18O transitions toward Orion BN/KL using the Submillimeter Wave Astronomy Satellite (SWAS). ... 3 NASA Goddard Space Flight Center, Greenbelt, MD 20771.
The Astrophysical Journal, 539:L87–L91, 2000 August 20 q 2000. The American Astronomical Society. All rights reserved. Printed in U.S.A.
OBSERVATIONS OF WATER VAPOR TOWARD ORION BN/KL 1
G. J. Melnick, M. L. N. Ashby,1 R. Plume,1 E. A. Bergin,1 D. A. Neufeld,2 G. Chin,3 N. R. Erickson,4 P. F. Goldsmith,5 M. Harwit,6 J. E. Howe,4 S. C. Kleiner,1 D. G. Koch,7 B. M. Patten,1 R. Schieder,8 R. L. Snell,4 J. R. Stauffer,1 V. Tolls,1 Z. Wang,1 G. Winnewisser,8 and Y. F. Zhang1 Received 2000 April 17; accepted 2000 June 20; published 2000 August 16
ABSTRACT We have obtained spectra of the rotational ground-state 110–101 556.936 GHz ortho-H216O and 110–101 547.676 GHz ortho-H218O transitions toward Orion BN/KL using the Submillimeter Wave Astronomy Satellite (SWAS). The ortho-H216O spectrum shows strong evidence for both a broad (Dv . 48 km s21 ) and a narrow (Dv . 7.5 km s21) component, while the ortho-H218O shows evidence for only a broad (Dv . 24 km s21) component. The broad component emission in both ortho-H216O and ortho-H218O arises primarily from gas heated within the low- and high-velocity outflows and shocked gas surrounding IRc2 in which the ortho-H216O and ortho-H218O fractional abundances are estimated to be 3.5 # 1024 and 7 # 1027, respectively. This finding provides further confirmation that water is efficiently and abundantly produced within warm shock-heated gas. We estimate that the hot core plus the compact ridge contribute &10% to the ortho-H216O integrated intensity within the SWAS beam. The narrow component seen in the ortho-H216O spectrum is best fitted by ortho-water emission from the extended ridge (ER) and the higher temperature core of the extended ridge (CER) with a common fractional abundance of 3.3 # 1028. The absence of any discernible narrow component in the ortho-H218O spectrum is used to set 3 j upper limits on the ortho-water fractional abundance within the ER of 7 # 1028 and within the CER of 5.2 # 1027. This implies that within the dense extended quiescent region, gas-phase water is neither a major repository of oxygen nor a major coolant in Orion BN/KL. Subject headings: ISM: abundances — ISM: individual (Orion) — ISM: molecules Orion BN/KL (e.g., Blake et al. 1987), the higher spectral resolution of SWAS permits us to use the line profiles to retrieve important dynamical information and help separate sources of the emission. In this Letter, we present the ground-state 110–101 556.936 GHz ortho-H216O and corresponding 110–101 547.676 GHz ortho-H218O spectra obtained toward Orion BN/KL. The inferred sources of this emission and the derived water abundances are also presented. An accompanying Letter by Snell et al. (2000) describes the results of SWAS water observations of the more extended gas surrounding Orion BN/KL.
