arXiv subjects

Motohide Tamura

Publications and source records attributed to Motohide Tamura.

At least 19 recordsLinked to original sources

arXiv2026

B-fields In STar-forming Region Observations (BISTRO): Technical Foundations for Quantitative 450/850 $μ$m JCMT SCUBA-2/POL-2 Polarimetry

We present five technical validations of SCUBA-2/POL-2 polarimetry on the James Clerk Maxwell Telescope at 450 and 850 $μ$m, using 174 archival fields in 69 molecular cloud regions and subsets appropriate to each test. Extended-source calibration avoids a deterministic shift of $Δβ=-1.05$ in the dust emissivity index $β$ that would result from calibrating extended emission using peak brightness. We modeled Stokes $I$ noise with additive and signal-dependent components. The measured median ratio of surface-brightness noise at 450/850 $μ$m is 25. Independent tests of Stokes $Q,U$ noise support approximately equal additive variances and weak error correlations, providing a basis for propagating polarization uncertainties. Corrections determined for individual fields reduce additive zero-level offsets of order $1$ mJy arcsec$^{-2}$ in the 450 $μ$m Stokes $Q$ and $U$ intensities. The three-step regression leaves a median combined residual amplitude of $0.07$ mJy arcsec$^{-2}$. Independent comparisons with SOFIA/HAWC+ at 154 and 214 $μ$m support the correction in strongly affected fields. We matched the 450 $μ$m maps to the modeled 850 $μ$m beam and the common $12.6''$-$300''$ range of angular scales. We compared three reductions of the same 450 $μ$m observations across 30 sources. Taking the median angular residual relative to the final product for each source, we find that beam matching reduces the median over sources from $6.3^\circ$ to $4.7^\circ$. After matching, 29 of the 30 sources have median residuals below $10^\circ$. We also quantified how residual $Q/U$ offsets propagate in ratios of polarization fractions, differences in spectral indices, and polarization angles. These validations provide procedures and quantitative references for interpreting POL-2 measurements and combining them with polarimetry at other wavelengths.

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arXiv2026

B-fields In STar-forming Region Observations (BISTRO): Dual-Wavelength Constraints on Size-Dependent Grain Alignment in Star-Forming Regions

We compared 450 and 850 $μ$m POL-2 observations from the James Clerk Maxwell Telescope at $12.6''$ resolution. The median absolute difference in polarization angle between bands is $6.7^\circ$ across 21 regions. For total intensity $I\proptoν^α$ and polarized intensity $PI\proptoν^γ$, 24-27 of 35 regions have $γ<α$, allowing for $3σ$ limits on weak polarized intensities. The median $γ-α$ is $-0.51$, ranging from $-0.57$ to $-0.45$ under these limits. The ratio of polarization fractions is $P_{450}/P_{850}=0.69\pm0.02_{\rm sys}\pm0.03_{\rm stat}$, with no detected dependence on the angle difference. We suggest that the dependence of alignment on grain size offers the simplest explanation of these findings. For optically thin components sharing a polarization angle and opacity law, temperature mixing reproduces the median $-0.51$ only if the warm dust is polarized less efficiently than the cold dust. If the cold dust is at 10 K or warmer and the warm dust at 40 K or cooler, the median requires a ratio of warm to cold effective polarization efficiencies $r\lesssim0.18$. We tested mixtures with opacity index $β=2$ in four fields against Herschel dust temperatures and Green Bank Ammonia Survey kinetic temperatures. We assumed equal gas and dust temperatures, ammonia tracing cold gas, and model ranges of 3-20 K for cold dust and 25-40 K for warm dust carrying at most half the mass. Mixtures matching both temperatures cannot reproduce $γ-α$ within $1σ$ in three fields, including OMC 2 tentatively, even for $r=0$. Allowing warm dust at 10-100 K and up to 95% of the mass permits solutions only for $r\leq0.041$. To explain these three fields by temperature mixing, we need wider model ranges and a very strong contrast in polarization efficiency. The origin of this contrast remains undetermined.

