We have detected the 110â101 pure rotational transition of water vapor toward comet C/1999 H1 using the. Submillimeter Wave Astronomy Satellite. Over the ...
The Astrophysical Journal, 539:L151–L154, 2000 August 20 q 2000. The American Astronomical Society. All rights reserved. Printed in U.S.A.
SUBMILLIMETER WAVE ASTRONOMY SATELLITE OBSERVATIONS OF WATER VAPOR TOWARD COMET C/1999 H1 (LEE) D. A. Neufeld,1 J. R. Stauffer,2 E. A. Bergin,2 S. C. Kleiner,2 B. M. Patten,2 Z. Wang,2 M. L. N. Ashby,2 G. Chin,3 N. R. Erickson,4 P. F. Goldsmith,5 M. Harwit,6 J. E. Howe,4 D. G. Koch,7 R. Plume,2 R. Schieder,8 R. L. Snell,4 V. Tolls,2 G. Winnewisser,8 Y. F. Zhang,2 and G. J. Melnick2 Received 1999 August 16; accepted 2000 June 23; 2000 August 16
ABSTRACT We have detected the 110–101 pure rotational transition of water vapor toward comet C/1999 H1 using the Submillimeter Wave Astronomy Satellite. Over the period 1999 May 19.01–23.69 UT, the average integrated antenna temperature was 1.79 5 0.03 K km s21 within a 39. 3 # 49. 5 (FWHM) elliptical beam. For an assumed ortho-to-para ratio of 3, we estimate the total water production rate as 8 # 10 28 s21 . This value lies approximately 50% above the value estimated by Biver et al. from contemporaneous radio observations of hydroxyl molecules. The observed line width of 1.8 km s21 (FWHM) is broader than the instrumental profile and suggests an intrinsic line width of about 1.4 km s21 (FWHM). The data, taken during a portion of every 97 minute spacecraft orbit over a 4.68 day period, provide no evidence for variability. Subject headings: comets: general — comets: individual (Lee (C/1999 H1)) estimates of the water production rate Q(H2O) derived from the integrated line intensities that we measured and compare them with a previously reported estimate that was based on contemporaneous observations of OH.
1. INTRODUCTION
Water ice has long been recognized as the primary volatile constituent of cometary nuclei, and water vapor as the principal gaseous constituent of cometary comae. Although the photodissociation products of water—hydroxyl radicals and oxygen atoms—are routinely detected by means of ground-based observations (e.g., A’Hearn 1982), the detection of water itself is made more difficult by absorption by the Earth’s atmosphere. Direct observations of water have therefore been limited to ground-based observations of high-lying (“hot”) vibrational bands (Mumma et al. 1995; Dello Russo et al. 2000), airplane observations of the n3 vibrational band (e.g., Mumma et al. 1986; Larson et al. 1989), and recent space-based observations carried out using the Infrared Space Observatory (e.g., Crovisier et al. 1997). The Submillimeter Wave Astronomy Satellite (SWAS; Melnick et al. 2000)—launched in 1998 December—provides us with a unique opportunity to observe cometary water by means of the lowest rotational transition of ortho-water, the 110–101 line near 557 GHz, with a beam that is typically comparable in size to the region of water vapor emission from cometary comae. Observations of the 110–101 transition yield an estimate of the water production rate that is entirely independent of those obtained by the less direct methods that are usually employed. In this Letter, we report the detection of water toward comet C/1999 H1 (Lee) by means of SWAS observations performed during the period of 1999 May 19–23. In § 2, we describe our observations, and in § 3 we present the water spectrum and integrated line luminosity thereby obtained. In § 4, we discuss
2. OBSERVATIONS AND DATA REDUCTION
