L. Paganini

MAPPING THE RELEASE OF VOLATILES IN THE INNER COMAE OF COMETS C/2012 F6 (LEMMON) AND C/2012 S1 (ISON) USING THE ATACAMA LARGE MILLIMETER/SUBMILLIMETER ARRAY

Results are presented from the first cometary observations using the Atacama Large Millimeter/Submillimeter Array (ALMA), including measurements of the spatially resolved distributions of HCN, HNC, H_2CO, and dust within the comae of two comets: C/2012 F6 (Lemmon) and C/2012 S1 (ISON), observed at heliocentric distances of 1.5 AU and 0.54 AU, respectively. These observations (with angular resolution ≈0.5), reveal an unprecedented level of detail in the distributions of these fundamental cometary molecules, and demonstrate the power of ALMA for quantitative measurements of the distributions of molecules and dust in the inner comae of typical bright comets. In both comets, HCN is found to originate from (or within a few hundred kilometers of) the nucleus, with a spatial distribution largely consistent with spherically symmetric, uniform outflow. By contrast, the HNC distributions are clumpy and asymmetrical, with peaks at cometocentric radii ~500-1000 km, consistent with release of HNC in collimated outflow(s). Compared to HCN, the H_2CO distribution in comet Lemmon is very extended. The interferometric visibility amplitudes are consistent with coma production of H_2CO and HNC from unidentified precursor material(s) in both comets. Adopting a Haser model, the H_2CO parent scale length is found to be a few thousand kilometers in Lemmon and only a few hundred kilometers in ISON, consistent with the destruction of the precursor by photolysis or thermal degradation at a rate that scales in proportion to the solar radiation flux.

C/2013 R1 (LOVEJOY) AT IR WAVELENGTHS AND THE VARIABILITY OF CO ABUNDANCES AMONG OORT CLOUD COMETS

We report production rates, rotational temperatures, and related parameters for gases in C/2013 R1 (Lovejoy) using the Near InfraRed SPECtrometer at the Keck Observatory, on six UT dates spanning heliocentric distances (R_h ) that decreased from 1.35 AU to 1.16 AU (pre-perihelion). We quantified nine gaseous species (H_2O, OH^*, CO, CH^4, HCN, C_2H_6, CH_3OH, NH_3, and NH_2) and obtained upper limits for two others (C_2H_2 and H_2CO). Compared with organics-normal comets, our results reveal highly enriched CO, (at most) slightly enriched CH_3OH, C_2H_6, and HCN, and CH_4 consistent with normal, yet depleted, NH_3, C_2H_2, and H_2CO. Rotational temperatures increased from ~50 K to ~70 K with decreasing R_h , following a power law in R_h of –2.0 ± 0.2, while the water production rate increased from 1.0 to 3.9 × 10^(28) molecules s^(–1), following a power law in R_h of –4.7 ± 0.9. The ortho-para ratio for H_2O was 3.01 ± 0.49, corresponding to spin temperatures (T_(spin)) ≥ 29 K (at the 1σ level). The observed spatial profiles for these emissions showed complex structures, possibly tied to nucleus rotation, although the cadence of our observations limits any definitive conclusions. The retrieved CO abundance in Lovejoy is more than twice the median value for comets in our IR survey, suggesting this comet is enriched in CO. We discuss the enriched value for CO in comet C/2013 R1 in terms of the variability of CO among Oort Cloud comets.

The Unexpectedly Bright Comet C-2012 F6 (Lemmon) Unveiled at Near-Infrared Wavelengths

