Keywords

Dust grains of crystalline silicate, which are rarely present in interstellar space, were found in cometary nuclei. These crystalline silicates are thought to have formed by annealing of amorphous silicate grains or direct condensation of gaseous materials near the Sun in the solar nebula (SN), and incorporated into cometary nuclei in the cold comet-forming region after radial transportation of grains in the SN. Abundances of the crystalline silicate dust grains were therefore expected to be smaller farther from the Sun. We aim to better understand the formation mechanism of minerals incorporated into comet 17P/Holmes based on its mineral abundances. To derive the mineral composition of comet 17P/Holmes, we applied a thermal emission model for cometary dust grains to mid-infrared spectra of comet 17P/Holmes taken with the Cooled Mid-Infrared Camera and Spectrograph mounted on the Subaru Telescope a few days later the great outburst in 2007 October. The resulting mass fraction of crystalline silicate, f_cry, and an olivine-to-pyroxene abundance ratio, f_OP, are f_cry = 0.31 ± 0.03 and f_OP = 1.20^+0.16/_−0.12, respectively. Based on a simple consideration of the mixing of dust grains originating in both the interstellar medium and SN, the minerals of 17P/Holmes formed by nonequilibrium condensation. This result is consistent with theoretical and experimental predictions for vaporization and condensation of olivine in the SN.

We performed a monitoring observation of a Jupiter-family comet, 17P/Holmes, during its 2014 perihelion passage to investigate its secular change in activity. The comet has drawn the attention of astronomers since its historic outburst in 2007, and this occasion was its first perihelion passage since then. We analyzed the obtained data using an aperture photometry package and derived the ${Af}\rho$ parameter, a proxy for the dust-production rate. We found that ${Af}\rho$ showed asymmetric properties with respect to the perihelion passage: it increased moderately from 100 cm at the heliocentric distance of r_h = 2.6–3.1 AU to a maximal value of 185 cm at r_h = 2.2 AU (near the perihelion) during the inbound orbit, while dropping rapidly to 35 cm at r_h = 3.2 AU during the outbound orbit. We applied a model for characterizing dust-production rates as a function of r_h and found that the fractional active area of the cometary nucleus had dropped from 20%–40% in 2008–2011 (around the aphelion) to 0.1%–0.3% in 2014–2015 (around the perihelion). This result suggests that a dust mantle would have developed rapidly in only one orbital revolution around the Sun. Although a minor eruption was observed on UT 2015 January 26 at r_h = 3.0 AU, the areas excavated by the 2007 outburst would be covered with a layer of dust (≲10 cm depth) which would be enough to insulate the subsurface ice and to keep the nucleus in a state of low activity.

This article reports a new optical observation of 17P/Holmes one orbital period after the historical outburst event in 2007. We detected not only a common dust tail near the nucleus but also a long narrow structure that extended along the position angle 2746 ± 01 beyond the field of view (FOV) of the Kiso Wide Field Camera, i.e., >02 eastward and >20 westward from the nuclear position. The width of the structure decreased westward with increasing distance from the nucleus. We obtained the total cross section of the long extended structure in the FOV, C_FOV = (2.3 ± 0.5) × 10¹⁰ m². From the position angle, morphology, and mass, we concluded that the long narrow structure consists of materials ejected during the 2007 outburst. On the basis of the dynamical behavior of dust grains in the solar radiation field, we estimated that the long narrow structure would be composed of 1 mm–1 cm grains having an ejection velocity of >50 m s⁻¹. The velocity was more than one order of magnitude faster than that of millimeter–centimeter grains from typical comets around a heliocentric distance r_h of 2.5 AU. We considered that sudden sublimation of a large amount of water-ice (≈10³⁰ mol s⁻¹) would be responsible for the high ejection velocity. We finally estimated a total mass of M_TOT = (4–8) × 10¹¹ kg and a total kinetic energy of E_TOT = (1–6) × 10¹⁵ J for the 2007 outburst ejecta, which are consistent with those of previous studies that were conducted soon after the outburst.

