Wednesday Colloquium


"Neutron stars with outbursts from superfluid crust"

Aleksander Kaminker ( ‎Ioffe Physical-Technical Institute, Saint Petersburg)

Thermal evolution of neutron stars (NSs) with variable internal heaters distributed over NS crust is discussed with special emphasis on the effects of crustal supefluidity. Superfluidity can significantly shorten and intensify the thermal surface emission from a star in response to an internal heater's outburst. This can be important for interpretation of observed outbursts of transiently accreting NSs in low-mass X-ray binaries (LMXBs) as well as of long outbursts of magnetars. Particular attention is paid to the so called quasi-persistent LMXBs where intense accretion overheats NS crusts. An analysis of the IGR J17480-2446 source shows a reasonable agreement between observations of long-term afterburst relaxation and theoretical models of relaxation in NSs with strong superfluidity and partly accreted (hybrid) crusts.


"From Particles to Fields, and Everything in Between: What is Chaos and What Does it Have to do with Black Hole Mergers?"

Nathan Leigh (Museum of Natural History, New York)

In this talk, we briefly review the concept of chaos in highly collisional gravitationally-bound systems of interacting particles. These concepts are then applied to the classic three- and four-body problems in Newtonian gravity. A statistical mechanics methodology is applied in order to derive analytic expressions for the properties of the outcomes of these interactions. This includes the orbital parameter distributions expected for populations of binary and triple star systems produced as a result of three- and four-body interactions. Such small-number collisional interactions occur regularly in star clusters throughout the Universe, across all mass scales. At higher cluster masses, such as globular and nuclear star clusters, stellar-mass black holes (BHs) are thought to be regular participants in these interactions. This is a key step that influences directly the interpretation of observations of BH-BH mergers using gravitational wave (GW) detectors, such as aLIGO. We discuss how the ever-growing number of observed BH-BH mergers can be used to constrain the astrophysical environments from which the detected GWs originate.


"Astrophysics of Cepheids -- results from the analysis of Cepheids in eclipsing binary systems"

Bogumił Pilecki (NCAC, Warsaw)

Cepheids are radially pulsating giants and supergiants, and form one of the most important classes of variable stars. They have proved to be very useful in many different areas of astrophysics. Cepheids are important distance indicators in the local Universe and key objects for testing the predictions of stellar evolution and stellar pulsation theories. A presence of Cepheids in eclipsing binary systems give us an opportunity to measure and study their physical parameters, including the mass, which we can hardly determine for single stars. The analysis of such systems is very challenging and new methods had to be developed to take full advantage of this configuration. These objects are also very rare and hard to find. In total, we have studied seven Cepheids in six eclipsing binary systems and found firm relations between such observed parameters as period mass and radius. The results based on evolution and pulsation models can be now compared with observations and challenged. The study resulted in finding evidence for binary interactions during the evolution and a non-pulsating object inside the instability strip. Our measurements provide strong constraints for solving the famous Cepheid mass discrepancy problem.