Herschel observations of EXtra-Ordinary Sources (HEXOS): Observations of H2O and its isotopologues towards Orion KL
G. J. Melnick, V. Tolls, D. A. Neufeld, E. A. Bergin, T. G. Phillips, S. Wang, N. R. Crockett, T. A. Bell, G.A. Blake, S. Cabrit, E. Caux, C. Ceccarelli, J. Cernicharo, C. Comito, F. Daniel, M.-L. Dubernet, M. Emprechtinger, P. Encrenaz, E. Falgarone, M. Gerin, T. F. Giesen, J. R. Goicoechea, P. F. Goldsmith, E. Herbst, C. Joblin, D. Johnstone, W. D. Langer, W.D. Latter, D. C. Lis, S. D. Lord, S. Maret, P. G. Martin, K. M. Menten, P. Morris, H. S. P. Muller, J. A. Murphy, V. Ossenkopf, L. Pagani, J. C. Pearson, M. Perault, R. Plume, S.-L. Qin, M. Salez, P. Schilke, S. Schlemmer, J. Stutzki, N. Trappe, F. F. S. van der Tak, C. Vastel, H. W. Yorke, S. Yu, J. Zmuidzinas
AAstronomy & Astrophysics manuscript no. Orion˙H2O˙Paper˙arXiv c (cid:13)
ESO 2018October 26, 2018
Herschel observations of EXtra-Ordinary Sources (HEXOS):Observations of H O and its isotopologues towards Orion KL (cid:63)
G. J. Melnick, V. Tolls, D. A. Neufeld, E. A. Bergin, T. G. Phillips, S. Wang, N. R. Crockett, T. A. Bell, G.A. Blake S. Cabrit, E. Caux, , C. Ceccarelli, J. Cernicharo, C. Comito, F. Daniel, , M.-L. Dubernet, , M. Emprechtinger, P. Encrenaz, E. Falgarone, M. Gerin, T. F. Giesen, J. R. Goicoechea, P. F. Goldsmith, E. Herbst, C. Joblin, , D. Johnstone, W. D. Langer W.D. Latter D. C. Lis, S. D. Lord, S. Maret, P. G. Martin, K. M. Menten, P. Morris, H. S. P. M¨uller, J. A. Murphy, V. Ossenkopf, , L. Pagani, J. C. Pearson, M. P´erault, R. Plume, S.-L. Qin, M. Salez, P. Schilke, , S. Schlemmer, J. Stutzki, N. Trappe, F. F. S. van der Tak, C. Vastel, , H. W. Yorke, S. Yu, and J. Zmuidzinas, (A ffi liations can be found after the references) Preprint online version: October 26, 2018
ABSTRACT
We report the detection of more than 48 velocity-resolved ground rotational state transitions of H
O, H
O, and H
O – most for the first time– in both emission and absorption toward Orion KL using
Herschel / HIFI. We show that a simple fit, constrained to match the known emissionand absorption components along the line of sight, is in excellent agreement with the spectral profiles of all the water lines. Using the measuredH
O line fluxes, which are less a ff ected by line opacity than their H O counterparts, and an escape probability method, the column densities ofH
O associated with each emission component are derived. We infer total water abundances of 7.4 × − , 1.0 × − , and 1.6 × − for the plateau,hot core, and extended warm gas, respectively. In the case of the plateau, this value is consistent with previous measures of the Orion-KL waterabundance as well as those of other molecular outflows. In the case of the hot core and extended warm gas, these values are somewhat higher thanwater abundances derived for other quiescent clouds, suggesting that these regions are likely experiencing enhanced water-ice sublimation from(and reduced freeze-out onto) grain surfaces due to the warmer dust in these sources. Key words.
