arXiv subjects

Fumi Yoshida

Publications and source records attributed to Fumi Yoshida.

At least 19 recordsLinked to original sources

arXiv2026

Opposition effect of comet 28P/Neujmin observed with Subaru Hyper Suprime-Cam

We present an observational study of the nucleus of comet 28P/Neujmin at a heliocentric distance exceeding 10 au, where coma contamination is effectively minimized. Observations were conducted in the $g$, $r$, and $y$ bands with the Hyper Suprime-Cam (HSC) on the 8.2-m Subaru Telescope. The measured colors, $g - r = 0.67\pm0.17$ and $r - y = 0.41\pm0.19$, yield a spectral index of $S' = 8.8\pm4.2\%/100$ nm, comparable to that of D-type asteroids. By incorporating new observational data at a phase angle $\alpha = 0.334^{\circ}$ with previous observations, we determined the phase function for the nucleus of 28P and confirmed an opposition surge at small phase angles. The derived opposition effect amplitude depends on the adopted phase coefficient, which is uncertain due to potential systematic effects in multi-apparition phase curves. Nevertheless, even under conservative assumptions, the opposition effect of 28P suggests a larger coherent backscattering contribution than is typical for C- and D-type asteroids. The Subaru HSC observations suggest that, although the nucleus color resembles that of D-type asteroids, the surface microstructure of comet 28P's nucleus likely differs from those of C- and D-type asteroids. Future single-apparition observations covering a wide phase angle range from near-opposition to larger angles, combined with polarimetric measurements, will be essential to definitively establish the physical mechanisms responsible for the opposition effects of cometary nuclei.

astro-ph.EP↗

arXiv2026

The first detection of an atmosphere on a trans-Neptunian object beyond Pluto

Trans-Neptunian objects (TNOs) in the outer Solar System are predominantly small, icy worlds long presumed to be atmosphereless except for the largest bodies. Until now, Pluto has been unique among TNOs in exhibiting a substantial atmosphere (nitrogen with trace methane and carbon monoxide) at microbar pressure levels. All other known TNOs, including ~ 1000-km-sized bodies such as Eris, Haumea, Makemake, and Quaoar, have shown no detectable atmospheres in stellar occultation observations, with surface pressure upper limits of order 1-100 nanobars. Here we report the first detection of an atmosphere around a TNO besides Pluto. A stellar occultation by the ~ 250-km-radius plutino (612533) 2002 XV93 on 10 January 2024 revealed a refractive signature, indicating the presence of a thin atmosphere. The derived surface pressure is 100-200 nanobars, i.e. approximately a hundred times lower than Pluto's and yet significantly above previous limits for other larger bodies. This discovery provides the first evidence that even a sub-1000-km TNO can retain an atmosphere, challenging current paradigms of volatile retention. Our findings indicate that a fraction of distant icy minor planets can exhibit atmospheres possibly caused by ongoing cryovolcanic activity or a recent impact event of a small icy object.

astro-ph.EP↗

arXiv2026

Color and Size Distributions of Small Jupiter Trojans

We conducted a two-band imaging survey observation using the Subaru Telescope and its wide-field camera, Suprime-Cam, to study the visible colors and size distribution of Jupiter's Trojan asteroids. The survey covered an area around Jupiter's L4 Lagrange point totaling 9.2 square degrees. We detected 120 Trojan asteroids in this survey. From these Trojan asteroids, we extracted 44 unbiased samples with absolute magnitudes in the $g^\prime$ band ranging from 12.9 to 16.9 mag (corresponding to diameter ranges of approximately $\sim$3 - 16 km assuming an albedo of 0.05) and analyzed their $g^\prime - i^\prime$ color and size distributions. Large Jupiter Trojan asteroids are known to be classified into two color groups, ''red'' and ''less red''. We found that such bimodality in the color distribution is absent for small Jupiter Trojan asteroids, which is consistent with previous studies. Previous studies have also shown that these two groups have different slopes in the magnitude distributions from each other, which was explained by conversion of red objects to less-red fragments through catastrophic disruptions. In contrast, we found that the size frequency distributions of our two sample groups divided by the color of $g^\prime - i^\prime$ = 0.7 (in AB magnitude) are quite similar. Our results can provide new insights into collisional evolution of color and size distribution of small Jupiter Trojans.

