Constraints on the spin-pole orientation, jet morphology, and rotation of interstellar comet 2I/Borisov with deep HST imaging

ABSTRACT We present high resolution, deep imaging of interstellar comet 2I/Borisov taken with the Hubble Space Telescope/Wide Field Camera 3 (HST/WFC3) on 2019 December 8 UTC and 2020 January 27 UTC (HST GO 16040, PI: Bolin) before and after its perihelion passage in combination with HST/WFC3 images taken on 2019 October 12 UTC and 2019 November 16 UTC (HST GO/DD 16009, PI: Jewitt) before its outburst and fragmentation of 2020 March, thus observing the comet in a relatively undisrupted state. We locate 1–2 arcsec long (2000–3000 km projected length) jet-like structures near the optocentre of 2I that appear to change position angles from epoch to epoch. With the assumption that the jet is located near the rotational pole supported by its stationary appearance on ∼10–100 h time frames in HST images, we determine that 2I’s pole points near α = 322 ± 10° and δ = 37 ± 10° (λ = 341° and β = 48°) and may be in a simple rotation state. Additionally, we find evidence for possible periodicity in the HST time-series light curve on the time-scale of ∼5.3 h with a small amplitude of ∼0.05 mag implying a lower limit on its b/a ratio of ∼1.5 unlike the large ∼2 mag light curve observed for 1I/‘Oumuamua. However, these small light-curve variations may not be the result of the rotation of 2I’s nucleus due to its dust-dominated light-scattering cross-section. Therefore, uniquely constraining the pre-Solar system encounter, pre-outburst rotation state of 2I may not be possible even with the resolution and sensitivity provided by HST observations.

Spitzer Space Telescope observations of bilobate comet 8P/Tuttle

Context. Comet 8P/Tuttle is a nearly isotropic comet whose physical properties are poorly known and might be different from those of ecliptic comets owing to their different origin. Two independent observations have shown that 8P/Tuttle has a bilobate nucleus. Aims. Our goal is to determine the physical properties of the nucleus (size, shape, thermal inertia, and albedo) and coma (water and dust) of 8P/Tuttle. Methods. We observed the inner coma of 8P/Tuttle with the infrared spectrograph and the infrared camera of the Spitzer Space Telescope. We obtained one spectrum (5–40 μm) on 2 November 2007 and a set of 19 images at 24 μm on 22–23 June 2008 sampling the rotational period of the nucleus. The data were interpreted using thermal models for the nucleus and the dust coma, and we considered two possible shape models of the nucleus derived from Hubble Space Telescope visible and Arecibo radar observations. Results. We favor a model for the nucleus shape that is composed of two 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° and Dec = +20 ± 5°. The thermal inertia of the nucleus lies in the range 0–100 J K−1 m−2 s−1∕2 and the R-band geometric albedo is 0.042 ± 0.008. The water production rate amounts to 1.1 ± 0.2 × 1028 molecules s−1 at 1.6 AU from the Sun pre-perihelion, which corresponds to an active fraction of ≈9%. At the same distance, the ɛfρ quantity amounts to 310 ± 34 cm, and it reaches 325 ± 36 cm at 2.2 AU post-perihelion. The dust grain temperature is estimated to be 258 ± 10 K, which is 37 K higher than the thermal equilibrium temperature at 1.6 AU. This indicates that the dust grains that contribute to the thermal infrared flux have a typical size of ≈10 μm. The dust spectrum exhibits broad emission around 10 μm (1.5σ confidence level) and 18 μm (5σ confidence level) that we attribute to amorphous pyroxene.

Optical observations of NEA 3200 Phaethon (1983 TB) during the 2017 apparition

Context. The near-Earth asteroid 3200 Phaethon (1983 TB) is an attractive object not only from a scientific viewpoint but also because of JAXA’s DESTINY+ target. The rotational lightcurve and spin properties were investigated based on the data obtained in the ground-based observation campaign of Phaethon. Aims. We aim to refine the lightcurves and shape model of Phaethon using all available lightcurve datasets obtained via optical observation, as well as our time-series observation data from the 2017 apparition. Methods. Using eight 12-m telescopes and an optical imager, we acquired the optical lightcurves and derived the spin parameters of Phaethon. We applied the lightcurve inversion method and SAGE algorithm to deduce the convex and non-convex shape model and pole orientations. Results. We analysed the optical lightcurve of Phaethon and derived a synodic and a sidereal rotational periods of 3.6039 h, with an axis ratio of a∕b = 1.07. The ecliptic longitude (λp) and latitude (βp) of the pole orientation were determined as (308°, −52°) and (322°, −40°) via two independent methods. A non-convex model from the SAGE method, which exhibits a concavity feature, is also presented.

