arXiv subjects

Keisuke Isogai

Publications and source records attributed to Keisuke Isogai.

At least 37 records · Page 2Linked to original sources

arXiv2024

The possible long-term periodic variability of the extremely luminous quasar WISE J090924.01+000211.1

The extremely luminous infrared galaxy (ELIRG), WISE J090924.01+000211.1 (hereafter; WISE J0909+0002, $z=1.87$) is an extraordinary object with a quasar aspect. This study performs monitoring observations of WISE J0909+0002 with the 105 cm Murikabushi telescope, Okayama and Akeno 50 cm telescopes/MITSuME ($g'$, $R_{\rm c}$, and $I_{\rm c}$ bands), and the SaCRA 55 cm telescope/MuSaSHI ($r$, $i$, and $z$ bands). We obtain the following results by combining the UV/optical light curves of the CRTS, Pan-STARRS, and ZTF archive data, and our observational data: (1) the light curves of WISE J0909+0002 present quasi-periodic (sinusoidal) oscillations with the rest-frame period of $\sim$ 660$-$689 day; (2) the structure functions of WISE J0909+0002 do not show a damped random walk (DRW) trend; (3) the mock DRW light curves present periodic-like trend on rare occasions in 10000 simulations; (4) the relativistic boost scenario is favored, since the relation between variability amplitude and power-law slope ratio is consistent with the theoretical prediction of this scenario, and a substantial parameter space exists between the inclination angles and the black hole mass; (5) the circumbinary disk model is difficult to explain the spectral energy distribution of our target; (6) the significant radio flux density of WISE J0909+0002 is not detected from the VLA FIRST Survey, thus the radio jet precession scenario is ruled out. From our results, the Doppler boost scenario is likely as a cause of the periodic variability, consequently the quasi-periodic oscillations in WISE J0909+0002 is possibly interpreted by a supermassive blackhole binary. Additional observations to investigate the continuity of the periodic trend would bring new insights into mechanisms of the quasi-periodic oscillations and/or ELIRGs.

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arXiv2024

Evolution of Accretion Disk Structure of the Black Hole X-ray Binary MAXI J1820$+$070 during the Rebrightening Phase

To understand the evolution of global accretion disk structure in the ``rebrightening'' phase of MAXI J1820$+$070, we perform a comprehensive analysis of its near infrared/optical/UV to X-ray spectral energy distribution (SED) utilizing data obtained by OISTER, Las Cumbres Observatory (LCO), Swift, NICER, and NuSTAR in 2019. Optical spectra observed with Seimei telescope in 2019 and 2020 are also analyzed. On the basis of the optical and X-ray light curves and their flux ratios, we divide the whole phase into 3 periods, Periods I (flux rise), II (decay), and III (dim). In the first 2 periods, the source stayed in the low/hard state (LHS), where the X-ray (0.3--30 keV) and optical/UV SED can be both fitted with power-law models. We interpret that the X-ray emission arises from hot corona via Comptonization, whereas the optical/UV flux is dominated by synchrotron radiation from the jets, with a partial contribution from the irradiated disk. The optical/UV power-law component smoothly connects to a simultaneous radio flux, supporting its jet origin. Balmer line profiles in the optical spectra indicate that the inner radius of an irradiated disk slightly decreased from $\sim 2\times 10^5 r_{\rm g}$ (Period I) to $\sim 1\times 10^5 r_{\rm g}$ (Period II), where $r_{\rm g}$ is the gravitational radius, implying a change of the hot corona geometry. In Period III, the SED can be reproduced by an advection-dominated accretion flow and jet emission. However, the double-peaked H$α$ emission line indicates that a cool disk remained at large radii.

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arXiv2024

Smart Kanata: A Framework for Autonomous Decision Making in Rapid Follow-up Observations of Cataclysmic Variables

Studying the early stages of transient events provides crucial information about the fundamental physical processes in cataclysmic variables (CVs). However, determining an appropriate observation mode immediately after the discovery of a new transient presents challenges due to significant uncertainties regarding its nature. We developed a framework designed for autonomous decision making in prompt follow-up observations of CVs using the Kanata 1.5-m telescope. The system, named Smart Kanata, first estimates the class probabilities of variable star types using a generative model. It then selects the optimal observation mode from three possible options based on the mutual information calculated from the class probabilities. We have operated the system for ~300 days and obtained 21 samples, among which automated observations were successfully performed for a nova and a microlensing event. In the time-series spectra of the nova V4370 Oph, we detected a rapid deepening of the absorption component of the H_alpha line. These initial results demonstrate the capability of Smart Kanata in facilitating rapid observations and improving our understanding of outbursts and eruptions of CVs and other galactic transients.

