arXiv subjects

Ali Pourmand

Publications and source records attributed to Ali Pourmand.

5 recordsLinked to original sources

arXiv2025

Observational Constraints on Chaplygin Gas Models in Non-Minimally Coupled Power Law $f(Q)$ Gravity with Quasars

In the framework of $f(Q)$ gravity, where gravity emerges from non-metricity $Q$, we explore the cosmological implications of its non-minimal coupling to matter. Inspired by the recent success of Chaplygin gas models in explaining dark energy, we consider a background fluid composed of baryonic matter, radiation, and a family of Chaplygin gas variants namely Generalized Chaplygin Gas (GCG), Modified Chaplygin Gas (MCG), and Variable Chaplygin Gas (VCG). We constrain these models with three recent observational datasets: Observational Hubble Data (OHD), Baryonic Acoustic Oscillation (BAO) measurements, and Quasi-Stellar Objects (QSO) data. For the QSO dataset, we propose an analytical expression for errors in comoving distance to circumvent the reliance on Monte Carlo simulations. Using kinematic diagnostics such as the deceleration and jerk parameters and Om diagnostic, we assess deviations of the proposed models from $\Lambda$CDM. Our joint analysis of the three datasets reveals that the transition redshift from a decelerated to an accelerated expansion of the universe for the GCG, MCG and VCG models is $0.620^{+0.018}_{-0.017}$, $0.537^{+0.017}_{-0.017}$ and $0.470^{+0.012}_{-0.012}$ respectively, indicating a departure from $\Lambda$CDM.

gr-qc↗

arXiv2025

Second Generation Planet Formation in Post-AGB Discs: Testing the Role of Gravitational Instability

Post-asymptotic giant branch (post-AGB) binary stars are evolved systems that host circumbinary discs formed through mass loss during late stage binary interactions. Their structural, morphological, kinematic, and chemical similarities to planet-forming discs suggest that these systems may act as sites of "second generation" planet formation. In this study, we assess whether the disc instability mechanism -- a proposed pathway for rapid, giant planet formation in some protoplanetary discs - can operate in post-AGB discs; motivated by their short lifetimes. Using the Toomre criterion under well motivated assumptions for disc structure and size, mass, and thermal properties, we assess the conditions for gravitational instability. We first benchmark our analytical framework using well studied protoplanetary disc systems (including HL Tauri, Elias 2-27, GQ Lupi) before applying the same analysis to observed post-AGB discs. We find that post-AGB discs are generally gravitationally stable at present, due primarily to their low masses. Using viscous disk theory, we find that the discs were stable against collapse even in the past, when their masses were potentially higher. In contrast, several protoplanetary discs analysed in the same way show that they likely experienced gravitationally unstable phases early on. We also find that higher viscosity parameters are better aligned with expected post-AGB disc lifetimes. Finally, we revisit the planet formation scenario proposed for the post-common envelope system NN Ser, first carried out by Schleicher and Dreizler and we show that gravitational instability could be feasible under specific, high disc mass assumptions. Overall, our results provide the first systematic theoretical assessment of gravitational instability in post-AGB discs, demonstrating that this mechanism is unlikely to dominate second generation planet formation in these systems.

astro-ph.EP↗

arXiv2024

Unified Rapid Mass Transfer

We present a method to obtain rapid mass loss rates in binary systems, specifically at the onset of MT episodes. The method unifies atmospheric (underflow) and $L_1$ stream (overflow) mass rates in a single continuous procedure. The method uses averaged 3D properties of the binaries, such as effective binary potential and effective binary acceleration, to both evolve the donor and obtain properties of the matter at the $L_1$ plane. In the case of underflow, we obtain atmospheric stratification. Our method can be used for binaries with an extensive range of mass ratios, $0.01 \le q \le 100$, and can also be applied to hot donors. The considered examples show that the MT rates obtained with this revised formalism always differ from the optically thin and optically thick MT rates widely used during the computations of binary evolution.

astro-ph.SR↗

arXiv2023

Properties of binary systems in a one-dimensional approximation

Evolutionary calculations for stars in close binary systems are in high demand to obtain better constraints on gravitational wave source progenitors, understand transient events from stellar interactions, and more. Modern one-dimensional stellar codes make use of the Roche lobe radius $R_{\rm L}$ concept in order to treat stars in binary systems. If the stellar companion is approaching its $R_{\rm L}$, mass transfer treatment is initiated. However, the effective acceleration also affects the evolution of a star in a close binary system. This is different from the gravity inside a single star, whether that single star is rotating or not. Here, we present numerically obtained tables of properties of stars in a binary system as a function of the effective potential: volume-equivalent radii of the equipotential surfaces, effective accelerations and the inverse effective accelerations averaged over the same equipotential surfaces, and the properties of the L1 plane cross-sections. The tables are obtained for binaries where the ratios of the primary star mass to the companion star mass are from $10^{-6}$ to $10^5$ and include equipotential surfaces up to the star's outer Lagrangian point. We describe the numerical methods used to obtain these quantities and report how we verified the numerical results. We also describe and verify the method to obtain the effective acceleration for non-point mass distributions. We supply a sample code showing how to use our tables to get the average effective accelerations in one-dimensional stellar codes.

astro-ph.SR↗

arXiv2022

Constraining Generalized Chaplygin Gas in Non-Minimally Coupled $f(Q)$ Cosmology using Quasars and $H(z)$ Data

In the current framework of Einstein's equations in general relativity (GR), gravity is described by the spacetime curvature. However, there are other descriptions where the origin of gravity can be understood through torsion and non-metricity $Q$. In this work, we discuss a modified theory of gravity namely $f(Q)$ gravity, which considers a non-linear extension of $Q$. In particular, we study the case where it is non-minimally coupled to matter. Motivated by the recent success of Chaplygin gas models in the explanation of dark energy, we assume a pressureless baryonic matter and a generalized Chaplygin gas as the background fluid. We constrain the proposed model using two different datasets: one for Hubble measurements and the other for quasars (which we calibrated) with Markov-Chain Monte Carlo (MCMC) methods. We employ kinematic tools such as deceleration and jerk parameters to determine deviations of the proposed model from $\Lambda$CDM. We establish that the transition redshift $z_T$ in the deceleration parameter $q$ is $0.607$ and $0.204$ with the two datasets respectively, therefore describing the universe's acceleration.

gr-qc↗