Jan E. Staff

Hydrodynamic Simulations of the Interaction between Giant Stars and Planets

We present the results of hydrodynamic simulations of the interaction between a 10 Jupiter mass planet and a red or asymptotic giant branch stars, both with a zeroage main sequence mass of 3.5 M⊙. Dynamic in-spiral timescales are of the order of few years and a few decades for the red and asymptotic giant branch stars, respectively. The planets will eventually be destroyed at a separation from the core of the giants smaller than the resolution of our simulations, either through evaporation or tidal disruption. As the planets in-spiral, the giant stars’ envelopes are somewhat puffed up. Based on relatively long timescales and even considering the fact that further inspiral should take place before the planets are destroyed, we predict that the merger would be difficult to observe, with only a relatively small, slow brightening. Very little mass is unbound in the process. These conclusions may change if the planet’s orbit enhances the star’s main pulsation modes. Based on the angular momentum transfer, we also suspect that this star-planet interaction may be unable to lead to large scale outflows via the rotation-mediated dynamo effect of Nordhaus and Blackman. Detectable pollution from the destroyed planets would only result for the lightest, lowest metallicity stars. We furthermore find that in both simulations the planets move through the outer stellar envelopes at Mach-3 to Mach-5, reaching Mach-1 towards the end of the simulations. The gravitational drag force decreases and the in-spiral slows down at the sonic transition, as predicted analytically.

The effect of a wider initial separation on common envelope binary interaction simulations

We present hydrodynamic simulations of the common envelope binary interaction between a giant star and a compact companion with an adaptive mesh refinement and a smooth particle hydrodynamics codes. These simulations mimic the parameters of one of the simulations by Passy et al., but start with a wider orbital separation to assess the influence of a larger initial orbital separation on the common envelope simulation outcome. We conclude that the post-common envelope separation is somewhat larger and the amount of unbound mass slightly greater when the initial separation is wide enough that the giant does not yet overflow or just overflows its Roche lobe. By setting our simulations in the context of those carried out in the past that contain at least one giant star, we conclude the following: the reason for the larger final orbital separation in simulations starting with a wider orbital separation has more to do with the expanded giant at the time of in-spiral and less to do with a larger amount of angular momentum. We also suggest that the large range in unbound mass for different simulations is difficult to explain and may have something to do with simulations that are not fully converged.

Hydrodynamic simulations of the interaction between an AGB star and a main-sequence companion in eccentric orbits

The Rotten Egg Nebula has at its core a binary composed of a Mira star and an A-type companion at a separation >10 au. It has been hypothesized to have formed by strong binary interactions between the Mira and a companion in an eccentric orbit during periastron passage ~800 years ago. We have performed hydrodynamic simulations of an asymptotic giant branch star interacting with companions with a range of masses in orbits with a range of initial eccentricities and periastron separations. For reasonable values of the eccentricity, we find that Roche lobe overflow can take place only if the periods are <<100 years. Moreover, mass transfer causes the system to enter a common envelope phase within several orbits. Since the central star of the Rotten Egg nebula is an AGB star, we conclude that such a common envelope phase must have lead to a merger, so the observed companion must have been a tertiary companion of a binary that merged at the time of nebula ejection. Based on the mass and timescale of the simulated disc formed around the companion before the common envelope phase, we analytically estimate the properties of jets that could be launched. Allowing for super-Eddington accretion rates, we find that jets similar to those observed are plausible, provided that the putative lost companion was relatively massive.

Considerations on the role of fall-back discs in the final stages of the common envelope binary interaction

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.

A MASSIVE PROTOSTAR FORMING BY ORDERED COLLAPSE OF A DENSE, MASSIVE CORE

We present 30 and 40 μm imaging of the massive protostar G35.20–0.74 with SOFIA-FORCAST. The high surface density of the natal core around the protostar leads to high extinction, even at these relatively long wavelengths, causing the observed flux to be dominated by that emerging from the near-facing outflow cavity. However, emission from the far-facing cavity is still clearly detected. We combine these results with fluxes from the near-infrared to mm to construct a spectral energy distribution (SED). For isotropic emission the bolometric luminosity would be 3.3 × 104 L ☉. We perform radiative transfer modeling of a protostar forming by ordered, symmetric collapse from a massive core bounded by a clump with high-mass surface density, Σcl. To fit the SED requires protostellar masses ~20-34 M ☉ depending on the outflow cavity opening angle (35°-50°), and Σcl ~ 0.4-1 g cm–2. After accounting for the foreground extinction and the flashlight effect, the true bolometric luminosity is ~(0.7-2.2) × 105 L ☉. One of these models also has excellent agreement with the observed intensity profiles along the outflow axis at 10, 18, 31, and 37 μm. Overall our results support a model of massive star formation involving the relatively ordered, symmetric collapse of a massive, dense core and the launching bipolar outflows that clear low-density cavities. Thus a unified model may apply for the formation of both low- and high-mass stars.