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arXiv2024

Low abundances of TiO and VO on the Dayside of KELT-9 b: Insights from Ground-Based Photometric Observations

We present ground-based photometric observations of secondary eclipses of the hottest known planet KELT-9b using MuSCAT2 and Sinistro. We detect secondary eclipse signals in $i$ and $z_{\rm s}$ with eclipse depths of $373^{+74}_{-75}$ ppm and $638^{+199}_{-178}$, respectively. We perform an atmospheric retrieval on the emission spectrum combined with the data from HST/WFC3, Spitzer, TESS, and CHEOPS to obtain the temperature profile and chemical abundances, including TiO and VO, which have been thought to produce temperature inversion structures in the dayside of ultra-hot Jupiters. While we confirm a strong temperature inversion structure, we find low abundances of TiO and VO with mixing ratios of $\rm{log(TiO)}=-7.80^{+0.15}_{-0.30}$ and $\rm{log(VO)}=-9.60^{+0.64}_{-0.57}$, respectively. The low abundances of TiO and VO are consistent with theoretical predictions for such an ultra-hot atmosphere. In such low abundances, TiO and VO have little effect on the temperature structure of the atmosphere. The abundance of ${\rm e}^{-}$, which serves as a proxy for ${\rm H}^{-}$ ions in this study, is found to be high, with $\rm{log(e^-)}=-4.89\pm{0.06}$. These results indicate that the temperature inversion in KELT-9 b's dayside atmosphere is likely not caused by TiO/VO, but rather by the significant abundance of ${\rm H}^{-}$ ions. The best-fit model cannot fully explain the observed spectrum, and chemical species not included in the retrieval may introduce modeling biases. Future observations with broader wavelength coverage and higher spectral resolution are expected to provide more accurate diagnostics on the presence and abundances of TiO/VO. These advanced observations will overcome the limitations of current data from HST and photometric facilities, which are constrained by narrow wavelength coverage and instrumental systematics.

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arXiv2024

MASTER OT J030227.28+191754.5: an unprecedentedly energetic dwarf nova outburst

We present a detailed study of the MASTER OT J030227.28+191754.5 outburst in 2021-2022, reaching an amplitude of 10.2 mag and a duration of 60 d. The detections of (1) the double-peaked optical emission lines, and (2) the early and ordinary superhumps, established that MASTER OT J030227.28+191754.5 is an extremely energetic WZ Sge-type dwarf nova (DN). Based on the superhump observations, we obtained its orbital period and mass ratio as 0.05986(1) d and 0.063(1), respectively. These are within a typical range of low-mass-ratio DNe. According to the binary parameters derived based on the thermal-tidal instability model, our analyses showed that (1) the standard disk model requires an accretion rate $\simeq$ 10$^{20}$ g s$^{-1}$ to explain its peak optical luminosity and (2) large mass was stored in the disk at the outburst onset. These cannot be explained solely by the impact of its massive ($\gtrsim$ 1.15 M$_\odot$) primary white dwarf implied by Kimura et al. (2023). Instead, we propose that the probable origin of this enormously energetic DN outburst is the even lower quiescence viscosity than other WZ Sge-type DNe. This discussion is qualitatively valid for most possible binary parameter spaces unless the inclination is low ($\lesssim 40^\circ$) enough for the disk to be bright explaining the outburst amplitude. Such low inclinations, however, would not allow detectable amplitude of early superhumps in the current thermal-tidal instability model. The optical spectra at outburst maximum showed the strong emission lines of Balmer, He I, and He II series whose core is narrower than $\sim 800$ km s$^{-1}$. Considering its binary parameters, a Keplerian disk cannot explain this narrow component, but the presumable origin is disk winds.

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arXiv2024

Detection of an Earth-sized exoplanet orbiting the nearby ultracool dwarf star SPECULOOS-3

Located at the bottom of the main sequence, ultracool dwarf stars are widespread in the solar neighbourhood. Nevertheless, their extremely low luminosity has left their planetary population largely unexplored, and only one of them, TRAPPIST-1, has so far been found to host a transiting planetary system. In this context, we present the SPECULOOS project's detection of an Earth-sized planet in a 17 h orbit around an ultracool dwarf of M6.5 spectral type located 16.8 pc away. The planet's high irradiation (16 times that of Earth) combined with the infrared luminosity and Jupiter-like size of its host star make it one of the most promising rocky exoplanet targets for detailed emission spectroscopy characterization with JWST. Indeed, our sensitivity study shows that just ten secondary eclipse observations with the Mid-InfraRed Instrument/Low-Resolution Spectrometer on board JWST should provide strong constraints on its atmospheric composition and/or surface mineralogy.

