arXiv subjects

Jessica Agarwal

Publications and source records attributed to Jessica Agarwal.

At least 37 records · Page 2Linked to original sources

arXiv2021

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

We study the development of activity in the incoming long-period comet C/2017 K2 over the heliocentric distance range 9 < r_H < 16 AU. The comet continues to be characterized by a coma of sub-millimeter and larger particles ejected at low velocity. In a fixed co-moving volume around the nucleus we find that the scattering cross-section of the coma is related to the heliocentric distance by a power law with heliocentric index $s = 1.14\pm0.05$. This dependence is significantly weaker than the inverse square variation of the insolation as a result of two effects. These are, first, the heliocentric dependence of the dust velocity and, second, a lag effect due to very slow-moving particles ejected long before the observations were taken. A Monte Carlo model of the photometry shows that dust production beginning at r_H ~ 35 AU is needed to match the measured heliocentric index, with only a slight dependence on the particle size distribution. Mass loss rates in dust at 10 AU are of order 1000 kg/s, while loss rates in gas may be much smaller, depending on the unknown dust to gas ratio. Consequently, the ratio of the non-gravitational acceleration to the local solar gravity may, depending on the nucleus size, attain values comparable to values found in short-period comets at much smaller distances. Non-gravitational acceleration in C/2017 K2 and similarly distant comets, while presently unmeasured, may limit the accuracy with which we can infer the properties of the Oort cloud from the orbits of long-period comets.

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arXiv2020

Component properties and mutual orbit of binary main-belt comet 288P/(300163) 2006 VW139

The binary asteroid 288P/(300163) is unusual both for its combination of wide-separation and high mass ratio and for its comet-like activity. It is not currently known whether there is a causal connection between the activity and the unusual orbit or if instead the activity helped to overcome a strong detection bias against such sub-arcsecond systems. We aim to find observational constraints discriminating between possible formation scenarios and to characterise the physical properties of the system components. We measured the component separation and brightness using point spread function fitting to high-resolution Hubble Space Telescope/Wide Field Camera 3 images from 25 epochs between 2011 and 2020. We constrained component sizes and shapes from the photometry, and we fitted a Keplerian orbit to the separation as a function of time. Approximating the components A and B as prolate spheroids with semi-axis lengths a$<$b and assuming a geometric albedo of 0.07, we find $a_A \leq$ 0.6 km, $b_A \geq$ 1.4 km, $a_B \leq$ 0.5 km, and $b_B \geq$ 0.8 km. We find indications that the dust production may have concentrated around B and that the mutual orbital period may have changed by 1-2 days during the 2016 perihelion passage. Orbit solutions have semi-major axes in the range of (105-109) km, eccentricities between 0.41 and 0.51, and periods of (117.3-117.5) days pre-perihelion and (118.5-119.5) days post-perihelion, corresponding to system masses in the range of (6.67-7.23) $\times$ 10$^{12}$ kg. The mutual and heliocentric orbit planes are roughly aligned. Based on the orbit alignment, we infer that spin-up of the precursor by the YORP effect led to the formation of the binary system. We disfavour (but cannot exclude) a scenario of very recent formation where activity was directly triggered by the break-up, because our data support a scenario with a single active component.

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arXiv2020

Thermophysical model for icy cometary dust particles

Context. Cometary dust particles are subjected to various forces after being lifted off the nucleus. These forces define the dynamics of dust, trajectories, alignment, and fragmentation, which, in turn, have a significant effect on the particle distribution in the coma. Aims. We develop a numerical thermophysical model that is applicable to icy cometary dust to study the forces attributed to the sublimation of ice. Methods. We extended the recently introduced synoptic model for ice-free dust particles to ice-containing dust. We introduced an additional source term to the energy balance equation accounting for the heat of sublimation and condensation. We use the direct simulation Monte Carlo approach with the dusty gas model to solve the mass balance equation and the energy balance equation simultaneously. Results. The numerical tests show that the proposed method can be applied for dust particles covering the size range from tens of microns to centimeters with a moderate computational cost. We predict that for an assumed ice volume fraction of 0.05, particles with a radius, r >> 1 mm, at 1.35 AU, may disintegrate into mm-sized fragments due to internal pressure build-up. Particles with r < 1 cm lose their ice content within minutes. Hence, we expect that only particles with r > 1cm may demonstrate sustained sublimation and the resulting outgassing forces.