ISM: abundances — ISM: molecules
1. Introduction
During its 6-year mission, the Submillimeter Wave AstronomySatellite (SWAS) surveyed more than 300 galactic sources andmore than 6800 lines-of-sight (Melnick et al. 2000a), yet noneproduced stronger water emission than the line of sight towardOrion-KL. The source of this emission was attributed primarilyto the chemistry and excitation accompanying the exceptionallypowerful outflows emanating from the BN / KL region (Harwitet al. 1998; Wright et al. 2000; Melnick et al. 2000b; Cernicharoetal. 2006; Lerate etal. 2006); however, many sources possessingphysical conditions favorable to the production of strong wateremission – e.g. high densities and temperatures – are known toexist close to KL and could very likely be significant contribu-tors to the water emission detected by ISO, SWAS, and
Odin .Unfortunately, with access to only the ground-state 1 − transition of ortho-H O and H O, even the velocity-resolvedSWAS and
Odin measurements were limited in what could beinferred about the various components giving rise to the strongwater emission.The availability of the
Herschel / HIFI instrument (deGraauwet al. 2010) with its extended frequency coverage and higher an-gular resolution, now permits a more detailed examination of theconditions responsible for the water emission toward Orion-KL. (cid:63)
Herschel is an ESA space observatory with science instruments pro-vided by European-led Principal Investigator consortia and with impor-tant participation from NASA. Also referred to simply as H O Here we report the detection of 21 H O, 15 H
O, and 12 H
Ovelocity-resolved lines toward this source obtained as part of theHEXOS program (Bergin et al. 2010).In this paper, we present an analysis of the sources of thewater emission based upon the lower-opacity lines of H
O. Wealso show that the approach taken in this analysis holds greatpromise when applied to the H O and H
O lines, which will bepursued in a future paper.
2. Observations and results
The HIFI observations presented here were carried out in Marchand April 2010 using the spectral scan dual beam switch (DBS)mode pointed towards Orion-KL α J = h m . s and δ J = − ◦ (cid:48) . (cid:48)(cid:48) . All observations were obtained with abeamsize of ∼ (22 / ν THz ) (cid:48)(cid:48) and reference beams approximately3 (cid:48) east and west, which is roughly orthogonal to the orienta-tion of the Orion molecular ridge (e.g., Ungerechts et al. 1997).However, water emission is extended in Orion (Snell et al. 2000)and the reference beam may contain some contamination froma narrow ( ∆ v ∼ − − ) component centered at ∼ − . We utilized the wide band spectrometer providing a spectralresolution of 1.1 MHz over a 4 GHz IF bandwidth. The data pre-sented here are from a range of HIFI bands obtained as part ofthe HEXOS program. These data were reduced and converted tosingle side band as described by (Bergin et al. 2010), with ad-ditional analysis performed at the CfA. In our study, we adopt auniform main beam e ffi ciency of 70%. a r X i v : . [ a s t r o - ph . GA ] J u l elnick et al.: Water in Orion Because of flux di ff erences between the H- and the V-polarizations, which are most likely due to the known pointingo ff set between the two beams, we use only the H-polarizationdata for our analysis. The spectra for all H O, H
O, and H
Olines were extracted from the more extended HEXOS spec-tral scan data using the JPL Spectral Line Catalog (Pickettet al. 1998) for identification. Finally, the continuum o ff set ap-propriate to each line was determined directly from emission-free spectral regions near each line.Figures 1 and 2 show the spectra of H O and H O plusH
O, respectively. These spectra span a broad range of exci-tation conditions, ranging in upper-level energies between 53 Kand more than 1000 K. All spectra have been examined for se-vere blending using the CLASS-Weeds tool (Maret et al. 2010),the JPL Spectral Line Catalog, or visual evidence of non-smoothwater line wings. Blended lines were excluded from the follow-ing analysis.
3. Analysis
The goals of the present e ff ort are twofold: (1) isolate the compo-nents giving rise to the water emission we detect; and, (2) modelthese components in a way that best reproduces the measuredline fluxes and profiles. To do this, we focus here on the observedH O lines. These lines have been detected over a broad rangeof excitation conditions with high signal-to-noise ratios and aremuch less a ff ected by optical depth e ff ects than their H O coun-terparts, making the analysis more straightforward. In addition,the O: O ratio is well known (i.e., ∼ O abundance to H
O abundance robust.Step 1 – isolate the components: The lines exhibit complexprofiles which we attribute to a combination of emission and ab-sorption components along the line of sight. To isolate what webelieve are the three predominant emission components withinthe HIFI beams – namely the plateau molecular outflow, the hotcore, and an extended region of gas composed of the compactridge plus the warmer, denser portion of the extended ridge nearKL (cf. Blake et al. 1987) – we adopt a line-fitting strategy thatfixes the well-established characteristics of these regions, such astheir v
LSR , and, in some cases, the typical line width, and leavesas free fitting parameters such quantities as the line strengths.In addition to the three emission components, we includethe e ff ects of absorption by foreground material in two dis-tinct kinematic components: a narrow component near 7 km s − ,and a broad component centered at an LSR velocity of − − . While the presence of these absorption components isclearly required to fit the observed water line profiles, particu-larly in the case of low-lying transitions of H O, the existenceof foreground absorbing material at these velocities has beenindependently confirmed by HIFI observations of HF (Phillipset al. 2010), OH + and H O + (Gupta et al. 2010), as well asCRIRES observations of the fundamental CO vibrational band(Beuther et al. 2010). The narrow component arises in quies-cent gas, while the broad, blueshifted component represents out-flowing material, presumably associated primarily with the LowVelocity Flow (Genzel & Stutzki 1989). For the lower-lying tran-sitions, these absorption components account for pronouncedasymmetries in the line shapes, as well as the absorption fea-ture close to the systemic source velocity (although we note herethat narrow line emission in the reference beam is potentially acontributor to this absorption feature observed in the very low-est transitions.) Even in the case of H O, transitions to the
Table 1.