QUARK-NOVAE IN LOW-MASS X-RAY BINARIES. II. APPLICATION TO G87–7 AND TO GRB 110328A

We propose a simple model explaining two outstanding astrophysical problems related to compact objects: (1) that of stars such as G87–7 (alias EG 50) that constitute a class of relatively low-mass white dwarfs (WDs) which nevertheless fall away from the C/O composition and (2) that of GRB 110328A/Swift J164449.3+57345 which showed spectacularly long-lived strong X-ray flaring, posing a challenge to standard gamma-ray burst models. We argue that both these observations may have an explanation within the unified framework of a quark-nova (QN) occurring in a low-mass X-ray binary (LMXB; neutron star (NS)-WD). For LMXBs, where the binary separation is sufficiently tight, ejecta from the exploding NS triggers nuclear burning in the WD on impact, possibly leading to Fe-rich composition compact WDs with mass 0.43 M ☉ 0.72 M ☉) experiencing the QN shock, degeneracy will not be lifted when carbon burning begins, and a sub-Chandrasekhar Type Ia supernova may result in our model. Under slightly different conditions and for pure He WDs (i.e., M WD < 0.43 M ☉), the WD is ablated and its ashes raining down on the quark star (QS) leads to accretion-driven X-ray luminosity with energetics and duration reminiscent of GRB 110328A. We predict additional flaring activity toward the end of the accretion phase if the QS turns into a black hole.

Hubble Space Telescope scale 3D simulations of MHD disc winds: a rotating two-component jet structure

We present the results of large scale, three-dimensional magneto-hydrodynamics simulations of disc-winds for different initial magnetic field configurations. The jets are followed from the source to 90 AU scale, which covers several pixels of HST images of nearby protostellar jets. Our simulations show that jets are heated along their length by many shocks. We compute the emission lines that are produced, and find excellent agreement with observations. The jet width is found to be between 20 and 30 AU while the maximum velocities perpendicular to the jet is found to be up to above 100 km/s. The initially less open magnetic field configuration simulations results in a wider, two-component jet; a cylindrically shaped outer jet surrounding a narrow and much faster, inner jet. These simulations preserve the underlying Keplerian rotation profile of the inner jet to large distances from the source. However, for the initially most open magnetic field configuration the kink mode creates a narrow corkscrew-like jet without a clear Keplerian rotation profile and even regions where we observe rotation opposite to the disc (counter-rotating). The RW Aur jet is narrow, indicating that the disc field in that case is very open meaning the jet can contain a counter-rotating component that we suggests explains why observations of rotation in this jet has given confusing results. Thus magnetized disc winds from underlying Keplerian discs can develop rotation profiles far down the jet that are not Keplerian.

QUARK-NOVAE IN LOW-MASS X-RAY BINARIES WITH MASSIVE NEUTRON STARS: A UNIVERSAL MODEL FOR SHORT-HARD GAMMA-RAY BURSTS

We show that several features reminiscent of short-hard gamma-ray bursts (GRBs) arise naturally when quark-novae (QNe) occur in low-mass X-ray binaries born with massive neutron stars (NS; ≥1.6 M ☉) and harboring a circumbinary disk (CD). Near the end of the first accretion phase, conditions are just right for the explosive conversion of the NS to a quark star (QS). In our model, the subsequent interaction of material from the NSs ejected crust with the CD explains the duration, variability, and near-universal nature of the prompt emission in short-hard GRBs. We also describe a statistical approach to ejecta breakup and collision to obtain the photon spectrum in our model, which turns out to be remarkably similar to the empirical Band function. We apply the model to the fluence and spectrum of GRB 000727, GRB 000218, and GRB980706A, obtaining excellent fits. Extended emission (EE; spectrum and duration) is explained by shock heating and ablation of the white dwarf by the highly energetic ejecta. Depending on the orbital separation when the QN occurs, we isolate interesting regimes within our model when both prompt emission and EE can occur. We find that the spectrum can carry signatures typical of Type Ib/c supernovae (SNe) although these should appear less luminous than normal type Ib/c SNe. Late X-ray activity is due to accretion onto the QS as well as its spin-down luminosity. Afterglow activity arises from the expanding shell of material from the shock-heated expanding CD. We find a correlation between the duration and spectrum of short-hard GRBs as well as modest hard-to-soft time evolution of the peak energy.

The effect of binding energy and resolution in numerical simulations of the common envelope binary interaction

The common envelope binary interaction remains one of the least understood phases in the evolution of compact binaries, including those that result in Type Ia supernovae and in mergers that emit detectable gravitational waves. In this work we continue the detailed and systematic analysis of 3D hydrodynamic simulations of the common envelope interaction aimed at understanding the reliability of the results. Our first set of simulations replicate the 5 simulations of Passy et al. (a 0.88Msun, 90Rsun RGB primary with companions in the range 0.1 to 0.9Msun) using a new AMR gravity solver implemented on our modified version of the hydrodynamic code Enzo. Despite smaller final separations obtained, these more resolved simulations do not alter the nature of the conclusions that are drawn. We also carry out 5 identical simulations but with a 2.0Msun primary RGB star with the same core mass as the Passy et al. simulations, isolating the effect of the envelope binding energy. With a more bound envelope all the companions in-spiral faster and deeper though relatively less gas is unbound. Even at the highest resolution, the final separation attained by simulations with a heavier primary is similar to the size of the smoothed potential even if we account for the loss of some angular momentum by the simulation. As a result we suggest that a ~2.0

The SOFIA Massive (SOMA) Star Formation Survey. I. Overview and First Results

Msun RGB primary may possibly end in a merger with companions as massive as 0.6Msun, something that would not be deduced using analytical arguments based on energy conservation.