arXiv subjects

Pierre Lesaffre

Publications and source records attributed to Pierre Lesaffre.

32 records · Page 2Linked to original sources

arXiv2016

Origin of CH+ in diffuse molecular clouds warm H2 and ion-neutral drift

This paper assesses the roles of the presence of warm H2, and the increased formation rate due to the ion-neutral drift. We performed ideal MHD simulations that include the heating and cooling of the multiphase ISM, and where we treat dynamically the formation of H2. In a post-processing step we compute the abundances of species at chemical equilibrium. We show that CH+ is efficiently formed at the edge of clumps, in regions where the H2 fraction is low, but nevertheless higher than its equilibrium value, and where the gas temperature is high. We show that warm and out of equilibrium H2 increases the integrated column densities of CH+ by one order of magnitude, up to values still 3-10 times lower than those observed in the diffuse ISM. We balance the Lorentz force with the ion-neutral drag to estimate the ion-drift velocities (vd). We find that the vd distribution peaks around 0.04 km s-1, and that high vd are too rare to have a significant statistical impact on the abundances of CH+. Compared to previous works, our multiphase simulations reduce the spread in vd, and our self-consistent treatment of the ionisation leads to much reduced vd. Nevertheless, our resolution study shows that this velocity distribution is not converged: the ion-neutral drift has a higher impact on CH+ at higher resolution. On the other hand, our ideal MHD simulations do not include ambipolar diffusion, which would yield lower drift velocities. Within these limitations, we conclude that warm H2 is a key ingredient in the efficient formation of CH+ and that the ambipolar diffusion has very little influence on the abundance of CH+, mainly due to the small drift velocities obtained. However, we point out that small-scale processes and other non-thermal processes not included in our MHD simulation may be of crucial importance, and higher resolution studies with better controlled dissipation processes are needed.

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arXiv2016

Radiative and mechanical feedback into the molecular gas in the Large Magellanic Cloud. I. N159W

We present Herschel SPIRE Fourier Transform Spectrometer (FTS) observations of N159W, an active star-forming region in the Large Magellanic Cloud (LMC). In our observations, a number of far-infrared cooling lines including CO(4-3) to CO(12-11), [CI] 609 and 370 micron, and [NII] 205 micron are clearly detected. With an aim of investigating the physical conditions and excitation processes of molecular gas, we first construct CO spectral line energy distributions (SLEDs) on 10 pc scales by combining the FTS CO transitions with ground-based low-J CO data and analyze the observed CO SLEDs using non-LTE radiative transfer models. We find that the CO-traced molecular gas in N159W is warm (kinetic temperature of 153-754 K) and moderately dense (H2 number density of (1.1-4.5)e3 cm-3). To assess the impact of the energetic processes in the interstellar medium on the physical conditions of the CO-emitting gas, we then compare the observed CO line intensities with the models of photodissociation regions (PDRs) and shocks. We first constrain the properties of PDRs by modelling Herschel observations of [OI] 145, [CII] 158, and [CI] 370 micron fine-structure lines and find that the constrained PDR components emit very weak CO emission. X-rays and cosmic-rays are also found to provide a negligible contribution to the CO emission, essentially ruling out ionizing sources (ultraviolet photons, X-rays, and cosmic-rays) as the dominant heating source for CO in N159W. On the other hand, mechanical heating by low-velocity C-type shocks with ~10 km/s appears sufficient enough to reproduce the observed warm CO.

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arXiv2016

3D radiative hydrodynamic simulations of protostellar collapse with H-C-O dynamical chemistry

Combining the co-evolving chemistry, hydrodynamics and radiative transfer is an important step for star formation studies. It allows both a better link to observations and a self-consistent monitoring of the magnetic dissipation in the collapsing core. Our aim is to follow a chemo-dynamical evolution of collapsing dense cores with a reduced gas-grain chemical network. We present the results of radiative hydrodynamic (RHD) simulations of 1 M$_\odot$ isolated dense core collapse. The physical setup includes RHD and dynamical evolution of a chemical network. To perform those simulations, we merged the multi-dimensional adaptive-mesh-refinement code RAMSES and the thermo-chemistry Paris-Durham shock code. We simulate the formation of the first hydro-static core (FHSC) and the co-evolution of 56 species describing mainly H-C-O chemistry. Accurate benchmarking is performed, testing the reduced chemical network against a well-establiched complex network. We show that by using a compact set of reactions, one can match closely the CO abundances with results of a much more complex network. Our main results are: (a) We find that gas-grain chemistry post-processing can lead to one order of magnitude lower CO gas-phase abundances compared to the dynamical chemistry, with strongest effect during the isothermal phase of collapse. (b) The free-fall time has little effect on the chemical abundances for our choice of the parameters. (c) Dynamical chemical evolution is required to describe the CO gas phase abundance as well as the CO ice formation for the mean grain size larger then 1$μ$m. (d) Furthermore, dust mean size and size distribution have a strong effect on chemical abundances and hence on the ionization degree and magnetic dissipation. We conclude that dust grain growth in the collapse simulations can be as important as coupling the collapse with chemistry.