astro-ph.EP↗

arXiv2025

Moving object detection from multi-depth images with an attention-enhanced CNN

One of the greatest challenges for detecting moving objects in the solar system from wide-field survey data is determining whether a signal indicates a true object or is due to some other source, like noise. Object verification has relied heavily on human eyes, which usually results in significant labor costs. In order to address this limitation and reduce the reliance on manual intervention, we propose a multi-input convolutional neural network integrated with a convolutional block attention module. This method is specifically tailored to enhance the moving object detection system that we have developed and used previously. The current method introduces two innovations. This first one is a multi-input architecture that processes multiple stacked images simultaneously. The second is the incorporation of the convolutional block attention module which enables the model to focus on essential features in both spatial and channel dimensions. These advancements facilitate efficient learning from multiple inputs, leading to more robust detection of moving objects. The performance of the model is evaluated on a dataset consisting of approximately 2,000 observational images. We achieved an accuracy of nearly 99% with AUC (an Area Under the Curve) of >0.99. These metrics indicate that the proposed model achieves excellent classification performance. By adjusting the threshold for object detection, the new model reduces the human workload by more than 99% compared to manual verification.

cs.CV↗

arXiv2025

A Possible Shutting-Down Event of Mass Accretion in An Active Galactic Nucleus at z~1.8

We present the discovery of a large gradual apparent fading event in optical and near-infrared wavelengths in a quasar at z=1.767 by a factor of 20-30 (in optical) over a period of ~20 years in the observed frame. This pronounced fading trend in brightness was first identified by comparing the magnitudes measured in the Subaru/Hyper Suprime-Cam (HSC) images with those in the Sloan Digital Sky Survey (SDSS) images for ~3x10^4 quasars spectroscopically identified by SDSS. We performed follow-up observations, including optical imaging and spectroscopy as well as near-infrared imaging, with >4m-class telescopes such as Subaru, GTC, Keck, and SOAR telescopes. We combine these new data with the archival data to examine the variability behavior over ~20 years in detail and even the longer-term trend of the variability over ~70 years in the observed frame. We find that (i) the AGN component likely faded by a factor of ~50 from the early 2000s to 2023 and (ii) the observed brightness decline is best explained by a substantial decrease in accretion rate rather than time-varying line-of-sight dust obscuration. These findings are derived from multi-component (time-varying AGN + constant galaxy) spectral energy distribution fitting over multi-epochs, which is well consistent with the optical spectra. The Eddington ratio decreases by a factor of ~50, from ~0.4 to ~0.008 if we use the black hole mass measured with the SDSS spectrum, which could be highly uncertain because of the very large variability. The total brightness is dominated by the host galaxy in the rest-frame optical wavelength rather than the AGN as of 2023.

astro-ph.GA↗

arXiv2025

Discovery and dynamics of a Sedna-like object with a perihelion of 66 au

Trans-Neptunian objects (TNOs) with large perihelion distances ($q > 60$ au) and semi-major axes ($a > 200$ au) provide insights into the early evolution of the solar system and the existence of a hypothetical distant planet. These objects are still rare and their detection is challenging, yet they play a crucial role in constraining models of solar system formation. Here we report the discovery of a Sedna-like TNO, 2023\,KQ$_{14}$, nicknamed `Ammonite', with $q = 66$ au, $a = 252$ au, and inclination $i=11^\circ$. Ammonite's orbit does not align with those of the other Sedna-like objects and fills the previously unexplained `$q$-gap' in the observed distribution of distant solar system objects. Simulations demonstrate that Ammonite is dynamically stable over 4.5 billion years. % with less than 1\% variation in its semi-major axis. Our analysis suggests that Ammonite and the other Sedna-like objects may have shared a primordial orbital clustering around 4.2 billion years ago. Furthermore, Ammonite's stable orbit favors larger orbits ($\sim$ 500 au) rather than closer ones for a large hypothetical planet in present-day trans-Neptunian space.

