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Register a new userCommon Envelope Shaping of Planetary Nebulae
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Added by
Guillermo Garcia-Segura
Publication date
2018
fields
Physics
and research's language is
English
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The morphology of planetary nebulae emerging from the common envelope phase of binary star evolution is investigated. Using initial conditions based on the numerical results of hydrodynamical simulations of the common envelope phase it is found that the shapes and sizes of the resulting nebula are very sensitive to the effective temperature of the remnant core, the mass-loss rate at the onset of the common envelope phase, and the mass ratio of the binary system. These parameters are related to the efficiency of the mass ejection after the spiral-in phase, the stellar evolutionary phase (i.e., RG, AGB or TP-AGB), and the degree of departure from spherical symmetry in the stellar wind mass loss process itself respectively. It is found that the shapes are mostly bipolar in the early phase of evolution, but can quickly transition to elliptical and barrel-type shapes. Solutions for nested lobes are found where the outer lobes are usually bipolar and the inner lobes are elliptical, bipolar or barrel-type, a result due to the flow of the photo-evaporated gas from the equatorial region. It is found that the lobes can be produced without the need for two distinct mass ejection events. In all the computations, the bulk of the mass is concentrated in the orbital or equatorial plane, in the form of a large toroid, which can be either neutral (early phases) or photoionized (late phases), depending of the evolutionary state of the system.
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We compute successfully the launching of two magnetic winds from two circumbinary disks formed after a common envelope event. The launching is produced by the increase of magnetic pressure due to the collapse of the disks. The collapse is due to internal torques produced by a weak poloidal magnetic field. The first wind can be described as a wide jet, with an average mass-loss rate of $sim 1.3 times 10^{-7}$ Moy and a maximum radial velocity of $sim 230$ kms. The outflow has a half-opening angle of $sim 20^{circ}$. Narrow jets are also formed intermittently with velocities up to 3,000 kms, with mass-loss rates of $sim 6 times 10^{-12} $ Moy during short periods of time. The second wind can be described as a wide X-wind, with an average mass-loss rate of $sim 1.68 times 10^{-7}$ Moy and a velocity of $sim 30$ kms. A narrow jet is also formed with a velocity of 250 kms, and a mass-loss rates of $sim 10^{-12} $ Moy. The computed jets are used to provide inflow boundary conditions for simulations of proto-planetary nebulae. The wide jet evolves into a molecular collimated outflow within a few astronomical units, producing proto-planetary nebulae with bipolar, elongated shapes, whose kinetic energies reach $sim 4 times 10^{45}$ erg at 1,000 years. Similarities with observed features in W43A, OH231.8+4.2, and Hen 3-1475 are discussed. The computed wide X-wind produces proto-planetary nebulae with slower expansion velocities, with bipolar and elliptical shapes, and possible starfish type and quadrupolar morphology.
Magnetic fields of order $10^1-10^2$ gauss that are present in the envelopes of red giant stars are ejected in common envelope scenarios. These fields could be responsible for the launching of magnetically driven winds in proto-planetary nebulae. Using 2D simulations of magnetized winds interacting with an envelope drawn from a 3D simulation of the common envelope phase, we study the confinement, heating, and magnetic field development of post-common envelope winds. We find that the ejected magnetic field can be enhanced via compression by factors up to $sim 10^4$ in circumbinary disks during the self-regulated phases. We find values for the kinetic energy of the order of $10^{46}$ erg that explain the large values inferred in proto-planetary nebula outflows. We show that the interaction of the formed circumbinary disk with a spherical, stellar wind produces a tapered flow that is almost indistinguishable from an imposed tapered flow. This increases the uncertainty of the origin of proto-planetary nebula winds, which could be either stellar, circumstellar (stellar accretion disk), circumbinary (circumbinary accretion disk), or a combination of all three. Within this framework, a scenario for self-collimation of weakly magnetized winds is discussed, which can explain the two objects where the collimation process is observationally resolved, HD 101584 and Hen 3-1475. An explanation for the equatorial, molecular hydrogen emission in CRL 2688 is also presented.
Nearly 50 post-common-envelope (post-CE) close binary central stars of planetary nebulae (CSPNe) are now known. Most contain either main sequence or white dwarf (WD) companions that orbit the WD primary in around 0.1-1.0 days. Only PN~G222.8-04.2 and NGC~5189 have post-CE CSPNe with a Wolf-Rayet star primary (denoted [WR]), the low-mass analogues of massive Wolf-Rayet stars. It is not well understood how H-deficient [WR] CSPNe form, even though they are relatively common, appearing in over 100 PNe. The discovery and characterisation of post-CE [WR] CSPNe is essential to determine whether proposed binary formation scenarios are feasible to explain this enigmatic class of stars. The existence of post-CE [WR] binaries alone suggests binary mergers are not necessarily a pathway to form [WR] stars. Here we give an overview of the initial results of a radial velocity monitoring programme of [WR] CSPNe to search for new binaries. We discuss the motivation for the survey and the associated strong selection effects. The mass functions determined for PN~G222.8-04.2 and NGC~5189, together with literature photometric variability data of other [WR] CSPNe, suggest that of the post-CE [WR] CSPNe yet to be found, most will have WD or subdwarf O/B-type companions in wider orbits than typical post-CE CSPNe (several days or months c.f. less than a day).
A current issue in the study of planetary nebulae with close binary central stars is the extent to which the binaries affect the shaping of the nebulae. Recent studies have begun to show a high coincidence rate between nebulae with large-scale axial or point symmetries and close binary stars. In addition, combined binary-star and spatio-kinematic modeling of the nebulae have demonstrated that all of the systems studied to date appear to have their central binary axis aligned with the primary axis of the nebula. Here we add two more systems to the list, the central stars and nebulae of NGC 6337 and Sp 1. We show both systems to be low inclination, with their binary axis nearly aligned with our line-of-sight. Their inclinations match published values for the inclinations of their surrounding nebulae. Including these two systems with the existing sample statistically demonstrates a direct link between the central binary and the nebular morphology. In addition to the systems inclinations we give ranges for other orbital parameters from binary modeling, including updated orbital periods for the binary central stars of NGC 6337 and Sp 1.
In 1997 Soker laid out a framework for understanding the formation and shaping of planetary nebulae (PN). Starting from the assumption that non-spherical PN cannot be formed by single stars, he linked PN morphologies to the binary mechanisms that may have formed them, basing these connections almost entirely on observational arguments. In light of the last decade of discovery in the field of PN, we revise this framework, which, although simplistic, can still serve as a benchmark against which to test theories of PN origin and shaping. Within the framework, we revisit the role of planets in shaping PN. Soker invoked a planetary role in shaping PN because there are not enough close binaries to shape the large fraction of non-spherical PN. In this paper we adopt a model whereby only ~20% of all 1-8 solar mass stars make a PN. This reduces the need for planetary shaping. Through a propagation of percentages argument, and starting from the assumption that planets can only shape mildly elliptical PN, we conclude, like in Soker, that ~20% of all PN were shaped via planetary and other substellar interactions but we add that this corresponds to only ~5% of all 1-8 solar mass stars. This may be in line with findings of planets around main sequence stars. PN shaping by planets is made plausible by the recent discovery of planets that have survived interactions with red giant branch (RGB) stars. Finally, we conclude that of the ~80% of 1-8 solar mass stars that do not make a PN, about one quarter do not even ascend the AGB due to interactions with stellar and substellar companions, while three quarters ascend the AGB but do not make a PN. Once these stars leave the AGB they evolve normally and can be confused with post-RGB, extreme horizontal branch stars. We propose tests to identify them.
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