Thermal Emission From The Dust Coma Of Comet Hale-Bopp And The Composition Of The Silicate Grains

The dust coma of comet Hale-Bopp was observed in the thermal infrared over a wide range in solar heating (R = 4.9–0.9 AU) and over the full wavelength range from 3 μm to 160 μm. Unusual early activity produced an extensive coma containing small warm refractory grains; already at 4.9 AU, the 10 μm silicate emission feature was strong and the color temperature was 30% above the equilibrium blackbody temperature. Near perihelion the high color temperature, strong silicate feature, and high albedo indicated a smaller mean grain size than in other comets. The 8–13 μm spectra revealed a silicate emission feature similar in shape to that seen in P/Halley and several new and long period comets. Detailed spectral structure in the feature was consistent over time and with different instruments; the main peaks occur at 9.3, 10.0 and 11.2 μm. These peaks can be identified with olivine and pyroxene minerals, linking the comet dust to the anhydrous chondritic aggregate interplanetary dust particles. Spectra at 16–40 μm taken with the ISO SWS displayed pronounced emission peaks due to Mg-rich crystalline olivine, consistent with the 11.2 μm peak. This revised version was published online in July 2006 with corrections to the Cover Date.     

Spectrocam-10 Thermal Infrared Observations Of The Dust In Comet C/1995 O1 Hale-Bopp

We describe 5 to 18 μm broadband images and R ∼ 100 spectra of comet Hale-Bopp taken with SpectroCam-10 on the 5 m Hale telescope during six runs between 1996 June and 1997 April. Our data show the gradual warming of dust grains as the comet approached perihelion. In June, the 10 μm silicate emission feature was already stronger than observed in any other comet, and it increased to about 3 times the continuum level near perihelion. Spectral structure attributed to a crystalline olivine grain component remained relatively constant, but other features associated with pyroxenes appeared to vary with heliocentric distance. This revised version was published online in July 2006 with corrections to the Cover Date.     

Isophot Observations Of Comet Hale-Bopp

Comet Hale-Bopp has been observed five times with ISOPHOT, the photometer on board the Infrared Space Observatory (ISO), four times before its perihelion passage at heliocentric distances of 4.92, 4.58, 2.93 and 2.81 AU, and at 3.91 AU postperihelion. Each time, multi-filter photometry covering the range between 3.6–175 μm with eight to ten filters was performed to sample the spectral energy distribution of the comet. These measurements were used to determine dust temperatures for the cometary coma. The evolution of the strength of the silicate feature can be followed in the data as well as the flux deficit at longer wavelengths. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermal Infrared Spectra of Comet Hale-Bopp at Heliocentric Distances of 4 and 2.9 AU

We present 10 and 20 μm spectra of comet Hale-Bopp taken at UKIRT on 1996 June 20 and 1996 September 29.The 10 μm spectra clearly show a strong silicate feature with peaks at 10.0 and 11.2 μm. The 20 μm spectrum on September 29 has strong excess flux relative to a blackbody and a peak near 19μm, in good agreement with the ISO SWS spectra obtained a week later. However, the 20 μm spectrum on June 20 has significantly lower flux than would be expected based on a blackbody extrapolation from the flux at 12.5 μm. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Infrared Spectral Energy Distribution And Polarization Of Comet C/1995 O1 (Hale–Bopp) During 1997

Comets, such as C/1995 O1 (Hale-Bopp), are important to studies of the origins of the solar system because they are believed to be frozen reservoirs of the most primitive pre-solar dust grains and ices. Here, we report 1.2–18.5 μm infrared (IR) spectrophotometric and polarimetric observations of comet Hale-Bopp. Our measurements of the spectral energy distribution (SED) and IR polarization near perhelion passage suggest that emission from the coma was dominated by scattering and thermal emission from sub-micron sized dust grains. Hale-Bopp's surprising brightness may have been largely a result of the properties of its coma grains rather than the size of its nucleus. The thermal emission continuum from the grains had a superheat of S = T_color/T_BB ≥ 1.7, the peak of the 10 μm silicate emission feature was 1.7 mags above the carbon grain continuum, and the albedo (reflectivity) of the grains was ≥ 0.4 at a scattering angles, θ ≥ 135° This revised version was published online in July 2006 with corrections to the Cover Date.     