astro-ph.EP↗

arXiv2025

An Extremely Deep Rubin Survey to Explore the Extended Kuiper Belt and Identify Objects Observable by New Horizons

A proposed Vera C. Rubin Observatory Deep Drilling micro-survey of the Kuiper Belt will investigate key properties of the distant solar system. Utilizing 30 hours of Rubin time across six 5-hour visits over one year starting in summer 2026, the survey aims to discover and determine orbits for up to 730 Kuiper Belt Objects (KBOs) to an $r$-magnitude of 27.5. These discoveries will enable precise characterization of the KBO size distribution, which is critical for understanding planetesimal formation. By aligning the survey field with NASA's {\it New Horizons} spacecraft trajectory, the micro-survey will facilitate discoveries for the mission operating in the Kuiper Belt. Modeling based on the Outer Solar System Origin Survey (OSSOS) predicts at least 12 distant KBOs observable with the {\it New Horizons} LOng Range Reconnaissance Imager (LORRI) and approximately three objects within 1~au of the spacecraft, allowing higher-resolution observations than Earth-based facilities. LORRI's high solar phase angle monitoring will reveal these objects' surface properties and shapes, potentially identifying contact binaries and orbit-class surface correlations. The survey could identify a KBO suitable for a future spacecraft flyby. The survey's size, depth, and cadence design will deliver transformative measurements of the Kuiper Belt's size distribution and rotational properties across distance, size, and orbital class. Additionally, the high stellar density in the survey field also offers synergies with transiting exoplanet studies.

astro-ph.EP↗

arXiv2025

Characterization of the Deep, Extended Kuiper Belt in the Galactic Disk

We propose a Roman Space Telescope survey to investigate fundamental properties of the distant solar system in the region of the Kuiper Belt where object characteristics and the size distribution are inaccessible from any other telescope. Our pointing is coincident with the search space accessible to NASA's New Horizons spacecraft meaning, that a discovered object sufficiently near the orbit of New Horizons would potentially be investigated by a close flyby. In addition, numerous objects expected to be discovered by this search can be observed in the distance by New Horizons allowing their surface properties and satellite systems to both be probed. As designed, this survey will discover and determine orbits for as many as 900 Kuiper Belt objects (KBOs), providing a unique opportunity for ground-breaking Kuiper Belt science. It will simultaneously: (1) Probe and characterize the deep Kuiper Belt by identifying objects as small as a few km and taking our understanding of the size distribution to a new level. This has implications for understanding the the standard model (the Streaming Instability) of KBO formation and elucidating crater formation physics on these icy bodies. (2) Open KBO rotation studies, in particular of those objects with long rotation periods,(3) Discover and characterize KBO binaries at large distances, important because their duplicity offers information about object densities at these distant locations from the Sun. (4) Shed light on the cratering history of KBOs and improving the dating of the surfaces of Arrokoth, Pluto and Charon in addition to helping to place the 32 distant KBOs New Horizons has observed in context. This project also has synergies with transiting exoplanet studies due to the stellar density of our search fields. Coupled with our timing requirements it is sensitive to discovery of hot Jupiters and hot Neptunes.

astro-ph.IM↗

arXiv2024

Resonant amplitude distribution of the Hilda asteroids and the free-floating planet flyby scenario

