An analysis of spectrophotometric observations of comets Austin (1982g) and Bradfield (1980t)

Post-perihelion observed emission fluxes at 388 nm (CN) and 516 nm (C₂) of the coma of comets Austin (1982g) and Bradfield (1980t) are analysed in the framework of the Haser model. Ratios of Haser model CN and C₂ parent production rates with expansion velocity show that each comet behaves normally. For comet Austin (1982g), the Q _CN/v and Q _c2/v values decrease with increase of heliocentric distance of comet. For an assumed %; activity of the total spherical surface area of the nucleus, the water vaporization theory coupled with derived water production rates from the International Ultraviolet Explorer H and OH flux data yields a nuclear diameter of about 6 km for comet Austin (1982g). For comet Bradfield (1980t), the derived nuclear diameter is expected to be of about 1 km. In each comet, the dust mass production rates as well as ratio of dust-to-gas mass production rates decrease with increase of heliocentric distance of comet.     

Chemical Composition Diversity Among 24 Comets Observed At Radio Wavelengths

We present a comparative study on molecular abundances in comets basedon millimetre/submillimetre observations made with the IRAM 30-m,JCMT, CSO and SEST telescopes. This study concerns a sample of 24comets (6 Jupiter-family, 3 Halley-family, 15 long-period) observedfrom 1986 to 2001 and 8 molecular species (HCN, HNC, CH₃CN,CH₃OH, H₂CO, CO, CS, H₂S). HCN was detected in all comets,while at least 2 molecules were detected in 19 comets. From the sub-sample of comets for which contemporary H₂O productionrates are available, we infer that the HCN abundance relative to water variesfrom 0.08% to 0.25%. With respect to other species, HCN is the moleculewhich exhibits the lowest abundance variation from comet to comet. Therefore,production rates relative to that of HCN can be used for a comparative study ofmolecular abundances in the 19 comets. It is found that: CH₃OH/HCN varies from ≤ 9 to 64; CO/HCN varies from ≤ 24 to 180; H₂CO/HCN varies between 1.6 and 10; and H₂S/HCN varies between 1.5 and 7.6. This study does not show any clear correlation between the relative abundancesand the dynamical origins of the comets, or their dust-to-gas ratios.     

A tale of two very different comets: ISO and MSX measurements of dust emission from 126P/IRAS (1996) and 2P/Encke (1997)

We present the characteristics of the dust comae of two comets, 126P/IRAS, a member of the Halley family (a near-isotropic comet), and 2P/Encke, an ecliptic comet. We have primarily used mid- and far-infrared data obtained by the ISOPHOT instrument aboard the Infrared Space Observatory (ISO) in 1996 and 1997, and mid-infrared data obtained by the SPIRIT III instrument aboard the Midcourse Space Experiment (MSX) in 1996. We find that the dust grains emitted by the two comets have markedly different thermal and physical properties. P/IRAS's dust grain size distribution appears to be similar to that of fellow family member 1P/Halley, with grains smaller than 5 microns dominating by surface area, whereas P/Encke emits a much higher fraction of big (20 μm and higher) grains, with the grain mass distribution being similar to that which is inferred for the interplanetary dust population. P/Encke's dearth of micron-scale grains accounts for its visible-wavelength classification as a “gassy” comet. These conclusions are based on analyses of both imaging and spectrophotometry of the two comets; this combination provides a powerful way to constrain cometary dust properties. Specifically, P/IRAS was observed preperihelion while 1.71 AU from the Sun, and seen to have a 15-arcmin long mid-infrared dust tail pointing in the antisolar direction. No sunward spike was seen despite the vantage point being nearly in the comet's orbital plane. The tail's total mass at the time was about 8×10⁹ kg. The spectral energy distribution (SED) is best fit by a modified greybody with temperature T=265±15 K and emissivity ε proportional to a steep power law in wavelength λ: ε∝λ^−α, where α=0.50±0.20(2σ). This temperature is elevated with respect to the expected equilibrium temperature for this heliocentric distance. The dust mass loss rate was between 150-600 kg/s (95% confidence), the dust-to-gas mass loss ratio was about 3.3, and the albedo of the dust was 0.15±0.03. Carbonaceous material is depleted in the comet's dust by a factor of 2-3, paralleling the C₂ depletion in P/IRAS's gas coma. P/Encke, on the other hand, observed while 1.17 AU from the Sun, had an SED that is best fit by a Planck function with T=270±15 K and no emissivity falloff. The dust mass loss rate was 70-280 kg/s (95% confidence), the dust-to-gas mass loss ratio was about 2.3, and the albedo of the dust was about 0.06±0.02. These conclusions are consistent with the strongly curved dust tail and bright dust trail seen by Reach et al. (2000; Icarus 148, 80) in their ISO 12-μm imaging of P/Encke. The observed differences in the P/IRAS and P/Encke dust are most likely due to the less evolved and insolated state of the P/IRAS nuclear surface. If the dust emission behavior of P/Encke is typical of other ecliptic comets, then comets are the major supplier of the interplanetary dust cloud.     

