Special Seminars



"A lack of classical Cepheids in the inner part of the Galactic disk"

Matsunaga Noriyuki 松永 典之 (まつなが のりゆき) (University of Tokyo )

Recent large-scale infrared surveys have been revealing stellar populations in the inner Galaxy seen through strong interstellar extinction in the disk. In particular, classical Cepheids with their period-luminosity and period-age relations are useful tracers of Galactic structure and evolution. Interesting groups of Cepheids reported recently include four Cepheids in the Nuclear Stellar Disk (NSD), about 200 pc around the Galactic Centre, found by Matsunaga et al. (2011, Nature, 477, 190) and those spread across the bulge region reported based on VVV data (Dekany et al. 2015, ApJ, 812, L29). In this talk, I'll talk our discovery of several classical Cepheids towards the bulge region and discuss the large impact of the reddening correction on distance estimates for these objects. Most of the other Cepheids discussed here are located signifficantly further than the Galactic Centre. This suggests a lack of Cepheids in the inner 3 kpc region of the Galactic disk except the NSD. I'll also present some results and plans of near-infrared spectroscopy of Cepheids.

Lecture at 11:15 am, Copernicus Center, Lecture Hall.


"The initial binary-star population distribution functions and dynamical population synthesis"

Pavel Kroupa (Helmholtz-Institut fuer Strahlen- und Kernphysik, Universitaet Bonn)

Most and probably all stars form in binary systems and some in higher-order systems. The distribution function of binaries at their birth is not observable because it evolves rapidly. Very young binaries evolve due to system-internal processes termed pre-main sequence eigenevolution, while older binaries evolve through tidal circularisation. Binary systems are also split apart into two single stars due to stellar-dynamical encounters in the birth embedded clusters, and some binaries merge as well. I will show how the initial distribution functions for periods, mass-ratios and eccentricities can be constrained for late-type stars by using observations of stars in the Galactic field and in star-forming regions. This leads to possibly universal distribution functions. Assuming all stars form as binaries in embedded star clusters we can then synthesize the population of binaries in a whole galaxy. The prediction of this dynamical population synthesis is that massive elliptical galaxies ought to have a low binary fraction (about 30 per cent) while star-forming dwarf galaxies ought to have a binary fraction of about 80 per cent, where the Galaxy has a binary fraction of about 55 per cent as is observed to be the case. This approach has much potential for further applications.

Lecture in the main lecture hall at NCAC will start at 11:15 am and will last 1,5 h.