Laboratoire Univers et Théories

LUTH

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Observatoire de Paris

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UFR Physique

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Université Paris Diderot, CNRS

Présentation

Le Laboratoire Univers et Théories (LUTH) est une unité mixte de recherche du CNRS, de l’Observatoire de Paris et de l’Université Paris Diderot. Le laboratoire regroupe plus de 80 personnes. Son activité de recherche concerne la théorie et la modélisation avancées des systèmes astrophysiques. Notre laboratoire met l’accent sur deux aspects de l’activité de recherche astrophysique : le calcul numérique intensif et la pluridisciplinarité.

La simulation numérique joue un rôle fondamental en astrophysique et a été à l’origine de progrès spectaculaires. Complément indispensable des approches purement observationnelles et théoriques, elle constitue une troisième voie de recherche dont la spécificité est l’utilisation heuristique de l’ordinateur.

La complexité des systèmes astrophysiques étudiés nécessite, une expertise sur des processus physiques multiples et sur de nombreuses méthodes en mathématiques, statistiques et numérique. Le LUTH est un lieu d’échange afin de permettre l’élaboration et le suivi de projets de modélisation ambitieux.

Les thématiques de recherches vont de la cosmologie, aux planètes extra solaires, en passant par l’étude des trous noirs et objets compacts, des phénomènes de hautes énergies, des phénomènes hydrodynamiques et de plasmas présents au sein des objets astrophysiques.

Le LUTH participe aux grands projets instrumentaux et observationnels de notre discipline tels que : ALMA, COROT, CTA, Euclid, Fermi, DEUS, GAIA, Herschel, Hess, Horizon, LMJ, Planck, TPF, XMIVI, etc.

Le LUTH participe à plusieurs expériences d’intérêt astrophysique comme l’étude des chocs radiatifs en utilisant les grands lasers comme le LULI.

L’enseignement et la formation par la recherche sont parmi les principaux objectifs du LUTH en participant à la formation des étudiants en astrophysique, en physique et en mathématiques tant au niveau du master d’astrophysique d’lle de France, que dans des cursus universitaires ou de grandes écoles.

Thèmes de recherche

Astrophysique Multi-Echelle (AME)

L’équipe AME a pour objectif principal la compréhension des échanges et des mélanges de matière dans l’Univers. Les recherches qui y sont menées vont de la physique fondamentale à l’expérimentation de laboratoire.

Phénomènes aux Hautes Energies (Equipe PHE)

L’équipe PHE se consacre à l’étude des sources astrophysiques aux hautes énergies et de la physique des milieux moléculaires hors équilibre thermodynamique.

Relativité et Objets Compacts (Equipe ROC)

Les thèmes de recherche de l'équipe ROC concernent la théorie et les tests de la gravitation, les ondes gravitationnelles, la formation et les propriétés des astres compacts (étoiles à neutrons, trous noirs). Le développement d'outils numériques ouverts et originaux y tient une place importante.

Cosmologie : structures et origines (Equipe COS)

L'activité de l'équipe COS couvre plusieurs sujets de recherche en cosmologie parmi lesquels l'étude de l'Energie Noire et ses empreintes sur la formation et évolution des grandes structures cosmiques, travaux qui sont réalisé à l'aide de simulations numériques a haute-performance.

[hal-03017404] Quantitative inference of the H2 column densities from 3 mm molecular emission: case study towards Orion B

Date: 20 jan 2021 - 14:13

Desc: Context. Based on the finding that molecular hydrogen is unobservable in cold molecular clouds, the column density measurements of molecular gas currently rely either on dust emission observation in the far-infrared, which requires space telescopes, or on star counting, which is limited in angular resolution by the stellar density. The (sub)millimeter observations of numerous trace molecules can be effective using ground-based telescopes, but the relationship between the emission of one molecular line and the H2 column density is non-linear and sensitive to excitation conditions, optical depths, and abundance variations due to the underlying physico- chemistry. Aims. We aim to use multi-molecule line emission to infer the H2 molecular column density from radio observations. Methods. We propose a data-driven approach to determine the H2 gas column densities from radio molecular line observations. We use supervised machine-learning methods (random forest) on wide-field hyperspectral IRAM-30m observations of the Orion B molecular cloud to train a predictor of the H2 column density, using a limited set of molecular lines between 72 and 116 GHz as input, and the Herschel-based dust-derived column densities as “ground truth” output.Results. For conditions similar to those of the Orion B molecular cloud, we obtained predictions of the H2 column density within a typical factor of 1.2 from the Herschel-based column density estimates. A global analysis of the contributions of the different lines to the predictions show that the most important lines are 13CO(1–0), 12CO(1–0), C18O(1–0), and HCO+(1–0). A detailed analysis distinguishing between diffuse, translucent, filamentary, and dense core conditions show that the importance of these four lines depends on the regime, and that it is recommended that the N2H+(1–0) and CH3OH(20–10) lines be added for the prediction of the H2 column density in dense core conditions. Conclusions. This article opens a promising avenue for advancing direct inferencing of important physical parameters from the molecular line emission in the millimeter domain. The next step will be to attempt to infer several parameters simultaneously (e.g., the column density and far-UV illumination field) to further test the method.

