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Register a new userPDR Emission from the Arched-Filaments and Nearby Positions
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We investigate the physical conditions of the gas, atomic and molecular, in the filaments in the context of Photo-Dissociation Regions (PDRs) using the KOSMA-PDR mode of clumpy clouds. We also compare the [CII] vs. [NII] integrated intensity predictions in Abel et al. 2005 for HII regions and adjacent PDRs in the Galactic disk, and check for their applicability under the extreme physical conditions present in the GC. Our preliminary results show that observed integrated intensities are well reproduced by the PDR model. The gas is exposed to a relatively low Far-UV field between 10^2 - 10^3 Draine fields. The total volume hydrogen density is well constrained between 10^4 - 10^5 cm^-3. The hydrogen ionization rate due to cosmic-rays varies between 10^-15 and 4x10^-15 s^-1, with the highest value ~ 10^-14 s^-1 found towards G0.07+0.04. Our results show that the line-of-sight contribution to the total distance of the filaments to the Arches Cluster is not negligible. The spatial distribution of the [CII]/[NII] ratio shows that the integrated intensity ratios are fairly homogeneously distributed for values below 10 in energy units. Calculations including variation on the [C/N] abundance ratio show that tight constraints on this ratio are needed to reproduce the observations.
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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.
We present the results of data analysis of the [CI] ($^{3}P_{1}$-$^{3}P_{0}$) emission from the $rho$ Ophiuchi A photon-dominated region (PDR) obtained in the ALMA ACA stand-alone mode with a spatial resolution of 2.6 (360 au). The [CI] emission shows filamentary structures with a width of $sim$1000 au, which are adjacent to the shell structure seen in the 4.5 $mu$m map. We found that the 4.5 $mu$m emission, C$^0$, and CO are distributed in this order from the excitation star (S1) in a complementary pattern. These results indicate that [CI] is emitted from a thin layer in the PDR generated by the excitation star, as predicted in the plane-parallel PDR model. In addition, extended [CI] emission was also detected, which shows nearly uniform integrated intensity over the entire field-of-view (1.6$times$1.6). The line profile of the extended component is different from that of the above shell component. The column density ratio of C$^0$ to CO in the extended component was $sim$2, which is significantly higher than those of Galactic massive star-forming regions (0.1-0.2). These results suggest that [CI] is emitted also from the extended gas with a density of $n_mathrm{H_2} sim 10^3$ cm$^{-3}$, which is not greatly affected by the excitation star.
Recent Herschel and ALMA observations of Photodissociation Regions (PDRs) have revealed the presence of a high thermal pressure (P ~ 10^7-10^8 K cm-3) thin compressed layer at the PDR surface where warm molecular tracer emission (e.g. CH+, SH+, high-J CO, H2,...) originate. These high pressures (unbalanced by the surrounding environment) and a correlation between pressure and incident FUV field (G0) seem to indicate a dynamical origin with the radiation field playing an important role in driving the dynamics. We investigate whether photoevaporation of the illuminated edge of a molecular cloud could explain these high pressures and pressure-UV field correlation. We developed a 1D hydrodynamical PDR code coupling hydrodynamics, EUV and FUV radiative transfer and time-dependent thermo-chemical evolution. We applied it to a 1D plane-parallel photoevaporation scenario where a UV-illuminated molecular cloud can freely evaporate in a surrounding low-pressure medium. We find that photoevaporation can produce high thermal pressures and the observed P-G0 correlation, almost independently from the initial gas density. In addition, we find that constant-pressure PDR models are a better approximation to the structure of photoevaporating PDRs than constant-density PDR models, although moderate pressure gradients are present. Strong density gradients from the molecular to the neutral atomic region are found, which naturally explain the large density contrasts (1-2 orders of magnitude) derived from observations of different tracers. The photoevaporating PDR is preceded by a low velocity shock (a few km/s) propagating into the molecular cloud. Photoevaporating PDR models offer a promising explanation to the recent observational evidence of dynamical effects in PDRs.
The standard cosmological model ($Lambda$CDM) predicts the existence of the cosmic web: a distribution of matter into sheets and filaments connecting massive halos. However, observational evidence has been elusive due to the low surface brightness of the filaments. Recent deep MUSE/VLT data and upcoming observations offer a promising avenue for Ly$alpha$ detection, motivating the development of modern theoretical predictions. We use hydrodynamical cosmological simulations run with the AREPO code to investigate the potential detectability of large-scale filaments, excluding contributions from the halos embedded in them. We focus on filaments connecting massive ($M_{200c}sim(1-3)times10^{12} M_odot$) halos at z=3, and compare different simulation resolutions, feedback levels, and mock-image pixel sizes. We find increasing simulation resolution does not substantially improve detectability notwithstanding the intrinsic enhancement of internal filament structure. By contrast, for a MUSE integration of 31 hours, including feedback increases the detectable area by a factor of $simeq$5.5 on average compared with simulations without feedback, implying that even the non-bound components of the filaments have substantial sensitivity to feedback. Degrading the image resolution from the native MUSE scale of (0.2)$^2$ per pixel to (5.3)$^2$ apertures has the strongest effect, increasing the detectable area by a median factor of $simeq$200 and is most effective when the size of the pixel roughly matches the width of the filament. Finally, we find the majority of Ly$alpha$ emission is due to electron impact collisional excitations, as opposed to radiative recombination.
We report on high spatial resolution observations, using the Australia Telescope Compact Array (ATCA), of ground-state OH masers. These observations were carried out toward 196 pointing centres previously identified in the Southern Parkes Large-Area Survey in Hydroxyl (SPLASH) pilot region, between Galactic longitudes of $334^{circ}$ and $344^{circ}$ and Galactic latitudes of $-2^{circ}$ and $+2^{circ}$. Supplementing our data with data from the MAGMO (Mapping the Galactic Magnetic field through OH masers) survey, we find maser emission towards 175 of the 196 target fields. We conclude that about half of the 21 non-detections were due to intrinsic variability. Due to the superior sensitivity of the follow-up ATCA observations, and the ability to resolve nearby sources into separate sites, we have identified 215 OH maser sites towards the 175 fields with detections. Among these 215 OH maser sites, 111 are new detections. After comparing the positions of these 215 maser sites to the literature, we identify 122 (57 per cent) sites associated with evolved stars (one of which is a planetary nebula), 64 (30 per cent) with star formation, two sites with supernova remnants and 27 (13 per cent) of unknown origin. The infrared colors of evolved star sites with symmetric maser profiles tend to be redder than those of evolved star sites with asymmetric maser profiles, which may indicate that symmetric sources are generally at an earlier evolutionary stage.
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