Binary central stars of planetary nebulae

This paper reviews our knowledge on binary central stars of planetary nebulae and presents some personal opinions regarding their evolution. Three types of interactions are distinguished: type I, where the binary companion induces the mass loss; type II, where it shapes the mass loss but does not enhance it; type III, where a wide orbit causes the centre of mass to move, leading to a spiral embedded in the wind. Surveys for binary central stars are discussed, and the separations are compared to the distribution for binary post-AGB stars. The effect of close binary evolution on nebular morphology is discussed. Post-common-envelope binaries are surrounded by thin, expanding disks, expelled in the orbital plane. Wider binaries give rise to much thicker expanding torii. Type I binary evolution predicts a wide distribution of masses of central stars, skewed towards low masses. Comparison with observed mass distributions suggests that this is unlikely to be the only channel leading to the formation of a planetary nebula. A new sample of compact Bulge nebulae shows about 40% of nebulae with binary-induced morphologies.Comment: Invited review, in 'Evolution and chemistry of symbiotic stars and related objects', Wierzba, August 2006. To appear in Baltic Astronom

Alignment of the Angular Momentum Vectors of Planetary Nebulae in the Galactic Bulge

We use high-resolution H {\alpha} images of 130 planetary nebulae (PNe) to investigate whether there is a preferred orientation for PNe within the Galactic Bulge. The orientations of the full sample have an uniform distribution. However, at a significance level of 0.01, there is evidence for a non-uniform distribution for those planetary nebulae with evident bipolar morphology. If we assume that the bipolar PNe have an unimodal distribution of the polar axis in Galactic coordinates, the mean Galactic position angle is consistent with 90{\deg}, i.e. along the Galactic plane, and the significance level is better than 0.001 (the equivalent of a 3.7{\sigma} significance level for a Gaussian distribution). The shapes of PNe are related to angular momentum of the original star or stellar system, where the long axis of the nebula measures the angular momentum vector. In old, low-mass stars, the angular momentum is largely in binary orbital motion. Consequently, the alignment of bipolar nebulae that we have found indicates that the orbital planes of the binary systems are oriented perpendicular to the Galactic plane. We propose that strong magnetic fields aligned along the Galactic plane acted during the original star formation process to slow the contraction of the star forming cloud in the direction perpendicular to the plane. This would have produced a propensity for wider binaries with higher angular momentum with orbital axes parallel to the Galactic plane. Our findings provide the first indication of a strong, organized magnetic field along the Galactic plane that impacted on the angular momentum vectors of the resulting stellar population.Comment: There are two effective parts. The main paper consists of the first 17 pages and includes 8 figures and 7 tables. The remaining 10 pages will be published as an online supplement that is made up of 4 multi-part figures. Accepted for publication in MNRAS Main Journa

The ionization structure of multiple shell planetary nebulae: I. NGC 2438

In recent times an increasing number of extended haloes and multiple shells around planetary nebulae have been discovered. These faint extensions to the main nebula trace the mass-loss history of the star, modified by the subsequent evolution of the nebula. Integrated models predict that some haloes may be recombining, and not in ionization equilibrium. But parameters such as the ionization state and thus the contiguous excitation process are not well known. The haloes are very extended, but faint in surface brightness - 10^3 times below the main nebula. The observational limits lead to the need of an extremely well studied main nebula, to model the processes in the shells and haloes of one object. NGC2438 is a perfect candidate to explore the physical characteristics of the halo. Long-slit spectroscopic data were obtained. These data are supplemented by imaging data from the HST archive, and archival VLA observations. The use of diagnostic diagrams draws limits for physical properties in the models. CLOUDY is used to model the nebular properties, and to derive a more accurate distance and ionized mass. We derive an accurate extinction E(B-V)=0.16, and distance of 1.9kpc. This puts the nebula behind the nearby open cluster M46. The low-excitation species are found to be dominated by clumps. The emission line ratios show no evidence for shocks. We find the shell in ionization equilibrium: a significant amount of UV radiation infiltrates the inner nebula. Thus the shell still seems to be ionized. The spatially resolved CLOUDY model supports the hypothesis that photoionization is the dominant process in this nebula, far out into the shell. Previous models predicted that the shell would be recombining, but this is not confirmed by the data. We note that these models used a smaller distance, and therefore different input parameters, than derived by us.Comment: Accepted for publication in A&A (13 pages, 15 figures, 8 tables

