Wednesday Colloquium


"Star-disc magnetospheric interaction and launching of outflows and jets"

Miljenko Cemeljic (NCAC, Warsaw)

I present results in numerical simulations of star-disc magnetospheric interaction. I performed resistive and viscous MHD simulations, in which a quasi-stationary state is reached. Sweeping through the parameter space, I computed the torque exerted on the star, and the mass and angular momentum flux expelled from the system with the stellar wind, outflows and jets launched from the magnetosphere. I show the trends found in such solutions and derive some hints on the analytical solutions for the magnetic thin accretion discs.


"Gravitational lensing of gravitational waves and precision cosmology"

朱宗宏 Zong-Hong Zhu (Wuhan University)

The standard siren approach of gravitational wave cosmology appeals to the direct luminosity distance estimation through the waveform signals from inspiralling double compact binaries, especially those with electromagnetic counterparts providing redshifts. It is limited by the calibration uncertainties in strain amplitude and relies on the fine details of the waveform. We will show the next generation detector, e.g., the Einstein Telescope, is expected to produce 10^4 −10^5 gravitational wave detections per year, 50−100 of which will be lensed. Then we report a waveform-independent strategy to achieve precise cosmography by combining the accurately measured time delays from strongly lensed gravitational wave signals with the images and redshifts observed in the electromagnetic domain. We demonstrate that just 10 such lensing systems can provide a Hubble constant uncertainty of 0.68% for a flat Lambda Cold Dark Matter universe in the era of third generation ground-based detectors.


"Multi-messenger Astronomy"

Nayantara Gupta (Raman Research Institute, Bangalore, India)

Our group at Raman Research Institute, Bangalore works on cosmic rays (Galactic and extragalactic), multi-wavelength modelling of AGN and neutrino astronomy. In this talk I select two topics from our current research activities. In the first part of my talk I will discuss about physics of extended jets of quasars. The radio, optical and X-ray emission from extended jets is difficult to explain with the popular inverse Compton emission model. I will discuss about the other possible scenarios. In the second part I will discuss about injection, propagation and composition of ultrahigh energy cosmic rays with Pierre Auger data. We have used the publicly available code CRPropa3 to infer some favourable scenarios of source distribution, spectral index of injected cosmic rays and their composition for the EBL model by Dominguez et al.


"Unexpected presence of hot gamma Doradus and A-F type hybrid pulsators"

Filiz Kahraman (NCAC, Warsaw)

Gamma Doradus and A-F type hybrid pulsators are supposed to have an effective temperature range of about 6500 – 7500 K. However, the high-quality Kepler data has revealed some gamma Doradus and A-F hybrid pulsators hotter than 7500 K. To drive the pulsation mechanism in these oscillating stars, a sufficient convective envelope is necessary. However, according to theory in these hot objects, the convective envelope should not be sufficient enough to excite the gamma Doradus type pulsations. To explain the unexpected situation of these objects, some opinions were proposed such as possible incorrect atmospheric parameters, rapid rotation, chemical peculiarity, and binarity. A detailed spectroscopic and photometric investigation is needed to clarify these opinions. Therefore, we present the conclusions of our comprehensive spectroscopic and photometric study of twenty-four hot gamma Doradus and A-F hybrid candidate stars. Consequently, we confirm that the hot gamma Doradus and A-F hybrid stars exist. The surface chemical peculiarity and the binarity suggestions don’t appear to be a possible answer for the existence of these objects. It has been suggested that hot gamma Doradus and hybrid stars might be fast rotating slowly pulsating B stars (Balona et al. 2016). The stars in our sample have high projected rotational velocity (on average 130 and 190 km/s for gamma Doradus and hybrid stars, respectively) but they don’t show B type spectral lines. The result of this study provides new information to improve the understanding of the pulsation mechanism that occurs in the gamma Doradus stars.