Wednesday Colloquium


"Compressible Small-Scale Kinematic Dynamos"

Michael Mond (Ben Gurion University Negev, Israel)

The origin of the magnetic fields in the universe is one of the fundamental issues in astrophysics. In the current-day paradigm, seed magnetic fields are amplified over many orders of magnitude by turbulent magnetic dynamo action. In particular, the small-scale dynamo gives rise to the amplification of small-scale fluctuations of the magnetic field due to the random stretching of the field by the turbulent flow. In this talk I will concentrate on the role of compressibility of the turbulent flow. Its effect on small-scale field amplification are investigate by employing the Kazantsev-Kraichnan theory for kinematic dynamos. It is shown that the sonic scale, defined as the scale where typical turbulent eddy velocity equals the speed of sound, imposes a robust structure on the growth process. Three as well as two dimensional cases will be discussed.


"Accretion disks with outflows and their implications on massive black hole growth in the early universe"

Xinwu Cao (Zhejiang University, Hangzhou, China (浙江大学))

We derive a global solution to a slim disk with radiation-driven outflows, and find that the outflows are driven from the disk surface if the dimensionless mass accretion rate is around 1.8 (in units of Eddington rate: mdot=Mdot_Edd = L_Edd/0.1c^2). The rate of the gas swallowed by the black hole (BH) is always limited to mdot∼1.8−1.9 even if the mass accretion rate at the outer edge of the disk is very high. This implies it rather difficult to grow a massive BH with billions of solar masses at z>7 via accretion from a stellar mass BH. We develop a self-consistent accretion disk model with magnetically driven outflows, in which most angular momentum of the disk is removed by the outflows. It is found that the SMBH with several billion solar masses discovered at z>7 may probably be grown through chaotic accretion predominantly driven by magnetic outflows from a stellar mass BH, when the disks are radiating at moderate luminosity (∼ 0:5 Eddington luminosity) with mild outflows. Most SMBHs are found to be spinning at moderate values of spin parameter a at high redshifts, which may imply only a small fraction of quasars having radio jets.


"Numerical studies of white dwarf explosions: type Ia supernovae and tidal disruption events"

Ataru Tanikawa (University of Tokyo, Graduate School of Arts and Sciences)

White dwarfs (WDs) are the final states of intermediate- and low-mass stars. They will just cool over a long time if alone. However, they can experience explosion due to nuclear burning triggered by mass accretion, tidal disruption, and so on. First, I will talk about three dimensional simulation of double-detonation explosion in double WD system, so-called the D6 model, for modeling type Ia supernovae (SNe Ia). The D6 model predicts that the heavier WD explodes, while the lighter WD does not. Our simulation has shown that the explosion of the heavier WD strips materials from the lighter WD. This stripped materials consist of carbon and oxygen, and may contribute to low-velocity ejecta components as observationally interred for several sub-luminous SNe Ia. Second, I will introduce our numerical simulations of tidal disruption events of WDs, and discuss about nuclear ignition mechanism of such WDs.


"Single-sided pulsators - a new type of oscillating stars in close binaries"

Gerald Handler (CAMK, Warsaw)

Oscillating stars in binary systems usually have their pulsation axis located perpendicular to the orbital plane. It has long been hypothesized, but never observationally confirmed, that the gravitational force of the companion can alter the orientation of the pulsation axis of an oscillating star. Here we report the discovery of "single-sided pulsators" (working title), stars that have pulsation amplitudes strongly enhanced in one hemisphere. We explain this by the presence of a close companion, that deforms the geometry of the pulsator and tilts its pulsation axis into the orbital plane. So far, we have discovered three such stars in data from NASA's TESS mission. Each one of them behaves differently. We present the (short) history of this new type of pulsating stars and summarize the knowledge gathered on them so far.