Context. In bright photodissociation regions (PDR) associated with massive star formation, the presence of dense “clumps” that are immersed in a less dense interclump medium is often proposed to ...explain the difficulty of models to account for the observed gas emission in high-excitation lines. Aims. We aim to present a comprehensive view of the modelling of the CO rotational ladder in PDRs, including the high-J lines that trace warm molecular gas at PDR interfaces. Methods. We observed the 12CO and 13CO ladders in two prototypical PDRs, the Orion Bar and NGC 7023 NW using the instruments onboard Herschel. We also considered line emission from key species in the gas cooling of PDRs (C+, O, and H2) and other tracers of PDR edges such as OH and CH+. All the intensities are collected from Herschel observations, the literature and the Spitzer archive and were analysed using the Meudon PDR code. Results. A grid of models was run to explore the parameter space of only two parameters: thermal gas pressure and a global scaling factor that corrects for approximations in the assumed geometry. We conclude that the emission in the high-J CO lines, which were observed up to Jup = 23 in the Orion Bar (Jup = 19 in NGC 7023), can only originate from small structures with typical thicknesses of a few 10−3 pc and at high thermal pressures (Pth ~ 108 K cm−3). Conclusions. Compiling data from the literature, we find that the gas thermal pressure increases with the intensity of the UV radiation field given by G0, following a trend in line with recent simulations of the photoevaporation of illuminated edges of molecular clouds. This relation can help to rationalise the analysis of high-J CO emission in massive star formation and provides an observational constraint for models which study stellar feedback on molecular clouds.
Context. It has been found from ISO, Spitzer, and Herschel observations that molecular hydrogen, H2, can form on warm grains. Numerical models of interstellar chemistry have failed to reproduce the ...observed formation rates of H2, which remains a difficulty when interpreting observations of photon-dominated regions (PDRs). Aims: We attempt to include as much experimental and theoretical information as possible to describe H2 formation in astrophysical environments to solve this problem. Methods: We modified our "Meudon PDR code" to include a detailed treatment of H2 formation mechanisms including: i) the Langmuir-Hinshelwood mechanism taking into account the contribution of the different sizes of dust grains in the diffusion processes; and ii) the Eley-Rideal mechanism. Results: We are able to form H2 even in regions where the dust temperature is higher than 25 K. We also show that formation by the Eley-Rideal mechanism can be a significant source of gas heating. We derive line intensities for various astrophysical conditions. Conclusions: Our approach results in a higher H2 formation rate than for the "standard" 3 × 10-17 nH n(H) cm3 s-1 expression.
Compared to the well-established powder bed fusion techniques, sinter-based additive manufacturing of titanium alloys remains extremely challenging. This technique involves three steps: (i) shaping ...of a part composed of metallic powders bound with polymeric binder (ii) debinding (iii) sintering. One main issue is that densification during the solid sintering is promoted by small powder particles whereas the latter have a high propensity to carbon and oxygen uptakes from the binder, which are detrimental to ductility of titanium parts. In this article, we report a unique in-depth characterization of solid sintering of titanium powders using in situ coupled micro-computed tomography (µCT) and X-Ray diffraction under synchrotron radiation at high temperature, combined with in situ environmental scanning electron microscopy (HT-eSEM). Evolution of global porosity, pore size distribution and interconnectivity as well as allotropic titanium phase transformation and precipitation of second-phase precipitates (titanium carbides) were determined, allowing a discussion on the densification/phase transformation relationship. This multi-scale in situ study of solid sintering was used to identify the effect of powder particle size on the contamination / densification trade-off. Carbon/oxygen uptakes clearly increase the α-to-β transus but do not affect significantly the final porosity of the sintered parts, which argues for a secondary role of the β phase on the sintering kinetics of titanium alloys. Reducing the powder particle size has a tremendous effect on both the densification kinetics and the final pores structure.
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The dislocation densities were measured on the same samples using transmission electron microscopy (TEM), scanning electron microscopy (electron channeling contrast imaging (ECCI) and ...high-angular-resolution-electron backscattered diffraction (HR-EBSD)), and X-ray diffraction (XRD). Notably, these different methods do not observe the same types of dislocations, i.e., statistically stored dislocations (SSDs) and/or geometrically necessary dislocations (GNDs). ECCI and TEM imaging are direct-measurement techniques, whereas HR-EBSD and XRD are indirect methods. Therefore, a quantitative comparison of the measurements obtained using these four techniques on undeformed and deformed duplex steels is proposed. For low deformation, where the dislocation density is quite small (1 − 5 × 1013 m−2), imaging methods are rather performant, whereas XRD measurements suffer from high uncertainty levels. HR-EBSD measurements show results that are in good agreement with the other methods for these deformation levels. For higher deformation levels (with dislocation densities above 1 − 3 × 1014 m−2), imaging methods are no longer relevant because of the increasing uncertainty arising from local contrast variation and overlapping of dislocations. The different results obtained highlight the necessity of taking a step back on each method used. Correctly defining what is to be measured (SSDs or GNDs), in which condition (solid material or thin plate) as well as the parameters (pixel size, area, etc.) and their bias is essential, especially if the objective is to use the measurement in a micromechanical model.
