arXiv subjects

Laurent Philippe

Publications and source records attributed to Laurent Philippe.

At least 19 recordsLinked to original sources

arXiv2024

Flux and fluence effects on the Vacuum-UV photodesorption and photoprocessing of CO$_2$ ices

CO$_2$ is a major component of the icy mantles surrounding dust grains in planet and star formation regions. Understanding its photodesorption is crucial for explaining gas phase abundances in the coldest environments of the interstellar medium irradiated by vacuum-UV (VUV) photons. Photodesorption yields determined experimentally from CO$_2$ samples grown at low temperatures (T=15~K) have been found to be very sensitive to experimental methods and conditions. Several mechanisms have been suggested for explaining the desorption of CO$_2$, O$_2$ and CO from CO$_2$ ices. In the present study, the cross sections characterizing the dynamics of photodesorption as a function of photon fluence (determined from released molecules in the gas phase) and of ice composition modification (determined in situ in the solid phase) are compared for the first time for different photon flux conditions (from 7.3$\times 10^{12}$~photon/s/cm$^2$ to 2.2$\times 10^{14}$~photon/s/cm$^2$) using monochromatic synchrotron radiation in the VUV range (on the DESIRS beamline at SOLEIL). This approach reveals that CO and O$_2$ desorption are decorrelated from that of CO$_2$. CO and O$_2$ photodesorption yields depend on photon flux conditions and can be linked to surface chemistry. By contrast, the phodesorption yield of CO$_2$ is independent of the photon flux conditions and can be linked to bulk ice chemical modification, consistently with an indirect desorption induced by electronic transition (DIET) process.

astro-ph.GA↗

arXiv2021

Photodesorption of acetonitrile CH3CN in UV-irradiated regions of the Interstellar Medium: an experimental evidence

Pure acetonitrile (CH3CN) and mixed CO:CH3CN and H2O:CH3CN ices have been irradiated at 15K with Vacuum UltraViolet (VUV) photons in the 7-13.6 eV range using synchrotron radiation. VUV photodesorption yields of CH3CN and of photo-products have been derived as a function of the incident photon energy. The coadsorption of CH3CN with CO and H2O molecules, which are expected to be among the main constituents of interstellar ices, is found to have no significant influence on the VUV photodesorption spectra of CH3CN, CHCN, HCN, CN and CH3. Contrary to what has generally been evidenced for most of the condensed molecules, these findings point toward a desorption process for which the CH3CN molecule that absorbs the VUV photon is the one desorbing. It can be ejected in the gas phase as intact CH3CN or in the form of its photo-dissociation fragments. Astrophysical VUV photodesorption yields, applicable to different locations, are derived and can be incorporated into astrochemical modeling. They vary from 0.67(+/-0.33).10^{-5} to 2.0(+/-1.0).10^{-5} molecule/photon for CH3CN depending on the region considered, which is high compared to other organic molecules such as methanol. These results could explain the multiple detections of gas phase CH3CN in different regions of the interstellar medium and are well-correlated to astrophysical observations of the Horsehead nebula and of protoplanetary disks (such as TW Hya and HD 163296).

astro-ph.GA↗

arXiv2021

Vacuum-UV photodesorption from compact Amorphous Solid Water : photon energy, isotopic and temperature effects

Vacuum-UV (VUV) photodesorption from water-rich ice mantles coating interstellar grains is known to play an important role in the gas-to-ice ratio in star- and planet-forming regions. Quantitative photodesorption yields from water ice are crucial for astrochemical models. We aim to provide the first quantitative photon-energy dependent photodesorption yields from water ice in the VUV. This information is important to understand the photodesorption mechanisms and to account for the variation of the yields under interstellar irradiation conditions. Experiments have been performed on the DESIRS beamline at the SOLEIL synchrotron, delivering tunable VUV light, using the SPICES (Surface Processes and ICES) set-up. Compact amorphous solid water ice (H$_2$O and D$_2$O) has been irradiated from 7 to 13.5 eV. Quantitative yields have been obtained by detection in the gas phase with mass-spectrometry for sample temperatures ranging from 15 K to 100 K. Photodesorption spectra of H$_2$O (D$_2$O), OH (OD), H$_2$ (D$_2$) and O$_2$ peak around 9-10 eV and decrease at higher energies. Average photodesorption yields of intact water at 15 K are 5 $\times$ 10$^{-4}$ molecule/photon for H$_2$O and 5 $\times$ 10$^{-5}$ molecule/photon for D$_2$O over the 7-13.5 eV range. The strong isotopic effect can be explained by a differential chemical recombination between OH (OD) and H (D) photofragments originating from lower kinetic energy available for the OH photofragments upon direct water photodissociation and/or possibly by an electronic relaxation process. It is expected to contribute to water fractionation during the building-up of the ice grain mantles in molecular clouds and to favor OH-poor chemical environment in comet-formation regions of protoplanetary disks. The yields of all the detected species except OH (OD) are enhanced above (70 $\pm$10) K, suggesting an ice restructuration at this temperature.