1. INTRODUCTION
The study of water is of interest since it has been predicted to be an important reservoir of gas-phase oxygen and, in some circumstances, one of the dominant gas coolants along with CO and H2 (e.g., Goldsmith & Langer 1978; Neufeld, Lepp, & Melnick 1995). To what extent and under what conditions these predictions are correct are questions that have motivated many observational studies. The Orion BN/KL region has been the focus of many of these because it is the closest site of highmass star formation and because it contains many of the physical conditions predicted to result in strong water emission, i.e., warm dense gas and the presence of shocks. The successful launches of the Infrared Space Observatory (ISO) and the Submillimeter Wave Astronomy Satellite (SWAS) permit the study of water absent the complications imposed by the atmosphere. ISO spectrometers covered the wavelength range from 2.4 to 197 mm with a maximum velocity resolution of approximately 10 km s21 for l ! 45 mm and ∼30–40 km s21 for l 1 45 mm. SWAS carries two heterodyne receivers capable of observing the ground-state rotational transitions of both orthowater (ortho-H216O) and ortho-isotopic water (ortho-H218O), with a spectral resolution of ≤1 km s21. Because multiple components lie along the line of sight to
2. OBSERVATIONS AND RESULTS
The observations reported here were acquired by SWAS during the periods 1998 December 20–24, 1999 January 6–18, 1999 August 29–October 15, and 2000 February 19–28. All data were obtained with the beam centered on the position s , d p 205722 0 37 00 (J2000), corresponding to a p 5 h 35 m14.5 the BN/KL region. Both the ortho-H216O and ortho-H218O observations were carried out by nodding the observatory between the source position and a reference position 17. 45 west of the source, selected to coincide with a region of no detectable 12 CO emission. The beam size at the frequency of both lines is 39. 3 # 49. 5. The spectra were reduced using the standard SWAS pipeline and have not been corrected for the SWAS mainbeam efficiency of 0.90. A more complete description of the instrument, observing modes, and data reduction methods is given in Melnick et al. (2000). The spectra are shown in Figure 1, and the peak and integrated intensities are summarized in Tables 1 and 2. The orthoH216O spectrum shows clear evidence for more than one emission component; a two-component Gaussian line (shown in Fig. 1) provides a good fit to the data and suggests that the SWAS beam encompasses both a broad-line (Dv * 20 km s21)
1 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138. 2 Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218. 3 NASA Goddard Space Flight Center, Greenbelt, MD 20771. 4 Department of Astronomy, University of Massachusetts, Amherst, MA 01003. 5 National Astronomy and Ionosphere Center, Department of Astronomy, Cornell University, Space Sciences Building, Ithaca, NY 14853-6801. 6 511 H Street SW, Washington, DC 20024; also Cornell University. 7 NASA Ames Research Center, Moffett Field, CA 94035. 8 I. Physikalisches Institut, Universita¨t zu Ko¨ln, Zu¨lpicher Strasse 77, D-50937 Ko¨ln, Germany.
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Vol. 539 TABLE 1 Observational Resultsa
Parameter ∗ A
Peak T (K) . . . . . . . . . . . . . ∫ TA∗ dv (K km s21) . . . . . .
o-H218O
o-H216O
0.24 5.8
3.98 132
a Summary of the SWAS Orion BN/KL o-H218O and o-H2O spectra.
and narrow-line (Dv ∼ 7–8 km s21) region. The ortho-H218O spectrum in Figure 1 is well fitted by a single, broad (Dv * 20 km s21) Gaussian curve. An upper limit to the peak and integrated intensities in any narrow-line ortho-H218O component is derived by fitting the residual to the best-fit single Gaussian curve with a second Gaussian curve whose LSR linecenter velocity and FWHM line width are constrained to match those of the narrow-line component in the ortho-H216O spectrum. The parameters for these Gaussian line fits are given in Tables 1 and 2. 3. WATER ABUNDANCE
3.1. Narrow-Line Component
Fig. 1.—SWAS spectra of the 110–101 556.936 GHz ortho-H216O line (top) and the 110–101 547.676 GHz ortho-H218O line (bottom). Also shown in both panels are the Gaussian fits to the narrow-line components (dotted lines) and the broad-line components (dashed lines). The composite two-Gaussian fits are shown as solid lines. The ortho-H218O spectrum also contains two lines that have been determined to lie in the image sideband: JK p 21–22 553.764 GHz vt p 1 CH3OH-A between 270 and 250 km s21 and JK p 31–32 553.768 GHz vt p 1 CH3OH-A between 30 and 50 km s21. Because these image sideband lines are not properly averaged, they appear smeared out in the signal sideband shown here. In this case, a first-order baseline was obtained by fitting the emission over the velocity interval from 280 to 80 km s21, excluding the ortho-H218O line emission within the interval from 275 to 245 and 225 to 55 km s21. The parameters of two-Gaussian fits are given in Tables 1 and 2.
