arXiv subjects

J. Le Bourlot

Publications and source records attributed to J. Le Bourlot.

31 records · Page 2Linked to original sources

arXiv2011

OH emission from warm and dense gas in the Orion Bar PDR

As part of a far-infrared (FIR) spectral scan with Herschel/PACS, we present the first detection of the hydroxyl radical (OH) towards the Orion Bar photodissociation region (PDR). Five OH rotational Lambda-doublets involving energy levels out to E_u/k~511 K have been detected (at ~65, ~79, ~84, ~119 and ~163um). The total intensity of the OH lines is I(OH)~5x10^-4 erg s^-1 cm^-2 sr^-1. The observed emission of rotationally excited OH lines is extended and correlates well with the high-J CO and CH^+ J=3-2 line emission (but apparently not with water vapour), pointing towards a common origin. Nonlocal, non-LTE radiative transfer models including excitation by the ambient FIR radiation field suggest that OH arises in a small filling factor component of warm (Tk~160-220 K) and dense (n_H~10^{6-7} cm^-3) gas with source-averaged OH column densities of ~10^15 cm^-2. High density and temperature photochemical models predict such enhanced OH columns at low depths (A_V<1) and small spatial scales (~10^15 cm), where OH formation is driven by gas-phase endothermic reactions of atomic oxygen with molecular hydrogen. We interpret the extended OH emission as coming from unresolved structures exposed to far-ultraviolet (FUV) radiation near the Bar edge (photoevaporating clumps or filaments) and not from the lower density "interclump" medium. Photodissociation leads to OH/H2O abundance ratios (>1) much higher than those expected in equally warm regions without enhanced FUV radiation fields.

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arXiv2010

The global dust SED: Tracing the nature and evolution of dust with DustEM

The Planck and Herschel missions are currently measuring the farIR-mm emission of dust, which combined with existing IR data, will for the first time provide the full SED of the galactic ISM dust emission with an unprecedented sensitivity and angular resolution. It will allow a systematic study of the dust evolution processes that affect the SED. Here we present a versatile numerical tool, DustEM, that predicts the emission and extinction of dust given their size distribution and their optical and thermal properties. In order to model dust evolution, DustEM has been designed to deal with a variety of grain types, structures and size distributions and to be able to easily include new dust physics. We use DustEM to model the dust SED and extinction in the diffuse interstellar medium at high-galactic latitude (DHGL), a natural reference SED. We present a coherent set of observations for the DHGL SED. The dust components in our DHGL model are (i) PAHs, (ii) amorphous carbon and (iii) amorphous silicates. We use amorphous carbon dust, rather than graphite, because it better explains the observed high abundances of gas-phase carbon in shocked regions of the interstellar medium. Using the DustEM model, we illustrate how, in the optically thin limit, the IRAS/Planck HFI (and likewise Spitzer/Herschel for smaller spatial scales) photometric band ratios of the dust SED can disentangle the influence of the exciting radiation field intensity and constrain the abundance of small grains relative to the larger grains. We also discuss the contributions of the different grain populations to the IRAS, Planck and Herschel channels. Such information is required to enable a study of the evolution of dust as well as to systematically extract the dust thermal emission from CMB data and to analyze the emission in the Planck polarized channels. The DustEM code described in this paper is publically available.

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arXiv2010

Gas morphology and energetics at the surface of PDRs: new insights with Herschel observations of NGC 7023

We investigate the physics and chemistry of the gas and dust in dense photon-dominated regions (PDRs), along with their dependence on the illuminating UV field. Using Herschel-HIFI observations, we study the gas energetics in NGC 7023 in relation to the morphology of this nebula. NGC 7023 is the prototype of a PDR illuminated by a B2V star and is one of the key targets of Herschel. Our approach consists in determining the energetics of the region by combining the information carried by the mid-IR spectrum (extinction by classical grains, emission from very small dust particles) with that of the main gas coolant lines. In this letter, we discuss more specifically the intensity and line profile of the 158 micron (1901 GHz) [CII] line measured by HIFI and provide information on the emitting gas. We show that both the [CII] emission and the mid-IR emission from polycyclic aromatic hydrocarbons (PAHs) arise from the regions located in the transition zone between atomic and molecular gas. Using the Meudon PDR code and a simple transfer model, we find good agreement between the calculated and observed [CII] intensities. HIFI observations of NGC 7023 provide the opportunity to constrain the energetics at the surface of PDRs. Future work will include analysis of the main coolant line [OI] and use of a new PDR model that includes PAH-related species.