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arXiv2015

H2 distribution during formation of multiphase molecular clouds

H2 is the simplest and the most abundant molecule in the ISM, and its formation precedes the formation of other molecules. Understanding the dynamical influence of the environment and the interplay between the thermal processes related to the formation and destruction of H2 and the structure of the cloud is mandatory to understand correctly the observations of H2. We perform high resolution MHD colliding flow simulations with the AMR code RAMSES in which the physics of H2 has been included. We compare the simulation results with various observations including the column densities of excited rotational levels. Due to a combination of thermal pressure, ram pressure and gravity, the clouds produced at the converging point of HI streams are highly inhomogeneous. H2 molecules quickly form in relatively dense clumps and spread into the diffuse interclump gas. This in particular leads to the existence of significant abundances of H2 in the diffuse and warm gas that lies in between clumps. Simulations and observations show similar trends, specially for the HI-to-H2 transition. The abundances of excited rotational levels, calculated at equilibrium in the simulations are very similar to the observed abundances inferred from FUSE results. This is a direct consequence of the presence of the H2 enriched diffuse and warm gas. Our simulations show that H2 rapidly forms in the dense clumps and, due to the complex structure of molecular clouds, quickly spreads at lower densities. Consequently a significant fraction of warm H2 exists in the low density gas. This warm H2 leads to column densities of excited rotational levels close to the observed ones likely revealing the complex intermix between the warm and the cold gas in molecular clouds. This suggests that the 2-phase structure of molecular clouds is an essential ingredient to fully understand molecular hydrogen in these objects.

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arXiv2015

Turbulent energy dissipation and intermittency in ambipolar diffusion magnetohydrodynamics

The dissipation of kinetic and magnetic energy in the interstellar medium (ISM) can proceed through viscous, Ohmic or ambipolar diffusion (AD). It occurs at very small scales compared to the scales at which energy is presumed to be injected. This localized heating may impact the ISM evolution but also its chemistry, thus providing observable features. Here, we perform 3D spectral simulations of decaying magnetohydrodynamic turbulence including the effects of AD. We find that the AD heating power spectrum peaks at scales in the inertial range, due to a strong alignment of the magnetic and current vectors in the dissipative range. AD affects much greater scales than the AD scale predicted by dimensional analysis. We find that energy dissipation is highly concentrated on thin sheets. Its probability density function follows a lognormal law with a power-law tail which hints at intermittency, a property which we quantify by use of structure function exponents. Finally, we extract structures of high dissipation, defined as connected sets of points where the total dissipation is most intense and we measure the scaling exponents of their geometric and dynamical characteristics: the inclusion of AD favours small sizes in the dissipative range.

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arXiv2013

Low velocity shocks: signatures of turbulent dissipation in diffuse irradiated gas

We examine the chemical and emission properties of mildly irradiated (G0=1) magnetised shocks in diffuse media (nH=10^2 to 10^4 /cm3) at low to moderate velocities (from 3 to 40 km/s). Results: The formation of some molecules relies on endoergic reactions. In J-shocks, their abundances are enhanced by several orders of magnitude for shock velocities as low as 7 km/s. Otherwise most chemical properties of J-type shocks vary over less than an order of magnitude between velocities from about 7 to about 30 km/s, where H2 dissociation sets in. C-type shocks display a more gradual molecular enhancement as the shock velocity increases. We quantify the energy flux budget (fluxes of kinetic, radiated and magnetic energies) with emphasis on the main cooling lines of the cold interstellar medium. Their sensitivity to shock velocity is such that it allows observations to constrain statistical distributions of shock velocities. We fit various probability distribution functions (PDFs) of shock velocities to spectroscopic observations of the galaxy-wide shock in Stephan's Quintet (SQ) and of a Galactic line of sight sampling diffuse molecular gas in Chamaeleon. In both cases, low velocities bear the greatest statistical weight and the PDF is consistent with a bimodal distribution. In the very low velocity shocks (below 5 km/s), dissipation is due to ion-neutral friction which powers H2 low energy transitions and atomic lines. In moderate velocity shocks (20 km/s and above), the dissipation is due to viscous heating and accounts for most of the molecular emission. In our interpretation a significant fraction of the gas on the line of sight is shocked (from 4% to 66%). For example, C+ emission may trace shocks in UV irradiated gas where C+ is the dominant carbon species.