Table 3 summarizes the major quiescent gas components within the SWAS beam toward Orion BN/KL. The similarity between the ortho-H216O line-center LSR velocity measured toward BN/KL, 10.2 km s21, and those measured in the more extended gas, 9–11 km s21 (Snell et al. 2000), suggest that the narrow-line component arises from within the extended ridge (ER) and the higher temperature core of the extended ridge (CER). The typical LSR velocities associated with the compact ridge of 7–8 km s21 and with the hot core of 3–6 km s21 (e.g., Blake et al. 1987) make it likely that emission from these regions is blended within the broad-line component. A Monte Carlo approach similar to that described in Ashby et al. (2000b) is used to assess the possible contribution from the ER and the CER as well as to set upper limits on the water abundance within these regions. We assume n(H 2 ) . 10 6 cm23 within the ER and CER and a gas temperature of approximately 40 K in the ER and 70 K in the CER (Bergin et al. 1994; Bergin, Snell, & Goldsmith 1996; Wang et al. 1993). The rate coefficients for collisions between ortho-H216O and ortho- and para-H2 applicable to the lowest five rotational energy levels of orthowater are used (Phillips, Maluendes, & Green 1996), and the ortho-to-para H2 ratio within each component is assumed to be the LTE value at the adopted gas temperature. The effects of a dust continuum are included by assuming that the dust and gas are coextensive and that Tdust p Tgas. To simulate the effect of a volume filling factor fv of less than unity, we multiply the dust mass opacity by fv. Thus, the model produces the proper
TABLE 2 Two-Component Gaussian Fitsa o-H218O Parameter 21
vLSR (km s ) . . . . . . . . . . . . DvFWHM (km s21) . . . . . . . .
Peak TA∗ (K) . . . . . . . . . . . . . ∫ TA∗ dv (K km s21) . . . . . . a
o-H216O
Narrow Componentb
Broad Component
Narrow Component
Broad Component
10.2 7.5 ≤0.05 ≤0.13
11.2 23.8 0.23 5.7
10.2 7.5 1.75 14.0
10.7 48.2 2.19 112.3
Summary of the SWAS Orion BN/KL o-H218O and o-H2O spectra. Upper limits to TA∗ and ∫ TA∗ dv in the narrow component are established by fitting a Gaussian to the residual from the o-H218O broad component under the constraint that the vLSR and Dv match those of the fit to the narrow component in o-H216O. The upper limits given are 3 j results. b
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TABLE 3 Quiescent Components within the SWAS Beam toward Orion BN/KLa Source Diameter (arcsec)
Gas Temperatureb (K)
vLSR
Parameter
(km s21)
DvFWHM (km s21)
n(H2) (cm23)
N(H2) (cm22)
fvc
ER . . . . . . . . . . . . . . . . . . CER . . . . . . . . . . . . . . . . . Compact ridge . . . . . . Hot core . . . . . . . . . . . .
*180 40 25 ∼10
30–40 (40) 70 (70) 80–140 (125) 150–250 (140)d
8–10 8–10 7–8 3–6
3–5 3–5 3–5 6–12
106 106 3 # 106 2 # 107
∼1023 ∼1023 ∼1023 1024
0.1 0.3 0.2 0.7
a
The major outflow components within the SWAS beam are discussed in the text. Range of observed temperatures along with the temperature assumed in our Monte Carlo models (in parentheses). Assumed to be approximately the ratio of [N(H2)/n(H2)] to the source diameter. d The maximum temperature for which rate coefficients for collisions between o-H216O and ortho- and para-H2 exist. b c
line component derived from this ortho-H218O transition are not highly sensitive to the assumed dust continuum.
