Merging in the common envelope and the origin of early R-type stars

Abstract

Context. Binary systems experiencing one or two common envelope episodes during the red giant branch or the Hertzsprung gap phases can produce a single star, evolving along the Hayashi track, as a final outcome. Even if these objects are expected to be very common in nature, a proper description of their evolution and physical properties is still missing. Moreover, this scenario (red giant merging scenario) has been invoked as the progenitor systems of early-R stars, by assuming that the physical conditions developed as a consequence of the cores merging could produce the mixing into the convective envelope of fresh carbon that was synthesized during the He-flash. Aims. We analyze in detail the red giant merging scenario to verify if the resulting star develops the physical conditions suitable for a dredge-up of C-enriched material from the core to the envelope. Methods. We performed 3D simulations of the merging stars, to check whether He is burnt efficiently during the formation of a selfsustained disk. We therefore did 1D computations of the accretion phase occurring after the merging and of the following evolution up to the settling of quiescent He-burning in the center. We adopted different assumptions on the amount of angular momentum transferred from the disk to the core and on the angular momentum transport. Results. Efficient He-burning does not occur during the merging, because a very high temperature (T > 10 8 K) at the disk/He-core interface develops only for a few minutes. Our computations show that the accretion process is the leading parameter in determining the final properties of the merged object. In particular, the thermal energy delivered by the accreted matter determines the heating of the whole newborn core, thus preventing the developing of highly degenerate physical conditions. This occurrence determines the onset of the He-burning with an He-flash milder and closer to the center, as compared to standard RGB stars. Rotation and different angular momentum transport efficiency plays a secondary role by determining the exact location of the first He-flash. In none of the computed models is material formed in the He-core mixed into the convective envelope, because the H-burning shell, which always active during the He-flashes and later on, acts as a barrier. Conclusions. In the red giant merging scenario, the physical conditions suitable for both a peculiar He-flash and the dredging-up of C-enriched material never occur. Our results speak against the possibility that such an evolutionary scenario could represent the progenitor system of early R-stars.

Dawes Review 6: The Impact of Companions on Stellar Evolution

“…A helium WD merging with a red giant, itself with a helium core, followed by mixing during helium ignition in a rapidly rotating star, may be the answer. Population synthesis models predicted that there are sufficient CE mergers to explain these stars (Izzard et al, 2007), and subsequent detailed modelling confirmed that in some cases, this may indeed be the case (Piersanti et al 2010; Zhang & Jeffery 2013). The former study adopted a two-pronged approach combining 3D hydrodynamics and 1D implicit codes; see Section 4.2.1).…”

THE EFFECTS OF ROTATION ONs-PROCESS NUCLEOSYNTHESIS IN ASYMPTOTIC GIANT BRANCH STARS

“…In particular, for the gravitational plus rotational potential we adopt the approximation by Kippenhahn & Thomas (1970), i.e. the radial component of the centrifugal force is averaged over a sphere (see Piersanti et al 2003Piersanti et al , 2010Tornambé & Piersanti 2013). In addition, a corrective term for the radiative temperature gradient is included in the energy transport equation (Endal & Sofia 1976).…”

Some Stars Are Totally Metal: A New Mechanism Driving Dust Across Star-Forming Clouds, and Consequences for Planets, Stars, and Galaxies

“…Of course, the two possibilities are not mutually exclusive; the stars probably represent the sites of new grain formation, which in turn help "seed" new populations, and the connection can become self-reinforcing! Moreover, alternative explanations for this subpopulation, including stellar evolution processes (mixing or production from rotation, convection, unusual AGB phases or He-flashes), stellar mergers or mass transfer, all appear to make predictions in serious conflict with the observations (Izzard et al 2007;Zamora et al 2009;Mocák et al 2009;Piersanti et al 2010). It is much more natural to assume the stars are simply "primordially" enhanced in the species most abundant in large grains (if a mechanism to do so exists); the other mechanisms above would still act, of course, and modify the abundances actually measured, but they would only have to explain secondary characteristics like the N enhancement in these stars believed to result from the CN cycle and He-burning (relatively easy to produce; see Zamora et al 2009).…”