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Register a new userDisintegration of Long-Period Comet C/2019 Y4 (ATLAS): I. Hubble Space Telescope Observations
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Near-Sun Comet C/2019 Y4 (ATLAS) is the first member of a long-period comet group observed to disintegrate well before perihelion. Here we present our investigation into this disintegration event using images obtained in a 3-day {it Hubble Space Telescope} (hst) campaign. We identify two fragment clusters produced by the initial disintegration event, corresponding to fragments C/2019 Y4-A and C/2019 Y4-B identified in ground-based data. These two clusters started with similar integrated brightness, but exhibit different evolutionary behavior. C/2019 Y4-A was much shorter-lived compared to C/2019 Y4-B, and showed signs of significant mass-loss and changes in size distribution throughout the 3-day campaign. The cause of the initial fragmentation is undetermined by the limited evidence but crudely compatible with either the spin-up disruption of the nucleus or runaway sublimation of sub-surface supervolatile ices, either of which would lead to the release of a large amount of gas as inferred from the significant bluing of the comet observed shortly before the disintegration. Gas can only be produced by the sublimation of volatile ices, which must have survived at least one perihelion passage at a perihelion distance of $q=0.25$~au. We speculate that Comet ATLAS is derived from the ice-rich interior of a non-uniform, kilometer-wide progenitor that split during its previous perihelion. This suggests that comets down to a few kilometers in diameter can still possess complex, non-uniform interiors that can protect ices against intense solar heating.
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We present an analysis of the photometric and spectroscopic observations of the split comet C/2019 Y4 (ATLAS). Observations were carried out on the 14th and 16th of April 2020 when the heliocentric distances of the comet were 1.212 and 1.174 au, its geocentric distances 0.998 and 0.991 au, and the phase angle 52.9{deg} and 54.5{deg}, respectively. The comet was observed with the 6-m BTA telescope of the Special Astrophysical Observatory (Russia) with the SCORPIO-2 multi-mode focal reducer. The narrow-band BC and RC cometary filters in the continuum were used. We identified numerous emissions of the CN, C2, C3, and NH2 molecules within the range of 3750-7100 {AA}. The C2/CN and C3/CN production rate ratios coincide with those of typical comets. Four fragments belonging to the coma were detected in both observational runs. We compared and analyzed temporal variations of the visual magnitudes, gas productivity, and dust colour. Based on our dynamical investigation of the orbits of comets C/1844 Y1 (Great comet) and C/2019 Y4 (ATLAS), we can claim that, with high probability, two comets do not have a common progenitor.
Comet 8P/Tuttle is a Nearly Isotropic Comet (NIC), whose physical properties are poorly known and could be different from those of Ecliptic Comets (EC) owing to their different origin. Two independent observations have shown that 8P has a bilobate nucleus. Our goal is to determine the physical properties of the nucleus (size, shape, thermal inertia, albedo) and coma (water and dust) of 8P/Tuttle. We observed the inner coma of 8P with the infrared spectrograph (IRS) and the infrared camera (MIPS) of the Spitzer Space Telescope (SST). We obtained one spectrum (5-40 $mu$m) on 2 November 2007 and a set of 19 images at 24 $mu$m on 22-23 June 2008 sampling the nucleus rotational period. The data were interpreted using thermal models for the nucleus and the dust coma, and considering 2 possible shape models of the nucleus derived from respectively Hubble Space Telescope visible and Arecibo radar observations. We favor a nucleus shape model composed of 2 contact spheres with respective radii of 2.7+/-0.1 km and 1.1+/-0.1 km and a pole orientation with RA=285+/-12 deg and DEC=+20+/-5 deg. The nucleus has a thermal inertia in the range 0-100 J/K/m^2/s^0.5 and a R-band geometric albedo of 0.042+/-0.008. The water production rate amounts to 1.1+/-0.2x10^28~molecules/s at 1.6 AU from the Sun pre-perihelion, which corresponds to an active fraction of 9%. At the same distance, the $epsilon f rho$ quantity amounts to 310+/-34 cm at 1.6~AU, and reaches 325+/-36 cm at 2.2~AU post-perihelion. The dust grain temperature is estimated to 258+/-10 K, which is 37 K larger than the thermal equilibrium temperature at 1.6 AU. This indicates that the dust grains contributing to the thermal infrared flux have a typical size of 10 $mu$m. The dust spectrum exhibits broad emissions around 10 $mu$m (1.5-sigma confidence level) and 18 $mu$m (5-sigma confidence level) that we attribute to amorphous pyroxene.