physics.chem-ph↗

arXiv2020

Multi-agent model for risk prediction in surgery

Risk management resulting from the actions and states of the different elements making up a operating room is a major concern during a surgical procedure. Agent-based simulation shows an interest through its interaction concepts, interactivity and autonomy of different simulator entities. We want in our study to implement a generator of alerts to listen the evolution of different settings applied to the simulator of agents (human fatigue, material efficiency, infection rate ...). This article presents our model, its implementation and the first results obtained. It should be noted that this study also made it possible to identify several scientific obstacles, such as the integration of different levels of abstraction, the coupling of species, the coexistence of several scales in the same environment and the deduction of unpredictable alerts. Case-based reasoning (CBR) is a beginning of response relative to the last lock mentioned and will be discussed in this paper.

cs.AI↗

arXiv2019

Vacuum ultraviolet photodesorption and photofragmentation of formaldehyde-containing ices

Non-thermal desorption from icy grains containing H$_2$CO has been invoked to explain the observed H$_2$CO gas phase abundances in ProtoPlanetary Disks (PPDs) and Photon Dominated Regions (PDRs). Photodesorption is thought to play a key role, however no absolute measurement of the photodesorption from H$_2$CO ices were performed up to now, so that a default value is used in the current astrophysical models. As photodesorption yields differ from one molecule to the other, it is crucial to experimentally investigate photodesorption from H$_2$CO ices. We measured absolute wavelength-resolved photodesorption yields from pure H$_2$CO ices, H$_2$CO on top of a CO ice (H$_2$CO/CO), and H$_2$CO mixed with CO ice (H$_2$CO:CO) irradiated in the Vacuum UltraViolet (VUV) range (7-13.6~eV). Photodesorption from a pure H$_2$CO ice releases H$_2$CO in the gas phase, but also fragments, such as CO and H$_2$. Energy-resolved photodesorption spectra, coupled with InfraRed (IR) and Temperature Programmed Desorption (TPD) diagnostics, showed the important role played by photodissociation and allowed to discuss photodesorption mechanisms. For the release of H$_2$CO in the gas phase, they include Desorption Induced by Electronic Transitions (DIET), indirect DIET through CO-induced desorption of H$_2$CO and photochemical desorption. We found that H$_2$CO photodesorbs with an average efficiency of $\sim 4-10 \times 10^{-4}$ molecule/photon, in various astrophysical environments. H$_2$CO and CO photodesorption yields and photodesorption mechanisms, involving photofragmentation of H$_2$CO, can be implemented in astrochemical codes. The effects of photodesorption on gas/solid abundances of H$_2$CO and all linked species from CO to Complex Organic Molecules (COMs), and on the H$_2$CO snowline location, are now on the verge of being unravelled.

astro-ph.GA↗

arXiv2019

Safe Disassociation of Set-Valued Datasets

Disassociation introduced by Terrovitis et al. is a bucketization based anonimyzation technique that divides a set-valued dataset into several clusters to hide the link between individuals and their complete set of items. It increases the utility of the anonymized dataset, but on the other side, it raises many privacy concerns, one in particular, is when the items are tightly coupled to form what is called, a cover problem. In this paper, we present safe disassociation, a technique that relies on partial-suppression, to overcome the aforementioned privacy breach encountered when disassociating set-valued datasets. Safe disassociation allows the $k^m$-anonymity privacy constraint to be extended to a bucketized dataset and copes with the cover problem. We describe our algorithm that achieves the safe disassociation and we provide a set of experiments to demonstrate its efficiency.

cs.DB↗

arXiv2018

Synchrotron radiation interaction with cryosorbed layers for astrochemical investigations

Photon-stimulated desorption (PSD) is a process of interest for the two seemingly unrelated topics of accelerator vacuum dynamics and astrochemistry. Here we present an approach to studying PSD of interstellar ice analogs, i.e. condensed films of molecules of astrophysical interest at cryogenic temperatures, using synchrotron radiation. We present results obtained in the VUV range on various pure and layered ices, focusing on elucidating the desorption mechanisms, and results in the X-ray range for H$_2$O.