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arXiv2010

Herschel/HIFI discovery of interstellar chloronium (H$_2$Cl$^+$)

We report the first detection of chloronium, H$_2$Cl$^+$, in the interstellar medium, using the HIFI instrument aboard the \emph{Herschel} Space Observatory. The $2_{12}-1_{01}$ lines of ortho-H$_2^{35}$Cl$^+$ and ortho-H$_2^{37}$Cl$^+$ are detected in absorption towards NGC~6334I, and the $1_{11}-0_{00}$ transition of para-H$_2^{35}$Cl$^+$ is detected in absorption towards NGC~6334I and Sgr~B2(S). The H$_2$Cl$^+$ column densities are compared to those of the chemically-related species HCl. The derived HCl/H$_2$Cl$^+$ column density ratios, $\sim$1--10, are within the range predicted by models of diffuse and dense Photon Dominated Regions (PDRs). However, the observed H$_2$Cl$^+$ column densities, in excess of $10^{13}$~cm$^{-2}$, are significantly higher than the model predictions. Our observations demonstrate the outstanding spectroscopic capabilities of HIFI for detecting new interstellar molecules and providing key constraints for astrochemical models.

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arXiv2010

Sensitivity analyses of dense cloud chemical models

Because of new telescopes that will dramatically improve our knowledge of the interstellar medium, chemical models will have to be used to simulate the chemistry of many regions with diverse properties. To make these models more robust, it is important to understand their sensitivity to a variety of parameters. In this article, we report a study of the sensitivity of a chemical model of a cold dense core, with homogeneous and time-independent physical conditions, to variations in the following parameters: initial chemical inventory, gas temperature and density, cosmic-ray ionization rate, chemical reaction rate coefficients, and elemental abundances. From the results of the parameter variations, we can quantify the sensitivity of the model to each parameter as a function of time. Our results can be used in principle with observations to constrain some parameters for different cold clouds. We also attempted to use the Monte Carlo approach with all parameters varied collectively. Within the parameter ranges studied, the most critical parameters turn out to be the reaction rate coefficients at times up to 4e5 yr and elemental abundances at later times. At typical times of best agreement with observation, models are sensitive to both of these parameters. The models are less sensitive to other parameters such as the gas density and temperature. The improvement of models will require that the uncertainties in rate coefficients of important reactions be reduced. As the chemistry becomes better understood and more robust, it should be possible to use model sensitivities concerning other parameters, such as the elemental abundances and the cosmic ray ionization rate, to yield detailed information on cloud properties and history. Nevertheless, at the current stage, we cannot determine the best values of all the parameters simultaneously based on purely observational constraints.

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arXiv2009

The ionization fraction gradient across the Horsehead edge: An archetype for molecular clouds

The ionization fraction plays a key role in the chemistry and dynamics of molecular clouds. We study the H13CO+, DCO+ and HOC+ line emission towards the Horsehead, from the shielded core to the UV irradiated cloud edge, i.e., the Photodissociation Region (PDR), as a template to investigate the ionization fraction gradient in molecular clouds. We analyze a PdBI map of the H13CO+ J=1-0 line, complemented with IRAM-30m H13CO+ and DCO+ higher-J line maps and new HOC+ and CO+ observations. We compare self-consistently the observed spatial distribution and line intensities with detailed depth-dependent predictions of a PDR model coupled with a nonlocal radiative transfer calculation. The chemical network includes deuterated species, 13C fractionation reactions and HCO+/HOC+ isomerization reactions. The role of neutral and charged PAHs in the cloud chemistry and ionization balance is investigated. The detection of HOC+ reactive ion towards the Horsehead PDR proves the high ionization fraction of the outer UV irradiated regions, where we derive a low [HCO+]/[HOC+]~75-200 abundance ratio. In the absence of PAHs, we reproduce the observations with gas-phase metal abundances, [Fe+Mg+...], lower than 4x10(-9) (with respect to H) and a cosmic-rays ionization rate of zeta=(5+/-3)x10(-17) s(-1). The inclusion of PAHs modifies the ionization fraction gradient and increases the required metal abundance. The ionization fraction in the Horsehead edge follows a steep gradient, with a scale length of ~0.05 pc (or ~25''), from [e-]~10(-4) (or n_e ~ 1-5 cm(-3)) in the PDR to a few times ~10(-9) in the core. PAH^- anions play a role in the charge balance of the cold and neutral gas if substantial amounts of free PAHs are present ([PAH] >10(-8)).

