Herbert Ho Bun Lau

University of Bonn | Uni Bonn · Argelander-Institute of Astronomy

We explore the final fates of massive intermediate-mass stars by computing detailed stellar models from the zero-age main sequence until near the end of the thermally pulsing phase. These super-asymptotic giant branch (super-AGB) and massive AGB star models are in the mass range between 5.0 and 10.0 M⊙ for metallicities spanning the range Z = 0.02–...

We use our new population synthesis code BONNFIRES to test how surface abundances predicted by rotating stellar models depend on the numerical treatment of rotational mixing, such as spatial resolution, temporal resolution and computation of mean molecular weight gradients. We find that even with identical numerical prescriptions for calculating th...

Powerful telescopes equipped with multi-fibre or integral field spectrographs combined with detailed models of stellar atmospheres and automated fitting techniques allow for the analysis of large number of stars. These datasets contain a wealth of information that require new analysis techniques to bridge the gap between observations and stellar ev...

The very bright red star HV2112 in the Small Magellanic Cloud could be a massive Thorne–Żytkow object (TŻO), a supergiant-like star with a degenerate neutron core. With a luminosity of over 105 L⊙, it could also be a super asymptotic giant branch (SAGB) star, a star with an oxygen/neon core supported by electron degeneracy and undergoing thermal pu...

We present a new grid of stellar models and nucleosynthetic yields for super-AGB stars with metallicities Z = 0.001 and 0.0001, applicable for use within galactic chemical evolution models. Contrary to more metal-rich stars where hot bottom burning is the main driver of the surface composition, in these lower metallicity models the effect of third...

BONNFIRES, a new generation of population synthesis code, can calculate nuclear reaction, various mixing processes and binary interaction in a timely fashion. We use this new population synthesis code to study the interplay between binary mass transfer and rotation. We aim to compare theoretical models with observations, in particular the surface n...

We have computed detailed evolution and nucleosynthesis models for super and massive asymptotic giant branch (AGB) stars over the mass range 6.5–9.0 M⊙ in divisions of 0.5 M⊙ with metallicities Z = 0.02, 0.008 and 0.004. These calculations, in which we find third dredge-up and hot bottom burning, fill the gap between existing low- and intermediate-...

The J-type carbon (J)-stars constitute 10-15% of the observed carbon stars in both our Galaxy and the Large Magellanic Cloud (LMC). They are characterized by strong 13C absorption bands with low 12C/13C ratios along with other chemical signatures peculiar for typical carbon stars, e.g. a lack of s-process enhancement. Most of the J-stars are dimmer...

Our main goals are to get a deeper insight into the evolution and final fates of intermediate-mass, extremely metal-poor (EMP) stars. We also aim to investigate their C, N, and O yields. Using the Monash University Stellar Evolution code we computed and analysed the evolution of stars of metallicity Z = 10^-5 and masses between 4 and 9 M_sun, from...

Tables evohrXXX.dat present the effective temperature (1st column) and surface luminosity (2nd column) for our model stars of metallicity Z=10-5 and masses 4M⊙ (evohr040.dat), 5M⊙ (evohr050.dat), 6M⊙ (evohr060.dat), 7M⊙ (evohr070.dat), 8M⊙ (evohr080.dat), and 9M⊙ (evohr090.dat). The corresponding data are plotted in Figure 1. Tables esurfXXX.dat p...

Aims: Our main goals are to get a deeper insight into the evolution and final fates of intermediate-mass, extremely metal-poor (EMP) stars. We also aim to investigate the C, N, and O yields of these stars. Methods: Using the Monash University Stellar Evolution code MONSTAR we computed and analysed the evolution of stars of metallicity Z = 10-5 and...

Certain carbon-enhanced metal-poor stars likely obtained their composition via pollution from some of the earliest generations of asymptotic giant branch stars and as such provide important clues to early Universe nucleosynthesis. Recently, Kinman et al. discovered that the highly carbon- and barium-enriched metal-poor star SDSS J1707+58 is in fact...

We present numerical simulations of internal gravity waves (IGW) in a star with a convective core and extended radiative envelope. We report on amplitudes, spectra, dissipation and consequent angular momentum transport by such waves. We find that these waves are generated efficiently and transport angular momentum on short timescales over large dis...

We propose that the observed misalignment between extrasolar planets and their hot host stars can be explained by angular momentum transport within the host star. Observations have shown that this misalignment is preferentially around hot stars, which have convective cores and extended radiative envelopes. This situation is amenable to substantial...

