arXiv subjects

Man-To Hui

Publications and source records attributed to Man-To Hui.

At least 37 records · Page 2Linked to original sources

arXiv2020

Observations of Disintegrating Long-Period Comet C/2019 Y4 (ATLAS) -- A Sibling of C/1844 Y1 (Great Comet)

We present a study of C/2019 Y4 (ATLAS) using Sloan $gri$ observations from mid-January to early April 2020. During this timespan, the comet brightened with a growth in the effective cross-section of $\left(2.0 \pm 0.1 \right) \times 10^{2}$ m$^{2}$ s$^{-1}$ from the beginning to $\sim$70 d preperihelion in late March 2020, followed by a brightness fade and the comet gradually losing the central condensation. Meanwhile, the comet became progressively bluer, and was even bluer than the Sun ($g - r \approx 0.2$) when the brightness peaked, likely due to activation of subterranean fresh volatiles exposed to sunlight. With the tailward-bias corrected astrometry we found an enormous radial nongravitational parameter, $A_{1} = \left(+2.25 \pm 0.13\right) \times 10^{-7}$ au d$^{-2}$ in the heliocentric motion of the comet. Taking all of these finds into consideration, we conclude that the comet has disintegrated since mid-March 2020. By no means was the split new to the comet, as we quantified that the comet had undergone another split event around last perihelion $\sim$5 kyr ago, during which its sibling C/1844 Y1 (Great Comet) was produced, with the in-plane component of the separation velocity $\gtrsim$1 m s$^{-1}$. We constrained that the nucleus of C/2019 Y4 before disintegration was $\gtrsim$60 m in radius, and has been protractedly ejecting dust grains of $\sim$10-40 $μ$m (assuming dust bulk density 0.5 g cm$^{-3}$) with ejection speed $\sim$30 m s$^{-1}$ in early March 2020 and increased to $\sim$80 m s$^{-1}$ towards the end of the month for grains of $\sim$10 $μ$m.

astro-ph.EP↗

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.

astro-ph.EP↗

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.

astro-ph.EP↗

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.

astro-ph.EP↗

arXiv2019

New Insights into Interstellar Object 1I/2017 U1 (`Oumuamua) from SOHO/STEREO Nondetections

Object 1I/2017 U1 (`Oumuamua) is the first interstellar small body ever discovered in the solar system. By the time of discovery, it had already passed perihelion. To investigate the behavior of `Oumuamua around perihelion, we searched for it in Solar and Heliospheric Observatory (SOHO) and Solar TErrestrial RElations Observatory (STEREO) images from early 2017 September (preperihelion), but did not detect it. The nondetection of `Oumuamua by STEREO renders more stringent constraints on its physical properties thanks to the extreme forward-scattering observing geometry. Assuming geometric albedo $p_V = 0.1$, the effective scattering cross-section of any dust coma was $\lesssim \left(2.1 \pm 0.2 \right) \times 10^{4}$ m$^{2}$. Assuming it behaved like a typical solar-system comet this would correspond to a total mass of $\lesssim 20 \pm 2$ kg, and a water production rate of $\lesssim \left(6.1 \pm 0.5 \right) \times 10^{25}$ s$^{-1}$ at heliocentric distance $r_{\rm H} = 0.375$ au. If scaled to post-discovery $r_{\rm H}$, the water production rate would be smaller than any of the previously reported upper limits by at least an order of magnitude. To exhibit the reported nongravitational motion with our default assumptions requires a nucleus bulk density $\lesssim$40 kg m$^{-3}$; higher bulk densities are possible for other assumptions. Alternatively, we show that thermal fracturing could have plausibly removed an inert surface layer between these observations and discovery, thus initiating activity after `Oumuamua left the field of view of STEREO.

astro-ph.EP↗

arXiv2019

Asteroid (3200) Phaethon: colors, phase curve, limits on cometary activity and fragmentation