We present high angular resolution Submillimeter Array observations of the outbursting Jupiter family comet 17P/Holmes on 2007 October 26–29, achieving a spatial resolution of 25, or ∼3000 km at the comet distance. The observations resulted in detections of the rotational lines CO 3–2, HCN 4–3, H¹³CN 4–3, CS 7–6, H₂CO 3_{1, 2}–2_{1, 1}, H₂S 2_{2, 0}–2_{1, 1}, and multiple CH₃OH lines, along with the associated dust continuum at 221 and 349 GHz. The continuum has a spectral index of 2.7 ± 0.3, slightly steeper than blackbody emission from large dust particles. From the imaging data, we identify two components in the molecular emission. One component is characterized by a relatively broad line width (∼1 km s⁻¹ FWHM) exhibiting a symmetric outgassing pattern with respect to the nucleus position. The second component has a narrower line width (<0.5 km s⁻¹ FWHM) with the line center redshifted by 0.1–0.2 km s⁻¹ (cometocentric frame), and shows a velocity shift across the nucleus position with the position angle gradually changing from 66° to 30° within the four days of observations. We determine distinctly different CO/HCN ratios for each of the components. For the broad-line component we find CO/HCN < 7, while in the narrow-line component, CO/HCN = 40 ± 5. We hypothesize that the narrow-line component originates from the ice grain halo found in near-nucleus photometry, believed to be created by sublimating recently released ice grains around the nucleus during the outburst. In this interpretation, the high CO/HCN ratio of this component reflects the more pristine volatile composition of nucleus material released in the outburst.

Comet 17P/Holmes underwent a massive outburst in 2007 October, brightening by a factor of almost a million in under 48 hr. We used infrared images taken by the Wide-Field Infrared Survey Explorer mission to characterize the comet as it appeared at a heliocentric distance of 5.1 AU almost 3 yr after the outburst. The comet appeared to be active with a coma and dust trail along the orbital plane. We constrained the diameter, albedo, and beaming parameter of the nucleus to 4.135 ± 0.610 km, 0.03 ± 0.01, and 1.03 ± 0.21, respectively. The properties of the nucleus are consistent with those of other Jupiter family comets. The best-fit temperature of the coma was 134 ± 11 K, slightly higher than the blackbody temperature at that heliocentric distance. Using Finson–Probstein modeling, we found that the morphology of the trail was consistent with ejection during the 2007 outburst and was made up of dust grains between 250 μm and a few cm in radius. The trail mass was ∼1.2–5.3 × 10¹⁰ kg.

A Jupiter-family comet, 17P/Holmes, underwent outbursts in 1892 and 2007. In particular, the 2007 outburst is known as the greatest outburst over the past century. However, little is known about the activity before the outburst because it was unpredicted. In addition, the time evolution of the nuclear physical status has not been systematically studied. Here, we study the activity of 17P/Holmes before and after the 2007 outburst through optical and mid-infrared observations. We found that the nucleus was highly depleted in its near-surface icy component before the outburst but that it became activated after the 2007 outburst. Assuming a conventional 1 μm sized grain model, we derived a surface fractional active area of 0.58% ± 0.14% before the outburst whereas the area was enlarged by a factor of ∼50 after the 2007 outburst. We also found that large (⩾1 mm) particles could be dominant in the dust tail observed around aphelion. Based on the size of the particles, the dust production rate was ≳170 kg s⁻¹ at a heliocentric distance of r_h = 4.1 AU, suggesting that the nucleus was still active around the aphelion passage. The nucleus color was similar to that of the dust particles and average for a Jupiter-family comet but different from that of most Kuiper Belt objects, implying that color may be inherent to icy bodies in the solar system. On the basis of these results, we concluded that more than 76 m of surface material was blown off by the 2007 outburst.

On 2007 October 29, the outbursting comet 17P/Holmes passed within 079 of a background star. We recorded the event using optical, narrowband photometry and detect a 3%–4% dip in stellar brightness bracketing the time of closest approach to the comet nucleus. The detected dimming implies an optical depth τ ≈ 0.04 at 15 from the nucleus and an optical depth toward the nucleus center τ_n < 13.3. At the time of our observations, the coma was optically thick only within ρ ≲ 001 from the nucleus. By combining the measured extinction and the scattered light from the coma, we estimate a dust red albedo p_d = 0.006 ± 0.002 at α = 16° phase angle. Our measurements place the most stringent constraints on the extinction optical depth of any cometary coma.

