arXiv subjects

H. Meheut

Publications and source records attributed to H. Meheut.

At least 19 recordsLinked to original sources

arXiv2026

Unlocking the dynamics of Young Stellar Objects: Time-Domain Interferometry with six 4-m class telescopes

The dynamics of the inner regions of young stellar objects (YSOs) is driven by a variety of physical phenomena, from magnetospheres and accretion to the dust sublimation rim and inner disk flows. These inner environments evolve on timescales of hours to days, exactly when bursts, dips, and rapid structural changes carry the most valuable information about star and planet formations, but remain hardly reachable with current facilities. A better reactive infrastructure with six or more telescopes, combined with alerts from large time-domain surveys (e.g., at the era of LSST/Rubin type facilities), and equipped with instruments spanning from the V-band to the thermal infrared (N), would provide the instantaneous uv-coverage and spectral diagnostics needed to unambiguously interpret and image these events as they happen. Such a world's first time-domain interferometric observatory would enable qualitatively new science: directly linking optical and infrared variability to spatially resolved changes in magnetospheric accretion, inner-disk geometry, and dust and gas dynamics in the innermost astronomical unit. Crucially, connecting these processes to outer-scale unresolved information from JWST, ALMA, and the ELT would yield a complete tomography of the planet-forming region.

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arXiv2025

A unique solution to overcome the barriers to planetesimal formation at low dust-to-gas ratio

In the incremental growth model, planetesimal formation constitutes the least understood step in the process of planetary formation. The two main difficulties in this regard are the collision/fragmentation and the drift barriers. Numerous solutions have been proposed to overcome these barriers, but often need a conjunction of processes to reach the conditions for planetesimal formation. We present numerical simulations, in which the protoplanetary disk turbulence is fully captured rather than modeled with a turbulent diffusion or turbulent viscosity parameter. When the turbulent cascade is taken into account, and in the case of weakly turbulent disks, not only can solid grains be highly concentrated in clusters, but their radial drift can also be slowed or even halted. These results open a unique path to planetesimal formation starting at disk canonical dust-to-gas ratio, namely Keplerian turbulence.

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arXiv2024

What trade-off for astronomy between greenhouse gas emissions and the societal benefits? A sociological approach

The threat posed to humanity by global warming has led scientists to question the nature of their activities and the need to reduce the greenhouse gas emissions from research. Until now, most studies have aimed at quantifying the carbon footprints and relatively less works have addressed the ways GHG emissions can be significantly reduced. A factor two reduction by 2030 implies to think beyond increases in the efficacy of current processes, which will have a limited effect, and beyond wishful thinking about large new sources of energy. Hence, choices among research questions or allocated means within a given field will be needed. They can be made in light of the perceived societal utility of research activities. Here, we addressed the question of how scientists perceive the impact of GHG reduction on their discipline and a possible trade-off between the societal utility of their discipline and an acceptable level of GHG emissions. We conducted 28 semi-directive interviews of French astrophysicists from different laboratories. Our most important findings are that, for most researchers, astronomy is considered to have a positive societal impact mainly regarding education but also because of the fascination it exerts on at least a fraction of the general public. Technological applications are also mentioned but with relatively less emphasis. The reduction of GHG emissions is believed to be necessary and most often reductions within the private-sphere have been achieved. However, the question of community-wide reductions in astrophysics research, and in particular the possible reductions of large facilities reveals much more contrasted opinions.

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arXiv2023

Gender and Precarity in Astronomy

Following the survey Well-being in astrophysics that was sent out in March 2021, to establish how astrophysics researchers, primarily in France, experience their career, some of the results were published in Webb et al. (2021). Here we further analyse the data to determine if gender can cause different experiences in astrophysics. We also study the impact on the well-being of temporary staff (primarily PhD students and postdocs), compared to permanent staff. Whilst more temporary staff stated that they felt permanently overwhelmed than permanent staff, the experiences in astrophysics for the different genders were in general very similar, except in one area. More than three times more females than males experienced harassment or discrimination, rising sharply for gender discrimination and sexual harassment, where all of those having experienced sexual harassment and who had provided their gender in the survey, were female. Further, as previously reported (Webb et al. 2021), 20% of the respondents had suffered mental health issues before starting their career in astrophysics. We found that whilst this group was split approximately equally with regards to males and females, the number rose sharply to almost 45% of astronomers experiencing mental health issues since starting in astrophysics. Of this population, there were 50% more females than males. This excess of females was almost entirely made up of the population of women that had been harassed or discriminated against.