astro-ph.GA↗

arXiv2018

X-ray photodesorption from water ice in protoplanetary disks and X-ray dominated regions

Water is the main constituent of interstellar ices, and it plays a key role in the evolution of many regions of the interstellar medium, from molecular clouds to planet-forming disks. In cold regions of the ISM, water is expected to be completely frozen out onto the dust grains. Nonetheless, observations indicate the presence of cold water vapor, implying that non-thermal desorption mechanisms are at play. Photodesorption by UV photons has been proposed to explain these observations, with the support of extensive experimental and theoretical work on ice analogues. In contrast, photodesorption by X-rays, another viable mechanism, has been little studied. The potential of this process to desorb key molecules, such as water, intact rather than fragmented or ionised, remains unexplored. We experimentally investigated X-ray photodesorption from water ice, monitoring all desorbing species. We find that desorption of neutral water is efficient, while ion desorption is minor. We derive for the first time yields that can be implemented in astrochemical models. These results open up the possibility of taking into account the X-ray photodesorption process in the modelling of protoplanetary disks or X-ray dominated regions.

astro-ph.GA↗

arXiv2017

Nitrile versus isonitrile adsorption at interstellar grain surfaces II. Carbonaceous aromatic surfaces

Almost 20 % of the ~ 200 different species detected in the interstellar and circumstellar media present a carbon atom linked to nitrogen by a triple bond. Of these 37 molecules, 30 are nitrile R-CN compounds, the remaining 7 belonging to the isonitrile R-NC family. How these species behave in their interactions with the grain surfaces is still an open question. In a previous work, we have investigated whether the difference between nitrile and isonitrile functional groups may induce differences in the adsorption energies of the related isomers at the surfaces of interstellar grains of various nature and morphologies. This study is a follow up of this work, where we focus on the adsorption on carbonaceous aromatic surfaces. The question is addressed by means of a concerted experimental and theoretical approach of the adsorption energies of CH$_3$CN and CH$_3$NC on the surface of graphite (with and without surface defects). The experimental determination of the molecule and surface interaction energies is carried out using temperature-programmed desorption (TPD) in an ultra-high vacuum (UHV) between 70 and 160 K. Theoretically, the question is addressed using first-principle periodic density functional theory (DFT) to represent the organised solid support. The adsorption energy of each compound is found to be very sensitive to the structural defects of the aromatic carbonaceous surface: these defects, expected to be present in a large numbers and great diversity on a realistic surface, significantly increase the average adsorption energies to more than 50\% as compared to adsorption on perfect graphene planes. The most stable isomer (CH$_3$CN) interacts more efficiently with the carbonaceous solid support than the higher energy isomer (CH$_3$NC), however.

astro-ph.GA↗

arXiv2017

Well-supported phylogenies using largest subsets of core-genes by discrete particle swarm optimization

The number of complete chloroplastic genomes increases day after day, making it possible to rethink plants phylogeny at the biomolecular era. Given a set of close plants sharing in the order of one hundred of core chloroplastic genes, this article focuses on how to extract the largest subset of sequences in order to obtain the most supported species tree. Due to computational complexity, a discrete and distributed Particle Swarm Optimization (DPSO) is proposed. It is finally applied to the core genes of Rosales order.

q-bio.PE↗

arXiv2017

Spectrally-resolved UV photodesorption of CH4 in pure and layered ices

Context. Methane is among the main components of the ice mantles of insterstellar dust grains, where it is at the start of a rich solid-phase chemical network. Quantification of the photon-induced desorption yield of these frozen molecules and understanding of the underlying processes is necessary to accurately model the observations and the chemical evolution of various regions of the interstellar medium. Aims. This study aims at experimentally determining absolute photodesorption yields for the CH4 molecule as a function of photon energy. The influence of the ice composition is also investigated. By studying the methane desorption from layered CH4:CO ice, indirect desorption processes triggered by the excitation of the CO molecules is monitored and quantified. Methods. Tunable monochromatic VUV light from the DESIRS beamline of the SOLEIL synchrotron is used in the 7 - 13.6 eV (177 - 91 nm) range to irradiate pure CH4 or layers of CH4 deposited on top of CO ice samples. The release of species in the gas phase is monitored by quadrupole mass spectrometry and absolute photodesorption yields of intact CH4 are deduced. Results. CH4 photodesorbs for photon energies higher than ~9.1 eV (~136 nm). The photodesorption spectrum follows the absorption spectrum of CH4, which confirms a desorption mechanism mediated by electronic transitions in the ice. When it is deposited on top of CO, CH4 desorbs between 8 and 9 eV with a pattern characteristic of CO absorption, indicating desorption induced by energy transfer from CO molecules. Conclusions. The photodesorption of CH4 from the pure ice in various interstellar environments is around 2.0 x 10^-3 molecules per incident photon. Results on CO-induced indirect desorption of CH4 provide useful insights for the generalization of this process to other molecules co-existing with CO in ice mantles.

