arXiv subjects

Man-To Hui

Publications and source records attributed to Man-To Hui.

46 records · Page 3Linked to original sources

arXiv2016

Resurrection of (3200) Phaethon in 2016

We present a study of the active asteroid (3200) Phaethon in the 2016 apparition using the STEREO spacecraft and compare the results with data from the previous two perihelia in 2009 and 2012. Once again, Phaethon brightened by $\sim$2 mag soon after its perihelion passage, contradicting expectations from the phase function of a macroscopic monolithic body. Subsequently, a short antisolar tail of $\sim$0°.1 in length was formed within $\sim$1 day and quickly disappeared. No trail was seen. Our syndyne-synchrone analysis indicates that the tail was comprised of submicron to micron particles and can be approximated by a synchrone coinciding with the outburst. We estimate that the outburst has released a mass of $\sim$10$^{4}$--10$^{5}$ kg, comparable to the two mass ejections in 2009 and 2012, and that the average mass-loss rate is $\sim$0.1--1 kg s$^{-1}$. The forward-scattering effect hinted at low level activity of Phaethon prior to the outburst, which increased the effective cross section by merely $\lesssim$1 km$^2$. Without the forward-scattering enhancement, detecting such activity at side-scattering phase angles is very difficult. The forward-scattering effect also reinforces that the ejected dust grains rather than gas emissions were responsible for the activity of Phaethon. Despite Phaethon's reactivation, it is highly unlikely that the Geminid meteoroid stream can be sustained by similar perihelion mass-loss events.

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arXiv2016

Constraints on Comet 332P/Ikeya-Murakami

Encke-type comet 332P/Ikeya-Murakami is experiencing cascading fragmentation events during its 2016 apparition. It is likely the first splitting Encke-type comet ever being observed. A nongravitational solution to the astrometry reveals a statistical detection of the radial and transverse nongravitational parameters, $A_{1} = \left(1.54 \pm 0.39\right) \times 10^{-8}$ AU day$^{-2}$, and $A_{2} = \left(7.19 \pm 1.92\right) \times 10^{-9}$ AU day$^{-2}$, respectively, which implies a nucleus erosion rate of $\left(0.91 \pm 0.17\right)$% per orbital revolution. The mass-loss rate likely has to be supported by a much larger fraction of an active surface area than known cases of short-period comets; it may be relevant to the ongoing fragmentation. We failed to detect any serendipitous pre-discovery observations of the comet in archival data from major sky surveys, whereby we infer that 332P used to be largely inactive, and is perhaps among the few short-period comets which have been reactivated from weakly active or dormant states. We therefore constrain an upper limit to the nucleus size as $2.0 \pm 0.2$ km in radius. A search for small bodies in similar orbits to that of 332P reveals comet P/2010 B2 (WISE) as the best candidate. From an empirical generalised Jupiter-family (Encke-type included) comet population model, we estimate the likelihood of chance alignment of the 332P--P/2010 B2 pair to be 1 in 33, a small number indicative of a genetic linkage between the two comets on a statistical basis. The pair possibly originated from a common progenitor which underwent a disintegration event well before the twentieth century.

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arXiv2016

Fragmentation Kinematics in Comet 332P/Ikeya-Murakami

We present initial time-resolved observations of the split comet 332P/Ikeya-Murakami taken using the Hubble Space Telescope. Our images reveal a dust-bathed cluster of fragments receding from their parent nucleus at projected speeds in the range 0.06 to 3.5 m s$^{-1}$ from which we estimate ejection times from October to December 2015. The number of fragments with effective radii $\gtrsim$20 m follows a differential power law with index $γ$ = -3.6$\pm$0.6, while smaller fragments are less abundant than expected from an extrapolation of this power-law. We argue that, in addition to losses due to observational selection, torques from anisotropic outgassing are capable of destroying the small fragments by driving them quickly to rotational instability. Specifically, the spin-up times of fragments $\lesssim$20 m in radius are shorter than the time elapsed since ejection from the parent nucleus. The effective radius of the parent nucleus is $r_e \le$ 275 m (geometric albedo 0.04 assumed). This is about seven times smaller than previous estimates and results in a nucleus mass at least 300 times smaller than previously thought. The mass in solid pieces, $2\times10^9$ kg, is about 4% of the mass of the parent nucleus. As a result of its small size, the parent nucleus also has a short spin-up time. Brightness variations in time-resolved nucleus photometry are consistent with rotational instability playing a role in the release of fragments.

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arXiv2016

The catastrophic fragmentation of Comet 332P (Ikeya-Murakami)

We describe 2016 January to April observations of the fragments of 332P/Ikeya-Murakami, a comet earlier observed in a 2010 October outburst (Ishiguro et al 2014). We present photometry of the fragments, and perform simulations to infer the time of breakup. We argue that the eastern-most rapidly brightening fragment ($F4$) best corresponds to the original nucleus, rather than the initial bright fragment $F1$. We compute radial and tangential non-gravitational parameters, $A_1 = (1.5 \pm 0.4) \times 10^{-8}$ AU day$^{-2}$ and $(7.2 \pm 1.9) \times 10^{-9}$ AU day$^{-2}$; both are consistent with zero at the $4σ$ level. Monte Carlo simulations indicate that the fragments were emitted on the outbound journey well after the 2010 outburst, with bright fragment $F1$ splitting in mid--2013 and the fainter fragments within months of the 2016 January recovery. Western fragment $F7$ is the oldest, dating from 2011. We suggest that the delayed onset of the splitting is consistent with a self-propagating crystallization of water ice.