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arXiv2026

Testing KMTNet--PRIME Optical--Near-Infrared Source-Color Constraints in KMT-2024-BLG-0211 and KMT-2024-BLG-1522

We present analyses of two 2024 microlensing events, KMT-2024-BLG-0211 and KMT-2024-BLG-1522, jointly observed by KMTNet in the optical and PRIME in the near-infrared. For KMT-2024-BLG-0211, the KMTNet--PRIME $I-H$ color provides a useful constraint on the angular source radius for a short-timescale finite-source event with a giant source. The event is consistent with a low-mass stellar lens, although the weak parallax constraint leaves the lens mass and distance uncertain. For KMT-2024-BLG-1522, we compare the results obtained from $V-I$, $I-H$, $V-H$, and $J-H$ color constraints. The microlensing parameters are nearly identical among the four analyses, yielding a robust binary-lens solution with nearly equal masses. However, the inferred source properties and lens physical parameters depend on the adopted source color because the different color estimates imply different angular source radii. Comparing the $(V-I)_{\rm KMT}$ versus $(I-H)_{\rm KMT,PRIME}$ plane shows that the inferred source colors lie off empirical color--color relations but within the scatter of observed stars. The prevalence of such deviations should be tested with a larger sample of KMTNet--PRIME events and this specific case can be further tested with future adaptive-optics follow-up, which can directly measure the lens--source relative proper motion and lens flux.

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arXiv2026

Resolved Structure and Orbital Motion of a Localized 1 au-scale Dust Accumulation in the Protoplanetary Disk around TW Hya

We present the results of high-resolution ($\sim1$~au) Atacama Large Millimeter/submillimeter Array (ALMA) observations of the TW~Hya protoplanetary disk in the Band 6 dust continuum, as well as the \ce{^13CO} and \ce{C^18O} J=2--1 emission lines. The primary focus of this study is to investigate the kinematics, internal morphology, and local gas environment of the prominent dust blob at a radius of 52~au. By comparing our 2021 data with archival observations from 2017, we detect the proper motion of the blob. The measured azimuthal velocity of 3.3$\pm$0.9~km~s$^{-1}$ is fully consistent with local Keplerian rotation. Combined with the lack of significant radial migration over the four-year baseline, this confirms that the structure is robustly co-moving with the disk system. Crucially, our high-resolution continuum map resolves the blob into a distinct double-peaked morphology separated by $\sim$1.7~au azimuthally. We robustly validate this double-peaked substructure by reproducing it in the independent 2017 dataset using a sparse-modeling image reconstruction technique. We discuss potential physical origins for this double-peaked morphology, including an inclined circumplanetary disk with an inner dust cavity, the roots of planet-induced spiral arms, or alternative hydrodynamic scenarios that do not involve an actively accreting planet such as the U-turn trajectory of secondary dust or a short-lived hydrodynamic gas vortex. We detect no compact gas emission counterparts associated with the continuum blob. Since these CO lines likely trace optically thick upper atmospheric layers, the absence of localized vertical gas perturbations suggests that if an embedded planet is responsible for the dust structure, its mass must be exceptionally low.

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arXiv2026

Near-infrared spectra of chemically peculiar stars and applications to kilonova spectra

Kilonova spectra provide information on rapid neutron-capture nucleosynthesis in neutron star mergers. However, it remains challenging to identify individual elements in kilonova spectra, primarily due to incomplete atomic data for heavy elements at near-infrared wavelengths. Recent work demonstrated that spectra of chemically peculiar stars offer excellent laboratory for studying kilonova spectra. In this paper, to expand our understanding of strong transitions of heavy elements at near-infrared wavelengths, we present high-resolution near-infrared spectra of 14 chemically peculiar stars covering a range of effective temperatures, surface gravities, and abundance patterns, obtained with Subaru/IRD. The Sr II, Ce III, and Gd III lines, previously suggested to be among the strongest transitions, are detected in the spectra of most of our targets, confirming their intrinsic importance. In addition, Eu II and Nd III lines are detected in several stars. By comparing observed equivalent widths and abundance ratios, we show that under a highly heavy-element-enriched abundance pattern, Eu II and Nd III lines may contribute to kilonova spectral features comparably to Sr II. Importantly, we find no unidentified lines systematically stronger than these transitions despite the larger stellar sample. These results further support the primary importance of the Sr II and Ce III transitions and provide new insights into the potential contribution of Eu II and Nd III.

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arXiv2026

The Nancy Grace Roman Space Telescope Coronagraph Community Participation Program

In preparation for the operational phase of the Nancy Grace Roman Space Telescope, NASA has created the Coronagraph Community Participation Program (CPP) to prepare for and execute Coronagraph Instrument technology demonstration observations. The CPP is composed of 7 small, US-based teams, selected competitively via the Nancy Grace Roman Space Telescope Research and Support Participation Opportunity, members of the Roman Project Team, and international partner teams from ESA, JAXA, CNES, and the Max Planck Institute for Astronomy. The primary goals of the CPP are to prepare simulation tools, target databases, and data reduction software for the execution of the Coronagraph Instrument observation phase. Here, we present the current status of the CPP and its working groups, along with plans for future CPP activities up through Roman's launch. We also discuss plans to potentially enable future commissioning of currently-unsupported modes.