We acquired near-infrared spectra of the Oort cloud comet C/2012 F6 (Lemmon) at three different heliocentric distances (R h) during the comets 2013 perihelion passage, providing a comprehensive measure of the outgassing behavior of parent volatiles and cosmogonic indicators. Our observations were performed pre-perihelion at R h = 1.2xa0AU with CRIRES (on 2013 February 2 and 4), and post-perihelion at R h = 0.75xa0AU with CSHELL (on March 31 and April 1) and R h = 1.74xa0AU with NIRSPEC (on June 20). We detected 10 volatile species (H2O, OH* prompt emission, C2H6, CH3OH, H2CO, HCN, CO, CH4, NH3, and NH2), and obtained upper limits for two others (C2H2 and HDO). One-dimensional spatial profiles displayed different distributions for some volatiles, confirming either the existence of polar and apolar ices, or of chemically distinct active vents in the nucleus. The ortho-para ratio for water was 3.31xa0± 0.33 (weighted mean of CRIRES and NIRSPEC results), implying a spin temperature >37xa0K at the 95% confidence limit. Our (3σ) upper limit for HDO corresponds to D/Hxa0< 2.45xa0× 10–3 (i.e., <16 Vienna Standard Mean Ocean Water, VSMOW). At R h = 1.2xa0AU (CRIRES), the production rate for water was Q(H2O) = 1.9xa0± 0.1xa0× 1029xa0s–1 and its rotational temperature was T rotxa0~ 69xa0K. At R h = 0.75xa0AU (CSHELL), we measured Q(H2O) = 4.6xa0± 0.6xa0× 1029xa0s–1 and T rot = 80xa0K on March 31, and 6.6xa0± 0.9xa0× 1029xa0s–1 and T rot = 100xa0K on April 1. At R h = 1.74xa0AU (NIRSPEC), we obtained Q(H2O) = 1.1xa0± 0.1xa0× 1029xa0s–1 and T rotxa0~ 50xa0K. The measured volatile abundance ratios classify comet C/2012 F6 as rather depleted in C2H6 and CH3OH, while HCN, CH4, and CO displayed abundances close to their median values found among comets. H2CO was the only volatile showing a relative enhancement. The relative paucity of C2H6 and CH3OH (with respect to H2O) suggests formation within warm regions of the nebula. However, the normal abundance of HCN and hypervolatiles CH4 and CO, and the enhancement of H2CO, may indicate a possible heterogeneous nucleus of comet C/2012 F6 (Lemmon), possibly as a result of radial mixing within the protoplanetary disk.

THE INNER COMA OF COMET C/2012 S1 (ISON) AT 0.53 AU AND 0.35 AU FROM THE SUN

Using long-slit spectroscopy at the NASA Infrared Telescope Facility, we extracted H{sub 2}O production rates and spatial profiles of gas rotational temperature and molecular column abundance in comet C/2012 S1 ISON, observed at heliocentric distances of 0.53 and 0.35 AU. These measurements uniquely probed the physical environment in the inner collisional coma of this comet during its first (and last) approach to the Sun since being emplaced in the Oort Cloud some 4.5 billion years ago. Our observations revealed a comet evolving on various timescales, both over hours and days. At 0.35 AU, ISON showed a considerable decrease in water production rate in less than 2 hr, likely declining from a major outburst. Our measured temperature spatial distributions reflect the competition between the processes that cause heating and cooling in the coma, and also provide insight about the prevalent mechanism(s) of releasing gas-phase H{sub 2}O. The observed temperatures suggest that the comet was likely ejecting icy material continuously, which sublimated in the coma and heated the ambient gas, augmenting fast H-atoms produced by H{sub 2}O photolysis. ISON adds to the very limited sample of comets for which spatial-spectral studies of water temperatures have been conducted. These studies are now feasible and can bemorexa0» extended to comets having a variety of gas production rates. Continued synergy of such observations with both space missions like Rosetta and with physical models is strongly encouraged in order to gain a deeper understanding of the processes in the inner collisional zone of the cometary coma.«xa0less

EN ROUTE TO DESTRUCTION: THE EVOLUTION IN COMPOSITION OF ICES IN COMET D/2012 S1 (ISON) BETWEEN 1.2 AND 0.34 AU FROM THE SUN AS REVEALED AT INFRARED WAVELENGTHS*

