Reiko Furusho

Spin Temperatures of Ammonia and Water Molecules in Comets

The nuclear spin temperature, which is derived from the ortho-to-para abundance ratio of molecules measured in cometary comae, is a clue to the formation conditions of cometary materials, especially the physical temperature at which the molecules were formed. In this paper we present new results for the nuclear spin temperatures of ammonia in comets Hale-Bopp (C/1995 O1) and 153P/Ikeya-Zhang based on observations of NH2 at 26 and 32 K, respectively. These results are similar to previous measurements in two other comets, and the nuclear spin temperatures of ammonia in the four comets are concentrated at about 30 K. We emphasize that the nuclear spin temperatures of water measured thus far have also been about 30 K. In particular, the spin temperatures of ammonia and water are equal to each other within ±1 σ error bars in the case of comet Hale-Bopp. These nuclear spin temperatures of ammonia and water were measured under quite different conditions (heliocentric distances and gas production rates). There is no clear trend between the nuclear spin temperatures and the heliocentric distances, the gas production rates, or the orbital periods of the comets. The possibilities of the ortho-to-para conversion in the coma and in the nucleus are discussed. The present data set implies that the ortho-to-para ratios were not altered after the molecules were incorporated into the cometary nuclei. It appears that cometary ammonia and water molecules formed on cold grains at about 30 K.

ORTHO-TO-PARA RATIOS OF WATER AND AMMONIA IN COMET C/2001 Q4 (NEAT): COMPARISON OF NUCLEAR SPIN TEMPERATURES OF WATER, AMMONIA, AND METHANE

Cold nuclear spin temperatures found in cometary molecules have been simply interpreted as the physical temperature when the population distribution among different nuclear spin isomers was determined through thermal equilibrium processes. However, the real meaning of cold nuclear spin temperatures is unclear due to a lack of experimental studies about the ortho-to-para ratios (OPRs) of molecules in cometary ice analogs. Here we report the nuclear spin temperatures (Tspin) of water and ammonia in comet C/2001 Q4 (NEAT). Measurements of the nuclear spin temperatures of these species and methane from previous work are all consistent with ~30 K. Consistency of the nuclear spin temperatures among different molecular species may suggest that OPRs (or abundance ratios of different nuclear spin isomers) of these molecules were last determined in thermal equilibrium. The obtained nuclear spin temperature of cometary ices is not consistent with molecular formation by hydrogen-atom addition reactions on cold grains, where the H atoms accreted from the gas phase onto grains. The condensation process on the grains might control the ortho-to-para ratios of the precometary ices, or conversion of OPRs within the ices might occur. The small diversity of the nuclear spin temperatures and lack of clear correlation between Tspin and chemical composition in several comets are consistent with the hypothesis that Tspin reflects the temperatures in the presolar nebula.

Nuclear Spin Temperature and Deuterium-to-Hydrogen Ratio of Methane in Comet C/2001 Q4 (NEAT)*

We carried out high-dispersion, spectroscopic observations of comet C/2001 Q4 (NEAT) in the near-infrared with the 8 m Subaru telescope and detected the R-branch emission series of the ν3 vibrational band of methane. The signal-to-noise ratio of the observed spectrum was sufficient to make the first determination of the nuclear spin temperature of methane, derived to be 33 K, which reflects the temperature of formation or condensation of molecules on cold grains. The upper limit of the CH3D/CH4 ratio was determined to be 0.04 (95% confidence limit), indicating the formation of methane in a dense molecular cloud at temperatures higher than about 30 K. On the basis of these observational results, we conclude that the Sun was born in a warm molecular cloud near 30 K, not in a cold dark cloud near 10 K, as is usually assumed.

The Thickness and Formation Age of the Surface Layer on Comet 9P/Tempel 1

Cometary nuclei are believed to contain important information on the condition of the solar nebula, but there is little observational data available on their interior structure. Our ground-based observations of NASAs Deep Impact event show that comet 9P/Tempel 1 has a surface layer consisting of small (submicron-sized) carbonaceous grains whose thickness is several tens of centimeters. This suggests that comet 9P/Tempel 1 contains, at several tens of centimeters of depth, material that has not metamorphosed since this comet left the trans-Neptunian region. This further implies that many short-period comets may maintain the components they had upon leaving the trans-Neptunian region at ~1 m of depth from the surface, even after numerous perihelion passages.

The 10 Micron Spectra of Comet C/2002 V1 (NEAT) and C/2001 RX14 (LINEAR)*

We have carried out mid-infrared 8–13 � m spectroscopic observations of C/2002 V1 (NEAT) and C/2001 RX14 (LINEAR) on 2003 January 10–11 UT using the Cooled Mid-Infrared Camera and Spectrometer (COMICS) on the 8.2 m Subaru Telescope. The spectra of C/2002 V1 showed the broad silicate feature with the 11.2 � m peak, indicating the presence of crystalline olivine grains. The spectra of C/2001 RX14 also showed the broad trapezoidal silicate feature. The silicate feature profile of C/2002 V1 is explained by a combination of small (0.1 � m) amorphous olivine and pyroxene, large (2.0 � m) amorphous silicate, and small crystalline forsterite grains, while that of C/2001 RX14 is explained by small and large amorphous silicate grains without crystalline silicate grains. The ubiquity of large grains and crystalline silicate grains among these Oort Cloud comets indicates processing of the primordial interstellar matter in the early solar system and incorporation into the region where these cometary nuclei were formed. Subject headings: comets: individual (C/2002 V1, C/2001 RX14) — infrared: solar system

