey of 66 proto-planetary
nebula candidates using two mid-IR cameras (MIRAC2 and Berkcam) at the NASA
Infrared Telescope Facility and the United Kingdom Infrared Telescope. The goal
of this survey is to determine the size, flux, and morphology of the mid-IR
emission regions, which sample the inner regions of the circumstellar dust
shells of proto-planetary nebulae. We imaged these proto-planetary nebulae with
narrow-band filters () at wavelengths of
notable dust features. With our typical angular resolution of 1\arcsec, we
resolve 17 sources, find 48 objects unresolved, and do not detect 1 source. For
several sources, we checked optical and infrared associations and positions of
the sources. In table format, we list the size and flux measurements for all
the detected objects and show figures of all the resolved sources. Images for
all the detected objects are available on line in FITS format from the
Astronomy Digital Image Library at the National Center for Supercomputing
Application. The proto-planetary nebula candidate sample includes, in addition
to the predominant proto-planetary nebulae, extreme asymptotic giant branch
stars, young planetary nebulae, a supergiant, and a luminous blue variable. We
find that dust shells which are cooler ( K) and brighter in the
infrared are more easily resolved. Eleven of the seventeen resolved sources are
extended and fall into one of two types of mid-IR morphological classes:
core/elliptical or toroidal. Core/elliptical structures show unresolved cores
with lower surface brightness elliptical nebulae. Toroidal structures show
limb-brightened peaks suggesting equatorial density enhancements. We argue that
core/ellipticals have denser dust shells than toroidals.Comment: 32 pages, 5 tables, 2 e/ps figures (fig3 is available through ADIL
[see text]), to be published in ApJS May 1999 issu
The Vega Debris Disk -- A Surprise from Spitzer
We present high spatial resolution mid- and far-infrared images of the Vega
debris disk obtained with the Multiband Imaging Photometer for Spitzer (MIPS).
The disk is well resolved and its angular size is much larger than found
previously. The radius of the disk is at least 43" (330 AU), 70"(543 AU), and
105" (815 AU) in extent at 24, 70 and 160 um, respectively. The disk images are
circular, smooth and without clumpiness at all three wavelengths. The radial
surface brightness profiles imply an inner boundary at a radius of 11"+/-2" (86
AU). Assuming an amalgam of amorphous silicate and carbonaceous grains, the
disk can be modeled as an axially symmetric and geometrically thin disk, viewed
face-on, with the surface particle number density following an r^-1 power law.
The disk radiometric properties are consistent with a range of models using
grains of sizes ~1 to ~50 um. We find that a ring, containing grains larger
than 180 um and at radii of 86-200 AU from the star, can reproduce the observed
850 um flux, while its emission does not violate the observed MIPS profiles.
This ring could be associated with a population of larger asteroidal bodies
analogous to our own Kuiper Belt. Cascades of collisions starting with
encounters amongthese large bodies in the ring produce the small debris that is
blown outward by radiation pressure to much larger distances where we detect
its thermal emission. The dust production rate is >~10^15 g/s based on the MIPS
results. This rate would require a very massive asteroidal reservoir for the
dust to be produced in a steady state throughout Vega's life. Instead, we
suggest that the disk we imaged is ephemeral and that we are witnessing the
aftermath of a large and relatively recent collisional event, and subsequent
collisional cascade.Comment: 13 pages, 17 figures, accepted for publication in ApJ. (Figures 2,
3a, 3b and 4 have been degraded to lower resolutions.
The dynamical evolution of the circumstellar gas around low-and intermediate-mass stars I: the AGB
We have investigated the dynamical interaction of low- and-intermediate mass
stars (from 1 to 5 Msun) with their interstellar medium (ISM). In this first
paper, we examine the structures generated by the stellar winds during the
Asymptotic Giant Branch (AGB) phase, using a numerical code and the wind
history predicted by stellar evolution. The influence of the external ISM is
also taken into account. We find that the wind variations associated with the
thermal pulses lead to the formation of transient shells with an average
lifetime of 20,000 yr, and consequently do not remain recorded in the density
or velocity structure of the gas. The formation of shells that survive at the
end of the AGB occurs via two main processes: shocks between the shells formed
by two consecutive enhancements of the mass-loss or via continuous accumulation
of the material ejected by the star in the interaction region with the ISM. Our
models show that the mass of the circumstellar envelope increases appreciably
due to the ISM material swept up by the wind (up to 70 % for the 1 Msun stellar
model). We also point out the importance of the ISM on the deceleration and
compression of the external shells. According to our simulations, large regions
(up to 2.5 pc) of neutral gas surrounding the molecular envelopes of AGB stars
are expected. These large regions of gas are formed from the mass-loss
experienced by the star during the AGB evolution.Comment: 43 pages, 15 figures. Accepted for publication in the Astrophysical
Journa
Anesthetic Management for Concomitant Correction of Congenital Cardiac Defects and Long "O" Ring Tracheal Stenosis: A Role for Heliox?: Report of 2 Cases.
We present 2 infants with the rare association of long congenital tracheal stenosis, ventricular septal defect, and pulmonary hypertension. We describe a step-by-step assessment of the patients and the necessary procedures for a successful concomitant repair of both cardiac and tracheal malformations. The use of a helium-oxygen mixture (heliox) for the induction of anesthesia and pre-cardiopulmonary bypass is discussed
Experimental charge density of LiBD4 from maximum entropy method
We report on maximum entropy method study of the experimental atomic and ionic charges of LiBD4 in its low-temperature orthorhombic phase. Synchrotron radiation x-ray powder diffraction data, neutron powder diffraction data, and density functional calculations were used. The atomic and ionic charges were determined for both experimental and theoretical results using the Bader analysis for atoms in molecules. The charge transfer from the Li cation to the BD4 anion is 0.86(+/- 9) e, which is in good agreement with the ab initio calculated value of 0.895 e. The experimental accuracy was determined considering the differences between results obtained for data collected at 10 and 90 K, different experimental setups (high-resolution diffractometer or image plate diffractometer), and different structural models used for the prior density distributions needed for accurate maximum entropy calculations (refined using only synchrotron radiation x-ray powder diffraction data or combined with neutron powder diffraction data)
- …