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arXiv2020

Outburst and Splitting of Interstellar Comet 2I/Borisov

We present Hubble Space Telescope observations of a photometric outburst and splitting event in interstellar comet 2I/Borisov. The outburst, first reported with the comet outbound at 2.8 AU (Drahus et al.~2020), was caused by the expulsion of solid particles having a combined cross-section about 100 sq. km and a mass in 0.1 mm sized particles about 2e7 kg. The latter corresponds to 1e-4 of the mass of the nucleus, taken as a sphere of radius 500 m. A transient ``double nucleus'' was observed on UT 2020 March 30 (about three weeks after the outburst), having a cross-section about 0.6 sq. km and corresponding dust mass 1e5 kg. The secondary was absent in images taken on and before March 28, and in images taken on and after April 03. The unexpectedly delayed appearance and rapid disappearance of the secondary are consistent with an origin through rotational bursting of one or more large (meter-sized) boulders under the action of outgassing torques, following their ejection from the main nucleus. Overall, our observations reveal that the outburst and splitting of the nucleus are minor events involving a negligible fraction of the total mass: 2I/Borisov will survive its passage through the planetary region largely unscathed.

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arXiv2020

Coma Anisotropy and the Rotation Pole of Interstellar Comet 2I/Borisov

Hubble Space Telescope observations of interstellar comet 2I/Borisov near perihelion show the ejection of large (>~100 um) particles at <~9 m/s speeds, with estimated mass-loss rates of ~35 kg/s. The total mass loss from comet Borisov corresponds to loss of a surface shell on the nucleus only ~0.4 m thick. This shell is thin enough to be susceptible to past chemical processing in the interstellar medium by cosmic rays, meaning that the ejected materials cannot necessarily be considered as pristine. Our high-resolution images reveal persistent asymmetry in the dust coma, best explained by a thermal lag on the rotating nucleus causing peak mass loss to occur in the comet nucleus afternoon. In this interpretation, the nucleus rotates with an obliquity of 30 deg (pole direction RA = 205 deg and Dec. = 52 deg). The subsolar latitude varied from -35 deg (southern solstice) at the time of discovery to 0 deg (equinox) in 2020 January, suggesting the importance of seasonal effects. Subsequent activity likely results from regions freshly activated as the northern hemisphere is illuminated for the first time.

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arXiv2020

GAUSS -- A Sample Return Mission to Ceres

The goal of Project GAUSS is to return samples from the dwarf planet Ceres. Ceres is the most accessible ocean world candidate and the largest reservoir of water in the inner solar system. It shows active cryovolcanism and hydrothermal activities in recent history that resulted in minerals not found in any other planets to date except for Earth's upper crust. The possible occurrence of recent subsurface ocean on Ceres and the complex geochemistry suggest possible past habitability and even the potential for ongoing habitability. Aiming to answer a broad spectrum of questions about the origin and evolution of Ceres and its potential habitability, GAUSS will return samples from this possible ocean world for the first time. The project will address the following top-level scientific questions: 1) What is the origin of Ceres and the origin and transfer of water and other volatiles in the inner solar system? 2) What are the physical properties and internal structure of Ceres? What do they tell us about the evolutionary and aqueous alteration history of icy dwarf planets? 3) What are the astrobiological implications of Ceres? Was it habitable in the past and is it still today? 4) What are the mineralogical connections between Ceres and our current collections of primitive meteorites? GAUSS will first perform a high-resolution global remote sensing investigation, characterizing the geophysical and geochemical properties of Ceres. Candidate sampling sites will then be identified, and observation campaigns will be run for an in-depth assessment of the candidate sites. Once the sampling site is selected, a lander will be deployed on the surface to collect samples and return them to Earth in cryogenic conditions that preserves the volatile and organic composition as well as the original physical status as much as possible.

