Showing papers by "François Levrier published in 2012"

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

Abstract: 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.

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

Abstract: Context. Observations have long demonstrated the molecular diversity of the di use interstellar medium (ISM). Only now, with the advent of high-performance computing, does it become possible for numerical simulations of astrophysical fluids to include a treatment of chemistry, in order to faithfully reproduce the abundances of the many observed species, and especially that of CO, which is used as a proxy for molecular hydrogen. When applying photon-dominated region (PDR) codes to describe the UV-driven chemistry of uniform density cloud models, it is found that the observed abundances of CO are not well reproduced. Aims. Our main purpose is to estimate the e ect 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”. Methods. We use results from a magnetohydrodynamical (MHD) simulation as a model for the density structures found in a turbulent di use 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. Results. 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 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[logN(CO)]=d[logN(H2)] ’ 3:07 0:73, we find d[logN(CO)]=d[logN(H2)] ’ 14 when assuming uniform density, and d[logN(CO)]=d[logN(H2)] ’ 5:2 when including density fluctuations.

Hydride spectroscopy of the diffuse interstellar medium: new clues on the gas fraction in molecular form and cosmic ray ionization rate in relation to H3+.

Abstract: The Herschel-guaranteed time key programme PRobing InterStellar Molecules with Absorption line Studies (PRISMAS)(1) is providing a survey of the interstellar hydrides containing the elements C, O, N, F and Cl. As the building blocks of interstellar molecules, hydrides provide key information on their formation pathways. They can also be used as tracers of important physical and chemical properties of the interstellar gas that are difficult to measure otherwise. This paper presents an analysis of two sight-lines investigated by the PRISMAS project, towards the star-forming regions W49N and W51. By combining the information extracted from the detected spectral lines, we present an analysis of the physical properties of the diffuse interstellar gas, including the electron abundance, the fraction of gas in molecular form, and constraints on the cosmic ray ionization rate and the gas density.

Synthetic observations of first hydrostatic cores in collapsing low-mass dense cores - II. Simulated ALMA dust emission maps

Abstract: Max-Planck-Institut fu¨r Astronomie, Ko¨nigstuhl 17, 69117 Heidelberg, GermanyReceived 20 july 2012; accepted 3 october 2012ABSTRACTContext. First hydrostatic cores are predicted by theories of star formation, but their existence has never been demonstrated con-vincingly by (sub)millimeter observations. Furthermore, the multiplicity at the early phases of the star formation process is poorlyconstrained.Aims.The purpose of this paper is twofold. First, we seek to provide predictions of ALMA dust continuum emission maps from earlyClass 0 objects. Second, we show to what extent ALMA will be able to probe the fragmentation scale in these objects.Methods.Following our previous paper (Commerc¸on et al. 2012, hereafter paper I), we post-process three state-of-the-art radiation-magneto-hydrodynamic 3D adaptive mesh refinement calculations to compute the emanating dust emission maps. We then producesynthetic ALMA observations of the dust thermal continuum from first hydrostatic cores.Results.We present the first synthetic ALMA observations of dust cont inuum emission from first hydrostatic cores. We analyze theresults given by the different bands and configurations and we discuss for which combi nations of the two the first hydrostatic coreswould most likely be observed. We also show that observing dust continuum emission with ALMA will help in identifying the physi-cal processes occurring within collapsing dense cores. If the magnetic field is playing a role, the emission pattern will show evidenceof a pseudo-disk and even of a magnetically driven outflow, which pure hydrodynamical calculations cannot reproduce.Conclusions.The capabilities of ALMA will enable us to make significant pr ogress towards understanding fragmentation at the earlyClass 0 stage and discovering first hydrostatic cores.Key words. Stars: low mass, formation - Magnetohydrodynamics (MHD), radiative transfer - Methods: numerical - Techniques:interferometric

Intense velocity-shears, magnetic fields and filaments in diffuse gas

Abstract: Abstract The dissipation of turbulence is a key process in the evolution of diffuse gas towards denser structures. The vast range of coupled scales and the variety of dissipative processes in interstellar turbulence make it a complex system to analyze. Observations now provide powerful statistics of the gas velocity field, density and magnetic field orientations, opening a rich field of investigation. On-going comparisons of the orientation of intense velocity-shears, magnetic field and tenuous filaments of matter in a turbulent high-latitude cloud are promising.

Synthetic observations of first hydrostatic cores in collapsing low-mass dense cores II. Simulated ALMA dust emission maps

Abstract: First hydrostatic cores are predicted by theories of star formation, but their existence has never been demonstrated convincingly by (sub)millimeter observations. Furthermore, the multiplicity at the early phases of the star formation process is poorly constrained. The purpose of this paper is twofold. First, we seek to provide predictions of ALMA dust continuum emission maps from early Class 0 objects. Second, we show to what extent ALMA will be able to probe the fragmentation scale in these objects. Following our previous paper (Commer\c{c}on et al. 2012, hereafter paper I), we post-process three state-of-the-art radiation-magneto-hydrodynamic 3D adaptive mesh refinement calculations to compute the emanating dust emission maps. We then produce synthetic ALMA observations of the dust thermal continuum from first hydrostatic cores. We present the first synthetic ALMA observations of dust continuum emission from first hydrostatic cores. We analyze the results given by the different bands and configurations and we discuss for which combinations of the two the first hydrostatic cores would most likely be observed. We also show that observing dust continuum emission with ALMA will help in identifying the physical processes occurring within collapsing dense cores. If the magnetic field is playing a role, the emission pattern will show evidence of a pseudo-disk and even of a magnetically driven outflow, which pure hydrodynamical calculations cannot reproduce. The capabilities of ALMA will enable us to make significant progress towards understanding fragmentation at the early Class 0 stage and discovering first hydrostatic cores.