Wednesday Colloquium


"New Scenario for IMBH Formation in Star Clusters - Tidal Disruption Events"

Mirosław Giersz (NCAC, Warsaw)

I will discuss recent developments in the new scenario for intermediate mass black hole (IMBH) formation in dense star clusters. In this scenario, an IMBH is formed as a result of a dynamical interactions of a hard binary, containing a stellar mass black hole (BH), with other stars and binaries. The mass of the BH constantly increases due to mergers with incoming stars and mass transfer from companions. The recent developments are connected with the relation of the new scenario to the well known runaway scenario and to the tidal capture scenario, just recently described in an excellent paper by Stone, Kupper & Ostrilker (2016). I will also discuss, with relation to the new scenario, observational imprints of the presence of IMBH in star clusters. These imprints are connected with particular features of electromagnetic and gravitational wave radiations and properties of binaries containing IMBH.


"The jet propagation in the external environment of the Long GRB Collapsar – The Rarefaction Process"

Konstantinos Sapountzis (Center for Theoretical Physics, Polish Academy of Sciences, Warsaw)

The widely accepted model for the Long Gamma Ray Burst progenitor is based on the core collapse of a Wolf-Rayet star (Collapsar) and the consequent launching of jets that penetrate the stellar envelope and continue their propagation in the interstellar medium. The transition from a high pressure environment that supports the jet (star interior) to an environment of reduced external pressure (interstellar medium) results in the formation of shock and weak discontinuities in the outflow body. These formations correspond to areas where significant energy conversions take place and they can give answers to the open questions of the prompt emission origin. The Rarefaction phenomenon is well studied for the newtonian and unmagnetized flows, but for the hyper-relativistic and magnetic dominated limit, as expected in the Collapsar model, the issue is open and only recently studies were performed mostly by means of numerical simulations. In order to clarify the physical interpretation of the magnetized phenomenon, we constructed a semi-analytical model consisting of the self similar solutions of the planar equations. This model provides valuable insight for the energetic conversions and the spatial scales they take place, while the scaling laws and estimations extend their validity also to the Collapsar outflow. At the second part, we present an algorithm we built based on the system characteristics that integrates the axisymmetric system and we study the implication that the axisymmetry induces. Finally, we conclude with the verification of our results by the use of other familiar numerical algorithms (PLUTO).


"Close binary central stars of planetary nebulae"

Brent Miszalski (SAAO, SA)

Planetary Nebulae (PNe) are a brief phase in the lives of low to intermediate mass stars where the atmosphere of an asymptotic giant branch star is ejected and ionised by the hot leftover core that eventually becomes a white dwarf. They appear in an extremely wide variety of shapes, but exactly how they are shaped into such a diverse range of morphologies is still highly uncertain despite over thirty years of vigorous debate. Binaries have long been thought to offer a solution to this vexing problem, but only now we are beginning to understand the important role that a binary channel, in particular common-envelope (CE) evolution, plays in the formation of a large fraction of PNe. I'll give an overview of results in this area and show how binary central stars are connected to many other aspects of stellar astrophysics. An important corollary of this work has been the much improved understanding of jets in PNe. I'll conclude the talk with highlights of results from observations with the Southern African Large Telescope (SALT) aimed at discovering more unusual binary central stars.


"Pulsar glitches, a tool to study the neutron star interior"

Danai Antonopoulou (NCAC, Warsaw)

Sudden spin-ups of a neutron star, called glitches, were first detected shortly after the discovery of the first pulsars. To date, about 500 glitches have been observed in over 100 sources. Glitches seem to be common across different types of neutron stars, like young, rotationally-powered radio and/or high-energy pulsars, magnetically-powered anomalous X-ray pulsars and RRATs - which are objects with intermittently pulsed radio emission. Because the spin-up and its subsequent relaxation are thought to be driven by an internal neutron superfluid component, their study provides one of the few direct probes into the dynamics and physics of the neutron star interior. I will present the process of detecting and measuring glitches, their characteristics, and some of the physical information we can extract from such observations.


"Cosmic butterflies: the product of tempestuous stellar marriages"

David Jones (Instituto de Astrofísica de Canarias)

Planetary nebulae are some of the most strikingly beautiful astrophysical phenomena known, gracing many a glossy-paged, coffee-table book and earning them the nickname "cosmic butterflies". It is now clear that a significant fraction of these objects originate from a binary evolutionary pathway, with some theories even going as far as to say that binarity may be a prerequisite for all but the most massive stars to form a planetary nebula. In this seminar, I will begin by outlining some fundamentals of close binary evolution and how they relate to the formation of planetary nebulae. I will then go on to discuss some of our most interesting results, what they can tell us about the common envelope phase of close binary evolution, and the growing connections between these systems and other binary phenomena including novae and supernovae type Ia.