Formation and Evolution of [Wolf-Rayet] Planetary Nebulae through a Late Thermal Pulse

Abstract

We present the first radiation-hydrodynamical simulations of the formation of a born-again planetary nebula (PN) triggered by a late thermal pulse (LTP). The 2D radiation-hydrodynamic simulations, performed with the {\sc pluto} code, have been consistently coupled to stellar evolution calculations using the Modules for Experiments in Stellar Astrophysics ({\sc mesa}) code. Very particularly the stellar evolution model uses (i) updated opacity tables for H-deficient, C-rich mixtures during the LTP, and (ii) a mass-loss prescription tailored for H-deficient [Wolf-Rayet]([WR])-type winds during the post-LTP phase. Our stellar model reproduces the nearly complete depletion of H expected after an LTP event, while matching the observed abundances and spectral types of iconic [WR]-type central stars of PNe. The simulations show for the first time that the H-deficient LTP ejecta forms a transient double-shell structure which, after $\sim$1000 yr, becomes fully mixed with the H-rich PN. The ejecta mass ($\sim3.4\times10^{-4}$~M$_\odot$) is too small to leave a lasting imprint on the nebular abundances, predicting H-rich PNe around [WR] central stars. The injection of LTP material into the hot bubble drives turbulence, clump formation, and enhanced mixing, providing an explanation to the larger expansion velocities and larger turbulent nebular structures of PNe with [WR] central stars compared to those with H-rich central stars. These results provide robust support for the born-again scenario as the origin of H-deficient [WR] central stars within H-rich PNe.

Figures (11)

• Figure 1: Evolution in the HR diagram of a $M_\mathrm{ZAMS}=1.7$ M$_\odot$ star model with $Z_\mathrm{ZAMS}$=0.02 that experiences an LTP event created with mesa. Different segments illustrate the evolutive phases experienced by the star. The square, triangle, and bullet symbols represent 500, 2000, and 4500 yr after the LTP event. The numbered star symbols correspond to the properties of the [WR] CSPN of NGC 40 (1), NGC 1501 (2), NGC 2371 (3), NGC 5189 (4), NGC 6905 (5), and PC 22 (6).
• Figure 2: Mass-loss rate ($\dot{M}$, top), stellar wind velocity ($\varv_\infty$, middle), and ionising photon rate ($Q$, bottom) time evolution. The onset of the LTP occurs at time $t= - 544$ yr, while the post-LTP phase begins at time $t=0$. The star symbol in the bottom panel is the predicted ionising flux from the PoWR model of Fig. \ref{['fig:spec_WO']}.
• Figure 3: Abundance mass fraction (top) and carbon-to-oxygen ($X_\mathrm{C}/X_\mathrm{O}$) abundance mass ratio (bottom) time evolution according to the stellar model used here. The vertical dashed lines at $t=-544$ yr represents the onset of the LTP event, while the vertical dotted lines at $t=0$ yr is the onset of the post-LTP phase.
• Figure 4: Synthetic optical spectrum calculated with the non-LTE stellar atmosphere code PoWR at 4500 yr of post-LTP evolution (solid line). The typical violet (VP), blue (BB), and red bump (RB) are labelled as well as other emission lines. The corresponding abundances are listed in the fourth column of Table \ref{['tab:analysis']}. For comparison, the spectrum of the CSPN of NGC 6905 (dashed line), originally presented by GG2022, is also shown. Note the contamination of the observed stellar spectrum by narrow nebular emission lines, most notably H$\beta$, H$\alpha$, and [O iii] $\lambda\lambda$4959,5007.
• Figure 5: Evolution of the number density ($n$ - upper panel) and temperature ($T$ - lower panel) during the post-AGB phase. Time is measured from the beginning of the post-AGB evolution, defined as the moment when the stellar effective temperature exceeds log$_{10}(T_{\mathrm{eff}}/\mathrm{K}) = 3.8$.