Jeremie J. Vaubaillon

Explosion of Comet 17P/Holmes as revealed by the Spitzer Space Telescope

An explosion on Comet 17P/Holmes occurred on 2007 October 23, projecting particulate debris of a wide range of sizes into the interplanetary medium. We observed the comet using the mid-Infrared Spectrograph (5–40 μm), on 2007 November 10 and 2008 February 27, and the imaging photometer (24 and 70 μm), on 2008 March 13, on board the Spitzer Space Telescope. The 2007 November 10 spectral mapping revealed spatially diffuse emission with detailed mineralogical features, primarily from small crystalline olivine grains. The 2008 February 27 spectra, and the central core of the 2007 November 10 spectral map, reveal nearly featureless spectra, due to much larger grains that were ejected from the nucleus more slowly. Optical images were obtained on multiple dates spanning 2007 October 27–2008 March 10 at the Holloway Comet Observatory and 1.5-m telescope at Palomar Observatory. The images and spectra can be segmented into three components: (1) a hemispherical shell fully 28′ on the sky in 2008 March, due to the fastest (262 m s^(−1)), smallest (2 μm) debris, with a mass 1.7 × 10^(12) g; (2) a ‘blob’ or ‘pseudonucleus’ offset from the true nucleus and subtending some 10′ on the sky, due to intermediate speed (93 m s^(−1)) and size (8 μm) particles, with a total mass 2.7 × 10^(12) g; and (3) a ‘core’ centered on the nucleus due to slower (9 m s^(−1)), larger (200 μm) ejecta, with a total mass 3.9 × 10^(12) g. This decomposition of the mid-infrared observations can also explain the temporal evolution of the millimeter-wave flux. The orientation of the leading edge of the ejecta shell and the ejecta ‘blob,’ relative to the nucleus, do not change as the orientation of the Sun changes; instead, the configuration was imprinted by the orientation of the initial explosion. The distribution and speed of ejecta implies an explosion in a conical pattern directed approximately in the solar direction on the date of explosion. The kinetic energy of the ejecta >10^(21) erg is greater than the gravitational binding energy of the nucleus. We model the explosion as being due to crystallization and release of volatiles from interior amorphous ice within a subsurface cavity; once the pressure in the cavity exceeded the surface strength, the material above the cavity was propelled from the comet. The size of the cavity and the tensile strength of the upper layer of the nucleus are constrained by the observed properties of the ejecta; tensile strengths on >10 m scale must be greater than 10 kPa (or else the ejecta energy exceeds the binding energy of the nucleus) and they are plausibly 200 kPa. The appearance of the 2007 outburst is similar to that witnessed in 1892, but the 1892 explosion was less energetic by a factor of about 20.

OBSERVATIONAL EVIDENCE FOR AN IMPACT ON THE MAIN-BELT ASTEROID (596) SCHEILA

An unexpected outburst was observed around (596) Scheila in 2010 December. We observed (596) Scheila soon after the impact using ground-based telescopes. We succeeded in the detection of a faint linear tail after 2011 February, which provides a clue to determine the dust ejection date. It is found that the dust particles ranging from 0.1-1 ?m to 100 ?m were ejected into the interplanetary space impulsively on December 3.5 ?1.0 day. The ejecta mass was estimated to be (1.5-4.9)?108?kg, suggesting that an equivalent mass of a 500-800?m diameter crater was excavated by the event. We also found that the shape of the light curve changed after the impact event probably because fresh material was excavated around the impact site. We conclude that a decameter-sized asteroid collided with (596) Scheila only eight days before the discovery.

Interpretation of (596) Scheila's Triple Dust Tails

Strange-looking dust cloud around asteroid (596) Scheila was discovered on 2010 December 11.44-11.47. Unlike normal cometary tails, it consisted of three tails and faded within two months. We constructed a model to reproduce the morphology of the dust cloud based on the laboratory measurement of high-velocity impacts and the dust dynamics. As a result, we succeeded in reproducing the peculiar dust cloud by an impact-driven ejecta plume consisting of an impact cone and downrange plume. Assuming an impact angle of 45 Degree-Sign , our model suggests that a decameter-sized asteroid collided with (596) Scheila from the direction of ({alpha}{sub im}, {delta}{sub im}) = (60 Degree-Sign , -40 Degree-Sign ) in J2000 coordinates on 2010 December 3. The maximum ejection velocity of the dust particles exceeded 100 m s{sup -1}. Our results suggest that the surface of (596) Scheila consists of materials with low tensile strength.

