Cometary Spectra Research Articles

Optical spectrophotometric monitoring of comet C/2006 W3 (Christensen) before perihelion

Context. Long period distant comet C/2006 W3 (Christensen) was discovered in 2006 and passed perihelion at 3.13 au in July 2009. It was a target for the Herschel Space Observatory, Spitzer Space Telescope, and ground-based radio telescopes. We enlarge the observational database with optical spectrophotometric ground base data.Aims. We performed optical monitoring of cometary activity and searched for possible emission lines in the C/2006 W3 (Christensen) spectrum in a wide range of heliocentric distances.Methods. We carried out pre-perihelion observations of the comet at heliocentric distances from 8.49 to 3.13 au with the 2 m and 6 m telescopes, both in imaging and spectroscopic modes, during the period 2006 to 2009.Results. No emissions were detected in the spectra at heliocentric distances of 8.49 and 6.25 au. Rich molecular emission spectra were obtained, when the comet was at a distance of 3.74 au from the Sun. The CN, C3 , C2 , CH, NH2 , CO+ , and, probably, CH+ emissions were assigned in the 3800–6800 A spectral range. Gas production rates or corresponding upper limits in the emissionless case were calculated using a Haser model. We derived the gas production rates of 1.03 × 1026 mol s-1 for CN, 4.12 × 1024 mol s-1 for C3 , 6.54 × 1025 mol s-1 for C2 , and 3.12 × 1025 mol s-1 for NH2 , which is matched with the CN:C3 :C2 :NH2 = 1.0:0.04:0.63:0.30 gas production ratios at a heliocentric distance of 3.74 au. The CO+ production rate of the comet, which was equal to 2 × 1024 mol s-1 , was also obtained. We quantified the evolution of the comet activity in a wide range of heliocentric distances with the Afρ technique. Heliocentric dependence of the Afρ parameters indicates that steep growth of the comet activity occurred around 3.96 au, almost simultaneously with the appearance of molecular emissions in the comet spectrum. The dust production rate was estimated from the Afρ reaching a maximal value of 380 kg s-1 at a distance of 3.13 au before perihelion passage.

Mid-infrared spectra of comet nuclei

Comet nuclei and D-type asteroids have several similarities at optical and near-IR wavelengths, including near-featureless red reflectance spectra, and low albedos. Mineral identifications based on these characteristics are fraught with degeneracies, although some general trends can be identified. In contrast, spectral emissivity features in the mid-infrared provide important compositional information that might not otherwise be achievable. Jovian Trojan D-type asteroids have emissivity features strikingly similar to comet comae, suggesting that they have the same compositions and that the surfaces of the Trojans are highly porous. However, a direct comparison between a comet and asteroid surface has not been possible due to the paucity of spectra of comet nuclei at mid-infrared wavelengths. We present 5–35 µm thermal emission spectra of comets 10P/Tempel 2, and 49P/Arend-Rigaux observed with the Infrared Spectrograph on the Spitzer Space Telescope. Our analysis reveals no evidence for a coma or tail at the time of observation, suggesting the spectra are dominated by the comet nucleus. We fit each spectrum with the near-Earth asteroid thermal model (NEATM) and find sizes in agreement with previous values. However, the NEATM beaming parameters of the nuclei, 0.74–0.83, are systematically lower than the Jupiter-family comet population mean of 1.03 ± 0.11, derived from 16- and 22-µm photometry. We suggest this may be either an artifact of the spectral reduction, or the consequence of an emissivity low near 16 µm. When the spectra are normalized by the NEATM model, a weak 10-µm silicate plateau is evident, with a shape similar to those seen in mid-infrared spectra of D-type asteroids. A silicate plateau is also evident in previously published Spitzer spectra of the nucleus of comet 9P/Tempel 1. We compare, in detail, these comet nucleus emission features to those seen in spectra of the Jovian Trojan D-types (624) Hektor, (911) Agamemnon, and (1172) Aneas, as well as those seen in the spectra of seven comet comae. The comet comae present silicate features with two distinct shapes, either trapezoidal, or more rounded, the latter apparently due to enhanced emission near 8 to 8.5 µm. The surfaces of Tempel 2, Arend-Rigaux, and Hektor best agree with the comae that present trapezoidal features, furthering the hypothesis that the surfaces of these targets must have high porosities in order to exhibit a spectrum similar to a comet coma. An emissivity minimum at 15 µm, present in the spectra of Tempel 2, Arend-Rigaux, Hektor, and Agamemnon, is also described, the origin of which remains unidentified. The compositional similarity between D-type asteroids and comets is discussed, and our data supports the hypothesis that they have similar origins in the early Solar System.

