Cometary Science Newsletter
- June 2021
- Michael S. P. Kelley (firstname.lastname@example.org)
Abstracts of articles in press or recently published. Limited to 3000 characters.
Six Outbursts of Comet 46P/Wirtanen
- University of Maryland
- Planetary Science Institute
- Auburn University
- University of Edinburgh, Royal Observatory, Edinburgh
- University of Washington
- University of Minnesota
- Department of Astronomy, California Institute of Technology
- Division of Physics, Mathematics, and Astronomy, California Institute of Technology
- Texas Tech University
- IPAC, California Institute of Technology
- Caltech Optical Observatories, California Institute of Technology
- University of St Andrews
- Niels Bohr Institute
- The Open University
- Universitá di Salerno
- Istituto Nazionale di Fisica Nucleare, Napoli, Italy
- Universität Hamburg
- University of Dundee
- Max Planck Institute for Astronomy
- Nagoya University
- Kyoto University
- Nicolaus Copernicus University
- Chungnam National University
- Universität Heidelberg
- Sharif University of Technology
- Universidad de Antofagasta
- Universidad Católica de la Santísima Concepción
- Isfahan University of Technology
- Keele University
- Universidad de Atacama
Cometary activity is a manifestation of sublimation-driven processes at the surface of nuclei. However, cometary outbursts may arise from other processes that are not necessarily driven by volatiles. In order to fully understand nuclear surfaces and their evolution, we must identify the causes of cometary outbursts. In that context, we present a study of mini-outbursts of comet 46P/Wirtanen. Six events are found in our long-term lightcurve of the comet around its perihelion passage in 2018. The apparent strengths range from −0.2 to −1.6 mag in a 5" radius aperture, and correspond to dust masses between ∼104 to 106 kg, but with large uncertainties due to the unknown grain size distributions. However, the nominal mass estimates are the same order of magnitude as the mini-outbursts at comet 9P/Tempel 1 and 67P/Churyumov-Gerasimenko, events which were notably lacking at comet 103P/Hartley 2. We compare the frequency of outbursts at the four comets, and suggest that the surface of 46P has large-scale (∼10–100 m) roughness that is intermediate to that of 67P and 103P, if not similar to the latter. The strength of the outbursts appear to be correlated with time since the last event, but a physical interpretation with respect to solar insolation is lacking. We also examine Hubble Space Telescope images taken about 2 days following a near-perihelion outburst. No evidence for macroscopic ejecta was found in the image, with a limiting radius of about 2-m.
The Planetary Science Journal (In press)
All Comets are Somewhat Hyperactive and the Implications Thereof
- University of Maryland, College Park, MD, USA.
We critically examine what hyperactivity on a comet entails, fully develop the A’Hearn Model for Hyperactivity based on the analyses of data collected for the Deep Impact encounter of comet 103P/Hartley 2, describe manifestations of hyperactivity suggested on many, if not all, comets, and give implications of hyperactivity for future cometary exploration. The A’Hearn model requires a highly volatile ice reservoir within a comet to undergo sublimation, escape the nucleus, and drive out less volatile ices along its path to the surface. Once in the coma, the less volatile ice eventually sublimates, creating a secondary source of that gas in the coma, which is generally displaced anti-sunward and not distributed symmetrically about the nucleus. The secondary source of gas increases the total production of the less volatile species in the coma, sometimes well above that expected if the total surface was undergoing sublimation. We argue that based on the simple assumptions of the A’Hearn model and the fact that several comets display one or more of the characteristics of hyperactivity detailed here, it is probable that nearly all comets experience some degree of hyperactivity. Of significance, the ice that is brought from deep within the nucleus into the coma via the process described by the A’Hearn model is the least thermally altered and is thus the most pristine ice in the comet. Therefore, it behooves future mission teams to consider cryogenically sampling coma ice, rather than or in addition to attempting a direct nucleus sample, for a better understanding of the unaltered ices and conditions present in the protoplanetary disk.
The Planetary Science Journal (Published)
DOI: 10.3847/PSJ/abf11f NASA ADS: 2021PSJ.....2...92S
Discovery of Carbon Monoxide in Distant Comet C/2017 K2 (PANSTARRS)
- European Southern Observatory, Chile
- Department of Earth, Planetary and Space Sciences, UCLA, USA
- Department of Physics and Astronomy, UCLA, USA
- Purple Mountain Observatory, Chinese Academy of Sciences, China
- East Asian Observatory, USA
- Department of Astronomy, University of Maryland, USA
- Institute of Astronomy and Astrophysics, Academia Sinica, Taiwan
Optical observations of the Oort cloud comet C/2017 K2 (PANSTARRS) show that its activity began at large heliocentric distances (up to 35 au), which cannot be explained by either the sublimation or the crystallization of water ice. Supervolatile sublimation, most likely of carbon monoxide (CO), has been proposed as a plausible driver of the observed mass loss. Here, we present the detection of the J = 2-1 rotational transition in outgassed CO from C/2017 K2 when at heliocentric distance rH = 6.72 au, using the James Clerk Maxwell Telescope. The CO line is blue-shifted by 0.20±0.03 km s^-1 with an area and width of 8.3±2.3 mK km s^-1 and 0.28±0.08 km s^-1, respectively. The CO production rate is Q(CO) = (1.6±0.5)×10^27 s^-1. These are the first observations of a gaseous species in C/2017 K2 and provide observational confirmation of the role of supervolatile sublimation in this comet.