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arXiv2012

UV-driven chemistry in simulations of the interstellar medium. I. Post-processed chemistry with the Meudon PDR code

Our main purpose is to estimate the effect of assuming uniform density on the line-of-sight in PDR chemistry models, compared to a more realistic distribution for which total gas densities may well vary by several orders of magnitude. A secondary goal of this paper is to estimate the amount of molecular hydrogen which is not properly traced by the CO (J = 1 -> 0) line, the so-called "dark molecular gas". We use results from a magnetohydrodynamical (MHD) simulation as a model for the density structures found in a turbulent diffuse ISM with no star-formation activity. The Meudon PDR code is then applied to a number of lines of sight through this model, to derive their chemical structures. It is found that, compared to the uniform density assumption, maximal chemical abundances for H2, CO, CH and CN are increased by a factor 2 to 4 when taking into account density fluctuations on the line of sight. The correlations between column densities of CO, CH and CN with respect to those of H2 are also found to be in better overall agreement with observations. For instance, at N(H2) > 2.10^{20} cm-2, while observations suggest that d[log N(CO)]=d[log N(H2)] = 3.07 +/- 0.73, we find d[log N(CO)]=d[log N(H2)] =14 when assuming uniform density, and d[log N(CO)]=d[log N(H2)] = 5.2 when including density fluctuations.

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arXiv2011

Ortho-H2 and the Age of Interstellar Dark Clouds

Interstellar dark clouds are the sites of star formation. Their main component, dihydrogen, exists under two states, ortho and para. H2 is supposed to form in the ortho:para ratio (OPR) of 3:1 and to subsequently decay to almost pure para-H2 (OPR <= 0.001). Only if the H2 OPR is low enough, will deuteration enrichment, as observed in the cores of these clouds, be efficient. The second condition for strong deuteration enrichment is the local disappearance of CO, which freezes out onto grains in the core formation process. We show that this latter condition does not apply to DCO+, which, therefore, should be present all over the cloud. We find that an OPR >= 0.1 is necessary to prevent DCO+ large-scale apparition. We conclude that the inevitable decay of ortho-H2 sets an upper limit of ~6 million years to the age of starless molecular clouds under usual conditions.

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arXiv2010

Simplified Hydrostatic Carbon Burning in White Dwarf Interiors

We introduce two simplified nuclear networks that can be used in hydrostatic carbon burning reactions occurring in white dwarf interiors. They model the relevant nuclear reactions in carbon-oxygen white dwarfs (COWDs) approaching ignition in Type Ia supernova (SN Ia) progenitors, including the effects of the main e-captures and β-decays that drive the convective Urca process. They are based on studies of a detailed nuclear network compiled by the authors and are defined by approximate sets of differential equations whose derivations are included in the text. The first network, N1, provides a good first order estimation of the distribution of ashes and it also provides a simple picture of the main reactions occurring during this phase of evolution. The second network, N2, is a more refined version of N1 and can reproduce the evolution of the main physical properties of the full network to the 5% level. We compare the evolution of the mole fraction of the relevant nuclei, the neutron excess, the photon energy generation and the neutrino losses between both simplified networks and the detailed reaction network in a fixed temperature and density parcel of gas.

astro-ph.SR↗

arXiv2010

Modules for Experiments in Stellar Astrophysics (MESA)

Stellar physics and evolution calculations enable a broad range of research in astrophysics. Modules for Experiments in Stellar Astrophysics (MESA) is a suite of open source libraries for a wide range of applications in computational stellar astrophysics. A newly designed 1-D stellar evolution module, MESA star, combines many of the numerical and physics modules for simulations of a wide range of stellar evolution scenarios ranging from very-low mass to massive stars, including advanced evolutionary phases. MESA star solves the fully coupled structure and composition equations simultaneously. It uses adaptive mesh refinement and sophisticated timestep controls, and supports shared memory parallelism based on OpenMP. Independently usable modules provide equation of state, opacity, nuclear reaction rates, and atmosphere boundary conditions. Each module is constructed as a separate Fortran 95 library with its own public interface. Examples include comparisons to other codes and show evolutionary tracks of very low mass stars, brown dwarfs, and gas giant planets; the complete evolution of a 1 Msun star from the pre-main sequence to a cooling white dwarf; the Solar sound speed profile; the evolution of intermediate mass stars through the thermal pulses on the He-shell burning AGB phase; the interior structure of slowly pulsating B Stars and Beta Cepheids; evolutionary tracks of massive stars from the pre-main sequence to the onset of core collapse; stars undergoing Roche lobe overflow; and accretion onto a neutron star. Instructions for downloading and installing MESA can be found on the project web site (http://mesa.sourceforge.net/).