Optical spectroscopy of comet C/2014 Q2 (Lovejoy) from the Mount Abu Infrared Observatory

Spectra of comet C/2014 Q2 (Lovejoy) were taken with a low resolution spectrograph mounted on the 0.5 m telescope at the Mount Abu Infrared Observatory (MIRO), India during January to May 2015 covering the perihelion and post-perihelion periods. The spectra showed strong molecular emission bands (C2, C3 and CN) in January, close to perihelion. We have obtained the scale lengths for these molecules by fitting the Haser model to the observed column densities. The variation of gas production rates and production rate ratios with heliocentric distance were studied. The extent of the dust continuum using the Af-rho parameter and its variation with the heliocentric distance were also investigated. The comet is seen to become more active in the post-perihelion phase, thereby showing an asymmetric behaviour about the perihelion.

Plasma code for astrophysical charge exchange emission at X-ray wavelengths

Charge exchange X-ray emission provides unique insights into the interactions between cold and hot astrophysical plasmas. Besides its own profound science, this emission is also technically crucial to all observations in the X-ray band, since charge exchange with the solar wind often contributes a significant foreground component that contaminates the signal of interest. By approximating the cross sections resolved to $n$ and $l$ atomic subshells, and carrying out complete radiative cascade calculation, we create a new spectral code to evaluate the charge exchange emission in the X-ray band. Comparing to collisional thermal emission, charge exchange radiation exhibits enhanced lines from large-$n$ shells to the ground, as well as large forbidden-to-resonance ratios of triplet transitions. Our new model successfully reproduces an observed high-quality spectrum of comet C/2000 WM1 (LINEAR), which emits purely by charge exchange between solar wind ions and cometary neutrals. It demonstrates that a proper charge exchange model will allow us to probe remotely the ion properties, including charge state, dynamics, and composition, at the interface between the cold and hot plasmas.

The survivability of phyllosilicates and carbonates impacting Stardust Al foils: Facilitating the search for cometary water

AbstractComet 81P/Wild 2 samples returned by NASA's Stardust mission provide an unequalled opportunity to study the contents of, and hence conditions and processes operating on, comets. They can potentially validate contentious interpretations of cometary infrared spectra and in situ mass spectrometry data: specifically the identification of phyllosilicates and carbonates. However, Wild 2 dust was collected via impact into capture media at ~6 km s−1, leading to uncertainty as to whether these minerals were captured intact, and, if subjected to alteration, whether they remain recognizable. We simulated Stardust Al foil capture conditions using a two‐stage light‐gas gun, and directly compared transmission electron microscope analyses of pre‐ and postimpact samples to investigate survivability of lizardite and cronstedtite (phyllosilicates) and calcite (carbonate). We find the phyllosilicates do not survive impact as intact crystalline materials but as moderately to highly vesiculated amorphous residues lining resultant impact craters, whose bulk cation to Si ratios remain close to that of the impacting grain. Closer inspection reveals variation in these elements on a submicron scale, where impact‐induced melting accompanied by reducing conditions (due to the production of oxygen scavenging molten Al from the target foils) has resulted in the production of native silicon and Fe‐ and Fe‐Si‐rich phases. In contrast, large areas of crystalline calcite are preserved within the calcite residue, with smaller regions of vesiculated, Al‐bearing calcic glass. Unambiguous identification of calcite impactors on Stardust Al foil is therefore possible, while phyllosilicate impactors may be inferred from vesiculated residues with appropriate bulk cation to Si ratios. Finally, we demonstrate that the characteristic textures and elemental distributions identifying phyllosilicates and carbonates by transmission electron microscopy can also be observed by state‐of‐the‐art scanning electron microscopy providing rapid, nondestructive initial mineral identifications in Stardust residues.

Absorption and Laser Induced Fluorescence Spectroscopy of Neutral Polycyclic Aromatic Hydrocarbons in Argon matrices

Polycyclic aromatic hydrocarbons (PAHs) are considered as plausible carriers for the extended red emission (ERE), a photoluminescent process associated with a wide variety of interstellar environments, as well as for broad emission band features seen in cometary spectra. We report the absorption spectra of phenanthrene, anthracene, fluoranthene, pentacene, pyrene, chrysene and triphenylene isolated at 10 K in solid argon matrices together with laser induced fluorescence (LIF) spectra at 355 nm of matrix-isolated anthracene and fluoranthene. LIF spectra are compared with the UV/blue fluorescence spectra of the Red Rectangle Nebula (RR). The LIF spectra measured in solid Ar matrices have been shifted to the predicted position of the PAH band emission in the gas phase for comparison with the astronomical observations (Fig. 1).