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arXiv2015

Bangs and Meteors from the Quiet Comet 15P/Finlay

Jupiter-family comet 15P/Finlay has been reportedly quiet in activity for over a century but has harbored two outbursts during its 2014/2015 perihelion passage. Here we present an analysis of these two outbursts using a set of cometary observations. The outbursts took place between 2014 Dec. 15.4--16.0 UT and 2015 Jan. 15.5--16.0 UT as constrained by ground-based and spacecraft observations. We find a characteristic ejection speed of $V_0=300$ to $650 \mathrm{m \cdot s^{-1}}$ for the ejecta of the first outburst and $V_0=550$ to $750 \mathrm{m \cdot s^{-1}}$ for that of the second outburst using a Monte Carlo dust model. The mass of the ejecta is calculated to be $M_\mathrm{d}=2$ to $3\times10^5 \mathrm{kg}$ for the first outburst and $M_\mathrm{d}=4$ to $5\times10^5 \mathrm{kg}$ for the second outburst, corresponds to less than $10^{-7}$ of the nucleus mass. The specific energy of the two outbursts is found to be $0.3$ to $2\times10^5 \mathrm{J \cdot kg^{-1}}$. We also revisit the long-standing puzzle of the non-detection of the hypothetical Finlayid meteor shower by performing a cued search using the 13-year data from the Canadian Meteor Orbit Radar, which does not reveal any positives. The Earth will pass the 2014/2015 outburst ejecta around 2021 Oct. 6 at 22 h UT to Oct. 7 at 1 h UT, with a chance for some significant meteor activity in the radio range, which may provide further clues to the Finlayid puzzle. A southerly radiant in the constellation of Ara will favor the observers in the southern tip of Africa.

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arXiv2015

Gone in a Blaze of Glory: the Demise of Comet C/2015 D1 (SOHO)

We present studies of C/2015 D1 (SOHO), the first sunskirting comet ever seen from ground stations over the past half century. The Solar and Heliospheric Observatory (SOHO) witnessed its peculiar light curve with a huge dip followed by a flareup around perihelion: the dip was likely caused by sublimation of olivines, directly evidenced by a coincident temporary disappearance of the tail. The flareup likely reflects a disintegration event, which we suggest was triggered by intense thermal stress established within the nucleus interior. Photometric data reveal an increasingly dusty coma, indicative of volatile depletion. A catastrophic mass loss rate of $\sim$10$^{5}$ kg s$^{-1}$ around perihelion was seen. Ground-based Xingming Observatory spotted the post-perihelion debris cloud. Our morphological simulations of post-perihelion images find newly released dust grains of size $a \gtrsim 10$ $μ$m in radius, however, a temporal increase in $a_{\min}$ was also witnessed, possibly due to swift dispersions of smaller grains swept away by radiation forces without replenishment. Together with the fading profile of the light curve, a power law dust size distribution with index $γ= 3.2 \pm 0.1$ is derived. We detected no active remaining cometary nuclei over $\sim$0.1 km in radius in post-perihelion images acquired at Lowell Observatory. Applying radial non-gravitational parameter, $\mathcal{A}_{1} = \left(1.209 \pm 0.118 \right) \times 10^{-6}$ AU day$^{-2}$, from an isothermal water-ice sublimation model to the SOHO astrometry significantly reduces residuals and sinusoidal trends in the orbit determination. The nucleus mass $\sim$10$^{8}$--10$^{9}$ kg, and the radius $\sim$50--150 m (bulk density $ρ_{\mathrm{d}} = 0.4$ g cm$^{-3}$ assumed) before the disintegration are deduced from the photometric data; consistent results were determined from the non-gravitational effects.

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arXiv2015

When Comets Get Old: A Synthesis of Comet and Meteor Observations of the Low Activity Comet 209P/LINEAR

It is speculated that some weakly active comets may be transitional objects between active and dormant comets. These objects are at a unique stage of the evolution of cometary nuclei, as they are still identifiable as active comets, in contrast to inactive comets that are observationally indistinguishable from low albedo asteroids. In this paper, we present a synthesis of comet and meteor observations of Jupiter-family comet 209P/LINEAR, one of the most weakly active comets recorded to-date. Images taken by the Xingming 0.35-m telescope and the Gemini Flamingo-2 camera are modeled by a Monte Carlo dust model, which yields a low dust ejection speed ($1/10$ of that of moderately active comets), dominance of large dust grains, and a low dust production of $0.4~\mathrm{kg \cdot s^{-1}}$ at 19~d after the 2014 perihelion passage. We also find a reddish nucleus of 209P/LINEAR that is similar to D-type asteroids and most Trojan asteroids. Meteor observations with the Canadian Meteor Orbit Radar (CMOR), coupled with meteoroid stream modeling, suggest a low dust production of the parent over the past few hundred orbits, although there are hints of a some temporary increase in activity in the 18th century. Dynamical simulations indicate 209P/LINEAR may have resided in a stable near-Earth orbit for $\sim 10^4$~yr, which is significantly longer than typical JFCs. All these lines of evidence imply that 209P/LINEAR as an aging comet quietly exhausting its remaining near surface volatiles. We also compare 209P/LINEAR to other low activity comets, where evidence for a diversity of the origin of low activity is seen.