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arXiv2020

Hubble Space Telescope Search for Activity in High Perihelion Objects

Solar system objects with perihelia beyond the orbit of Jupiter ($q >$ 5 AU) are too cold for water ice to generate an appreciable coma via sublimation. Despite this, numerous high perihelion objects (HPOs) including many comets and recently escaped Kuiper belt objects (``Centaurs'') are observed to be active out at least to the orbit of Saturn ($q \sim$ 10 AU). Peak equilibrium temperatures at 10 AU ($\sim$125 K), while far too low to sublimate water ice, are sufficient to sublimate super-volatiles such as CO and CO$_2$ ice. Temperatures at 10 AU are also high enough to trigger the rapid crystallization of exposed amorphous ice, thus constituting another possible driver of distant activity. While supervolatile ices can sublimate strongly (as $r_H^{-2}$) to at least Kuiper belt (30 AU) distances, crystallization is an exponential function of temperature that cannot be sustained much beyond $\sim$10 AU. The heliocentric dependence of the activity thus suggests an observational test. If activity in high perihelion objects is triggered by crystallization, then no examples of activity should be found with perihelia $q >>$ 10 AU. If, on the other hand, activity is due to free sublimation of exposed supervolatile ices, or another cause, then distant activity might be detected. We obtained sensitive, high resolution Hubble Space Telescope observations of HPOs to search for activity beyond the crystallization zone. No examples of activity were detected in 53 objects with $q >$ 15 AU, consistent with the crystallization trigger hypothesis. However, sensitivity limits are such that we cannot reject the alternative hypothesis that mass loss is driven by the sublimation of supervolatile ices. We also searched for binary companions in our sample, finding none and setting an empirical 3$σ$ limit to the binary fraction of $<8$\%.

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arXiv2020

Scattering, absorption, and thermal emission by large cometary dust particles: Synoptic numerical solution

Context: Remote light scattering and thermal infrared observations provide clues about the physical properties of cometary and interplanetary dust particles. Identifying these properties will lead to a better understanding of the formation and evolution of the Solar System. Aims: We present a numerical solution for the radiative and conductive heat transport in a random particulate medium enclosed by an arbitrarily shaped surface. The method will be applied to study thermal properties of cometary dust particles. Methods: The recently introduced incoherent Monte Carlo radiative transfer method developed for scattering, absorption, and propagation of electromagnetic waves in dense discrete random media is extended for radiative heat transfer and thermal emission. The solution is coupled with the conductive Fourier transport equation that is solved with the finite-element method. Results: The proposed method allows the synoptic analysis of light scattering and thermal emission by large cometary dust particles consisting of submicrometer-sized grains. In particular, we show that these particles can sustain significant temperature gradients resulting in the superheating factor phase function observed for the coma of comet 67P/Churyumov-Gerasimenko.

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arXiv2020

The Nucleus of Interstellar Comet 2I/Borisov

We present high resolution imaging observations of interstellar comet 2I/Borisov (formerly C/2019 Q4) obtained using the Hubble Space Telescope. Scattering from the comet is dominated by a coma of large particles (characteristic size 0.1 mm) ejected anisotropically. Convolution modeling of the coma surface brightness profile sets a robust limit to the spherical-equivalent nucleus radius r_n < 0.5 km (geometric albedo 0.04 assumed). We obtain an independent constraint based on the non-gravitational acceleration of the nucleus, finding r_n > 0.2 km (nucleus density 500 kg/m3 assumed). The profile and the non-gravitational constraints cannot be simultaneously satisfied if density < 25 kg/m3; the nucleus of comet Borisov cannot be a low density fractal assemblage of the type proposed elsewhere for the nucleus of 1I/'Oumuamua. We show that the spin-up timescale to outgassing torques, even at the measured low production rates, is comparable to or shorter than the residence time in the Sun's water sublimation zone. The spin angular momentum of the nucleus should be changed significantly during the current solar fly-by. Lastly, we find that the differential interstellar size distribution in the 0.5 mm to 100 m size range can be represented by power laws with indices < 4 and that interstellar bodies of 100 m size scale strike Earth every one to two hundred million years.

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arXiv2019

Tomographic inversion of gravity gradient field for a synthetic Itokawa model

This article investigates reconstructing the internal mass density of a numerical asteroid model using the gradient of a simulated gravity field as synthetic measurement data. Our goal is to advance the mathematical inversion methodology and find feasibility constraints for the resolution, noise and orbit selection for future space missions. We base our model on the shape of the asteroid Itokawa as well as on the recent observations and simulation studies which suggest that the internal density varies, increasing towards the center, and that the asteroid may have a detailed structure. We introduce randomized multiresolution scan algorithm which might provide a robust way to cancel out bias and artifact effects related to the measurement noise and numerical discretization. In this scheme, the inverse algorithm can reconstruct details of various sizes without fixing the exact resolution {\em a priori}, and the randomization minimizes the effect of discretization on the solution. We show that the adopted methodology provides an advantageous way to diminish the surface bias of the inverse solution. The results also suggest that a noise level below 80 Eotvos will be sufficient for the detection of internal voids and high density anomalies, if a sparse set of measurements can be obtained from a close-enough distance to the target.

