Observations of photometric profiles of two planetary nebulae

It is noted that the Abel integral of a Gaussian function is a Gaussian function. This special property is used in order to develop an analytic method for studying photometric profile in Planetary Nebulae. Monochromatic observations have been made at the Haute-Provence Observatory using the RCA-CCD (320×512 pixels) mounted at theF/6 Newtonian focus of the 1.2 m telescope. Among the observed nebulae, two of them (NGC 6778 and NGC 7354) are regular enough to test the photometric profile method. The nebular emissivity per volume unit is then obtained for H⁺ and O⁺⁺ ions.     

Formation of a filamentary structure in planetary nebulae

Intricate filamentary structure and multiple shell-like appearance are very common phenomena in Planetary Nebulae.In addition, recent observations also indicate that the individual filaments present in these objects can have larger velocities than the adjacent smooth background (Pascoli, 1992 PASP 104, 350 and paper quoted therein).We have hypothesized that non linear hydrodynamical processes existing within the nebular gas are, possibly, responsible for these structures. As a matter of fact, it is argued that such a characteristic morphology, reinterpreted as a intermingled network of solitary waves or solitons, can be spontaneously generated in Planetary Nebulae as soon as one assumes that the nebular gas is permeated by a weak magnetic field whose strength is about 10^–5 to 10^–4 gauss.Main results of this work and further comments will be subsequently published in Ap&SS.     

The Interaction of Stellar Winds with Their Environment: Theory and Modelling

This paper contains an introductionary review of the theory of stellar wind bubbles. I describe the basic difference between radiative and adiabatic bubbles, the effects a central photo-ionizing source may have and in some more detail how these concepts can be used to explain the formation of Planetary Nebulae. This revised version was published online in July 2006 with corrections to the Cover Date.     

Bipolar outflows and the evolution of stars

Hypersonic bipolar outflows are a ubiquitous phenomena associated with both young and highly evolved stars. Observations of Planetary Nebulae, the nebulae surrounding Luminous Blue Variables such as η Carinae, Wolf Rayet bubbles, the circumstellar environment of SN 1987A and Young Stellar Objects all reveal high velocity outflows with a wide range of shapes. In this paper the current state of our theoretical understanding of these outflows is reviewed. Beginning with Planetary Nebulae considerable progress has been made in understanding bipolar outflows as the result of stellar winds interacting with the circumstellar environment. In what has been called the “Generalized Wind Blown Bubble” (GWBB) scenario, a fast tenuous wind from the central star expands into a ambient medium with an aspherical (toroidal) density distribution. Inertial gradients due to the gaseous torus quickly lead to an expanding prolate or bipolar shell of swept-up gas bounded by strong shock waves. Numerical simulations of the GWBB scenario show a surprisingly rich variety of gasdynamical behavior, allowing models to recover many of the observed properties of stellar bipolar outflows including the development of collimated supersonic jets. In this paper the physics behind the GWBB scenario is reviewed in detail and its strengths and weakness are considered. Alternative models involving MHD processes are also examined. Applications of these models to each of the principle classes of stellar bipolar outflow (YSO, PNe, LBV, SN87A) are then reviewed. Outstanding issues in the study of bipolar outflows are considered as are those questions which arise when the outflows are viewed as a single class of phenomena occurring across the HR diagram.     

Spectral analyses of WR-type central stars of planetary nebulae

So far, the evolution of post-AGB stars is not fully understood. In particular the formation of hydrogen-deficient and hydrogen-free Central Stars of Planetary Nebulae (CSPN) is unsettled. New evolution models, which allow for the consistent treatment of the physics of late thermalpulses, promise new insights to the formation of these stars. In this paper we summarize the results of non-LTE analyses of CSPN with wind. By comparing these results with the predictions of the new evolutionmodels, open questions concerning the evolution of the stars might be answered. In addition we discuss the driving mechanism of the winds of Wolf-Rayet CSPN. New models, which account for millions of iron lines, support the assumptions that these winds are driven by radiation.     

