Jun Takahashi

We present two-epoch JWST NIRSpec and MIRI observations of the young protostar EC 53 (V371 Ser), a periodically variable source with well-characterized quiescent and burst phases. The spectra in both epochs show absorption in the CO overtone (∼2.3 μ m) and fundamental (∼4.6 μ m) bands and the H ₂ O stretching (∼2.7 μ m) and bending (∼6.0 μ m) modes. We also obtained high-resolution ( R ≈ 45,000) IGRINS spectra during the burst to constrain the CO overtone line profiles. Local thermodynamic equilibrium (LTE) slab modeling yields gas temperatures of ∼1800 K (CO overtone) and ∼1200 K (CO fundamental), consistent with the overtone tracing hotter gas at smaller radii. The H ₂ O stretching-mode absorption shows no compelling evidence for variability, and the current JWST CO overtone data do not provide a robust constraint on overtone variability. In contrast, the CO fundamental and H ₂ O bending-mode features weaken by a factor of ∼2 during the burst, which is most naturally explained by continuum changes rather than large variations in absorbing gas. To quantify continuum dilution, we introduce a “relative veiling” that treats the quiescent spectrum as an internal reference and measures the change in the continuum excess between the two epochs. This formalism yields burst-to-quiescent hot-continuum ratios of 2.9 ± 0.2 for the CO overtone and 1.71 ± 0.11 for the CO fundamental. Using a viscous-disk prescription, these imply representative accretion-rate ratios of ∼3.6 and ∼2.0, respectively. The differing ratios suggest that inner-disk regions traced at different temperatures, and thus radii, respond differently across the burst cycle, consistent with episodic mass buildup in the inner disk during quiescence followed by more efficient transport through the innermost disk onto the protostar during the burst.

On behalf of the project team, I report our inclusive design workshop at the Nishi-Harima Astronomical Observatory (NHAO). In the workshop, participants including people with visual or hearing impairments were asked to propose ideas to improve astronomical activities in the observatory. Some of the proposed ideas are presented. Benefits from inclusive design are also discussed.

We present independent polarimetric observations of the interstellar object 3I/ATLAS, including the first near-infrared polarimetric measurements. Using imaging polarimeters, we measured the degree of linear polarization from the visible R _C band (0.64 μ m) to the near-infrared K _s band (2.25 μ m), and investigated its dependence on solar phase angle (polarization phase curve, PPC) and wavelength (polarization color curve, PCC). We confirm that the PPC of 3I/ATLAS differs significantly from those of typical solar system comets, showing an unusually large polarization amplitude. This PPC shows no significant change in the R _C band across perihelion passage, despite the perihelion lying within the water snow line. This indicates that the unusual polarimetric behavior of 3I/ATLAS is unlikely to be driven by transient volatile activity, but instead reflects intrinsic optical properties of refractory dust particles. The PCC increases with wavelength over 0.6–1.2 μ m and peaks at 1.5–2.0 μ m, suggesting that the dominant scattering units are dust aggregates composed of submicron-sized monomers, broadly consistent with interstellar dust and solar-system cometary aggregates. Taken together, our results indicate that 3I/ATLAS preserves polarimetric properties characteristic of a primitive cometary planetesimal formed in another planetary system, with a refractory dust composition that differs from that typically observed among solar system comets, despite sharing a similar size scale of the aggregate building blocks.