We report on a multi-observatory campaign to examine asteroid 3200 Phaethon during its December 2017 close approach to Earth, in order to improve our measurements of its fundamental parameters, and to search for surface variations, cometary activity and fragmentation. The mean colors of Phaethon are B-V = 0.702 +/- 0.004, V-R = 0.309 +/- 0.003, R-I = 0.266 +/- 0.004, neutral to slightly blue, consistent with previous classifications of Phaethon as a F-type or B-type asteroid. Variations in Phaethon's B-V colors (but not V-R or R-I) with observer sub-latitude are seen and may be associated with craters observed by the Arecibo radar. High cadence photometry over phases from 20 to 100 degrees allows a fit to the values of the HG photometric parameters; H = 14.57 +/- 0.02, 13.63 +/- 0.02, 13.28 +/- 0.02, 13.07 +/- 0.02; G = 0.00 +/- 0.01, -0.09 +/- 0.01, -0.10 +/- 0.01, -0.08 +/- 0.01 in the BVRI filters respectively; the negative G values are consistent with other observations of F type asteroids. Light curve variations were seen that are also consistent with concavities reported by Arecibo, indicative of large craters on Phaethon's surface whose ejecta may be the source of the Geminid meteoroid stream. A search for gas/dust production set an upper limit of 0.06 +/- 0.02 kg/s when Phaethon was 1.449 AU from the Sun, and 0.2 +/- 0.1 kg/s at 1.067 AU. A search for meter-class fragments accompanying Phaethon did not find any whose on-sky motion was not also consistent with background main belt asteroids.

astro-ph.EP↗

arXiv2019

New Active Asteroid (6478) Gault

Main-belt asteroid (6478) Gault was observed to show cometary features in early 2019. To investigate the cause, we conducted {\it BVR} observations at Xingming Observatory, China, from 2019 January to April. The two tails were formed around 2018 October 26--November 08, and 2018 December 29--2019 January 08, respectively, and consisted of dust grains of $\gtrsim$20 $μ$m to 3 mm in radius ejected at a speed of $0.15 \pm 0.05$ m s$^{-1}$ and following a broken power-law size distribution bending at grain radius $\sim$70 $μ$m (bulk density 1 g cm$^{-3}$ assumed). The total mass of dust within a $10^4$ km-radius aperture around Gault declined from $\sim$$9 \times 10^6$ kg since 2019 January at a rate of $2.28 \pm 0.07$ kg s$^{-1}$, but temporarily surged around 2019 March 25, because Earth thence crossed the orbital plane of Gault, within which the ejected dust was mainly distributed. No statistically significant colour or short-term lightcurve variation was seen. Nonetheless we argue that Gault is currently subjected to rotational instability. Using the available astrometry, we did not detect any nongravitational acceleration in the orbital motion of Gault.

astro-ph.EP↗

arXiv2019

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

We present a photometric and dynamical study of comet C/2010 U3 (Boattini), which was seen active in prediscovery data as early as 2005 November at a new inbound record heliocentric distance $r_{\rm H} = 25.8$ au. Two outburst events around 2009 and 2017 were observed. The coma and tail of the comet consist of dust grains of $\sim$10 $μ$m in radius, ejected protractedly at speeds $\lesssim$50 m s$^{-1}$ near the subsolar point, and are subjected to the Lorentz force, solar gravitation and radiation pressure force altogether. The prolonged activity indicates that sublimation of supervolatiles (e.g., CO, CO$_2$) is at play, causing a net mass-loss rate $\gtrsim$1 kg s$^{-1}$. To sustain the mass loss, the nucleus radius has to be $\gtrsim$0.1 km. The color of the cometary dust, similar to other long-period comets, is redder than the solar colors, but we also observed potential color variations when the comet was at $10 < r_{\rm H} < 15$ au, concurrent with the onset of crystallisation of amorphous water ice, if at all. Using publicly available and our refined astrometric measurements, we estimated the precise trajectory of the comet and propagated it backward to its previous perihelion. We found that the comet has visited the planetary region $1.96 \pm 0.04$ Myr ago, with barycentric perihelion distance $q = 8.364 \pm 0.004$ 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.

