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

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 Model for the Strings of Eta Carinae

We propose a model based on ionization shadows to explain the formation of the long and narrow strings of Eta Carinae. Five strings are known, all located along the symmetry axis outside the Homunculus. The model assumes that each string is formed in a shadow behind a dense clump near the symmetry axis. The surrounding gas is ionized first, becomes much hotter, and compresses the gas in the shadow. This leads to the formation of a radial, dense, long, and narrow region, i.e., a string. Later the neutral material in the strings is ionized, and becomes brighter. Still later it re-expands, and we predict that in about 200 years the strings will fade. The condition for the model to work is that the ionization front, due to the diffuse ionizing recombination radiation of the surrounding gas, proceeds into the shadow at a velocity slower than the compression speed. From that we get a condition on the mass loss rate of the mass loss episode that formed the strings, which should be less than 10^{-4} Mo/year. The model can also explain the strings in the planetary nebula NGC 6543.Comment: 8 pages; Submitted to A&

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

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 Simple Measurement of Turbulence in Cores of Galaxy Clusters

Using a simple model, we study the effects of turbulence on the motion of bubbles produced by AGN jet activities in the core of a galaxy cluster. We focus on the turbulence with scales larger then the size of the bubbles. We show that for a bubble pair with an age of ~10^8 yr, the projected angle between the two vectors from the cluster center to the two bubbles should be ~> 90 degree and the ratio of their projected distances from the cluster center should be ~< 2.5, if the velocity and scale of the turbulence are ~250 km s^-1 and ~20 kpc, respectively. The positions of the bubbles observed in the Perseus cluster suggest that the turbulent velocity is ~>100 km s^-1 for the cluster.Comment: Accepted for publication in ApJ

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