number of dust continuum photons even though, by construction, the model assumes fv p 1. This is a reasonable simplification given that the dust continuum emission is thin. A more complete description of the dust properties and the inclusion of dust emission in our Monte Carlo analysis can be found in Ashby et al. (2000a). The model parameters are summarized in Table 3. Assuming that the water abundance is the same in the ER and CER and that [o-H216O]/[o-H218O] p [16O]/[18O] p 500 at the Galactocentric distance of Orion (Wilson & Rood 1994), we compute the fractional abundance x needed to produce the total integrated intensity in the ortho-H216O narrow-line component. These values are given in Table 4. A firm upper limit to x(o-H216O) in the ER and CER is set by the absence of a detectable ortho-H218O narrow component. Table 4 also presents the upper limits derived under the assumption that (1) x(o-H216O) is 500 times that of the x(o-H218O) needed to reproduce the upper limit to the ortho-H218O narrow-component integrated intensity and (2) the entire ortho-H218O narrowcomponent upper limit is due to the ER and CER individually. It is evident that the ER is the dominant narrow-line water emission component in the SWAS beam, largely because of the high beam filling factor of the ER relative to the CER. The presence of a far-infrared/submillimeter dust continuum will depopulate the lower 101 level relative to the no-dust case by radiatively pumping molecules to higher levels. Thus, the ortho-water abundance necessary to reproduce a given line flux will decrease when a dust continuum is excluded. We have estimated the magnitude of this effect by reexamining our derived ortho-H218O upper limits in the absence of dust and find that the ortho-H218O upper limits given in Table 4 decrease by 10% and 20% for the ER and the CER, respectively. This implies that upper limits to the water abundance in the narrow-
3.2. Broad-Line Component Both the width and the intensity of the broad-line component of the ortho-H216O and ortho-H218O lines argue for emission originating in the low-velocity flow (or “18 km s21” flow), the high-velocity flow (or “plateau”), and the high-velocity shocked gas. The low-velocity flow is centered on IRc2 and extends almost 300 northeast-southwest, roughly along the dense quiescent ridge. This outflow has Dv FWHM of ∼18 km s21 that is clearly traced in the low-velocity H2 O masers (Genzel et al. 1981). The high-velocity flow, which is approximately perpendicular to the low-velocity flow, has a Dv FWHM . 20–50 km s21 and is prominent in the millimeter lines of CO. The CO emission suggests a weakly bipolar structure extending approximately 5200 from a position ∼100 north of IRc2 (see Masson et al. 1987 and references therein). This high-velocity flow collides with the surrounding quiescent gas starting about 300 from the dynamical center, creating a region of shocked molecular gas cooling in transitions of H2, high-J CO, and H2O among a variety of other infrared and submillimeter lines. Strong support for the outflows 1 shock origin of the broadline water emission was obtained using the long-wavelength spectrometer (LWS) on board ISO. Harwit et al. (1998) observed eight water emission lines between 71 and 125 mm within a 750 beam centered on Orion BN/KL. The association of these lines with the outflows and shocked gas region is based on their excitation requirements and line widths. All of the observed transitions arise from energy levels whose upper state lies between ∼300 and 800 K above the ground rotational state; such temperatures are more characteristic of the shock-heated
TABLE 4 Results for Narrow-Line Componenta Parameter
x(o-H218O)
TA∗ (K)
∫ TA∗ dv (K km s21)
x(o-H216O)
TA∗ (K)
∫ TA∗ dv (K km s21)
Best-Fit Abundances Derived from o-H216O and Assuming the Same Fractional Abundances in ER and CER ER . . . . . . . . . . . CER . . . . . . . . . Total . . . . . .
6.6 (211) 6.6 (211)
0.010 0.002 0.012
0.072 0.010 0.082
3.3 (28) 3.3 (28)
1.48 0.17 1.65
13.0 1.1 14.1
3 j Upper Limit Fractional Abundances Derived from o-H218Ob and Assuming That Only the Single Region Contributes ER . . . . . . . . . . . CER . . . . . . . . .
1.3 (210) 1.0 (29)
0.020 0.026
0.14 0.13
6.5 (28) 5.2 (27)
2.1 0.61
19.0 3.9
a Peak and integrated intensities have been computed for a main-beam efficiency of 0.90. b x(o-H218O) { [o-H218O]/[H2]; x(o-H216O) { [o-H216O]/[H2].