High-resolution spectra of comet C/2013 R1 (Lovejoy)

Context. High-resolution spectra of comets permit deriving the physical properties of the coma. In the optical range, relative production rates can be computed, and information about isotopic ratios and the origin of oxygen atoms can be obtained.Aims. The main objective of the work presented here was to obtain information about the chemical composition of comet C/2013 R1 (Lovejoy), a bright and long-period comet that passed perihelion (0.81 au) on 22 December 2013.Methods. We used the HARPS-North echelle spectrograph at the 3.5 m telescope TNG to obtain high-resolution spectra of comet C/2013 R1 (Lovejoy) in the optical range immediately after its perihelion passage during four consecutive nights in the period December 23 to 26, 2013.Results. Our results demonstrate the ability of HARPS-North to efficiently obtain cometary spectra. Very faint emission lines, such as those of 15 NH2 , have been detected, leading to a rough estimate of the 14 N/15 N ratio in NH2 . The 12 C/13 C ratio was measured in the C2 lines and is equal to 80 ± 30. The oxygen lines were studied as well (green to red line intensity ratios and widths), confirming that H2 O is the main parent molecule that photodissociates to produce oxygen atoms. This suggests that this comet has a high CO2 abundance. Relative production rates for C2 and NH2 were computed, but we found no significant deviation from a typical NH2 /C2 ratio.

Spectrum of comet C/2009 P1 (Garradd) in the optical wavelength range

Moderate-resolution spectra of comet C/2009 P1 (Garradd) in the wavelength range from 380 to 540 nm have been analyzed. The spectra were acquired, when the comet was at the distances 1.96 and 1.36 AU from the Sun and the Earth, respectively. The observations were fulfilled in the prime focus of the 6-m BTA telescope (Special Astrophysical Observatory, Russian Academy of Sciences) with the use of the SCORPIO focal reducer. Molecular emissions of C2, C3, CN, CN2, CH, CH+, and CO+ in the cometary coma were identified. In total, 148 emission features were detected. The gas production rate of molecules was determined with the Haser model. The ratios of the gas production rates are log[Q(C2/Q(CN)] = 0.26 and log[Q(C3/Q(CN)] = −1.09. The gas production rates of C2 and C3 molecules are 4.84 × 1026 and 2.16 × 1025 mol/s, respectively.

CXO X-ray spectroscopy of comets and abundances of heavy ions in the solar wind

X-rays from comets originate in charge exchange between heavy ions of the solar wind and cometary species. Spectra of nine comets observed by the Chandra X-ray Observatory (CXO) are analyzed using the time-dependent instrument sensitivity and the energy-dependent spectral resolution. X-ray emissions are extracted from the spectra in the range of 150–1100eV using the χ2-fitting. Production of X-rays varies in the observed comets by a factor of 500 from 4.4×1013ergs−1 in Comet 73P to 2.2×1016ergs−1 in Comet Ikeya–Zhang. The measured solar wind flow varies within a factor of 20, being the weakest in Comet 73P and the strongest in 9P/Tempel 1. The retrieved X-ray line intensities vary within a factor of 5×104. These lines above 300eV are attributed to emissions of the H- and He-like ions, and laboratory data on the excitation cross sections for these emissions (Greenwood et al. [2000]. Astrophys. J. 533, L175–L178) are used to convert the observed emissions into abundances of heavy ions in the solar wind. Continuity equations for charge exchange in comets are solved analytically and result in relationships between the X-ray emissions and the ion fluxes. The flux of O7+ scaled to 1AU varies within a factor of 35 with a mean value of 1.6×104cm−2s−1. The retrieved ratios of O8+/O7+, C6+/C5+, Ne10+/Ne9+, C6+/O7+, N6+/O7+, and Ne9+/O7+ demonstrate significant variations, while their mean values for O, C, and N agree with those recommended by Schwadron and Cravens (Schwadron and Cravens [2000]. Astrophys. J. 544, 558–566) for the slow and fast solar wind. (Data on Ne9+ and Ne10+ are lacking in Scwadron and Cravens (Schwadron and Cravens [2000]. Astrophys. J. 544, 558–566).) The results are compared with the ion ratios from Bodewits et al. (Bodewits et al. [2007]. Astron. Astrophys. 469, 1183–1195) that were obtained from the same CXO spectra of comets, and some significant differences are briefly discussed. CXO X-ray spectroscopy of comets is a diagnostic tool to study the composition of the solar wind and its variations.