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arXiv2026

KMT-2026-BLG-0083L: A Two-Jovian-Planet System Orbiting an M Dwarf Discovered by Microlensing

We present the analysis of the microlensing event KMT-2026-BLG-0083, which was independently detected by the KMTNet, OGLE, and PRIME surveys and also monitored by the DREAMS survey. The combined data provide dense coverage of two distinct short-duration anomalies in the light curve that cannot be reproduced by a standard binary-lens model. Independent analyses of the two anomalies indicate that each is produced by a planetary companion to the lens, motivating a triple-lens interpretation. The modeling yields two pairs of degenerate solutions arising from the well-known inner--outer degeneracy, with each pair exhibiting two local solutions depending on whether the source passes above or below the distant planetary companion. The preferred model indicates two giant planets with mass ratios $q_2=(5.25\pm 0.08)\times 10^{-3}$ and $q_3=(3.04\pm 0.24)\times10^{-3}$ orbiting a common host. Bayesian analysis indicates that the host is an M-dwarf star with a mass of $0.48^{+0.36}_{-0.28}~M_\odot$, hosting two giant planets with masses of $2.65^{+1.96}_{-1.55}~M_{\rm J}$ and $1.54^{+1.14}_{-0.90}~M_{\rm J}$. The projected planet--host separations are $10.6^{+1.7}_{-2.2}$~au and $1.7^{+0.3}_{-0.3}$~au, placing the inner planet near the host's snow line and the outer planet at a substantially larger separation. Thus, KMT-2026-BLG-0083L becomes the seventh confirmed multiple-planet system discovered through gravitational microlensing, providing another example of a cold giant planetary system orbiting a subsolar-mass star.

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arXiv2026

A Search for helium in the atmospheres of three sub-Neptunes and a super-Earth around M-dwarfs

Thousands of sub-Neptunes have been discovered mainly through space-based surveys such as Kepler and TESS. Their bulk compositions and internal structures are thought to reflect their formation and evolutionary pathways, and atmospheric observations provide constraints on these processes. The near-infrared helium triplet is a potential tracer of extended, escaping H/He atmospheres. Recent models that include geometric effects suggest that planets orbiting nearby late M dwarfs may offer favorable conditions for detecting this signal. Nevertheless, helium has been reported for only three planets around M dwarfs to date. We conducted high-resolution transmission spectroscopy of three sub-Neptunes (TOI-2136b, TOI-654b, and LP 791-18c) and a super-Earth (TOI-1634b) orbiting M dwarfs with the InfraRed Doppler (IRD) spectrograph on the Subaru Telescope. We find no statistically significant helium absorption in any target; accordingly, we derive 95% confidence upper limits on the helium line depth of 1.36%, 0.60%, 2.07%, and 3.00%, and on the equivalent width of 7.3, 2.1, 7.4, and 9.1 mÅ, for TOI-2136b, TOI-1634b, TOI-654b, and LP 791-18c, respectively. We further explored constraints on the upper-atmospheric temperature and mass-loss rate by comparing these results with isothermal Parker-wind models. While we have compared with self-consistent ATES models of primordial H/He atmospheres spanning a range of assumed X-ray luminosities, changes in the assumed XUV flux do not appear to account for the non-detections. The results suggest that these planets have metal-enriched H/He primary atmospheres or non-primordial atmospheres, such as water-rich envelopes. Future observations of other absorption lines, such as Lyman-$α$, H-$α$, and H$_2$O, may provide further constraints on these atmospheres.