A taxonomic survey of comet composition 1985-2004 using CCD spectroscopy

A summary is presented of our spectroscopic survey of comets extending for roughly 19 years from 1985 to 2004 comprising data for 92 comets of which 50 showed good emissions. All data were re-analyzed using consistent reduction techniques. Our observations of comets over several apparitions and comets observed over an extended period indicate no major changes in compositional classification. To our regret, no major unidentified cometary features were found in our surveyed spectral region of 5200-10400 Å. Absolute production rates for the dominant parent molecule H₂O and the daughter species C₂, NH₂ and CN are determined within the limits of the Haser model as are values for the dust continuum, Afρ. From these data, production rate ratios are calculated for C₂/H₂O, NH₂/H₂O, CN/H₂O and Afρ/H₂O. Excluding the odd Comets Yanaka (1988r), 43P/Wolf-Harrington and 19P/Borrelly, with unusual spectra, our set of comets exhibited relatively uniform composition. Detailed analyses of our data resulted in four taxonomic classes:

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Comets of typical composition (∼70%); exhibiting typical ratios with respect to water of C₂, NH₂, and CN.

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Tempel 1 type (∼22%); having a deficiency in C₂ but normal NH₂ abundance.

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G-Z type (∼6%); having both low C₂ and NH₂ ratios.

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The unusual object Yanaka (1988r) (∼2%?); no detectable C₂ or CN emission but normal NH₂.

It is uncertain whether there is a clear separation between the comets of typical composition and those with C₂ depletion, or whether the latter consists of a group showing a continuum of decreasing C₂/CN ratios. Our spectroscopic investigations result in a visual record of the various compositional classes, which are illustrated in a number of figures. Production rate comparisons with the comet photometry program of Schleicher and A'Hearn [A'Hearn, M.F., and 4 colleagues, 1995. Icarus 118, 223-270] for 13 comets in common yielded good agreement once the different scale lengths are taken into account. An investigation into the possible origin of our compositional groups with respect to dynamical families of comets shows that the Halley family exhibits essentially no C₂ depletion. These objects were presumably formed in the region of Saturn and Uranus and scattered into the Oort cloud. Comets formed in the space near Neptune, responsible for the scattered Kuiper Belt show a mixture of “typical” and C₂ depleted objects, while we associate comets formed in-situ in the classical Kuiper belt with our C₂ depleted group.     

Spectroscopy of comet P/Schaumasse

Column density profiles for CN, C₃, C₂ and NH have been determined from a long-slit CCD spectrum of periodic comet P/Schaumasse (1992x). Comparisons of these profiles with Haser models indicate that the ratios of the CN, C₃ and C₂ production rates are typical for a short-period comet. Although the scale lengths for NH and its parent species are uncertain, the results indicate that the production rate for NH is much greater than for either C₂ or CN.     