The thermal emission of Centaurs and trans-Neptunian objects at millimeter wavelengths from ALMA observations

The sensitivity of ALMA makes it possible to detect thermal mm/submm emission from small and/or distant solar system bodies at the sub-mJy level. While the measured fluxes are primarily sensitive to the objects’ diameters, deriving precise sizes is somewhat hampered by the uncertain effective emissivity at these wavelengths. Following recent work presenting ALMA data for four trans-Neptunian objects (TNOs) with satellites, we report on ALMA 233 GHz (1.29 mm) flux measurements of four Centaurs (2002 GZ32, Bienor, Chiron, Chariklo) and two other TNOs (Huya and Makemake), sampling a range of sizes, albedos, and compositions. These thermal fluxes are combined with previously published fluxes in the mid/far infrared in order to derive their relative emissivity at radio (mm/submm) wavelengths, using the Near Earth Asteroid Standard Model (NEATM) and thermophysical models. We reassess earlier thermal measurements of these and other objects – including Pluto/Charon and Varuna – exploring, in particular, effects due to non-spherical shape and varying apparent pole orientation whenever information is available, and show that these effects can be key for reconciling previous diameter determinations and correctly estimating the spectral emissivities. We also evaluate the possible contribution to thermal fluxes of established (Chariklo) or claimed (Chiron) ring systems. For Chariklo, the rings do not impact the diameter determinations by more than ~5%; for Chiron, invoking a ring system does not help in improving the consistency between the numerous past size measurements. As a general conclusion, all the objects, except Makemake, have radio emissivities significantly lower than unity. Although the emissivity values show diversity, we do not find any significant trend with physical parameters such as diameter, composition, beaming factor, albedo, or color, but we suggest that the emissivity could be correlated with grain size. The mean relative radio emissivity is found to be 0.70 ± 0.13, a value that we recommend for the analysis of further mm/submm data.

Spherical albedo of a Lommel-Seeliger scattering ellipsoidal asteroid

We compute the spherical albedo for a Lommel-Seeliger scattering ellipsoidal asteroid with a realistic disk-integrated phase function. The spherical (or Bond) albedo gives the ratio of the fluxes incident on and scattered by an asteroid. Thus, it plays a key role in the determination of the flux absorbed and afterwards thermally emitted by the asteroid at longer wavelengths. We provide extensive computations for the spherical albedo of low-albedo and moderate-albedo asteroids by utilizing the analytical disk-integrated brightness of a Lommel-Seeliger ellipsoid. In doing so, we utilize realistic triaxial models of known asteroids as well as idealistic prolate or oblate models of substantial elongation or flatness, respectively. We show that the spherical albedos can vary significantly as a function of the rotational pole orientation, rotational phase, and the triaxial ellipsoidal shape: variations of the order of 5-10% are realistic, with a tendency to grow with increasing elongation or flatness of the shape.

Incomplete pole orientation of kinetochores in complex meiotic metaphase I configurations delays metaphase–anaphase transition in Secale

To prevent unbalanced chromosome segregation, meiotic metaphase I – anaphase I transition is carefully regulated by delaying anaphase until all kinetochores are well oriented (anaphase checkpoint) in mammals and insects. In plants this has not yet been established. In heterozygotes of two reciprocal translocations of Secale cereale, with one chromosome replaced by its two telocentric arms, anaphase delay was correlated with the orientation of the kinetochores of the complex of five chromosomes. The terminal kinetochores of the half chromosomes were readily elongated and pole oriented. Chains of five chromosomes with all five kinetochores orienting on alternate poles where the first to start anaphase. Kinetochores of two adjacent chromosomes when oriented on the same pole were partly shielded and less well pole directed. Anaphase was delayed. Cells with this configuration accumulated during anther development. Kinetochores in metacentric chromosomes lacking chiasmata in one arm (in trivalents and bivalents) were slightly better pole oriented and delayed anaphase less. Release of chromatid cohesion as triggered by kinetochore stretch is apparently delayed by inadequate exposition and pole orientation of the kinetochores. It is a mild form of an anaphase checkpoint, in normal material synchronizing bivalent segregation.