3D/BIELA AND THE ANDROMEDIDS: FRAGMENTING VERSUS SUBLIMATING COMETS

Comet 3D/Biela broke up in 1842/1843 and continued to disintegrate in the returns of 1846 and 1852. When meteor storms were observed in November of 1872 and 1885, it was surmised that those showers were the debris from that breakup. This could have come from one of two sources: (1) the initial separation of fragments near aphelion or (2) the continued disintegration of the fragments afterward. Alternatively, the meteoroids could simply have come from water vapor drag when the fragments approached perihelion (option 3). We investigated the source of the Andromedid storms by calculating the dynamical evolution of dust ejected in a normal manner by water vapor drag in the returns from 1703 to 1866, assuming that the comet would have remained similarly active over each return. In addition, we simulated the isotropic ejection of dust during the initial fragmentation event at aphelion in December of 1842. We conclude that option 2 is the most likely source of meteoroids encountered during the 1872 and 1885 storms, but this accounts for only a relatively small amount of mass lost in a typical comet breakup.

The 2005 Draconid outburst

Aims. We report the flux profile and mean orbit for meteoroids associated with an unexpected activity outburst from the Draconid meteor shower on 8 October, 2005. The primary aim is to define the characteristics of the outburst and establish the age of the associated meteoroids. Methods. Radar data from the outburst are used to define the flux profile and mass distribution for Draconid meteoroids at small meteoroid masses, while visual data are used to define the ZHR profile at larger masses. The radar recorded both single station and orbital data permitting orbits for many individual Draconid radar echoes as well as determination of a mean shower radiant. Results. The peak activity was centered at 16.1 hrs UT, (/to = 195°42 (J2000) solar longitude) with noticeably heightened radar activity lasting for a total of more than three hours. Based on the distribution of amplitudes for underdense Draconid echoes, the mean mass distribution index at masses of 10 -6 kg was found to be 2.0 ± 0.1. The equivalent hourly-binned radar ZHR was in excess of 150, while visual observations in the same intervals produced ZHRs of 40. The apparent radiant of the outburst was α = 256.9 ± 2.2°, δ = +56.6 ± 1.8°. Numerical modelling of radar-sized Draconids show that a significant number of meteoroids from the 1946 perihelion passage of 21 P/Giacobini-Zinner encountered the Earth over the interval 195°.23 λ ⊙ < 196°0 in 2005, centred about 195°50 Conclusions. The shower was rich in faint meteors, and therefore showed higher activity in radar data than in visual data alone. The duration of the outburst was very similar to past returns, while the mean radar stream orbit was somewhat different than previous measurements (the radiant differed by 6° in right ascension and 2° in declination) and also from the expected distribution of orbital elements for modelled 1946 meteoroids encountered in 2005.

MINOR PLANET 2008 ED69 AND THE KAPPA CYGNID METEOR SHOWER

Until recently, the kappa Cygnids (IAU#12) were considered an old shower, because the meteors were significantly dispersed in node, radiant, and speed, despite being 28-38° inclined. In 1993, an outburst of kappa Cygnids was observed, which implied that this meteoroid stream was relatively young, instead. At least some dust was still concentrated in dust trailets. Until now, no active comet parent body was known, however, and the wide 22° dispersion of nodes was difficult to explain. This work reports that a minor planet has been discovered that has the right orbital dynamics to account for the kappa Cygnids. Minor planet 2008 ED69 is intrinsically bright, with H = 16.7 ± 0.3, and moves in a highly inclined orbit (i = 36.3°). With one node near Jupiters orbit, the perihelion distance, longitude of perihelion, and node quickly change over time, but in a manner that keeps dust concentrated for a long period of time. The stream is more massive than the remaining body, and a form of fragmentation is implicated. A break-up, leaving a stream of meteoroids and at least the one remaining fragment 2008 ED69, can account for the observed dispersion of the kappa Cygnids in Earths orbit, if the formation epoch is about 2-3 nutation cycles ago, dating to around 4000-1600 BC. Most of that debris now passes close to the orbit of Venus, making the kappa Cygnids a significant shower on Venus.

Spitzer Space Telescope Observations and the Particle Size Distribution of Comet 73P/Schwassmann-Wachmann 3

Infrared observations of the broken comet 73P/Schwassmann-Wachmann 3 were performed at 24 μm with the Spitzer Space Telescope in 2006 May. The image reveals the fragments and the meteoroid stream associated with the comet. Heavy numerical simulation of the production and evolution of the meteoroids ejected by the comet during the last century, coupled with a model of thermal emission of cometary grains, allows us to determine the dust size distribution. As found by in situ measurements performed during spacecraft encounters with 1P/Halley and 81P/Wild 2, the size distribution for 73P shows a plateau for the mass range 10–10 to 10–7 kg. Except for this mass bin, the cumulative mass index is in the range 0.85-1, so the size distribution has the same slope for dust and meteoroids separately, but with a significantly different normalization such that there are more large particles than would be expected from an extrapolation of the small-particle size distribution. The abundance of meteoroids with masses larger than 10–5 kg is poorly determined by in situ measurements due to the small number of detections; infrared observations provide a unique way to constrain it. The stream is mainly composed of particles ejected during the 1995 and 2001 passages. We show that the dust production rate increased by a factor of 11 during the 1995 outburst.