All the data presented here were obtained using the standard nodding observational mode described by Melnick et al. (2000). The spectra were collected within observational segments of duration about 38 minutes in each of 70 consecutive 97 minute orbits over the period of 1999 May 19.01–23.69 UT. During this period, comet C/Lee was approaching the Sun at heliocentric distances R in the range of 1.24–1.18 AU and at geocentric distances D in the range of 0.836–0.915 AU. The Earth-comet-Sun angle ranged from 547. 1 to 567. 3. At the average geocentric distance of 0.875 AU, the angular size of the elliptical SWAS beam (39. 3 # 49. 5 FWHM) corresponds to a projected size of 125,000 # 170,000 km. For each observation, the SWAS beam was centered on the position of the cometary nucleus, as predicted by the ephemeris given in Minor Planet Circular 34421. The data were reduced using the standard SWAS pipeline (Melnick et al. 2000). The frequency scale was corrected to the (changing) rest frame of the cometary nucleus by removing the Doppler shifts associated with the spacecraft orbital velocity and the cometary geocentric velocity. 3. RESULTS
In Figure 1, we show the overall water spectrum obtained by co-adding data from the entire 4.68 day observing period (with a total observing time of 16.03 hr on-source). The integrated brightness temperature for the 110–101 water line is 1.79 5 0.03 (1 j) K km s21. None of the other SWAS target lines was detected, and the observations place 3 j upper limits of 0.1 K km s21 on the integrated antenna temperatures for the 13 CO J p 5–4, O2 (3, 3)–(1, 2), and C i 3P1–3P0 lines9 (and,
1
Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218. 2 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138. 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 20024-2725; 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.
9 Given the excitation requirements for these other SWAS target transitions, our failure to detect them was entirely expected and does not place interesting constraints on the composition of the coma.
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Fig. 1.—Water vapor spectrum obtained toward comet C/1999 H1 Lee on 1999 May 19.01–23.69 by SWAS. The spectrum shows the 110–101 pure rotational transition of water vapor near 557 GHz. The velocity scale is in the (changing) rest frame of the cometary nucleus.
indeed, for any other line that happens to fall in the SWAS bandpass). 4. WATER PRODUCTION RATE
We have modeled the water production rate from comet C/Lee on the basis of a spherically symmetric radial outflow model with an assumed water density profile n(H 2 O) p
Q(H 2 O) exp [2r/( vtH 2 O )] 4pr 2 v
p 796
Q 29 exp [2r10 /( v 5t5 )] cm23, r102 v 5
(1)
where Q(H 2 O) p 10 29 Q 29 molecules s21 is the water production rate, r p 10 10 r10 cm is the distance from the cometary nucleus, v p v 5 km s21 is the radial outflow velocity, and tH 2 O p 10 5 t5 s is the water lifetime. The destruction rate of cometary water depends on solar activity. Using the model of Budzien, Festou, & Feldman (1994), we estimate a water lifetime of t5 p 0.73 during the period that the observations were carried out. This value implies that the total number of water molecules in the coma is N(H 2 O) p Q(H 2 O)tH 2 O ∼ 7.3 # 10 33 Q 29 (R/AU) 2. For an assumed outflow velocity v 5 p 0.85(R/AU)21/2 (Budzien et al. 1994), the outflowing water vapor is distributed with a scale length (vtH 2 O p 55,000 km) that is comparable to the radius of the SWAS beam. The first comprehensive model for the excitation of pure rotational lines of water within cometary comae was that of Bockele´e-Morvan (1987, hereafter B87), which included the effects of (1) radiative pumping by solar infrared radiation via vibrational bands, (2) excitation by means of inelastic H2OH2O collisions, and (3) radiative trapping in rotational transitions of water. The latter effect is particularly important for the 110–101 transition (and other transitions originating in the ground states of ortho- or para-water, 101 and 0 00), the optical 21 depth in the 110–101 line being 1.7r10 Q 29 v22 5 . Optical depth effects therefore reduce the effective spontaneous radiative rate for the 110–101 transition and increase the 110 level population throughout the region sampled by the SWAS beam, but the
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Fig. 2.—Derived water production rate as a function of time during the observing period. The error bars reflect the 1 j statistical uncertainties in the measured line fluxes; they do not include uncertainties in calibration or modeling, but nevertheless they provide a useful estimate of relative uncertainties.