astro-ph.EP↗

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.

astro-ph.EP↗

arXiv2018

In Search of Recent Disruption of (3200) Phaethon: Model Implication and Hubble Space Telescope Search

Near-Earth asteroid (3200) Phaethon is notable for its association to a strong annual meteor shower, the Geminids, indicative of one or more episodes of mass ejection in the past. The mechanism of Phaethon's past activity is not yet understood. Here we present a Hubble Space Telescope (HST) search of meter-sized fragments in the vicinity of Phaethon, carried out during Phaethon's historic approach to the Earth in mid-December of 2017. Numerical simulation conducted to guide HST's pointing also show that the dynamical evolution of Phaethon-originated particles is quick, as ejected materials take no longer than $\sim250$ yr to spread to the entire orbit of Phaethon. Our search was completed down to 4-meter-class limit (assuming Phaethon-like albedo) and was expected to detect 0.035% particles ejected by Phaethon in the last several decades. The negative result of our search capped the total mass loss of Phaethon over the past few dozen orbits to be $10^{12}$ kg at $3σ$ level, taking the best estimates of size power-law from meteor observations and spacecraft data. Our result also implies a millimeter-sized dust flux of $<10^{-12} \mathrm{m^{-2} s^{-1}}$ within 0.1 au of Phaethon, suggesting that any Phaethon-bound mission is unlikely to encounter dense dust clouds.

astro-ph.EP↗

arXiv2018

Is the Cometary Nucleus Extraction Technique Reliable?

It depends. Our experiment reveals that, given an optically thin coma, generally, the smaller the signal ratio of nucleus to coma, the less reliable is the cometary nucleus-extraction technique. We strongly suggest the technique only be applied to cases where the nucleus signal occupies $\gtrsim$10% of the total signal wherein the bias is no more than a few percent. Otherwise there is probably no way to debias results from this technique in reality, since its reliability is highly sensitive to entangling complications, including the coma profile, and the point-spread function (PSF).

astro-ph.EP↗

arXiv2018

Two Hyperbolic Baldheads in the Solar System: 2017 U7 and 2018 C2

We present a study of two newly discovered heliocentric hyperbolic objects -- 2017 U7 and 2018 C2. Both are possibly thermally evolved comets. Observations of the latter in 2018 March from Xingming Observatory revealed that it has a color similar to those of the long-period comets, Trojans, and D-type asteroids: $m_B - m_V = 0.75 \pm 0.03$, $m_V - m_R = 0.41 \pm 0.02$, and $m_R - m_I = 0.37 \pm 0.03$. A possible extremely faint coma of $\sim$9$"$ across was observed. The radial profile of 2018 C2 in comparison with those of the field stars helps confirm its cometary nature. Based on our convolution model, its mass-loss rate is estimated to be $0.7 \pm 0.2$ kg s$^{-1}$. Assuming geometric albedo $p_R = 0.04$, its effective radius is $4.4 \pm 0.5$ km, which means that the fraction of active area is merely $\sim$10$^{-5}$-10$^{-4}$. Our N-body dynamical simulations show that both objects are most likely dynamically old members from the Oort cloud. 2017 U7 has a $\sim$60% chance to escape the solar system in the 1.5 Myr following its current perihelion passage due to a moderate close encounter with Jupiter in 2020 May. On the condition that no disintegration occurs, 2018 C2 will revisit the inner solar system 0.13-0.14 Myr later, with perihelion distance $\sim$2 AU.

astro-ph.EP↗

arXiv2018

Finding Long Lost Lexell's Comet: The Fate of the First Discovered Near-Earth Object

