arXiv subjects

Kohei Miyakawa

Publications and source records attributed to Kohei Miyakawa.

10 recordsLinked to original sources

arXiv2026

DOLPHIN (Device of Line Photometry in Nasmyth): A Switchable Multiband Photometer for the ISAS 1.3-m Telescope

We present the design, construction, and commissioning of DOLPHIN (Device of Line Photometry in Nasmyth), a compact instrument for switchable broad- and narrow-band photometry at the Nasmyth focus of the ISAS 1.3-m telescope. DOLPHIN was developed as a cost-conscious platform for differential measurements of selected spectral features with modest-aperture telescopes. A motorized eight-position filter wheel provides access to Johnson-Bessell B, V, and R bands and narrow-band Na D and H$α$ filters. All filters share a common optical path and CMOS detector, and adjacent positions can be switched in approximately 0.8 s. The system uses commercially available components and provides an approximately 10-arcmin field of view. Laboratory characterization shows that the detector response is linear to within 0.1% below 10,000 ADU, with a conversion factor of 0.81 $e^-$/ADU, read noise of 9 $e^-$, and pixel-response non-uniformity of 0.37%. We also measure the in-system transmission profiles of all filters and identify a broader-than-specified Na D passband. Commissioning observations of HD 189733 are used to validate filter-sequence operation, image calibration, and relative photometric stability. The sequence used 1-s exposures in B, V, and R and 20-s exposures in Na D and H$α$. The unbinned RMS ranges from 0.34% in Na D to 0.94% in B; after eight-point binning, the Na D/R ratio reaches 0.16%. Positive inter-band correlations that increase with bin size demonstrate removal of part of the low-frequency common-mode noise. Recovery of the known broadband transit provides an end-to-end validation of the observing and reduction system. These results establish DOLPHIN as a practical platform for differential narrow-band monitoring and define the instrumental limitations to be addressed in future observations.

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arXiv2026

The NUV transit of XO-3 b

Near-UV (NUV) measurements of exoplanet transits offer a means to probe atmospheric escape, cloud formation, and planetary magnetic fields. We examine a 2024 XMM-Newton Optical Monitor NUV observation of the transit of XO-3~b, a massive hot Jupiter on an eccentric orbit with a previously observed abnormally large NUV-absorbing atmosphere. We analyze this NUV data jointly with a concurrent ground-based optical observation and all TESS transit observations, and find a NUV transit depth of $R_{p,NUV}/R_{\star} = 0.1371^{+0.016}_{-0.019}$, which is 30-70% deeper than the optical transit. Although the optical transits do not show signs of transit timing variations, the transit center in the NUV is $22^{+13}_{-11}$ minutes late compared to the optical ephemeris. We investigate atmospheric escape as a potential explanation of the properties of this NUV transit by examining X-ray data from XMM-Newton, characterizing the X-ray luminosity of XO-3 for the first time and estimating an extremely small mass-loss rate of $\sim10^4$ g/s ($\sim10^{-19}$ M$_{\text{jup}}$/yr). Finally, we investigate the likelihood of an NUV-absorbent bow-shock by estimating the magnetic field of the planet. While such a mechanism is capable of producing NUV transit offsets on the order of tens of minutes, our analytic approximations predict an early rather than late transit, indicating a need for further magnetohydrodynamic simulations.

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arXiv2024

JASMINE image simulator for high-precision astrometry and photometry

JASMINE is a Japanese planned space mission that aims to reveal the formation history of our Galaxy and discover habitable exoEarths. For these objectives, the JASMINE satellite performs high-precision astrometric observations of the Galactic bulge and high-precision transit monitoring of M-dwarfs in the near-infrared (1.0-1.6 microns in wavelength). For feasibility studies, we develop an image simulation software named JASMINE-imagesim, which produces realistic observation images. This software takes into account various factors such as the optical point spread function (PSF), telescope jitter caused by the satellite's attitude control error (ACE), detector flat patterns, exposure timing differences between detector pixels, and various noise factors. As an example, we report a simulation for the feasibility study of astrometric observations using JASMINE-imagesim. The simulation confirms that the required position measurement accuracy of 4 mas for a single exposure of 12.5-mag objects is achievable if the telescope pointing jitter uniformly dilutes the PSF across all stars in the field of view. On the other hand, the simulation also demonstrates that the combination of realistic pointing jitter and exposure timing differences in the detector can significantly degrade accuracy and prevent achieving the requirement. This means that certain countermeasures against this issue must be developed. This result implies that this kind of simulation is important for mission planning and advanced developments to realize more realistic simulations help us to identify critical issues and also devise effective solutions.

