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IRCS/Subaru Observations of Water in the Inner Coma of Comet 73P-B/Schwassmann-Wachmann 3: Spatially Resolved Rotational Temperatures and Ortho-Para Ratios

Comet 73P-B/Schwassmann-Wachmann 3 was observed with IRCS/Subaru at geocentric distance of 0.074 AU on UT 10 May 2006. Multiple H2O emission lines were detected in non-resonant fluorescence near 2.9 microns. No significant variation in total H2O production rate was found during the (3 hour) duration of our observations. H2O rotational temperatures and ortho-to-para abundance ratios were measured for several positions in the coma. The temperatures extracted from two different time intervals show very similar spatial distributions. For both, the rotational temperature decreased from ~110 K to ~90 K as the projected distance from the nucleus increased from ~5 to ~30 km. We see no evidence for OPR change in the coma. The H2O ortho-para ratio is consistent with the statistical equilibrium value (3.0) for all spatially resolved measurements. This implies a nuclear spin temperature higher than ~ 45 K.


Introduction
Water presents a paradigm for the release of parent 1 volatiles in comets. As the dominant volatile species in the nuclei of active comets, sublimation of water controls the release of other molecules within ~3 AU from the Sun. Understanding the excitation of H 2 O and other molecules in cometary comae is absolutely essential for determining accurate abundances of sublimated parent species, thereby characterizing the volatile composition of the comet nucleus.
Here we present results obtained from a detailed spectroscopic analysis of H 2 O "hot-band" emission from the inner coma of comet 73P-B/Schwassmann-Wachmann 3 (hereafter, SW3-B) 3 . The principal goal of this work is to investigate the (possible) 1 The term "parent" (or "native") relates to molecules that have sublimated directly from the comet nucleus. Some molecules present as ice in the nucleus can also be released from extended sources (e.g. icy or dust grains) in the comet coma (escaping atmosphere). Chemical "splitting" (photodissociation, photoionization, etc) of parent molecules in the coma gives rise to "daughter" species (e.g. OH from H 2 O). 2 First detected in the Kuiper Airborne Observatory spectra of 1P/Halley (Mumma et al. 1986. 3 The Jupiter Family comet 73P/Schwassmann-Wachmann 3 (orbital period of ~5.4 yr.) split into three components ("A", "B", and "C") during its 1995 apparition and more components were found later (Sekanina 2005 (Crovisier 1984;Mumma et al. 1986Mumma et al. , 1987Mumma et al. , 1993. This (somewhat controversial) hypothesis implies that the OPR might have cosmogonic significance, since comet nuclei preserve a chemical record from the formative stages of our planetary system.
Section 2 gives the essential physics background (including defining terminology) for the excitation of water in cometary comae and the "hot-band" fluorescence mechanism. Section 3 describes our observations. Section 4 presents our detections of H 2 O in non-resonant fluorescence. Section 5 describes the spatially resolved measurements of H 2 O rotational temperature and OPR. Section 6 presents time-resolved H 2 O production rates, followed by a summary of results.
active in 2006 displaying frequent outbursts and fragmentations (see Weaver et al. 2006). Various components of SW3 have been investigated at multiple wavelengths. Near-IR (2-5 µm) molecular studies focused primarily on the organic parent volatile composition of fragments "B" and "C" , Kobayashi et al. 2007).

Physics Background
When a comet nucleus enters the inner solar system, sublimation processes are activated by sunlight. Sublimated molecules leave the nucleus with vibrational and rotational temperatures expected to be equal to the nucleus surface temperature, i.e. about 150-200 K at 1 AU. Except within a few hundred meters from the nucleus, collisional rates are too small to maintain the initial vibrational population, so this population decays radiatively to the ground vibrational state (Weaver & Mumma 1984 The H 2 O molecule is organized into two distinct spin species depending on whether the nuclear spin vectors of its (identical) hydrogen atoms are parallel (ortho "ladder") or anti-parallel (para "ladder"). The ratio between the total populations of ortho and para states (i.e. the OPR) is temperature dependent if we assume that the given OPR was realized in thermal equilibrium. Thus, nuclear spin temperatures (T spin ) can be derived from measured OPRs. Para states are increasingly favored at low spin temperatures, while at T spin > ~50 K the OPR reaches the statistical equilibrium value of 3.0 (for additional details see Mumma et al. 1987, Figure 4;Crovisier 2006;Bonev et al. 2007, Figure 1).
An important problem in chemical physics and in cometary science is whether or not water molecules undergo nuclear spin conversion during their long residence in the interior of a comet nucleus or during and after sublimation. If realized, such processes would alter the "primordial" OPR, leading to spin temperatures that differ from the chemical formation temperature of cometary H 2 O. For an isolated H 2 O molecule in the coma, nuclear spin conversion is strongly forbidden for either radiative or (neutralneutral) collisional transitions, leading to the prediction that the ortho-to-para ratio should be invariant with distance from the nucleus. Bonev et al. 2007 recently applied a method for testing nuclear spin conversion in the inner cometary coma, through spatially-resolved OPR measurements about the nucleus (the same technique is used in the present work).

