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Register a new userIs the Sulphur Anomaly in Planetary Nebulae Caused by the s-Process?
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Motivated by unexplained observations of low sulphur abundances in planetary nebulae (PNe) and the PG1159 class of post asymptotic giant branch (AGB) stars, we investigate the possibility that sulphur may be destroyed by nucleosynthetic processes in low-to-intermediate mass stars during stellar evolution. We use a 3 Msun, Z=0.01 evolutionary sequence to examine the consequences of high and low reaction rate estimates of neutron captures onto sulphur and neighbouring elements. In addition, we have tested high and low rates for the neutron producing reactions C13(alpha,n)O16 and Ne22(alpha,n)Mg25. We vary the mass width of a partially mixed zone (PMZ), which is responsible for the formation of a C13 pocket and is the site of the C13(alpha,n)O16 neutron source. We test PMZ masses from zero up to an extreme upper limit of the entire He-intershell mass at 10^-2 Msun. We find that the alternative reaction rates and variations to the partially mixed zone have almost no effect on surface sulphur abundances and do not reproduce the anomaly. To understand the effect of initial mass on our conclusions, 1.8 Msun and 6 Msun evolutionary sequences are also tested with similar results for sulphur abundances. We are able to set a constraint on the size of the PMZ, as PMZ sizes that are greater than half of the He-intershell mass (in the 3 Msun model) are excluded by comparison with neon abundances in planetary nebulae. We compare the 1.8 Msun models intershell abundances with observations of PG1159-035, whose surface abundances are thought to reflect the intershell composition of a progenitor AGB star. We find general agreement between the patterns of F, Ne, Si, P, and Fe abundances and a very large discrepancy for sulphur where our model predicts abundances that are 30-40 times higher than is observed in the star.
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We present deep, high-resolution (R~40000) UVES at VLT spectrophotometric data of the planetary nebula NGC 3918. This is one of the deepest spectra ever taken of a planetary nebula. We have identified and measured more than 700 emission lines and, in particular, we have detected very faint lines of several neutron-capture elements (s-process elements: Kr, Xe and Rb) that enable us to compute their chemical abundances with unprecedented accuracy, thus constraining the efficiency of the s-process and convective dredge-up.
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.
Fast outflows and their interaction with slow shells (generally known as the fossil circumstellar envelope of asymptotic giant branch stars) play an important role in the structure and kinematics of protoplanetary and planetary nebulae (pPNe, PNe). To properly study their effects within these objects, we also need to observe the intermediate-temperature gas, which is only detectable in the far-infrared (FIR) and submillimetre (submm) transitions. We study the physical conditions of the outflows presented in a number of pPNe and PNe, with a focus on their temperature and excitation states. We carried out Herschel/HIFI observations in the submm lines of 12CO in nine pPNe and nine PNe and complemented them with low-J CO spectra obtained with the IRAM 30m telescope and taken from the literature. The spectral resolution of HIFI allows us to identify and measure the different nebular components in the line profiles. The comparison with large velocity gradient (LVG) model predictions was used to estimate the physical conditions of the warm gas in the nebulae, such as excitation conditions, temperature, and density. We found high kinetic temperatures for the fast winds of pPNe, typically reaching between 75 K and 200 K. In contrast, the high-velocity gas in the sampled PNe is colder, with characteristic temperatures between 25 K and 75 K, and it is found in a lower excitation state. We interpret this correlation of the kinetic temperature and excitation state of fast outflows with the amount of time elapsed since their acceleration (probably driven by shocks) as a consequence of the cooling that occurred during the pPN phase.
Planetary nebulae (PNe) represent the near endpoints of evolution for stars of initial mass $sim$1-8 $M_odot$, wherein the envelope of an asymptotic giant branch (AGB) star becomes photodissociated and ionized by high-energy radiation from a newly emerging white dwarf that was the progenitor stars core. It is increasingly evident that most PNe are descended from binary systems. PNe hence provide unique insight into a diverse range of astrophysical phenomena, including the influence of companion stars on the late stages of stellar evolution; stellar wind interactions and shocks; the physics and chemistry of photoionized plasmas and photon-dominated regions (PDRs); and enrichment of the ISM in the products of intermediate-mass stellar nucleosynthesis. We describe specific examples of the potential impact of the ngVLA in each of these areas.
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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