Thibault Boulet (Institute of Astrophysics and Space Sciences, University of Porto, Portugal)
Understanding the Milky Way's formation and evolution requires precise stellar age determination across its components. Recent advances in asteroseismology, spectroscopy, stellar modeling, and machine learning, along with all-sky surveys, have provided reliable stellar age estimates. We aim to furnish accurate age assessments for the Main Red Star Sample within the APOGEE DR17 catalogue. Leveraging asteroseismic age constraints, we employ machine learning to achieve this goal. We explore optimal non-asteroseismic stellar parameters, including Teff, L, [C/N], [Mg/Ce], [α/Fe], U(LSR), and 'Z' vertical height from the Galactic plane, to predict ages via categorical gradient boost decision trees. Our model, trained on merged samples from TESS and APOKASC-2 catalogs, achieves a median fractional age error of 20.8%, with a relative difference between the learning curves of 4.77%. For stars older than 3 Gyr, the error ranges from 7% to 23%; for those between 1 and 3 Gyr, it is 26% to 28%; and for stars younger than 1 Gyr, it is 43%. Applied to 125,445 stars, our analysis confirms the flaring of the young Galactic disc and reveals an age gradient among the youngest Galactic plane stars. We also identify two groups of metal-poor ([Fe/H] < -1 dex) and young (Age < 2 Gyr) stars exhibiting peculiar chemical abundances and halo kinematics. One of these groups is likely a remnant of the third gas infall episode that started around 2.7 Gyr ago.
Henryka Netzel (CAMK, PAN, Warsaw)
Our perception of RR Lyrae stars and classical Cepheids have changed dramatically in recent years. Precise photometric observations from ground-based surveys and space missions have revealed a plethora of additional periodicities and modulation phenomena that occur alongside the dominant simple radial pulsations. Some of these phenomena have been explained with satisfactory hypotheses, while the nature of others remains a mystery. Although most discoveries come from photometric observations of RR Lyrae stars and classical Cepheids, the most recent findings originate from spectroscopic time-series observations of classical Cepheids. I will summarize the latest advances on the pulsation properties of classical pulsators and present the newest developments in the field, focusing on spectroscopic observations.
Tomasz Krajewski (CAMK, Warsaw)
Even though, naked singularities are perceived by many scientists only as strictly academic curiosities, solutions of the Einstein equation representing them are not hard to obtain. The well-known Reissner-Nordström (RN) metric is an example that can model black holes as well as naked singularities. It turns out that the RN metric is not only the solution of the Einstein equation coupled to the Maxwell electrodynamics, as it was obtained for the first time more than a century ago, but is also the solution in certain modified theories of gravity. In this talk we will go beyond well-studied accretion onto black holes described by the Kerr metric and investigate the analogous process in the background of the RN one. We will consider both regimes, i.e. when the charge parameter Q is smaller than the mass M of the black hole or when it is greater than M and the metric describes the naked singularity. We will discover new, exciting phenomena like repulsive gravity, levitating atmospheres and much more. Our findings may have applications to astrophysics of the active galactic nuclei (AGNs) with their powerful jets and outflows
Shilpa Sarkar (Harish-Chandra Research Institute (HRI), India)
Accretion is one of the most efficient processes by which the gravitational potential energy of matter can be converted into radiation. This phenomenon provides us with an explanation of the huge amount of energy liberated and high luminosities observed in Active Galactic Nuclei, X-ray binaries, etc. Therefore, modelling of these accretion flows is necessary to understand the underlying physical processes present in these systems. The soup of protons and electrons in these ionised flows are bound together by weak Coulomb force. Additionally, in most of the astrophysical cases, the infall timescales are much shorter. This makes the species settle down into two different temperature distributions, hence, the name two- temperature flows. However, this theory suffers from a serious problem of degeneracy. Compared to one- temperature flows, there is one more variable in the two-temperature system -- the extra temperature. However, there is no increase in the number of equations of motion. Thus, no unique solution exists, for a given set of constants of motion; or in other words, the system is degenerate! Different values of Tp/Te ratio supplied at any boundary, would generate different kinds of solutions with drastically different topologies as well as spectra. In addition, there is no known principle dictated by plasma physics which may constrain the relation between these two-temperatures. This degeneracy is irrespective of the type of the central object and is generic to two-temperature flows. We propose for the first time, an entropy maximisation formulation using the first principles. Using this methodology, we were able to constrain degeneracy and a unique solution with maximum entropy was selected following the second law of thermodynamics. Thereafter, we analysed the spectrum of these unique solutions for different accreting systems like black holes and neutron stars.
