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Register a new userSpatial variation of the cooling lines in the Orion Bar from Herschel/PACS
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Jeronimo Bernard-Salas
Publication date
2011
fields
Physics
and research's language is
English
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We present spatially resolved Herschel/PACS observations of the Orion Bar. We have characterise the emission of the far-infrared fine-structure lines of [CII] (158um), [OI] (63 and 145um), and [NII] (122um) that trace the gas local conditions. The observed distribution and variation of the lines are discussed in relation to the underlying geometry and linked to the energetics associated with the Trapezium stars. These observations enable us to map the spatial distribution of these fine-structure lines with a spatial resolution between 4 and 11 and covering a total square area of about 120x105. The spatial profile of the emission lines are modelled using the radiative transfer code Cloudy. We find that the spatial distribution of the [CII] line coincides with that of the [OI] lines. The [NII] line peaks closer to the ionising star than the other three lines, but with a small region of overlap. We can distinguish several knots of enhanced emission within the Bar indicating the presence of an inhomogenous and structured medium. The emission profiles cannot be reproduced by a single photo-dissociation region, clearly indicating that, besides the Bar, there is a significant contribution from additional photo-dissociation region(s) over the area studied. The combination of both the [NII] and [OI] 145um lines can be used to estimate the [CII] emission and distinguish between its ionised or neutral origin. We have calculated how much [CII] emission comes from the neutral and ionised region, and find that at least 82% originates from the photo-dissocciation region. Together, the [CII] 158um and [OI] 63 and 145um lines account for 90% of the power emitted by the main cooling lines in the Bar (including CO, H2, etc...), with [OI] 63um alone accounting for 72% of the total.
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Context: The north-west photo-dissociation region (PDR) in the reflection nebula NGC 7023 displays a complex structure. Filament-like condensations at the edge of the cloud can be traced via the emission of the main cooling lines, offering a great opportunity to study the link between the morphology and energetics of these regions. Aims: We study the spatial variation of the far-infrared fine-structure lines of [C II] (158 um) and [O I] (63 and 145 um). These lines trace the local gas conditions across the PDR. Methods: We used observations from the Herschel/PACS instrument to map the spatial distribution of these fine-structure lines. The observed region covers a square area of about 110 x 110 with an angular resolution that varies from 4 to 11. We compared this emission with ground-based and Spitzer observations of H2 lines, Herschel/SPIRE observations of CO lines, and Spitzer/IRAC 3.6 um images that trace the emission of polycyclic aromatic hydrocarbons. Results: The [C II] (158 um) and [O I] (63 and 145 um) lines arise from the warm cloud surface where the PDR is located and the gas is warm, cooling the region. We find that although the relative contribution to the cooling budget over the observed region is dominated by [O I]63 um (>30%), H2 contributes significantly in the PDR (35%), as does [C II]158 um outside the PDR (30%). Other species contribute little to the cooling ([O I]145 um 9%, and CO 4%). The [O I] maps resolve these condensations into two structures and show that the peak of [O I] is slightly displaced from the molecular H2 emission. The size of these structures is about 8 (0.015 pc) and in surface cover about 9% of the PDR emission. Finally, we did not detect emission from [N II]122 um, suggesting that the cavity is mostly filled with non-ionised gas.
We report Herschel/PACS photometric observations at 70 {mu}m and 160 {mu}m of LRLL54361 - a suspected binary protostar that exhibits periodic (P=25.34 days) flux variations at shorter wavelengths (3.6 {mu}m and 4.5 {mu}m) thought to be due to pulsed accretion caused by binary motion. The PACS observations show unprecedented flux variation at these far-infrared wavelengths that are well cor- related with the variations at shorter wavelengths. At 70 {mu}m the object increases its flux by a factor of six while at 160{mu}m the change is about a factor of two, consistent with the wavelength dependence seen in the far-infrared spectra. The source is marginally resolved at 70 {mu}m with varying FWHM. Deconvolved images of the sources show elongations exactly matching the outflow cavities traced by the scattered light observations. The spatial variations are anti-correlated with the flux variation indicating that a light echo is responsible for the changes in FWHM. The observed far-infrared flux variability indicates that the disk and en- velope of this source is periodically heated by the accretion pulses of the central source, and suggests that such long wavelength variability in general may provide a reasonable proxy for accretion variations in protostars.