emission rate per water molecule is only diminished in the inner coma once the level populations reach LTE and the transition becomes “effectively thick.” According to the B87 model, the combined effects of radiative trapping in the 110–101 transition and collisional excitation drive the level populations to LTE within a collisional zone of radius only ∼1000 km. Thus, for observations with the large SWAS beam, the second and third effects described above have only a small effect on the line flux predicted by the B87 model.10 However, based on measurements of the electron abundance in comet P/Halley, Xie & Mumma (1992) have argued that the B87 model significantly underestimates the importance of collisional excitation by hot electrons in the coma, an effect that, instead of being negligible as assumed by B87, increases the total excitation rate of the 110–101 transition in the outer coma by a factor of ∼4 above what was assumed in B87 (Xie & Mumma 1992, Fig. 4a). This view was supported by observations of methanol in comets Austin, Levy, and Swift-Tuttle (Bockele´e-Morvan et al. 1994a, 1994b), which revealed rotational populations consistent with a collisional zone of substantially larger size than that predicted by B87. Unfortunately, detailed models have not yet been constructed for the water emission from cometary comae in which the collisional excitation of water by both neutrals and electrons is included. Such models would require a detailed treatment of the temperature and density of both water and electrons within the cometary coma and are beyond the scope of this Letter. However, the results of Xie & Mumma (1992, Fig. 4a) do suggest that radiative pumping by solar infrared radiation still dominates throughout most of the SWAS beam (average projected radius ∼7.5 # 10 4 km), even when electron impact excitation is included. Accordingly, we have used a simple radiative pumping model to obtain an approximate value for the water production rate in comet C/Lee as described below, while recognizing that collisional effects may limit the accuracy of the result. 10 Indeed, the results for the 110–101 line luminosity presented by B87 for two water production rates, Q29 p 0.2 and Q29 p 2, are in exact agreement with a pure radiative pump model for the case Q29 p 0.2. For the higher water production rate of Q29 p 2, the predicted line luminosity falls 20% below the prediction of the optically thin pure radiative pump model.
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NEUFELD ET AL.
In the optically thin limit,11 Crovisier’s (1984) analysis of the radiative pumping of water implies a pump rate z for the 110–101 transition of 5.7 # 1025 (R/AU)22 s21 per water molecule, given an assumed ortho-to-para ratio of 3. Using the oscillator strengths given in the HITRAN database (Rothman et al. 1987) and assuming that the infrared solar radiation can be described by a blackbody at 5770 K, we estimate that this pump rate should be increased by ∼30% to account for the excitation in vibrational bands not included by Crovisier, viz., the n2 1 n3, n1 1 n3, and n1 bands in declining order of importance. A pure radiative pump model therefore implies a total 110–101 luminosity of L 557 p hnzN(H 2 O) p 2.0 # 10 15 Q 29 ergs s21, where n p 557 GHz is the transition frequency and z p 7.3 # 1025 (R/AU)22 s21 is our adopted value of the radiative pump rate. The corresponding line flux received at Earth is thus F557 p L 557 /(4pD2 ) p 7.0 # 10213 (D/AU)22 Q 29 ergs cm22 s21.