Jupiter-family Comet D/1770 L1 (Lexell) was the first discovered Near-Earth Object (NEO), and passed the Earth on 1770 Jul 1 at a recorded distance of 0.015 au. The comet was subsequently lost due to unfavorable observing circumstances during its next apparition followed by a close encounter with Jupiter in 1779. Since then, the fate of D/Lexell has attracted interest from the scientific community, and now we revisit this long-standing question. We investigate the dynamical evolution of D/Lexell based on a set of orbits recalculated using the observations made by Charles Messier, the comet's discoverer, and find that there is a $98\%$ chance that D/Lexell remains in the Solar System by the year of 2000. This finding remains valid even if a moderate non-gravitational effect is imposed. Messier's observations also suggest that the comet is one of the largest known near-Earth comets, with a nucleus of $\gtrsim 10$ km in diameter. This implies that the comet should have been detected by contemporary NEO surveys regardless of its activity level if it has remained in the inner Solar System. We identify asteroid 2010 JL$_{33}$ as a possible descendant of D/Lexell, with a $0.8\%$ probability of chance alignment, but a direct orbital linkage of the two bodies has not been successfully accomplished. We also use the recalculated orbit to investigate the meteors potentially originating from D/Lexell. While no associated meteors have been unambiguously detected, we show that meteor observations can be used to better constrain the orbit of D/Lexell despite the comet being long lost.

astro-ph.EP↗

arXiv2017

Prediscovery Observations and Orbit of Comet C/2017 K2 (PANSTARRS)

We present a study of comet C/2017 K2 (PANSTARRS) using prediscovery archival data taken from 2013 to 2017. Our measurements show that the comet has been marginally increasing in activity since at least 2013 May (heliocentric distance of $r_{\mathrm{H}} = 23.7$ AU pre-perihelion). We estimate the mass-loss rate during the period 2013--2017 as $\overline{\dot{M}} \approx \left(2.4 \pm 1.1 \right) \times 10^{2}$ kg s$^{-1}$, which requires a minimum active surface area of $\sim$10--10$^2$ km$^{2}$ for sublimation of supervolatiles such as CO and CO$_2$, by assuming a nominal cometary albedo $p_V = 0.04 \pm 0.02$. The corresponding lower limit to the nucleus radius is a few kilometers. Our Monte Carlo dust simulations show that dust grains in the coma are $\gtrsim0.5$ mm in radius, with ejection speeds from $\sim$1--3 m s$^{-1}$, and have been emitted in a protracted manner since 2013, confirming estimates by Jewitt et al. (2017). The current heliocentric orbit is hyperbolic. Our N-body backward dynamical integration of the orbit suggests that the comet is most likely (with a probability of $\sim$98\%) from the Oort spike. The calculated median reciprocal of the semimajor axis 1 Myr ago was $a_{\mathrm{med}}^{-1} = \left( 3.61 \pm 1.71 \right) \times 10^{-5}$ AU$^{-1}$ (in a reference system of the solar-system barycentre).

astro-ph.EP↗

arXiv2017

A Comet Active Beyond the Crystallization Zone

We present observations showing in-bound long-period comet C/2017 K2 (PANSTARRS) to be active at record heliocentric distance. Nucleus temperatures are too low (60 K to 70 K) either for water ice to sublimate or for amorphous ice to crystallize, requiring another source for the observed activity. Using the Hubble Space Telescope we find a sharply-bounded, circularly symmetric dust coma 10$^5$ km in radius, with a total scattering cross section of $\sim$10$^5$ km$^2$. The coma has a logarithmic surface brightness gradient -1 over much of its surface, indicating sustained, steady-state dust production. A lack of clear evidence for the action of solar radiation pressure suggests that the dust particles are large, with a mean size $\gtrsim$ 0.1 mm. Using a coma convolution model, we find a limit to the apparent magnitude of the nucleus $V >$ 25.2 (absolute magnitude $H >$ 12.9). With assumed geometric albedo $p_V$ = 0.04, the limit to the nucleus circular equivalent radius is $<$ 9 km. Pre-discovery observations from 2013 show that the comet was also active at 23.7 AU heliocentric distance. While neither water ice sublimation nor exothermic crystallization can account for the observed distant activity, the measured properties are consistent with activity driven by sublimating supervolatile ices such as CO$_2$, CO, O$_2$ and N$_2$. Survival of supervolatiles at the nucleus surface is likely a result of the comet's recent arrival from the frigid Oort cloud.