In some recent work, we provided a quantitative explanation for the number asymmetry of Jupiter Trojans by hypothesizing a free-floating planet (FFP) flyby into the Solar System. In support of that explanation, this paper examines the influence of the same FFP flyby on the Hilda asteroids, which orbit stably in the 3:2 mean motion resonance with Jupiter. The observed Hilda population exhibits two distinct resonant patterns: (1) a lack of Hildas with resonant amplitudes < 40 deg. at eccentricities < 0.1; (2) a nearly complete absence of Hildas with amplitudes < 20 deg., regardless of eccentricity. Previous models of Jupiter migration and resonance capture could account for the eccentricity distribution of Hildas but have failed to replicate the unusual absence of those with the smallest resonant amplitudes, which theoretically should be the most stable. Here we report that the FFP flyby can trigger an extremely rapid outward migration of Jupiter, causing a sudden shift in the 3:2 Jovian resonance. Consequently, Hildas with varying eccentricities would have their resonant amplitudes changed by different degrees, leading to the observed resonant patterns. We additionally show that, in our FFP flyby scenario, these patterns are consistently present across different resonant amplitude distributions of primordial Hildas arising from various formation models. We also place constraints on the potential parameters of the FFP, suggesting it should have an eccentricity of 1-1.3 or larger, an inclination up to 30 deg. or higher, and a minimum mass of about 50 Earth masses.

astro-ph.EP↗

arXiv2024

Candidate Distant Trans-Neptunian Objects Detected by the New Horizons Subaru TNO Survey

We report the detection of 239 trans-Neptunian Objects discovered through the on-going New Horizons survey for distant minor bodies being performed with the Hyper Suprime-Cam mosaic imager on the Subaru Telescope. These objects were discovered in images acquired with either the r2 or the recently commissioned EB-gri filter using shift and stack routines. Due to the extremely high stellar density of the search region down stream of the spacecraft, new machine learning techniques had to be developed to manage the extremely high false positive rate of bogus candidates produced from the shift and stack routines. We report discoveries as faint as r2$\sim26.5$. We highlight an overabundance of objects found at heliocentric distances $R\gtrsim70$~au compared to expectations from modelling of the known outer Solar System. If confirmed, these objects betray the presence of a heretofore unrecognized abundance of distant objects that can help explain a number of other observations that otherwise remain at odds with the known Kuiper Belt, including detections of serendipitous stellar occultations, and recent results from the Student Dust Counter on-board the New Horizons spacecraft.

astro-ph.EP↗

arXiv2024

Diffraction modelling of a 2023 March 5 stellar occultation by subkilometer-sized asteroid (98943) 2001 CC21

We present an analysis of a stellar occultation event caused by a near-Earth asteroid (98943) 2001 CC21, an upcoming flyby target in the Hayabusa2 extended mission, on March 5, 2023. To accurately determine the asteroid's shape from diffraction-affected light curves, we developed a novel data reduction technique named the Diffracted Occultation's United Simulator for Highly Informative Transient Explorations (DOUSHITE). Using DOUSHITE-generated synthetic models, we derived constraints on (98943) 2001 CC21's shadow shape from the single-chord occultation data. Our results suggest a significant elongation of the shadow with an axis ratio of $b/a = 0.37\pm0.09$. This shape can be crucial for planning Hayabusa2's high-speed flyby to optimise the limited imaging opportunities.

astro-ph.EP↗

arXiv2024

A deep analysis for New Horizons' KBO search images

Observation datasets acquired by the Hyper Suprime-Cam (HSC) on the Subaru Telescope for NASA's New Horizons mission target search were analyzed through a method devised by JAXA. The method makes use of Field Programmable Gate arrays and was originally used to detect fast-moving objects such as space debris or near-Earth asteroids. Here we present an application of the method to detect slow-moving Kuiper Belt Objects (KBOs) in the New Horizons target search observations. A cadence that takes continuous images of one HSC field of view for half a night fits the method well. The observations for the New Horizons Kuiper Belt Extended Mission (NH/KEM) using HSC began in May 2020, and are ongoing. Here we show our result of the analysis of the dataset acquired from May 2020 through June 2021 that have already passed the proprietary period and are open to the public. We detected 84 KBO candidates in the June 2020 and June 2021 datasets, when the observation field was close to opposition.

