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Young-Jun Choi

Publications and source records attributed to Young-Jun Choi.

At least 19 recordsLinked to original sources
arXiv2026

On-Orbit Calibration of Danuri/PolCam. II. Radiometric Calibration

Danuri, South Korea's first lunar orbiter, was launched on August 5, 2022, and has successfully operated its two-year nominal mission phase. The wide-angle Polarimetric Camera (PolCam) onboard Danuri is the first instrument to conduct global polarimetric observations from lunar orbit. This paper presents the comprehensive radiometric calibration pipeline for PolCam's on-orbit data, consisting of dark current removal, smear correction, and flat-fielding. Notably, PolCam's raw data exhibit severe smear artifacts induced by the frame-transfer CCD architecture, which significantly degrade both radiometric fidelity and the accuracy of polarimetric measurements. These smear artifacts have been effectively mitigated through a rigorous correction algorithm, restoring data quality to a level sufficient for scientific analysis and facilitating the precise derivation of the degree of linear polarization (DoLP). Finally, we present representative examples of polarimetric measurements to validate calibration performance. Although the current calibration focuses on restoring data quality for qualitative scientific analysis, these results clearly demonstrate the expected inverse relationship between intensity and polarization. The absolute photometric calibration required for quantitative DoLP analysis is reserved for a subsequent publication.

astro-ph.IM↗
arXiv2026

Bottom of the Spectrum of Complete K\"ahler Metrics from Finite-Mass Plurisubharmonic Exhaustions

Let $\Omega\subset\mathbb{C}^{n}$ be a bounded domain, and let $\rho:\Omega\to[-1,0)$ be a smooth strictly plurisubharmonic exhaustion function. We consider the logarithmic potential $g=-\log(-\rho)$ and the associated complete K\"ahler metric $\omega=dd^{c}g$. We prove that if $\rho$ satisfies the finite weighted Monge--Amp\`ere mass condition $\int_{\Omega}(-\rho)^{\varepsilon}(dd^{c}\rho)^{n}<+\infty$ for every $\varepsilon>0$, then the bottom of the spectrum of the Laplace--Beltrami operator of $(\Omega,\omega)$ satisfies $\lambda_{0}(\Delta_{\omega},\Omega)=n^{2}$. The lower bound follows from the standard estimate applied to $g$, together with the inequality $|\partial g|_{\omega}^{2}\le 1$. For the reverse inequality, for each $\alpha>n/2$, we set $f=(-\rho)^{\alpha}$ and prove that $f\in W^{1,2}(\Omega,\omega)$ if and only if $\int_{\Omega}(-\rho)^{2\alpha-n}(dd^{c}\rho)^{n}<+\infty$. Under the finite weighted Monge--Amp\`ere mass condition, this allows us to let $\alpha\downarrow n/2$ in the Rayleigh quotient and obtain the upper bound $\lambda_{0}(\Delta_{\omega},\Omega)\le n^{2}$. As an application, Cegrell's theorem gives a smooth strictly plurisubharmonic exhaustion with finite Monge--Amp\`ere mass on every bounded hyperconvex domain; the associated complete K\"ahler metric constructed from this exhaustion therefore satisfies $\lambda_{0}(\Delta_{\omega},\Omega)=n^{2}$.

math.CV↗
arXiv2026

Bottom of the spectrum of complete noncompact Kähler manifolds

We present a survey on the bottom of the spectrum of the Hodge Laplacian on complete noncompact Kähler manifolds, with particular emphasis on Kähler hyperbolic manifolds and bounded symmetric domains. We also discuss theorems regarding the upper bounds for the bottom of the spectrum under Ricci and bisectional curvature assumptions, along with rigidity results for manifolds attaining the maximal bottom of the spectrum. Throughout the article, we propose several open problems.