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arXiv2014

New Active Asteroid 313P/Gibbs

We present initial observations of the newly-discovered active asteroid 313P/Gibbs (formerly P/2014 S4), taken to characterize its nucleus and comet-like activity. The central object has a radius $\sim$0.5 km (geometric albedo 0.05 assumed). We find no evidence for secondary nuclei and set (with qualifications) an upper limit to the radii of such objects near 25 m, assuming the same albedo. Both aperture photometry and a morphological analysis of the ejected dust show that mass-loss is continuous at rates $\sim$0.2 to 0.4 kg s$^{-1}$, inconsistent with an impact origin. Large dust particles, with radii $\sim$50 to 100 $μ$m, dominate the optical appearance. At 2.4 AU from the Sun, the surface equilibrium temperatures are too low for thermal or desiccation stresses to be responsible for the ejection of dust. No gas is spectroscopically detected (limiting the gas mass loss rate to $<$1.8 kg s$^{-1}$). However, the protracted emission of dust seen in our data and the detection of another episode of dust release near perihelion, in archival observations from 2003, are highly suggestive of an origin by the sublimation of ice. Coincidentally, the orbit of 313P/Gibbs is similar to those of several active asteroids independently suspected to be ice sublimators, including P/2012 T1, 238P/Read and 133P/Elst-Pizarro, suggesting that ice is abundant in the outer asteroid belt.

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arXiv2014

Where are the Mini Kreutz-family Comets?

The Kreutz family of sungrazing comets contains over 2~000 known members, many of which are believed to be under $\sim 100$~m sizes (\textit{mini~comets}) and have only been studied at small heliocentric distances ($r_\mathrm{H}$) with space-based SOHO/STEREO spacecraft. To understand the brightening process of mini Kreutz comets, we conducted a survey using CFHT/MegaCam at moderate $r_\mathrm{H}$ guided by SOHO/STEREO observations. We identify two comets that should be in our search area but are not detected, indicating that the comets have either followed a steeper brightening rate within the previously-reported rapid brighten stage (the \textit{brightening~burst}), or the brightening burst starts earlier than expected. We present a composite analysis of pre-perihelion light-curves of five Kreutz comets that covered to $\sim 1$~AU. We observe a significant diversity in the light-curves that can be used to grossly classify them into two types: C/Ikeya-Seki and C/SWAN follow the canonical $r_\mathrm{H}^{-4}$ while the others follow $r_\mathrm{H}^{-7}$. In particular, C/SWAN seems to have undergone an outburst ($Δm>5$~mag) or a rapid brightening ($n\gtrsim11$) between $r_\mathrm{H}=1.06$~AU to $0.52$~AU, and shows hints of structural/compositional differences compared to other bright Kreutz comets. We also find evidence that the Kreutz comets as a population lose their mass less efficiently than the dynamically new comet, C/ISON, and are relatively devoid of species that drive C/ISON's activity at large $r_\mathrm{H}$. Concurrent observations of C/STEREO in different wavelengths also suggest that a blue-ward species such as CN may be the main driver for brightening burst instead of previously-thought sodium.

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arXiv2014

An Early Look of Comet C/2013 A1 (Siding Spring): Breathtaker or Nightmare?

The dynamically new comet, C/2013 A1 (Siding Spring), is to make a close approach to Mars on 2014 October 19 at 18:30 UT at a distance of 40+/-1 Martian radius. Such extremely rare event offers a precious opportunity for the spacecrafts on Mars to closely study a dynamically new comet itself as well as the planet-comet interaction. Meanwhile, the high speed meteoroids released from C/Siding Spring also pose a threat to physically damage the spacecrafts. Here we present our observations and modeling results of C/Siding Spring to characterize the comet and assess the risk posed to the spacecrafts on Mars. We find that the optical tail of C/Siding Spring is dominated by larger particles at the time of the observation. Synchrone simulation suggests that the comet was already active in late 2012 when it was more than 7 AU from the Sun. By parameterizing the dust activity with a semi-analytic model, we find that the ejection speed of C/Siding Spring is comparable to comets such as the target of the Rosetta mission, 67P/Churyumov-Gerasimenko. Under nominal situation, the simulated dust cone will miss the planet by about 20 Martian radius. At the extreme ends of uncertainties, the simulated dust cone will engulf Mars, but the meteoric influx at Mars is still comparable to the nominal sporadic influx, seemly indicating that intense and enduring meteoroid bombardment due to C/Siding Spring is unlikely. Further simulation also suggests that gravitational disruption of the dust tail may be significant enough to be observable at Earth.

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