Cometary Activity beyond the Planets

Recent observations show activity in long-period comet C/2017 K2 at heliocentric distances beyond the orbit of Uranus. With this as motivation, we constructed a simple model that takes a detailed account of gas transport modes and simulates the time-dependent sublimation of supervolatile ice from beneath a porous mantle on an incoming cometary nucleus. The model reveals a localized increase in carbon monoxide (CO) sublimation close to heliocentric distance r H = 150 au (local blackbody temperature ∼23 K), followed by a plateau and then a slow increase in activity toward smaller distances. This localized increase occurs as heat transport in the nucleus transitions between two regimes characterized by the rising temperature of the CO front at larger distances and nearly isothermal CO at smaller distances. As this transition is a general property of sublimation through a porous mantle, we predict that future observations of sufficient sensitivity will show that inbound comets (and interstellar interlopers) will exhibit activity at distances far beyond the planetary region of the solar system.

C/2010 U3 (Boattini): A Bizarre Comet Active at Record Heliocentric Distance

<p>We report an identification of long-period comet C/2010 U3 (Boattini) active at a new record inbound heliocentric distance of <em>r</em><sub>H</sub> ≈ 26 au. Two outburst events around 2009 and 2017 were observed. The dust morphology of the coma and tail cannot be explained unless the Lorentz force, solar gravitation, and solar radiation pressure force are all taken into account. Optically dominant dust grains have radii of ~10 μm and are ejected protractedly at speeds ≤50 m s<sup>−1</sup> near the subsolar point. The prolonged activity indicates that sublimation of supervolatiles (e.g., CO, CO<sub>2</sub>) is at play. Similar to other long-period comets, the colour of the cometary dust is redder than the solar colours. We also observed potential colour variations when the comet was at 10 < <em>r</em><sub>H</sub> < 15 au, concurrent with the onset of crystallisation of amorphous water ice, if any. Using publicly available and our refined astrometric measurements, we estimated the precise trajectory of the comet, propagated it backward to its previous perihelion, and found that the comet visited the planetary region ~2 Myr ago at perihelion distance <em>q</em> ≈ 8 au. Thus, C/2010 U3 (Boattini) is almost certainly a dynamically old comet from the Oort cloud, and the observed activity cannot be caused by retained heat from the previous apparition. The detailed study is presented in Hui et al. (2019, AJ, 157, 162).</p>

The outbursts of comet 29P/SW-1 and C/2020 R4 (ATLAS)

<p>We present an analysis of the dynamic dust coma of Centaur 29P/Schewassmann-Wachmann 1 and long-period comet C/2020 R4 (Atlas). Comet P/SW1 has exhibited a considerable level of activity (so-called outbursts) since its discovery in 1925. In 2011, we found the morphology of comet P/SW 1 showed dust features in the form of jets, spirals and shells. This year, we have obtained multi-wavelength observations of 29P/SW 1's dust coma at Lulin observatory and we will present preliminary results including the morphological analysis and the colour (B-V, V-R, and R-I) investigation of the dust coma when comet is at quiet and active period.</p> <p>At least three outbursts found within two weeks from comet Atlas is unusual for long-period comet. However, we didn’t find any new jet features and fragments from the morphology of comet Atlas. Except for the color investigation during the outburst, we will also give the dust/gas production rates in the comparison between pre- and post-outbursts.</p>