math.DG↗
arXiv2026

Kähler Hyperbolicity Modulus for Simply-connected Kähler Hyperbolic manifolds

This paper investigates the Kähler hyperbolicity modulus on complete Kähler manifolds, with a particular focus on hyperconvex domains and bounded strongly pseudoconvex domains. Our main result establishes a lower bound for the Kähler hyperbolicity modulus in terms of the boundary behavior of the gradient length of a plurisubharmonic function. As applications, we compute the Kähler hyperbolicity modulus for bounded symmetric domains. Furthermore, we obtain lower bounds for the Kähler hyperbolicity modulus on bounded strongly pseudoconvex domains equipped with Kähler-Einstein metrics or Bergman metrics.

math.CV↗
arXiv2026

On-Orbit Calibration of Danuri/PolCam. I. Geometric Calibration

The wide-angle Polarimetric Camera (PolCam) onboard South Korea's first lunar orbiter, Danuri, is a pioneering instrument designed to conduct the first global polarimetric and high-phase-angle survey of the Moon. Precise geometric calibration is critical for this mission, particularly due to PolCam's highly oblique viewing geometry, which introduces significant topographic distortion. We present a comprehensive on-orbit geometric calibration that relies on 160,256 tie points derived from matching features between PolCam images and the well-orthorectified global map of the Kaguya Multiband Imager (MI). This dataset allows us to address two fundamental challenges: (1) the accurate reconstruction of the observation time for each line of an observation strip via a simple linear model, and (2) the refinement of the precise camera model, geometric model for PolCam optics. Our optimization method for these two challenges transforms the 2D image coordinates of identified features into 3D lunar coordinates and minimizes the reprojection error against the reference coordinates provided by the Kaguya MI map. From the refined observation time and camera model, we compute the precise longitude, latitude, and elevation of each pixel of an observed image. These estimated 3D coordinates are then used to generate orthorectified images, the final product of the geometric calibration. The resulting calibration achieves a geometric precision comparable to that of previous lunar orbiters and establishes the foundational framework necessary to produce geometrically-corrected data products of PolCam.

astro-ph.EP↗
arXiv2025

Continuity of solutions to complex Monge-Ampère equations on compact Kähler spaces

We prove the continuity of bounded solutions to complex Monge-Ampère equations on reduced, locally irreducible compact Kähler spaces. This in particular implies that any singular Kähler-Einstein potentials constructed in \cite{EGZ09} and \cite{Tsuji88, TianZhang06, ST17} are continuous. We also provide an affirmative answer to a conjecture in \cite{EGZ09} by showing that a resolution of any compact normal Kähler space satisfies the continuous approximation property. Finally, we settle the continuity of the potentials of the weak Kähler-Ricci flows \cite{ST17, GLZ20} on compact Kähler varieties with log terminal singularities.

math.DG↗
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

On the Kähler-hyperbolicity of bounded symmetric domains

In this paper, we characterize the Kähler-hyperbolicity length of a bounded symmetric domain, defined by its rank and genus, as a unique constant determined by a constant gradient length of a special Bergman potential. Additionally, we establish a characterization of the lower bound of $L^\infty$ norm of the gradient length of any Bergman potential.

math.CV↗
arXiv2024

An application of adjoint ideal sheaves to injectivity and extension theorems

This note reviews the authors' approach to Fujino's conjecture, i.e. the injectivity theorem for lc pairs on compact Kähler manifolds, via the use of adjoint ideal sheaves coupled with the associated residue computations in their previous work. Using only the techniques and results obtained from that study and under a slightly stronger positivity assumption, a "qualitative" extension result is obtained. Such extension result guarantees the existence of a global holomorphic extension $F$ of any holomorphic section $f$ on some $σ$-lc centres of the given lc pair. The extension $F$ can be shown to take values in the corresponding adjoint ideal sheaf, even though it comes without any $L^2$ estimate of $F$ in terms of $f$. Moreover, the proof invokes and implies no vanishing theorem in general.