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arXiv2008

On the chemistry and distribution of HOC+ in M82: More evidence for extensive PDRs

The molecular gas composition in the inner 1 kpc disk of the starburst galaxy M82 resembles that of Galactic Photon Dominated Regions (PDRs). In particular, large abundances of the reactive ions HOC+ and CO+ have been measured in the nucleus of this galaxy. To investigate the origin of the large abundances of reactive ions in M82, we have completed our previous 30m HOC+ J=1-0 observations with the higher excitation HCO+ and HOC+ J=4-3 and 3-2 rotational lines. In addition, we have obtained with the IRAM Plateau de Bure Interferometer (PdBI) a 4" resolution map of the HOC+ 1-0 emission, the first ever obtained in a Galactic or extragalactic source. Our HOC+ interferometric image shows that the emission of the HOC+ 1-0 line is mainly restricted to the nuclear disk, with the maxima towards the E. and W. molecular peaks. In addition, line excitation calculations imply that the HOC+ emission arises in dense gas. Therefore, the HOC+ emission is arising in the dense PDRs embedded in the M82 nuclear disk, rather than in the intercloud phase and/or wind. We have improved our previous chemical model of M82 by (i) using the new version of the Meudon PDR code, (ii) updating the chemical network, and (iii) considering two different types of clouds (with different thickness). Most molecular observations (HCO+, HOC+, CO+, CN, HCN, H3O+) are well explained assuming that ~ 87 % of the mass of the molecular gas is forming small clouds (Av=5 mag) while only ~ 13 % of the mass is in large molecular clouds (Av=50 mag). Such small number of large molecular clouds suggests that M82 is an old starburst, where star formation has almost exhausted the molecular gas reservoir.

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arXiv2008

Molecular hydrogen in the circumstellar environments of Herbig Ae/Be stars probed by FUSE

We observed molecular hydrogen around a sample of pre-main sequence stars in order to better characterize their gaseous CS environments. We analyzed the FUSE (Far Ultraviolet Spectroscopic Explorer) spectra of a sample of Herbig Ae/Be stars (HAeBes) covering a broad spectral range, including the main-sequence A5 star Beta-Pictoris. To better diagnose the origin of the detected H2 and its excitation conditions, we used a model of a photodissociation region. Our analysis demonstrates that the excitation of H2 is clearly different around most of the HAeBes compared to the interstellar medium. Moreover, the characteristics of H2 around Herbig Ae and Be stars give evidence for different excitation mechanisms. For the most massive stars of our sample (B8 to B2 type), the excitation diagrams are reproduced well by a model of photodissociation regions (PDR). Our results favor an interpretation in terms of large CS envelopes, remnants of the molecular clouds in which the stars were formed. On the other hand, the group of Ae stars (later than B9 type) known to possess disks is more inhomogeneous. In most cases, when CS H2 is detected, the lines of sight do not pass through the disks. The excitation conditions of H2 around Ae stars cannot be reproduced by PDR models and correspond to warm and/or hot excited media very close to the stars. In addition, no clear correlation has been found between the ages of the stars and the amount of circumstellar H2. Our results suggest structural differences between Herbig Ae and Be star environments. Herbig Be stars do evolve faster than Ae stars, and consequently, most Herbig Be stars are younger than Ae ones at the time we observe them. It is thus more likely to find remnants of their parent cloud around them.

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arXiv2008

Multi-wavelength observations of a nearby multi-phase interstellar cloud

High-resolution spectroscopic observations (UV HST/STIS and optical) are used to characterize the physical state and velocity structure of the multiphase interstellar medium seen towards the nearby (170 pc) star HD102065, located behind the tail of a cometary-shaped, infrared cirrus-cloud, in the area of interaction between the Sco-Cen OB association and the Local Bubble. We analyze interstellar components present along the line of sight by fitting multiple transitions from CO, CH, CH+, C I, S I, Fe I, Mg I, Mg II, Mn II, P II, Ni II, C II, N I, O I, Si III, C IV, and Si IV. The absorption spectra are complemented by H I, CO and C II emission-line spectra, H$_2$ column-densities derived from FUSE spectra, and IRAS images. Gas components of a wide range of temperatures and ionization states are detected along the line of sight. Most of the hydrogen column-density is in cold, diffuse, molecular gas at low LSR velocity. This gas is mixed with traces of warmer molecular gas traced by H2 in the J>2 levels, in which the observed CH+ must be formed. We also identify three distinct components of warm gas at negative velocities down to -20 km/s. The temperature and gas excitation are shown to increase with increasing velocity shift from the bulk of the gas. Hot gas at temperatures of several 10^5 K is detected in the most negative velocity component in the highly-ionized species. This hot gas is also detected in very strong lines of less-ionized species (Mg II, Si II* and C II*) for which the bulk of the gas is cooler. We relate the observational results to evidence for dynamical impact of the Sco-Cen stellar association on the nearby ISM. We propose a scenario where the cirrus cloud has been hit a few 10^5 yr ago by a supernova blast wave originating from the Lower Centaurus Crux group of the Sco-Cen association.

