I am a PhD student in the Time-domain astronomy group led by Ondřej Pejcha at Charles University in Prague. My current interests lie in the field of theoretical astrophysics, with a particular focus on (binary) stars and high-energy phenomena. Before entering the field of astronomy, I worked in the field of continuum mechanics under the supervision of Vít Průša.
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Dynamics of baryon ejection in magnetar giant flares
Giant flares (GFs) are extremely energetic and rare bursts of gamma-ray radiation associated with magnetars. In a recent paper (Cehula, Thompson & Metzger 2023), we explored the impact of a magnetar GF on the neutron star crust, and the associated potential baryon mass ejection. We considered that sudden magnetic energy dissipation creates a thin high-pressure shell above a portion of the neutron star surface, driving a relativistic shockwave into the crust and heating a fraction of these layers to sufficiently high energies to ultimately become unbound along directions unconfined by the magnetic field.
For an initial shell pressure corresponding to the dissipation of a magnetic field of strength 10^15.5--10^16 G, we found ejecta of mass 10^25--10^26 g with asymptotic velocities compatible with the ejecta properties inferred from the radio afterglow of the December 2004 GF from SGR 1806-20. The conditions are met in the outflow for heavy element r-process nucleosynthesis via the alpha-rich freeze-out mechanism. Given an energetic GF rate of roughly once per century in the Milky Way, we found that magnetar GFs could contribute an appreciable heavy r-process source that tracks star formation. We predicted that GFs are accompanied by short minutes long, luminous (10^39 ergs/s) optical transients powered by r-process decay (“nova brevis”), akin to scaled-down kilonovae.
Theory of mass transfer in binary stars
Calculation of the mass-transfer (MT) rate Mdot of a Roche-lobe overflowing star is a fundamental task in binary star evolution theory. Most of the existing MT prescriptions are based on a common set of assumptions that combine optically-thick and optically-thin regimes with different flow geometries.
I developed a new model of MT based on the assumption that the Roche potential sets up a nozzle converging on the inner Lagrangian point and that the gas flows mostly along the axis connecting both stars, see Cehula & Pejcha (2023, MNRAS, 524, 471–490). In this paper, we derived a set of 1D hydrodynamic equations governing the gas flow. We obtained algebraic solution for the polytropic equation of state (EOS), which gives Mdot within a factor of 0.9 to 1.0 of existing optically-thick prescriptions and which reduces to the existing optically-thin prescription for isothermal gas. For a realistic EOS, we found that Mdot differs by up to a factor of 4 from existing models. We illustrated the effects of our new MT model on 30 Msun low-metallicity star undergoing intensive thermal time-scale MT and found that it is more likely to become unstable to L2 overflow and common-envelope evolution than for existing MT prescriptions. Our model provides a framework for including additional physics such as radiation or magnetic fields.
Origami-like structures made of light activated shape memory polymers
Light activated shape memory polymers (LASMPs) are smart materials with the ability of remembering a deformed state (temporary shape) due to exposure to UV light and returning to the permanent (original) shape by the exposure to UV light with a different wavelength. Regarding the shape of smart materials frequent inspiration is taken from the art of origami, the Japanese art of folding paper into various shapes.
I developed a code for computer modelling of such origami-like structures made of LASMPs, see Cehula & Průša (2020, Int. J. Eng. Sci. 150, 103235). The code is a modification to the state-of-the-art code for simulations of elastic origami-like structures developed by Liu & Paulino (2018, Origami, 7, 1167-1182). The original code is called MERLIN2, my modification is called MERLIN2.LUX.