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تسجيل مستخدم جديدShocks and UV radiation around low-mass protostars: the Herschel-PACS legacy
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Far-infrared spectroscopy reveals gas cooling and its underlying heating due to physical processes taking place in the surroundings of protostars. These processes are reflected in both the chemistry and excitation of abundant molecular species. Here, we present the Herschel-PACS far-IR spectroscopy of 90 embedded low-mass protostars from the WISH (van Dishoeck et al. 2011), DIGIT (Green et al. 2013), and WILL surveys (Mottram et al. 2017). The $5times5$ spectra covering the $sim50times50$ field-of-view include rotational transitions of CO, H$_2$O, and OH lines, as well as fine-structure [O I] and [C II] in the $sim$50-200 $mu$m range. The CO rotational temperatures (for $J_mathrm{u}geq14)$ are typically $sim$300 K, with some sources showing additional components with temperatures as high as $sim$1000 K. The H$_2$O / CO and H$_2$O / OH flux ratios are low compared to stationary shock models, suggesting that UV photons may dissociate some H$_2$O and decrease its abundance. Comparison to C shock models illuminated by UV photons shows good agreement between the line emission and the models for pre-shock densities of $10^5$ cm$^{-3}$ and UV fields 0.1-10 times the interstellar value. The far-infrared molecular and atomic lines are the unique diagnostic of shocks and UV fields in deeply-embedded sources.
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Recent observations from Herschel allow the identification of important mechanisms responsible for the heating of gas surrounding low-mass protostars and its subsequent cooling in the far-infrared (FIR). Shocks are routinely invoked to reproduce some
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We present 70 and 160 micron Herschel science demonstration images of a field in the Orion A molecular cloud that contains the prototypical Herbig-Haro objects HH 1 and 2, obtained with the Photodetector Array Camera and Spectrometer (PACS). These ob
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significant sample of deeply-embedded low-mass young stellar objects in order to characterize shocks and the possible role of ultraviolet radiation in the immediate protostellar environment. Herschel/PACS spectral maps of 22 objects in the Perseus molecular cloud were obtained as part of the `William Herschel Line Legacy survey. Line emission from H$_mathrm{2}$O, CO, and OH is tested against shock models from the literature. Observed line ratios are remarkably similar and do not show variations with source physical parameters. Observations show good agreement with the shock models when line ratios of the same species are compared. Ratios of various H$_mathrm{2}$O lines provide a particularly good diagnostic of pre-shock gas densities, $n_mathrm{H}sim10^{5}$ cm$^{-3}$, in agreement with typical densities obtained from observations of the post-shock gas. The corresponding shock velocities, obtained from comparison with CO line ratios, are above 20 km,s$^{-1}$. However, the observations consistently show one-to-two orders of magnitude lower H$_mathrm{2}$O-to-CO and H$_mathrm{2}$O-to-OH line ratios than predicted by the existing shock models. The overestimated model H$_mathrm{2}$O fluxes are most likely caused by an overabundance of H$_mathrm{2}$O in the models since the excitation is well-reproduced. Illumination of the shocked material by ultraviolet photons produced either in the star-disk system or, more locally, in the shock, would decrease the H$_mathrm{2}$O abundances and reconcile the models with observations. Detections of hot H$_mathrm{2}$O and strong OH lines support this scenario.
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