astro-ph.GA↗

arXiv2017

Confinement effects on the nuclear spin isomer conversion of H$_2$O

The mechanism for interconversion between the nuclear spin isomers (NSI) of H$_2$O remains shrouded in uncertainties. The temperature dependence displayed by NSI interconversion rates for H$_2$O isolated in an Argon matrix provides evidence that confinement effects are responsible for the dramatic increase in their kinetics with respect to the gas phase, providing new pathways for o-H$_2$O $\leftrightarrow$ p-H$_2$O conversion in endohedral compounds. This reveals intramolecular aspects of the interconversion mechanism which may improve methodologies for the separation and storage of NSI en route to applications in nuclear magnetic resonance spectroscopy and imaging. It may also improve astronomers' ability to use their relative abundance in the interstellar medium as proxies, thereby providing a valuable "astronomical clock".

physics.chem-ph↗

arXiv2016

Nitrile versus isonitrile adsorption at interstellar grains surfaces: I - Hydroxylated surfaces

Almost 20% of the ~ 200 different species detected in the interstellar and circumstellar media present a carbon atom linked to nitrogen by a triple bond. Among these 37 molecules, 30 are nitrile R-CN compounds, the remaining seven belonging to the isonitrile R-NC family. How these species behave in presence of the grain surfaces is still an open question. In this contribution we investigate whether the difference between nitrile and isonitrile functional groups may induce differences in the adsorption energies of the related isomers at the surfaces of interstellar grains of different nature and morphologies. The question was addressed by means of a concerted experimental and theoretical study of the adsorption energies of CH3CN and CH3NC on the surface water ice and silica. The experimental determination of the molecule - surface interaction energies was carried out using temperature programmed desorption (TPD) under an ultra-high vacuum (UHV) between 70 and 160 K. Theoretically, the question was addressed using first principle periodic density functional theory (DFT) to represent the organized solid support. The most stable isomer (CH3CN) interacts more efficiently with the solid support than the higher energy isomer (CH3NC) for water ice and silica. Comparing with the HCN and HNC pair of isomers, the simulations show an opposite behaviour, in which isonitrile HNC are more strongly adsorbed than nitrile HCN provided that hydrogen bonds are compatible with the nature of the model surface. The present study confirms that the strength of the molecule surface interaction between isomers is not related to their intrinsic stability but instead to their respective ability to generate different types of hydrogen bonds.

astro-ph.GA↗

arXiv2016

Binary Particle Swarm Optimization versus Hybrid Genetic Algorithm for Inferring Well Supported Phylogenetic Trees

The amount of completely sequenced chloroplast genomes increases rapidly every day, leading to the possibility to build large-scale phylogenetic trees of plant species. Considering a subset of close plant species defined according to their chloroplasts, the phylogenetic tree that can be inferred by their core genes is not necessarily well supported, due to the possible occurrence of problematic genes (i.e., homoplasy, incomplete lineage sorting, horizontal gene transfers, etc.) which may blur the phylogenetic signal. However, a trustworthy phylogenetic tree can still be obtained provided such a number of blurring genes is reduced. The problem is thus to determine the largest subset of core genes that produces the best-supported tree. To discard problematic genes and due to the overwhelming number of possible combinations, this article focuses on how to extract the largest subset of sequences in order to obtain the most supported species tree. Due to computational complexity, a distributed Binary Particle Swarm Optimization (BPSO) is proposed in sequential and distributed fashions. Obtained results from both versions of the BPSO are compared with those computed using an hybrid approach embedding both genetic algorithms and statistical tests. The proposal has been applied to different cases of plant families, leading to encouraging results for these families.

cs.AI↗

arXiv2016

UV photodesorption of methanol in pure and CO-rich ices: desorption rates of the intact molecule and of the photofragments