Fixed and varied parameters in water-line fits
Peak T ∗ A v LSR
FWHMSource (K) (km s − ) (km s − )Plateau . . . . . . . . . . . . . . . . . . . Varied + + + − † Narrow absorption . . . . . . . . Varied + − † The FWHM was constrained to vary between only 2 and 8 km s − . ground states of ortho- or para-H O (i.e., 2 − , 1 − and 1 − ) are a ff ected by foreground absorption. Indeed, inthe 1 − and 2 − H O transitions, where the contin-uum brightness temperature is greatest, the blueshifted absorp-tion feature can cause the observed antenna temperature to dipbelow the continuum level.Thus, fits to all lines were made using the expression:Fitted Line ( T ∗ A ) = (cid:16) Continuum O ff set + G plat + G hc + G ewg (cid:17) × Exp [ − (G na + G ba )] , (1)where G plat , G hc , G ewg , G na and G ba are Gaussian componentsrepresenting the plateau, hot core, extended warm gas, narrowabsorbing feature, and broad absorbing feature, respectively.Table 1 provides the fit parameters fixed by previous measure-ments and those that were allowed to vary, unconstrained, in or-der to obtain the best fit to the line profiles.Step 2 – model the H O emission components: The resultsof Step 1 are a set of best-fit integrated intensities for each com-ponent and transition, including the absorption features, that sumto reproduce the line flux and profile for each ortho- and paraH
O line. In this paper, we focus on the emission componentsonly; analysis of the physical conditions associated with the ab-sorption components will be undertaken following the results ofa soon-to-be-completed water map toward Orion-KL. To assesshow the H
O line strengths constrain the water abundance ineach component, the equilibrium level populations of all H Oortho and para rotational levels of the ground vibrational statewith energies E / k up to 2000 K have been calculated using anescape probability method that includes the necessary e ff ectsof radiative excitations due to dust emission embedded withineach component. It is assumed that the water molecules see 4 π steradians of dust emission from within each component. Thevelocity gradient for each transition is assumed to be equal to ∆ v n (H ) / N (H ), where the line width, ∆ v, for each line for eachcomponent is taken from the best fit in Step 1, and n (H ) and N (H ) are the volume and column densities of H , respectively.The rate coe ffi cients for collisions between ortho- and para-H and ortho- and para-H O calculated by Faure et al. (2007) areused, and the H ortho-to-para ratio is assumed to be the LTEvalue at the gas temperature of each component. Finally, the cal-culations incorporate the beam size and aperture e ffi ciency ap-propriate to each transition.More than 90% of the presently observed H O total line flux(and >
98% of the H
O and H
O total line flux) lies in transi-tions with E upper ≤
600 K. Thus, we focus our modeling e ff ortsprimarily on reproducing the flux and profiles for these transi-tions. The H density, gas and dust temperatures, source size,and ortho- and para-H O column densities were varied to bestmatch the inferred line fluxes for each emission component. Thevalues yielding the best fit to the data are provided in Table 2.
Table 2.
