The unexpectedly bright comet C/2012 F6 (Lemmon) unveiled at near-infrared wavelengths

Journal Article · Wed Jan 01 04:00:00 EST 2014 · Astronomical Journal (New York, N.Y. Online)

DOI:https://doi.org/10.1088/0004-6256/147/1/15· OSTI ID:22340044

Paganini, Lucas; DiSanti, Michael A.; Mumma, Michael J.; Villanueva, Geronimo L.; Bonev, Boncho P. ^[1]; Keane, Jacqueline V.; Meech, Karen J. ^[2]; Gibb, Erika L. ^[3]; Boehnhardt, Hermann ^[4]

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1. Goddard Center for Astrobiology, NASA GSFC, MS 690, Greenbelt, MD 20771 (United States)
2. Institute for Astronomy, University of Hawaii, Honolulu, HI 96822 (United States)
3. Department of Physics and Astronomy, University of Missouri, St. Louis, MO 63121 (United States)
4. Max-Planck-Institut für Sonnensystemforschung, D-37191, Katlenburg-Lindau (Germany)

We acquired near-infrared spectra of the Oort cloud comet C/2012 F6 (Lemmon) at three different heliocentric distances (R {sub h}) during the comet's 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 {sub h} = 1.2 AU with CRIRES (on 2013 February 2 and 4), and post-perihelion at R {sub h} = 0.75 AU with CSHELL (on March 31 and April 1) and R {sub h} = 1.74 AU with NIRSPEC (on June 20). We detected 10 volatile species (H{sub 2}O, OH* prompt emission, C{sub 2}H{sub 6}, CH{sub 3}OH, H{sub 2}CO, HCN, CO, CH{sub 4}, NH{sub 3}, and NH{sub 2}), and obtained upper limits for two others (C{sub 2}H{sub 2} 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 3.31 ± 0.33 (weighted mean of CRIRES and NIRSPEC results), implying a spin temperature >37 K at the 95% confidence limit. Our (3σ) upper limit for HDO corresponds to D/H < 2.45 × 10{sup –3} (i.e., <16 Vienna Standard Mean Ocean Water, VSMOW). At R {sub h} = 1.2 AU (CRIRES), the production rate for water was Q(H{sub 2}O) = 1.9 ± 0.1 × 10{sup 29} s{sup –1} and its rotational temperature was T {sub rot} ∼ 69 K. At R {sub h} = 0.75 AU (CSHELL), we measured Q(H{sub 2}O) = 4.6 ± 0.6 × 10{sup 29} s{sup –1} and T {sub rot} = 80 K on March 31, and 6.6 ± 0.9 × 10{sup 29} s{sup –1} and T {sub rot} = 100 K on April 1. At R {sub h} = 1.74 AU (NIRSPEC), we obtained Q(H{sub 2}O) = 1.1 ± 0.1 × 10{sup 29} s{sup –1} and T {sub rot} ∼ 50 K. The measured volatile abundance ratios classify comet C/2012 F6 as rather depleted in C{sub 2}H{sub 6} and CH{sub 3}OH, while HCN, CH{sub 4}, and CO displayed abundances close to their median values found among comets. H{sub 2}CO was the only volatile showing a relative enhancement. The relative paucity of C{sub 2}H{sub 6} and CH{sub 3}OH (with respect to H{sub 2}O) suggests formation within warm regions of the nebula. However, the normal abundance of HCN and hypervolatiles CH{sub 4} and CO, and the enhancement of H{sub 2}CO, may indicate a possible heterogeneous nucleus of comet C/2012 F6 (Lemmon), possibly as a result of radial mixing within the protoplanetary disk.

• OSTI ID: 22340044
• Journal Information: Astronomical Journal (New York, N.Y. Online), Journal Name: Astronomical Journal (New York, N.Y. Online) Journal Issue: 1 Vol. 147; ISSN 1538-3881
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
AMMONIA
CARBON MONOXIDE
COMETS
DEGASSING
DISTRIBUTION
EMISSION
ETHANE
HEAVY WATER
HYDROCYANIC ACID
ICE
INDICATORS
INFRARED SPECTRA
METHANE
METHANOL
NEBULAE
ONE-DIMENSIONAL CALCULATIONS
PLANETS
PROTOPLANETS
SATELLITES