2,312 research outputs found
Spherical Planetary Nebulae
By examining their mass loss history and their distribution in the galaxy I
argue that spherical planetary nebulae (PNe) form a special group among all
planetary nebulae. The smooth surface brightness of most spherical PNe suggests
that their progenitors did not go through a final intensive wind (FIW, also
termed superwind) phase. While 70 per cent of the PNe of all other PNe groups
are closer to the galactic center than the sun is, only 30 per cent of
spherical PNe are. These, plus the well known high scale height above the
galactic plane of spherical PNe, suggest that the progenitors of spherical PNe
are low mass stars having low metallicity. Although many stars have these
properties, only about 10 per cent of all PNe are spherical. By comparing the
galactic distribution of spherical PNe to the metallicity evolution in the
galaxy, I find that the critical metallicity above which no spherical PNe are
formed is [Fe/H] ~ -0.4. I explain this as well as other properties of
spherical PNe in the context of the companion model for shaping PNe, arguing
that spherical PNe are formed from stars which had no close companion, stellar
or substellar, orbiting them.Comment: 10 pages + 1 table(ps) and 1 figure(ps); Submitted to MNRA
A Phenomenological Model for the Extended Zone Above AGB Stars
I suggest the existence of an extended zone above the surface of asymptotic
giant branch (AGB), as well as similar stars experiencing high mass loss rates.
In addition to the escaping wind, in this zone there are parcels of gas that do
not reach the escape velocity. These parcels of dense gas rise slowly and then
fall back. The wind and bound gas exist simultaneously to distances of ~100AU.
I term this region the effervescent zone. In this phenomenological study I find
that the density of the bound material in the effervescent zone falls as
~r^{-5/2}, not much faster than the wind density. The main motivation to
propose the effervescent model is to allow wide binary companions to influence
the morphology of the descendant planetary nebulae (PN) by accreting mass from
the effervescent zone. Accretion from the effervescent zone is more efficient
than accretion from the wind in forming an accretion disk around the companion.
The companion might then blow two jets that will shape the descendant PN.Comment: New Astronomy, in pres
Bubbles in Planetary Nebulae and Clusters of Galaxies: Instabilities at Bubble Fronts
I study the stability of off-center low-density more or less spherical (fat)
bubbles in clusters of galaxies and in planetary nebulae (PNs) to
Rayleigh-Taylor (RT) instability. As the bubble expands and decelerates, the
interface between the low-density bubble's interior and the dense shell formed
from the accreted ambient medium is RT-stable. If, however, in a specific
direction the density decreases such that this segment is accelerated by the
pressure inside the bubble, then this accelerated region is RT-unstable. The
outermost region, relative to the center of the system, is the most likely to
become unstable because there the density gradient is the steepest. Using
simple analytical analysis, I find that off-center fat bubbles in PNs are much
less stable than in clusters. In PNs bubbles become unstable when they are very
small relative to their distance from the center; they can be stabilized
somewhat if the mass loss rate from the stellar progenitor decreases for a
time, such that the negative density gradient is much shallower. In clusters
fat bubbles become unstable when their size is comparable to their distance
from the center. I discuss some implications of this instability in clusters
and in PNs.Comment: New Astronomy, in press; a third in a series of 3 paper
The role of thermal pressure in jet launching
I present and discuss a unified scheme for jet launching that is based on
stochastic dissipation of the accretion disk kinetic energy, mainly via shock
waves. In this scheme, termed thermally-launched jet model, the kinetic energy
of the accreted mass is transferred to internal energy, e.g., heat or magnetic
energy. The internal energy accelerates a small fraction of the accreted mass
to high speeds and form jets. For example, thermal energy forms a pressure
gradient that accelerates the gas. A second acceleration stage is possible
wherein the primary outflow stretches magnetic field lines. The field lines
then reconnect and accelerate small amount of mass to very high speeds. This
double-stage acceleration process might form highly relativistic jets from
black holes and neutron stars. The model predicts that detail analysis of
accreting brown dwarfs that launch jets will show the mass accretion rate to be
larger than 10^{-9}-10^{-8} Mo/year, which is higher than present claims in the
literature.Comment: To appear in the proceedings of Star-disk interaction in young stars,
Grenoble 2007, ed. J. Bouvie
Pairs of Bubbles in Planetary Nebulae and Clusters of Galaxies
I point to an interesting similarity in the morphology and some
non-dimensional quantities between pairs of X-ray-deficient bubbles in clusters
of galaxies and pairs of optical-deficient bubbles in planetary nebulae (PNs).
This similarity leads me to postulate a similar formation mechanism. This
postulate is used to strengthen models for PN shaping by jets (or collimated
fast winds: CFW). The presence of dense material in the equatorial plane
observed in the two classes of bubbles constrains the jets and CFW activity in
PNs to occur while the AGB star still blows its dense wind, or very shortly
after. I argue that only a stellar companion can account for such jets and CFW.Comment: PASP, in pres
Are jets rotating at the launching?
I argue that the Doppler shift asymmetries observed in some young stellar
object (YSO) result from the interaction of the jets with the circumstellar
gas, rather than from jets' rotation. The jets do rotate, but at a velocity
much below claimed values. During the meeting I carefully examined new claims,
and found problems with the claimed jets' rotation. I will challenge any future
observation that will claim to detect jet rotation in YSOs that requires the
jets (and not a wind) to be launched from radii much larger than the accreting
stellar radius. I conclude that the most likely jets' launching mechanism
involves a very efficient dynamo in the inner part of the accretion disk, with
jets' launching mechanism that is similar to solar flares (coronal mass
ejection).Comment: to be published in Astrophysical Outflows and Associated Accretion
Phenomena, eds. E. M. de Gouveia Dal Pino, A. Raga (IUA JD 7 at the XXVIIth
IAU General Assembly). Proceedings: IAU Highlights of Astronomy,
Editor-in-Chief: I. F Corbet
Local Circumstellar Magnetic Fields Around Evolved Stars
I argue that the presence of magnetic fields around evolved stars, e.g.,
asymptotic giant branch stars, and in PNe, does not necessarily imply that the
magnetic field plays a global dynamical role in shaping the circumstellar
envelope. Instead, I favor magnetic fields with small coherence lengths, which
result from stellar magnetic spots or from jets blown by an accreting
companion. Although the magnetic field does not play a global role in shaping
the circumstellar envelope, it may enhance local motion (turbulent) via
magnetic tension and reconnection. The locally strong magnetic tension may
enforce coherence flow which may favor the masing process.Comment: Submitted to MNRA
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