L'index des normes et prix agricoles donne des ordres de grandeur de marge, duree, prix d'achat, tarif, montant, seuil de rentabilite, resultats economiques... pour 70 rubriques qui visent a couvrir ...l'ensemble de l'activite agricole: aliments du betail, aliments a la ferme, amortissement, analyse financiere, approvisonnements divers, assurances, avances aux cultures, etc.
L'index des normes et prix agricoles donne des ordres de grandeur de marge, duree, prix d'achat, tarif, montant, seuil de rentabilite, resultats economiques... pour 70 rubriques qui visent a couvrir ...l'ensemble de l'activite agricole: aliments du betail, aliments a la ferme, amortissement, analyse financiere, approvisonnements divers, assurances, avances aux cultures, etc
In bright photodissociation regions (PDRs) associated to 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.
We aim at presenting a comprehensive view of the modeling of the CO rotational ladder in PDRs, including the high-J lines that trace warm molecular gas at PDR interfaces.
We observed the
CO and
CO ladders in two prototypical PDRs, the Orion Bar and NGC 7023 NW using the instruments onboard
. We also considered line emission from key species in the gas cooling of PDRs (C
, O, H
) 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 are analyzed using the Meudon PDR code.
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 J
=23 in the Orion Bar (J
=19 in NGC 7023), can only originate from small structures of typical thickness of a few 10
pc and at high thermal pressures (
~ 10
K cm
).
Compiling data from the literature, we found that the gas thermal pressure increases with the intensity of the UV radiation field given by
, following a trend in line with recent simulations of the photoevaporation of illuminated edges of molecular clouds. This relation can help rationalising the analysis of high-J CO emission in massive star formation and provides an observational constraint for models that study stellar feedback on molecular clouds.
We report the first detection of the ground-state rotational transition of the methylidyne cation CH+ towards the massive star-forming region DR 21 with the HIFI instrument onboard the Herschel ...satellite. The line profile exhibits a broad emission line, in addition to two deep and broad absorption features associated with the DR 21 molecular ridge and foreground gas. These observations allow us to determine a 12CH+J = 1–0 line frequency of ν = 835 137 ± 3 MHz, in good agreement with a recent experimental determination. We estimate the CH+ column density to be a few 1013 cm-2 in the gas seen in emission, and >1014 cm-2 in the components responsible for the absorption, which is indicative of a high line of sight average abundance CH+ /H > 1.2 × 10-8. We show that the CH+ column densities agree well with the predictions of state-of-the-art C-shock models in dense UV-illuminated gas for the emission line, and with those of turbulent dissipation models in diffuse gas for the absorption lines.
PDF