We report production rates for H2O and eight trace molecules (CO, C_2H_6, CH_4, CH_3OH, NH_3, H_2CO, HCN, C_2H_2) in the dynamically new, Sun-grazing Comet C/2012 S1 (ISON), using high-resolution spectroscopy at Keck II and the NASA IRTF on 10 pre-perihelion dates encompassing heliocentric distances R_h = 1.21–0.34 AU. Measured water production rates spanned two orders of magnitude, consistent with a long-term heliocentric power law Q(H_2O) ∝ R_h^(-3.1±0.1). Abundance ratios for CO, C_2H_6, and CH_4 with respect to H_2O remained constant with R_h and below their corresponding mean values measured among a dominant sample of Oort Cloud comets. CH_3OH was also depleted for R_h > 0.5 AU, but was closer to its mean value for R_h ≤ 0.5 AU. The remaining four molecules exhibited higher abundance ratios within 0.5 AU: for R_h > 0.8 AU, NH_3 and C_2H_2 were consistent with their mean values while H_2CO and HCN were depleted. For R_h < 0.5 AU, all four were enriched, with NH_3, H_2CO, and HCN increasing most. Spatial profiles of gas emission in ISON consistently peaked sunward of the dust continuum, which was asymmetric antisunward and remained singly peaked for all observations. NH_3 within 0.5 AU showed a broad spatial distribution, possibly indicating its release in the coma provided that optical depth effects were unimportant. The column abundance ratio NH_2/H_2O at 0.83 AU was close to the typical NH/OH from optical wavelengths, but was higher within 0.5 AU. Establishing its production rate and testing its parentage (e.g., NH_3) require modeling of coma outflow.

THE VOLATILE COMPOSITION OF COMET C/2003 K4 (LINEAR) AT NEAR-IR WAVELENGTHS—COMPARISONS WITH RESULTS FROM THE NANÇAY RADIO TELESCOPE AND FROM THE ODIN, SPITZER, AND SOHO SPACE OBSERVATORIES

We observed comet C/2003 K4 (LINEAR) using NIRSPEC at the Keck Observatory on UT 2004 November 28, when the comet was at 1.28 AU from the Sun (post-perihelion) and 1.38 AU from Earth. We detected six gaseous species (H2O, OH*, C2H6, CH3OH, CH4, and HCN) and obtained upper limits for three others (H2CO, C2H2, and NH3). Our results indicate a water production rate of (1.72 ± 0.18) × 1029 molecules s−1, in reasonable agreement with production rates from SOHO (on the same day), Odin (one day earlier), and Nancay (about two weeks earlier). We also report abundances (relative to water) for seven trace species: CH3OH (∼1.8%), CH4 (∼0.9%), and C2H6 (∼0.4%) that were consistent with mean values among Oort cloud (OC) comets, while NH3 (<0.55%), HCN (∼0.07%), H2CO (<0.07%), and C2H2 (<0.04%) were “lower” than the mean values in other OC comets. We extracted inner-coma rotational temperatures for four species (H2O, C2H6, CH3OH, and CH4), all of which are consistent with 70 K (within 1σ). The extracted ortho-para ratio for water was 3.0 ± 0.15, corresponding to spin temperatures larger than 39 K (at the 1σ level) and agreeing with those obtained with the Spitzer Space Telescope at the 2σ level.

Thermal Physics of the Inner Coma: ALMA Studies of the Methanol Distribution and Excitation in Comet C/2012 K1 (PanSTARRS)

We present spatially and spectrally-resolved observations of CH

ALMA Mapping of Rapid Gas and Dust Variations in Comet C/2012 S1 (ISON): New Insights into the Origin of Cometary HNC

_3

AN INFRARED SEARCH FOR HDO IN COMET D/2012 S1 (ISON) AND IMPLICATIONS FOR iSHELL

OH emission from comet C/2012 K1 (PanSTARRS) using The Atacama Large Millimeter/submillimeter Array (ALMA) on 2014 June 28-29. Two-dimensional maps of the line-of-sight average rotational temperature (

Pre- and post-perihelion observations of C/2009 P1 (Garradd): Evidence for an oxygen-rich heritage?

T_{rot}