•Different measurement techniques lead to different dislocation density measurement.•XRD measurement are more adequate for high density values (>1014m−2).•Imaging techniques are more adequate for densities less than(<1014–15 m−2.•EBSD/HR-EBSD are the most versatile techniques and accurate for the larger range.•Combining EBSD/HR-EBSD and ECCI seems ideal to describe a dislocation population.
We present the revised "Meudon" model of photon-dominated region (PDR) code, available on the Web under the GNU Public License. General organization of the code is described down to a level that ...should allow most observers to use it as an interpretation tool with minimal help from our part. Two grids of models, one for low-excitation diffuse clouds and one for dense highly illuminated clouds, are discussed, and some new results on PDR modelization highlighted.
Context.
Massive stars form within dense clumps inside giant molecular clouds (GMCs). Finding appropriate chemical tracers of the dense gas (
n
(H
2
) > several 10
4
cm
−3
or
A
V
> 8 mag) and linking ...their line luminosity with the star formation rate is of critical importance.
Aims.
Our aim is to determine the origin and physical conditions of the HCN-emitting gas and study their relation to those of other molecules.
Methods.
In the context of the IRAM 30m ORION-B large program, we present 5 deg
2
(~250 pc
2
) HCN, HNC, HCO
+
, and CO
J
=1–0 maps of the Orion B GMC, complemented with existing wide-field C
I
492 GHz maps, as well as new pointed observations of rotationally excited HCN, HNC, H
13
CN, and HN
13
C lines. We compare the observed HCN line intensities with radiative transfer models including line overlap effects and electron excitation. Furthermore, we study the HCN/HNC isomeric abundance ratio with updated photochemical models.
Results.
We spectroscopically resolve the HCN
J
= 1–0 hyperfine structure (HFS) components (and partially resolved
J
= 2−1 and 3−2 components). We detect anomalous HFS line intensity (and line width) ratios almost everywhere in the cloud. About 70% of the total HCN
J
= 1−0 luminosity,
L
′(HCN
J
= 1−0) = 110 K km s
−1
pc
−2
, arises from
A
V
< 8 mag. The HCN/CO
J
= 1−0 line intensity ratio, widely used as a tracer of the dense gas fraction, shows a bimodal behavior with an inflection point at
A
V
< 3 mag typical of translucent gas and illuminated cloud edges. We find that most of the HCN
J
= 1−0 emission arises from extended gas with
n
(H
2
) < 10
4
cm
−3
, and even lower density gas if the ionization fraction is χ
e
≥ 10
−5
and electron excitation dominates. This result contrasts with the prevailing view of HCN
J
= 1−0 emission as a tracer of dense gas and explains the low-
A
V
branch of the HCN/CO
J
= 1−0 intensity ratio distribution. Indeed, the highest HCN/CO ratios (~ 0.1) at
A
V
< 3 mag correspond to regions of high C
I
492 GHz/CO
J
= 1−0 intensity ratios (>1) characteristic of low-density photodissociation regions. The low surface brightness (≲ 1 K km s
−1
) and extended HCN and HCO
+
J
= 1−0 emission scale with
I
FIR
– a proxy of the stellar far-ultraviolet (FUV) radiation field – in a similar way. Together with CO
J
= 1−0, these lines respond to increasing
I
FIR
up to
G
0
≃ 20. On the other hand, the bright HCN
J
= 1−0 emission (> 6 K km s
−1
) from dense gas in star-forming clumps weakly responds to
I
FIR
once the FUV field becomes too intense (
G
0
> 1500). In contrast, HNC
J
= 1−0 and C
I
492 GHz lines weakly respond to
I
FIR
for all
G
0
. The different power law scalings (produced by different chemistries, densities, and line excitation regimes) in a single but spatially resolved GMC resemble the variety of Kennicutt-Schmidt law indexes found in galaxy averages.
Conclusions.
Given the widespread and extended nature of the C
I
492 GHz emission, as well as its spatial correlation with that of HCO
+
, HCN, and
13
CO
J
= 1−0 lines (in this order), we argue that the edges of GMCs are porous to FUV radiation from nearby massive stars. Enhanced FUV radiation favors the formation and excitation of HCN on large scales, not only in dense star-forming clumps, and it leads to a relatively low value of the dense gas mass to total luminosity ratio,
α
(HCN) = 29
M
⊙
/(K km s
−1
pc
2
) in Orion B. As a corollary for extragalactic studies, we conclude that high HCN/CO
J
= 1−0 line intensity ratios do not always imply the presence of dense gas, which may be better traced by HNC than by HCN.
