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)
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.