math.CV↗
arXiv2024

An injectivity theorem on snc compact Kähler spaces: an application of the theory of harmonic integrals on log-canonical centers via adjoint ideal sheaves

Let $(X,D)$ be a log-canonical (lc) pair, in which $X$ is a compact Kähler manifold and $D$ is a reduced snc divisor, and let $F$ be a holomorphic line bundle on $X$ equipped with a smooth metric $h_F = e^{-φ_F}$. Via the use of the adjoint ideal sheaves (constructed from $φ_F$ and $D$) and the associated residue morphisms, sections of $K_D \otimes \left. F\right|_D$ on $D$ (as well as those of $K_X \otimes D \otimes F$ on $X$) can be related to the $F$-valued holomorphic top-forms on each lc center of $(X,D)$ by an inductive use of a certain residue exact sequence derived from the adjoint ideal sheaves. The theory of harmonic integrals is valid on each lc center (which is compact Kähler), so this provides a pathway to apply the techniques in harmonic theory to the possibly singular Kähler space $D$. To illustrate the use of such apparatus in problems concerning lc pairs, we prove a Kollár-type injectivity theorem for the cohomology on $D$ when $F$ is semi-positive. This in turn also solves the conjecture by Fujino on the injectivity theorem for the compact Kähler lc pair $(X,D)$, providing an alternative proof of a recent result by Cao and Păun.

math.CV↗
arXiv2024

Injectivity theorems for higher direct images under proper K\"ahler morphisms on snc spaces

Let $X$ be a complex manifold, and let $Y$ and $D$ be two reduced simple-normal-crossing (snc) divisors on $X$ with no common irreducible components. Given a proper locally K\"ahler morphism $\pi \colon X \to \Delta$ from $X$ to a complex analytic space $\Delta$, we prove Fujino's conjecture on the injectivity theorem in the relative setting in a generalized form. Specifically, we establish an injectivity result for the higher direct images under $\pi$ for the lc pairs $(X, D)$ as well as $(Y, D_Y)$, where $D_Y := D \cap Y$. As an application, this result immediately implies the injectivity theorem on holomorphically convex K\"ahler manifolds with reduced snc divisors. The main technique in the proof consists of the theory of harmonic integrals together with residue formulae associated with adjoint ideal sheaves, which are developed from our previous work for the absolute case (where $\Delta$ is a point and $X$ is compact). Additionally, we make use of the Takegoshi harmonic forms to deal with the non-compactness of $X$.

math.CV↗
arXiv2024

Curvature of higher direct images of sheaves of twisted holomorphic forms

This paper investigates the curvature properties of higher direct images $ R^qf_*Ω_{X/S}^p(E)$, where $f: X\rightarrow S$ is a family of compact Kähler manifolds equipped with a hermitian vector bundle $E \rightarrow X$. We derive a general curvature formula and explore several special cases, including those where $p + q = n$, $q = 0$, and $p = n$, with $E$ being a line bundle. Furthermore, the paper examines the curvature in the context of fiberwise hermitian flat cases, families of Hermite-Einstein vector bundles, and applications to moduli spaces and Weil-Petersson metrics, providing some insight into their geometric and analytical properties.

math.CV↗
arXiv2023

Photometry of the Didymos system across the DART impact apparition

On 26 September 2022, the Double Asteroid Redirection Test (DART) spacecraft impacted Dimorphos, the satellite of binary near-Earth asteroid (65803) Didymos. This demonstrated the efficacy of a kinetic impactor for planetary defense by changing the orbital period of Dimorphos by 33 minutes (Thomas et al. 2023). Measuring the period change relied heavily on a coordinated campaign of lightcurve photometry designed to detect mutual events (occultations and eclipses) as a direct probe of the satellite's orbital period. A total of 28 telescopes contributed 224 individual lightcurves during the impact apparition from July 2022 to February 2023. We focus here on decomposable lightcurves, i.e. those from which mutual events could be extracted. We describe our process of lightcurve decomposition and use that to release the full data set for future analysis. We leverage these data to place constraints on the post-impact evolution of ejecta. The measured depths of mutual events relative to models showed that the ejecta became optically thin within the first ~1 day after impact, and then faded with a decay time of about 25 days. The bulk magnitude of the system showed that ejecta no longer contributed measurable brightness enhancement after about 20 days post-impact. This bulk photometric behavior was not well represented by an HG photometric model. An HG1G2 model did fit the data well across a wide range of phase angles. Lastly, we note the presence of an ejecta tail through at least March 2023. Its persistence implied ongoing escape of ejecta from the system many months after DART impact.

