arXiv subjects

Yoshiharu Shinnaka

Publications and source records attributed to Yoshiharu Shinnaka.

12 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 $α= 0.334^{\circ}$ 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.

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arXiv2026

A post-perihelion constraint on the CO$_{2}$/H$_{2}$O ratio of interstellar comet 3I/ATLAS from [O I] forbidden lines

We present high-resolution optical spectroscopy of interstellar comet 3I/ATLAS (C/2025 N1) obtained with the High Dispersion Spectrograph mounted on the Subaru Telescope on UT 2026 January 7, when the comet was on its outbound trajectory at a heliocentric distance of $r_{\mathrm{h}} = 2.87$ au. The spectra cover the forbidden atomic oxygen lines, [O~I], at 557.7, 630.0, and 636.4 nm. The [O~I] red-doublet intensity ratio $I_{630.0}/I_{636.4} = 2.91 \pm 0.21$ matches the optically thin branching ratio ($\sim$3; \citealt{StoreyZeippen2000}), indicating that optical-depth effects are small and that our relative flux calibration is reliable. We measure a green-to-red [O~I] intensity ratio of $G/R = I_{557.7}/(I_{630.0} + I_{636.4}) = 0.339 \pm 0.027$. This value is higher than those of most Solar System comets at similar heliocentric distances, but comparable to that of the interstellar comet 2I/Borisov. From the measured $G/R$ ratio in 3I/ATLAS, we estimate the CO$_2$/H$_2$O abundance ratio under the assumption that H$_2$O and CO$_2$ are the dominant parents of O($^1$S) and O($^1$D), with other oxygen-bearing species expected to have a smaller influence under typical conditions (e.g., \citealt{FestouFeldman1981}). The derived ratio is significantly lower than the extremely CO$_2$-rich composition reported from infrared observations on the inbound trajectory at $r_{\mathrm{h}} \sim 3.3$ au, yet higher than typical values measured for comets in the Solar System. Together with published pre- and post-perihelion measurements, our result indicates that the CO$_2$/H$_2$O ratio decreased substantially across perihelion.

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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.

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arXiv2021

Detection of $^7$Be II in the Classical Nova V5669 Sgr (Nova Sagittarii 2015 No.3)

We report the new detection of $^7$Be II in the ultraviolet spectra of V5669 Sgr during its early decline phase ($+24$ and $+28$ d). We identified three blue-shifted absorption systems in our spectra. The first two, referred to as low- and high-velocity components, were noticeably identified among H I Balmer, Na I D, and Fe II whose lower energies of transients are low ($<4$ eV). The third absorption component was identified among N II, He I, and C II lines whose lower energy levels are relatively high (9--21 eV). The absorption lines of $^7$Be II at $3130.583$ Å, and $3132.228$ Å were identified as the first and second components in our observations. No evidence suggested the existence of Li I at 6708 Å in any velocity components. The estimated number density ratio of lithium relative to hydrogen, which was finally produced by this object using the equivalent widths of $^7$Be and Ca II K, $N({\rm ^{7}Li})/N({\rm H})_{\rm final}$ is $4.0\pm0.7\times10^{-6}$. This value is an order of magnitude lower than the average observed values for classical novae wherein $^7$Be has been detected, and is comparable to the most optimistic value of theoretical predictions.

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arXiv2021

Mid-infrared observations of the nucleus of comet P/2016 BA14 (PANSTARRS)

We present mid-infrared observations of comet P/2016 BA14 (PANSTARRS), which were obtained on UT 2016 March 21.3 at heliocentric and geocentric distances of 1.012 au and 0.026 au, respectively, approximately 30 hours before its closest approach to Earth (0.024 au) on UT 2016 March 22.6. Low-resolution ($λ$/$Δλ$~250) spectroscopic observations in the N-band and imaging observations with four narrow-band filters (centered at 8.8, 12.4, 17.7 and 18.8 $μ$m) in the N- and Q-bands were obtained using the Cooled Mid-Infrared Camera and Spectrometer (COMICS) mounted on the 8.2-m Subaru telescope atop Maunakea, Hawaii. The observed spatial profiles of P/2016 BA14 at different wavelengths are consistent with a point-spread function. Owing to the close approach of the comet to the Earth, the observed thermal emission from the comet is dominated by the thermal emission from its nucleus rather than its dust coma. The observed spectral energy distribution of the nucleus at mid-infrared wavelengths is consistent with a Planck function at temperature T~350 K, with the effective diameter of P/2016 BA14 estimated as ~0.8 km (by assuming an emissivity of 0.97). The normalized emissivity spectrum of the comet exhibits absorption-like features that are not reproduced by the anhydrous minerals typically found in cometary dust coma, such as olivine and pyroxene. Instead, the spectral features suggest the presence of large grains of phyllosilicate minerals and organic materials. Thus, our observations indicate that an inactive small body covered with these processed materials is a possible end state of comets.

