A study of four multipolar planetary nebulae in the Galactic bulge

We present infrared (IR) spectroscopic and optical morphological analyses of four multipolar young planetary nebulae (PNe) located in the Galactic bulge (GB) to investigate their dust characteristics and complex multi-lobed structures. Hubble Space Telescope high-resolution images of the nebulae (H 1-8, H 1-43, K 5-4, and M 3-14) reveal that these objects have interlaced multi-lobed features, indicating that their formation process is complex. Spitzer IR spectroscopic measurements of three of the young PNe show that these nebulae have unidentified IR emission bands and broad silicate features, suggesting the existence of a mixed-chemistry dust environment around these objects; such an environment, if present, may be caused by the last thermal pulse of the final asymptotic giant branch phase or be related to the thick tori produced by the interactions of central binaries. To find a potential connection between the multi-lobed shapes and central stars (CSs) of these nebulae, we employed Transiting Exoplanet Survey Satellite (TESS) monitoring to check whether the CSs of the objects exhibit photometric variations. Analysis of TESS observations of the four young PNe shows that the CS of H 1-43 exhibits a periodic photometric variation of 20.88 hr; no regular brightness variations are detected for the other three nebulae. To study and differentiate the multipolar nebulae in the Galactic disk (GD) and GB, a statistical analysis was performed on the properties of these nebulae. The binary fraction of multipolar PN CSs in the GD region is estimated to be 23-40 $%$, which is significantly larger than the binary fraction of the PN CSs reported in other studies. This strongly supports the hypothesis that binary interactions play an important role in the formation of multipolar PNe. Analyses of the spectral energy distributions of the objects show that their IR luminosities, dust temperatures, and mean emission measures are higher than the averages for normal PNe, which may indicate that they are young.