Reactivity and survivability of glycolaldehyde in simulated meteorite impact experiments.

Sugars of extraterrestrial origin have been observed in the interstellar medium (ISM), in at least one comet spectrum, and in several carbonaceous chondritic meteorites that have been recovered from the surface of the Earth. The origins of these sugars within the meteorites have been debated. To explore the possibility that sugars could be generated during shock events, this paper reports on the results of the first laboratory impact experiments wherein glycolaldehyde, found in the ISM, as well as glycolaldehyde mixed with montmorillonite clay, have been subjected to reverberated shocks from ~5 to >25GPa. New biologically relevant molecules, including threose, erythrose and ethylene glycol, were identified in the resulting samples. These results show that sugar molecules can not only survive but also become more complex during impact delivery to planetary bodies.

Rotational variation of daughter species production rates in Comet 103P/Hartley: Implications for the progeny of daughter species and the degree of chemical heterogeneity

We present analysis of high spectral resolution optical spectra of Comet 103P/Hartley taken during its Fall 2010 apparition. These spectra include transitions belonging to CN, C2, CH, NH2, and OI. We measure production rates and mixing ratios from these spectra. We find evidence for large changes in production rates (factors of a few) over the course of a nucleus rotation, in agreement with other measurements. We also measure variability with rotational phase in the CN/H2O and C2/CN ratios, which has not been previously reported for any comet. There may also be variability in the NH2/H2O ratio with rotational phase, but this trend is not as clear as for CN/H2O. We interpret the changing mixing ratios as due to H2O and C2 being released primarily from the icy grain halo, while the CN parent molecule comes directly from the nucleus. There is evidence that the CH/CN ratio is higher pre-perihelion than post-perihelion. We conclude that the observed CN and NH2 abundances are consistent with HCN and NH3 being the dominant parent molecules for these species. The C2 and CH abundances are higher than those of candidate parent molecules (C2H2 and CH4 respectively), so there must be another source for these molecules in 103P’s coma. Carbonaceous dust grains could serve as this source.

Modeling of nitrogen compounds in cometary atmospheres: Fluorescence models of ammonia (NH3), hydrogen cyanide (HCN), hydrogen isocyanide (HNC) and cyanoacetylene (HC3N)

We developed full cascade fluorescence models for NH3, HCN and HNC, and a new band model for the ν1 ro-vibrational band of HC3N. The models are based on ab-initio spectral databases containing millions of spectral lines and also include extremely precise spectral information contained in several high-resolution spectral databases. Using these new models we derive detailed cascade maps for these species, and obtain realistic fluorescence efficiencies applicable to high-resolution infrared spectra. The new models permit accurate synthesis of line-by-line spectra for a wide range of rotational temperatures. We validated the models by comparing simulated emissions of these nitrogen species with measured spectra of comet C/2007 W1 (Boattini) acquired with high-resolution infrared spectrometers at high altitude sites. The new models accurately describe the complex emission spectrum, providing distinct rotational temperatures and production rates at greatly improved accuracy compared with results derived from earlier fluorescence models. In addition, we made use of the completeness and scope of the new databases to investigate possible HCN↔HNC radiative isomerization mechanisms, obtaining estimates of conversion efficiencies under typical cometary conditions.

Spectral studies of comet C/2001 Q4 (NEAT)

The paper presents the results of the spectral observations of comet C/2001 Q4 (NEAT) acquired with the Zeiss-600 telescope of the Andrushevka astronomical observatory in May 2004. The spectrum of the comet was obtained in the range of 3600–8200 A. We identified a number of emission features in the spectrum of comet C/2001 Q4 (NEAT). The emission bands of C2, C3, CN, CH, NH2, H2O+ were detected in the spectrum of the comet, and their intensities were determined. The ratios of gas-production rates Q(C2)/Q(CN) = 0.23, Q(C3)/Q(CN) = −0.79, and Q(NH2)/Q(CN) = −0.029 were determined with the Haser model.

