arXiv subjects

Saoud Baouche

Publications and source records attributed to Saoud Baouche.

3 recordsLinked to original sources

arXiv2026

Efficient Interstellar Grain Growth from High Sticking Coefficients on Amorphous Carbon Dust

Cosmic dust is the solid phase of the interstellar medium (ISM), classically assumed to be composed of carbonaceous and silicate grains with size distributions spanning $\sim 5~\AA$ to $\sim 1~\mu$m (Weingartner & Draine 2001, Draine & Li 2007, Hensley & Draine 2023). While it constitutes at most order-of-magnitude $\mathbf{1\%}$ of the ISM mass, dust is second only to stars in importance for the observable properties of galaxies (Zavala et al. 2021). Large uncertainties in the efficiency of grain growth obfuscate the relative contribution of the two dominant sources of dust in the Universe: direct production from evolved stars versus gas-phase accretion in the ambient ISM (Feldmann 2015, Esmerian & Gnedin 2022, Esmerian & Gnedin 2024). Advances in supercomputers have only recently allowed us to move beyond simple, idealized predictions of dust grain growth efficiencies (Leitch-Devlin & Williams 1985) with atomistic dynamical calculations (Bossion et al. 2024). We show that small carbon dust grains can grow significantly on timescales much shorter than the age of the universe and, in some ISM phases, comparable to the lifetimes of giant molecular clouds. Specifically, we perform molecular dynamics simulations of an amorphous carbon (a-C) grain surface impacted by gas-phase atoms of cosmologically abundant elements with realistic interstellar conditions, finding high ($\gtrsim 0.2$) sticking coefficients for all non-inert elements at all relevant gas and grain temperatures. We present the results of experiments conducted on similar dust candidate materials that support our theoretical calculations. Our results therefore confirm that the process of gas-phase accretion onto grains is likely an efficient mechanism for the growth of interstellar dust mass on astrophysical timescales, and plausibly central to the evolutionary life-cycle of interstellar grains at all cosmic epochs. (abridged)

astro-ph.GA↗

arXiv2020

A new multi-beam apparatus for the study of surface chemistry routes to formation of complex organic molecules in space

A multi-beam ultra-high vacuum apparatus is presented. In this article we describe the design and construction of a new laboratory astrophysics experiment -- VErs de NoUvelles Synthèses (VENUS) -- that recreates the solid-state non-energetic formation conditions of complex organic molecules in dark clouds and circumstellar environments. The novel implementation of four operational differentially-pumped beam lines will be used to determine the feasibility and the rates for the various reactions that contribute to formation of molecules containing more than six atoms. Data are collected by means of Fourier transform infrared spectroscopy and quadrupole mass spectrometry. The gold-coated sample holder reaches temperatures between 7 and 400 K. The apparatus was carefully calibrated and the acquisition system was developed to ensure that experimental parameters are recorded as accurately as possible. A great effort has been made to have the beam lines converge towards the sample. Experiments have been developed to check the beam alignment using reacting systems of neutral species (NH$_3$, H$_2$CO). Preliminary original results were obtained for the NO+H system, which shows that chemistry occurs only in the very first outer layer of the deposited species, that is the chemical layer and the physical layer coincide. This article illustrates the characteristics, performance, and future potential of the new apparatus in view of the forthcoming launch of the James Webb Space Telescope. We show that VENUS will have a major impact through its contributions to surface science and astrochemistry.

astro-ph.IM↗

arXiv2017

Deuteration of ammonia with D atoms on oxidized partly ASW covered graphite surface

The deuteration of ammonia by D atoms has been investigated experimentally in the sub-monolayer regime on realistic analogues of interstellar dust grain surfaces. About 0.8 monolayer of solid NH3 was deposited on top of an oxidized graphite surface held at 10 K, partly covered with ASW ice. Ammonia ice is subsequently exposed to D atoms for different exposure times using a differentially pumped beam-line. The deuteration experiments of ammonia were carried out by mass spectroscopy and temperature programmed desorption (TPD) technique. The experimental results showed the formation of three isotopologue ammonia species by direct exothermic H-D substitution surface reactions: NH3+D->NH2D+H, NH2D+D->NHD2+H, and NHD2+D->ND3+H. The formation of the deuterated isotopologues NH2D, NHD2, and ND3 at low surface temperature (10 K) is likely to occur through quantum tunneling process on the oxidized graphite surface. A kinetic model taking into account the diffusion of D atoms on the surface is developed to estimate the width and the hight of the activation energy barriers for the successive deuteration reactions of ammonia species by D atoms. Identical control experiments were performed using CH3OH and D atoms. The deuteration process of solid methanol is ruled by H abstraction and D addition mechanism, and is almost five orders of magnitude faster than ammonia deuteration process.

astro-ph.IM↗