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arXiv2009

A comparison of local simulations and reduced models of MRI-induced turbulence

We run mean-field shearing-box numerical simulations with a temperature-dependent resistivity and compare them to a reduced dynamical model. Our simulations reveal the co-existence of two quasi-steady states, a `quiet' state and an `active' turbulent state, confirming the predictions of the reduced model. The initial conditions determine on which state the simulation ultimately settles. The active state is strongly influenced by the geometry of the computational box and the thermal properties of the gas. Cubic domains support permanent channel flows, bar-shaped domains exhibit eruptive behaviour, and horizontal slabs give rise to infrequent channels. Meanwhile, longer cooling time-scales lead to higher saturation amplitudes.

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arXiv2009

MRI channel flows and their parasites

Local simulations of the magnetorotational instability (MRI) in accretion disks can exhibit recurrent coherent structures called channel flows. The formation and destruction of these structures may play a role in the development and saturation of MRI-induced turbulence, and consequently help us understand the time-dependent accretion behaviour of certain astrophysical objects. Previous investigations have revealed that channel solutions are attacked by various parasitic modes, foremost of which is an analogue of the Kelvin-Helmholtz instability. We revisit these instabilities and show how they relate to the classical instabilities of plasma physics, the kink and pinch modes. However, we argue that in most cases channels emerge from developed turbulence and are eventually destroyed by turbulent mixing, not by the parasites. The exceptions are the clean isolated channels which appear in systems near criticality or which emerge from low amplitude initial conditions. These structures inevitably achieve large amplitudes and are only then destroyed, giving rise to eruptive behaviour.

astro-ph.HE↗

arXiv2006

Critical Mass Transfer in Double-Degenerate Type Ia Supernovae

Doubly-degenerate binary systems consisting of two white dwarfs both composed of carbon and oxygen and close enough that mass is transferred from the less massive to the more massive are possible progenitors of type Ia supernovae. If the mass transfer rate is slow enough that the accreting white dwarf can reach 1.38 solar masses then it can ignite carbon degenerately at its centre. This can lead to a thermonuclear runaway and thence a supernova explosion. However if the accretion rate is too high the outer layers of the white dwarf heat up too much and carbon ignites there non-degenerately. A series of mild carbon flashes can then propagate inwards and convert the carbon to neon relatively gently. There is no thermonuclear runaway and no supernova. We have determined the critical rate for accretion and find it to be generally about two-fifths of the Eddington rate. In the case of actual bianry stars the mass transfer rate falls off as mass is transferred so we find that this rate can be very much exceeded at the onset of mass transfer. We determine combinations of white dwarf masses that can lead to central degenerate carbon ignition.

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arXiv2006

Cosmological Implications of the Second Parameter of Type Ia Supernovae

Theoretical models predict that the initial metallicity of the progenitor of a Type Ia supernova (SN Ia) affects the peak of the supernova light curve. This can cause a deviation from the standard light curve calibration employed when using SNe Ia as standardizable distance candles and, if there is a systematic evolution of the metallicity of SN Ia progenitors, could affect the determination of cosmological parameters. Here we show that this metallicity effect can be substantially larger than has been estimated previously, when the neutronisation in the immediate pre-explosion phase in the CO white dwarf is taken into account, and quantitatively assess the importance of metallicity evolution for determining cosmological parameters. We show that, in principle, a moderate and plausible amount of metallicity evolution could mimic a lambda-dominated, flat Universe in an open, lambda-free Universe. However, the effect of metallicity evolution appears not large enough to explain the high-z SN Ia data in a flat Universe, for which there is strong independent evidence, without a cosmological constant. We also estimate the systematic uncertainties introduced by metallicity evolution in a lambda-dominated, flat Universe. We find that metallicity evolution may limit the precision with which Omega_m and w can be measured and that it will be difficult to distinguish evolution of the equation of state of dark energy from metallicity evolution, at least from SN Ia data alone.

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