REVIEW
Far-IR Excited OH Lines from Orion KL Outflows
Astrophysics · Submitted 3 Mar 2006 · Published version · 32 references
Pith's verdict: Genuinely new far-IR OH detections; the derived abundance is plausible but sits on a simple shell model, so treat X(OH) as a factor-of-a-few estimate.
T0 · AI review
Why Pith says this
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Referee report
The paper
The paper claims
Excited OH toward Orion KL comes from outflow gas, not the hot core.
The reading
What carries the argument
The load-bearing object is the set of far-infrared OH $\Lambda$-doublet rotational lines in the $^2\Pi_{3/2}$ and $^2\Pi_{1/2}$ ladders; a $\Lambda$-doublet is the closely spaced parity pair of each OH rotational level, and the lines connecting these levels respond differently to collisions and to radiative pumping by dust. The argument is carried by a nonlocal non-LTE radiative transfer code applied to the first twenty OH rotational levels, with the region represented as a spherical $25''$ shell expanding at 25 km s$^{-1}$ around a uniform, optically thick $10''$ core at 150 K. In that geometry the balance between collisional excitation and far-infrared dust pumping decides whether a line is seen in absorption, emission, or P-Cygni shape, and the small spontaneous emission rates and line strengths of cross-ladder transitions keep those lines optically thin and sensitive to the gas velocity field.
Extended reading notes
Core claim
The paper's central claim is that most of the excited OH detected toward Orion KL arises from the outflow (plateau) spectral component, not from the hot core. The evidence includes P-Cygni and self-absorbed line profiles, a mean expansion velocity of $25\pm5$ km s$^{-1}$ inferred from the velocity separation between emission and absorption peaks, and a declination raster in which the OH lines weaken by at least a factor of five outside the central position. Under a spherical-shell radiative transfer model, the best single-component fit gives $X(\mathrm{OH})=(0.5{-}1.0)\times10^{-6}$, $T_k\sim100$ K, and $n(\mathrm{H_2})\simeq5\times10^5\ \mathrm{cm^{-3}}$. The paper also shows that OH excitation is heavily coupled to the strong far-infrared dust continuum of the inner hot core and of the outflow itself, so some lines appear in emission while others appear in absorption; the optically thin cross-ladder transitions are especially sensitive to the outflow velocity field.
Load-bearing premise
The derived temperature, density, and abundance depend on assuming that the gas and dust temperatures are equal everywhere and that the outflow is a smooth spherical expanding shell around a uniform core, whereas the real region is clumpy and inhomogeneous.
Editorial extensions
If this is right
- Far-infrared OH lines can serve as a thermometer and densitometer for the warm outflowing gas, fixing $T_k\sim100$ K and $n(\mathrm{H_2})\sim5\times10^5\ \mathrm{cm^{-3}}$ in the Orion plateau.
- The high OH abundance, $X(\mathrm{OH})\sim10^{-6}$, puts a substantial reservoir of oxygen in the outflow, consistent with the oxygen-rich chemistry seen in the outflow-cloud interaction region.
- Because far-infrared line and continuum opacity in the plateau hides most hot-core OH emission, far-IR OH observations of embedded regions will tend to trace outflowing gas rather than the central core.
- OH P-Cygni and self-absorbed profiles give an expansion velocity of about $25\pm5$ km s$^{-1}$, matching the low-velocity outflow rather than the high-velocity component.
- Only the $\sim119\ \mu\mathrm{m}$ ground-state line shows emission above 100 km s$^{-1}$, implying that the high-velocity outflow makes little contribution to the excited OH lines.
Reading between the lines
- A testable extension the paper leaves implicit is to combine this OH abundance with simultaneous $\mathrm{H_2O}$ line measurements from the same survey to close the oxygen budget in the outflow, showing how much oxygen is locked in OH versus water.
- The same ratio of cross-ladder absorption to intra-ladder emission could be used elsewhere as a rough outflow diagnostic without high angular resolution, as long as a bright far-infrared continuum source lies behind the flow.
- If future high-resolution far-infrared spectroscopy resolves the plateau's clumpiness, the reported $X(\mathrm{OH})$ is likely to be a beam-averaged lower limit to the local abundance inside dense clumps.
- The assumed equality of gas and dust temperatures is the most fragile part of the model; observations of OH lines with widely different upper-level energies could test whether $T_k$ tracks $T_{\mathrm{dust}}$ in the outflow.
