The ionised and molecular mass of post-common-envelope planetary nebulae. The missing mass problem

Abstract

Context. Most planetary nebulae (PNe) show beautiful, axisymmetric morphologies despite their progenitor stars being essentially spherical. Close binarity is widely invoked to help eject an axisymmetric nebula, after a brief phase of engulfment of the secondary within the envelope of the Asymptotic Giant Branch (AGB) star, known as the common envelope (CE). The evolution of the AGB would thus be interrupted abruptly, its still quite massive envelope being rapidly ejected to form the PN, which a priori would be more massive than a PN coming from the same star, were it single. Aims. We aim at testing this hypothesis by investigating the ionised and molecular masses of a sample consisting of 21 post-CE PNe, roughly one fifth of the known total population of these objects, and comparing them to a large sample of 'regular' (i.e. not known to arise from close-binary systems) PNe. Methods. We have gathered data on the ionised and molecular content of our sample from the literature, and carried out molecular observations of several previously unobserved objects. We derive the ionised and molecular masses of the sample by means of a systematic approach, using tabulated, dereddened Hβ fluxes for finding the ionised mass, and 12 CO J=2−1 and J=3−2 observations for estimating the molecular mass. Results. There is a general lack of molecular content in post-CE PNe. Our observations only reveal molecule-rich gas around NGC 6778, distributed into a low-mass, expanding equatorial ring lying beyond the ionised broken ring previously observed in this nebula. The only two other objects showing molecular content (from the literature) are NGC 2346 and NGC 7293. Once we derive the ionised and molecular masses, we find that post-CE PNe arising from Single-Degenerate (SD) systems are just as massive, on average, as the 'regular' PNe sample, whereas post-CE PNe systems arising from Double-Degenerate (DD) systems are considerably more massive, and show substantially larger linear momenta and kinetic energy than SD systems and 'regular' PNe. Reconstruction of the CE of four objects, for which a wealth of data on the nebulae and complete orbital parameters are available, further suggests that the mass of SD nebulae actually amounts to a very small fraction of the envelope of their progenitor stars. This leads to the uncomfortable question of where the rest of the envelope is and why we cannot detect it in the stars' vicinity, thus raising serious doubts on our understanding of these intriguing objects.

Common Envelope to Explosion Delay time Distribution (CEEDTD) of Type Ia Supernovae

“…The high mass loss rate at the end of the AGB might be long, up to ≈10 5 yr (e.g., Corradi et al 2003;Michaely & Perets 2019;Igoshev et al 2020;Santander-García et al 2021). I quantify the relevant times from the study by Corradi et al (2003) of halos of PNe.…”

Common envelope to explosion delay time of Type Ia supernovae

“…The high mass loss rate at the end of the AGB might be long, up to ≈ 10 5 yr (e.g., Corradi et al 2003;Michaely & Perets 2019;Igoshev, Perets, & Michaely 2020;Santander-García et al 2021). I quantify the relevant times from the study by Corradi et al (2003) of halos of PNe.…”

Chemistry of nebulae around binary post-AGB stars: A molecular survey of mm-wave lines

“…Additionally, photodissociation effects in AGB envelopes only appear in outer layers. The comparison of our results with PNe is more difficult, because these objects present large differences in the molecular gas content (Huggins et al 1996;Santander-García et al 2021), which is also present in young PNe (Bujarrabal et al 1988(Bujarrabal et al , 1992. This fact is most likely due to molecular photodissociation, which strongly depends on the different evolutionary state of each source (Bachiller et al 1997a,b;Bublitz et al 2019;Ziurys et al 2020).…”