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arXiv2026

The Roman Coronagraph Community Participation Program: calibration strategy for the Mueller matrix using on-sky sources

The Nancy Grace Roman Space Telescope Coronagraph Instrument will provide space-based polarimetric observations of circumstellar disks and exoplanetary systems. Accurate reconstruction of the linear polarization fraction requires calibration of the instrumental Mueller matrix using polarized and weakly polarized standard stars. We constructed a candidate catalog by combining published optical polarimetry with Gaia DR3 astrometry and photometry and selected separate samples for coronagraphic calibration observations and observations using a neutral-density filter. Precursor $VRI$-band polarimetry of 18 faint candidates was obtained with HONIR on the 1.5-m Kanata telescope. The wavelength dependence of their normalized Stokes parameters was modeled using the Serkowski law to predict their polarization properties in \cgi\ Bands~1 and 4, and 2 sets of 3 calibrators were selected for the 2 calibration scenarios. We then estimated the achievable LPF reconstruction accuracy using Monte Carlo simulations that include uncertainties in the calibrator polarization properties, photometric noise, and residual detector-response errors. A dithered observing configuration was also simulated to reduce differential detector-response errors among the calibrators. The current estimates indicate LPF reconstruction errors at the few-percentage-point level, with a small bias arising from treating a weakly polarized calibrator as unpolarized. Finally, we present progress toward an end-to-end test using \texttt{corgisim} and \texttt{corgiDRP}, including successful processing of simulated datasets from Level~1 through Level~2b.

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arXiv2026

Discovery of an Eccentric Hot Super-Jupiter Leaving the Transiting Geometry of the Early-A-type star TOI-1355

Hot Jupiters orbiting hot stars ($T_\mathrm{eff} > 7000$ K) are suggested to have experienced high-eccentricity migration, often evidenced by the tendency for misaligned orbits, despite their circular orbits. In this paper, we present the discovery of TOI-1355 b: an eccentric ($e\sim0.22$) hot Jupiter with a mass of $m_{\mathrm{p}}\sim5.8M_J$ and a radius of $R_{\mathrm{p}}\sim 1.4R_J$ orbiting an A-type star with a period of about $2.17$ days, identified from the TESS transit survey and subsequent follow-up observations. We measured the stellar parameters using the data from the high-resolution spectrograph Seimei/GAOES-RV and obtained the planetary parameters from the photometric data acquired by TESS and ground-based telescopes. This is one of the rare eccentric hot Jupiters around hot stars. This system could be undergoing high-eccentricity migration. We detected nodal precession by measuring the change in its impact parameter. This implies that its transit will no longer be observable from the middle of 2033. Nevertheless, TOI-1355 b is anticipated to be a compelling target for future atmospheric observations, given the hint of atmospheric variability detected in this study.

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arXiv2026

The Roman Coronagraph Community Participation Program: Observation planning and data reduction for polarimetric mode

Reflected-light polarimetry of exoplanets constrains and resolves degeneracies in atmospheric properties, while polarized light observations of debris disks enable the characterization of dust-grain properties. The best-effort polarimetric mode of the Roman Coronagraph Instrument will be able to perform multi-wavelength observations of planetary systems using both the Hybrid Lyot Coronagraph (HLC) and the Shaped Pupil Coronagraph (SPC). This paper presents an overview of observation planning, simulations, and data reduction procedures for the polarimetric mode of the Roman Coronagraph. As an initial test of simulation and data reduction, a dataset of polarimetric observing sequences for the debris disk HD 172555 in HLC mode was generated using corgisim with estimated observation parameters, and data reduction was performed using corgidrp, incorporating all relevant noise factors and calibration products. Currently, mock calibration products are used in corgidrp; these will be replaced with simulated calibration products in future updates

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arXiv2026

Discovery of an Unbound Flyby Companion of UBC 63: In the Immediate Aftermath of a Close Encounter

We re-investigate the open cluster UBC 63 using the Gaia DR3 data and show that, rather than being a single cluster as previously classified, it is a compelling candidate for a double cluster undergoing an unbound flyby interaction. A GMM decomposition performed in the 5D astrometric space reveals the two statistically distinct components, namely UBC 63A (98 members, Age = 21 $\pm$ 4 Myr) and UBC 63B (148 members, Age = 562 $\pm$ 43 Myr). A significant age difference of $Δ\mathrm{Age} = 541 \pm 43$ Myr between the clusters, rules out coeval formation. Their 3D separation of $60 \pm 29$ pc at the birth-epoch of the younger cluster, indicates that the clusters might have originated from the same molecular cloud complex. At present, the system exhibits a 3D separation of $26 \pm 8$ pc, with a relative velocity of $3.60 \pm 1.80$ km s$^{-1}$. Orbital integrations and \textit{N}-body simulations of the pair suggest that the systems had a close encounter, reaching a separation of $7 \pm 2$ pc only $\sim$~6 Myr ago and predict a rapid divergence to a separation of $491 \pm 213$ pc within the next $\sim$100 Myr. The low escape velocity ($V_{\rm esc} = 0.51 \pm 0.12$ km s$^{-1}$) of the system compared to the relative 3D velocity indicates that they are gravitationally unbound. Their low tidal factors, elongated structures and populations extending beyond the Jacobi radii may reflect a strong transient tidal interaction between the clusters.