Mid-Infrared Spectro-Imaging Observations Of Comet Hale-Bopp

Comet C/1995 O1 (Hale-Bopp) was observed with camiras, the Saclay mid-infrared camera, mounted on the Nordic Optical Telescope (Roque de los Muchachos, La Palma), from April 14th to April 25th, 1997. This observing run was part of the European campaign devoted to Hale-Bopp observations from the Canary Islands (PI R. West). camiras spectro-imaging capabilities, achieved with a Circular Variable Filter (of spectral resolution R ≈ 50), were used to obtain spectra of the comet in the N atmospheric window (8–13 μm) over a large field of view (52″ × 78″), at a spatial resolution of ∼1 arcsec. Data were also collected at shorter wavelengths (in the L and M bands). The silicate dust feature around 10 μm is clearly apparent in the data. The shape of the feature varies continuously according to the position in the field. A dust model developed at Saclay in the framework of β-Pictoris dust disk studies was used to interpret the observations. The presence of both olivine and pyroxene with a high degree of crystallinity is needed to account for the observations. A change in the size distribution of the grains allows to reproduce the spatial change of the silicate feature according to the position in the field. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Visual Light Curve Of C/1995 O1 (Hale-Bopp) From Discovery To Late 1997

We present a light curve of C/1995 O1 (Hale-Bopp) compiledfrom more than 3000 visual observations of the comet made by members of the The Astronomer Group world-wide. These observations cover the period from discovery through to the end of 1997. The light curve shows that the rate of brightening of the comet varied widely at different times, with rapid rates of brightening at high heliocentric distance pre-perhelion and a comparably rapid post-perihelion fade. There is no evidence that the comet was suffering a large photometric outburst when first discovered, although a small outburst can be identified at perihelion. At least five difficult brightening regimes can be identified in the light curve between discovery and perihelion. From 2.5 AU to perihelion the rate of brightening with decreasing heliocentric distance was typical for “fairly” new comets(n ∼ 3.5, where “n” is the power law exponent of the heliocentric distance), although this was preceded by a period of very slow brightening with n ∼ 1 from r ∼ 4.0 AU to r ∼ 2.8 AU and followed by an initially more rapid brightening which appears to be related to the on-set of rapid water sublimation activity. We derive the light curve parameters at different stages of the comet's apparition. This revised version was published online in July 2006 with corrections to the Cover Date.     

Near-Infrared Imaging Spectrophotometry Of Comet C/1995 O1 (Hale-Bopp) Near Perihelion

We present 1- to 5-μm broadband and CVF images of comet Hale-Bopp taken 1997 February 10.5 UT, 50 days before perihelion. All the images exhibit a nonspherical coma with a bright “ridge” in the direction of the dust tail approximately 10″ from the coma. Synthetic aperture spectrophotometry implies that the optically important grains are of a radius ≤0.4 μm; smallest radius for any comet seen to date. The variation of the integrated surface brightness with radial distance from the coma (ρ) in all the images closely follows the “steady state” ρ⁻¹ model for comet dust ablation (Gehrz and Ney, 1992). The near-infrared colors taken along the dust tail are not constant implying the dust grain properties vary with coma distance. This revised version was published online in July 2006 with corrections to the Cover Date.     

Estimating the Size of Hale-Bopp's Nucleus

A variety of independent methods have been used to estimate the size of the nucleus of comet Hale-Bopp. Several groups have analyzed optical and infrared images of the comet and claim to detect the signature of the nucleus, despite the presence of a strong coma. A detection of the nucleus was also claimed during mm- and cm-wave observations of Hale-Bopp shortly before perihelion. A team of observers detected the occultation of a star by the nucleus of Hale-Bopp in October 1996. The maximum observed gas production rate of the comet near perihelion can be used to place a lower limit on the size of the nucleus. This paper critically reviews the many different methods used to constrain the size of Hale-Bopp's nucleus. All of the techniques are affected by systematic errors that can be difficult to quantify precisely. Nevertheless, the available evidence strongly suggests that the nucleus of Hale-Bopp has an effective radius of at least 15 km and is probably in the range 20–35 km. Thus, the prodigious gas and production rates from this comet are naturally explained by its unusually large size. This revised version was published online in July 2006 with corrections to the Cover Date.     

