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Register a new userPast and future of the central double-degenerate core of Henize 2-428
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It has been suggested that SNe Ia could be produced in the condition of the violent merger scenario of the double-degenerate model, in which a thermonuclear explosion could be produced when the merging of double carbon-oxygen white dwarfs (CO WDs) is still ongoing. It has been recently found that the nucleus of the bipolar planetary nebula Henize 2-428 consists of double CO WDs that have a total mass of ~1.76Msun, a mass ratio of ~1 and an orbital period of ~4.2 hours, which is the first and only discovered progenitor candidate of SNe Ia predicted by the violent merger scenario. In this work, we aim to reproduce the evolutionary history of the central double CO WDs of Henize 2-428. We find that the planetary nebula Henize 2-428 may originate from a primordial binary that have a ~5.4Msun primary and a ~2.7Msun secondary with an initial orbital period of ~15.9 days. The double CO WDs are formed after the primordial binary experiencing two Roche-lobe overflows and two common-envelope ejection processes. According to our calculations, it takes about ~840 Myr for the double CO WDs to merge and form an SN Ia driven by the gravitational wave radiation after their birth. To produce the current status of Henize 2-428, a large common-envelope parameter is needed. We also estimate that the rate of SNe Ia from the violent merger scenario is at most 2.9*10-4 yr-1, and that the delay time is in the range of ~90 Myr to the Hubble time.
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We present a study of Hen 2-155 and Hen 2-161, two planetary nebulae which bear striking morphological similarities to other planetary nebulae known to host close-binary central stars. Both central stars are revealed to be photometric variables while spectroscopic observations confirm that Hen 2-155 is host to a double-eclipsing, post-common-envelope system with an orbital period of 3h33m making it one of the shortest period binary central stars known. The observations of Hen 2-161 are found to be consistent with a post-common-envelope binary of period ~1 day. A detailed model of central star of Hen 2-155, is produced, showing the nebular progenitor to be a hot, post-AGB remnant of approximately 0.62 Msol, consistent with the age of the nebula, and the secondary star to be an M dwarf whose radius is almost twice the expected ZAMS radius for its mass. In spite of the small numbers, all main-sequence companions, of planetary nebulae central stars, to have had their masses and radii constrained by both photometric and spectroscopic observations have also been found to display this inflation. The cause of the inflation is uncertain but is probably related to rapid accretion, immediately before the recent common-envelope phase, to which the star has not yet thermally adjusted. The chemical composition of both nebulae is also analysed, showing both to display elevated abundance discrepancy factors. This strengthens the link between elevated abundance discrepancy factors and close binarity in the nebular progenitor.
The nucleus of the planetary nebula Hen2-428 is a short orbital-period (4.2h), double-lined spectroscopic binary, whose status as a potential supernova type Ia progenitor has raised some controversy in the literature. With the aim of resolving this debate, we carried out an in-depth reanalysis of the system. Our approach combines a refined wavelength calibration, thorough line-identifications, improved radial-velocity measurements, non-LTE spectral modeling, as well as multi-band light-curve fitting. Our results are then discussed in view of state-of-the-art stellar evolutionary models. Besides systematic zero-point shifts in the wavelength calibration of the OSIRIS spectra which were also used in the previous analysis of the system, we found that the spectra are contaminated with diffuse interstellar bands. Our Voigt-profile radial velocity fitting method, which considers the additional absorption of these diffuse interstellar bands, reveals significantly lower masses ($M_1=0.66pm0.11M_odot$ and $M_2=0.42pm0.07M_odot$) than previously reported and a mass ratio that is clearly below unity. Our spectral and light curve analyses lead to consistent results, however, we find higher effective temperatures and smaller radii than previously reported. Moreover, we find that the red-excess that was reported before to prove to be a mere artifact of an outdated reddening law that was applied. Our work shows that blends of HeII 5412A with diffuse interstellar bands have led to an overestimation of the previously reported dynamical masses of Hen2-428. The merging event of Hen2-428 will not be recognised as a supernova type Ia, but most likely leads to the formation of a H-deficient star. We suggest that the system was formed via a first stable mass transfer episode, followed by common envelope evolution, and it is now composed of a post-early AGB star and a reheated He-core white dwarf.
The origin of the progenitors of type Ia supernovae (SNe Ia) is still uncertain. The core-degenerate (CD) scenario has been proposed as an alternative way for the production of SNe Ia. In this scenario, SNe Ia are formed at the final stage of common-envelope evolution from a merger of a carbon-oxygen white dwarf (CO WD) with the CO core of an asymptotic giant branch companion. However, the birthrates of SNe Ia from this scenario are still not well determined. In this work, we performed a detailed investigation on the CD scenario based on a binary population synthesis approach. The SN Ia delay times from this scenario are basically in the range of 90Myr-2500Myr, mainly contributing to the observed SNe Ia with short and intermediate delay times although this scenario can also produce some old SNe Ia. Meanwhile, our work indicates that the Galactic birthrates of SNe Ia from this scenario are no more than 20% of total SNe Ia due to more careful treatment of mass transfer. Although the SN Ia birthrates in the present work are lower than those in Ilkov & Soker, the CD scenario cannot be ruled out as a viable mechanism for the formation of SNe Ia. Especially, SNe Ia with circumstellar material from this scenario contribute to 0.7-10% of total SNe Ia, which means that the CD scenario can reproduce the observed birthrates of SNe Ia like PTF 11kx. We also found that SNe Ia happen systemically earlier for a high value of metallicity and their birthrates increase with metallicity.
We consider a class of toy models where a spatially flat universe is filled with a perfect fluid. The dynamics is found exactly for all these models. In one family, the perfect fluid is of the phantom type and we find that the universe is first contracting and then expanding while the dynamics is always accelerated. In a second family, the universe is first in an accelerated expansion stage, then in a decelerated expansion stage until it reaches a turning point after which it contracts in a decelerated way (increasing contraction rate) followed by another accelerated stage (decreasing contraction rate). We also consider the possibility to embed this perfect fluid in a realistic cosmology. The first family cannot be viable in a conventional big bang universe and requires a rebound in the very early universe. The second family is viable in the range $0<1+w_{DE,0}lesssim 0.09$ for a spatially closed universe with a curvature satisfying current bounds. Though many of the models in this family cannot be distinguished today from a universe dominated by a cosmological constant, the present accelerated expansion is transient and these universes will reach a turning point in the future before entering a contraction phase.
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