On the aging of comets

This study is based primarily on the calculations of comet orbits over ~ 10⁶ years for 160 short-period comets by Harold F. Levison and Martin J. Duncan from which there are calculated “ablation AGES”. There are positive statistical correlations (having many deviations) with radial nongravitational forces, comet activity measures, and dust-to-gas ratios in the spectra, in the sense that comets of greater “AGES” tend to be less active and to show less dust in their spectra than comets of lesser “AGES”.     

Composition of Comets: Observations and Models

We analyze the chemical composition and abundances of comets based on in situ measurements of Comet 1P/Halley and remote sensing observations of several recent bright comets including Hale-Bopp (C/1995 O1) and Hyakutake (C/1996 B2), in light of the elemental abundances of the solar system. Nitrogen is underabundant in comets relative to the solar system because nitrogen tends to be in N₂, which is chemically relatively inert. While many details remain uncertain, some gross features are emerging. The abundance of water : silicates: carbonaceous molecules (CO, CO₂, and hydrocarbons) by mass is approximately 1 : 1 : 1. Furthermore, the mass abundance of ice : dust (silicates and hydrocarbon polycondensates) is about1 : 1. We compare a list of identified comet molecules with molecules detected in the interstellar medium, although a comparison with their relative abundances, particularly in the ice phase, would be more meaningful. However, ice-phase abundances are not yet available. One can expect a variation of the abundances of carbon-bearing molecules in comets to be associated with their place of origin in the solar nebula. However, we also note that comets are heterogeneous. Thus, observed differences may be related to the place of origin, heterogeneity of the nucleus, or acquired through evolution. The molecular and elemental compositions of the coma are most likely not the same as those in the nucleus. This is particularly true for volatile ices and their gases and for the dust-to-ice and dust-to-gas ratios. Analyses must carefully consider the three sources of gas: Water from the surface of the nucleus, gases more volatile than water from the interior of the nucleus, and gases from the sublimation of the dust distributed in the coma. Topography on the surface of the nucleus may cause important evolutionary differences in the dust-to-gas mass ratio. Relatively inactive areas on the surface of the nucleus are probably associated with convex topography. Gas sublimated from convex areas (hills and mountains) diverges more strongly relative to gas sublimated from concave areas, which can entrain dust more efficiently. Thus, the entrainment of dust from convex areas is poor and dust may fall back to the surface of the nucleus creating a dust mantle, which further inhibits outgassing.     

Spectropolarimetry of comets Austin and Churyumov-Gerasimenko

Spectropolarimetric observations from 5000 to 8000 Å have been obtained for comets P/Austin (1982g) and P/Churyumov-Gerasimenko (1982f). The observations were spaced over phase angles of 50–125° for comet Austin and 10–40° for comet Churyumov-Gerasimenko. The use of spectropolarimetry allowed an evaluation of continuum polarization without molecular line contamination. Especially for comet Churyumov-Gerasimenko, the curve of polarization versus phase angle resembles curves for asteroids, where the polarization is negative (electric vector maximum parallel to the scattering plane) for phase angles less than 20° and the most negative polarization is from ?1 to ?2%. The negative polarization at backscattering angles may be due to multiple scattering in agglomerated grains, as assumed for asteroids, or to Mie scattering by small dielectric particles. If multiple scattering is important in comet dust, polarization measurements may imply a low albedo, less than 0.08. The polarization of comet Austin remained steady during a large change in the dust production rate. Both comets increased continuum flux by a factor of 2 near perihelion. The continuum of comet Churyumov-Gerasimenko had the shape of the solar spectrum with derivations less than 5%. The equivalent width of spectral features of C₂, NH₂, and O varied as r^?2.     

Photometric studies of comet C/2009 P1 (Garradd) before the perihelion

The results of the photometric observations of comet C/2009 P1 (Garradd) performed at the 60-cm Zeiss-600 telescope of the Terskol observatory have been analyzed. During the observations, the comet was at the heliocentric and geocentric distances of 1.7 and 2.0 AU, respectively. The CCD images of the comet were obtained in the standard narrowband interference filters suggested by the International research program for comet Hale-Bopp and correspondingly designated the “Hale-Bopp (HB) set.” These filters were designed to isolate the BC (λ4450/67 Å), GC (λ5260/56 Å) and RC (λ7128/58 Å) continua and the emission bands of C₂ (λ5141/118 Å), CN (λ3870/62 Å), and C₃ (λ4062/62 Å). From the photometric data, the dust production rate of the comet and its color index and color excess were determined. The concentration of C₂, CN, and C₃ molecules and their production rates along the line of sight were estimated. The obtained results show that the physical parameters of the comet are close to the mean characteristics typical of the dynamically new comets.     