Context . Previous fast-modulating polarimeters have successfully achieved precisions on the order of parts per million (ppm). However, they lack imaging capability, which restricts their scientific application. Aims . We developed POlarimeter for Precision Observations (POPO), an instrument designed to provide either a high precision (≲10 ppm) or a high time resolution (≲1 s), along with imaging capability. In this study, we evaluated the system characteristics and performance of POPO, focusing on the observations of bright point sources. Methods . Test observations were conducted with POPO mounted on the Cassegrain focus of the 2.0-m Nayuta telescope. Bright (Results . Under nearly constant conditions, the best precision of Stokes measurement was 5 ppm. We found the precision to be 2.5 times the photon noise limit. In addition, even with a time resolution of 1 s, a precision sufficient to detect 1% polarization was achieved for stars brighter than 10 mag. The night-to-night stability was recorded as ≲10 ppm. The accuracy of the linear polarization degree ( P ) was evaluated to be ≲0.1% for strongly polarized stars ( P ≈ several %) when observed at several position angles of the instrument. POPO successfully confirmed the rotational variation in Vesta’s polarization (Δ P of ~0.06% to ~0.08%). Conclusions . POPO is suitable for studying small polarimetric variation in bright objects. Furthermore, POPO demonstrated the ability to perform high-time-resolution polarimetry.

We conducted contemporaneous optical and near-infrared polarimetric and spectroscopic observations of C/2023 A3 (Tsuchinshan-ATLAS, hereafter T-A) from 2024 October 16 to December 17, covering a wide range of phase angles (20°–123°) and wavelengths (0.5–2.3 μm). We paid special attention to gas contamination in the dust polarization using these data. As a result, we find the maximum polarization degree P max = 31.21 % ± 0.05 %, 33.52% ± 0.06%, 35.12% ± 0.01%, 37.57% ± 0.01%, and 35.35% ± 0.01% in the Rc, Ic, J, H, and Ks bands, respectively. Although dust polarization shows a red slope at shorter wavelengths and can peak around 1.6 μm, the phase angle at which maximum polarization occurs exhibits less dependence on wavelength ( α max ∼ 90 ° –95°). Although only a few historically bright comets, such as West, Halley, and Hale–Bopp, have undergone such extensive dust-polarization observations, our measurements are generally consistent with those of two comets that possibly originated from the Oort Cloud (West and Halley). From these results, we conjecture that the optical properties and growth processes of dust in the presolar nebula, which formed these cometary nuclei, were likely uniform.

We conducted contemporaneous optical and near-infrared polarimetric and spectroscopic observations of C/2023 A3 (Tsuchinshan-ATLAS, hereafter T-A) from 2024 October 16 to December 17, covering a wide range of phase angles (20-123 deg) and wavelength (0.5-2.3 um). We paid special attention to gas contamination in the dust polarization using these data. As a result, we find the maximum polarization degree $P_max=31.21 +- 0.05 %, 33.52 +- 0.06 %, 35.12 +- 0.01 %, 37.57 +- 0.01 %, and 35.35 +- 0.01 % in the Rc-, Ic-, J-, H-, and Ks-bands, respectively. Although dust polarization shows a red slope at shorter wavelengths and can peak around 1.6 um, the phase angle at which maximum polarization occurs exhibits less dependence on wavelength (alpha_max ~ 90 deg - 95 deg). Although only a few historically bright comets, such as West, Halley, and Hale-Bopp, have undergone such extensive dust-polarization observations, our measurements are generally consistent with those of two comets that possibly originated from the Oort Cloud (West and Halley). From these results, we conjecture that the optical properties and growth processes of dust in the presolar nebula, which formed these cometary nuclei, were likely uniform.

Context. (4015) Wilson-Harrington (hereafter, WH) was discovered as a comet in 1949 but has a dynamical property consistent with that of a near-Earth asteroid. Although there is a report that the 1949 activity is associated with an ion tail, the cause of the activity has not yet been identified. Aims. This work aims to reveal the mysterious comet-like activity of the near-Earth asteroid. Methods. We conducted new polarimetric observations of WH from May 2022 to January 2023, reanalyses of the photographic plate images taken at the time of its discovery in 1949, and dust tail simulation modelings, where the dust terminal velocity and ejection epoch are taken into account. Results. We found that this object shows polarization characteristics similar to those of low-albedo asteroids. We derived the geometric albedo ranging from p_V = 0.076 ± 0.010 to p_V = 0.094 ± 0.018 from our polarimetry (the values vary depending on the data used for fitting and the slope-albedo relationship coefficients). In addition, the 1949 image showed an increase in brightness around the nucleus. Furthermore, we found that the color of the tail is consistent with sunlight, suggesting that the 1949 activity is associated with dust ejection. From the dust tail analysis, ~9 × 10⁵ kg of material was ejected episodically at a low velocity equivalent to or even slower than the escape velocity. Conclusions. We conclude that WH is most likely an active asteroid of main belt origin and that the activity in 1949 was likely triggered by mass shedding due to fast rotation.

