National Astronomical Observatory of Japan

Abstract In this work, we present the proof of concept of a characterization of the temperature
dependence of the quality factor Q(T ) for a massive silicon substrate suspended in a
cryogenic environment using the Gentle Nodal Suspension (GeNS) method. Measurements
were conducted in a low-vibration cryostat, analyzing three distinct resonant vibrational
modes of the sample. Quality factor values and associated uncertainties were extracted
using multiple statistical approaches. The results highlight the suitability of the
experimental method and confirm crystalline silicon as a promising material for last-stage
test mass suspensions in cryogenic conditions, as required in next-generation gravitational
wave detectors. A comparison between experimental data and FEM simulations confirms
the accuracy of the setup. This study, part of the broader development of monolithic
silicon suspensions, demonstrates the effectiveness of the GeNS technique for assessing
mechanical dissipation in large-scale substrates.

Abstract The secondary i - and s -processes that follow the r -process in collapsar outflows, characterized by low electron fraction Y e and decelerated expansion, can serve as a major source of rare earth element production. We systematically evaluated the contributions of the r -, i -, and s -processes to the production of rare earth elements in collapsar outflows. A sensitivity analysis of nucleosynthesis yields with respect to ( n , γ ) reaction rates involving unstable nuclei near the line of stability highlights dozens of key reactions critical for the synthesis of thulium ( Z = 69) and lutetium ( Z = 71). These specific ( n , γ ) reactions, however, have a negligible impact on the yields predicted in magnetohydrodynamically driven jets or binary neutron star mergers. Our nucleosynthesis models indicate that collapsars can produce elevated Tm/Eu and Lu/Eu abundance ratios, potentially serving as observational signatures distinguishing collapsar nucleosynthesis from others. The robustness of these predictions, however, is highly sensitive to uncertainties in the relevant nuclear reaction rates. We therefore emphasize the need for precise future experimental measurements of the ( n , γ ) cross sections for key unstable nuclei. Reducing these nuclear-physics uncertainties will allow collapsar models to predict Tm/Eu and Lu/Eu ratios with greater confidence, thereby improving our interpretation of observed abundances in r -process-enhanced metal-poor stars.

Abstract 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>g</mml:mi> <mml:mo accent="false">′</mml:mo> </mml:math> band ranging from 12.9 to 16.9 mag (corresponding to diameter ranges of approximately ∼3–16 km assuming an albedo of 0.05) and analyzed their <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>g</mml:mi> <mml:mo accent="false">′</mml:mo> <mml:mo>−</mml:mo> <mml:mi>i</mml:mi> <mml:mo accent="false">′</mml:mo> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>g</mml:mi> <mml:mo accent="false">′</mml:mo> <mml:mo>−</mml:mo> <mml:mi>i</mml:mi> <mml:mo accent="false">′</mml:mo> </mml:math> = 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.

In light of the latest Planck and Atacama Cosmology Telescope (P-ACT) joint results on the primordial scalar power spectrum, we show that the R 2 inflation model extended with a non-minimally coupled scalar field χ — namely the χ-extended R 2 inflation model — can naturally accommodate a larger spectral index ns and a small positive running αs at cosmic microwave background (CMB) scales, both of which are consistent with the latest P-ACT constraints. This is because the χ field contributes a blue-tilted component to the primordial power spectrum, which both modifies the large-scale power and, as a result, significantly enhances power on small scales. The deviation of the ns and αs from the single field R 2 inflation is related to the non-minimal coupling constant ξ. The consequent enhancement in the primordial power spectrum can be large enough to lead to the formation of primordial black holes (PBHs) of mass ≲1020 g as dark matter candidates. Furthermore, future observations of the small-scale power spectrum, CMB spectral distortions, and stochastic gravitational waves will provide decisive tests of this model and its predictions for PBHs. We stress its strong connection to the seesaw mechanism for the generation of the observed small masses.

The circumgalactic medium (CGM) surrounding galaxies is believed to be a significant reservoir of baryons, yet its total mass remains poorly constrained. We present a novel approach to probe the CGM of the Andromeda galaxy (M31) using fast radio bursts (FRBs) from the CHIME/FRB Catalog 2. By comparing the dispersion measures (DMs) of 171 FRBs whose sightlines intersect M31's halo (within $r_{\rm vir}= 302\,\mathrm{kpc}$) to a control sample of 684 FRBs, we estimate the DM contribution from M31's CGM. We find evidence for an excess DM of $δ\mathrm{DM} = 5.9$--$59.6\,\mathrm{pc\,cm^{-3}}$ in the inner halo ($\approx 0$--$151\,\mathrm{kpc}$) and $δ\mathrm{DM} = 25.7$--$64.6\,\mathrm{pc\,cm^{-3}}$ in the outer halo ($\approx 151$--$302\,\mathrm{kpc}$). Using a generalized halo model parameterized by a closure radius $r_{\mathrm{close}}$, we constrain the baryon distribution and infer a best-fit value for $r_{\mathrm{close}}$ to be $9.2^{+9.9}_{-4.9}\,r_{\rm vir}$ ($1σ$), corresponding to a total CGM mass of $M_{b,\mathrm{halo}} = 18.6^{+7.9}_{-8.4} \times 10^{10}\,M_\odot$. Our results suggest that M31 may harbor a substantial fraction of its cosmic baryon budget in diffuse, ionized gas. This work demonstrates the potential of FRBs as a powerful tool for studying the CGM of nearby galaxies, with future larger samples expected to provide tighter constraints on the baryon content of galactic halos.