Aims.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. Methods. A number of benchmark models have been created, covering low and high gas densities $n = 10^3,10^{5.5}$ cm-3 and far ultraviolet intensities χ = 10, 105 in units of the Draine field (FUV: 6 < $h\,\nu$ < 13.6 eV). 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 self consistently by solving the thermal balance, thus testing the modeling of the heating and cooling mechanisms accounting for the detailed energy balance throughout the clouds. Results.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 such as the effect of PAHs on the electron density or the temperature dependence of the dissociation of CO by cosmic ray induced secondary photons, 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. Nevertheless, the remaining spread in the computed observables such as the atomic fine-structure line intensities serves as a warning that there is still a considerable uncertainty when interpreting astronomical data with our models.
Context. The early stages of low-mass star formation are likely to be subject to intense ionization by protostellar energetic MeV particles. As a result, the surrounding gas is enriched in molecular ...ions, such as HCO+ and N2H+. Nonetheless, this phenomenon remains poorly understood for Class 0 objects. Recently, based on Herschel observations taken as part of the key programme Chemical HErschel Surveys of Star forming regions (CHESS), a very low HCO+/N2H+ abundance ratio of about three to four, has been reported towards the protocluster OMC-2 FIR4. This finding suggests a cosmic-ray ionization rate in excess of 10-14 s-1, much higher than the canonical value of ζ = 3 × 10-17 s-1 (value expected in quiescent dense clouds). Aims. We aim to assess the specificity of OMC-2 FIR4, we have extended this study to a sample of sources in low- and intermediate mass. More specifically, we seek to measure the HCO+/N2H+ abundance ratio from high energy lines (J ≥ 6) towards this source sample in order to infer the flux of energetic particles in the warm and dense gas surrounding the protostars. Methods. We have used observations performed with the Heterodyne Instrument for the Far-Infrared spectrometer on board the Herschel Space Observatory towards a sample of nine protostars. Results. We report HCO+/N2H+ abundance ratios in the range of five up to 73 towards our source sample. The large error bars do not allow us to conclude whether OMC-2 FIR4 is a peculiar source. Nonetheless, an important result is that the measured HCO+/N2H+ ratio does not vary with the source luminosity. At the present time, OMC-2 FIR4 remains the only source where a high flux of energetic particles is clearly evident. More sensitive and higher angular resolution observations are required to further investigate this process.
Aims. We present spectroscopic observations obtained with the infrared Spitzer Space Telescope, which provide insight into the H2 physics and gas energetics in photodissociation regions (PDRs) of low ...to moderate far-ultraviolet (FUV) fields and densities. Methods. We analyze data on six well known Galactic PDRs (L1721, California, N7023E, Horsehead, rho Oph, N2023N), sampling a poorly explored range of excitation conditions (χ ~ 5−103), relevant to the bulk of molecular clouds in galaxies. Spitzer observations of H2 rotational lines are complemented with H2 data, including ro-vibrational line measurements, obtained with ground-based telescopes and ISO, to constrain the relative contributions of ultraviolet pumping and collisions to the H2 excitation. The data analysis is supported by model calculations with the Meudon PDR code. Results. The observed column densities of rotationally excited H2 are observed to be much higher than PDR model predictions. In the lowest excitation PDRs, the discrepancy between the model and the data is about one order of magnitude for rotational levels J ≥ 3. We discuss whether an enhancement in the H2 formation rate or a local increase in photoelectric heating, as proposed for brighter PDRs in former ISO studies, may improve the data-model comparison. We find that an enhancement in the H2 formation rates reduces the discrepancy, but the models still fall short of the data. Conclusions. This large disagreement suggests that our understanding of the formation and excitation of H2 and/or of PDRs energetics is still incomplete. We discuss several explanations, which could be further tested using the Herschel Space Telescope.
Context.FUV radiation strongly affects the physical and chemical state of molecular clouds, from protoplanetary disks to entire galaxies. Aims.The solution of the FUV radiative transfer equation can ...be complicated if the most relevant radiative processes such us dust scattering and gas line absorption are included, and have realistic (non-uniform) properties, i.e. if optical properties are depth dependent. Methods.We have extended the spherical harmonics method to solve for the FUV radiation field in externally or internally illuminated clouds taking into account gas absorption and coherent, nonconservative and anisotropic scattering by dust grains. The new formulation has been implemented in the Meudon PDR code and thus it will be publicly available. Results.Our formalism allows us to consistently include: $(i)$ varying dust populations and $(ii)$ gas lines in the FUV radiative transfer. The FUV penetration depth rises for increasing dust albedo and anisotropy of the scattered radiation (e.g. when grains grow towards cloud interiors). Conclusions.Illustrative models of illuminated clouds where only the dust populations are varied confirm earlier predictions for the FUV penetration in diffuse clouds (AV < 1). For denser and more embedded sources (AV > 1) we show that the FUV radiation field inside the cloud can differ by orders of magnitude depending on the grain properties and growth. Our models reveal significant differences regarding the resulting physical and chemical structures for steep vs. flat extinction curves towards molecular clouds. In particular, we show that the photochemical and thermal gradients can be very different depending on grain growth. Therefore, the assumption of uniform dust properties and averaged extinction curves can be a crude approximation to determine the resulting scattering properties, prevailing chemistry and atomic/molecular abundances in ISM clouds or protoplanetary disks.
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