Context. Ch-type asteroids are distinctive among other dark asteroids in that they exhibit deep negative polarization branches (NPBs). Nevertheless, the physical and compositional properties that cause their polarimetric distinctiveness are less investigated. Aims. We aim to investigate the polarimetric uniqueness of Ch-type asteroids by making databases of various observational quantities (i.e., spectroscopic and photometric properties as well as polarimetric ones) of dark asteroids. Methods. We conducted an intensive polarimetric survey of 52 dark asteroids (including 31 Ch-type asteroids) in the R_C band to increase the size of polarimetric samples. The observed data were compiled with previous polarimetric, spectroscopic, and photometric archival data to find their correlations. Results. We find remarkable correlations between these observed quantities, particularly the depth of NPBs and their spectroscopic features associated with the hydrated minerals. The amplitude of the opposition effect in photometric properties also shows correlations with polarimetric and spectral properties. However, these observed quantities do not show noticeable correlations with the geometric albedo, thermal inertia, and diameter of asteroids. Conclusions. Based on the observational evidence, we arrive at our conclusion that the submicrometer-sized structures (fibrous or flaky puff pastry-like structures in phyllosilicates) in the regolith particles could contribute to the distinctive NPBs of hydrated asteroids.

Context.Sunlight scattered from the surface of an airless body is generally partially polarized, and the corresponding polarization state includes information about the scattering surface, such as albedo, surface grain sizes, composition, and taxonomic types. Most polarimetric studies on airless bodies thus far have focused on optical wavelengths (λ≲ 1 µm).Aims.We conducted polarimetry of two large airless bodies, the Dawn mission targets (1) Ceres and (4) Vesta, in the near-infrared region. We further investigated the change in the polarimetric phase curves over the wavelengths expected from previous works.Methods.We used the Nishiharima Infrared Camera (NIC) installed at the Nishi-Harima Astronomical Observatory (NHAO) to observe these objects at multiple geometric configurations in theJ,H, andK_sbands (λ ~1.2–2.3 µm).Results.Polarimetric parameters were determined and compared with previously reported experimental results. In particular, Vesta exhibits a characteristic change in the negative polarization branch as the wavelength increases to theK_sband, which we interpret as an indication of the dominant existence ofD~ 10–20 µm particles. Our approach is supported by empirical reasoning and coincides well with an independent, theory-driven approach based on thermal modeling.Conclusions.This work demonstrates how near-infrared polarimetry can be utilized to quantitatively determine the particle size of airless objects. This finding will have important implications for asteroid taxonomy and regolith evolution.

This work explores characteristics of the negative polarization branch (NPB), which occurs in scattered light from rough surfaces, with particular focus on the effects of fine particles. Factors such as albedo, compression, roughness, and the refractive index are considered to determine their influence on the NPB. This study compiles experimental data and lunar observations to derive insights from a wide array of literature. Employing our proposed methodology, we estimate the representative grain sizes on the lunar surface to be D ~ 1–2 µm, with D ≲ 2–4 µm, consistent with observed grain size frequency distributions in laboratory settings for lunar fines. Considering Mars, we propose that the finest particles are likely lacking (D ≫ 10 µm), which matches previous estimations. This study highlights the potential of multiwavelength, particularly near-infrared, polarimetry for precisely gauging small particles on airless celestial bodies. The conclusions provided here extend to cross-validation with grain sizes derived from thermal modeling, asteroid taxonomic classification, and regolith evolution studies.