Comet 17P/Holmes underwent the largest cometary outburst in recorded history on UT 2007 October 23, releasing massive quantities of dust and gas. We used the Canada–France–Hawaii Telescope to obtain wide-field images of 17P/Holmes on 15 dates over a period of three months following the outburst and employ them here to examine the subsequent activity of the nucleus and the nature of the ejecta closest to the nucleus. Through aperture photometry we observed the inner coma (within 2500 km of the nucleus) to fade from an apparent magnitude of 11.7 mag to 17.6 mag, corresponding to absolute magnitudes of 8.1 mag and 12.4 mag, between UT 2007 November 6 and 2008 February 12. A second much smaller outburst occurred on UT 2007 November 12, three weeks after the original outburst, suggesting that the nucleus remained unstable. The surface brightness profile of the inner coma was consistently shallow relative to the expected steady-state profile, and showed a persistent brightness enhancement within ∼5000 km of the nucleus. We propose that sublimating ice grains created an ice grain halo around the nucleus, while fragmenting grains were responsible for the shallow surface brightness profile.

We performed an image search for "Comet Holmes," using the Yahoo! Web search engine, on 2010 April 1. Thousands of images were returned. We astrometrically calibrated—and therefore vetted—the images using the Astrometry.net system. The calibrated image pointings form a set of data points to which we can fit a test-particle orbit in the solar system, marginalizing over image dates and detecting outliers. The approach is Bayesian and the model is, in essence, a model of how comet astrophotographers point their instruments. In this work, we do not measure the position of the comet within each image, but rather use the celestial position of the whole image to infer the orbit. We find very strong probabilistic constraints on the orbit, although slightly off the Jet Propulsion Lab ephemeris, probably due to limitations of our model. Hyperparameters of the model constrain the reliability of date meta-data and where in the image astrophotographers place the comet; we find that ∼70% of the meta-data are correct and that the comet typically appears in the central third of the image footprint. This project demonstrates that discoveries and measurements can be made using data of extreme heterogeneity and unknown provenance. As the size and diversity of astronomical data sets continues to grow, approaches like ours will become more essential. This project also demonstrates that the Web is an enormous repository of astronomical information, and that if an object has been given a name and photographed thousands of times by observers who post their images on the Web, we can (re-)discover it and infer its dynamical properties.

We present time-resolved photometric observations of the Jupiter family comet 17P/Holmes during its dramatic 2007 outburst. The observations, from the orbiting Solar Mass Ejection Imager (SMEI), provide the most complete measure of the whole-coma brightness, free from the effects of instrumental saturation and with a time resolution well matched to the rapid brightening of the comet. The light curve is divided into two distinct parts. A rapid rise between the first SMEI observation on UT 2007 October 24 06h 37m (mid-integration) and UT 2007 October 25 is followed by a slow decline until the last SMEI observation on UT 2008 April 6 22h 16m (mid-integration). We find that the rate of change of the brightness is reasonably well described by a Gaussian function having a central time of UT 2007 October 24.54 ± 0.01 and a full width at half-maximum of 0.44 ± 0.02 days. The maximum rate of brightening occurs some 1.2 days after the onset of activity. At the peak, the scattering cross-section grows at 1070 ± 40 km² s⁻¹ while the (model-dependent) mass loss rates inferred from the light curve reach a maximum at 3 × 10⁵ kg s⁻¹. The integrated mass in the coma lies in the range (2–90) × 10¹⁰ kg, corresponding to 0.2%–10% of the nucleus mass, while the kinetic energy of the ejecta is (0.7–30) megatonnes TNT. The particulate coma mass could be contained within a shell on the nucleus of thickness 1–60 m. This is also the approximate distance traveled by conducted heat in the century since the previous outburst of 17P/Holmes. This coincidence is consistent with, but does not prove, the idea that the outburst was triggered by the action of conducted heat, possibly through the crystallization of buried amorphous ice.