We report the results of a search for molecular oxygen (O2) toward the Orion Bar, a prominent photodissociation region at the southern edge of the HII region created by the luminous Trapezium stars. We observed the spectral region around the frequency of the O2 N_J = 3_3 - 1_2 transition at 487 GHz and the 5_4 - 3_4 transition at 774 GHz using the Heterodyne Instrument for the Far Infrared on the Herschel Space Observatory. Neither line was detected, but the 3sigma upper limits established here translate to a total line-of-sight O2 column density < 1.5 10^16 cm^-2 for an emitting region whose temperature is between 30K and 250 K, or < 1 10^16 cm^-2 if the O2 emitting region is primarily at a temperature of ~< 100 K. Because the Orion Bar is oriented nearly edge-on relative to our line of sight, the observed column density is enhanced by a factor estimated to be between 4 and 20 relative to the face-on value. Our upper limits imply that the face-on O2 column density is less than 4 10^15 cm^-2, a value that is below, and possibly well below, model predictions for gas with a density of 10^4 - 10^5 cm^-3 exposed to a far ultraviolet flux 10^4 times the local value, conditions inferred from previous observations of the Orion Bar. The discrepancy might be resolved if: (1) the adsorption energy of O atoms to ice is greater than 800 K; (2) the total face-on Av of the Bar is less than required for O2 to reach peak abundance; (3) the O2 emission arises within dense clumps with a small beam filling factor; or, (4) the face-on depth into the Bar where O2 reaches its peak abundance, which is density dependent, corresponds to a sky position different from that sampled by our Herschel beams.
The abundance of CH+ and OH and excitation are predicted to be enhanced by the presence of vibrationally excited H2 or hot gas (~500-1000 K) in PDRs with high incident FUV radiation field. The excitation may also originate in dense gas (>10^5 cm-3) followed by nonreactive collisions. Previous observations suggest that the CH+ and OH correlate with dense and warm gas, and formation pumping contributes to CH+ excitation. We examine the spatial distribution of the CH+ and OH emission in the Orion Bar to establish their physical origin and main formation and excitation mechanisms. We present spatially sampled maps of the CH+ J=3-2 transition at 119.8 {mu}m and the OH {Lambda}-doublet at 84 {mu}m in the Orion Bar over an area of 110x110 with Herschel (PACS). We compare the spatial distribution of these molecules with those of their chemical precursors, C+, O and H2, and tracers of warm and dense gas. We assess the spatial variation of CH+ J=2-1 velocity-resolved line profile observed with Herschel (HIFI). The OH and CH+ lines correlate well with the high-J CO emission and delineate the warm and dense molecular region. While similar, the differences in the CH+ and OH morphologies indicate that CH+ formation and excitation are related to the observed vibrationally excited H2. This indicates that formation pumping contributes to the excitation of CH+. Interestingly, the peak of the rotationally excited OH 84 {mu}m emission coincides with a bright young object, proplyd 244-440, which shows that OH can be an excellent tracer of UV-irradiated dense gas. The spatial distribution of CH+ and OH revealed in our maps is consistent with previous modeling studies. Both formation pumping and nonreactive collisions in a UV-irradiated dense gas are important CH+ J=3-2 excitation processes. The excitation of the OH {Lambda}-doublet at 84 {mu}m is mainly sensitive to the temperature and density.
Using the Texas Echelon Cross Echelle Spectrograph (TEXES) we mapped emission in the H_2 v = 0-0 S(1) and S(2) lines toward the Orion Bar PDR at 2 resolution. We also observed H_2 v = 0-0 S(4) at selected points toward the front of the PDR. Our maps cover a 12 by 40 region of the bar where H_2 ro-vibrational lines are bright. The distributions of H_2 0-0 S(1), 0-0 S(2), and 1-0 S(1) line emission agree in remarkable detail. The high spatial resolution (0.002 pc) of our observations allows us to probe the distribution of warm gas in the Orion Bar to a distance approaching the scale length for FUV photon absorption. We use these new observational results to set parameters for the PDR models described in a companion paper (Draine et al. 2005, in prep). The best-fit model can account for the separation of the H_2 emission from the ionization front and the intensities of the ground state rotational lines as well as the 1-0 S(1) and 2-1 S(1) lines. This model requires significant adjustments to the commonly used values for the dust UV attenuation cross section and the photoelectric heating rate.
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