The common envelope interaction is thought to be the gateway to all evolved compact binaries and mergers. Hydrodynamic simulations of the common envelope interaction between giant stars and their companions are restricted to the dynamical, fast, in-spiral phase. They find that the giant envelope is lifted during this phase, but remains mostly bound to the system. At the same time, the orbital separation is greatly reduced, but in most simulations it levels off at values larger than measured from observations. We conjectured that during the post-in-spiral phase the bound envelope gas will return to the system. Using hydrodynamic simulations, we generate initial conditions for our simulation that result in a fall-back disk with total mass and angular momentum in line with quantities from the simulations of Passy et al. We find that the simulated fall-back event reduces the orbital separation efficiently, but fails to unbind the gas before the separation levels off once again. We also find that more massive fall-back disks reduce the orbital separation more efficiently, but the efficiency of unbinding remains invariably very low. From these results we deduce that unless a further energy source contributes to unbinding the envelope (such as was recently tested by Nandez et al.), all common envelope interactions would result in mergers. On the other hand, additional energy sources are unlikely to help, on their own, to reduce the orbital separation. We conclude by discussing our dynamical fall-back event in the context of a thermally-regulated post-common envelope phase.

We carry out magnetohydrodynamical simulations with FLASH of the formation of a single, a tight binary (a ∼2.5 AU) and a wide binary star (a ∼45 AU). We study the outflows and jets from these systems to understand the contributions the circumstellar and circumbinary discs have on the efficiency and morphology of the outflow. In the single star and tight binary case we obtain a single pair of jets launched from the system, while in the wide binary case two pairs of jets are observed. This implies that in the tight binary case the contribution of the circumbinary disc on the outflow is greater than that in the wide binary case. We also find that the single star case is the most efficient at transporting mass, linear and angular momentum from the system, while the wide binary case is less efficient (∼50%, ∼33%, ∼42% of the respective quantities in the single star case). The tight binary's efficiency falls between the other two cases (∼71%, ∼66%, ∼87% of the respective quantities in the single star case). By studying the magnetic field structure we deduce that the outflows in the single star and tight binary star case are magnetocentrifugally driven, whereas in the wide binary star case the outflows are driven by a magnetic pressure gradient.

We present observations of three FU Orionis objects (hereafter, FUors) with nonredundant aperturemask interferometry (NRM) at 1.59 µm and 2.12 µm that probe for binary companions on the scale of the protoplanetary disk that feeds their accretion outbursts. We do not identify any companions to V1515 Cyg or HBC 722, but we do resolve a close binary companion to V1057 Cyg that is at the diffraction limit (ρ = 58.3 ± 1.4 mas or 30 ± 5 AU) and currently much fainter than the outbursting star (∆K ′ = 3.34 ± 0.10 mag). Given the flux excess of the outbursting star, we estimate that the mass of the companion (M ∼ 0.25M ⊙ ) is similar to or slightly below that of the FUor itself, and therefore it resembles a typical T Tauri binary system. Our observations only achieve contrast limits of ∆K ′ ∼ 4 mag, and hence we are only sensitive to companions that were near or above the pre-outburst luminosity of the FUors. It remains plausible that FUor outbursts could be tied to the presence of a close binary companion. However, we argue from the system geometry and mass reservoir considerations that these outbursts are not directly tied to the orbital period (i.e., occurring at periastron passage), but instead must only occur infrequently.

Magnetic fields are known to play a crucial role in the star formation process, particularly in the formation of jets and outflows from protostellar discs. The magnetic field structure in star forming regions is not always uniform and ordered, often containing regions of magnetic turbulence. We present grid-based, magneto-hydrodynamical simulations of the collapse of a 1 M cloud core, to investigate the influence of complex magnetic field structures on outflow formation, morphology and efficiency. We compare three cases: a uniform field, a partially turbulent field and a fully turbulent field, with the same magnetic energy in all three cases. We find that collimated jets are produced in the uniform-field case, driven by a magneto-centrifugal mechanism. Outflows also form in the partially turbulent case, although weaker and less collimated, with an asymmetric morphology. The outflows launched from the partially turbulent case carry the same amount of mass as the uniform-field case but at lower speeds, having only have 71% of the momentum of the uniform-field case. In the case of a fully turbulent field, we find no significant outflows at all. Moreover, the turbulent magnetic field initially reduces the accretion rate and later induces fragmentation of the disc, forming multiple protostars. We conclude that a uniform poloidal component of the magnetic field is necessary for the driving of jets.

We present observations of disc-bearing stars in Upper Scorpius (US) and Upper Centaurus-Lupus (UCL) with moderate resolution spectroscopy in order to determine the influence of multiplicity on disc persistence after ∼ 5−20 Myr. Discs were identified using infra-red (IR) excess from the Wide-field Infra-red Survey Explorer (WISE) survey. Our survey consists of 55 US members and 28 UCL members, using spatial and kinematic information to assign a probability of membership. Spectra are gathered from the ANU 2.3m telescope using the Wide Field Spectrograph (WiFeS) to detect radial velocity variations that indicate the presence of a companion. We identify 2 double-lined spectroscopic binaries, both of which have strong IR excess. We find the binary fraction of disc-bearing stars in US and UCL for periods up to 20 years to be 0.06 0.07 0.02 and 0.13 0.06 0.03 respectively. Based on the multiplicity of field stars, we obtain an expected binary fraction of ∼ 0.12 0.02 0.01 . The determined binary fractions for discbearing stars does not vary significantly from the field, suggesting that the overall lifetime of discs may not differ between single and binary star systems.