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arXiv2008

The role of carbon grains in the deuteration of H2

Aims: The production of molecular hydrogen and its deuterated forms onto carbonaceous dust grains is investigated in detail. The goal of this study is to estimate the importance of the chemistry occuring on grain surfaces for the deuteration of H2. Furthermore, we aim to find a robust and general surface chemical model which can be used in different astrophysical environments. Methods: Surface processes are described for the cases of graphitic and amorphous--carbon grains, where laboratory work is available. Langmuir--Hinshelwood as well as Eley--Rideal surface chemistries are included in the model and their relative contributions are highlighted. Analytic expressions are derived for H2, HD, and D2 formation efficiencies for both type of grains. Rate equations are tested against stochastic methods. Results: As expected, rate equations and stochastic methods diverge for grain sizes lower than a critical value acrit. For grain sizes below this critical value, D2 formation decreases to favour HD formation. The formation efficiencies of H2 and D2 can be calculated by adding a correction factor to the rate equations methods. We found that because of the presence of chemisorbed sites, which can store atoms to form molecules up to high grain temperatures, the formation efficiency of HD and D2 is very high compared to models where only physisorption sites are taken into account. When considering a realistic distribution of dust grains, we found that the formation rate of H2 and HD is enhanced by an order of magnitude if small grains are taken into account. The processes described in this paper, that allow a strong enhancement of the deuterated forms of molecular hydrogen, could explain the high degree of deuterium fractionation observed in protostellar environments.

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arXiv2007

A PDR-Code Comparison Study

We present a comparison between independent computer codes, modeling the physics and chemistry of interstellar photon dominated regions (PDRs). Our goal was to understand the mutual differences in the PDR codes and their effects on the physical and chemical structure of the model clouds, and to converge the output of different codes to a common solution. A number of benchmark models have been created, covering low and high gas densities and far ultraviolet intensities. The benchmark models were computed in two ways: one set assuming constant temperatures, thus testing the consistency of the chemical network and photo-processes, and a second set determining the temperature selfconsistently. We investigated the impact of PDR geometry and agreed on the comparison of results from spherical and plane-parallel PDR models. We identified a number of key processes governing the chemical network which have been treated differently in the various codes, and defined a proper common treatment. We established a comprehensive set of reference models for ongoing and future PDR model bench-marking and were able to increase the agreement in model predictions for all benchmark models significantly.

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arXiv2006

Detection of CO+ in the nucleus of M82

We present the detection of the reactive ion CO+ towards the prototypical starburst galaxy M82. This is the first secure detection of this short-lived ion in an external galaxy. Values of [CO+]/[HCO+]>0.04 are measured across the inner 650pc of the nuclear disk of M82. Such high values of the [CO+]/[HCO+] ratio had only been previously measured towards the atomic peak in the reflection nebula NGC7023. This detection corroborates that the molecular gas reservoir in the M82 disk is heavily affected by the UV radiation from the recently formed stars. Comparing the column densities measured in M82 with those found in prototypical Galactic photon-dominated regions (PDRs), we need \~20 clouds along the line of sight to explain our observations. We have completed our model of the molecular gas chemistry in the M82 nucleus. Our PDR chemical model successfully explains the [CO+]/[HCO+] ratios measured in the M~82 nucleus but fails by one order of magnitude to explain the large measured CO+ column densities (~1--4x10^{13} cm^{-2}). We explore possible routes to reconcile the chemical model and the observations.

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arXiv2005

Molecular hydrogen formation on grain surfaces

We reconsider H2 formation on grain surfaces. We develop a rate equation model which takes into account the presence of both physisorbed and chemisorbed sites on the surface, including quantum mechanical tunnelling and thermal diffusion. In this study, we took into consideration the uncertainties on the characteristics of graphitic surfaces. We calculate the H2 formation efficiency with the Langmuir Hinshelwood and Eley Rideal mechanisms, and discuss the importance of these mechanisms for a wide range of grain and gas temperatures. We also develop a Monte Carlo simulation to calculate the H2 formation efficiency and compare the results to our rate equation models. Our results are the following: (1) Depending on the barrier against chemisorption, we predict the efficiency of H2 formation for a wide range of grain and gas temperatures. (2) The Eley-Rideal mechanism has an impact on the H2 formation efficiency at high grain and gas temperatures. (3) The fact that we consider chemisorption in our model makes the rate equation and Monte Carlo approaches equivalent.

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