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arXiv2019

Modelling DESTINY+ interplanetary and interstellar dust measurements en route to the active asteroid (3200) Phaethon

The JAXA/ISAS spacecraft DESTINY$^+$ will be launched to the active asteroid (3200) Phaethon in 2022. Among the proposed core payload is the DESTINY+ Dust Analyzer (DDA) which is an upgrade of the Cosmic Dust Analyzer flown on the Cassini spacecraft to Saturn (Srama et al. 2011). We use two up-to-date computer models, the ESA Interplanetary Meteoroid Engineering Model (IMEM, Dikarev et al. 2005), and the interstellar dust module of the Interplanetary Meteoroid environment for EXploration model (IMEX; Sterken2013 et al., Strub et al. 2019) to study the detection conditions and fluences of interplanetary and interstellar dust with DDA. Our results show that a statistically significant number of interplanetary and interstellar dust particles will be detectable with DDA during the 4-years interplanetary cruise of DESTINY+. The particle impact direction and speed can be used to descriminate between interstellar and interplanetary particles and likely also to distinguish between cometary and asteroidal particles.

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arXiv2018

Distant Comet C/2017 K2 and the Cohesion Bottleneck

Distant long-period comet C/2017 K2 has been outside the planetary region of the solar system for 3 Myr, negating the possibility that heat retained from the previous perihelion could be responsible for its activity. This inbound comet is also too cold for water ice to sublimate and too cold for amorphous water ice, if present, to crystallize. C/2017 K2 thus presents an ideal target in which to investigate the mechanisms responsible for activity in distant comets. We have used Hubble Space Telescope to study the comet in the pre-perihelion distance range 13.8 to 15.9 AU. The coma maintains a logarithmic surface brightness gradient $m = -1.010\pm$0.004, consistent with steady-state mass loss. The absence of a radiation pressure swept tail indicates that the effective particle size is large (0.1 mm) and the mass loss rate is $\sim$200 kg s$^{-1}$, remarkable for a comet still beyond the orbit of Saturn. Extrapolation of the photometry indicates that activity began in 2012.1, at 25.9$\pm$0.9 AU, where the blackbody temperature is only 55 K. This large distance and low temperature suggest that cometary activity is driven by the sublimation of a super-volatile ice (e.g.~CO), presumably preserved by K2's long-term residence in the Oort cloud. The mass loss rate can be sustained by CO sublimation from an area $\lesssim 2$ km$^2$, if located near the hot sub-solar point on the nucleus. However, while the drag force from sublimated CO is sufficient to lift millimeter sized particles against the gravity of the cometary nucleus, it is 10$^2$ to 10$^3$ times too small to eject these particles against inter-particle cohesion. Our observations thus require either a new understanding of the physics of inter-particle cohesion or the introduction of another mechanism to drive distant cometary mass loss. We suggest thermal fracture and electrostatic supercharging in this context.

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arXiv2018

Interpretation of the phase functions measured by the OSIRIS instrument for Comet 67P/Churyumov-Gerasimenko

We show that the scattering phase functions of the coma and the nucleus of the comet 67P/Churyumov-Gerasimenko measured by the Rosetta/OSIRIS instrument can be reproduced by a particle model involving clustered densely packed submicrometer-sized grains composed of organic material and larger micrometer-sized silicate grains. The simulated and measured coma phase functions suggest that near the nucleus scattering is dominated by large particles, and the size distribution of dust particles varies with time and/or local coma environment. Further, we show that the measured nucleus phase function is consistent with the coma phase function by modelling a nucleus-sized object consisting of the same particles that explain the coma phase functions.