Comet Grains: Their IR Emission and Their Relation to ISm Grains

Comets and the chondritic porous interplanetary dust particles (CP IDPs) that they shed in their comae are reservoirs of primitive solar nebula materials. The high porosity and fragility of cometary grains and CP IDPs, and anomalously high deuterium contents of highly fragile, pyroxene-rich Cluster IDPs imply these aggregate particles contain significant abundances of grains from the interstellar medium (ISM). IR spectra of comets (3–40 μm) reveal the presence of a warm (near-IR) featureless emission modeled by amorphous carbon grains. Broad andnarrow resonances near 10 and 20 microns are modeled by warm chondritic (50% Feand 50% Mg) amorphous silicates and cooler Mg-rich crystalline silicate minerals, respectively. Cometary amorphous silicates resonances are well matched by IRspectra of CP IDPs dominated by GEMS (0.1 μm silicate spherules) that are thought to be the interstellar Fe-bearing amorphous silicates produced in AGB stars. Acid-etched ultramicrotomed CP IDP samples, however, show that both the carbon phase (amorphous and aliphatic) and the Mg-rich amorphous silicate phase in GEMS are not optically absorbing. Rather, it is Fe and FeS nanoparticles embedded in the GEMS that makes the CP IDPs dark. Therefore, CP IDPs suggest significant processing has occurred in the ISM. ISM processing probably includes in He⁺ ion bombardment in supernovae shocks. Laboratory experiments show He⁺ ion bombardment amorphizes crystalline silicates, increases porosity, and reduces Fe into nanoparticles. Cometary crystalline silicate resonances are well matched by IR spectra of laboratory submicron Mg-rich olivine crystals and pyroxene crystals. Discovery of a Mg-pure olivine crystal in a Cluster IDP with isotopically anomalous oxygen indicates that a small fraction of crystalline silicates may have survived their journey from AGB stars through the ISM to the early solar nebula. The ISM does not have enough crystalline silicates (<5%), however, to account for the deduced abundance of crystalline silicates in comet dust. An insufficient source of ISMMg-rich crystals leads to the inference that most Mg-rich crystals in comets are primitive grains processed in the early solar nebula prior to their incorporation into comets. Mg-rich crystals may condense in the hot (～1450 K), inner zones of the early solar nebula and then travel large radial distances out to the comet-forming zone. On the other hand, Mg-rich silicate crystals may be ISM amorphous silicates annealed at ～1000 K and radially distributed out to the comet-forming zone or annealed in nebular shocks at ～5-10 AU. Determining the relative abundance of amorphous and crystalline silicatesin comets probes the relative contributions of ISM grains and primitive grains to small, icy bodies in the solar system. The life cycle of dust from its stardust origins through the ISM to its incorporation into comets is discussed.     

Infrared Spectroscopy of Comet Hale-Bopp

High resolution (λ/δλ ∼ 20,000) spectra of comet C/1995 O1 (Hale-Bopp) in the 2–5 μm region were obtained during UT 2–5 March 1997 using CSHELL at the NASA Infrared Telescope Facility (IRTF) on Mauna Kea. The heliocentric and geocentric distances of the comet were ∼1.1 AU and ∼1.5 AU,respectively. We detected emission lines of the gas-phase molecules H₂O, ₄, C₂H₆, C₂H₂, HCN, and CO and derived absolute production rates and relative abundances for all species. We also used the 2-dimensional nature of the CSHELL data to investigate the spatial distribution of the molecules and find evidence that CO was derived at least partly from an extended source in the coma. This revised version was published online in July 2006 with corrections to the Cover Date.     