Observations of the inner coma of C/1995O1 (Hale-Bopp)—gas and dust production

Hale-Bopp (C/1995 O1) was the most productive recent comet observed in terms of gas and dust output. Since its discovery in 1995 at a distance of 7.14 AU from the Sun, the comet has been well observed, revealing the dynamics of a rare and large comet. Hale-Bopp showed strong emissions of the principle cometary gases CN, C₃, and C₂, as well as an abundance of dust. The production rates of these gases were found to be 1.45×10²⁸, 1.71×10²⁸, and , respectively, with dust production, in terms of Afρ, , as measured in the green continuum (5260 Å). The observations for this paper are presented in two groups spanning 10 days each, one group centered near 32 days prior to and the other 21 days after perihelion. The averages of dust and gas production rates show a slightly higher value for each prior to perihelion than after perihelion, consistent with a possible peak in production a few weeks prior to perihelion passage.     

Observations of C2 Molecules in the Coma of Comets at Mitaka

In order to monitor the various phenomena of comets, we continuously made the near-nucleus imaging observations of comets at National Astronomical Observatory, Mitaka. Here we report on the results of the observations of two spectacular comets. One is the analysis of photometry of C₂ emission images in the coma of comet Hyakutake (C/1996 B2). By comparing the production rates of C₂ radicals with C₂H₂ molecules, we conclude that the most C₂ radicals were originated from C₂H₂, while some of them might be produced from other parents. The second item is the analysis of the morphology of C₂ emission images in the coma of comet Hale-Bopp(C/1995 O1). By applying a ring masking technique, we detected asymmetrical distribution of the C₂ molecules. The results of these two comets might suggest that a portion of C₂ radicals were formed by the disintegration of the small organic dust grains, like the so-called CHON particles. This revised version was published online in July 2006 with corrections to the Cover Date.     

Comet C/2010X1 (Elenin). Unrealized expectations

We present the analysis of the photometric and spectroscopic data obtained for comet C/2010 X1 (Elenin) when it was at a distance of 2.92 AU from the Sun. The observations were made at the prime focus of the 6-m BTA telescope with the SCORPIO focal reducer. The magnitude of the comet, measured in the R _c -band with an 9?? aperture radius amounted to 16?8 ± 0?1. The computed dust production rate was estimated to be about 6 kg/s. The cometary coma manifested the emissions in the (0?C0) band of the CN molecule violet system, and a number of emission band heads of the C₃ molecule. The gas production rate of the molecules is determined using the Haser model and amounts to 1.41 × 10²⁴ and 4.20 × 10²³ molecules per second for CN and C₃, respectively. The ratio of gas production rates log[Q(C₃)/Q(CN)] is equal to ?0.85, which is close to the mean value, determined for a significant number of comets. A normalized gradient of the cometary dust reflectivity, calculated for the 4430?C6840 ? spectral range amounts to 14.3 ± 1.2%.     