The dust trail complex of comet 79P/du Toit-Hartley and meteor outbursts at Mars

Aims. Meteoroid trails ejected during past perihelion passages of the Mars-orbit-intersecting comet 79P/du Toit-Hartley have the potential of generating meteor outbursts in the Martian atmosphere. Depending on timing and intensity, the effects of these outbursts may be detectable by instrumentation operating in the vicinity of Mars. We aim to generate predictions for meteor activity in the martian atmosphere related to that comet; to search for evidence, in planetary mission data, that such activity took place; and to make predictions for potentially detectable future activity. Methods. We have modelled the stream by integrating numerically the states of particle ensembles, each ensemble representing a trail of meteoroids ejected from the comet during 39 perihelion passages from 1803, and propagated them forward in time, concentrating on those particles that physically approach Mars in the recent past and near future. Results. We find several instances where meteor outbursts of low to moderate intensity may have taken place at Mars since 1997. A search through Mars Global Surveyor (MGS) radio science data during two periods in 2003 and 2005 when data coverage was available showed that a plasma layer did indeed form in the martian ionosphere for a period of a few hours in April 2003 as a direct consequence of the predicted outburst. The apparent failure to identify such an event in 2005 could be due to those meteoroids ablating lower in the atmosphere or that the cometary dust follows a different particle size distribution than what was assumed. Our study highlights the need for further theoretical modelling of the response of the martian ionosphere to a time-variable meteoroid flux, observations of the comet itself and, most importantly, regular monitoring of the martian ionosphere during future outbursts predicted by our model.

Association of individual meteors with their parent bodies

Context. The main problem in establishing a parent body for a meteoroid stream is the choice of a reliable meteoroid stream identification method. There are several identification methods based on three components: a dynamical similarity function, a threshold value, and meteoroid stream search algorithm. Aims. The French Meteor Network, developed in the CABERNET project (PODET-MET), will soon provide a large amount of meteor observation data aiming to establish a parent body for each observed meteor. We therefore aim to obtain the value of the upper limit to the criteria that we can later use for data provided by the French Meteor Network. Methods. We tested four D-criteria, using artificial data sets for which the parent body is known. We obtained threshold values and applied them to the Armagh Observatory meteor database. A detailed comparison is made between a similarity function based on the orbital elements and the function defined by quasi-invariants. Results. We detected major meteoroid streams in the Armagh Observatory meteor database. A few meteors were also found to be associated with the asteroid 2005 UW6 – an asteroids not considered as a possible parent body for Taurid complex before. However, the problem of finding the appropriate threshold value that would work with all meteoroid streams is still open.

Spectral properties of the largest asteroids associated with Taurid Complex

Context. The Taurid Complex is a massive stream of material in the inner part of the Solar System. It contains objects spanning the range of 10 −6 ‐10 3 m, considered by some authors to have a common cometary origin. The asteroids belonging to Taurid Complex are on Apollo type orbit, with most of them being flagged as potent ially hazardous asteroids. In this context, understanding the nature and the origin of this asteroidal population is not only of sc ientific interest but also of practical importance. Aims. We aim to investigate the surface mineralogy of the asteroids associated with Taurid Complex using visible and near-infrared spectral data. Compositional linking between these asteroids and meteorites can be derived based on the obtained spectra. Methods. We obtained spectra of six of the largest asteroids (2201, 4183, 4486, 5143, 6063, and 269690) associated with Taurid complex. The observations were made with the IRTF telescope equipped with the spectro-imager SpeX. Their taxonomic classification is made using Bus-DeMeo taxonomy. The asteroid spectra are compared with the meteorite spectra from the Relab database. Mineralogical models were applied to determine their surface composition. All the spectral analysis is made in the context of the already published physical data. Results. Five of the objects studied in this paper present spectral ch aracteristics similar to the S taxonomic complex. The spectra of ordinary chondrites (spanning H, L, and LL subtypes) are the best matches for these asteroid spectra. The asteroid (269690) 1996 RG3 presents a flat featureless spectrum which could be assoc iated to a primitive C-type object. The increased reflectanc e above 2.1 microns constrains its geometrical albedo to a value around 0.03. Conclusions. While there is an important dynamical grouping among the Taurid Complex asteroids, the spectral data of the largest objects do not support a common cometary origin. Furthermore, there are significant variations between the spectra acqu ired until now.