Wavelength dependent photodesorption rates have been determined using synchrotron radiation, for condensed pure and mixed methanol ice in the 7 -- 14 eV range. The VUV photodesorption of intact methanol molecules from pure methanol ices is found to be of the order of 10$^{-5}$ molecules/photon, that is two orders of magnitude below what is generally used in astrochemical models. This rate gets even lower ($<$ 10$^{-6}$ molecules/photon) when the methanol is mixed with CO molecules in the ices. This is consistent with a picture in which photodissociation and recombination processes are at the origin of intact methanol desorption from pure CH$_3$OH ices. Such low rates are explained by the fact that the overall photodesorption process is dominated by the desorption of the photofragments CO, CH$_3$, OH, H$_2$CO and CH$_3$O/CH$_2$OH, whose photodesorption rates are given in this study. Our results suggest that the role of the photodesorption as a mechanism to explain the observed gas phase abundances of methanol in cold media is probably overestimated. Nevertheless, the photodesorption of radicals from methanol-rich ices may stand at the origin of the gas phase presence of radicals such as CH$_3$O, therefore opening new gas phase chemical routes for the formation of complex molecules.

astro-ph.GA↗

arXiv2013

Indirect ultraviolet photodesorption from CO:N2 binary ices - an efficient grain-gas process

UV ice photodesorption is an important non-thermal desorption pathway in many interstellar environments that has been invoked to explain observations of cold molecules in disks, clouds and cloud cores. Systematic laboratory studies of the photodesorption rates, between 7 and 14 eV, from CO:N2 binary ices, have been performed at the DESIRS vacuum UV beamline of the synchrotron facility SOLEIL. The photodesorption spectral analysis demonstrates that the photodesorption process is indirect, i.e. the desorption is induced by a photon absorption in sub-surface molecular layers, while only surface molecules are actually desorbing. The photodesorption spectra of CO and N2 in binary ices therefore depend on the absorption spectra of the dominant species in the subsurface ice layer, which implies that the photodesorption efficiency and energy dependence are dramatically different for mixed and layered ices compared to pure ices. In particular, a thin (1-2 ML) N2 ice layer on top of CO will effectively quench CO photodesorption, while enhancing N2 photodesorption by a factors of a few (compared to the pure ices) when the ice is exposed to a typical dark cloud UV field, which may help to explain the different distributions of CO and N2H+ in molecular cloud cores. This indirect photodesorption mechanism may also explain observations of small amounts of complex organics in cold interstellar environments.

astro-ph.GA↗

arXiv2013

Wavelength-dependent UV photodesorption of pure N2 and O2 ices

Ultraviolet photodesorption of molecules from icy interstellar grains can explain observations of cold gas in regions where thermal desorption is negligible. This non-thermal desorption mechanism should be especially important where UV fluxes are high. N2 and O2 are expected to play key roles in astrochemical reaction networks, both in the solid state and in the gas phase. Measurements of the wavelength-dependent photodesorption rates of these two infrared-inactive molecules provide astronomical and physical-chemical insights into the conditions required for their photodesorption. Tunable radiation from the DESIRS beamline at the SOLEIL synchrotron in the astrophysically relevant 7 to 13.6 eV range is used to irradiate pure N2 and O2 thin ice films. Photodesorption of molecules is monitored through quadrupole mass spectrometry. Absolute rates are calculated by using the well-calibrated CO photodesorption rates. Strategic N2 and O2 isotopolog mixtures are used to investigate the importance of dissociation upon irradiation. N2 photodesorption mainly occurs through excitation of the b^1Pi_u state and subsequent desorption of surface molecules. The observed vibronic structure in the N2 photodesorption spectrum, together with the absence of N3 formation, supports that the photodesorption mechanism of N2 is similar to CO, i.e., an indirect DIET (Desorption Induced by Electronic Transition) process without dissociation of the desorbing molecule. In contrast, O2 photodesorption in the 7 - 13.6 eV range occurs through dissociation and presents no vibrational structure. Photodesorption rates of N2 and O2 integrated over the far-UV field from various star-forming environments are lower than for CO. Rates vary between 10E-3 and 10E-2 photodesorbed molecules per incoming photon.

astro-ph.GA↗

arXiv2013

Computational investigations of folded self-avoiding walks related to protein folding

Various subsets of self-avoiding walks naturally appear when investigating existing methods designed to predict the 3D conformation of a protein of interest. Two such subsets, namely the folded and the unfoldable self-avoiding walks, are studied computationally in this article. We show that these two sets are equal and correspond to the whole $n$-step self-avoiding walks for $n\leqslant 14$, but that they are different for numerous $n \geqslant 108$, which are common protein lengths. Concrete counterexamples are provided and the computational methods used to discover them are completely detailed. A tool for studying these subsets of walks related to both pivot moves and proteins conformations is finally presented.

q-bio.BM↗