Aperture Polarimetry And Photometry Of Comet Hale-Bopp

We present results of polarimetric and photometric observations of bright comet C/1995 O1 (Hale-Bopp) obtained at the 0.7 m telescope of Kharkov University Observatory from June 18, 1996 to April 24, 1997. The IHW and HB comet filters were used. The C₂ and C₃ production rates for Hale-Bopp are more than one order of magnitude larger and the dust production rates are more than two orders of magnitude larger than the Halley ones at comparable distances. Hence, Hale-Bopp was one of the most dusty comets. The average UC-BC and BC-RC colours of the dust were −0.02 and 0.13 mag, respectively. The polarization of comet Hale-Bopp at small phase angles of 4.8–13.0° was in good agreement with the date for comet P1/Halley at the same phase angles in spite of the fact that the heliocentric distances of comments differed nearly twice. However, at intermediate phase angles of 34–49° the polarization of comet Hale-Bopp was significantly larger than the polarization of the other dusty comets. It is the first case of such a large difference found in the continuum polarization of comets. The wavelength dependence of polarization for Hale-Bopp was steeper than for other dusty comets. The observed degree of polarization for the anti-sunward side of the coma was permanently higher than that for the sunward shell side. The polarization phase dependence of Hale-Bopp is discussed and compared with the polarization curves for other dusty comets. The peculiar polarimetric properties of comet Hale-Bopp are most likely caused by an over-abundance of small or/and absorbing dust particles in the coma. This revised version was published online in July 2006 with corrections to the Cover Date.     

Comet C/2002 T7 (LINEAR): Polarimetric and photometric studies

We present the results of polarimetric and photometric observations of dynamically new comet C/2002 T7 (LINEAR) at phase angles from 6° to 26°. During the observations, the comet was at a distance of 2.7–1.3 AU from the Sun and 1.7–2.0 AU from the Earth. The aperture polarimetry was made with the 2.6-m Shain telescope and the 1.25-m AZT-11 telescope of the Crimean Astrophysical Observatory and with the 0.7-m telescope of the Astronomical Institute of the Kharkiv National University during the period from November 21, 2003, to February 21, 2004. The wideband UBVRI and WRC (λ7228/1142 Å) filters and the narrowband GC (λ5260/56 Å) filter were used. The photometric observations of the comet were carried out on February 21, 2004, with narrowband filters isolated the BC (λ4845/65 Å) and RC (λ6840/90 Å) continuum and the C₂ emission (λ5140/90 Å). The phase-angle dependence of linear polarization of the comet has been obtained, and its parameters, such as the minimal polarization P _min = ?1.63%, the phase angle of the minimal polarization α_min = 10.6°, the inversion angle α_inv = 22.7°, and the slope of the phase curve at the inversion angle h = 0.24% per degree, were found. From the photometric observations, the following quantities have been obtained: the column density of molecules C₂ in the line of sight logN (C₂) = ?9.15 mol/cm² and their production rate log Q (C₂) = 27.11 mol/s, the spectral gradient of reflectivity for the dust S′(BC, RC) ≈ 3%/1000 Å, and the dust production parameter Afρ equal to 371 and 273 cm for the blue and red continuum ranges, respectively. According to these results, the physical parameters of comet C/2002 T7 are close to the average characteristics of typical dusty comets.     

The molecular composition of Comet C/2007 W1 (Boattini): Evidence of a peculiar outgassing and a rich chemistry

We measured the chemical composition of Comet C/2007 W1 (Boattini) using the long-slit echelle grating spectrograph at Keck-2 (NIRSPEC) on 2008 July 9 and 10. We sampled 11 volatile species (H₂O, OH^∗, C₂H₆, CH₃OH, H₂CO, CH₄, HCN, C₂H₂, NH₃, NH₂, and CO), and retrieved three important cosmogonic indicators: the ortho-para ratios of H₂O and CH₄, and an upper-limit for the D/H ratio in water. The abundance ratios of almost all trace volatiles (relative to water) are among the highest ever observed in a comet. The comet also revealed a complex outgassing pattern, with some volatiles (the polar species H₂O and CH₃OH) presenting very asymmetric spatial profiles (extended in the anti-sunward hemisphere), while others (e.g., C₂H₆ and HCN) showed particularly symmetric profiles. We present emission profiles measured along the Sun-comet line for all observed volatiles, and discuss different production scenarios needed to explain them. We interpret the emission profiles in terms of release from two distinct moieties of ice, the first being clumps of mixed ice and dust released from the nucleus into the sunward hemisphere. The second moiety considered is very small grains of nearly pure polar ice (water and methanol, without dark material or apolar volatiles). Such grains would sublimate only very slowly, and could be swept into the anti-sunward hemisphere by radiation pressure and solar-actuated non-gravitational jet forces, thus providing an extended source in the anti-sunward hemisphere.     