(2)
For observations with SWAS, this line flux corresponds to an integrated antenna temperature of
E
TA∗ dv p ea fH 2 O
F557 Al p 2.6fH 2 O Q 29 (D/AU)22 K km s21, 2k (3)
where A p 2954 cm2 is the area of the SWAS primary mirror, ea p 0.66 is the aperture efficiency, l p 0.0538 cm is the transition wavelength, and fH 2 O is the effective fraction of water emission that is encompassed by the SWAS beam (given formally by ∫ PI dQ/ ∫ I dQ, where P is the beam power pattern and I is the line intensity). Using the measured beam pattern for the SWAS antenna (Melnick et al. 2000) and the water density profile given by equation (1), we estimate the average value of fH 2 O as 0.62 during our observations of comet C/Lee. We then find that the integrated antenna temperature of 1.79 K km s21 measured by SWAS toward comet C/Lee implies a value of 8 # 10 28 s21 for the average water production rate12 during the period of 1999 May 19.01–23.69 UT.13 We conser11 The optical depth in the strongest vibrational lines responsible for pumping 22 is only ∼0.008Q29r21 10 v5 , so our neglect of optical depth effects on the pumping rate is entirely justified. 12 The estimate given previously by Bergin et al. (1999) was larger than this value because we adopted Crovisier’s (1984) original estimate of the radiative pump rate rather than the larger value adopted here. 13 The result given here for the total water production rate is based on an assumed ortho-to-para ratio of 3. Our estimate of the production rate for orthowater alone is 6 # 1028 s21. Since we have only observed emission from orthowater, our results place no constraint on the para-water production rate. Thus, if the actual ortho-to-para ratio is less than 3, the total water production rate is larger than the estimate given above. However, measurements by Crovisier et al. (1997) of the ortho-to-para ratio for water molecules in comet C/1995 O1 (Hale-Bopp) yielded a result of 2.45 5 0.10, fairly close to 3 and corresponding to a spin temperature of ∼25 K. If we had adopted this value for comet C/1999 H1 (Lee) instead of 3, our estimate of the total water production rate would have increased by only 6%.
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vatively estimate that an overall uncertainty of ∼10% in the water production rate results from possible errors in the absolute flux calibration and from statistical errors in the measured antenna temperature. Larger uncertainties result from the model used to interpret the data and, in particular, its neglect of collisional excitation by hot electrons. As discussed above, the results presented by Xie & Mumma (1992) suggest that such effects are modest, but in the absence of detailed models, it is not possible to give a meaningful quantitative estimate of their magnitude. The total water production rate of 8 # 10 28 s21 derived above is approximately 50% higher than that estimated by Biver et al. (1999) from contemporaneous radio observations of hydroxyl molecules. A discrepancy of this magnitude is perhaps unsurprising given the likely model dependence of both estimates. Our estimate of the water production rate neglects the effects of electron-impact excitation, while estimates derived from OH observations are presumably quite dependent on the assumed outflow geometry. The high sensitivity of SWAS to water vapor in the coma of comet C/Lee means that useful estimates for Q(H 2 O) can be obtained using just a subset of the data. Results are shown in Figure 2 for individual observing periods of duration ∼8 hr (five orbits), along with error bars that reflect the 1 j statistical uncertainties in the measured line fluxes (typically ∼510% for an 8 hr observing period). These error bars do not include uncertainties in calibration or modeling, but nevertheless they provide a useful estimate of relative uncertainties. Variations in the derived water production rates can be explained entirely by measurement errors, and there is no evidence for real variations in Q(H 2 O). In particular, searches for periodicity using both the Lomb-normalized periodogram method and a method based on Fourier analysis yielded no evidence for periodicity in the range of 3.25 hr–2.35 days (i.e., from the Nyquist critical frequency to one-half the total baseline). The measured line width of 1.84 km s21 (FWHM) is slightly broader than the instrumental line width (1.19 km s21 FWHM; see Melnick et al. 2000), suggesting that the cometary water line has an intrinsic width of ∼1.4 km s21. This value is in acceptable agreement with the outflow velocity of 0.85(R/AU)21/2 km s21 ∼ 0 .77 km s 21 assumed above, for which the expected line FWHM would be 0.77Î2 ∼ 1.1 km s 21. To within the accuracy with which the line centroid can be determined, there is no velocity offset between the water line emission and the rest frame of the cometary nucleus.
We thank G. Williams for his help in providing cometary ephemerides, and we gratefully acknowledge helpful discussions with H. Weaver and M. Mumma. We thank the anonymous referee for many insightful comments. This work was supported by NASA’s SWAS 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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