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arXiv2026

Revealing α-Element's Past with Subaru/IRD: Oxygen Abundance of 35 Very Metal-Poor Stars from Near-IR OH lines

Oxygen abundances in very and extremely metal-poor (V/EMP) stars provide critical constraints on early massive stars' nucleosynthesis. An Oxygen abundance analysis is presented for 35 V/EMP stars (-4.0<[Fe/H]< -1.5) using near-infrared H-band OH vibro-rotational lines from high-resolution Subaru/IRD spectra. To examine the reliability of these NIR OH lines, the results are compared with the abundances obtained from the 3D/NLTE-insensitive forbidden [OI] 6300Å line using archival high-resolution optical spectra. After homogeneously rederiving stellar parameters and 1D/NLTE Fe abundances using Gaia photo-astrometry and literature optical Fe equivalent width data, oxygen abundance from OH and [OI] lines is determined through 1D/LTE spectral synthesis. A sensitivity analysis confirms that near-IR OH lines are highly sensitive to the adopted temperature compared to the forbidden line. A temperature-dependent discrepancy between the tracers is identified: in cool red giants (Teff <4600 K), OH-based abundances are systematically lower than [OI]-based abundance by 0.05 to 0.25 dex, while warmer red giants show higher OH-based abundances as expected from 3D effects. Despite this systematic offset, the numerous measurable NIR OH lines yield significantly smaller random abundance errors than that of the single, weak [OI] line. Leveraging this statistical precision, an empirical calibration as a function of Teff, log g, [Fe/H], and [C/Fe] is derived to align the 1D/LTE OH abundances onto the [OI] scale. Applying this correction substantially reduces the scatter and temperature dependence in the [O/Fe] versus [Fe/H] plane and flattens the trend, bringing the results into fairly good agreement with Galactic chemical evolution models.

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arXiv2026

Four Cold Giant Planets Discovered by High-Cadence Microlensing Surveys

We report the discovery of four cold giant planets identified through the analysis of microlensing events detected by high-cadence surveys: OGLE-2016-BLG-0261, KMT-2025-BLG-0026, KMT-2025-BLG-0030, and KMT-2025-BLG-2272. The planetary signals appear as short-duration anomalies in the light curves and are well described by binary-lens single-source models with mass ratios between the lens components of order $q \sim 10^{-3}$. Finite-source effects are securely measured in three out of four events, enabling determinations of the angular Einstein radius. A Bayesian analysis incorporating the measured event timescale and angular Einstein radius yields host masses of $\sim 0.07$--$0.6~M_\odot$ and companion masses of $\sim 0.2$--$2.5~M_{\rm J}$, confirming that all companions lie in the giant-planet regime. The projected separations are ~ 0.7--6 au, placing all planets at or beyond the snow lines of their host stars. The inferred lens distances span $\sim 6.6$--$7.9$ kpc, with all systems consistent with bulge lenses. These detections expand the sample of cold giant planets from homogeneous high-cadence surveys and highlight the sensitivity of microlensing to planetary systems beyond the snow line, providing further constraints on the occurrence and properties of giant planets around low-mass stars.

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arXiv2026

Combining a Diffraction-Limited Coronagraph with Fiber Nulling: A Demonstration of Serially Coupling Different Nullers

We present experimental results of an efficient small-IWA ($\sim$1 $λ/D$) high contrast imaging approach realized by co-optimizing a coronagraph front-end with a fiber nulling 2nd stage. The setup includes the one-dimensional diffraction-limited coronagraph (1DDLC) and Parity Fiber Nuller (PFN). The 1DDLC has promising features (binary nuller, small inner working angles (IWAs)). Although the 1DDLC has the 2nd/4th-order sensitivity to spectral bandwidth and tilt aberrations, it outputs stellar leak due to wavelengths other than the design wavelength only as a flat wavefront on the Lyot-stop plane, preserving the same complex amplitude profile as an on-axis point source. The PFN after the 1DDLC erases the leak from the 1DDLC. For the wavelength 6% less than the coronagraph's design-center wavelength, we confirmed the contrast mitigation ability of $3.5\times10^{-5}$, which is about 1/20 times the value of the case with only 1DDLC, suggesting that the combined system works robustly against the broad spectral bandwidth. Future work needs to address the demonstration of the anticipated broadband robustness for the contrast level lower than about $10^{-5}$.