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arXiv2020

Possibility of Concentration of Non-volatile Species near the Surface of Comet 67P/Churyumov-Gerasimenko

The cometary materials are thought to be the reservoir of primitive materials in the Solar System. The recent detection of glycine and CH$_3$NH$_2$ by the ROSINA mass spectrometer in the coma of 67P/Churyumov-Gerasimenko suggests that amino acids and their precursors may have been formed in an early evolutionary phase of the Solar System. We investigate the evolution of cometary interior considering the evaporation process of water followed by the concentration of non-volatile species. We develop a Simplified Cometary Concentration Model (SCCM) to simulate the evaporation and concentration processes on the cometary surface.We use 67P/Churyumov-Gerasimenko as the benchmark of SCCM. We investigate the depth of the layer where non-volatile species concentrate after the numerous passages of perihelion after the formation of the Solar System. As a result, the SCCM explains the observed production rates of water and CH$_3$NH$_2$ at 100 comet years. SCCM results suggest that the non-volatile species would concentrate at depths between 0 and 100cm of comet surface within 10 comet years. Our results also suggest that the non-volatile species would concentrate several meters beneath the surface before it hit the early Earth. This specific mass of non-volatile species may provide unique chemical condition to the volcanic hot spring pools.

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arXiv2020

High-resolution optical spectroscopic observations of comet 21P/Giacobini-Zinner in Its 2018 apparition

Comet 21P/Giacobini-Zinner is a peculiar comet from the viewpoints of the chemical and physical properties of its dust grains. We conduct optical high-resolution spectroscopic observations of the comet. The intensity ratios of forbidden oxygen lines (at 557.7, 630.0, and 636.4 nm) and ortho-to-para abundance ratios (OPRs) of water cations (H$_2$O$^+$) and amidogen radicals (NH$_2$) are obtained while only the upper limit for $^{14}$N/$^{15}$N in the amidogen radical is restricted. The OPRs of H$_2$O$^+$ and NH$_2$ are similar to those of other comets, although the real meaning of these OPRs is still debated. Based on the observation of the forbidden emission lines of oxygen atoms, it can be concluded that the comet is depleted in CO$_2$. In consideration with the depletion in other highly volatile species found in the near-infrared region and the presence of complex organics in comet 21P/Giacobini-Zinner, this comet might form in a warmer region in the solar nebula compared with other comets.

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arXiv2019

Unidentified Infrared Emission Features in Mid-infrared Spectrum of Comet 21P/Giacobini-Zinner

Comet 21P/Giacobini-Zinner (hereafter, comet 21P/G-Z) is a Jupiter-family comet and a parent comet of the October Draconids meteor shower. If meteoroids originating from a Jupiter-family comet contain complex organic molecules, such as amino acids, they are essential pieces of the puzzle regarding the origin of life on Earth. We observed comet 21P/G-Z in the mid-infrared wavelength region using the Cooled Mid-infrared Camera and Spectrometer (COMICS) on the 8.2 m Subaru Telescope on UT 2005 July 5. Here, we report the unidentified infrared (UIR) emission features of comet 21P/G-Z, which are likely due to complex organic molecules (both aliphatic and aromatic hydrocarbons), and the thermal emission from amorphous/crystalline silicates and amorphous carbon grains in its mid-infrared low-resolution spectrum. The UIR features at ~8.2 micron, ~8.5 micron, and ~11.2 micron found in the spectrum of comet 21P/G-Z could be attributed to polycyclic aromatic hydrocarbons (or hydrogenated amorphous carbons) contaminated by N- or O-atoms, although part of the feature at ~11.2 micron comes from crystalline olivine. The other feature at ~9.2 micron might originate from aliphatic hydrocarbons. Comet 21P/G-Z is enriched in complex organic molecules. Considering that the derived mass fraction of crystalline silicates in comet 21P/G-Z is typical of comets, we propose that the comet originated from a circumplanetary disk of giant planets (similar to Jupiter and Saturn) where was warmer than the typical comet-forming region (5-30 au from the Sun) and was suitable for the formation of complex organic molecules. Comets from circumplanetary disks might be enriched in complex organic molecules, such as comet 21P/G-Z, and may have provided pre-biotic molecules to ancient Earth by direct impact or meteor showers.