A new model for theν1vibrational band of HCN in cometary comae, with application to three comets

Aims. Hydrogen cyanide (HCN) radiates effectively at infrared wavelengths in cometary atmospheres, and a new quantum-band model is needed to properly interpret high-resolution spectra. HCN spectra of comets 8P/Tuttle, C/2007 W1 (Boattini), and C/2008 Q3 (Garradd) have been recorded by our team using the high-resolution CRyogenic InfraRed Echelle Spectometer (CRIRES) at the Very Large Telescope (VLT), ultimately posing an excellent test for our newly developed model. Methods. We developed a quantum-band model for the ν1 fundamental of HCN using the latest spectroscopic parameters available and with it retrieved HCN in the above mentioned three comets. For each comet, we sampled several lines of HCN in the spectral region near 3 μm, and retrieved molecular production rates, mixing ratios, and rotational temperatures. Results. When compared to other comets, 8P/Tuttle is relatively depleted in HCN, while C/2007 W1 (Boattini) appears to be enriched and C/2008 Q3 (Garradd) normal. The spatial profile of HCN observed in 8P/Tuttle is symmetric, consistent with isotropic outgassing from the nucleus, while in comet C/2007 W1 we observed an asymmetric excess of HCN in the anti-solar direction. We investigated the HCN-CN parentage by comparing our production rate ratios (HCN/H2O) with those of CN/OH derived at optical wavelengths. In comet C/2007 W1 the two mixing ratios are comparable, while in 8P/Tuttle our derived HCN abundance is too low to support the HCN molecule as the only parent of the CN radical.

HIGHLY DEPLETED ETHANE AND MILDLY DEPLETED METHANOL IN COMET 21P/GIACOBINI-ZINNER: APPLICATION OF A NEW EMPIRICAL ν2-BAND MODEL FOR CH3OH NEAR 50 K

We obtained infrared spectra of Comet 21P/Giacobini-Zinner (hereafter 21P/GZ) using NIRSPEC at Keck II on UT 2005 June 3, approximately one month before perihelion, that simultaneously sampled H2O, C2H6, and CH3OH. Our production rate for H2O (3.885?? 0.074?? 1028?molecules?s?1) was consistent with that measured during other apparitions of 21P/GZ as retrieved from optical, infrared, and radio observations. Our analysis also provided values for rotational temperature (T rot?= 51?? 3?K) and the abundance ratio of ortho and para spin populations for water (OPR?= 2.99?? 0.23, implying a spin temperature exceeding 50?K). Six Q-branches in the ?7 band of C2H6 provided a production rate (5.27?? 0.90?? 1025?s?1) that corresponded to an abundance ratio of 0.136?? 0.023?? 10?2 relative to H2O, confirming the previously reported strong depletion of C2H6 from IR observations during the 1998 apparition, and in qualitative agreement with the depletion of C2 known from optical studies of 21P/GZ. For CH3OH, we applied our recently published quantum model for the ?3 band to obtain a rotational temperature (48+10/?7?K) consistent with that obtained for H2O. In addition, we developed a new empirical model for the CH3OH ?2 band, based on observations of Comet 8P/Tuttle with NIRSPEC. The application of our ?2 model to 21P/GZ yielded a production rate in agreement with that obtained from the ?3 band. Combining results from both ?2 and ?3 bands provided a production rate (47.5?? 4.4??1025?s?1) that corresponded to an abundance ratio of 1.22?? 0.11?? 10?2 relative to H2O in 21P/GZ, indicating mild depletion of CH3OH. Together with observations of 21P/GZ in 1998, our study provides a measure of primary volatile production rates for this Jupiter family comet over multiple apparitions using high-resolution IR spectroscopy.

A high-resolution infrared spectral survey of 103P/Hartley 2 on the night of the EPOXI closest approach

We obtained high-resolution (λ/Δλ∼28,000) infrared spectra of Comet 103P/Hartley 2 on UT 2010 November 4.6 using the NIRSPEC spectrometer at the W.M. Keck Observatory. Here we present spectra of Hartley 2 between 2.832 and 3.639μm (3531–2748cm−1), representing the most complete high-resolution infrared survey of a Jupiter-family comet to date in this wavelength region. We have tabulated rest frequencies, line fluxes, line signal-to-noise ratios and line widths for all detected emissions. Fluorescence models, published line lists and laboratory spectra were used to obtain molecular assignments for detected emissions. Multiple lines of the following species were detected in Hartley 2: H2O, OH, CH3OH, C2H6, HCN, C2H2, H2CO, NH3 and NH2. All identified species seen in this survey have been previously detected in comets. There were 364 distinct emission features present in these spectra, of which 36 were unidentified. We compare the spectrum of Hartley 2 to chemically different Jupiter-family Comets 73P/Schwassmann–Wachmann 3-B and 17P/Holmes in order to obtain additional information on the characteristics of unknown lines through the comparison of relative line fluxes for corresponding emissions in these comets. For the strongest unidentified emissions, additional information was also obtained through a comparison of their spatial distributions in the coma to that of known emission features in Hartley 2. This spectral survey of Hartley 2 provides detailed information about its overall volatile chemistry, provides a comparison to past and future high-resolution infrared datasets, and further characterizes the most promising spectral regions for future molecular searches in comets.