astro-ph.IM↗

arXiv2024

JASMINE: Near-Infrared Astrometry and Time Series Photometry Science

Japan Astrometry Satellite Mission for INfrared Exploration (JASMINE) is a planned M-class science space mission by the Institute of Space and Astronautical Science, the Japan Aerospace Exploration Agency. JASMINE has two main science goals. One is the Galactic archaeology with Galactic Center Survey, which aims to reveal the Milky Way's central core structure and formation history from Gaia-level (~25 $μ$as) astrometry in the Near-Infrared (NIR) Hw-band (1.0-1.6 $μ$m). The other is the Exoplanet Survey, which aims to discover transiting Earth-like exoplanets in the habitable zone from NIR time-series photometry of M dwarfs when the Galactic center is not accessible. We introduce the mission, review many science objectives, and present the instrument concept. JASMINE will be the first dedicated NIR astrometry space mission and provide precise astrometric information of the stars in the Galactic center, taking advantage of the significantly lower extinction in the NIR. The precise astrometry is obtained by taking many short-exposure images. Hence, the JASMINE Galactic center survey data will be valuable for studies of exoplanet transits, asteroseismology, variable stars and microlensing studies, including discovery of (intermediate mass) black holes. We highlight a swath of such potential science, and also describe synergies with other missions.

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arXiv2023

An Earth-sized Planet around an M5 Dwarf Star at 22 pc

We report on the discovery of an Earth-sized transiting planet ($R_p=1.015\pm0.051\,R_\oplus$) in a $P=4.02$ day orbit around K2-415 (EPIC 211414619), an M5V star at 22 pc. The planet candidate was first identified by analyzing the light curve data by the K2 mission, and is here shown to exist in the most recent data from TESS. Combining the light curves with the data secured by our follow-up observations including high-resolution imaging and near infrared spectroscopy with IRD, we rule out false positive scenarios, finding a low false positive probability of $2\times 10^{-4}$. Based on IRD's radial velocities of K2-415, which were sparsely taken over three years, we obtain the planet mass of $3.0\pm 2.7\,M_\oplus$ ($M_p<7.5\,M_\oplus$ at $95\,\%$ confidence) for K2-415b. Being one of the lowest mass stars ($\approx 0.16\,M_\odot$) known to host an Earth-sized transiting planet, K2-415 will be an interesting target for further follow-up observations, including additional radial velocity monitoring and transit spectroscopy.

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arXiv2022

Color Dependence of the Transit Detectability for Young Active M-dwarfs

We investigate the planetary transit detectability in the presence of stellar rotational activity from light curves for young M-dwarfs and estimate improvements of the detection at near-infrared (NIR) wavelengths. Making maps of the transit signal detection efficiency over the orbital period and planetary radius with light curves of members of four clusters, Hyades, Praesepe, Pleiades, and Upper Scorpius observed by the K2 mission, we evaluate the detectability for the rotation period and modulation semi-amplitude. We find that the detection efficiency remarkably decreases to about 20% for rapidly rotators with P_{rot} <= 1 d and the lack of planets in Pleiades is likely due to the high fraction of rapidly rotating M-dwarfs. We also evaluate the improvements of the planet detection with NIR photometry via tests using mock light curves assuming that the signal amplitude of stellar rotation decreases at NIR wavelengths. Our results suggest that NIR photometric monitoring would double relative detection efficiency for transiting planetary candidates with P_{rot} <= 1 d and find planets around M-dwarfs with approximately 100 Myr missing in the past transit surveys from the space.

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arXiv2021

Wavelength Dependence of Activity-Induced Photometric Variations for Young Cool Stars in Hyades

We investigate photometric variations due to stellar activity which induce systematic radial-velocity errors (so-called "jitter") for the four targets in the Hyades open cluster observed by the K2 mission (EPIC 210721261, EPIC 210923016, EPIC 247122957, and EPIC 247783757). Applying Gaussian process regressions to the K2 light curves and the near-infrared (NIR) light curves observed with the IRSF 1.4-m telescope, we derive the wavelength dependences of the photometric signals due to stellar activity. To estimate the temporal variations in the photometric variability amplitudes between the two observation periods of K2 and IRSF, separated by more than 2 years, we analyze a number of K2 targets in Hyades that have also been observed in Campaigns 4 and 13 and find a representative variation rate over 2 years of 38%pm71%. Taking this temporal variation into account, we constrain projected sizes and temperature contrast properties of the starspots in the stellar photosphere to be approximately 10% and 0.95, respectively. These starspot properties can induce relatively large differences in the variability amplitude over different observational passbands, and we find that radial-velocity jitter may be more suppressed in the NIR than previously expected. Our result supports profits of on-going exoplanet search projects that are attempting to detect or confirm young planets in open clusters via radial-velocity measurements in the NIR.

astro-ph.SR↗

arXiv2021

Two Bright M Dwarfs Hosting Ultra-Short-Period Super-Earths with Earth-like Compositions