NICMOS/HST Post-Perihelion Images of Comet Hale-Bopp in Outburst

Near-infrared images of comet Hale-Bopp (C/1995 O1) were obtained from NICMOS/HST on UT August 27–28, 1997, when the comet emerged from the 50 degree solar elongation limit at 2.99 AU from Earth. Diffraction-limited images were obtained with camera 2 filters centered at 1.87, 1.90, 2.04 and 2.22 μm with ∼0.2″ resolution (0.076″/pixel; 165 km/pixel). Over the 1.7-hour baseline of observation, a recent (<7 hours) outburst is seen in the form of an expanding spiral arm with a projected expansion velocity of ∼80 m/s. Other asymmetric features include a jet emanating from the nucleus and several static linear features. Comparisons of the flux distribution in the 2.04 and 2.22 μm filters indicate that the region near the nucleus exhibits a slight, ∼3%, water ice absorption. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Infrared Spectrum of Comet Hele–Bopp as Seen by the Infrared Space Observatory

Spectra of comet C/1995 O1 (Hale-Bopp) were obtained with the Infrared Space Observatory (ISO) at medium resolution with the grating spectrometer in the photometer (PHT-S) and/or at high resolution with the short wavelength spectrometer (SWS) and long wavelength spectrometer (LWS) in April 1996 (Crovisier et al., 1996), September–October 1996 (Crovisier et al., 1997a, b) and December 1997, at distances from the Sun of 4.6, 2.9 and 3.9 AU, respectively. For the first time, high-resolution spectra of a comet covering the entire 2.4 to 200 μm spectral range were obtained. The vibrational bands of H₂O, CO₂ and CO are detected in emission with PHT-S. Relative production rates of 100:22:70 are derived for H₂O:CO₂:CO at 3 AU pre-perihelion. H₂O is observed at high spectral resolution in the ν₃ group of bands around 2.7 μm and the ν₂ group around 6 μm with SWS, and in several rotational lines in the 100–180 μm region with LWS. The high signal-to-noise ratio of the ν₃ band observed on September–October 1996 allows accurate determinations of the water rotational temperature (28 K) and of its ortho-to-para ratio(2.45 ± 0.10, which significantly differs from the high temperature limit and corresponds to a spin temperature of 25 K). Longward of 6 μm the spectrum is dominated by dust thermal continuum emission, upon which broad emission features are superimposed. The wavelengths of the emission peaks correspond to those of Mg-rich crystalline olivine (forsterite). In the September–October 1996 spectra, emission features at 45 and 65 μm and possible absorption at 2.9–3.2 μm suggest that grains of water ice were present at 3 AU from the Sun. The observations made post-perihelion in late December 1997 led to the detections of H₂O, CO₂ and CO at 3.9 AU from the Sun (Figures 1 and 2). The production rates were ≈3.0 × 10²⁸,3.5 × 10²⁸ and ≈1.5 × 10²⁹ s^-1, respectively. This corresponds to H₂O:CO₂:CO = 100:110:500 and confirms that at such distances from the Sun, cometary activity is dominated by sublimation of CO and CO₂ rather than by H₂O. This revised version was published online in July 2006 with corrections to the Cover Date.     

2–16 μm spectroscopy of micron-sized enstatite (Mg,Fe)2Si2O6 silicates from primitive chondritic meteorites

We present mid-infrared spectra from individual enstatite silicate grains separated from primitive type 3 chondritic meteorites. The 2–16 μm transmission spectra were taken with microspectroscopic Fourier-transform infrared (FT-IR) techniques as part of a project to produce a data base of infrared spectra from minerals of primitive meteorites for comparison with astronomical spectra. In general, the wavelength of enstatite bands increases with the proportion of Fe. However, the wavelengths of the strong En₁₀₀ bands at 10.67 and 11.67 decrease with increasing Fe content. The 11.67-μm band exhibits the largest compositional wavelength shift (twice as large as any other). Our fits of the linear dependence of the pyroxene peaks indicate that crystalline silicate peaks in the 10-μm spectra of Herbig AeBe stars, HD 179218 and 104237, are matched by pyroxenes of En₉₀₋₉₂ and En₇₈₋₈₀, respectively. If these simplistic comparisons with the astronomical grains are correct, then the enstatite pyroxenes seen in these environments are more Fe-rich than are the forsterite (Fo₁₀₀) grains identified in the far-infrared which are found to be Mg end-member grains. This differs from the general composition of type 3 chondritic meteoritic grains in which the pyroxenes are more Mg-rich than are the olivines from the same meteorite.     