Cometary Activity Begins at Kuiper Belt Distances: Evidence from C/2017 K2

<p class="p1">We discuss the development of activity in the extraordinary, distant long-period comet C/2017 K2 over the heliocentric distance range<span class="Apple-converted-space"> </span>9 < r<sub>H</sub> < 16 AU.<span class="Apple-converted-space">  C/2017 K2 is an incoming long-period comet with a period so long (~ 3 Myr) that no heat from the previous perihelion can be retained; we can be sure that the observed mass-loss is driven by the current insolation and not by a thermal lag.  </span>The comet is characterized by a steady-state coma of sub-millimeter and larger particles ejected at low (4 m/s) velocity, filling a roughly spheroidal coma with a characteristic scale of 80,000 km.<span class="Apple-converted-space">  </span>In a fixed, co-moving volume around the nucleus we find that the scattering cross-section of the coma, C, is related to the heliocentric distance by a power law, C ~ r<sub>H</sub><sup>-s</sup>, with heliocentric index s = 1.14+/-0.05. This dependence is significantly weaker than the r<sub>H</sub><sup>-2</sup>, variation of the<span class="Apple-converted-space"> </span>insolation as a result of two effects.<span class="Apple-converted-space">  </span>These are, first, the heliocentric dependence of the dust velocity and, second, a lag effect due to very slow-moving<span class="Apple-converted-space"> </span>particles ejected long before the observations were taken. <span class="Apple-converted-space">  </span>A Monte Carlo<span class="Apple-converted-space"> </span>model of the photometry shows that dust production beginning at r<sub>H</sub> ~ 35 AU is needed to match the measured heliocentric index, with only a slight dependence on the particle size distribution.<span class="Apple-converted-space">  </span>Dust mass loss rates at 10 AU are of order dM/dt ~ 10<sup>3 </sup>a<sub>1</sub> kg/s, where 0.1 < a<sub>1</sub> < 1 is the effective particle radius expressed in millimeters.</p> <p class="p1">The expulsion of submillimeter and larger grains, beginning at Kuiper belt distances, is likely the result of the sublimation of near-surface supervolatile ice (probably CO, as suggested by the recent detection of this molecule at 6.7 AU; Yang et al. Ap. J. Letters, in press). Water ice is involatile over the observed distance range and even the energy and gas release triggered by the crystallization of amorphous ice, if present, cannot produce activity at 35 AU.  Comet C/2017 K2 will reach perihelion near Mars' orbit in December 2022.  </p> <p class="p1"> </p> <p class="p1">This work is described in D. Jewitt, Y. Kim. M. Mutchler, J. Agarwal, J. Li and H. Weaver (2021).  Astronomical Journal, 161:188 (11pp) </p>

The Dazzling Comet C/2020 F3 (NEOWISE): The Comet of The Century

The newfound long period comet NEOWISE has the potential to be one of the brightest comets ever seen. The comet which was visible to the naked eye is considered as the comet of the century. This icy object was discovered on March 27, 2020. In this paper, we dive into some fascinating facts about NEOWISE comet that make such comet unique.

A surprise in the updated list of stellar perturbers of long-period comet motion

Context. The second Gaia data release (Gaia DR2) provided us with the precise five-parameter astrometry for 1.3 billion of sources. As stars passing close to the Solar System are thought to influence the dynamical history of long-period comets, we update and extend the list of stars that could potentially perturb the motion of these comets. Aims. We announce a publicly available database containing an up-to-date list of stars and stellar systems potentially perturbing the motion of long-period comets. We add new objects and revise previously published lists. Special emphasis is placed on stellar systems. A discussion of mass estimation is included. Methods. Using the astrometry, preferably from Gaia DR2, augmented with data from other sources, we calculate nominal spatial positions and velocities for each star. To filter studied objects on the basis of their nominal minimum heliocentric distances we numerically integrate the motion of stars under the Galactic potential and their mutual interactions. Results. We announce the updated list of stellar perturbers of cometary motion, including the masses of perturbers along with the publicly available database interface. These data are ready to be used with the observed long-period comets orbits to study an individual influence of a whole sample of perturbers, or specific stars, on a dynamical past or future of a specific comet. New potential perturbers were added; there are 138 more than in the previously published sources. Conclusions. We demonstrate that a new set of prospective perturbers is an important tool in studies of cometary dynamics. The use of our data changes the results of the past and future cometary motion analysis. We point out a puzzling object in our list, star ALS 9243. The Gaia DR2 astrometry suggests a very close encounter of this star with the Sun; however, its astrophysical parameters result in a completely different current distance of ALS 9243 and its high mass.