We present observations of two bright M dwarfs (TOI-1634 and TOI-1685: $J=9.5-9.6$) hosting ultra-short period (USP) planets, identified by the TESS mission. The two stars are similar in temperature, mass, and radius ($T_\mathrm{eff}\,\approx\,3500$ K, $M_\star\,\approx\,0.45-0.46\,M_\odot$, and $R_\star\approx 0.45-0.46\,R_\odot$), and the planets are both super-Earth-sized ($1.25\,R_\oplus<R_p<2.0\,R_\oplus$). For both systems, light curves from the ground-based photometry exhibit planetary transits, whose depths are consistent with those by the TESS photometry. We also refine the transit ephemerides based on the ground-based photometry, finding the orbital periods of $P=0.9893436\pm0.0000020$ day and $P=0.6691416\pm0.0000019$ day for TOI-1634b and TOI-1685b, respectively. Through intensive radial velocity (RV) observations using IRD on the Subaru 8.2m telescope, we confirm the planetary nature of the TOIs, and measure their masses: $10.14\pm0.95\,M_\oplus$ and $3.43\pm0.93\,M_\oplus$ for TOI-1634b and TOI-1685b, respectively, when the observed RVs are fitted with a single-planet circular-orbit model. Combining those with the planet radii of $R_p=1.749\pm 0.079\,R_\oplus$ (TOI-1634b) and $1.459\pm0.065\,R_\oplus$ (TOI-1685b), we find that both USP planets have mean densities consistent with an Earth-like internal composition, which is typical for small USP planets. TOI-1634b is currently the most massive USP planet in this category, and it resides near the radius valley, which makes it a benchmark planet in the context of discussing the size limit of rocky planet cores as well as testing the formation scenarios for USP planets. Excess scatter in the RV residuals for TOI-1685 suggests the presence of a possible secondary planet or unknown activity/instrumental noise in the RV data, but further observations are required to check those possibilities.

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arXiv2021

Joint Analysis of Multicolor Photometry: A New Approach to Constrain the Nature of Multiple-Star Systems Hosting Exoplanet Candidates

We present a new method to assess the properties of transiting planet candidates by multicolor photometry. By analyzing multicolor transit/eclipse light curves and apparent magnitudes of the target in parallel, this method attempts to identify the nature of the system and provide a quantitative constraint on the properties of unresolved companion(s). We demonstrate our method by observing the six systems hosting candidate transiting planets, identified by the K2 mission (EPIC 206036749, EPIC 206500801, EPIC 210513446, EPIC 211800191, EPIC 220621087, and EPIC 220696233). Applying our analysis code to the six targets, we find that EPIC 206036749, EPIC 210513446, and EPIC 211800191 are likely to be triple-star systems including eclipsing binaries, and EPIC 220696233 is likely a planetary system, albeit further observations are required to confirm the nature. Additionally, we confirm that the systematic errors in the derived system parameters arising from adopting specific isochrone models and observing instruments (passbands) are relatively small. While this approach alone is not powerful enough to validate or refute planet candidates, the technique allows us to constrain the properties of resolved/unresolved companions, and prioritize the planet candidates for further follow-up observations (e.g., radial-velocity measurements).

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arXiv2018

Exoplanets around Low-mass Stars Unveiled by K2

We present the detection and follow-up observations of planetary candidates around low-mass stars observed by the K2 mission. Based on light-curve analysis, adaptive-optics imaging, and optical spectroscopy at low and high resolution (including radial velocity measurements), we validate 16 planets around 12 low-mass stars observed during K2 campaigns 5-10. Among the 16 planets, 12 are newly validated, with orbital periods ranging from 0.96-33 days. For one of the planets (K2-151b) we present ground-based transit photometry, allowing us to refine the ephemerides. Combining our K2 M-dwarf planets together with the validated or confirmed planets found previously, we investigate the dependence of planet radius $R_p$ on stellar insolation and metallicity [Fe/H]. We confirm that for periods $P\lesssim 2$ days, planets with a radius $R_p\gtrsim 2\,R_\oplus$ are less common than planets with a radius between 1-2$\,R_\oplus$. We also see a hint of the "radius valley" between 1.5 and 2$\,R_\oplus$ that has been seen for close-in planets around FGK stars. These features in the radius/period distribution could be attributed to photoevaporation of planetary envelopes by high-energy photons from the host star, as they have for FGK stars. For the M dwarfs, though, the features are not as well defined, and we cannot rule out other explanations such as atmospheric loss from internal planetary heat sources, or truncation of the protoplanetary disk. There also appears to be a relation between planet size and metallicity: those few planets larger than about 3 $R_\oplus$ are found around the most metal-rich M dwarfs.

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