Cometary Science Newsletter
- Issue
- 56
- Month
- November 2019
- Editor
- Michael S. P. Kelley (msk@astro.umd.edu)
Refereed Articles
Abstracts of articles in press or recently published. Limited to 3000 characters.
29P/Schwassmann–Wachmann 1, A Centaur in the Gateway to the Jupiter-family Comets
- University of Central Florida, Florida Space Institute, USA
- Lunar and Planetary Laboratory, The University of Arizona, USA
- Planetary Science Institute, USA
- University of Texas at Austin, Department of Aerospace Engineering and Engineering Mechanics, USA
- California State University San Bernardino, Department of Physics, USA
Jupiter-family comets (JFCs) are the evolutionary products of trans-Neptunian objects (TNOs) that evolve through the giant planet region as Centaurs and into the inner solar system. Through numerical orbital evolution calculations following a large number of TNO test particles that enter the Centaur population, we have identified a short-lived dynamical Gateway, a temporary low-eccentricity region exterior to Jupiter through which the majority of JFCs pass. We apply an observationally based size distribution function to the known Centaur population and obtain an estimated Gateway region population. We then apply an empirical fading law to the rate of incoming JFCs implied by the the Gateway region residence times. Our derived estimates are consistent with observed population numbers for the JFC and Gateway populations. Currently, the most notable occupant of the Gateway region is 29P/Schwassmann–Wachmann 1 (SW1), a highly active, regularly outbursting Centaur. SW1's present-day, very-low-eccentricity orbit was established after a 1975 Jupiter conjunction and will persist until a 2038 Jupiter conjunction doubles its eccentricity and pushes its semimajor axis out to its current aphelion. Subsequent evolution will likely drive SW1's orbit out of the Gateway region, perhaps becoming one of the largest JFCs in recorded history. The JFC Gateway region coincides with a heliocentric distance range where the activity of observed cometary bodies increases significantly. SW1's activity may be typical of the early evolutionary processing experienced by most JFCs. Thus, the Gateway region, and its most notable occupant SW1, are critical to both the dynamical and physical transition between Centaurs and JFCs.
The Astrophysical Journal Letters (Published)
DOI: 10.3847/2041-8213/ab3fb3 NASA ADS: 2019ApJ...883L..25S arXiv: 1908.04185
The contribution of Centaur-emitted dust to the interplanetary dust distribution
- Space Sciences Lab., U.C. Berkeley
Interplanetary dust grains originate from a variety of source bodies, including comets, asteroids, and Edgeworth-Kuiper Belt objects. Centaurs, generally defined as those objects with orbits that cross the outer planets, have occasionally been observed to exhibit cometary-like outgassing at distances beyond Jupiter, implying that they may be an important source of dust grains in the outer Solar system. Here, we use an interplanetary dust grain dynamics model to study the behaviour and equilibrium distribution of Centaur-emitted interplanetary dust grains. We focus on the five Centaurs with the highest current mass-loss rates: 29P/Schwassmann-Wachmann 1, 166P/2001 T4, 174P/Echeclus, C/2001 M10, and P/2004 A1, which together comprise 98 per cent of the current mass-loss from all Centaurs. Our simulations show that Centaur-emitted dust grains with radii, s < 2 micron have median lifetimes consistent with Poynting-Robertson drag lifetimes, while grains with radii, s > 2 micron have median lifetimes much shorter than their P-R drag lifetimes, suggesting that dynamical interactions with the outer planets are effective in scattering larger grains, in analogy to the relatively short lifetimes of Centaurs themselves. Equilibrium density distributions of grains emitted from specific Centaurs show a variety of structure including local maxima in the outer Solar system and azimuthal asymmetries, depending on the orbital elements of the parent Centaur. Finally, we compare the total Centaur interplanetary dust density to dust produced from Edgeworth-Kuiper Belt objects, Jupiter-family comets, and Oort Cloud comets, and conclude that Centaur-emitted dust may be an important component between 5 and 15 au, contributing approximately 25 per cent of the local interplanetary dust density at Saturn.