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arXiv2022

Well-being in French Astrophysics

It has become clear that early career astrophysics researchers (doctoral researchers, post-docs, etc) have a very diverse appreciation of their career, with some declaring it the best job that you can have and others suffering from overwork, harrassment and stress from the precarity of their job, and associated difficulties. In order to establish how astrophysics researchers, primarily in France, experience their career, we sent out a survey to understand the impact that their job has on their well-being. 276 people responded to the survey. Whilst around half of the respondents expressed pleasure derived from their career, it is clear that many (early career) researchers are suffering due to overwork, with more than a quarter saying that they work in excess of 50 hours per week and 2\% in excess of 90 h per week. Almost 30\% professed to having suffered harrassment or discrimination in the course of their work. Further, whilst only 20\% had suffered mental health issues before starting their career in astrophysics, $\sim$45\% said that they suffered with mental health problems since starting in astrophysics. Here we provide results from the survey as well as possible avenues to explore and a list of recommendations to improve (early) careers in astrophysics.

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arXiv2020

The asymmetric inner disk of the Herbig Ae star HD 163296 in the eyes of VLTI/MATISSE: evidence for a vortex?

Context. The inner few au region of planet-forming disks is a complex environment. High angular resolution observations have a key role in understanding the disk structure and the dynamical processes at work. Aims. In this study we aim to characterize the mid-infrared brightness distribution of the inner disk of the young intermediate-mass star HD 163296, from VLTI/MATISSE observations. Methods. We use geometric models to fit the data. Our models include a smoothed ring, a flat disk with inner cavity, and a 2D Gaussian. The models can account for disk inclination and for azimuthal asymmetries as well. We also perform numerical hydro-dynamical simulations of the inner edge of the disk. Results. Our modeling reveals a significant brightness asymmetry in the L-band disk emission. The brightness maximum of the asymmetry is located at the NW part of the disk image, nearly at the position angle of the semimajor axis. The surface brightness ratio in the azimuthal variation is $3.5 \pm 0.2$. Comparing our result on the location of the asymmetry with other interferometric measurements, we confirm that the morphology of the $r<0.3$ au disk region is time-variable. We propose that this asymmetric structure, located in or near the inner rim of the dusty disk, orbits the star. For the physical origin of the asymmetry, we tested a hypothesis where a vortex is created by Rossby wave instability, and we find that a unique large scale vortex may be compatible with our data. The half-light radius of the L-band emitting region is $0.33\pm 0.01$ au, the inclination is ${52^\circ}^{+5^\circ}_{-7^\circ}$, and the position angle is $143^\circ \pm 3^\circ$. Our models predict that a non-negligible fraction of the L-band disk emission originates inside the dust sublimation radius for $μ$m-sized grains. Refractory grains or large ($\gtrsim 10\ μ$m-sized) grains could be the origin for this emission.

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arXiv2020

SPHERE+: Imaging young Jupiters down to the snowline

SPHERE (Beuzit et al,. 2019) has now been in operation at the VLT for more than 5 years, demonstrating a high level of performance. SPHERE has produced outstanding results using a variety of operating modes, primarily in the field of direct imaging of exoplanetary systems, focusing on exoplanets as point sources and circumstellar disks as extended objects. The achievements obtained thus far with SPHERE (~200 refereed publications) in different areas (exoplanets, disks, solar system, stellar physics...) have motivated a large consortium to propose an even more ambitious set of science cases, and its corresponding technical implementation in the form of an upgrade. The SPHERE+ project capitalizes on the expertise and lessons learned from SPHERE to push high contrast imaging performance to its limits on the VLT 8m-telescope. The scientific program of SPHERE+ described in this document will open a new and compelling scientific window for the upcoming decade in strong synergy with ground-based facilities (VLT/I, ELT, ALMA, and SKA) and space missions (Gaia, JWST, PLATO and WFIRST). While SPHERE has sampled the outer parts of planetary systems beyond a few tens of AU, SPHERE+ will dig into the inner regions around stars to reveal and characterize by mean of spectroscopy the giant planet population down to the snow line. Building on SPHERE's scientific heritage and resounding success, SPHERE+ will be a dedicated survey instrument which will strengthen the leadership of ESO and the European community in the very competitive field of direct imaging of exoplanetary systems. With enhanced capabilities, it will enable an even broader diversity of science cases including the study of the solar system, the birth and death of stars and the exploration of the inner regions of active galactic nuclei.