Best-fit radiative transfer model parameters for Orion H
O emission components ∆ v T gas n (H ) T dust † N (ortho / para-H O) Total InferredSource (km s − ) (K) (cm − ) (K) θ source (cm − ) H O Abundance ‡ Extended warm gas . . . . . . . 2 − ×
30 20 (cid:48)(cid:48) × (ortho) / × (para) 1.6 × − Plateau . . . . . . . . . . . . . . . . . . 20 −
34 188 2 × (cid:48)(cid:48) × (ortho) / × (para) 7.4 × − Hot core . . . . . . . . . . . . . . . . . 10 150 1 ×
180 5 (cid:48)(cid:48) × (ortho) / × (para) 1.0 × − † Greybody fit to the Orion continuum of the form: B ˜ ν ( T dust ) × (0 . ν ) . , where ˜ ν is wavelength in wavenumbers. ‡ Assumes O / O = N (H ) = × , 1 × , and 1 × cm − in a 30 (cid:48)(cid:48) beam for the extended ridge, plateau, and hot core, respectively (Blake et al. 1987). The line profiles resulting from the radiative transfer model cal-culations for the emission components and Step 1 line-fits to theabsorption components are shown as the red curves superposedon the observed spectra in Fig. 1. The models summarized inTable 2 provide a remarkably good match to the data, thoughthe deviation between the models and the observed spectra forthe higher-energy H
O transitions clearly illustrates the short-comings of single-value models for each component as smallamounts of hotter gas are not accounted for.The physical conditions summarized in Table 2 have alsobeen used to model the H
O lines with E upper ≤
400 K. To do so,the column densities of ortho- and para-H
O are assumed to be500 times greater than those of H
O, the line fluxes calculated,and then applied using the best-fit H
O plateau line widths de-termined using Eqn. 1. For the hot core and extended warm gasregion, the H
O widths were assumed to be twice those of theH
O, and the absorption components are unchanged. The re-sults of this simple approach are shown as the superposed redcurves on the relevant H
O spectra in Fig. 2. The potential fora more careful analysis of the H
O and H
O lines is illus-trated by how well the constrained fits match the other line pro-files, shown as the superposed brown curves in Fig. 2. A moredetailed model will be presented in a future paper.
4. Discussion
Modeling of the rich spectrum of H
O lines toward Orion-KL reveals several things. First, the relatively high H O abun-dance associated with the plateau is consistent with elevated wa-ter abundances measured previously toward KL (cf. Cernicharoet al. 2006) as well as toward a number of other molecular out-flows (cf. Franklin et al. 2008). This is most likely the result ofa combination of H O-ice sublimated and sputtered from grainsurfaces and H O formed e ffi ciently in the gas phase via neutral-neutral reactions favored in hotter portions of the plateau. Theinferred water abundance for the plateau given in Table 2 isless than that cited in some larger-beamsize studies (e.g., Harwitet al. 1998, Melnick et al. 2000b), and may be due to the ex-clusion of more extended regions where the outflows encounterthe surrounding quiescent material (cf. Genzel & Stutzki 1989).These shock-heated regions, which are particularly prominent inH emission, can subject virtually all of the a ff ected gas to tem-peratures in excess 1000 K, thus facilitating the neutral-neutralreactions that e ffi ciently produce H O.Second, the water abundances inferred for the hot core andextended warm gas are more than an order of magnitude greaterthan that inferred toward other quiescent regions (cf. Melnick &Bergin 2005). This is likely the result of enhanced sublimationof water-ice from, and reduced freeze-out onto, the warm dustgrains present within both regions. It should be noted that the T A * ( K ) H O 1 -0 T A * ( K ) H O 1 -1 T A * ( K ) H O 2 -1 T A * ( K ) H O 2 -1 T A * ( K ) H O 2 -2 T A * ( K ) H O 2 -2 T A * ( K ) H O 3 -3 v LSR (km s -1 ) T A * ( K ) H O 3 -2 H O 3 -3 H O 3 -3 H O 4 -4 H O 4 -4 H O 5 -4 H O 5 -5 v LSR (km s -1 )H O 6 -6 Fig. 1. H O lines toward Orion-KL in order of increasing up-per level energy. The superposed red curves show the line pro-files resulting from our radiative transfer modeling of the emis-sion components and line fits to the absorption components.The labels in the upper left corner of each plot list the species,the transition, the transition rest frequency, and the upper-levelenergy. The vertical dashed line denotes the 9 km s − sys-temic velocity of the cloud. The 2 − , 4 − , 6 − ,and 6 − spectra are omitted due to blending with otherlines or a low signal-to-noise ratio. We also note possible blend-ing of the 4 − line with CH OH (12 − ) and H CO(18 − ), both of which lie within 27 km s − of the H Oline. T A * ( K ) H O 1 -0 T A * ( K ) H O 1 -1 T A * ( K ) H O 2 -1 T A * ( K ) H O 2 -1 T A * ( K ) H O 2 -2 T A * ( K ) H O 2 -2 v LSR (km s -1 ) T A * ( K ) H O 3 -2 H O 3 -3 H O 4 -4 H O 3 -4 H O 4 -4 H O 4 -3 H O 5 -4 v LSR (km s -1 )H O 5 -5 H O 5 -4 H O 6 -6 H O 6 -5 H O 6 -7 H H O 7 -8 v LSR (km s -1 )H O 7 -7 H O 1 - 0 H O 1 - 1 H O 2 - 1 H O 2 - 2 H O 2 - 2 v LSR (km s -1 )H O 3 - 2 H O 3 - 3 H O 3 - 3 H O 4 - 4 H O 4 - 4 H O 5 - 6 v LSR (km s -1 )H O 5 - 5 Fig. 2.