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TABLE 5 Broad-Line Component: Illustrative Resultsa Parameter
x(o-H218O)
Compact ridge . . . . . . . . . . . . . . . . . . Hot core . . . . . . . . . . . . . . . . . . . . . . . . Outflows 1 shocked gasb . . . . . . Total . . . . . . . . . . . . . . . . . . . . . . . . . .
2.0 (28) 2.0 (27) 7.0 (27)
∫ TA∗ dv (K km s21) 0.76 0.28 4.43 5.5
x(o-H216O) 1.0 (25) 1.0 (24) 3.5 (24)
∫ TA∗ dv (K km s21) 5.13 0.51 99 105
a
Integrated intensities have been computed for a main-beam efficiency of 0.90. Assumes an emitting region in the o-H216O and o-H218O lines 1000 in diameter (see text). b
gas than any other extended region. In addition, the Gaussian line widths of ∼35 km s21 FWHM (after correction for the LWS Fabry-Perot profiles) are broader than those measured from all but the outflows 1 shocked gas. To estimate the ortho-H218O and ortho-H216O abundance within these regions, we use the profiles of postshock velocity, H2 density, and temperature given by Kaufman & Neufeld (1996, hereafter KN). This model, which was designed to reproduce the observed line-strength ratios of H2 and high-J CO in Orion BN/KL, implies a shock velocity vs of 37 km s21, a preshock H2 density of 105 cm23, and a peak postshock temperature Tpeak of ∼3000 K. However, because the flux from the 110–101 transition as well as the transitions observed by Harwit et al. (1998) become relatively insensitive to temperature for T k Eu /k, and since Tpeak 1 500 K for vs 1 12 km s21, the KN profiles suffice to estimate the SWAS-observed H2O emission from all outflows within our beam. The KN model, modified by the assumption that n(H2 O)/n(H2) ∼ 5 # 1024, was used by Harwit et al. to reproduce the far-infrared water line flux in Orion. The free parameters in this model are the water abundance, the ratio of the actual surface area of the shock(s) to the projected area of the beam at the distance of the source (what KN term the projection parameter F), and the size of the emitting region. We adopt an ortho-H216O abundance of 3.5 # 1024, consistent with the total water abundance used by Harwit et al. and an ortho-H216O : para-H216O ratio of 3 : 1, along with the value of F derived by Harwit et al. of ∼1.05. We also assume that [o-H216O]/[o-H218O] p 500. The size of the emitting region is not known, but there is strong evidence that the size is ≥750. First, the assumption that the water emission filled the 750 ISO LWS beam resulted in a good fit to the far-infrared water line flux. Second, a map of the Orion BN/ KL region in the 4 32–3 03 40.6909 mm ortho-H216O line using the ISO short-wavelength spectrometer (SWS) 17 00 # 40 00 beam (G. J. Melnick, M. Harwit, & D. A. Neufeld 2000, in preparation) shows the emission to be extended over 700–900. Since the 110–101 transition can be excited in gas considerably cooler than both the LWS lines and the SWS line, it is reasonable to assume that the SWAS lines arise from a region at least as large as the ISO-observed lines. We solve for the emission from the lowest 179 rotational states of ortho-H216O using the rate coefficients for ortho-H216OHe collisions from Green, Maluendes, & McLean (1993), multiplied by 1.348 to account for the different reduced mass when H2 is the collision partner. Because the large-velocity gradient (LVG) approximation applies to the postshocked region, a much faster converging LVG (vs. Monte Carlo) approach is used to solve for the total emission. Assuming that the emitting region is 1000 in diameter, we derive the results summarized in Table 5. The compact ridge and hot core may also contribute to the
H2O flux that we measure. Estimates of the integrated intensity assuming x(o-H216O) p 1024 for the hot core and 1025 for the compact ridge (Cernicharo et al. 1999a, 1999b) are also given in Table 5. The measured continuum within the SWAS beam is 0.23 K. From dust continuum emission maps at 450 and 790 mm (Goldsmith, Bergin, & Lis 1997), we estimate that the ER, which is well matched to the SWAS beam, contributes nearly 50% to the total continuum flux. The remaining continuum is likely generated within compact sources such as IRc2. Thus, in our analysis of the hot core, we assume that most of the remaining continuum flux, or ∼0.1 K at 540 mm, is generated solely within this region. This model also matches the 179 mm continuum intensity of 4.4 K in a 609 beam (Werner 1982). 4. DISCUSSION