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arXiv2016

Gaps, rings, and non-axisymmetric structures in protoplanetary disks - Emission from large grains

Dust grains with sizes around (sub)mm are expected to couple only weakly to the gas motion in regions beyond 10 au of circumstellar disks. In this work, we investigate the influence of the spatial distribution of such grains on the (sub)mm appearance of magnetized protoplanetary disks. We perform non-ideal global 3D magneto-hydrodynamic stratified disk simulations including particles of different sizes (50 micron to 1 cm), using a Lagrangian particle solver. We calculate the spatial dust temperature distribution, including the dynamically coupled submicron-sized dust grains, and derive ideal continuum re-emission maps of the disk through radiative transfer simulations. Finally, we investigate the feasibility to observe specific structures in the thermal re-emission maps with the Atacama Large Millimeter/submillimeter Array (ALMA). The pressure bump close to the outer edge of the dead-zone leads to particle trapping in ring structures. More specifically, vortices in the disk concentrate the dust and create an inhomogeneous distribution of the solid material in the azimuthal direction. The large-scale disk perturbations are preserved in the (sub)mm re-emission maps. The observable structures are very similar to those expected to result from planet-disk interaction. The larger dust particles increase the brightness contrast between gap and ring structures. We find that rings, gaps and the dust accumulation in the vortex could be traced with ALMA down to a scale of a few astronomical units in circumstellar disks located in nearby star-forming regions. Finally, we present a brief comparison of these structures with those recently found with ALMA in the young circumstellar disks of HL Tau and Oph IRS 48.

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arXiv2015

Angular momentum transport and large eddy simulations in magnetorotational turbulence: the small Pm limit

Angular momentum transport in accretion discs is often believed to be due to magnetohydrodynamic turbulence mediated by the magnetorotational instability. Despite an abundant literature on the MRI, the parameters governing the saturation amplitude of the turbulence are poorly understood and the existence of an asymptotic behavior in the Ohmic diffusion regime is not clearly established. We investigate the properties of the turbulent state in the small magnetic Prandtl number limit. Since this is extremely computationally expensive, we also study the relevance and range of applicability of the most common subgrid scale models for this problem. Unstratified shearing boxes simulations are performed both in the compressible and incompressible limits, with a resolution up to 800 cells per disc scale height. The latter constitutes the largest resolution ever attained for a simulation of MRI turbulence. In the presence of a mean magnetic field threading the domain, angular momentum transport converges to a finite value in the small Pm limit. When the mean vertical field amplitude is such that β, the ratio between the thermal and magnetic pressure, equals 1000, we find α~0.032 when Pm approaches zero. In the case of a mean toroidal field for which β=100, we find α~0.018 in the same limit. Both implicit LES and Chollet-Lesieur closure model reproduces these results for the α parameter and the power spectra. A reduction in computational cost of a factor at least 16 (and up to 256) is achieved when using such methods. MRI turbulence operates efficiently in the small Pm limit provided there is a mean magnetic field. Implicit LES offers a practical and efficient mean of investigation of this regime but should be used with care, particularly in the case of a vertical field. Chollet-Lesieur closure model is perfectly suited for simulations done with a spectral code.

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arXiv2014

Planet-vortex interaction:How a vortex can shepherd a planetary embryo

Context: Anticyclonic vortices are considered as a favourable places for trapping dust and forming planetary embryos. On the other hand, they are massive blobs that can interact gravitationally with the planets in the disc. Aims: We aim to study how a vortex interacts gravitationally with a planet which migrates toward it or a planet which is created inside the vortex. Methods: We performed hydrodynamical simulations of a viscous locally isothermal disc using GFARGO and FARGO-ADSG. We set a stationary Gaussian pressure bump in the disc in a way that RWI is triggered. After a large vortex is established, we implanted a low mass planet in the outer disc or inside the vortex and allowed it to migrate. We also examined the effect of vortex strength on the planet migration and checked the validity of the final result in the presence of self-gravity. Results: We noticed regardless of the planet's initial position, the planet is finally locked to the vortex or its migration is stopped in a farther orbital distance in case of a stronger vortex. For the model with the weaker vortex, we studied the effect of different parameters such as background viscosity, background surface density, mass of the planet and different planet positions. In these models, while the trapping time and locking angle of the planet vary for different parameters, the main result, which is the planet-vortex locking, remains valid. We discovered that even a planet with a mass less than 5 * 10^{-7} M_{\star} comes out from the vortex and is locked to it at the same orbital distance. For a stronger vortex, both in non-self-gravitated and self-gravitating models, the planet migration is stopped far away from the radial position of the vortex. This effect can make the vortices a suitable place for continual planet formation under the condition that they save their shape during the planetary growth.

