arXiv subjects

Gaël Servignat

Publications and source records attributed to Gaël Servignat.

4 recordsLinked to original sources

arXiv2026

A study of the dimer-trimer crossover in a driven three-component Fermi gas

We develop an Effective Field Theory (EFT) for a system with three distinguishable atomic species and present a variational calculation of the two and three-body binding energies in vacuum and in the presence of a single Fermi sea. Specifically, we consider the case where the interaction between first two atomic species is externally driven so as to produce a (non-universal) closed-channel dimer whose coupling can be controlled independently of all other interactions. We then model the remaining interactions as a contact interaction between the dimer and a third atomic species which forms the medium. We derive analytical expressions for the dimer and trimer binding energies in vacuum and in medium, and in the latter case we predict a crossing between the dimer and trimer branches as a function of the atom-dimer scattering length, analogous to the usual polaron and molecule problem. Furthermore, we show that the position of this crossing can be controlled by varying the atom-atom coupling that results from the external drive and we discuss the implications of these findings.

cond-mat.quant-gas↗

arXiv2024

ROXAS: a new pseudospectral non-linear code for general relativistic oscillations of fast rotating isolated neutron stars

Next-generation gravitational wave detectors are expected to increase their sensitivity in the kHz band where binary neutron star remnants are expected to emit. In this context, robust predictions of oscillation modes of the post-merger object are desirable. To that end, we present here our code \texttt{ROXAS} that is aimed at simulating isolated rotating neutron stars. It is based on our previously published formalism relying on primitive variables, along with the extended conformal-flatness condition formulation of Einstein equations. The equations are written under a well-balanced formulation. The code uses pseudospectral methods for both the metric and the hydrodynamics. Since standard test beds were already presented in a previous paper, we here mainly focus on presenting the numerical ingredients of \texttt{ROXAS}, as well as on the frequency extraction for axisymmetric and non-axisymmetric modes. We compare our results with semi-analytic perturbative methods. Spherically symmetric modes are also recovered. The code currently supports fast, rigidly rotating isolated neutron stars, described with a cold equation of state. A notable feature of the code is its ability to follow the surface of the star at every step. It is very lightweight and can be run on office computers.

gr-qc↗

arXiv2024

One- and two-argument equation of state parametrizations with continuous sound speed for neutron star simulations

We describe two fitting schemes that aim to represent the high-density part of realistic equations of state for numerical simulations such as neutron star oscillations. The low-density part of the equation of state is represented by an arbitrary polytropic crust, and we propose a generic procedure to stitch any desired crust to the high-density fit, which is performed on the internal energy, pressure and sound speed of barotropic equations of state that describe cold neutron stars in $β$-equilibrium. An extension of the fitting schemes to equations of state with an additional compositional argument is proposed. In particular we develop a formalism that ensures the existence of a $β$-equilibrium at low densities. An additional feature of this low-density model is that it can be, in principle, applied to any parametrization. The performance of the fits is checked on mass, radius and tidal deformability as well as on the dynamical radial oscillation frequencies. To that end, we use a pseudospectral isolated neutron star evolution code based on a non-conservative form of the hydrodynamical equations. A comparison to existing parametrizations is proposed, as far as possible, and to published radial frequency values in the literature. The static and dynamic quantities are well reproduced by the fitting schemes. Our results suggest that, even though the radius is very sensitive to the choice of the crust, this choice has little influence on the oscillation frequencies of a neutron star.

astro-ph.HE↗

arXiv2022

A new formulation of general-relativistic hydrodynamic equations using primitive variables

We present the derivation of hydrodynamical equations for a perfect fluid in General Relativity, within the 3+1 decomposition of spacetime framework, using only primitive variables. Primitive variables are opposed to conserved variables, as defined in the widely used Valencia formulation of the same hydrodynamical equations. The equations are derived in a covariant way, so that they can be used to describe any configuration of the perfect fluid. Once derived, the equations are tested numerically. We implement them in an evolution code for spherically symmetric self-gravitating compact objects. The code uses pseudospectral methods for both the metric and the hydrodynamics. First, convergence tests are performed, then the frequencies of radial modes of polytropes are recovered with and without the Cowling approximation, and finally the performance of our code in the black hole collapse and migration tests are described. The results of the tests and the comparison with a reference corecollapse and neutron star oscillations code suggests that not only our code can handle very strong gravitational fields, but also that this new formulation helps gaining a significant amount of computational time in hydrodynamical simulations of smooth flows in General Relativity.

gr-qc↗