A sequence of events, dominated by two outbursts and ending with the preperihelion disintegration of comet C/2017 S3, is examined. The onset times of the outbursts are determined with high accuracy from the light curve of the nuclear condensation before it disappeared following the second outburst. While the brightness of the condensation was declining precipitously, the total brightness continued to grow in the STEREO-As HI1 images until two days before perihelion. The red magnitudes measured in these images refer to a uniform cloud of nuclear fragments, 2200 km^2 in projected area, that began to expand at a rate of 76 m s^(-1) at the time of the second outburst. A tail extension, detected in some STEREO-A images, consisted of dust released far from the Sun. Orbital analysis of the ground-based observations shows that the comet had arrived from the Oort Cloud in a gravitational orbit. Treating positional residuals as offsets of a companion of a split comet, we confirm the existence of the cloud of radiation-pressure driven millimeter-sized dust grains emanating from the nucleus during the second outburst. We detect a similar, but compact and much fainter cloud (or a sizable fluffy dust aggregate fragment) released at the time of the first outburst. --- The debris would make a sphere of 140 m across and its kinetic energy is equivalent to the heat of crystallization liberated by 100 tons of amorphous water ice. Ramifications for short-lived companions of the split comets and for 1I `Oumuamua are discussed.
We present polarization images of Comet ISON (C/2012 S1) taken with the Hubble Space Telescope (HST) on UTC 2013 May 8 (rh = 3.81 AU, Delta = 4.34 AU), when the phase angle was alpha = 12.16 degrees. This phase angle is approximately centered in the negative polarization branch for cometary dust. The region beyond 1000 km from the nucleus shows a negative polarization amplitude of p% -1.6%. Within 1000 km of the nucleus, the polarization position angle rotates to be approximately perpendicular to the scattering plane, with an amplitude p% +2.5%. Such positive polarization has been observed previously as a characteristic feature of cometary jets, and we show that Comet ISON does indeed harbor a jet-like feature. These HST observations of Comet ISON represent the first visible light, imaging polarimetry with sub-arcsecond spatial resolution of a Nearly Isotropic Comet (NIC) beyond 3.8 AU from the Sun at a small phase angle. The observations provide an early glimpse of the properties of the cometary dust preserved in this Oort-cloud comet.
We present Hubble Space Telescope observations of the active asteroid (and Geminid stream parent) 3200 Phaethon when at its closest approach to Earth (separation 0.07 AU) in 2017 December. Images were recorded within $sim$1degr~of the orbital plane, providing extra sensitivity to low surface brightness caused by scattering from a large-particle trail. We placed an upper limit to the apparent surface brightness of such a trail at 27.2 magnitudes arcsecond$^{-2}$, corresponding to an in-plane optical depth $le 3times10^{-9}$. No co-moving sources brighter than absolute magnitude 26.3, corresponding to circular equivalent radius $sim$12 m (albedo 0.12 assumed), were detected. Phaethon is too hot for near-surface ice to survive. We briefly consider the thermodynamic stability of deeply-buried ice, finding that its survival would require either a very small (regolith-like) thermal diffusivity ($< 10^{-8}$ m$^2$ s$^{-1}$), or the unexpectedly recent injection of Phaethon (timescale $lesssim$ 10$^6$ yr) into its present orbit, or both.
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