I work with numerous colleagues around the world. In particular, at the AIfA I work with the following fine people on various interesting topics.
Herbert Lau : Principle author of the new single/binary star population synthesis code BONNFIRES.
Most stars do not live alone. Many of them evolve next to a companion star in a binary star system. [Click for more]
A binary system is bound by the gravity of its star which orbit one another much like the Earth orbits theSun. Stars grow in size during their evolution, so for most binarysystem, the stars will interact with their companion through gravityat some point in their lifetime. When the radii of the stars aresufficiently large that the outer envelope of the stars are no longergravitationally bound to the stars, masses will then be transferredonto the other star or lost from the system. Stars will then be spunup or spun down depending on whether mass is accreted or lostrespectively.
Stars can be born rapidly rotating or spun up through mass accretion.Rotation induces deeper chemical mixing in stars, so the observedsurface abundances in rapidly rotating stars should be more enhancedin elements produced by nuclear burning. Howver, both rotationalmixing and binary interaction transfer nuclear-processed materials tothe surface, either from its own interior nuclear burning region orfrom its more massive companion, so it is sometimes notstraightforward to distinguish from one case to the other. Anunderstanding of rotational mixing, binary interaction, and theinterplay between rotation and binarity through transfer of angularmomentum and mass are crucial are crucial in stellar astrophysics.
Fabian Schneider : Binary stars and the IMF, statistics of massive binary stellar populations.
The initial mass function (IMF) is important for various fields of astrophysics: it determinesobservable properties of stellar systems, the chemical enrichment of the interstellar mediumand supernova rates. [Click for more]
Furthermore we can learn much about star formation if we know theIMF. However, to measure the IMF directly is impossible because we can only observe thecurrent mass distribution of stars. It is therefore of utmost importance to know systematiceffects which may cause variations in the observed present day mass function (PDMF). In thiswork we explore the impact of stellar and binary evolution on the PDMF by means of a rapidbinary evolution code. We find that wind mass loss flattens the PDMF significantly at thehighest masses observed. Mass transfer due to Roche lobe overflow (RLOF), stellar mergersand the concomitant rejuvenation can shape the PDMF by large amounts. These effectstogether with those due to unresolved binaries also flatten the observed mass function andhence the inferred IMF. Star formation histories with a recent starburst act like an amplifierfor evolutionary effects. If mass transfer is highly conservative the inferred power law indexΓ of the IMF may change by ΔΓ ≈ 1 towards a flatter IMF for masses between ~8 and 30 Msunand towards a steeper IMF for masses >30 Msun. If we switch the mass transfer efficiency toa minimum these deviations are still of the order of ΔΓ≈0.2 to 0.3. We conclude that binaryinteractions need to be taken into account to determine the IMF and provide a method tocorrect for all mentioned effects. See Fabian's website for more information.
Denise Keller : Planetary Nebulae and the link with Binarity.
Planetary nebulae (PNe) are crucial in the understanding of theevolution of low- and intermediate-mass stars. [Click for more]
These stars haveinitial masses less than 8 solar masses and are the most common typeof stars in the Universe. Mostly all intermediate-mass stars gothrough the PN phase before they turn into white dwarfs (Abell andGoldreich 1966). It is the only phase which connects the asymptoticgiant branch (AGB) and the white dwarf stage of a single star.
Bystudying PNe we learn more about the nucleosynthesis, mixing andmass-loss processes in a giant branch star and the elements which arereturned to the interstellar medium. PNe are unique laboratories foratomic, nuclear and stellar physics. Nevertheless, single starevolution of AGB stars cannot thoroughly explain the various observedshapes and structures, especially bipolar outflows, of PNe. So binarystar evolution has to be considered. In fact, there are 12-21% of allPNe observed to have a close binary system in the center (Miszalski2009). These are formed by a common envelope phase. So there are twomajor mechanisms to form a planetary nebula: wind mass loss and commonenvelope ejection. With the binary-star population nucleosynthesiscode binary_c/nucsyn of Prof. Robert Izzard one can constrain thebinary fraction of PN central stars from theory and compare thefindings with the results from observations. Population synthesis isthe best tool to study the characteristics of PNe in a statisticalmanner. Especially the nucleosynthesis of PNe has never before beenstudied with a stellar population synthesis code.
The J-type carbon stars constitute a significant fraction (10 to 15%) ofthe observed carbon stars in our Galaxy and the Large MagellanicCloud. [Click for more]
They are characterized by their low 12C 13C(.10) ratiossignifying enhancement of 13C, along with other peculiar chemicalsignatures like the lack of s-process enhancement unlike normal(TP-AGB) carbon stars. There are also surveys estimating luminositiesof these objects (eg. Morgan et al. 2003 in the LMC) which show thatmost of them are dimmer than typical AGB stars, indicating they arelow-mass less evolved objects – contrary to hot bottom burning (AGB)models. As an new approach to understanding the observedpeculiarities in these mysterious objects, a binary model involvingnovae systems is investigated, where the WD companion (secondary) canevolve with such anomalous features, due to re-accretion of ejecta ofthe novae that can significantly alter its evolution and observedchemical abundances. Depending on the binary separation, thissecondary can evolve as a dimmer subgiant or merge with the WD into abright AGB star, enhanced in isotopes like 13C – significantlyproduced in novae (Jose & Hernanz 1998). A detailed populationsynthesis study is performed to estimate the number of systems inwhich such nova pollution can cause enough 13C enhancement to evolvethe WD companion with the peculiar features observed for the J-typecarbon stars.
• Telephone 
• Fax 