Original abstract
As part of the first far-IR line survey towards Orion KL, we present the detection of seven new rotationally excited OH Lambda-doublets (at 48, 65, 71, 79, 98 and 115 um). Observations were performed with the Long Wavelength Spectrometer (LWS) Fabry-Perots on board the Infrared Space Observatory (ISO). In total, more than 20 resolved OH rotational lines, with upper energy levels up to 620 K, have been detected at an angular and velocity resolutions of 80$'' and 33 km s^-1 respectively. OH line profiles show a complex behavior evolving from pure absorption, P-Cygni type to pure emission. We also present a large scale 6' declination raster in the OH ^2\Pi_3/2 J=5/2^+-3/2^- and ^2\Pi_3/2 J=7/2^-5/2^+ lines (at 119.441 and 84.597 um) revealing the decrease of excitation outside the core of the cloud. From the observed profiles, mean intrinsic line widths and velocity offsets between emission and absorption line peaks we conclude that most of the excited OH arises from Orion outflow(s), i.e. the ``plateau'' component. We determine an averaged OH abundance relative to H_2 of X(OH)=(0.5-1.0)x10^-6, a kinetic temperature of 100 K and a density of n(H_2)=5x10^5 cm^-3. Even with these conditions, the OH excitation is heavily coupled with the strong dust continuum emission from the inner hot core regions and from the expanding flow itself.
The compiled record
The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.
The abstract, sentence by sentence
As part of the first far-IR line survey towards Orion KL, we present the detection of seven new rotationally excited OH Lambda-doublets (at 48, 65, 71, 79, 98 and 115 um). Observations were performed with the Long Wavelength Spectrometer (LWS) Fabry-Perots on board the Infrared Space Observatory (ISO). In total, more than 20 resolved OH rotational lines, with upper energy levels up to 620 K, have been detected at an angular and velocity resolutions of 80$'' and 33 km s^-1 respectively. OH line profiles show a complex behavior evolving from pure absorption, P-Cygni type to pure emission. We also present a large scale 6' declination raster in the OH ^2\Pi_3/2 J=5/2^+-3/2^- and ^2\Pi_3/2 J=7/2^-5/2^+ lines (at 119.441 and 84.597 um) revealing the decrease of excitation outside the core of the cloud. From the observed profiles, mean intrinsic line widths and velocity offsets between emission and absorption line peaks we conclude that most of the excited OH arises from Orion outflow(s), i.e. the ``plateau'' component. We determine an averaged OH abundance relative to H_2 of X(OH)=(0.5-1.0)x10^-6, a kinetic temperature of 100 K and a density of n(H_2)=5x10^5 cm^-3. Even with these conditions, the OH excitation is heavily coupled with the strong dust continuum emission from the inner hot core regions and from the expanding flow itself.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | As part of the first far-IR line survey towards Orion KL, we present the detection of seven new rotationally excited OH Lambda-doublets (at 48, 65, 71, 79, 98 and 115 um). | result 0.58 | 0.97 | 0.08 | asserted 1.00 |
| s1 | Observations were performed with the Long Wavelength Spectrometer (LWS) Fabry-Perots on board the Infrared Space Observatory (ISO). | method 1.00 | 0.87 | 0.01 | asserted 0.99 |
| s2 | In total, more than 20 resolved OH rotational lines, with upper energy levels up to 620 K, have been detected at an angular and velocity resolutions of 80$'' and 33 km s^-1 respectively. | result 0.97 | 0.95 | 0.07 | asserted 1.00 |
| s3 | OH line profiles show a complex behavior evolving from pure absorption, P-Cygni type to pure emission. | result 1.00 | 0.91 | 0.06 | asserted 1.00 |
| s4 | We also present a large scale 6' declination raster in the OH ^2\Pi_3/2 J=5/2^+-3/2^- and ^2\Pi_3/2 J=7/2^-5/2^+ lines (at 119.441 and 84.597 um) revealing the decrease of excitation outside the core of the cloud. | result 0.50 | 0.96 | 0.06 | asserted 1.00 |
| s5 | From the observed profiles, mean intrinsic line widths and velocity offsets between emission and absorption line peaks we conclude that most of the excited OH arises from Orion outflow(s), i.e. the ``plateau'' component. | interpretation 0.96 | 0.96 | 0.64 | asserted 0.85 |
| s6 | We determine an averaged OH abundance relative to H_2 of X(OH)=(0.5-1.0)x10^-6, a kinetic temperature of 100 K and a density of n(H_2)=5x10^5 cm^-3. | result 1.00 | 0.96 | 0.19 | asserted 1.00 |
| s7 | Even with these conditions, the OH excitation is heavily coupled with the strong dust continuum emission from the inner hot core regions and from the expanding flow itself. | interpretation 0.91 | 0.79 | 0.09 | asserted 1.00 |
What was examined, and what was not
- arXiv listing abstract v1
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Figures
Figures reproduced from arXiv:astro-ph/0603077 by the authors.
References
Discussion
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