Discovery of a Fast-expanding Shell in the Inside-out Born-again Planetary Nebula HuBi 1 through High-dispersion Integral Field Spectroscopy

Abstract

HuBi 1 has been proposed to be member of the rare class of born-again planetary nebulae (PNe), i.e., its central star experienced a very late thermal pulse and ejected highly processed material at high speeds inside the old hydrogen-rich PN. In this Letter we present GTC MEGARA integral field spectroscopic observations of the innermost regions of HuBi 1 at high spectral resolution ≃16 km s−1 and multi-epoch subarcsecond images obtained ≃12 yr apart. The analysis of these data indicates that the inner regions of HuBi 1 were ejected ≃200 yr ago and expand at velocities ≃300 km s−1, in excellent agreement with the born-again scenario. The unprecedented tomographic capabilities of the GTC MEGARA high-dispersion observations used here reveal that the ejecta in HuBi 1 has a shell-like structure, in contrast to the disrupted disk and jet morphology of the ejecta in other born-again PNe.

Formation and fate of the born-again planetary nebula HuBi 1

“…The results of our simulations for HuBi 1 suggest that it is more accurate to assume that the H-poor material was ejected inside the old PN in an explosive event with velocities close as those currently observed. Run B predicts that if this is the case, the material ejected during the VLTP in HuBi 1 must have occurred ∼100 yr, which is rather consistent with the kinematical age estimated by Rechy-García et al (2020). Run B also suggests that the small difference in velocity between the VLTP and the pVLTP does not allow the shell to experience the formation of hydrodynamical instabilities, in particular, Rayleigh-Taylor.…”

“…The simulation of Run A reaches a radius for the inner shell of 0.05 pc after 600 yr of evolution, which is notably different to the age of 200 yr proposed by Rechy-García et al (2020). The model can neither reproduce the reported expansion velocities of the inner shell in HuBi 1, regardless of the injection of a pVLTP wind 20 times faster, which can not provide sufficient kinetic energy to accelerate the shell up to the observed velocities (∼300 km s −1 ; Rechy-García et al 2020), whereas the momentum provided by the radiation pressure is negligible.…”

“…Indeed Figure 1 in Guerrero et al (2018) suggests that this is the case. To explore the velocity structure of this emission, we have examined the GTC MEGARA integral field spectroscopic data published recently by our team (Rechy-García et al 2020). The results are illustrated in Figure 10, a colour-composite picture of HuBi 1 in the [N ] 𝜆𝜆6548,6584 emission lines where the green colour corresponds to [N ] emission at the systemic velocity of HuBi 1, the blue colour to the receding structure in the systemic velocity range from −55 to −45 km s −1 , and the red colour to the approaching structure in the systemic velocity range from +57 to +66 km s −1 .…”

“…To assess both scenarios we ran two simulations. Run A will be performed with 𝑣 VLTP1 = 20 km s −1 and Run B with 𝑣 VLTP2 = 300 km s −1 , similar to what is observed for the H-poor ejecta in HuBi 1 (Rechy-García et al 2020). No ionizing photon flux will be considered during this phase.…”

Planetary nebulae with Wolf-Rayet-type central stars -- II. Dissecting the compact planetary nebula M2-31 with GTC MEGARA

“…High-dispersion IFS observations of PNe provide 2D spatial information on their kinematics allowing to search for hidden spatiokinematic components that can be missed using long-slit echelle observations. The capabilities of this tomographic techniques have been demonstrated by our group for HuBi 1, where a shell-like structure has been revealed for its born-again ejecta (Rechy-García et al 2020), and for NGC 2392, where it has been possible to obtain an image of its jet for the first time, providing detailed information on its morphology and kinematics (Guerrero et al 2021).…”