3- to 14-μm Spectroscopy of Comet C/1999 T1 (McNaught-Hartley)

We report spectroscopy of Comet C/1991 T1 (McNaught-Hartley) at 3-13 μm on January 31.62 and February 1.7 2001 UT (delta=1.29 AU, r=1.40 AU) using the broadband array spectrograph system on the IRTF. The spectrum showed a silicate emission feature extending about 20% above the continuum. Two emission features at 10.3 and 11.2 μm appeared above the silicate band, the latter seemingly indicative of crystalline olivine. The 10.3-μm feature is only a 1-2 sigma detection but if real could indicate the presence of hydrated silicates. The color temperature at 8-13 μm was 260±10 K, approximately 6% above the blackbody radiative equilibrium temperature of 235 K. The magnitude at [N] was 3.13±0.02. On the second night, the comet had brightened slightly ([N]=2.98±0.02) and the two prominent emission features were absent, although the silicate emission feature maintained its trapezoidal shape with shoulders at 9.5 and 11.2 μm.     

Ultraviolet observations of comet Hale-Bopp

Ultraviolet spectroscopy and imaging of comet Hale-Bopp (C/1995 O1) were obtained from a variety of space platforms from shortly after the discovery of the comet through perihelion passage. Observations with the International Ultraviolet Explorer (IUE) and the Hubble Space Telescope (HST) spanned the range of 6.8 to 2.7 AU pre-perihelion, but IUE was decommissioned in September 1996 and HST was precluded from near-perihelion observations because of its solar avoidance constraint. In September 1996, observations were made by the Extreme Ultraviolet Explorer (EUVE) that showed the presence of soft X-rays offset from the optical center of the coma and provided a sensitive spectroscopic upper limit to the Ne/O abundance ratio. During the perihelion period NASA mounted a successful campaign of four sounding rockets that were launched at the White Sands Missile Range, New Mexico, between March 25 and April 8, 1997. The payloads included long-slit spectroscopy and ultraviolet imaging polarimetry. In addition, Hale-Bopp was observed near perihelion by ultraviolet instruments on orbiting spacecraft that were designed for solar or terrestrial observations. Observations with HST, using the Space Telescope Imaging Spectrograph (STIS), installed during the February 1997 servicing mission, resumed in August 1997. Intercomparison of the ultraviolet observations and comparison with the results on gas composition and activity from ground-based visible, infrared and radio observations may permit the resolution of many discrepant results present in the literature. This revised version was published online in July 2006 with corrections to the Cover Date.     

Long-term Evolution of the Outgassing of Comet Hale-Bopp From Radio Observations

C/1995 O1 (Hale-Bopp) has been observed on a regular basis since August 1995 at millimetre and submillimetre wavelengths using IRAM, JCMT, CSO and SEST radio telescopes. The production rates of eight molecular species (CO, HCN, CH₃OH, H₂CO,H₂S, CS, CH₃CN,HNC) have been monitored as a function of heliocentric distance(r_h from 7 AU pre-perihelion to 4 AU post-perihelion. As comet Hale-Bopp approached and receded from the Sun, these species displayed different behaviours. Far from the Sun, the most volatile species were found in general relatively more abundant in the coma. In comparison to other species, HNC, H₂CO and CS showed a much steeper increase of the production rate with decreasing r_h. Less than 1.5 AU from the Sun, the relative abundances were fairly stable and approached those found in other comets near 1 AU. The kinetic temperature of the coma, estimated from the relative intensities of the CH₃OH and CO lines, increased with decreasing r_h, from about10 K at 7 AU to 110 K around perihelion. The expansion velocity of the gaseous species, derived from the line shapes, also increased with a law close tor_h ³. This revised version was published online in July 2006 with corrections to the Cover Date.     

Infrared observations of Comet Kohoutek (1973f)

Comet Kohoutek has been observed at wavelengths between 1.25 μm and 12.5 μm before and after perihelion passage extending to comet-Sun distance 1 AU. The luminosity and the variation of brightness with comet-Sun distance in the infrared are extraordinarily similar to those of Comet Ikeya-Seki (1965f). Apart from an apparent “silicate” emission feature near 10 μm, the spectrum of the comet between 3.5 μm and 12.5 μm is close to that expected from emission by grey particles. Hotter particles and scattered sunlight produce the bulk of the 1.25- to 3.5 μm emission.     