Abstract Background stars observed through a circumstellar disk provide valuable benchmarks for investigating the disk’s extinction properties. The HD 163296 system is an excellent case study due to its large disk, the clearly visible extinction effects in JWST/NIRCam data, and the presence of numerous background sources within or around its disk. We present the measured contrasts and astrometry of sources surrounding HD 163296 from Cycle 1 JWST/NIRCam coronagraphic observations, which will serve as a useful reference for future studies of the disk’s extinction characteristics.

Abstract We present results from coordinated multiwavelength observations of the microquasar SS 433, obtained with XRISM, optical telescopes (Seimei, LCO, Tomo-e Gozen, MITSuME), and the kSIRIUS near-infrared camera during 2024 April and 2025 March. The XRISM exposures amounted to ${\sim}200$ ks in 2024 and ~100 ks in 2025, corresponding to different combinations of orbital and precessional phases. With XRISM/Resolve’s high spectral resolution and large effective area, we clearly resolved numerous emission lines even in short time segments, achieving improved accuracy in Doppler-shift measurements relative to earlier observations. The simultaneously obtained X-ray and optical Doppler shifts suggest a possible tendency for the optical emission to lag slightly behind the X-rays. In the Resolve data, the Doppler shifts of the two jet components exhibited apparent asymmetries, with jet speeds fluctuating around ${\sim}0.26 \pm 0.01c$ in 2024 and ${\sim}0.30 \pm 0.01c$ in 2025. The velocity variations indicated modulations on a timescale of ~6.3 d, with a phase offset of about $-90^{\circ }$ relative to the nutation cycle. The observed line widths and flux of the approaching and receding jets appear consistent with the expected geometrical effects, indicating systematically larger line widths in the inner regions of the jets, as proposed by Shidatsu et al. (2025, PASJ, 77, 1313). Optical light curves show flares of ~400 s in 2024 and ~1600 s in 2025, with amplitudes up to ~15% during out-of-eclipse intervals, while the XRISM/Xtend light curves show no significant variability within the overlapping intervals and given the statistical uncertainties. Near-infrared photometry in 2024, obtained during an out-of-eclipse interval at a different epoch from the optical observations, showed no flare-like variability, and the X-ray band also remained constant within uncertainties. These coordinated observations provide a foundation for future XRISM studies aimed at probing the dynamical properties of the relativistic jets in SS 433.

Abstract Complete catalogs of molecular clouds in the Milky Way allow analysis of the molecular medium and the star formation properties of the Milky Way that closely follows the method used for nearby galaxies. We explore whether the big dip in the radial distribution of molecular gas in the Milky Way is peculiar and find several other galaxies with similar patterns, all with similar morphological classifications of YClxxGnR, indicating a clearly defined, long bar leading to a grand-design spiral. This category is fairly rare among galaxies in the PHANGS sample, but all galaxies with this classification have some evidence for dips in the radial distribution of CO emission. The lengths of the bars correlate with the extents of the dips. The Milky Way and the other galaxies with dips have similar stellar masses and star formation rates, both lying near the high ends of the distributions for all PHANGS galaxies.

Abstract We investigate the outcomes of collisions between Mars-sized bodies through smooth particle hydrodynamics simulations, focusing on the transitions among merging, hit-and-run, and catastrophic disruption. By systematically varying impact velocity, angle, and mass ratio, we characterize the dependence of collision outcomes on geometric and energetic parameters. A new analytic model is developed using characteristic energies—particularly the energy deposited in overlapping regions of the colliding bodies—to accurately describe the mass of the largest and second largest remnants. The model successfully reproduces simulation results across a broad range of impact conditions and improves on previous models by better capturing the transitions between merging, hit-and-run, and disruption. We also derive outcome formulas averaged over impact-parameter-weighted angular distributions, enabling more realistic applications to integrated modeling of planet formation. The model further shows consistency with outcomes from dust aggregate collision simulations, highlighting its utility for modeling collisional processes not only for large planetesimals but also for smaller bodies.

Abstract We introduce a new photometric catalog of RR Lyrae (RRL) variables (∼300,000) mainly based on data available in public datasets. We also present the largest and most homogeneous spectroscopic dataset of RRLs and blue horizontal branch (BHB) stars ever collected. This includes radial velocity measurements (∼16,000) and iron abundances (Δ S method for 8140 RRLs, plus 547 from literature). Elemental abundances based on high-resolution spectra are provided for 487 RRLs and 64 BHB stars. We identified candidate RRLs associated with the main Galactic components and their iron distribution function (IDF) becomes more metal rich when moving from the halo ([Fe/H] = −1.56) to the thick disk (TCD; [Fe/H] = −1.47) and thin disk (TND; [Fe/H] = −0.73). Furthermore, halo RRLs and RRLs in retrograde orbits are α enhanced ([ α /Fe]=0.27, σ = 0.18), while TCD RRLs are either α enhanced ([Fe/H] ≤ −1.0) or α poor ([Fe/H] &gt; −1.0), and TND RRLs are mainly α poor ([ α /Fe] = −0.01, σ = 0.20). We also identified RRLs associated with the main stellar streams—Gaia–Sausage–Enceladus (GSE); Sequoia, Helmi, and Sagittarius—and we found that their IDFs are quite similar to halo RRLs. However, GSE RRLs lack the metal-poor/metal-rich tails and their α -element distribution is quite compact. The iron radial gradient in Galactocentric distance for TND, TCD, and halo RRLs is negative and it decreases from −0.026, to −0.010, and to −0.002 dex kpc −1 . The iron radial gradient based on dry halo (halo without substructures) RRLs is, within the errors, equal to the global halo. We also found a strong similarity between iron and [ α /Fe] radial gradients of Milky Way RRLs and M31 globular clusters throughout the full range of galactocentric distances covered by the two samples.