DETECTION OF PARENT H2O AND CO2 MOLECULES IN THE 2.5-5 μm SPECTRUM OF COMET C/2007 N3 (LULIN) OBSERVED WITH AKARI

Comet C/2007 N3 (Lulin) was observed with the Japanese infrared satellite AKARI in the near-infrared at a post-perihelion heliocentric distance of 1.7 AU. Observations were performed with the spectroscopic (2.5-5.0 μm) and imaging (2.4, 3.2, and 4.1 μm) modes on 2009 March 30 and 31 UT, respectively. AKARI images of the comet exhibit a sunward crescent-like shape coma and a dust tail extended toward the anti-solar direction. The 4.1 μm image (CO/CO2 and dust grains) shows a distribution different from the 2.4 and 3.2 μm images (H2O and dust grains). The observed spectrum shows distinct bands at 2.66 and 4.26 μm, attributed to H2O and CO2, respectively. This is the fifth comet in which CO2 has been directly detected in the near-infrared spectrum. In addition, CO at 4.67 μm and a broad 3.2-3.6 μm emission band from C-H bearing molecules were detected in the AKARI spectrum. The relative abundance ratios CO2/H2O and CO/H2O derived from the molecular production rates are ~4%-5% and <2%, respectively. Comet Lulin belongs to the group that has relatively low abundances of CO and CO2 among all observed comets.

Heliocentric Dependence of Sodium Emission of Comet Hale-Bopp (C/1995O1)

A sequential spectroscopic observation of comet C/1995 O1 (Hale-Bopp) was performed from 1996 September to 1997 May. After the first positive detection in early 1997 February, the development of the neutral sodium emission was monitored during about 4 months around the perihelion passage. The ratio of the sodium D-line emission relative to the continuum emission strength varied with the days from perihelion. The sodium D-line emission was weak relative to the continuum near the perihelion because of the Swings effect. We corrected the Swings effect, the phase angle dependence of continuum emission, and the differences in geocentric distance. We found that the corrected sodium-to-continuum ratio varied with the dependence of r-k; k = 5-6, r denotes a heliocentric distance of the comet. Our result supports the hypothesis of thermal desorption of sodium atoms from dust grains.

Heliocentric Dependence of the Sodium Emission of Comet 153P/Ikeya-Zhang

A low-dispersion spectroscopic monitor of comet 153P/Ikeya-Zhang was carried out from 2002 February through May. The sodium emission was derived mainly during March, when the heliocentric distance of the comet was between 0.511 and 0.764 AU. The number of the produced sodium atoms relative to the total cross section of dust grains follows a power law of r-5.1±1.0, where r is the heliocentric distance. This fact strongly suggests that the sodium is released by a thermal desorption mechanism, not by the photosputtering mechanism from cometary grains. The derived potential barrier for the release of the sodium is 0.49 ± 0.10 eV, which is about half of that in the case of the Moon.

PHOTOMETRIC MEASUREMENTS OF H2O ICE CRYSTALLINITY ON TRANS-NEPTUNIAN OBJECTS*

We present a measurement of H2O ice crystallinity on the surface of trans-neptunian objects (TNOs) with near-infrared narrow-band imaging. The newly developed photometric technique allows us to efficiently determine the strength of an 1.65-um absorption feature in crystalline H2O ice. Our data for three large objects, Haumea, Quaoar, and Orcus, which are known to contain crystalline H2O ice on the surfaces, show a reasonable result with high fractions of the crystalline phase. It can also be pointed out that if the H2O-ice grain size is larger than ~20 um, the crystallinities of these objects are obviously below 1.0, which suggest the presence of the amorphous phase. Especially, Orcus exhibits a high abundance of amorphous H2O ice compared to Haumea and Quaoar, possibly indicating a correlation between bulk density of the bodies and surface crystallization degree. We also found the presence of crystalline H2O ice on Typhon and 2008 AP129, both of which are smaller than the minimum size limit for inducing cryovolcanism as well as a transition from amorphous to crystalline through the thermal evolution due to the decay of long-lived isotopes.

Spin Temperature of Ammonia Determined from NH2 in Comet C/2001 A2 (LINEAR)

The ortho-to-para ratio (OPR) of a cometary molecule is one of primordial character in comets. The OPR which is characterized by a spin temperature, is thought to reflect the formation conditions of the molecule. In this paper we show the high-dispersion spectrum of cometary NH2 in Comet C/2001 A2 (LINEAR), from which the OPR of NH2 is determined based on the fluorescence excitation model. Since the NH2 is a photodissociation product of cometary ammonia, we applied the permutation group theory to the whole reaction system (i.e. the photodissociation reaction of ammonia to NH2 and H) in order to derive the OPR of ammonia from that of NH2.