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arXiv2024

Simultaneous multicolour transit photometry of hot Jupiters HAT-P-19b, HAT-P-51b, HAT-P-55b, and HAT-P-65b

Accurate physical parameters of exoplanet systems are essential for further exploration of planetary internal structure, atmospheres, and formation history. We aim to use simultaneous multicolour transit photometry to improve the estimation of transit parameters, to search for transit timing variations (TTVs), and to establish which of our targets should be prioritised for follow-up transmission spectroscopy. We performed time series photometric observations of 12 transits for the hot Jupiters HAT-P-19b, HAT-P-51b, HAT-P-55b, and HAT-P-65b using the simultaneous four-colour camera MuSCAT2 on the Telescopio Carlos Sánchez. We collected 56 additional transit light curves from TESS photometry. To derive transit parameters, we modelled the MuSCAT2 light curves with Gaussian processes to account for correlated noise. To derive physical parameters, we performed EXOFASTv2 global fits to the available transit and radial velocity data sets, together with the Gaia DR3 parallax, isochrones, and spectral energy distributions. To assess the potential for atmospheric characterisation, we compared the multicolour transit depths with a flat line and a clear atmosphere model. We consistently refined the transit and physical parameters. We improved the orbital period and ephemeris estimates, and found no evidence for TTVs or orbital decay. The MuSCAT2 broadband transmission spectra of HAT-P-19b and HAT-P-65b are consistent with previously published low-resolution transmission spectra. We also found that, except for HAT-P-65b, the assumption of a planetary atmosphere can improve the fit to the MuSCAT2 data. In particular, we identified HAT-P-55b as a priority target among these four planets for further atmospheric studies using transmission spectroscopy.

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arXiv2024

Optical and soft X-ray light-curve analysis during the 2022 eruption of U Scorpii: structural changes in the accretion disk

We present our optical photometric observations of the 2022 eruption of the recurrent nova U Scorpii (U Sco) using 49,152 data points over 70 d following the optical peak. We have also analyzed its soft X-ray (0.3--1 keV) light curve by the Neil Gehrels Swift Observatory. During the 2022 eruption, the optical plateau stage started 13.8--15.0 d and ended 23.8--25.0 d after the optical peak. The soft X-ray stage started 14.6--15.3 d and ended 38.7--39.5 d after the optical peak. Both stages started later and had shorter durations, and the soft X-ray light curve peaked earlier and was less luminous compared to those during the U Sco 2010 eruption. These points suggest that there were differences in the envelope mass between the different cycles of the nova eruption. Furthermore, we have analyzed the optical eclipses during the 2022 eruption. The primary eclipse was first observed 10.4--11.6 d after the optical peak, earlier than the beginning of the optical plateau stage. This sequence of events can be explained by the receding ejecta photosphere associated with the expanding nova ejecta. We have determined the ingress and egress phases of the primary eclipses and estimated the outer radius of the optical light source centered at the white dwarf (WD). During the optical plateau stage, the source radius remained $\sim$1.2 times larger than the Roche volume radius of the primary WD, being close to the L1 point. When the optical plateau stage ended, the source radius drastically shrank to the tidal truncation radius within a few orbital periods. This previously unresolved phenomenon can be interpreted as a structural change in U Sco where the temporarily expanded accretion disk due to the nova wind returned to a steady state.

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arXiv2024

Probing the origin of the two-component structure of broad line region by reverberation mapping of an extremely variable quasar

The physical origins of quasar components, such as the broad line region (BLR) and dust torus, remain under debate. To gain insights into them, we focused on Changing-State Quasars (CSQs) which provide a unique perspective through structural changes associated with accretion disk state transitions. We targeted SDSS J125809.31+351943.0, an extremely variable CSQ, to study its central core structure and kinematics. We conducted reverberation mapping with optical spectroscopy to explore the structure of the BLR and estimate the black hole mass. The results from H$β$ reverberation mapping indicated a black hole mass of $10^{9.64^{+0.11}_{-0.20}}\rm{M_\odot}$. Additionally, we analyzed variations in the optical to X-ray spectral indices, $α_{\rm{ox}}$, before and after the state transition, to investigate the accretion disk. These variations in $α_{\rm{ox}}$ and the Eddington ratio (from 0.4 \% to 2.4 \%) exhibitied behavior similar to state transitions observed in X-ray binary systems. Spectral analysis of H$β$ revealed a predominantly double-peaked profile during dim periods, transitioning to include a single-peaked component as the quasar brightened, suggesting that H$β$ contains a mixture of two components. Each of these components has its distinct characteristics: the first is a double-peaked profile that remains stable despite changes in the accretion rate, while the second is a variable single-peaked profile. Using time lags from reverberation mapping, we estimated the spatial relationships between these BLR components, the accretion disk, and the dust torus. Our results suggest that the BLR consists of two distinct components, each differing in location and origin.