Ensemble properties of comets in the Sloan Digital Sky Survey

We present the ensemble properties of 31 comets (27 resolved and 4 unresolved) observed by the Sloan Digital Sky Survey (SDSS). This sample of comets represents about 1 comet per 10 million SDSS photometric objects. Five-band (u, g, r, i, z) photometry is used to determine the comets’ colors, sizes, surface brightness profiles, and rates of dust production in terms of the Afρ formalism. We find that the cumulative luminosity function for the Jupiter Family Comets in our sample is well fit by a power law of the form N(<H) ∝ 10^{(0.49±0.05)H} for H < 18, with evidence of a much shallower fit N(<H) ∝ 10^{(0.19±0.03)H} for the faint (14.5 < H < 18) comets. The resolved comets show an extremely narrow distribution of colors (0.57 ± 0.05 in g ? r for example), which are statistically indistinguishable from that of the Jupiter Trojans. Further, there is no evidence of correlation between color and physical, dynamical, or observational parameters for the observed comets.     

Compositional and physical results for Rosetta's new target Comet 67P/Churyumov-Gerasimenko from narrowband photometry and imaging

We present compositional and physical results of Comet 67P/Churyumov-Gerasimenko, the new target of ESA's Rosetta mission. A total of 16 nights of narrowband photometry were obtained at Lowell Observatory during the 1982/83 and 1995/96 apparitions, along with one night of imaging near perihelion in 1996. These data encompass an interval of −61 to +118 days from perihelion, corresponding to a range of heliocentric distances before perihelion from 1.48 to 1.34 AU, and an outbound range from 1.30 to 1.86 AU. Production rates were determined for OH, NH, CN, C₃, and C₂, along with A(θ)fρ, a proxy of the dust production. Water production, based on OH, has a steep () power-law rH-dependence post-perihelion and the minor species are somewhat less steep ( to −4), while the dust is quite shallow (), possibly due to a lingering population of large, slow-moving grains. All species exhibit larger production rates after perihelion, with water having a ∼2×pre/post-perihelion asymmetry, while minor species and dust have larger asymmetries. These asymmetries imply a strong seasonal effect and probable high obliquity of the rotational axis, along with one or more isolated source regions coming into sunlight near perihelion. Peak water production (which occurred about 1 month after perihelion) was and, when combined with a standard water vaporization model, implies an effective active area on the surface of the nucleus of ∼1.5-2.2 km² or an active fraction of only about 3-4%. Abundances of carbon-chain molecules yield a classification of slightly “depleted” in the A'Hearn et al. [A'Hearn, M.F., Millis, R.L., Schleicher, D.G., Osip, D.J., Birch, P.V., 1995. Icarus 118, 223-270] database. The peak dust production (as measured by A(θ)fρ, and uncorrected for phase angle) was ∼450 cm, while the color of the dust is moderately reddened, and the mean radial profile has a power-law slope of −1.3. Large night-to-night variability is also present, presumably due to the source region(s) rotating in and out of sunlight along with effects due to the use of differently sized apertures. A strong sunward radial feature was detected in images obtained near perihelion, along with a significant asymmetry between the two perpendicular directions from the Sun/tail line. These features may be the result of a mid-latitude source region sweeping out a cone with each rotation, which we are viewing from the side and where the sunward radial feature is one edge of the cone seen in projection. When combined with other constraints on the pole orientation, a possible pole solution is found having an obliquity of about 134° at an RA of about 223° and a Dec of −65°, with a source region located near +50° and in overall agreement with the photometric results. In comparison to the original Rosetta target Comet 46P/Wirtanen, Comet Churyumov-Gerasimenko has essentially the same peak water production but a peak dust production about 3 times greater than does Wirtanen based on A(θ)fρ (i.e., if one assumes that the properties of the dust grains are similar) (cf. Farnham and Schleicher [1998. Astron. Astrophys. 335, L50-L55]).     