Monthly Notices of the Royal Astronomical Society (Published)
Comet 66P/du Toit: Not a near-Earth main belt comet
- European Southern Observatory, Alonso de Còrdova 3107, Vitacura, Casilla 19001, Santiago, Chile
- Space sciences, Technologies & Astrophysics Research (STAR) Institute, Université de Liège, 4000 Liège, Belgium
- Oukaimeden Observatory, High Energy Physics and Astrophysics Laboratory, Cadi Ayyad University, Marrakech, Morocco
- EXOTIC Lab, UR Astrobiology, AGO Department, University of Liège, 4000 Liège, Belgium
- Koyama Astronomical Observatory, Kyoto Sangyo University, Motoyama, Japan
- Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ 85719, USA
Main belt comets (MBCs) are a peculiar class of volatile-containing objects with comet-like morphology and asteroid-like orbits. However, MBCs are challenging targets to study remotely due to their small sizes and the relatively large distance they are from the Sun and the Earth. Recently, a number of weakly active short-period comets have been identified that might originate in the asteroid main belt. Among all of the known candidates, comet 66P/du Toit has been suggested to have one of the highest probabilities of coming from the main belt.
The main goal of this study is to investigate the physical properties of 66P via spectroscopic and imaging observations to constrain its formation conditions. In particular, the isotopic abundance ratio and the ortho-to-para ratio (OPR) of gaseous species can be derived via high-resolution spectroscopy, which is sensitive to the formation temperature of the nucleus.
We obtained medium and high-resolution spectra of 66P from 300-2500 nm with the X-shooter and the UVES instruments at the Very Large Telescope in July 2018. We also obtained a series of narrow-band images of 66P to monitor the gas and dust activity between May and July 2018 with TRAPPIST-South. In addition, we applied a dust model to characterize the dust coma of 66P and performed dynamical simulations to study the orbital evolution of 66P.
We derive the OPR of ammonia (NH_3) in 66P to be 1.08±0.06, which corresponds to a nuclear spin temperature of ~34 K. We computed the production rates of OH, NH, CN, C_3, and C_2 radicals and measured the dust proxy, Afρ. The dust analysis reveals that the coma can be best-fit with an anisotropic model and the peak dust production rate is about 55 kg s^{-1} at the perihelion distance of 1.29 au. Dynamical simulations show that 66P is moderately asteroidal with the capture time, t_{cap} ~ 10^4 yr.
Our observations demonstrate that the measured physical properties of 66P are consistent with other typical short-period comets and differ significantly from other MBCs. Therefore, 66P is unlikely to have a main belt origin.
Astronomy & Astrophysics (In press)
DOI: 10.1051/0004-6361/201936469 arXiv: 1909.13331
Extremely low linear polarization of comet C/2018 V1 (Machholz–Fujikawa–Iwamoto)
- Kyung Hee University, Republic of Korea.
- Far Eastern Federal University, Russia.
- Institute of Applied Astronomy of RAS, Russia.
We measured the degree of linear polarization P of comet C/2018 V1 (Machholz-Fujikawa-Iwamoto) with the broadband Johnson V filter in mid-November of 2018. Within a radius of ρ ≈ 17,000 km of the inner coma, we detected an extremely low linear polarization at phase angles α ≈ 83° – 91.2° and constrained the polarization maximum to P_max ≈ (6.8 ± 1.8)%. This is the lowest Pmax ever measured in a comet. Using model agglomerated debris particles, we reproduced the polarimetric response of comet C/2018 V1. Four retrieved refractive indices closely match what was experimentally found in Mg-rich silicates with little or no iron content. Moreover, the size distribution of the agglomerated debris particles appears in good quantitative agreement with the in situ findings of comet 1P/Halley. The dust model of polarization of comet C/2018 V1 suggests a strongly negative polarization with amplitude |P_min| ≈ 5% – 7%; whereas, an interpretation based on gaseous emission requires no negative polarization at small phase angles. This dramatic difference could be used to discriminate gaseous-emission and dust explanations in low-Pmax comets in future.
Icarus (Published)
DOI: 10.1016/j.icarus.2019.113453
Imaging Polarimetry and Photometry of Comet 21P/Giacobini-Zinner
- Far Eastern Federal University, Russia.
- Kyung Hee University, Republic of Korea.
- Astronomical Observatory, Taras Shevchenko National University of Kyiv, Ukraine.
We report results of polarimetric observations of comet 21P/Giacobini-Zinner made at phase angles, α ≈ 76–78°, between 10 and 17 of September, 2018, and compare them with previous measurements. We find significant variations in the polarimetric signals that appear consistent with those reported previously. These variations and subsequent modeling suggest that the particles in the coma are replenished within a period of approximately one day. This period is significantly shorter for highly absorbing carbonaceous particles than for non-absorbing Mg-rich silicate particles. Such a difference in the relative abundances of these components can lead to variations in the polarization response of the coma. The strong positive polarization in the subsolar direction suggests a large relative abundance of carbonaceous material, which may be an indicator of jet-type activity.
Icarus (Published)