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arXiv2013

Multiple spiral patterns in the transitional disk of HD 100546

Protoplanetary disks around young stars harbor many structures related to planetary formation. Of particular interest, spiral patterns were discovered among several of these disks and are expected to be the sign of gravitational instabilities leading to giant planets formation or gravitational perturbations caused by already existing planets. In this context, the star HD100546 presents some specific characteristics with a complex gas and dusty disk including spirals as well as a possible planet in formation. The objective of this study is to analyze high contrast and high angular resolution images of this emblematic system to shed light on critical steps of the planet formation. We retrieved archival images obtained at Gemini in the near IR (Ks band) with the instrument NICI and processed the data using advanced high contrast imaging technique taking advantage of the angular differential imaging. These new images reveal the spiral pattern previously identified with HST with an unprecedented resolution, while the large-scale structure of the disk is mostly erased by the data processing. The single pattern at the southeast in HST images is now resolved into a multi-armed spiral pattern. Using two models of a gravitational perturber orbiting in a gaseous disk we attempted to bring constraints on the characteristics of this perturber assuming each spiral being independent and we derived qualitative conclusions. The non-detection of the northeast spiral pattern observed in HST allows to put a lower limit on the intensity ratio between the two sides of the disk, which if interpreted as forward scattering yields a larger anisotropic scattering than derived in the visible. Also, we found that the spirals are likely spatially resolved with a thickness of about 5-10AU. Finally, we did not detect the candidate forming planet recently discovered in the Lp band, with a mass upper limit of 16-18 MJ.

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arXiv2013

Quasi-periodic oscillations from Rossby Wave Instability

We study the Rossby wave instability model of high-frequency quasi-periodic oscillations (QPO) of microquasars. We show ray-traced light curves of QPO within this model and discuss perspectives of distinguishing alternative QPO models with the future Large Observatory For X-ray Timing (LOFT) observations.

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arXiv2013

Flux Modulation from the Rossby Wave Instability in microquasars accretion disks: toward a HFQPO model

Context. There have been a long string of efforts to understand the source of the variability observed in microquasars, especially concerning the elusive High-Frequency Quasi-Periodic Oscillation. These oscillations are among the fastest phenomena that affect matter in the vicinity of stellar black holes and therefore could be used as probes of strong-field general relativity. Nevertheless, no model has yet gained wide acceptance. Aims. The aim of this article is to investigate the model derived from the occurrence of the Rossby wave instability at the inner edge of the accretion disk. In particular, our goal here is to demonstrate the capacity of this instability to modulate the observed flux in agreement with the observed results. Methods. We use the AMRVAC hydrodynamical code to model the instability in a 3D optically thin disk. The GYOTO ray-tracing code is then used to compute the associated light curve. Results. We show that the 3D Rossby wave instability is able to modulate the flux well within the observed limits.We highlight that 2D simulations allow us to obtain the same general characteristics of the light curve as 3D calculations. With the time resolution we adopted in this work, three dimensional simulations do not give rise to any new observable features that could be detected by current instrumentation or archive data.

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arXiv2013

How strong are the Rossby vortices?

The Rossby wave instability, associated with density bumps in differentially rotating discs, may arise in several different astrophysical contexts, such as galactic or protoplanetary discs. While the linear phase of the instability has been well studied, the nonlinear evolution and especially the saturation phase remain poorly understood. In this paper, we test the non-linear saturation mechanism analogous to that derived for wave-particle interaction in plasma physics. To this end we perform global numerical simulations of the evolution of the instability in a two-dimensional disc. We confirm the physical mechanism for the instability saturation and show that the maximum amplitude of vorticity can be estimated as twice the linear growth rate of the instability. We provide an empirical fitting formula for this growth rate for various parameters of the density bump. We also investigate the effects of the azimuthal mode number of the instability and the energy leakage in the spiral density waves. Finally, we show that our results can be extrapolated to 3D discs.

