arXiv subjects

Kanon Nakazawa

Publications and source records attributed to Kanon Nakazawa.

4 recordsLinked to original sources

arXiv2026

Constraints on the Crystallinity of Water Ice in Planet-forming Disks from Infrared Scattered-Light Spectra

The crystallinity of water ice not only records the thermal history experienced by an astronomical body, but also affects the composition of forming planets by controlling the trapping of volatile materials in amorphous ice and their subsequent transport. An additional structure within the 3~$\rm μm$ water-ice absorption band, known as the Fresnel feature, may serve as a diagnostic of ice crystallinity. Recent observations with the James Webb Space Telescope have detected a Fresnel peak in a debris disk and in Trans-Neptunian Objects (TNOs). Here, we propose a portable expression that translates the observed Fresnel peak strength into the degree of crystallinity of icy grains in debris disks. Our formula targets scattered light at around 90$^{\circ}$ angles, which are easily accessible for spatially resolved debris disks regardless of the inclination angle. Applying this expression, we derive the degree of crystallinity of a debris disk around HD 181327 to be 10-20%. We also study the Fresnel feature in protoplanetary disks and find that it is generally weaker than in debris disks even for the same crystallinity. We then analyzed a scattered light spectrum of the protoplanetary disk around d216-0939, which shows a weak crystalline feature, and inferred a crystallinity of $\sim$50%. We conclude that the Fresnel feature is a reliable observational tracer for ice crystallinity, and future near-IR spectroscopic observations will be crucial to elucidate the crystalline ice evolution.

astro-ph.EP↗

arXiv2026

Sulfur Enrichment in Close-in Exoplanet Atmospheres Induced by Pebble Drift across the Salt Line

Observations of JWST have revealed that several close-in exoplanets have sulfur-rich atmospheres through SO$_2$ detections. Atmospheric sulfur is often thought to originate from solid accretion during planet formation, whereas recent simultaneous detections of SO$_2$ and NH$_3$ challenge this conventional scenario. In this study, we propose that ammonium salts, such as NH$_4$SH tentatively detected in comets and molecular clouds, play a significant role in producing sulfur-rich disk gases, which serve as the ingredient of giant planet atmospheres. We simulated the radial transport of dust containing volatile ices and ammonium salts, along with the dissociation, sublimation, and recondensation of these materials, thereby predicting the atmospheric chemical structures and transmission spectra of planets inheriting these compositions. Assuming that ammonium salts sequester 20% of the elemental nitrogen and sulfur budgets, our results reveal that they enhance sulfur and nitrogen abundances in disk gases to 2-10 times the solar values near the salt dissociation line. Photochemical simulations demonstrate that SO$_2$, NS, H$_2$S, NO, and NH$_3$ become the dominant N and S chemical species in the atmospheres on planets that inherited the gas compositions inside H$_2$O snowline. SO$_2$ features clearly appear in the infrared transmission spectra when the salt-bearing grains enhance the sulfur abundance of disk gas by pebble drift. Our model provides a novel scenario that explains the SO$_2$ detected in some exoplanet atmospheres solely from disk gas accretion. Volatile-element ratios, particularly N/S and C/O, would provide a key to disentangle our scenario from the conventional solid-accretion scenario.

astro-ph.EP↗

arXiv2025

Nitrogen transport in protoplanetary disks by ammonium salts: a possible origin of Jupiter's nitrogen enrichment

Atmospheric compositions preserve the history of planet formation processes. Jupiter has the remarkable feature of being uniformly enriched in various elements compared to the Sun, including highly volatile elements such as nitrogen and noble gases. Radial transport of volatile species by amorphous ice in the solar nebula is one mechanism that explains Jupiter's volatile enrichment, but the low entrapment efficiency of nitrogen into amorphous ice is an issue. We propose an alternative mechanism of delivering nitrogen to Jupiter: radial transport of semi-volatile ammonium salts in the solar nebula. Ammonium salts have been identified in 67P/Churyumov-Gerasimenko and can potentially compensate for the comet's nitrogen depletion compared to the Sun. We simulate the radial transport and dissociation of ammonium salts carried by dust in a protoplanetary disk, followed by the accretion of the gas and NH$_3$ vapor by a protoplanet, as well as the delivery of nitrogen to the planetary atmosphere from the salt-containing planetary core that undergoes dilution. We find that when the dust contains 10-30 wt% ammonium salts, the production of NH$_3$ vapor in the inner disk (~ 3 au) by dissociated salts and the incorporation of the salt-derived NH$_3$ through core formation and subsequent gas accretion by the protoplanet result in a planetary nitrogen enrichment consistent with the observations of Jupiter. Ammonium salts may thus play a vital role in developing the atmospheric composition of planets forming in the inner disk. Combining our model with future observations of the bulk compositions and isotopes of comets and other primordial bodies will help to further elucidate the elemental transport to the gas giants and ice giants in the solar system.

astro-ph.EP↗

arXiv2021

Modeling Early Clustering of Impact-induced Ejecta Particles Based on Laboratory and Numerical Experiments

A projectile impact onto a granular target produces an ejecta curtain with the heterogeneous material distribution. Understanding how the heterogeneous pattern forms is potentially important for understanding how crater rays form. Previous studies predicted that the pattern formation is induced by inelastic collisions of ejecta particles in the early stages of crater formation and is terminated by the ejecta's expanding motion. In this study, we test this prediction based on a hyper-velocity impact experiment together with N-body simulations where the trajectories of inelastically colliding granular particles are calculated. Our laboratory experiment suggests that pattern formation is already completed on a timescale comparable to the geometrical expansion of the ejecta curtain, which is ~ 10 microseconds in our experiment. Our simulations confirm the previous prediction that the heterogeneous pattern grows through initial inelastic collisions of particle clusters and subsequent geometric expansion with no further cluster collisions. Furthermore, to better understand the two-stage evolution of the mesh pattern, we construct a simple analytical model that assumes perfect coalescence of particle clusters upon collision. The model shows that the pattern formation is completed on the timescale of the system's expansion independently of the initial conditions. The model also reproduces the final size of the clusters observed in our simulations as a function of the initial conditions. It is known that particles in the target are ejected at lower speeds with increased distance to the impact point. The difference in the ejection speed of the particles may result in the evolution of the mesh pattern into rays.

astro-ph.EP↗