astro-ph.EP↗

arXiv2024

The New Horizons Extended Mission Target: Arrokoth Search and Discovery

Following the Pluto fly-by of the New Horizons spacecraft, the mission provided a unique opportunity to explore the Kuiper Belt in-situ. The possibility existed to fly-by a Kuiper Belt object (KBO) as well as to observe additional objects at distances closer than are feasible from earth-orbit facilities. However, at the time of launch no KBOs were known about that were accessible by the spacecraft. In this paper we present the results of 10 years of observations and three uniquely dedicated efforts -- two ground-based using the Subaru Suprime Camera, the Magellan MegaCam and IMACS Cameras, and one with the Hubble Space Telescope -- to find such KBOs for study. In this paper we overview the search criteria and strategies employed in our work and detail the analysis efforts to locate and track faint objects in the galactic plane. We also present a summary of all of the KBOs that were discovered as part of our efforts and how spacecraft targetability was assessed, including a detailed description of our astrometric analysis which included development of an extensive secondary calibration network. Overall, these efforts resulted in the discovery of 89 KBOs including 11 which became objects for distant observation by New Horizons and (486958) Arrokoth which became the first post-Pluto fly-by destination.

astro-ph.EP↗

arXiv2023

Photometry and Polarimetry of 2010 XC$_{15}$: Observational Confirmation of E-type Near-Earth Asteroid Pair

Asteroid systems such as binaries and pairs are indicative of physical properties and dynamical histories of the Small Solar System Bodies. Although numerous observational and theoretical studies have been carried out, the formation mechanism of asteroid pairs is still unclear, especially for near-Earth asteroid (NEA) pairs. We conducted a series of optical photometric and polarimetric observations of a small NEA 2010 XC$_{15}$ in 2022 December to investigate its surface properties. The rotation period of 2010 XC$_{15}$ is possibly a few to several dozen hours and color indices of 2010 XC$_{15}$ are derived as $g-r=0.435\pm0.008$, $r-i=0.158\pm0.017$, and $r-z=0.186\pm0.009$ in the Pan-STARRS system. The linear polarization degrees of 2010 XC$_{15}$ are a few percent at the phase angle range of 58$^{\circ}$ to 114$^{\circ}$. We found that 2010 XC$_{15}$ is a rare E-type NEA on the basis of its photometric and polarimetric properties. Taking the similarity of not only physical properties but also dynamical integrals and the rarity of E-type NEAs into account, we suppose that 2010 XC$_{15}$ and 1998 WT$_{24}$ are of common origin (i.e., asteroid pair). These two NEAs are the sixth NEA pair and first E-type NEA pair ever confirmed, possibly formed by rotational fission. We conjecture that the parent body of 2010 XC$_{15}$ and 1998 WT$_{24}$ was transported from the main-belt through the $ν_6$ resonance or Hungaria region.

astro-ph.EP↗

arXiv2023

The invasion of a free floating planet and the number asymmetry of Jupiter Trojans

This paper extends our previous study (Li et al. 2023) of the early evolution of Jupiter and its two Trojan swarms by introducing the possible perturbations of a free floating planet (FFP) invading the Solar System. In the framework of the invasion of a FFP, we aim to provide some new scenarios to explain the number asymmetry of the L4 and L5 Jupiter Trojans, and some other observed features. We investigate two different cases: (i) The indirect case, where Jupiter experiences a scattering encounter with the FFP and jumps outwards at a speed that is much higher than that considered in(Li et al. 2023), resulting in a change in the numbers of the L4 (N4) and L5 (N5) Trojans swarms. (ii) The direct case, in which the FFP traverses the L5 region and affects the stability of the local Trojans. In the indirect case, the outward migration of Jupiter can be fast enough to make the L4 islands disappear temporarily, inducing a resonant amplitude increase of the local Trojans. After the migration is over, the L4 Trojans come back to the re-appeared and enlarged islands. As for the L5 islands, they always exist but expand even more considerably. Since the L4 swarm suffers less excitation in the resonant amplitude than the L5 swarm, more L4 Trojans are stable and could survive to the end. In the direct case, the FFP could deplete a considerable fraction of the L5 Trojans, while the L4 Trojans at large distances are not affected and all of them could survive. Both the indirect and direct cases could result in a number ratio of R45=N4/N5~1.6 that can potentially explain the current observations. The latter has the advantage of producing the observed resonant amplitude distribution. For achieving these results, we propose that the FFP should have a mass of at least of a few tens of Earth masses and its orbital inclination is allowed to be as high as 40 degrees.