The mid-infrared spectrum of the zodiacal and exozodiacal light

The zodiacal light is the dominant source of the mid-infrared sky brightness seen from Earth, and exozodiacal light is the dominant emission from planetary and debris systems around other stars. We observed the zodiacal light spectrum with the mid-infrared camera ISOCAM over the wavelength range 5-16 μm and a wide range of orientations relative to the Sun (solar elongations 68°-113°) and the ecliptic (plane to pole). The temperature in the ecliptic ranged from 269 K at solar elongation 68° to 244 K at 113°, and the polar temperature, characteristic of dust 1 AU from the Sun, is 274 K. The observed temperature is exactly as expected for large (>10 μm radius), low-albedo (<0.08), rapidly-rotating, gray particles 1 AU from the Sun. Smaller particles (<10 μm radius) radiate inefficiently in the infrared and are warmer than observed. We present theoretical models for a wide range of particle size distributions and compositions; it is evident that the zodiacal light is produced by particles in the 10-100 μm radius range. In addition to the continuum, we detect a weak excess in the 9-11 μm range, with an amplitude of 6% of the continuum. The shape of the feature can be matched by a mixture of silicates: amorphous forsterite/olivine provides most of the continuum and some of the 9-11 μm silicate feature, dirty crystalline olivine provides the red wing of the silicate feature (and a bump at 11.35 μm), and a hydrous silicate (montmorillonite) provides the blue wing of the silicate feature. The presence of hydrous silicate suggests the parent bodies of those particles were formed in the inner solar nebula. Large particles dominate the size distribution, but at least some small particles (radii ∼1 μm) are required to produce the silicate emission feature. The strength of the feature may vary spatially, with the strongest features being at the lowest solar elongations as well as at high ecliptic latitudes; if confirmed, this would imply that the dust properties change such that dust further from the Sun has a weaker silicate feature. To compare the properties of zodiacal dust to dust around other main sequence stars, we reanalyzed the exozodiacal light spectrum for β Pic to derive the shape of its silicate feature. The zodiacal and exozodiacal spectra are very different. The exozodiacal spectra are dominated by cold dust, with emission peaking in the far-infrared, while the zodiacal spectrum peaks around 20 μm. We removed the debris disk continuum from the spectra by fitting a blackbody with a different temperature for each aperture (ranging from 3.7″ to 27″); the resulting silicate spectra for β Pic are identical for all apertures, indicating that the silicate feature arises close to the star. The shape of the silicate feature from β Pic is nearly identical to that derived from the ISO spectrum of 51 Oph; both exozodiacal features are very different from that of the zodiacal light. The exozodiacal features are roughly triangular, peaking at 10.3 μm, while the zodiacal feature is more boxy, indicating a different mineralogy.     

Near-Infrared Photometric And Polarimetric Observations Of Comet Hale-Bopp

Near-infrared photometric and polarimetric observations of comet Hale-Bopp (1995 O1) using KONIC (Kiso Observatory Near-Infrared Camera) are reported. Observations were carried out on March 18 UT and April 26 UT 1997, when the heliocentric distances of the comet were 0.94 and 1.02 AU, and the phase angles were 48.5 deg and 32.9 deg, respectively. In the J, H, and K′ bands, we obtained linear polarization of the near-nucleus region of 16.4 ± 1.2, 18.8 ± 1.3, and 15.1 ± 0.9 percent on March 18UT and 7.1 ± 1.1, 8.9 ± 1.0, and 6.9 ± 0.6 percent on April 26, respectively. These values were higher than those observed for 1P/Halley. The maximum polarization was found at H band on both dates. Polarization maps showed higher polarization regions toward the anti-solar direction in the J and H bands. No distinct correlation was found between high polarization regions and bright regions. The projected expansion velocity of the arc structure of the dust jet was 375 ± 35.7 m/s on 17–19 March. The periodicity was found to be 11.1 ± 2.8 hours. This revised version was published online in July 2006 with corrections to the Cover Date.