Magnetic fields in Quasar 3C147 on milliarcsecond scales

The R Coronae Borealis (RCB) stars represent a rare, or short-lived stage of low and intermediate mass stellar evolution. They are important in the context of mass-loss anddust formation on the Asymptotic Giant Branch.These stars are defined by their large declines in brightness at irregular intervals caused by dust formation.There are two major evolutionary models for the origin of RCB stars, the Double Degenerate and the Final Helium Shell Flash.In the final flash model, RCB stars lie at the centers of old Planetary Nebulae (PN). Here I discuss possible relationships with the [WC] stars.     

Shaping of Planetary Nebulae by Dust Driven AGB Winds

We have investigated the interacting winds model (IWM) in which the shapes of elliptical Planetary Nebulae (PNe) can be explained by the asymmetric mass loss produced by a rotating AGB star. The mass loss mechanism is based on a dust driven wind calculated for stationary situations. Already for small rotation rates of the AGB star we get a significantly angle-dependent mass loss which is concentrated towards the equatorial plane. This pole to equator density variation in the space surrounding the star influences the shape of the later developed PN. We compare these theoretical shapes with observed PNe and for some objects with well known quantities our model can fit the observations quite well. This revised version was published online in July 2006 with corrections to the Cover Date.     

Photometry of the open cluster NGC 6996 in the North America Nebula

Thirty stars in the direction of the open cluster NGC 6996 are measured in the Vilnius photometric system. Photometric spectral types, absolute magnitudes, interstellar reddenings, extinctions, and distances are determined for most of them. Fifteen stars are suspected to be cluster members. Their mean distance is 620±30 pc and mean extinctionA _v is 1.74 mag. The extinction within the cluster is variable. The age of the cluster is of the order of 10⁸ yr and this excludes the possibility that it is evolutionary related with the North America and Pelican Nebulae complex. The distance of this complex 550 pc is confirmed.     

Multiwavelength study of M33's giant H ii regions NGC 588 and NGC 592

We present a detailed multiwavelength photometric study of giant H  ii regions NGC 592 and NGC 588 in the nearby small spiral galaxy M33. We use data taken with the Wide Field and Planetary Camera 2 (WFPC2) on board the Hubble Space Telescope ( HST ). We detect several massive stars in both ionizing clusters. Six Wolf–Rayet (WR) stars are known to exist within those regions and we are able to constrain their physical properties by comparing their photometry to the latest grid of model atmospheres for WR stars of the nitrogen sequence (WN subclass). We estimate the age and mass of both regions by fitting our photometry to models of integrated stellar populations.     

Hydrodynamic models of Planetary Nebulae

This article tries to give a short review of the hydrodynamic modelling of Planetary Nebulae to date. I start by discussing the types of observations the modelling aims to reproduce and then discuss the results of different types of models. First the simplified models in which evolutionary changes in the stellar properties are neglected, then the evolutionary models in which these effects are taken into account. The simplified models can reproduce single shell nebulae with the right kinematic properties. The evolutionary models can reproduce multiple shells and some of the kinematic and morphological differences between various ions. I also briefly discuss some of the difficulties which have not been addressed and/or solved by the numerical modelling, such as the point-symmetric nebulae, some of the collimated phenomena, and clumpiness.Abbreviations AGB Asymptotic Giant Branch - PN Planetary Nebulae - YSO Young Stellar Object     

Chemical abundance behavior of type I planetary nebulae

In this work 13 Planetary Nebulae have been classified as Type I according to Peimbert's criteria (Peimbert, 1978). These objects have been added to a previous sample (Maciel and Faúndez-Abans, 1985) and diagrams of O/H versus N/H, S/H, Ne/H and Ar/H, as well as N/H versus S/H, Ne/H and Ar/H have been drawn. All of them exhibit a tendency for linear correlation; moreover, the behavior of O and N versus Ar and S are very similar, with approximately the same slope. When the excitation class parameter was included in the diagrams, no clear tendency can be discerned, for any class.     