astro-ph.EP↗
arXiv2023

On an injectivity theorem for log-canonical pairs with analytic adjoint ideal sheaves

As an application of the residue functions corresponding to the lc-measures developed by the authors, the proof of the injectivity theorem on compact Kähler manifolds for plt pairs by Matsumura is improved in this article to allow multiplier ideal sheaves of plurisubharmonic functions with neat analytic singularities in the coefficients of the relevant cohomology groups. With the use of a refined version of analytic adjoint ideal sheaves, a plan towards a solution to the generalised version of Fujino's conjecture (i.e. an injectivity theorem on compact Kähler manifolds for lc pairs with multiplier ideal sheaves) is laid down and, in addition to the result for plt pairs, a proof for lc pairs in dimension 2, which is also an improvement of Matsumura's result, is given.

math.CV↗
arXiv2022

A characterization of the unit ball by a Kähler-Einstein potential

We will show that a universal covering of a compact Kähler manifold with ample canonical bundle is the unit ball if it admits a global potential function of the Kähler-Einstein metric whose gradient length is a minimal constant. As an application, we will extend the Wong-Rosay theorem to a complex manifold without boundary.

math.CV↗
arXiv2021

FOSSIL: I. The Spin Rate Limit of Jupiter Trojans

Rotation periods of 53 small (diameters $2 < D < 40$ km) Jupiter Trojans (JTs) were derived using the high-cadence light curves obtained by the FOSSIL phase I survey, a Subaru/Hyper Suprime-Cam intensive program. These are the first reported periods measured for JTs with $D < 10$ km. We found a lower limit of the rotation period near 4 hr, instead of the previously published result of 5 hr (Ryan et al. 2017; Szabo et al. 2017, 2020) found for larger JTs. Assuming a rubble-pile structure for JTs, a bulk density of 0.9 gcm$^{-3}$ is required to withstand this spin rate limit, consistent with the value $0.8-1.0$ gcm$^{-3}$ (Marchis et al. 2006; Mueller et al. 2010; Buie et al. 2015; Berthier et al. 2020) derived from the binary JT system, (617) Patroclus-Menoetius system.

astro-ph.EP↗
arXiv2021

Extension with log-canonical measures and an improvement to the plt extension of Demailly--Hacon--Paun

With a view to proving the conjecture of "dlt extension" related to the abundance conjecture, a sequence of potential candidates for replacing the Ohsawa measure in the Ohsawa-Takegoshi $L^2$ extension theorem, called the "lc-measures", which hopefully could provide the $L^2$ estimate of a holomorphic extension of any suitable holomorphic section on a subvariety with singular locus, are introduced in the first half of the paper. Based on the version of $L^2$ extension theorem proved by Demailly, a proof is provided to show that the lc-measure can replace the Ohsawa measure in the case where the classical Ohsawa-Takegoshi $L^2$ extension works, with some improvements on the assumptions on the metrics involved. The second half of the paper provides a simplified proof of the result of Demailly-Hacon-Paun on the "plt extension" with the superfluous assumption "$\operatorname{supp} D \subset \operatorname{supp}(S+B)$" in their result removed. Most arguments in the proof are readily adopted to the "dlt extension" once the $L^2$ estimates with respect to the lc-measures of holomorphic extensions of sections on subvarieties with singular locus are ready.

math.CV↗