796 research outputs found
Departure from Axisymmetry in Planetary Nebulae
Many planetary nebulae (PNe) exhibit distinctly non-axisymmetric structure in
either (i) the shape of the nebula, or (ii) in the off-centered position of the
illuminating star. By examining a large number of well resolved images of PNe
we estimate that about 30-50 percents of all PNe exhibit distinctly
non-axisymmetric structure. In this paper, we discuss how such departures from
axisymmetry can arise from the binary nature of the progenitors of the PNe. The
scenarios include (a) relatively close binaries with eccentric orbits, and (b)
longer orbital period systems with either circular or eccentric orbits. In
order to assess the fraction of PNe whose non-axisymmetric morphologies are
expected to arise in binary systems, we have carried out a detailed population
synthesis study. The expected deviations from axisymmetry are classified for
each binary and the results tabulated. We find that about 25 percents of
elliptical and 30-50 percents of bipolar PNe are expected to acquire
non-axisymmetric structure from binary interactions.Comment: 15 pages + 4 tables; Submitted to Ap
The evolution of M 2-9 from 2000 to 2010
M 2-9, the Butterfly nebula, is an outstanding representative of extreme
aspherical flows. It presents unique features such as a pair of high-velocity
dusty polar blobs and a mirror-symmetric rotating pattern in the inner lobes.
Imaging monitoring of the evolution of the nebula in the past decade is
presented. We determine the proper motions of the dusty blobs, which infer a
new distance estimate of 1.3+-0.2 kpc, a total nebular size of 0.8 pc, a speed
of 147 km/s, and a kinematical age of 2500 yr. The corkscrew geometry of the
inner rotating pattern is quantified. Different recombination timescales for
different ions explain the observed surface brightness distribution. According
to the images taken after 1999, the pattern rotates with a period of 92+-4 yr.
On the other hand, the analysis of images taken between 1952 and 1977 measures
a faster angular velocity. If the phenomenon were related to orbital motion,
this would correspond to a modest orbital eccentricity (e=0.10+-0.05), and a
slightly shorter period (86+-5 yr). New features have appeared after 2005 on
the west side of the lobes and at the base of the pattern. The geometry and
travelling times of the rotating pattern support our previous proposal that the
phenomenon is produced by a collimated spray of high velocity particles (jet)
from the central source, which excites the walls of the inner cavity of M 2-9,
rather than by a ionizing photon beam. The speed of such a jet would be
remarkable: between 11000 and 16000 km/s. The rotating-jet scenario may explain
the formation and excitation of most of the features observed in the inner
nebula, with no need for additional mechanisms, winds, or ionization sources.
All properties point to a symbiotic-like interacting binary as the central
source of M 2-9.Comment: Accepted for publication on Astronomy and Astrophysics (10 pages, 8
figures
Accretion onto the Companion of Eta Carinae During the Spectroscopic Event. IV. the Disappearance of Highly Ionized Lines
We show that the rapid and large decrease in the intensity of high-ionization
emission lines from the Eta Carinae massive binary system can be explained by
the accretion model. These emission lines are emitted by material in the nebula
around the binary system that is being ionized by radiation from the hot
secondary star. The emission lines suffer three months long deep fading every
5.54 year, assumed to be the orbital period of the binary system. In the
accretion model, for ~70 day the less massive secondary star is accreting mass
from the primary wind instead of blowing its fast wind. The accretion event has
two effects that substantially reduce the high-energy ionizing radiation flux
from the secondary star. (1) The accreted mass absorbs a larger fraction of the
ionizing flux. (2) The accreted mass forms a temporarily blanked around the
secondary star that increases its effective radius, hence lowering its
effective temperature and the flux of high energy photons. This explanation is
compatible with the fading of the emission lines at the same time the X-ray is
declining to its minimum, and with the fading being less pronounced in the
polar directions.Comment: ApJ, in pres
Inflating Fat Bubbles in Clusters of Galaxies by Precessing Massive Slow Jets
We conduct hydrodynamical numerical simulations and find that precessing
massive slow jets can inflate fat bubbles, i.e., more or less spherical
bubbles, that are attached to the center of clusters of galaxies. To inflate a
fat bubble the jet should precess fast. The precessing angle should be
large, or change over a large range (depending also on other parameters), where is the symmetry
axis. The constraints on the velocity and mass outflow rate are similar to
those on wide jets to inflate fat bubbles. The velocity should be v_j \sim
10^4 \kms, and the mass loss rate of the two jets should be 2 \dot M_j
\simeq 1-50 \dot M_\odot \yr^{-1} . These results, and our results from a
previous paper dealing with slow wide jets, support the claim that a large
fraction of the feedback heating in cooling flow clusters and in the processes
of galaxy formation is done by slow massive jets.Comment: Accepted for publication in MNRA
Are jets in symbiotic stars driven by magnetic fields?