Modele de champ magnetique fossile dans les nebuleuses planetaires

Résumé Jusqu'à présent, le champ magnétique dans les nébuleuses planétaires n'a été l'objet que de quelques rares études assez sommaires (Terzian, 1982) et son origine reste confuse.Des modèles ont été construits, mais la liaison entre le champ magnétique de l'étoile centrale et celui qui est, semble-t-il, associé à la nébuleuse n'est pas claire (Gurzadyan, 1970). Cependant, nous montrons ici qu'un lien direct peut être établi en considérant le transport des lignes du champ magnétique stellaire par la matière qui a été éjectée de l'étoile centrale.Ainsi, connaissant l'ordre de grandeur du champ magnétique régnant au voisinage de la nébuleuse, on peut en déduire une valeur approximative au voisinage de l'étoile ou inversement.

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A model of fossil magnetic field within planetary nebulae shells

Up to now, only a few studies have been made of the magnetic field in Planetary Nebulae (Terzian, 1982) The existence of such a field is suggested by the typical structure of some nebulae (NGC 650-1, NGC 7293, etc.). However, its origin remains unknown, although some models have been built (Gurzadyan, 1970). The connection between the magnetic field of the central star and the nebular shell is not yet elucidated. The aim of this paper is to point out a possible removal magnetic field line within the ejected shell. Such a field is able to maintain a relative stability of secondary structures in planetary nebulae (Louise, 1981, 1982).

     

Interactive Winds in Low-Mass Evolved Stars

Hypersonic flows occur in the late evolutionary stages of low to intermediate mass stars (M _ZAMS < 8M⊙). We present, from an observational point of view, the story of the matter ejected and shaped by different stellar winds. From the end of the Asymptotic Giant Branch to the Planetary Nebulae phases, the temperature and density both of the star and of the gas and dust envelope show drastic variations over a few thousand of years. We present multiwavelength (X-ray to radio) signatures of the outflows, observed through spectra and images of the consecutive phases, and discuss some open questions (bipolar morphology, microstructures,...). This revised version was published online in July 2006 with corrections to the Cover Date.     

Statistical and Physical Study of One-Sided Planetary Nebulae

We have investigated the spatial orientation of one-sided Planetary Nebulae. Most of them if not all are interacting with the interstellar medium. Seventy percent of the nebulae in our sample have inclination angles larger than 45^° to the galactic plane and 30% of the inclination angles are less than 45^°. Most of the selected objects are old, evolved planetary nebulae with large dimensions, and not far away from the galactic plane. Seventy-five percent of the objects are within 160 pc from the galactic plane. The enhanced concavity arc can be explained physically as a result of the `planetary nebulae – interstellar matter' interaction. We have discussed the possible effect of the interstellar magnetic field in the concavity regions. This revised version was published online in July 2006 with corrections to the Cover Date.     

PNE: The Key to Chemical Evolution in BCDGs

Planetary Nebulae have proven to be an essential key to understand the long term chemical enrichment of the interstellar medium due to low mass stars. They allow to study the original abundances of the star, and the effect of the star on the interstellar medium. Blue compact dwarf galaxies are known to host violent star formation in very heavy element depleted environments. They also show traces of past star formation (Doublier et al., 1999; 2001), including AGB starsand red giants which are responsible for long term enrichment. However, models fails to reproduce the low metallicities observed if those stars are taken into account. We observed PNe in BCDGs, and made a comparative study of the abundances in the HII regions and of the PNe. This revised version was published online in September 2006 with corrections to the Cover Date.     