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arXiv2012

Toward a model for HFQPOs in Microquasars

There have been a long string of efforts to understand the source of the variability observed in microquasars but no model has yet gained wide acceptance, especially concerning the elusive High-Frequency Quasi-Periodic Oscillation (HFQPO). We first list the constraints arising from observations and how that translates for an HFQPO model. Then we present how a model based on having the Rossby Wave Instability (RWI) active in the disk could answer those constraints.

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arXiv2012

Dust-trapping Rossby vortices in protoplanetary disks

One of the most challenging steps in planet formation theory is the one leading to the formation of planetesimals of kilometre size. A promising scenario involves the existence of vortices able to concentrate a large amount of dust and grains in their centres. Up to now this scenario has been studied mostly in 2D razor thin disks. A 3D study including, simultaneously, the formation and resulting dust concentration of the vortices with vertical settling, was still missing. The Rossby wave instability self-consistently forms 3D vortices, which have the unique quality of presenting a large scale vertical velocity in their centre. Here we aim to study how this newly discovered effect can alter the dynamic evolution of the dust. We perform global 3D simulations of the RWI in a radially and vertically stratified disk using the code MPI-AMRVAC. After the growth phase of the instability, the gas and solid phases are modelled by a bi-fluid approach, where the dust is considered as a fluid without pressure. Both the drag force of the gas on the dust and the back-reaction of the dust on the gas are included. Multiple grain sizes from 1mm to 5cm are used with a constant density distribution. We obtain in a short timescale a high concentration of the largest grains in the vortices. Indeed, in 3 rotations the dust-to-gas density ratio grows from 10^-2 to unity leading to a concentration of mass up to that of Mars in one vortex. The presence of the radial drift is also at the origin of a dust pile-up at the radius of the vortices. Lastly, the vertical velocity of the gas in the vortex causes the sedimentation process to be reversed, the mm size dust is lifted and higher concentrations are obtained in the upper layer than in the mid-plane.

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arXiv2012

Formation and long-term evolution of 3D vortices in protoplanetary discs

In the context of planet formation, anticyclonic vortices have recently received lots of attention for the role they can play in planetesimals formation. Radial migration of intermediate size solids toward the central star may prevent their growth to larger solid grains. On the other hand, vortices can trap the dust and accelerate this growth, counteracting fast radial transport. Multiple effects have been shown to affect this scenario, such as vortex migration or decay. The aim of this paper is to study the formation of vortices by the Rossby wave instability and their long term evolution in a full three dimensional protoplanetary disc. We use a robust numerical scheme combined with adaptive mesh refinement in cylindrical coordinates, allowing to affordably compute long term 3D evolutions. We consider a full disc stratified both radially and vertically that is prone to formation of vortices by the Rossby wave instability. We show that the 3D Rossby vortices grow and survive over hundreds of years without migration. The localized overdensity which initiated the instability and vortex formation survives the growth of the Rossby wave instability for very long times. When the vortices are no longer sustained by the Rossby wave instability, their shape changes toward more elliptical vortices. This allows them to survive shear-driven destruction, but they may be prone to elliptical instability and slow decay. When the conditions for growing Rossby wave-related instabilities are maintained in the disc, large-scale vortices can survive over very long timescales and may be able to concentrate solids.

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arXiv2012

Rossby Wave Instability in three dimensional discs

The Rossby wave instability (RWI) is a promising mechanism for producing large-scale vortices in protoplanetary discs. The instability operates around a density bump in the disc, and the resulting vortices may facilitate planetesimal formation and angular momentum transfer in the disc dead zone. Most previous works on the RWI deal with two-dimensional (height-integrated) discs. However, vortices may have different dynamical behaviours in 3D than in 2D. Recent numerical simulations of the RWI in 3D global discs by Meheut et al. have revealed intriguing vertical structure of the vortices, including appreciable vertical velocities. In this paper we present a linear analysis of the RWI in 3D global models of isothermal discs. We calculate the growth rates of the Rossby modes (of various azimuthal wave numbers m = 2 - 6) trapped around the fiducial density bump and carry out 3D numerical simulations to compare with our linear results. We show that the 3D RWI growth rates are only slightly smaller than the 2D growth rates, and the velocity structures seen in the numerical simulations during the linear phase are in agreement with the velocity eigenfunctions obtained in our linear calculations. This numerical benchmark shows that numerical simulations can accurately describe the instability. The angular momentum transfer rate associated with Rossby vortices is also studied.

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