Left:
Same as Fig. 1, except showing the H
O spectra toward Orion-KL / Hot Core in order of increasing upper level energy.The red curves superposed on the H
O spectra with upper-level energies less than 400 K result from our radiative transfer modelof the emission components and line fits to the absorption components. The brown curves show the best 5-component fit resultingfrom the procedure described in Section 3. The 2 − , 3 − , 6 − , 7 − , and 7 − spectra have been omitted dueto blending with other lines or a low signal-to-noise ratio. Right: H O spectra. The brown curves superposed on the spectra showthe best 5-component fit resulting from the procedure described in Section 3. The 2 − and 2 − spectra have been omitteddue to blending with other lines or a low signal-to-noise ratio.gas and dust temperatures inferred for the extended warm gasshould be viewed as lower limits given the probable presence ofboth water-line and continuum emission in the reference beam.Finally, the H O ortho-to-para ratio inferred for all threeemission components is consistent with a ratio of 3:1. A ratio ofgreater than 3:1 is likely the consequence of the rather simplemodel adopted for each component or residual inaccuracies inthe water collisional rate coe ffi cients, or both. Acknowledgements.
HIFI has been designed and built by a consortium ofinstitutes and university departments from across Europe, Canada and theUnited States under the leadership of SRON Netherlands Institute for SpaceResearch, Groningen, The Netherlands and with major contributions fromGermany, France and the US. Consortium members are: Canada: CSA,U.Waterloo; France: CESR, LAB, LERMA, IRAM; Germany: KOSMA,MPIfR, MPS; Ireland, NUI Maynooth; Italy: ASI, IFSI-INAF, OsservatorioAstrofisico di Arcetri- INAF; Netherlands: SRON, TUD; Poland: CAMK,CBK; Spain: Observatorio Astronmico Nacional (IGN), Centro de Astrobiologa(CSIC-INTA). Sweden: Chalmers University of Technology - MC2, RSS &GARD; Onsala Space Observatory; Swedish National Space Board, StockholmUniversity - Stockholm Observatory; Switzerland: ETH Zurich, FHNW; USA:Caltech, JPL, NHSC. Support for this work was provided by NASA through an award issued by JPL / Caltech. CSO is supported by the NSF, award AST-0540882.
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Appendix
Table 3. Best-Fit H
O Integrated Line Intensities † Emission Component ExtendedUpper-Level Plateau Hot Core Warm GasTransition Frequency Energy (cid:82) T ∗ A dv (cid:82) T ∗ A dv (cid:82) T ∗ A dv (GHz) (K) (K-km s − ) (K-km s − ) (K-km s − )1 − . . . . . . . . . 1101.70 52.9 315.4 7.6 50.81 − . . . . . . . . . 547.68 60.5 199.0 8.2 34.62 − . . . . . . . . . 994.68 100.6 423.3 60.1 10.72 − . . . . . . . . . 1655.87 113.6 574.9 0.0 71.52 − . . . . . . . . . 745.32 136.4 276.6 66.3 17.72 − . . . . . . . . . 1633.48 192.0 220.6 10.1 6.43 − . . . . . . . . . 1095.63 248.7 444.9 70.2 5.23 − . . . . . . . . . 1181.39 248.7 178.8 50.4 1.83 − . . . . . . . . . 1894.32 294.6 65.7 34.7 2.43 − . . . . . . . . . 1136.70 303.3 424.4 65.3 11.84 − . . . . . . . . . 1605.96 395.4 46.6 38.6 0.04 − †† . . . . . . . 1188.86 452.4 118.5 57.5 0.25 − . . . . . . . . . 1003.28 595.9 0.0 6.9 2.05 − . . . . . . . . . 1815.85 727.6 22.2 32.8 10.06 − . . . . . . . . . 1800.47 865.0 41.5 36.9 0.0 † The line fitting procedure is described in Section 3. †† Some flux attributed to this transition may be due to the CH OH (12 − ) and H CO (18 − )transitions, both of which lie within 27 km s − of the H O 4 −13