The region within 29 of Orion BN/KL encompasses an extraordinary variety of physical conditions, ranging from quiescent cool gas to outflows whose collisions with the surrounding molecular cloud often raise temperatures above 1000 K. The impact of these conditions on the gas-phase chemistry and water-line emission is predicted to be similarly diverse, and evidence that this is the case is clearly seen in the SWAS spectra. Within the quiescent gas of the ER and CER, we determine that x(o-H216O) is approximately a few times 1028, consistent with the values measured for more than a dozen spatial positions along the ridge and adjacent to BN/KL (Snell et al. 2000). These results are in some contrast to the conclusions of several studies that infer x(H2O) ∼ 1026 to 1025 in the ER (see Cernicharo et al. 1999a, 1999b). Is it possible that we have greatly underestimated the true water abundance? It might be argued that line scattering by extended foreground gas is causing us to underestimate the true line intensities and, thus, the true water abundances. The absence of self-absorption in the SWAS water spectra toward BN/ KL or in any of the neighboring positions argues against line scattering being important in this case. Nonetheless, even if it is significant for ortho-H216O, the 500 times lower optical depth in the ortho-H218O transition would leave this line largely unaffected, and the upper limits to the ortho-water abundance derived from the ortho-H218O data for the ER and CER would stand. Yet x(o-H216O) that is approximately a few times 1028 is 1–2 orders of magnitude below the predictions of timedependent chemical models for well-shielded molecular gas that has been chemically evolving for more than 103 yr (e.g., Lee, Bettens, & Herbst 1996). It is suggested that an increase in the gas-phase C/O ratio, as might result from the preferential depletion of oxygen-bearing species onto grains, could substantially reduce the gas-phase oxygen available to form H2O (see Bergin et al. 2000). Similarly, turbulence that cycles gas between the shielded cloud interior and a photodissociating
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surface could maintain a chemical youth that would achieve the lower fractional abundances required by the SWAS observations (see Chie`ze & Pineau des Foreˆts 1989). Our identification of the outflows 1 shocked gas as the primary sources of the broad-line emission is in good agreement with the earlier findings of Harwit et al. (1998) as well as with the recent results of Wright et al. (2000). The growing correlation of strong broad H2 O emission with regions of outflows and shocks further supports the claim that shock-heated gas contains very high water abundances. Most of this water is formed in situ via a sequence of neutral-neutral chemical reactions favored at gas temperatures above ∼300 K (Elitzur & de Jong 1973; Elitzur & Watson 1978), while between a few percent and perhaps as much as 30% of the water vapor may result from the evaporation of water ice from grain mantles. The factor of 2 difference in the ortho-H216O and orthoH218O line widths is most likely due to the diminishing mass of gas moving at velocities greater than 25 km s21; whereas
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the ortho-H216O column density remains high enough in the v 1 25 km s21 gas to produce detectable line flux, the orthoH218O column density has become too low to produce appreciable emission. The gas temperatures in both the hot core and the compact ridge exceed the 90 K sublimation temperature of water ice, and thus these regions may also contain x(H2 O) * 1025. However, owing to its small size and high dust continuum, we find that the hot core contributes negligibly to the water emission within the SWAS beam. Similarly, the compact ridge, although somewhat larger, probably contributes no more than about 5%–10% of the ortho-H216O emission that we detect. This work was supported by NASA contract NAS5-30702. R. Schieder and G. Winnewisser would like to acknowledge the generous support provided by the DLR through grants 50 0090 090 and 50 0099 011.
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