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arXiv2018

Mid-infrared spectroscopic observations of comet 17P/Holmes immediately after its great outburst in October 2007

Dust grains of crystalline silicate, which are rarely presented in interstellar space, were found in cometary nuclei. These crystalline silicates are thought to have formed by annealing of amorphous silicate grains or direct condensation of gaseous materials near the Sun in the solar nebula, and incorporated into cometary nuclei in the cold comet-forming region after radial transportation of grains in the solar nebula. Abundances of the crystalline silicate dust grains were therefore expected to be smaller farther from the Sun. We aim to better understand the formation mechanism of minerals incorporated into comet 17P/Holmes based on its mineral abundances. To derive the mineral composition of comet 17P/Holmes, we applied a thermal emission model for cometary dust grains to mid-infrared spectra of comet 17P/Holmes taken with the Cooled Mid-Infrared Camera and Spectrograph (COMICS) mounted on the Subaru Telescope a few days later the great outburst in October 2007. The resulting mass fraction of crystalline silicate, f_ cry, and an olivine-to-pyroxene abundance ratio, f_OP, are f_cry = 0.31 +/- 0.03 and f_OP = 1.20 +0.16/-0.12, respectively. Based on a simple consideration of the mixing of dust grains originating in both the interstellar medium and solar nebula, the minerals of 17P/Holmes formed by non-equilibrium condensation. This result is consistent with theoretical and experimental predictions for vaporization and condensation of olivine in the solar nebula.

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arXiv2018

Inversion angle of phase-polarization curve of near-Earth asteroid (3200) Phaethon

The linear polarization degree (referred to the scattering plane, P_r) as a function of the solar phase angle, α, of solar system objects is a good diagnostic to understand the scattering properties of their surface materials. We report Pr of Phaethon over a wide range of α from 19.1 deg to 114.3 deg in order to better understanding properties of its surface materials. The derived phase-polarization curve shows that the maximum of P_r, P_max, is >42.4% at α >114.3 deg, a value significantly larger than those of the moderate albedo asteroids (P_max ~9%). The phase-polarization curve classifies Phaethon as B-type in the polarimetric taxonomy, being compatible with the spectral property. We compute the geometric albedo, p_v, of 0.14 +/- 0.04 independently by using an empirical slope-albedo relation, and the derived p_v is consistent with previous results determined from mid-infrared spectra and thermophysical modeling. We could not find a fit to the period in our polarimetric data in the range from 0 up to 7.208 hr (e.g., less than twice the rotational period) and found significant differences between our P_r during the 2017 approach to the Earth and that of the 2016. These results imply that Phaethon has a region with different properties for light scattering near its orbital pole.

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arXiv2017

Isotopic ratios in outbursting comet C/2015 ER61

Isotopic ratios in comets are critical to understanding the origin of cometary material and the physical and chemical conditions in the early solar nebula. Comet C/2015 ER61 (PANSTARRS) underwent an outburst with a total brightness increase of 2 magnitudes on the night of 2017 April 4. The sharp increase in brightness offered a rare opportunity to measure the isotopic ratios of the light elements in the coma of this comet. We obtained two high-resolution spectra of C/2015 ER61 with UVES/VLT on the nights of 2017 April 13 and 17. At the time of our observations, the comet was fading gradually following the outburst. We measured the nitrogen and carbon isotopic ratios from the CN violet (0,0) band and found that $^{12}$C/$^{13}$C=100 $\pm$ 15, $^{14}$N/$^{15}$N=130 $\pm$ 15. In addition, we determined the $^{14}$N/$^{15}$N ratio from four pairs of NH$_2$ isotopolog lines and measured $^{14}$N/$^{15}$N=140 $\pm$ 28. The measured isotopic ratios of C/2015 ER61 do not deviate significantly from those of other comets.

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arXiv2017

Mid-infrared Spectroscopic Observations of the Dust-forming Classical Nova V2676 Oph

The dust-forming nova V2676 Oph is unique in that it was the first nova to provide evidence of C_2 and CN molecules during its near-maximum phase and evidence of CO molecules during its early decline phase. Observations of this nova have revealed the slow evolution of its lightcurves and have also shown low isotopic ratios of carbon (12C/13C) and nitrogen (14N/15N) in its nova envelope. These behaviors indicate that the white dwarf (WD) star hosting V2676 Oph is a CO-rich WD rather than an ONe-rich WD (typically larger in mass than the former). We performed mid-infrared spectroscopic and photometric observations of V2676 Oph in 2013 and 2014 (respectively 452 and 782 days after its discovery). No significant [Ne II] emission at 12.8 micron was detected at either epoch. These provided evidence for a CO-rich WD star hosting V2676 Oph. Both carbon-rich and oxygen-rich grains were detected in addition to an unidentified infrared feature at 11.4 micron originating from polycyclic aromatic hydrocarbon molecules or hydrogenated amorphous carbon grains in the envelope of V2676 Oph.

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