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The chemical composition of CO-rich comet C/2009 P1 (Garradd) at Rh = 2.4 and 2.0 AU before perihelion

We quantified ten parent volatiles in comet C/2009 P1 (Garradd) before perihelion, through high-dispersion infrared spectra acquired with CRIRES at ESO's VLT on UT 2011 August 07 (Rh = 2.4 AU) and September 17-21 (Rh = 2.0 AU). On August 07, water was searched but not detected at an upper limit (3{\sigma}) of 2.1 \times 10^28 s-1, while ethane was detected with a production rate of 6.1 \times 10^26 s-1 (apparent mixing ratio>2.90%). On September 17-21, the mean production rate for water was 8.4 \times 10^28 s-1, and abundance ratios (relative to water) of detected trace species were: CO (12.51%), CH3OH (3.90%), CH4 (1.24%), C2H6 (1.01%) and HCN (0.36%). Upper limits (3{\sigma}) to abundances for four minor species were: NH3 (1.55%), C2H2 (0.13%), HDO (0.89%) and OCS (0.20%). Given the relatively large heliocentric distance, we explored the effect of water not being fully sublimated within our FOV and identified the 'missing' water fraction needed to reconcile the retrieved abundance ratios with the mean values found for"organics-normal". The individual spatial profiles of parent volatiles and the continuum displayed rather asymmetric outgassing. Indications of H2O and CO gas being released in different directions suggest different active vents and/or the possible existence of polar and apolar ice aggregates in the nucleus. The high fractional abundance of CO identifies comet C/2009 P1 as a CO-rich comet.


Introduction
Until 2005, cometary nuclei were thought to be primordial remnants from the giant planets' accretion zone (then viewed as 5-30 AU from the proto-Sun), but subsequent dynamical studies challenge this view and demonstrate the need for an alternative metric of cometary origins (Morbidelli et al. 2008;Levison et al. 2010;Walsh et al. 2011).
Those origins are most directly inferred from the properties of native materials (dust, ice) in the nucleus.
Since 1985, measurements of nucleus composition have revealed major diversity amongst comets, based on crystallinity of silicates and/or the chemistry of native ices.
The interpretation, however, is partly linked to questions of orbital evolution. A comet making its first apparition after ejection from the long-term storage reservoir (Kuiper when it displayed a circular (15˝ diameter) coma with a visual magnitude ~17. Its orbit is inclined to the ecliptic by 106.2°, and the Tisserand parameter (T j ) and original semimajor axis 2 are -0.432 and 2564.1 AU, respectively. Together, these classify C/2009 P1 as a nearly isotropic (long period) comet from the OC reservoir.
In this Letter, we present results obtained from near IR observations of comet C/2009 P1 at the Very Large Telescope. These results, along with our current characterization of its organic-rich gas production and favorable astrometric positioning, establish C/2009 P1 as a prime target for astronomical observations through early 2012.

Observations
We observed comet C/2009 P1 with CRIRES at the Very Large Telescope (VLT) located in the Atacama desert (Chile), on UT 2011 August 07 and on five consecutive nights spanning September 17-21 3 . The Cryogenic high-Resolution InfraRed Echelle Spectrograph (CRIRES) provided a spectral resolution (λ/∆λ) of about 50,000 using an entrance slit of 0.4 ′′ (in width) and a spatial coverage of 40 ′′ (in length) (Käufl et al. 2004). Weather conditions were optimal during our September observations, with low wind speeds of 3 m s -1 (5 m s -1 ), relative humidity of 5% (10%) and water vapor of 1.2 mm (2.1 mm) on the first two nights (remaining nights). Seeing was in the range 0.5 ′′ - Cometary spectra were acquired in a four-step sequence (ABBA) with an integration time of 60 (or 120) seconds per step, nodding the telescope along the slit by 11 ′′ between the A and B position. We followed our standard procedures for initial data reduction and analysis of the individual echelle orders (DiSanti et al. 2001;Villanueva et al. 2011a), which included flat fielding, removal of high dark current pixels and cosmicray hits, spatial and spectral rectification, and spatial registration of individual A and B beams. After combining the A and B beams from the difference frames, we extracted a spectrum from the sum of 15 spatial rows (1.29 ′′ ) centered on the nucleus (this also assists in removing the residual background).
The simultaneous measurement of H 2 O or its direct proxy (OH*) in each instrument setting permitted accurate characterization of the abundance ratio for each trace species. Six primary volatiles (H 2 O, CO, C 2 H 6 , CH 4 , HCN, CH 3 OH) were securely detected, and upper limits were retrieved for NH 3 , C 2 H 2 , OCS, and HDO. As mentioned in our IAU Circular (Paganini et al. 2011) and shown in Figure 1, detections of eight prominent CO emission lines enabled robust retrieval of a (high) CO/H 2 O ratio.
Detections of other primary volatiles are shown in Figure 2. Several unidentified lines are seen in these spectra, however we defer a complete analysis to a future publication. Figure 3 shows the temporal development of specific production rates.  Table 1 for each setting, date, and volatile species. In the following subsections we highlight our major findings.