BD+30 3639: The Infrared Spectrum During Post-AGB Stellar Evolution

We present a radiative-transfer calculation which reproduces the infrared spectrum of the planetary nebula BD~+30$^{\circ}$3639. We calculate the transfer process through absorption and scattering in a spherical-symmetric multi-grain dust shell. The emission of transiently heated particles is taken into account, as well as polycyclic aromatic hydrocarbons. We obtain an acceptable fit to most of the spectrum, including the PAH infrared bands. At submillimetre wavelengths the observed emission is larger than the model predicts, indicating that large dust conglomerates (``f{}luffy grains'') may be needed as an additional constituent. The fit favours a distance of $\ge 2 \,$kpc, which implies that BD~+30$^\circ$3639 has evolved from a massive progenitor of several solar masses. A low dust-to-gas mass ratio is found in the ionised region. The calculations yield an original mass-loss rate of 2\times10^{-4} \msolar \peryr on the Asymptotic Giant Branch. Using this mass-loss rate, we calculate how the infrared spectrum has evolved during the post-AGB evolution. We show in particular the evolution of the IRAS colours during the preceding post-AGB evolution.Comment: accepted for publication in MNRAS. LaTeX, 15 pages, hardcopy and 8 figures available from [email protected] or [email protected]

Structure and shaping processes within the extended atmospheres of AGB stars

We present recent studies using the near-infrared instrument AMBER of the VLT Interferometer (VLTI) to investigate the structure and shaping processes within the extended atmosphere of AGB stars. Spectrally resolved near-infrared AMBER observations of the Mira variable S Ori have revealed wavelength-dependent apparent angular sizes. These data were successfully compared to dynamic model atmospheres, which predict wavelength-dependent radii because of geometrically extended molecular layers. Most recently, AMBER closure phase measurements of several AGB stars have also revealed wavelength-dependent deviations from 0/180 deg., indicating deviations from point symmetry. The variation of closure phase with wavelength indicates a complex non-spherical stratification of the extended atmosphere, and may reveal whether observed asymmetries are located near the photosphere or in the outer molecular layers. Concurrent observations of SiO masers located within the extended molecular layers provide us with additional information on the morphology, conditions, and kinematics of this shell. These observations promise to provide us with new important insights into the shaping processes at work during the AGB phase. With improved imaging capabilities at the VLTI, we expect to extend the successful story of imaging studies of planetary nebulae to the photosphere and extended outer atmosphere of AGB stars.Comment: 6 pages, Proc. of "Asymmetric Planetary Nebulae V", A.A. Zijlstra, F. Lykou, I. McDonald, and E. Lagadec (eds.), Jodrell Bank Centre for Astrophysics, Manchester, UK, 201

A new HCN maser in IRAS 15082-4808

We have identified a new vibrational HCN maser at 89.087 GHz in the asymptotic giant branch (AGB) star IRAS 15082-4808, a maser which is thought to trace the innermost region of an AGB envelope. The observations of this maser at three epochs are presented: two positive detections and one null detection. The line profile has varied between the positive detections, as has the intensity of the maser. The major component of the maser is found to be offset by -2.0+/-0.9 km/s with respect to the systemic velocity of the envelope, as derived from the 88.631 GHz transition of HCN. Similar blueshifts are measured in the other 9 sources where this maser has been detected. Maser variability with pulsation phase has been investigated for the first time using the 10 stars now available. Comparisons with AGB model atmospheres constrain the position of the formation region of the maser to the region between the pulsation shocks and the onset of dust acceleration, between 2 and 4 stellar radii.Comment: 11 pages, 10 figures, accepted MNRAS, minor correction to equation

Mode switching in the nearby Mira-like variable R Doradus

We discuss visual observations spanning nearly 70 years of the nearby semiregular variable R Doradus. Using wavelet analysis, we show that the star switches back and forth between two pulsation modes having periods of 332 days and about 175 days, the latter with much smaller amplitude. Comparison with model calculations suggests that the two modes are the first and third radial overtone, with the physical diameter of the star making fundamental mode pulsation unlikely. The mode changes occur on a timescale of about 1000 d, which is too rapid be related to a change in the overall thermal structure of the star and may instead be related to weak chaos. The Hipparcos distance to R Dor is 62.4 +/- 2.8 pc which, taken with its dominant 332-day period, places it exactly on the period-luminosity relation of Miras in the Large Magellanic Cloud. Our results imply first overtone pulsation for all Miras which fall on the P-L relation. We argue that semiregular variables with long periods may largely be a subset of Miras and should be included in studies of Mira behaviour. The semiregulars may contain the immediate evolutionary Mira progenitors, or stars may alternate between periods of semiregular and Mira behaviour.Comment: 12 pages, latex with figures, accepted by MNRA