The Astrophysical Journal Letters (In press)
Monitoring the negative polarization in Comet 29P/Schwassmann–Wachmann during quiescence
- Far Eastern Federal University, Russia
- Institute of Applied Astronomy of RAS, Russia
- Humanitas College, Kyung Hee University, South Korea
- Planetary Exploration Research Center, Chiba Institute of Technology, Japan
- Space Science Institute, USA
- Department of Astronomy and Space Science, Kyung Hee University, South Korea.
During five nights between February 3 and 10, 2021, we monitored the degree of linear polarization in Comet 29P/Schwassmann–Wachmann with the broadband R filter. It is the first polarimetric investigation of this comet measured during a period of quiescent activity. We found a nearly constant negative polarization P_Q ≈ –2.1% at α ≈ 9.5° that is somewhat stronger than in other comets on average; however, it appears similar to that of Comet 67P/Churyumov–Gerasimenko. Modeling the polarization in Comet 29P/Schwassmann–Wachmann suggests its coma consists of at least two components, whose chemical composition and size distribution appear similar to that of other comets. Further polarimetric measurements are necessary in order to infer the shape of its branch of negative polarization and, hence, better constrain the microphysical properties of its dust.
DOI: 10.1016/j.icarus.2021.114536 NASA ADS: 2021Icar..36614536K
Observations of distant comet C/2011 KP36 (Spacewatch): Photometry, spectroscopy, and polarimetry
- Astronomical Institute of the Slovak Academy of Sciences, Slovak Republic
- Main Astronomical Observatory of the National Academy of Sciences, Ukraine
- Astronomical Observatory of Taras Shevchenko National University of Kyiv, Ukraine
- University of Maryland, College Park, Maryland, USA
- Crimean Astrophysical Observatory, Nauchnij, Crimea
- Special Astrophysical Observatory of the Russian Academy of Science, Russia
The main objective of our study is to obtain new observational results for the active long-period comet C/2011 KP36 (Spacewatch). This comet has unusual orbital properties and moves at distances larger than 5 au from the Sun.
We carried out extensive observations of comet C/2011 KP36 (Spacewatch) at the 6m BTA telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences with the focal reducer SCORPIO-2. We obtained quasi-simultaneous long-slit spectra in the visible, as well as photometric and linear polarimetric images with the g-sdss and r-sdss filters on November 25 2016 when the heliocentric and geocentric distances of the comet were 5.06 au and 4.47 au, respectively. We modeled the behavior of the color and polarization in the coma, considering the dust as an ensemble of polydisperse nonspherical particles.
Two strong jet-like structures in solar and antisolar directions and two short and narrow jet features in the perpendicular direction were revealed in the coma. Our simulations showed that the latter two jets originated from the same active area. We determined the orientation of the rotation axis of the nucleus and the position of three active areas. High activity of the comet was characterized by Afrho values of 1065 +/- 11 cm in the g-sdss filter and 1264 +/- 17 cm in the r-sdss filter. The comet was found to be rich in CO+, while there was no clear detection of CN, C3, C2, and N2+. The dust color g-r varies over the coma from about 0.2^m to 0.7^m, and the linear polarization degree from about -1.0% to -6% at the phase angle 9.6 deg. The color of the nucleus of comet C/2011 KP36 (Spacewatch) is ultrared, B-R = 1.9^m +/- 0.3^m.
The high variability of the observed characteristics over the coma of comet C/2011 KP36 (Spacewatch) indicates significant and variable activity of the nucleus with, probably, numerous small active areas. Together with the three identified large active areas, they are characterized by different combinations of water ice, CO2 ice, and refractory dust and sizes of their particles, which are in the micron-size range.
Astronomy & Astrophysics (Published)
An Update of the Correlation between Polarimetric and Thermal Properties of Cometary Dust
- Technical University of Braunschweig, Germany
- University of Maryland, USA
- Max Planck Institute for Solar System Research, Germany
We present a possible correlation between the properties of scattered and thermal radiation from dust and the principal dust characteristics responsible for this relationship, and therefrom to gain insights into comet evolution. To this end, we use the NASA/PDS archival polarimetric data on cometary dust in the Red (0.62-0.73 μm) and K (2.00-2.39 μm) domains to leverage the relative excess of the polarisation degree of a comet to the average trend at the given phase angle (P_excess) as a metric of the dust’s scattered light characteristics. The flux excess of silicate emissions to the continuum around 10 μm (F_Si/F_cont) is adopted from previous studies as a metric of the dust’s MIR feature. The two metrics show a positive correlation when P_excess is measured in the K domain (Spearman’s rank correlation coefficient ρ = 0.71+0.10-0.19). No significant correlation was identified in the Red domain (ρ = 0.13+0.16-0.15). The gas-rich comets have systematically weaker F_Si/F_cont than the dust-rich ones, yet both groups retain the same overall tendency with different slope values. The observed positive correlation between the two metrics indicates that composition is a peripheral factor in characterising the dust’s polarimetric and silicate emission properties. The systematic difference in F_Si/F_cont for gas-rich versus dust-rich comets would rather correspond with the difference in their dust size distribution. Hence, our results suggest that the current MIR spectral models of cometary dust, which search for a minimum χ2 fit by considering various dust properties simultaneously, should prioritise the dust size and porosity over the composition. With light scattering being sensitive to different size scales in two wave-bands, we expect the K-domain polarimetry to be sensitive to the properties of dust aggregates, such as size and porosity, which might have been influenced by evolutionary processes. On the other hand, the Red-domain polarimetry reflects the characteristics of sub-μm constituents in the aggregate.
Astronomy & Astrophysics (In press)