Zara Osborn

We present a systematic test of formalisms for common envelope evolution by forward modeling observable post-common-envelope binaries (PCEBs). We compare predictions from the α -formalism and the Two-stage and SCATTER formalisms against observed PCEBs, including wide binaries with ultramassive white dwarfs and central binaries of planetary nebulae. The angular-momentum-based SCATTER formalism does not predict populations that match the complete observed population, even with adjustments to its parameters. We take this as indicative of fundamental challenges with using the orbital angular momentum balance to predict common envelope outcomes. The energy-based α - and hybrid Two-stage formalisms both well replicate the observed population. α _CE ∼ 0.2–0.3 can match current observations, in agreement with previous works. Recombination energy is necessary, but only a fraction of it (∼10%–40%) can contribute in order to predict IK Peg–like binaries with ultramassive white dwarfs at the correct orbital periods. Our work suggests energy-based formalisms remain the most accurate for predicting common envelope outcomes, but more observations can constrain the recombination contribution and how these outcomes systematically vary with the donor mass.

Close companions influence stellar evolution through tidal interactions, mass transfer, and mass loss effects. While such companions are detected around young stellar objects, main-sequence stars, red giants, and compact objects, direct observational evidence of close-in companions around asymptotic giant branch (AGB) stars has remained elusive. Here, we present (sub)millimeter time-domain imaging spectroscopy revealing the Keplerian motion of a close-in companion around the AGB star pi1 Gruis. The companion, slightly more massive than the AGB star, is likely a main-sequence star. Unlike more evolved stars with companions at comparable distances, pi1 Gru's companion follows a circular orbit, suggesting an eccentricity-generating mechanism late- or post-AGB. Our analysis suggests that model-predicted circularization rates may be underestimated. Our results highlight the potential of multi-epoch (sub)millimeter interferometry in detecting the Keplerian motion of close companions to giant stars and open avenues for our understanding of tidal interaction physics and binary evolution.

Context. The explosive burning that drives nova eruptions results in unique nucleosynthesis that heavily over-produces certain isotopes relative to the solar abundance. However, novae are often ignored when considering the chemical evolution of our Galaxy due to their low ejecta masses. Galactic chemical evolution studies including novae are rare and have previously relied upon simplified treatments for the behaviour of nova populations. Aims. In this work, we use previously computed synthetic nova populations and the galactic chemical evolution code OMEGA+ to assess the impact that novae have on the evolution of stable elemental and isotopic abundances. Methods. We combine populations of novae computed using the binary population synthesis code binary_c with the galactic chemical evolution code OMEGA+ and detailed, white dwarf mass-dependent nova yields to model the nucleosynthetic contributions of novae to the evolution of the Milky Way. We consider three different nova yield profiles, each corresponding to a different set of nova yield calculations. Results. We examine which nova sites contribute most to which isotopes. Despite novae from low-mass white dwarfs (WDs) dominating nova ejecta contributions, we find that novae occurring on massive WDs are still able to contribute significantly to many isotopes, particularly those with high mass numbers. We find that novae can produce up to 35% of the Galactic ¹³C and ¹⁵N mass by the time the model Galaxy reaches [Fe/H] = 0, and earlier in the evolution of the Galaxy (between [Fe/H] = −2 and −1) novae may have been the dominant source of ¹⁵N. Predictions for [¹³C/Fe], [¹⁵N/Fe], ¹²C/¹³C, and ¹⁴N/¹⁵N abundances ratios vary by up to 0.2 dex at [Fe/H] = 0 and by up to 0.7 dex in [¹⁵N/Fe] and ¹⁴N/¹⁵N between [Fe/H] = −2 and −1 (corresponding approximately to Galactic ages of 170 Myr and 1 Gyr in our model). The Galactic evolution of other stable isotopes (excluding Li) is not noticeably affected by including novae. For most isotopes, agreement is generally good between the three different yield profiles we consider. Isotopes where agreement is relatively poor include: ³He (especially at high M_WD), ⁷Li, ¹⁸O, ¹⁸F, and the > 1.3 M_⊙ regime of ²⁹Si, ³³S, ³⁴S, ³⁵Cl, and ³⁶Ar.

Aluminium-26 is a radioactive isotope which can be synthesized within asymptotic giant branch (AGB) stars, primarily through hot bottom burning. Studies exploring ²⁶Al production within AGB stars typically focus on single-stars; however, observations show that low- and intermediate-mass stars commonly exist in binaries. We use the binary population synthesis code binary_c to explore the impact of binary evolution on ²⁶Al yields at solar metallicity both within individual AGB stars and a low/intermediate-mass stellar population. We find the key stellar structural condition achieving most ²⁶Al overproduction is for stars to enter the thermally-pulsing AGB (TP-AGB) phase with small cores relative to their total masses, allowing those stars to spend abnormally long times on the TP-AGB compared to single-stars of identical mass. Our population with a binary fraction of 0.75 has an ²⁶Al weighted population yield increase of 25% compared to our population of only single-stars. Stellar-models calculated from the Mt Stromlo/Monash Stellar Structure Program, which we use to test our results from binary_c and closely examine the interior structure of the overproducing stars, support our binary_c results only when the stellar envelope gains mass after core-He depletion. Stars which gain mass before core-He depletion still overproduce ²⁶Al, but to a lesser extent. This introduces some physical uncertainty into our conclusions as 55% of our ²⁶Al overproducing stars gain envelope mass through stellar wind accretion onto pre-AGB objects. Our work highlights the need to consider binary influence on the production of ²⁶Al.