CO prompt emission as a CO2 marker in comets and planetary atmospheres

Observations of CO emissions in the visible and near-infrared (NIR) have been rare for comets, and no measurements from orbiters are currently available in the visible for the dayglows of Mars or Venus. Analysis of the ultraviolet CO(a–X) Cameron bands from Mars Express dayglow observations supports the conclusion that these bands have very high rotational temperatures, some thousands of kelvins. The most plausible source for the CO rotational excitation is its generation by CO2 photodissociation. Recent laboratory measurements investigating the photodissociation of CO2 in the extreme ultraviolet (EUV) reveal strong emissions in the visible and NIR region by the triplet CO(a′, d, e) states, which we take to be a primary source for the UV CO(a–X) Cameron bands. Thus, detection of visible emissions from the triplet CO states in planetary dayglows and comets provides an upper limit to the CO2 density. The presence of CO high rotational excitation along with the intense visible and NIR band emissions should be considered as a practical way by which planetary dayglow and cometary spectra provide information on the presence of CO2. Finally, we report on existing observations of cometary atmospheres and estimate the altitude for the emitting layer of the CO triplet states in Mars and Venus.

ORTHO-TO-PARA ABUNDANCE RATIO OF WATER ION IN COMET C/2001 Q4 (NEAT): IMPLICATION FOR ORTHO-TO-PARA ABUNDANCE RATIO OF WATER

The ortho-to-para abundance ratio (OPR) of cometary molecules is considered to be one of the primordial characteristics of cometary ices, and contains information concerning their formation. Water is the most abundant species in cometary ices, and OPRs of water in comets have been determined from infrared spectroscopic observations of H2O rovibrational transitions so far. In this paper, we present a new method to derive OPR of water in comets from the high-dispersion spectrum of the rovibronic emission of H2O+ in the optical wavelength region. The rovibronic emission lines of H2O+ are sometimes contaminated by other molecular emission lines but they are not affected seriously by telluric absorption compared with near-infrared observations. Since H2O+ ions are mainly produced from H2O by photoionization in the coma, the OPR of H2O+ is considered to be equal to that of water based on the nuclear spin conservation through the reaction. We have developed a fluorescence excitation model of H2O+ and applied it to the spectrum of comet C/2001 Q4 (NEAT). The derived OPR of water is 2.54+0.32 – 0.25, which corresponds to a nuclear spin temperature (T spin) of 30+10 – 4 K. This is consistent with the previous value determined in the near-infrared for the same comet (OPR = 2.6 ± 0.3, T spin = 31+11 – 5 K).

A QUANTUM BAND MODEL OF THE ν3FUNDAMENTAL OF METHANOL (CH3OH) AND ITS APPLICATION TO FLUORESCENCE SPECTRA OF COMETS

Methanol (CH3OH) radiates efficiently at infrared wavelengths, dominating the C-H stretching region in comets, yet inadequate quantum-mechanical models have imposed limits on the practical use of its emission spectra. Accordingly, we constructed a new line-by-line model for the 3 fundamental band of methanol at 2844 / cm (3.52 micron) and applied it to interpret cometary fluorescence spectra. The new model permits accurate synthesis of line-by-line spectra for a wide range of rotational temperatures, ranging from 10 K to more than 400 K.We validated the model by comparing simulations of CH3OH fluorescent emission with measured spectra of three comets (C/2001 A2 LINEAR, C/2004 Q2 Machholz and 8P/Tuttle) acquired with high-resolution infrared spectrometers at high-altitude sites. The new model accurately describes the complex emission spectrum of the nu3 band, providing distinct rotational temperatures and production rates at greatly improved confidence levels compared with results derived from earlier fluorescence models. The new model reconciles production rates measured at infrared and radio wavelengths in C/2001 A2 (LINEAR). Methanol can now be quantified with unprecedented precision and accuracy in astrophysical sources through high-dispersion spectroscopy at infrared wavelengths

Exploration of spectra of comet 81P/Wild 2 observed in March–April 2010

Exploration of spectra of comet 81P/Wild 2 observed in March–April 2010