[in2p3-01447669] First limits on the very-high energy gamma-ray afterglow emission of a fast radio burst

Date: 20 jan 2021 - 11:16

Desc: Aims. Following the detection of the fast radio burst FRB150418 by the SUPERB project at the Parkes radio telescope, we aim to search for very-high energy gamma-ray afterglow emission.Methods. Follow-up observations in the very-high energy gamma-ray domain were obtained with the H.E.S.S. imaging atmospheric Cherenkov telescope system within 14.5 h of the radio burst.Results. The obtained 1.4 h of gamma-ray observations are presented and discussed. At the 99% C.L. we obtained an integral upper limit on the gamma-ray flux of Φγ(E > 350 GeV) < 1.33 × 10-8 m-2 s-1. Differential flux upper limits as function of the photon energy were derived and used to constrain the intrinsic high-energy afterglow emission of FRB 150418.Conclusions. No hints for high-energy afterglow emission of FRB 150418 were found. Taking absorption on the extragalactic background light into account and assuming a distance of z = 0.492 based on radio and optical counterpart studies and consistent with the FRB dispersion, we constrain the gamma-ray luminosity at 1 TeV to L < 5.1 × 1047 erg/s at 99% C.L.

[hal-02188268] Dipolar tidal effects in scalar-tensor theories

Date: 20 jan 2021 - 07:27

Desc: The inclusion of finite-size effects in the gravitational waveform templates allows one not only to constrain the internal structure of compact objects, but to test deviations from general relativity. Here, we address the problem of tidal effects in massless scalar-tensor theories. We introduce the scalar-type tidal Love numbers that relate the time-varying scalar dipole moment to the induced scalar tidal field. We compute the leading-order scalar tidal contribution in the conservative dynamics and for the first time in the wave generation for quasicircular orbits. Importantly, we show that, in a system dominated by dipolar emission, such tidal effects may be detectable by third generation detectors such as the Einstein Telescope.

[hal-01827985] Horizon Surface Gravity in Corotating Black Hole Binaries

Date: 20 jan 2021 - 07:26

Desc: For binary systems of corotating black holes, the zeroth law of black hole mechanics states that the surface gravity is constant over each component of the horizon. Using the approximation of a conformally flat spatial metric, we compute sequences of quasi-equilibrium initial data for corotating black hole binaries with irreducible mass ratios in the range . For orbits outside the innermost stable one, the surface gravity is found to be constant on each component of the apparent horizon at the sub-percent level. We compare those numerical results to the analytical predictions from post-Newtonian theory at the fourth (4PN) order and from black hole perturbation theory to linear order in the mass ratio. We find a remarkably good agreement for all mass ratios considered, even in the strong-field regime. In particular, our findings confirm that the domain of validity of black hole perturbative calculations appears to extend well beyond the extreme mass-ratio limit.

[hal-01554830] Neutral gas heating by X-rays in primitive galaxies: Infrared observations of the blue compact dwarf I Zw 18 with Herschel

Date: 20 jan 2021 - 07:24

Desc: Context. The neutral interstellar medium of galaxies acts as a reservoir to fuel star formation. The dominant heating and cooling mechanisms in this phase are uncertain in extremely metal-poor star-forming galaxies. The low dust-to-gas mass ratio and low polycyclic aromatic hydrocarbon abundance in such objects suggest that the traditional photoelectric effect heating may not be effective. Aims. Our objective is to identify the dominant thermal mechanisms in one such galaxy, I Zw 18 (1/30Z⊙), assess the diagnostic value of fine-structure cooling lines, and estimate the molecular gas content. Even though molecular gas is an important catalyst and tracer of star formation, constraints on the molecular gas mass remain elusive in the most metal-poor galaxies. Methods. Building on a previous photoionization model describing the giant H ii region of I Zw 18-NW within a multi-sector topology, we provide additional constraints using, in particular, the [C ii] 157 μm and [O i] 63 μm lines and the dust mass recently measured with the Herschel Space Telescope. Results. The heating of the H i region appears to be mainly due to photoionization by radiation from a bright X-ray binary source, while the photoelectric effect is negligible. Significant cosmic ray heating is not excluded. Inasmuch as X-ray heating dominates in the H i gas, the infrared fine-structure lines provide an average X-ray luminosity of order 4 × 1040 erg s-1 over the last few 104 yr in the galaxy. The upper limits to the [Ne v] lines provide strong constraints on the soft X-ray flux arising from the binary. A negligible mass of H2 is predicted. Nonetheless, up to ~107 M⊙ of H2 may be hidden in a few sufficiently dense clouds of order ≲5 pc (≲0.05′′) in size. Regardless of the presence of significant amounts of H2 gas, [C ii] and [O i] do not trace the so-called “CO-dark gas”, but they trace the almost purely atomic medium. Although the [C ii]+[O i] to total infrared ratio in I Zw 18 is similar to values in more metal-rich sources (~1%), it cannot be safely used as a photoelectric heating efficiency proxy. This ratio seems to be kept stable owing to a correlation between the X-ray luminosity and the star formation rate. Conclusions. X-ray heating could be an important process in extremely metal-poor sources. The lack of photoelectric heating due to the low dust-to-gas ratio tends to be compensated for by the larger occurrence and power of X-ray binaries in low-metallicity galaxies. We speculate that X-ray heating may quench star formation. Key words: X-rays: binaries / infrared: ISM / photon-dominated region (PDR) / galaxies: ISM / galaxies: individual: I Zw 18 / galaxies: star formation⋆ Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.