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arXiv2026

Japan's Possible Contributions for Coronagraph of the Habitable Worlds Observatory (HWO)

In this paper, we describe Japan's possible contributions for coronagraph of the Habitable Worlds Observatory (HWO) based on our independent study. We are considering to contribute to the HOW coronagraph by science and hardware, based on Japan's experience for the SPICA coronagraph instrument, contributions to the Nancy Grace Roman Space Telescope, and SCExAO for the Subaru telescope. Currently, hardware contributions of various scales, from large-scale to small components, are considered. As an example of the large-scale hardware case, the optical and mechanical layout of the entire infrared coronagraph is presented. Several individual high-contrast technologies are also briefly introduced, for which research is ongoing in Japan. In discussions, it is pointed out that both the inner working angle (IWA) and sensitivity are particularly critical for the NIR coronagraph. In this situation, dedicated observations of a small number of targets close to the solar system can be one of key science program in this situation, and designing consolidating science objectives, requirements, observation targets, and survey plans is important. It is essential to push the development of advanced coronagraphs that provide small IWAs. On the other hand, it is also necessary to prepare solutions that adopt more robust coronagraphs in parallel. How to coexist visible and NIR coronagraphs within constraints of volume, mass, budget etc. is an important issue. The international sharing for the coronagraph development should be carefully decided by international agreement. Although all of our studies may not be realized in contributions to the first generation of HWO instruments, we are considering Japan's multigenerational participation in the HWO to maximize outcomes of the HWO.

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arXiv2026

EPISODE II: Variability in the CO and H$_2$O rovibrational absorption lines in a periodically variable protostar EC 53

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 ($\sim$2.3 $μ$m) and fundamental ($\sim$4.6 $μ$m) bands and the H$_2$O stretching ($\sim$2.7 $μ$m) and bending ($\sim$6.0 $μ$m) modes. We also obtained high-resolution ($R\approx45{,}000$) IGRINS spectra during the burst to constrain the CO overtone line profiles. LTE slab modeling yields gas temperatures of $\sim$1800 K (CO overtone) and $\sim$1200 K (CO fundamental), consistent with the overtone tracing hotter gas at smaller radii. The H$_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 $\sim$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 $2.9\pm0.2$ for the CO overtone and $1.71\pm0.11$ for the CO fundamental. Using a viscous-disk prescription, these imply representative accretion-rate ratios of $\sim$3.6 and $\sim$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.

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arXiv2026

The mass of TOI-1883 b: A low density super-Neptune in the ridge regime transiting an early-M dwarf

Recent large-scale transit surveys conducted by space telescopes such as Kepler and TESS have revealed a vast number of exoplanets, uncovering the diversity of their population. One of the remarkable findings is the presence of a deficiency region in the period-radius distribution of short-period (< 10 days) Neptune-sized planets (4-8 Earth radii). This region is classified into the Neptune desert (< 3.2 days), the ridge (3.2-5.7 days), and the savanna (> 5.7 days) based on orbital period, each likely reflecting distinct evolutionary pathways. In this study, we used the InfraRed Doppler (IRD) instrument on the Subaru Telescope to determine the mass of the super-Neptune TOI-1883 b, which resides in the ridge region (P ~ 4.51 days) orbiting an M dwarf. We measured a planetary mass of Mp = 13.7 +6.8/-6.5 Earth masses and a mean density of 0.4 +0.3/-0.2 g cm^-3, with 3-sigma upper limits of 34.1 Earth masses, and 5-sigma upper limits of 47.7 Earth masses. These results suggest that TOI-1883 b is likely a low density super-Neptune. We also find that the boundary of the Neptune desert defined by planets orbiting FGK-type stars exhibits a similar distribution for planets around M-type stars. According to the population-based argument of Bourrier et al. (2025), this suggests that TOI-1883 b may have undergone disk-driven migration to reach its current orbit and experienced early atmospheric photoevaporation driven by strong stellar XUV irradiation. The derived planetary mass is comparable to or exceeds the conventional critical core mass. We suggest that the high metallicity of the host star ([Fe/H] = 0.32 +/- 0.18) may have suppressed the onset of runaway gas accretion. Furthermore, TOI-1883 b has a high Transmission Spectroscopy Metric (TSM > 140), making it an excellent target for future atmospheric characterization via transmission spectroscopy.

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