We discuss the classification and orientation of planetary nebulae that interact with the interstellar medium throughout the Milky Way. A sample of 117 confirmed interacting planetary nebulae is used for this purpose. Our results indicate that the majority of interacting objects are located close to the Galactic plane, and ∼77% of them are located inside the Galactic thin disk. One third of the sample is less than 100 parsec from the Galactic plane and thus may interact with molecular and cold neutral clouds. There is a tendency for the planetary nebula interaction region to be parallel to the Galactic plane. We found that ∼73% of interacting planetary nebulae have inclination angles (defined as the angles that join the planetary nebula centroid and the interaction area or bow shock with the Galactic plane) larger than 45 • and ∼38% larger than 70 • , which highlights the possible effect of interstellar magnetic fields. While it is sometime believed that the interaction preferentially occurs in old planetary nebulae, our analysis indicates that the majority of observed planetary nebulae are in the mid stage of their evolution. The mean inclination angle, Galactic height, linear size, and dynamical age are estimated for each stage of interaction. The results indicate strong correlations between the mean inclination angle and the above parameters.
In the present paper, we discuss the classification of planetary nebulae that show interaction with the interstellar medium in terms of Galactic population. Furthermore, we investigate the case of interaction of each object in the framework of the isothermal and adiabatic shock models and we derive the mean critical, leading, and stopping densities of the nebulae located in the Galactic thin and thick disks. A sample of 34 objects with available proper motions, radial velocities, and reliable distances in the literature was used for these purposes. The results show that 16 and 10 objects are classified as Galactic thin-disk and thick-disk populations, respectively. Seven objects are classified as probable thin-disk populations, and one as a probable thick-disk population. The flow and cooling time of the post-shock nebular gas are derived and the cooling/flow time ratio shows a linear correlation versus the Galactic height of the interacting planetary nebulae. The results reveal that there is a tendency for the interacting planetary nebulae belonging to the thin disk to follow an isothermal shock model, whilst those belonging to the thick disk to follow an adiabatic shock model.
Binary companions to asymptotic giant branch (AGB) stars are an important aspect of their evolution. Few AGB companions have been detected, and in most cases it is difficult to distinguish between main-sequence and white dwarf companions. Detection of photometric flickering, a tracer of compact accretion disks around white dwarfs, can help identify the nature of these companions. In this work, we searched for flickering in four AGB stars suggested to have likely accreting companions. We found no signs for flickering in two targets: R Aqr and V1016 Cyg. Flickering was detected in the other two stars: Mira and Y Gem. We investigated the true nature of Mira's companion using three different approaches. Our results for Mira strongly suggest that its companion is a white dwarf.
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