astro-ph.EP↗

arXiv2017

The Main Belt Comets and Ice in the Solar System

We review the evidence for buried ice in the asteroid belt; specifically the questions around the so-called Main Belt Comets (MBCs). We summarise the evidence for water throughout the Solar System, and describe the various methods for detecting it, including remote sensing from ultraviolet to radio wavelengths. We review progress in the first decade of study of MBCs, including observations, modelling of ice survival, and discussion on their origins. We then look at which methods will likely be most effective for further progress, including the key challenge of direct detection of (escaping) water in these bodies.

astro-ph.EP↗

arXiv2017

Split Active Asteroid P/2016 J1 (PANSTARRS)

We present a photometric and astrometric study of the split active asteroid P/2016 J1 (PANSTARRS). The two components (hereafter J1-A and J1-B) separated either $\sim$1500 days (2012 May to June) or 2300 days (2010 April) prior to the current epoch, with a separation speed $V_{\mathrm{sep}} = 0.70 \pm 0.02$ m s$^{-1}$ for the former scenario, or $0.83 \pm 0.06$ m s$^{-1}$ for the latter. Keck photometry reveals that the two fragments have similar, Sun-like colors which are comparable to the colors of primitive C- and G-type asteroids. With a nominal comet-like albedo, $p_{R} = 0.04$, the effective, dust-contaminated cross sections are estimated to be 2.4 km$^{2}$ for J1-A, and 0.5 km$^{2}$ for J1-B. We estimate that the nucleus radii lie in the range $140 \lesssim R_{\mathrm{N}} \lesssim 900$ m for J1-A and $40 \lesssim R_{\mathrm{N}} \lesssim 400$ m, for J1-B. A syndyne-synchrone simulation shows that both components have been active for 3 to 6 months, by ejecting dust grains at speeds $\sim$0.5 m s$^{-1}$ with rates $\sim$1 kg s$^{-1}$ for J1-A and 0.1 kg s$^{-1}$ for J1-B. In its present orbit, the rotational spin-up and devolatilization times of 2016 J1 are very small compared to the age of the solar system, raising the question of why this object still exists. We suggest that ice that was formerly buried within this asteroid became exposed at the surface, perhaps via a small impact, and that sublimation torques then rapidly drove it to break-up. Further disintegration events are anticipated due to the rotational instability.

astro-ph.EP↗

arXiv2016

Non-Gravitational Acceleration of the Active Asteroids

Comets can exhibit non-gravitational accelerations caused by recoil forces due to anisotropic mass loss. So might active asteroids. We present an astrometric investigation of 18 active asteroids in search of non-gravitational acceleration. Statistically significant (signal-to-noise ratio (SNR) $> 3$) detections are obtained in three objects: 313P/Gibbs, 324P/La Sagra and (3200) Phaethon. The strongest and most convincing detection ($>$7$σ$ in each of three orthogonal components of the acceleration), is for the $\sim$1 km diameter nucleus of 324P/La Sagra. A 4.5$σ$ detection of the transverse component of the acceleration of 313P/Gibbs (also $\sim$1 km in diameter) is likely genuine too, as evidenced by the stability of the solution to the rejection or inclusion of specific astrometric datasets. We also find a 3.4$σ$ radial-component detection for $\sim$5 km diameter (3200) Phaethon, but this detection is more sensitive to the inclusion of specific datasets, suggesting that it is likely spurious in origin. The other 15 active asteroids in our sample all show non-gravitational accelerations consistent with zero. We explore different physical mechanisms which may give rise to the observed non-gravitational effects, and estimate mass-loss rates from the non-gravitational accelerations. We present a revised momentum-transfer law based on a physically realistic sublimation model for future work on non-gravitational forces, but note that it has little effect on the derived orbital elements.

astro-ph.EP↗