Detection of H 2 O in Non-Resonant Fluorescence
Infrared H 2 O emission from the coma of comet SW3-B was observed on UT 10 May 2006 with the 8-meter Subaru telescope atop Mauna Kea, Hawaii. Spectra of high resolution (λ/δλ ~ 10-12,000) were acquired with the Infrared Camera and Spectrograph (IRCS; Kobayashi et al. 2000). Based on these data, Kobayashi et al. (2007;hereafter Paper I) measured the organic volatile composition of SW3-B. In this second paper, we present a detailed spectroscopic study of H 2 O "hot-band" emission. Algorithms for data processing, flux calibration (based on observation of standard star, see Paper I), and spectral extraction were developed specifically for our comet observations. These algorithms, along with recent substantial improvements are described elsewhere (Bonev 2005

4.
Rotational Temperature and Ortho-Para Ratio

Methodology
The rotational temperature and OPR can be determined from the relative intensities of the detected H 2 O lines (see Bonev et al. 2007. Some ortho and para transitions that are closely spaced in frequency appear blended at the resolution of our spectra. In order to use all H 2 O lines (including ortho-para blends) in the analysis, we developed an iterative algorithm for finding T rot and OPR. For each iteration: 1. The rotational temperature T rot (i) is obtained for an assumed OPR(i) value  and LA + ' settings satisfy this criterion. We extracted five separate measurements of T rot corresponding to different projected distances from the intensity peak 5 , as indicated on

Spatially-resolved Measurements of the H 2 O Ortho-Para Ratio.
Our OPR measurements are shown as diamonds on Figure 3a (LA + ') and Figure   3b (LA -). Stochastic error bars (1σ) are shown for each measurement. For each setting, the weighted mean ortho-para ratio (<OPR>) is indicated as a solid line and its stochastic and systematic uncertainties (discussed in §5.4) are shown as dotted and dashed lines respectively. For LA + ' we obtain <OPR> = 3.28 ± 0.25, while for LAwe obtain <OPR> = 3.05 ± 0.26 (systematic errors). Assuming that the OPR did not change during the measurement interval, combining LA + ' and LAyields a mean weighted value <OPR> = 3.17 ± 0.20. Within the uncertainty, we conclude that the ortho-to-para ratio in SW3-B 5 Emission from a parent volatile commonly peaks at the nucleus.
is consistent with the statistical equilibrium value of 3.0, with T spin > ~45K. We see no evidence for OPR change in the coma.

Discussion of T rot and OPR Results
The T rot and OPR measurements test the quality of extant H 2 O fluorescence models. The bottom panels in Figure 1a, 1b, and 1c show modeled H 2 O spectra, for the LA + ', LA 0 , and LAsetting respectively. These models are calculated for OPR = 3.0 and T rot is taken from the analysis of LA + ' and LA -(T rot = 110 K is adopted for LA 0 ). The overall correlations between modeled and observed spectra are > 95% for the nucleuscentered extracts in LA + ' and LA -. Two of the brightest lines sampled in the LA 0 setting (at rest frequencies ~3394 and ~3373 cm -1 ) cannot be adequately modeled. The flux to gfactor ratio (F/g) for these lines was also abnormally high in several other comets, suggesting significantly underestimated fluorescence efficiency and/or blends with unknown lines.
Systematic sources of error in T rot and (especially) the OPR are quantified by the line-byline scatter in the flux to g-factor ratio (F/g) (see insets in Figures 1a-1c) and might include (slight) inaccuracies in predicted line frequencies, g-factors, and/or corrections for telluric extinction. In addition, while each T rot measurement provides an "effective" value for a given aperture, the actual temperature varies radially from the comet nucleus.
Some of the scatter in (line-by-line) F/g values and in the OPR spatially-resolved measurements (Fig. 3a, 3b) might be attributed to this effect, if the relative intensities of H 2 O lines deviate from those predicted by assuming a unique (for a given aperture) temperature.

