604 research outputs found
Cold Feedback in Cooling-Flow Galaxy Clusters
We put forward an alternative view to the Bondi-driven feedback between
heating and cooling of the intra-cluster medium (ICM) in cooling flow galaxies
and clusters. We adopt the popular view that the heating is due to an active
galactic nucleus (AGN), i.e. a central black hole accreting mass and launching
jets and/or winds. We propose that the feedback occurs with the entire cool
inner region (5-30 kpc). A moderate cooling flow does exist here, and
non-linear over-dense blobs of gas cool fast and are removed from the ICM
before experiencing the next major AGN heating event. Some of these blobs may
not accrete on the central black hole, but may form stars and cold molecular
clouds. We discuss the conditions under which the dense blobs may cool to low
temperatures and feed the black hole.Comment: 6 pages, no figures, to appear in the Proceedings of "Heating vs.
Cooling in Galaxies and Clusters of Galaxies", August 2006, Garching
(Germany
Is Mira a magneto-dusty rotator?
We investigate the possibility that a magnetic field may be present in the
star Ceti (hereafter, Mira) and that the field plays a role in the star's
mass loss. The model presented here is an application of an earlier derived
theory that has been successfully employed for intermediate and high-mass
evolved stars, and is now extended to the low-mass end. The modelling shows
that it is possible to obtain a hybrid magnetohydrodynamic-dust-driven wind
scenario for Mira, in which the role of a magnetic field in the equatorial
plane of the star is dynamically important for producing a stellar wind. The
wind velocity and the temperatures obtained from the model appear consistent
with findings elsewhere.Comment: 5 pages, 2 figure
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
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
A hybrid steady-state magnetohydrodynamic dust-driven stellar wind model for AGB stars
We present calculations for a magnetised hybrid wind model for Asymptotic
Giant Branch (AGB) stars. The model incorporates a canonical Weber-Davis (WD)
stellar wind with dust grains in the envelope of an AGB star. The resulting
hybrid picture preserves traits of both types of winds. It is seen that this
combination requires that the dust-parameter () be less than unity
in order to achieve an outflow. The emergence of critical points in the wind
changes the nature of the dust-driven outflow, simultaneously, the presence of
a dust condensation radius changes the morphology of the magnetohydrodynamic
(MHD) solutions for the wind. In this context, we additionally investigate the
effect of having magnetic-cold spots on the equator of an AGB star and its
implications for dust formation; which are seen to be consistent with previous
findings.Comment: 15 pages, 9 figure
An HI shell-like structure associated with nova V458 Vulpeculae?
We report the radio detection of a shell-like HI structure in proximity to,
and probably associated with, the nova V458 Vul. High spectral resolution
observation with the Giant Metrewave Radio Telescope has made it possible to
study the detailed kinematics of this broken and expanding shell. Unlike the
diffuse Galactic HI emission, this is a single velocity component emission with
significant clumping at ~ 0.5' scales. The observed narrow line width of ~ 5
km/s suggests that the shell consists of mostly cold gas. Assuming a distance
of 13 kpc to the system, as quoted in the literature, the estimated HI mass of
the nebula is about 25 M_sun. However, there are some indications that the
system is closer than 13 kpc. If there is a physical association of the HI
structure and the nova system, the asymmetric morphology and the off-centred
stellar system indicates past strong interaction of the mass loss in the
asymptotic giant branch phase with the surrounding interstellar medium. So far,
this is the second example, after GK Per, of a large HI structure associated
with a classical nova.Comment: 6 pages, 2 table, 3 figures. Accepted for publication in MNRAS
Letters. The definitive version will be available at
http://www.blackwell-synergy.com
A Possible Hidden Population of Spherical Planetary Nebulae
We argue that relative to non-spherical planetary nebulae (PNs), spherical
PNs are about an order of magnitude less likely to be detected, at distances of
several kiloparsecs. Noting the structure similarity of halos around
non-spherical PNs to that of observed spherical PNs, we assume that most
unobserved spherical PNs are also similar in structure to the spherical halos
around non-spherical PNs. The fraction of non-spherical PNs with detected
spherical halos around them, taken from a recent study, leads us to the claim
of a large (relative to that of non-spherical PNs) hidden population of
spherical PNs in the visible band. Building a toy model for the luminosity
evolution of PNs, we show that the claimed detection fraction of spherical PNs
based on halos around non-spherical PNs, is compatible with observational
sensitivities. We use this result to update earlier studies on the different PN
shaping routes in the binary model. We estimate that ~30% of all PNs are
spherical, namely, their progenitors did not interact with any binary
companion. This fraction is to be compared with the ~3% fraction of observed
spherical PNs among all observed PNs. From all PNs, ~15% owe their moderate
elliptical shape to the interaction of their progenitors with planets, while
\~55% of all PNs owe their elliptical or bipolar shapes to the interaction of
their progenitors with stellar companions.Comment: AJ, in pres
Can Planets Influence the Horizontal Branch Morphology?
As stars which have planetary systems evolve along the red giant branch and
expand, they interact with the close planets. The planets deposit angular
momentum and energy into the red giant stars' envelopes, both of which are
likely to enhance mass loss on the red giant branch. The enhanced mass loss
causes the star to become bluer as it turns to the horizontal branch. I propose
that the presence of planetary systems, through this mechanism, can explain
some anomalies in horizontal branch morphologies. In particular, planetary
systems may be related to the ``second parameter'', which determines the
distribution of horizontal branch stars on the Hertzsprung-Russel diagram. The
proposed scenario predicts that surviving massive planets or brown dwarfs orbit
many of the extreme blue horizontal branch stars, at orbital periods of tens
days.Comment: 21 pages, preprint, uses aasms4.st
Abundance Anomalies in the X-ray Spectra of the Planetary Nebulae NGC 7027 and BD +30 363
We revisit Chandra observations of the planetary nebulae NGC 7027 and BD +30 3639 in order to address the question of abundance anomalies in the X-ray emitting gas. Enhanced abundances relative to solar of magnesium (Mg) for NGC 7027 and neon (Ne) for BD +30 3639 are required to fit their X-ray spectra, whereas observations at optical and infrared wavelengths show depleted Mg and Ne in these systems. We attribute the enhancement of Mg in NGC 7027 in the X-ray, relative to the optical, to the depletion of Mg onto dust grains within the optical nebula. For BD +30 3639, we speculate that the highly enhanced Ne comes from a WD companion, which accreted a fraction of the wind blown by the asymptotic giant branch progenitor, and went through a nova-like outburst which enriched the X-ray emitting gas with Ne
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