Spectroscopy of Comet Hale-Bopp in the infrared

High resolution infrared spectra of Comet C/1995 O1 (Hale-Bopp) were obtained during 2-5 March 1997 UT from the NASA Infrared Telescope Facility on Mauna Kea, Hawaii, when the comet was at r≈1.0 AU from the Sun pre-perihelion. Emission lines of CH₄, C₂H₆, HCN, C₂H₂, CH₃OH, H₂O, CO, and OH were detected. The rotational temperature of CH₄ in the inner coma was T_rot=110±20 K. Spatial profiles of CH₄, C₂H₆, and H₂O were consistent with release solely from the nucleus. The centroid of the CO emission was offset from that of the dust continuum and H₂O. Spatial profiles of the CO lines were much broader than those of the other molecules and asymmetric. We estimate the CO production rate using a simplified outflow model: constant, symmetric outflow from the peak position. A model of the excitation of CO that includes optical depth effects using an escape probability method is presented. Optical depth effects are not sufficient to explain the broad spatial extent. Using a parent+extended-source model, the broad extent of the CO lines can be explained by CO being produced mostly (∼90% on 5 March) from an extended source in the coma. The CO rotational temperature was near 100 K. Abundances relative to H₂O (in percent) were 1.1±0.3 (CH₄), 0.39±0.10 (C₂H₆), 0.18±0.04 (HCN), 0.17±0.04 (C₂H₂), 1.7±0.5 (CH₃OH), and 37-41 (CO, parent+extended source). These are roughly comparable to those obtained for other long-period comets also observed in the infrared, though CO appears to vary.     

Dynamical capture and physical decay of short-period comets

The brightness evolution of short-period comets is discussed in connection with their physical lifetimes. It is shown that changes in the fraction of the free-subliming area of the nuclear surface may be more important than mass decrease in determining brightness variations. The decrease in the activity of short-period comets caused by the buildup of a dust mantle may be interrupted—and partially reversed—by dust blowoffs that leave exposed areas of fresh ices. Short-period comets may thus be subject to random brightness fluctuations that make quite uncertain any derivation of their physical lifetime based on comparisons of their absolute brightness at different apparitions. As an alternate procedure, the numerical integration of the whole sample of short-period comet orbits carried out by A. Carusi, L.Kresák, E. Perozzi and G. B. Valsecchi (1984, Long-Term Evolution of Short-Period Comets. Istituto Astrofisica Spaziale Internal Report 12, Rome) is used to draw conclusions about the transfer rate of their perihelia from Jupiter's region to the region of the terrestrial planets (heliocentric distances<1.5 AU). It is found that about one short-period comet per century reaches the region of the terrestrial planets. From this result and under the assumption of a steady-state comet population, an average lifetime of the order of 6 × 10³ years (～10³ revolutions) is derived for a typical kilometer-sized short-period comet of perihelion distance q ～ 1 AU. Such a rather long comet lifetime, as compared to some previous derivations, is consistent with the survival of some periodic comets on small-q orbits of long dynamical time scales.     

Ultraviolet absorption studies of H2O and other species in comet comae with satellite telescope-spectrometers

Ultraviolet absorption by H₂O and other species in the comae of comets could be detected by studying, with satellite telescope-spectrometers, the occultation of hot stars by comets. Such observations could produce the first direct detection of H₂O, the fundamental parent molecule in comet comae, and give measures of molecular level populations. The first instrument suitable for such observations will be the High Resolution Spectrograph on Space Telescope and, therefore, we consider its capabilities. We have used a Haser model to estimate the molecular column densities and to predict equivalent widths for lines of H₂O, OH, CO, and O as functions of time and angular distance from a comet with a high H₂O production rate. We have determined the minimum detectable equivalent widths, and therefore, the maximum angular separation from such a comet at which H₂O, OH, and CO could be studied. A conservative, statistical estimate shows that comets with high water production rates should pass near enough to about 10 to 100 stars suitable for absorption studies of the $\text{C}\text{}\text{X}$ band of H₂O (1240 Å). Estimated equivalent widths for CO, OH, and the resonance lines of C and O indicate that these species may also be detected.