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تسجيل مستخدم جديدObserving a column-dependent zeta in dense interstellar sources: the case of the Horsehead Nebula
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Context: Observations of small carbon-bearing molecules such as CCH, C4H, c-C3H2, and HCO in the Horsehead Nebula have shown these species to have higher abundances towards the edge of the source than towards the center. Aims: Given the determination of a wide range of values for zeta (s-1), the total ionization rate of hydrogen atoms, and the proposal of a column-dependent zeta(N_H), where N_H is the total column of hydrogen nuclei, we desire to determine if the effects of zeta(N_H) in a single object with spatial variation can be observable. We chose the Horsehead Nebula because of its geometry and high density. Method: We model the Horsehead Nebula as a near edge-on photon dominated region (PDR), using several choices for zeta, both constant and as a function of column. The column-dependent zeta functions are determined by a Monte Carlo model of cosmic ray penetration, using a steep power-law spectrum and accounting for ionization and magnetic field effects. We consider a case with low-metal elemental abundances as well as a sulfur-rich case. Results: We show that use of a column-dependent zeta(N_H) of 5(-15) s-1 at the surface and 7.5(-16) s-1 at Av = 10 on balance improves agreement between measured and theoretical molecular abundances, compared with constant values of zeta.
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Micro-physical processes on interstellar dust surfaces are tightly connected to dust properties (i.e. dust composition, size and shape) and play a key role in numerous phenomena in the interstellar medium (ISM). The large disparity in physical condit
ions (i.e. density, gas temperature) in the ISM triggers an evolution of dust properties. The analysis of how dust evolves with the physical conditions is a stepping-stone towards a more thorough understanding of interstellar dust. The aim of this paper is to highlight dust evolution in the Horsehead Nebula PDR region. We use Spitzer/IRAC (3.6, 4.5, 5.8 and 8 {mu}m), Spitzer/MIPS (24 {mu}m) together with Herschel/PACS (70 and 160 {mu}m) and Herschel/SPIRE (250, 350 and 500 {mu}m) to map the spatial distribution of dust in the Horsehead over the entire emission spectral range. We model dust emission and scattering using the THEMIS interstellar dust model together with the 3D radiative transfer code SOC. We find that the nano-grains dust-to-gas ratio in the irradiated outer part of the Horsehead is 6 to 10 times lower than in the diffuse ISM. Their minimum size is 2 to 2.25 times larger than in the diffuse ISM and the power-law exponent of their size distribution, 1.1 to 1.4 times lower than in the diffuse ISM. Regarding the denser part of the Horsehead, it is necessary to use evolved grains (i.e. aggregates, with or without an ice mantle). It is not possible to explain the observations using grains from the diffuse medium. We therefore propose the following scenario to explain our results. In the outer part of the Horsehead, all the nano-grains have not yet had time to re-form completely through photo-fragmentation of aggregates and the smallest of the nano-grains that are sensitive to the radiation field are photo-destroyed. In the inner part of the Horsehead, grains most likely consist of multi-compositional, mantled aggregates.
Aims. Our goal is to complete the inventory of S-bearing molecules and their abundances in the prototypical photodissociation region (PDR) the Horsehead nebula to gain insight into sulphur chemistry in UV irradiated regions. Based on the WHISPER mill
imeter (mm) line survey, our goal is to provide an improved and more accurate description of sulphur species and their abundances towards the core and PDR positions in the Horsehead. Methods. The Monte Carlo Markov chain (MCMC) methodology and the molecular excitation and radiative transfer code RADEX were used to explore the parameter space and determine physical conditions and beam-averaged molecular abundances. Results. A total of 13 S-bearing species (CS, SO, SO2, OCS, H2CS - both ortho and para - HDCS, C2S, HCS+, SO+, H2S, S2H, NS and NS+) have been detected in the two targeted positions. This is the first detection of SO+ in the Horsehead and the first detection of NS+ in any PDR. We find a differentiated chemical behaviour between C-S and O-S bearing species within the nebula. The C-S bearing species C2S and o-H2CS present fractional abundances a factor grater than two higher in the core than in the PDR. In contrast, the O-S bearing molecules SO, SO2, and OCS present similar abundances towards both positions. A few molecules, SO+, NS, and NS+, are more abundant towards the PDR than towards the core, and could be considered as PDR tracers. Conclusions. This is the first complete study of S-bearing species towards a PDR. Our study shows that CS, SO, and H2S are the most abundant S-bearing molecules in the PDR with abundances of a few 1E-9. We recall that SH, SH+, S, and S+ are not observable at the wavelengths covered by the WHISPER survey. At the spatial scale of our observations, the total abundance of S atoms locked in the detected species is < 1E-8, only ~0.1% of the cosmic sulphur abundance.
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Using large scale maps in C18O(2-1) and in the continuum at 1.2mm obtained at the IRAM-30m antenna with the Heterodyne Receiver Array (HERA) and MAMBO2, we investigated the morphology and the velocity field probed in the inner layers of the Horsehead
nebula. The data reveal a non--self-gravitating (m/mvir = 0.3) filament of dust and gas (the neck, diameter = 0.15-0.30 pc) connecting the Horsehead western ridge, a Photon-Dominated Region illuminated by sigmaOri, to its parental cloud L1630. Several dense cores are embedded in the ridge and the neck. One of these cores appears particularly peaked in the 1.2 mm continuum map and corresponds to a feature seen in absorption on ISO maps around 7 micr. Its cdo emission drops at the continuum peak, suggestive of molecular depletion onto cold grains. The channel maps of the Horsehead exhibit an overall north-east velocity gradient whose orientation swivels east-west, showing a somewhat more complex structure than was recently reported by cite{pound03} using BIMA CO(1-0) mapping. In both the neck and the western ridge, the material is rotating around an axis extending from the PDR to L1630 (angular velocity=1.5-4.0 km/s). Moreover, velocity gradients along the filament appear to change sign regularly (3 km/s/pc, period=0.30 pc) at the locations of embedded integrated intensity peaks. The nodes of this oscillation are at the same velocity. Similar transverse cuts across the filament show a sharp variation of the angular velocity in the area of the main dense core. The data also suggest that differential rotation is occurring in parts of the filament. We present a new scenario for the formation and evolution of the nebula and discuss dense core formation inside the filament.
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