The R Coronae Borealis Connection

The R Coronae Borealis (RCB) stars are rare hydrogen-deficient carbon-rich supergiants which undergo spectaculardeclines in brightness of up to 8 magnitudes at irregular intervals as dust forms along the line of sight.Understanding the RCB stars is a key test for any theory whichaims to explain hydrogen deficiency in post-Asymptotic Giant Branch (AGB) stars. There are two major evolutionary models for the origin of RCB stars: the Double Degenerate and the Final Helium ShellFlash. In the final flashmodel, there is a close relationship between RCB stars and Planetary Nebulae (PNe). The connection between RCB stars and PNe has recentlybecome stronger, since the central stars of three old PNe (Sakurai's Object, V605 Aql and FG Sge) have had observedoutbursts that transformed them from hot evolved central stars into cool giants with the spectral properties of an RCB star.     

The evolution of BD+60°2522: with mass loss and overshooting

The ionizing star BD+60°2522 is known as the central star of Bubble Nebulae NGC 7635—wind-blown bubble created by the interaction of the stellar wind of BD+60°2522 (O6.5 IIIef, V=8.7 mag, mass loss rate 10^−5.76 M _⊙/year) with the ambient interstellar medium. From the evolutionary calculations for the star with mass loss and overshooting, we find that the initial mass of the star is 60M _⊙, its present age is 2.5×10⁶ years, and the present mass is 45M _⊙.     

Young massive star clusters in M51

A search for young massive star clusters (YMCs) in the nearby face-on spiral galaxy M51 (NGC 5194) has been carried out using UBV CCD images from the prime focus camera on the Lick 3-m Shane telescope. The YMC population is found to be quite rich with a specific U -band luminosity     consistent with the high current star formation rate (SFR) of this galaxy. The brightest clusters have     far brighter than any young clusters currently known in the Milky Way and even surpassing the luminosity of the R136 cluster in the 30 Dor complex in the Large Magellanic Cloud. A few of the YMCs are examined on archive images from the Wide Field Planetary Camera (WF/PC2) on board the Hubble Space Telescope ( HST ), confirming their cluster nature and providing estimates of their effective radii of 2–3 pc. The number of YMCs in M51 is compatible with extrapolation of a power-law luminosity function with exponent ∼−2 from a Milky Way-like population of open clusters. Both the SFR and T _L ( U ) of M51 are similar to those of other cluster-rich spiral galaxies like NGC 1313 and M83.     

The Mass Function of Young Star Clusters in the "Antennae" Galaxies

We determine the mass function of young star clusters in the merging galaxies known as the "Antennae" (NGC 4038/9) from deep images taken with the Wide Field Planetary Camera 2 on the refurbished Hubble Space Telescope. This is accomplished by means of reddening-free parameters and a comparison with stellar population synthesis tracks to estimate the intrinsic luminosity and age, and hence the mass, of each cluster. We find that the mass function of the young star clusters (with ages less, similar160 Myr) is well represented by a power law of the form psi&parl0;M&parr0;~M-2 over the range 104 less, similarM less, similar106 M middle dot in circle. This result may have important implications for our understanding of the origin of globular clusters during the early phases of galactic evolution.     

Colour gradients in globular clusters

Radial colour gradients in the globular clusters NGC 2808, NGC 1851, NGC 5139, NGC 5139, NGC 6388, NGC 6441, NGC 6541, NGC 6723, and NGC 7099 were searched byB-V and DDO photoelectric photometry, using a set of circular diaphragms. Two of the observed globular clusters, NGC 5139 (Cen) and NGC 7099 (M30), showedB-V and C(45–48) colour gradients; the colour C(42–45) is essentially constant in these clusters, which means that the physical reason for those gradients cannot be attributed to chemical composition variations, since C(42–45) is a good metal abundance indicator.B-V colours in the external regions of NGC 5139 and NGC 7099 were obtained by synthesizing the observed HR diagrams of these clusters. The ages and the relaxation times for NGC 5139 and NGC 7099 indicate that mass segregation must have taken place in both clusters, which is probably related to the observed colour gradients.