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arXiv2018

Hubble Space Telescope Observations of 3200 Phaethon At Closest Approach

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$1\degr~of the orbital plane, providing extra sensitivity to low surface brightness caused by scattering from a large-particle trail. We placed an upper limit to the apparent surface brightness of such a trail at 27.2 magnitudes arcsecond$^{-2}$, corresponding to an in-plane optical depth $\le 3\times10^{-9}$. 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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arXiv2018

Models of Rosetta/OSIRIS 67P dust coma phase function

The phase function of the dust coma of comet 67P has been determined from Rosetta/OSIRIS images \citep{Bertini17}. This function show a deep minimum at phase angles near 100$^\circ$, and a strong backscattering enhancement. These two properties cannot be reproduced by regular models of cometary dust, most of them based on wavelength-sized and randomly-oriented aggregate particles. We show, however, that an ensamble of oriented elongated particles of a wide variety of aspect ratios, with radii $r \gtrsim$10 $μ$m, and whose long axes are perpendicular to the direction of the solar radiation, are capable of reproducing the observed phase function. These particles must be absorbing, with an imaginary part of the refractive index of about 0.1 to match the expected geometric albedo, and with porosity in the 60-70\% range.

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arXiv2018

Nucleus of active asteroid 358P/Pan-STARRS (P/2012 T1)

The dust emission from active asteroids is likely driven by collisions, fast rotation, sublimation of embedded ice, and combinations of these. Characterising these processes leads to a better understanding of their respective influence on the evolution of the asteroid population. We study the role of fast rotation in the active asteroid 358P (P 2012/T1). We obtained two nights of deep imaging of 358P with SOAR/Goodman and VLT/FORS2. We derived the rotational light curve from time-resolved photometry and searched for large fragments and debris > 8 mm in a stacked, ultra-deep image. The nucleus has an absolute magnitude of m_R=19.68, corresponding to a diameter of 530 m for standard assumptions on the albedo and phase function of a C-type asteroid. We do not detect fragments or debris that would require fast rotation to reduce surface gravity to facilitate their escape. The 10-hour light curve does not show an unambiguous periodicity.

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arXiv2018

The Nucleus of Active Asteroid 311P/(2013 P5) PANSTARRS

The unique inner-belt asteroid 311P/PANSTARRS (formerly P/2013 P5) is notable for its sporadic, comet-like ejection of dust in nine distinct epochs spread over $\sim$250 days in 2013. This curious behavior has been interpreted as the product of localized, equator-ward landsliding from the surface of an asteroid rotating at the brink of instability. We obtained new Hubble Space Telescope observations to directly measure the nucleus and to search for evidence of its rapid rotation. However, instead of providing photometric evidence for rapid nucleus rotation, our data set a lower limit to the lightcurve period, $P \ge$ 5.4 hour. The dominant feature of the lightcurve is a V-shaped minimum, $\sim$0.3 magnitudes deep, that is suggestive of an eclipsing binary. Under this interpretation, the time-series data are consistent with a secondary/primary mass ratio, $m_s/m_p \sim$ 1:6, a ratio of separation/primary radius, $r/r_p \sim$ 4 and an orbit period $\sim$0.8 days. These properties lie within the range of other asteroid binaries that are thought to be formed by rotational breakup. While the lightcurve period is long, centripetal dust ejection is still possible if one or both components rotates rapidly ($\lesssim$ 2 hour) and has a small lightcurve variation because of azimuthal symmetry. Indeed, radar observations of asteroids in critical rotation reveal "muffin-shaped" morphologies which are closely azimuthally symmetric and which show minimal lightcurves. Our data are consistent with 311P being a close binary in which one or both components rotates near the centripetal limit. The mass loss in 2013 suggests that breakup occurred recently and could even be on-going. A search for fragments that might have been recently ejected beyond the Hill sphere reveals none larger than effective radius $r_e \sim$ 10 m.

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arXiv2017

Evidence for the formation of comet 67P/Churyumov-Gerasimenko through gravitational collapse of a bound clump of pebbles

The processes that led to the formation of the planetary bodies in the Solar System are still not fully understood. Using the results obtained with the comprehensive suite of instruments on-board ESA's Rosetta mission, we present evidence that comet 67P/Churyumov-Gerasimenko likely formed through the gentle gravitational collapse of a bound clump of mm-sized dust aggregates ("pebbles"), intermixed with microscopic ice particles. This formation scenario leads to a cometary make-up that is simultaneously compatible with the global porosity, homogeneity, tensile strength, thermal inertia, vertical temperature profiles, sizes and porosities of emitted dust, and the steep increase in water-vapour production rate with decreasing heliocentric distance, measured by the instruments on-board the Rosetta spacecraft and the Philae lander. Our findings suggest that the pebbles observed to be abundant in protoplanetary discs around young stars provide the building material for comets and other minor bodies.

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