We have computed the evolution of Z = 10-5 stars of masses between 4 and 9 Msolar, from their main sequence till the late TP-(S)AGB phase. We use a recent Monash version of the Mount Stromlo Stellar Evolution code, in which molecular opacities include the effects of variable C=O abundances ratio, [1]. By computing hundreds (or thousands) of thermal...

Super Asymptotic Giant Branch Stars (Super AGBs) lie in the mass range 6-11 M⊙, which bridges the divide between low/intermediate mass AGB and massive stars. During the thermally pulsing phase of evolution competition between hot bottom burning (HBB) and third dredge up (3DU) events determine the stellar yields. Obtaining these yields is far more c...

The literature is rich in analysis and results related to thermally pulsing-asymptotic giant branch (TP-AGB) stars, but the problem of the instabilities that arise and cause the divergence of models during the late stages of their evolution is rarely addressed. We investigate the physical conditions, causes and consequences of the interruption in t...

We explore the production of and stellar yields of 7Li for a range of mass and metallicity. We test the effect of varying the mass loss rate and well as the mixing length parameter on the production of lithium. There is a short duration of strong enhancement of Lithium in the early AGB phase, before any thermal pulses. Higher mass-loss rate, partic...

Super asymptotic giant branch (SAGB) stars, in a mass range between 5 and 11 M⊙, have a similar evolution to high-mass AGB stars but are hot enough to ignite carbon in their cores non-degenerately during the early AGB phase. However, they could have a different nucleosynthesis signature because of the higher temperature at the bottom of intershell...

Models of rapidly rotating massive stars at low metallicities show significantly different evolution and higher metal yields compared to non-rotating stars. We estimate the spin-down time-scale of rapidly rotating non-convective stars supporting an α–Ω dynamo. The magnetic dynamo gives rise to mass-loss in a magnetically controlled stellar wind and...

The stellar origin of gamma-ray bursts can be explained by the rapid release of energy in a highly collimated, extremely relativistic jet. This in turn appears to require a rapidly spinning highly magnetised stellar core that collapses into a magnetic neutron star or a black hole within a relatively massive envelope. We hypothesize a binary star mo...

V605 Aquilae is today widely assumed to have been the result of a final helium shell flash occurring on a single post-asymptotic giant branch star. The fact that the outbursting star is in the middle of an old planetary nebula and that the ejecta associated with the outburst is hydrogen deficient supports this diagnosis. However, the material eject...

The stellar origin of gamma-ray bursts can be explained by the rapid release of energy in a highly collimated, extremely relativistic jet. This in turn appears to require a rapidly spinning highly magnetised stellar core that collapses into a magnetic neutron star or a black hole within a relatively massive envelope. They appear to be associated wi...

We have investigated the possibility that binary evolution is involved in the formation of the planetary nebula Abell 58. In particular, we assume a neon nova is responsible for the observed high oxygen and neon abundances of the central hydrogen-deficient knot of the H-deficient planetary nebula Abell 58 and the ejecta from the explosion are mixed...

We investigate the behaviour of asymptotic giant branch (AGB) stars between metallicities Z= 10−4 and 10−8. We determine which stars undergo an episode of flash-driven mixing, where protons are ingested into the intershell convection zone, as they enter the thermally pulsing AGB phase and which undergo third dredge-up. We find that flash-driven mix...

We use the Cambridge stellar evolution code stars to model the evolution of 5 and 7 M⊙ zero-metallicity stars. With enhanced resolution at the hydrogen- and helium-burning shell in the asymptotic giant branch (AGB) phases, we are able to model the entire thermally pulsing AGB (TP-AGB) phase. The helium luminosities of the thermal pulses are signifi...

We use the Cambridge stellar evolution code stars to model the evolution and nucleosynthesis of zero-metallicity intermediate-mass stars. We investigate the effect of duplicity on the nucleosynthesis output of these systems and the potential abundances of the secondaries. The surfaces of zero-metallicity stars are enriched in CNO elements after sec...

The first generation of stars, commonly referred to as Population-III stars or zero-metallicity stars, should have the composition after Big Bang nucleosynthesis. Hydrogen burning proceeds via the proton-proton chain rather than the CNO cycle owing to absence of CNO elements. It is much more inefficient and is less temperature dependent. So zero-me...

The Cambridge STARS code is used to model the evolution and nucleosynthesis of binary zero- metallicity low to intermediate mass stars. The surfaces of these stars are enriched in CNO ele- ments after second dredge up. During binary interaction metals can be released from these stars and the secondary enriched in CNO. The observed abundances of HE...