Rotational temperatures
We retrieved accurate rotational temperatures for H 2 O, CO, C 2 H 6 , HCN, and CH 3 OH by performing excitation analyses of extracted and predicted intensities for spectral lines of a given molecule. Predicted intensities were obtained from our custom fluorescence models (Villanueva 2011b;Villanueva 2011a, and references therein). A rotational temperature of 50 K is consistent with the average temperature from these five volatiles, although some minor differences are observed (Table 1).
On 2011 August 07 (R h = 2.4 AU), water was searched but not detected at an upper limit (3σ) of 2.1 × 10 28 s -1 , while ethane was detected with a production rate of 6.1 × 10 26 s -1 . When compared with measurements of C 2 H 6 in mid-September, the production of ethane in August is consistent with insolation-limited outgassing (varying as R h -2.2 ). The upper limit for water suggests a mixing ratio for ethane > 2.9% (3σ), which far exceeds the abundance ratio measured in late September. The likely explanation is that water sublimation was less fully activated at 2.4 AU, compared with 2.0 AU.

D. Schleicher 2011 (private communication) reports water production rates at (R h )
2.49 AU (July 28) and 1.97 AU (Sept. 24) that are larger than our values on nearby dates.
If we adjust the sublimated water fraction in mid-September to achieve "normal" abundances of C 2 H 6 (~ 0.52%) and/or HCN (~ 0.23%), the water production rate would increase by about 60-90%, resulting in abundance ratios of CO ~ 7.26%, C 2 H 6 ~ 0.59%, CH 4 ~ 0.72%, HCN ~ 0.21%, and CH 3 OH ~ 2.26%. If the 'missing' fraction were pure water ice, this increase would reconcile water production rates estimated in the infrared to those obtained in the optical. An even larger 'missing' fraction is required at 2.4 AU, but AKARI observed an increase in mixing ratios for CO 2 beyond 2.5 AU (Ootsubo et al. 2011), demonstrating the decreasing efficiency of water ice sublimation at heliocentric distances beyond 2.5 AU.

Spatial profiles
During our September observations we oriented the slit along the extended comet-Sun projected radius vector (Position Angle, P.A., ~ 96º). The Solar Phase angle of 29.5º (Table 1) indicates that the sub-solar point was well placed within the nucleus hemisphere seen from Earth.
During the first night (UT 2011 Sept 17, see Fig. 4a), the (asymmetric) water profile displayed strong enhancement in the sunward direction (here, we use "sunward" to mean the directed projection on the sky plane of the comet-sun vector). The profile of ethane was also enhanced in the sunward direction, but less so than that of water. The methane profile was symmetric about the nucleus, and displayed a steep slope in both directions with a secondary minimum at ~ 2000 km sunward.
On  Valuable lessons are also taken from observations towards young stellar objects

Conclusions
Among the organic compounds measured, we find a relative enhancement of organic volatiles in C/2009 P1 (Garradd), compared to other OC comets in our IR survey.
Among the as-measured abundance ratios, CO is significantly enhanced, CH 4 is toward the high end of values found to date, CH 3 OH is somewhat enhanced, and C 2 H 6 and HCN are normal to slightly enhanced. Our abundance of CH 4 and HCN were similar to values found in other CO-rich comets (excepting C/1996 B2, which had a lower amount of CH 4 ), while CH 3 OH and C 2 H 6 have similar to enhanced abundances among these comets.
In this work, we presented abundances ratios (in percent) relative to water. If we consider the possible effect of water not being fully sublimated within our field-of-view (FOV) at the observed heliocentric distance (~ 2 AU), and adjust the water fraction to achieve "normal" abundances of C 2 H 6 and HCN, the water production rate would increase by ~ 60-90%. Our non-detection of water in 2011 August could be related to this effect. If this added fraction were pure water ice, its addition would bring the abundance ratios of most other minor species (i.e., C 2 H 6 , CH 4 , CH 3 OH and HCN) into agreement with values in the "organics-normal" group Bockelée-Morvan et al. 2004;DiSanti and Mumma 2008). However, the CO abundance would decrease to about 7.26%, which is still higher than that seen in most OC comets. Possible explanations for a high CO composition suggest accretion in the outer region of the proto-planetary disk that underwent minor (if any) thermal processing before incorporation into the nucleus. Upcoming measurements at smaller heliocentric distances will provide additional tests of this issue.  c. Growth factor.
d. Global production rate, after applying a measured growth factor (and an error of ± 0.1) to the nucleus-centered (NC) production rate.
e. Assumed rotational temperature based on September observations. f. For molecules whose growth factor is not measured directly, we adopt GF = (1.5).
g. We tabulate the retrieved T rot and confidence limits. Measured temperatures are consistent with 53 K for all molecules. We adopted T rot = 50 K when calculating the NC production rates for all species whose T rot determination was not possible. This is indicated as (50). h. The stated molecular abundance ratios use the water production rate from September 17.
j. Retrieval of the water production rate is based on OH* emission lines, a direct proxy for water (Bonev et al. 2006    Profiles are discussed in Section 3.3.