5.1
Temperature Variation in the Inner Coma.
There are a number of model predictions for the temperature structure of cometary comae (for example see the reviews of Combi et al. 2004, Gombozi et al. 1986, andreferences therein;Combi et al. 1999;Combi 1989;Ip 1989;Combi and Smyth 1988;Bockelee-Morvan & Crovisier 1987;Marconi and Mendis 1982). Our T rot measurements might be valuable for qualitative 6 comparison with these models, particularly for the innermost ~30 km from the nucleus. In this coma region, the radial temperature gradient is predicted to be only weakly dependent on the gas production rate (Bockelee-Morvan & . Gas temperature profiles modeled for heliocentric distances of ~ 1 AU (Combi & Smyth 1988;Combi et al. 1999) predict temperatures of ~ 100 K near the nucleus and a temperature decrease (attributed mainly to near-adiabatic cooling) of several tens of degrees, similar to the observations reported here. Neutral-neutral collisions become less frequent with increasing nucleocentric distance, and electron collisions then become the controlling agent for thermalizing the H 2 O rotational distribution (10 3 -10 4 km, Xie and Mumma 1992). Depending on both gas production rate and solar UV flux density, the temperature is predicted to increase in outer regions of the 6 There are two main reasons for the qualitative nature of the comparison. First, the T rot measurements (Fig. 3a, 3b) do not correspond to unique cometocentric distances. Instead, each measurement is an "effective" temperature for the given range of projected distances from the peak gas intensity and for the full range of cometocentric distances sampled in the corresponding gas column along the line of sight. Second, models typically assume only water release directly from the nucleus. We cannot rule out that SW3-B had a secondary source of sublimating H 2 O molecules in near-nucleus coma, since the comet was especially active, releasing chunks of bulk material from its surface.
coma due to heating from thermalized H-atoms, created by water photolysis. However, the SW3 observations do not sample those outer (> 10 2 km) regions.

5.2
Ortho-Para Ratio Consistent with Statistical Equilibrium. Dello

Search for Temporal Variations in the H 2 O Production Rate
Paper I reported a rotational temperature (only for a nucleus-centered spectral extract) and a water production rate (1.9 ± 0.2 x 10 28 molecules s -1 ), that were extracted after combining the spectra from all three instrument settings used during the night.
Here, we search for temporal variability in Q(H 2 O) during the ~3 hour time frame of our observations. Our methodology for retrieving Q(H 2 O) has been described in detail in DiSanti et al. (2001DiSanti et al. ( , 2006, Bonev et al. 2006 and references therein. We measured the H 2 O production rates by analyzing separately each individual setting, first extracting a rotational temperature ( §4) and then total production rates from the lines measured. For LA + ' and LAwe used the T rot values retrieved for the nucleus-centered extracts (107 K and 110 K, respectively). The transitions in LA 0 do not constrain T rot well because of insufficient spread in rotational excitation. For this setting, we adopted the temperature found in the LAsetting (observed immediately after LA 0 , see Table 1), and we verified that the result for Q(H 2 O) measured in LA 0 does not change significantly if the temperature found in LA + ' is used instead. We also verified that the Q(H 2 O) measurements in LAand LA + ' are not affected by varying T rot within ranges that exceed its systematic uncertainty of 4 K.
The production rates extracted from each individual setting are shown in Table 1.
The measurement in LA 0 and LAdeviate by less than 1σ from the result reported in Paper I, while the measurement in LA + ' is slightly lower, although definitive evidence for variability cannot be argued given the size of the measurement error.
The present results do not support significant variability in water production within the ~3 hour span of our observation, even though the comet was observed after an outburst and the gas productivity was found to decrease by a factor of ~1.