We compare two scenarios to launch jets -- formation by MHD processes or
formation by thermal pressure in the boundary layer (BL) -- with respect to
their compatibility with observational data of jets in symbiotic stars,
especially in the well studied jet source MWC 560. Finally, we discuss points
of further research to be done.Comment: 4 pages, accepted for publication in A & A Letter
Planets and Axisymmetric Mass Loss
Bipolar planetary nebulae (PNe), as well as extreme elliptical PNe are formed
through the influence of a stellar companion. But half of all PN progenitors
are not influenced by any stellar companion, and, as I show here, are expected
to rotate very slowly on reaching the upper asymptotic giant branch; hence they
expect to form spherical PNe, unless they are spun-up. But since most PNe are
not spherical, I argue that about 50 percents of AGB stars are spun-up by
planets, even planets having a mass as low as 0.01 times the mass of Jupiter,
so they form elliptical PNe. The rotation by itself will not deform the AGB
wind, but may trigger another process that will lead to axisymmetric mass loss,
e.g., weak magnetic activity, as in the cool magnetic spots model. This model
also explains the transition from spherical to axisymmetric mass loss on the
upper AGB. For such low mass planets to substantially spin-up the stellar
envelope, they should enter the envelope when the star reaches the upper AGB.
This "fine-tuning" can be avoided if there are several planets on average
around each star, as is the case in the solar system, so that one of them is
engulfed when the star reaches the upper AGB.Comment: 8 pages, 1 figure. To appear in the proceedings of the conference,
"Post-AGB Objects (proto-planetary nebulae) as a Phase of Stellar Evolution",
Torun, Poland, July 5-7, 2000, eds. R. Szczerba, R. Tylenda, and S.K. Gorn
Accretion onto the Companion of Eta Carinae During the Spectroscopic Event: II. X-Ray Emission Cycle
We calculate the X-ray luminosity and light curve for the stellar binary
system Eta Carinae for the entire orbital period of 5.54 years. By using a new
approach we find, as suggested before, that the collision of the winds blown by
the two stars can explain the X-ray emission and temporal behavior. Most X-ray
emission in the 2-10 \kev band results from the shocked secondary stellar
wind. The observed rise in X-ray luminosity just before minimum is due to
increase in density and subsequent decrease in radiative cooling time of the
shocked fast secondary wind. Absorption, particularly of the soft X-rays from
the primary wind, increases as the system approaches periastron and the shocks
are produced deep inside the primary wind. However, absorption can not account
for the drastic X-ray minimum. The 70 day minimum is assumed to result from the
collapse of the collision region of the two winds onto the secondary star. This
process is assumed to shut down the secondary wind, hence the main X-ray
source. We show that this assumption provides a phenomenological description of
the X-ray behavior around the minimum.Comment: The Astrophysical Journal, in pres
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
Why a Single-Star Model Cannot Explain the Bipolar Nebula of Eta Carinae
I examine the angular momentum evolution during the 1837-1856 Great Eruption
of the massive star Eta Carinae. I find that the new estimate of the mass blown
during that eruption implies that the envelope of Eta Car substantially
spun-down during the 20 years eruption. Single-star models, most of which
require the envelope to rotate close to the break-up velocity, cannot account
for the bipolar nebula (the Homunculus) formed from matter expelled in that
eruption. The kinetic energy and momentum of the Homunculus further constrains
single-star models. I discuss how Eta Car can fit into a unified model for the
formation of bipolar lobes where two oppositely ejected jets inflate two lobes
(or bubbles). These jets are blown by an accretion disk, which requires stellar
companions in the case of bipolar nebulae around stellar objects.Comment: ApJ, in press. New references and segments were adde
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