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Fallback in Bipolar Planetary Nebulae?

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Fallback in Bipolar Planetary Nebulae? Research in Astron. Astrophys. Vol.0 (20xx) No.0, 000–000 Research in http://www.raa-journal.org http://www.iop.org/journals/raa Astronomy and (LATEX: ms2021-0130.tex; printed on September 10, 2021; 2:47) Astrophysics Fallback in bipolar Planetary Nebulae ? Willem A. Baan1;2;3, Hiroshi Imai2;4 and Gabor Orosz2;3;5 1 Netherlands Institute for Radio Astronomy ASTRON, 79901 PD Dwingeloo, the Netherlands, [email protected] 2 Center for General Education, Institute for Comprehensive Education, Kagoshima University, 1-21-30 Korimoto, Kagoshima 890-0065, Japan 3 Xinjiang Astronomical Observatory, 150 Science 1-Street, Urumqi, Xinjiang 830011, P.R. China 4 Amanogawa Galaxy Astronomy Research Center, Graduate School of Science and Engineering, Kagoshima University, 1-21-35 Korimoto, Kagoshima 890-0065, Japan 5 School of Natural Sciences, University of Tasmania, Private Bag 37, Hobart, Tasmania 7001, Australia Received 20xx month day; accepted 20xx month day Abstract The subclass of bipolar Planetary Nebulae (PN) exhibits well-defined low- power outflows and some show shock-related equatorial spiderweb structures and hour- glass structures surrounding these outflows. These structures are distinctly different from the phenomena associated with spherical and elliptical PN and suggest a non-standard way to simultaneously energise both kind of structures. This paper presents evidence from the published literature on bipolar PN Hb 12 and other sources in support of an alterna- tive scenario for energising these structures by means of accretion from material shells deposited during earlier post-AGB and pre-PN evolutionary stages. In addition to ener- gising the bipolar outflow, a sub-Eddington accretion scenario could hydrodynamically explain the spiderweb and outer hourglass structures as oblique shockwaves for guiding the accreting material into the equatorial region of the source. Estimates of the accre- tion rate resulting from fallback-related spherical accretion could indeed help to drive a low-power outflow and contribute to the total luminosity of these sources. Key words: ISM: planetary nebulae: general – ISM: planetary nebulae: individual Hb 12, Hen 2-104, MyCn 18 – ISM: jets and outflows – Stars: evolution 1 INTRODUCTION Planetary Nebulae (PNe) represent the post-evolution stages of medium mass stars leaving the arXiv:2107.14269v3 [astro-ph.SR] 9 Sep 2021 Asymptotic Giant Branch (AGB) before its stellar core eventually turns into a White Dwarf (WD) (Balick & Frank 2002). At the end of AGB evolution a superwind depletes the AGB envelope un- til the stellar structure changes, the photospheric radius shrinks, and the stellar effective temperature rises (de Marco 2009). The further evolution of PNe is then affected by the occurrence of multiple ‘late and very-late thermal pulse cycles’ at the stellar core (Balick & Frank 2002), the fallback of AGB shells (Perinotto et al. 2004; Chen et al. 2016), and the ionisation of surrounding material by the central star, which make the PN move back and forth across the Hertzsprung-Russell (HR) diagram (Werner & Herwig 2006). Radiation hydrodynamical modelling of AGB-to-PN evolution shows how the central-star mass and the (post-)AGB mass-loss history determine the shape and the kinematics of the resulting planetary nebula (Perinotto et al. 2004; Schonberner et al. 2005). 2 W.A. Baan, H. Imai & G. Orosz The complexity of the evolutionary processes of PNe can provide for the wide variety in colour and shape of spherical and elliptical PNe. However, the existence of spherical shells and ionization fronts does not explain any of the structures observed in bipolar PN (Balick & Frank 2002). The small number of bipolar (elongated) PN structures indicate the existence of a directed outflow that has been described as a stellar wind emanating from the inner layers of the stellar structure and blowing out along the symmetry axis (Mellema & Frank 1995; Balick & Frank 2002). A few sources exhibit a well-defined ‘hourglass’ axi-symmetric structure that defines the interaction boundaries of an expanding outflow with the surrounding circumstellar material (Sahai et al. 1999). Alternatively, bipolar sources have been associated with a companion star in order to create a system asymmetry or have been explained by accretion in a symbiotic binary system (SySt), as in the case of the Twin Jet Nebula (Corradi et al. 2011). One out of five PNe have indeed been confirmed to host a binary companion (Miszalski et al. 2015) but the evidence for a ‘binary hypothesis’ is not yet very strong as compared with the ‘single star hypothesis’ for PNe (de Marco 2009). Two of the prominent hourglass sources, Hb 12 and MyCn 18, show a Spiderweb arc structure in the equatorial plane of the system (perpendicular to the hourglass) that has (following tradition) been ex- plained by repeated stellar wind outflows (Clark et al. 2014; Sahai et al. 1999), by sequential ejections resulting from