astro-ph.EP↗

arXiv2022

Simultaneous Multicolor Photometry of the DESTINY$^{+}$ target asteroid (3200) Phaethon

Accurate estimation of brightness of (3200) Phaethon up to lower phase angles are essential for planning of the on-board camera of the DESTINY$^{+}$ mission. We have carried out intensive observations of Phaethon in the optical wavelength ($g$, $r$, and $i$) with the TriCCS camera on the Seimei 3.8 m telescope in October and November, 2021. We derived the absolute magnitude $H_\mathrm{V}$ and the slope parameter $G$ of Phaethon as $H_\mathrm{V}=14.23\pm0.02$ and $G=0.040\pm0.008$ from multiple photometric observations including lower phase angles down to $\sim$9$^{\circ}$ with the $H$-$G$ model. Using the $H_\mathrm{V}$ value and the geometric albedo of Phaethon derived in previous polarimetric studies, we estimated that the Phaethon's diameter is within a range of 5.22 to 6.74 km, which is consistent with radar and occultation observations. With the linear model, we derived $H_\mathrm{V}=14.65\pm0.02$, which corresponds to a diameter range of 4.30 to 5.56 km. Our simultaneous tricolor lightcurves of Phaethon indicate that no rotational spectral variations larger than 0.018 and 0.020 mag in the g-r and r-i colors, possibly related to inhomogeneity of the surface material and/or structure, are seen at the 2021 apparition.

astro-ph.EP↗

arXiv2022

Asymmetry in the number of L4 and L5 Jupiter Trojans driven by jumping Jupiter

Context. More than 10000 Jupiter Trojans have been detected so far. They are moving around the L4 and L5 triangular Lagrangian points of the Sun-Jupiter system and their distributions can provide important clues to the early evolution of the Solar System. Aims. The number asymmetry of the L4 and L5 Jupiter Trojans is a longstanding problem. We aim to test a new mechanism in order to explain this anomalous feature by invoking the jumping-Jupiter scenario. Methods. First, we introduce the orbital evolution of Jupiter caused by the giant planet instability in the early Solar System. In this scenario, Jupiter could undergo an outward migration at a very high speed. We then investigate how such a jump changes the numbers of the L4 (N4) and L5 (N5) Trojans. Results. The outward migration of Jupiter can distort the co-orbital orbits near the Lagrangian points, resulting in L4 Trojans being more stable than the L5 ones. We find that, this mechanism could potentially explain the unbiased number asymmetry of N4/N5~1.6 for the known Jupiter Trojans. The uncertainties of the system parameters, e.g. Jupiter's eccentricity and inclination, the inclination distribution of Jupiter Trojans, are also taken into account and our results about the L4/L5 asymmetry have been further validated. However, the resonant amplitudes of the simulated Trojans are excited to higher values compared to the current population. A possible solution is that collisions among the Trojans may reduce their resonant amplitudes.

astro-ph.EP↗

arXiv2022

(3200) Phaethon Polarimetry in the Negative Branch: New Evidence for the Anhydrous Nature of the DESTINY+ Target Asteroid

We report on the first polarimetric study of (3200) Phaethon, the target of JAXA's DESTINY$^+$ mission, in the negative branch to ensure its anhydrous nature and to derive an accurate geometric albedo. We conducted observations at low phase angles (Sun-target-observer angle, alpha = 8.8-32.4 deg) from 2021 October to 2022 January and found that Phaethon has a minimum polarization degree $P_{min}$ = -1.3 +- 0.1 %, a polarimetric slope h = 0.22 +- 0.02 % deg$^{-1}$, and an inversion angle alpha$_0$ = 19.9 +- 0.3 deg. The derived geometric albedo is $p_V$ = 0.11 (in the range of 0.08-0.13). These polarimetric properties are consistent with anhydrous chondrites, and contradict hydrous chondrites and typical cometary nuclei.

astro-ph.EP↗