a thermal nuclear runaway (Kwok & Hsia 2007), or by episodes of activity during earlier evolutionary stages (Vaytet et al. 2009). However, there is no clear explanation for a simultaneous out- flow along the polar axis and the structure in the equatorial plane. Similarly, intermittent jets or winds blown by an accreting binary system have also been invoked to explain both the inner arc structure of the Red Rectangle (HD 44179) and its (perpendicular) outer structure (Soker 2005; Bujarrabal et al. 2016). Since the bipolar PNe do not show any ionised shells that are characteristic of evolved PN, they seem to be either at a very early evolutionary stage or at the end of PN evolution. The suggestion has indeed been made for these to be low-excitation and young sources that only recently started ionising their surrounding shells and represent a short transition stage toward full development (Clark et al. 2014). The evolution of PN is known to be strongly affected by fallback by material deposited in the surrounding interstellar environment by AGB and PN evolutionary activity. However, no clear scenario has been established to recognise when fallback actually occurs. While fallback may happen unnoticed observationally when the accretion rate is very low and streams of fallback material may leak into the atmosphere of the central star of the PN, a definite accretion signature should be established when the accretion rate become high. When the accretion rate becomes higher because of the availability of accretion material, sub-Eddington accretion may happen where the low luminosity of the PN source cannot stagnate the accretion flow. The power generated during this (possibly short-lived) accretion could still be sufficient to energise activity in the PN. Although existing explanations for bipolar PN sources have traditionally relied on nuclear activity of the source itself, an accretion scenario should also be considered to drive an outflow along the polar axis and also explain any structures close to the equatorial plane. In this paper, we seek qualitative evidence from the published literature that would support the possibility of semi-spherical sub-Eddington accretion unto a bipolar source such as PN Hb 12. The origin of the accreting material may be the ejecta during earlier AGB stages that have been deposited in the surrounding medium of a post-AGB environment of a bipolar PN. Alternatively, accretion could also occur during or at the end of the PN evolutionary path with diminished/ceased stellar activity but the accretion rate from the ISM may not be sufficient during these stages. The evidence for an accretion scenario in a bipolar source such as PN Hb 12 will set the stage for further research of bipolar PN. After describing the structural evidence of bipolar PN Hb 12 in Section 2, available observational evidence is presented in Section 3 that would directly support or be consistent with an accretion-outflow scenario. 2 THE BIPOLAR STRUCTURE OF PN HB 12 The bipolar Matryoshka Nebula PN Hb 12 has been very well studied and provides specific fundamental insights into the workings of a bipolar PN (Hora & Latter 1996; Welch et al. 1999; Kwok & Hsia 2007; Vaytet et al. 2009; Clark et al. 2014). Figure 1(left) presents a composite optical/infrared Hubble image Fallback in bipolar pre-PN 3 Fig. 1: (Left) A composite optical/infrared HST image shows the inner structure of the Matryoshka Nebula Hubble Hb 12 with the [N II](λ6584 µm) emission and the NICMOS F160W image showing shocked [Fe II](λ1.6435 µm) emission in the boundary of the bipolar Outflow. Photo: ESA/NASA/J. Schmidt. (Right) HST NICMOS F160W image from the HST Legacy Archive showing the detailed [Fe II] emission structures of the Spiderweb. The arrows point to the Spiderweb arcs detected in NICMOS F220W H2 (λ2.1214 µm) data. The equatorial arcs are integrated with an outer and larger bipolar Hourglass structure. The field of view is 4000x4000. Image from (Clark et al. 2014). of the central structure of Hb 12, which shows a hourglass-shaped ‘bipolar outflow’ channels seen in narrow band [N II] (λ 6584) observations (Balick 2003; Kwok & Hsia 2007). The axis of the hourglass outflow structure in PN Hb 12 with concentric ring structures in its boundary is tipped clockwise by 10deg and with the upper part tilted away from the plane of the sky by 25deg (Vaytet et al. 2009). In addition, prominent infrared [Fe II] (λ1.6435 µm) line emission shows the two edge section of an equa- torial and axi-symmetric Spiderweb structure together with the inner sections of a larger scale ‘Outer Hourglass’ structure (Clark et al. 2014). Figure 1 (right) again shows the detailed [Fe II] equatorial Spiderweb structure surrounding the star and the continuation towards a bipolar Outer Hourglass struc- ture that is less collimated than the (inner) outflow hourglass structure. This Outer Hourglass (shock) structure is expected become less prominent with distance and will close beyond the stagnation point of the bipolar outflows; this closure has not yet been observed in Hb 12.
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