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arXiv2024

Intermediate-luminosity Type IIP SN 2021gmj: a low-energy explosion with signatures of circumstellar material

We present photometric, spectroscopic and polarimetric observations of the intermediate-luminosity Type IIP supernova (SN) 2021gmj from 1 to 386 days after the explosion. The peak absolute V-band magnitude of SN 2021gmj is -15.5 mag, which is fainter than that of normal Type IIP SNe. The spectral evolution of SN 2021gmj resembles that of other sub-luminous supernovae: the optical spectra show narrow P-Cygni profiles, indicating a low expansion velocity. We estimate the progenitor mass to be about 12 Msun from the nebular spectrum and the 56Ni mass to be about 0.02 Msun from the bolometric light curve. We also derive the explosion energy to be about 3 x 10^{50} erg by comparing numerical light curve models with the observed light curves. Polarization in the plateau phase is not very large, suggesting nearly spherical outer envelope. The early photometric observations capture the rapid rise of the light curve, which is likely due to the interaction with a circumstellar material (CSM). The broad emission feature formed by highly-ionized lines on top of a blue continuum in the earliest spectrum gives further indication of the CSM at the vicinity of the progenitor. Our work suggests that a relatively low-mass progenitor of an intermediate-luminosity Type IIP SN can also experience an enhanced mass loss just before the explosion, as suggested for normal Type IIP SNe.

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arXiv2024

Migration and Evolution of giant ExoPlanets (MEEP) I: Nine Newly Confirmed Hot Jupiters from the TESS Mission

Hot Jupiters were many of the first exoplanets discovered in the 1990s, but in the decades since their discovery, the mysteries surrounding their origins remain. Here, we present nine new hot Jupiters (TOI-1855 b, TOI-2107 b, TOI-2368 b, TOI-3321 b, TOI-3894 b, TOI-3919 b, TOI-4153 b, TOI-5232 b, and TOI-5301 b) discovered by NASA's TESS mission and confirmed using ground-based imaging and spectroscopy. These discoveries are the first in a series of papers named the Migration and Evolution of giant ExoPlanets (MEEP) survey and are part of an ongoing effort to build a complete sample of hot Jupiters orbiting FGK stars, with a limiting Gaia $G$-band magnitude of 12.5. This effort aims to use homogeneous detection and analysis techniques to generate a set of precisely measured stellar and planetary properties that is ripe for statistical analysis. The nine planets presented in this work occupy a range of masses (0.55 Jupiter masses (M$_{\rm{J}}$) $<$ M$_{\rm{P}}$ $<$ 3.88 M$_{\rm{J}}$) and sizes (0.967 Jupiter radii (R$_{\rm{J}}$) $<$ R$_{\rm{P}}$ $<$ 1.438 R$_{\rm{J}}$) and orbit stars that range in temperature from 5360 K $<$ Teff $<$ 6860 K with Gaia $G$-band magnitudes ranging from 11.1 to 12.7. Two of the planets in our sample have detectable orbital eccentricity: TOI-3919 b ($e = 0.259^{+0.033}_{-0.036}$) and TOI-5301 b ($e = 0.33^{+0.11}_{-0.10}$). These eccentric planets join a growing sample of eccentric hot Jupiters that are consistent with high-eccentricity tidal migration, one of the three most prominent theories explaining hot Jupiter formation and evolution.

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arXiv2023

Spectra of V1405 Cas at the very beginning indicate a low-mass ONeMg white dwarf progenitor

The lowest possible mass of ONeMg white dwarfs (WDs) has not been clarified despite its importance in the formation and evolution of WDs. We tackle this issue by studying the properties of V1405 Cas (Nova Cassiopeiae 2021), which is an outlier given a combination of its very slow light-curve evolution and the recently reported neon-nova identification. We report its rapid spectral evolution in the initial phase, covering 9.88, 23.77, 33.94, 53.53, 71.79, and 81.90 hours after the discovery. The first spectrum is characterized by lines from highly-ionized species, most noticeably He II and N III. These lines are quickly replaced by lower-ionization lines, e.g., N II, Si II, and O I. In addition, Al II (6237 Å) starts emerging as an emission line at the second epoch. We perform emission-line strength diagnostics, showing that the density and temperature quickly decrease toward later epochs. This behavior, together with the decreasing velocity seen in H$α$, H$β$, and He I, indicates that the initial nova dynamics is reasonably well described by an expanding fireball on top of an expanding photosphere. Interestingly, the strengths of the N III and Al II indicate large abundance enhancement, pointing to an ONeMg WD progenitor as is consistent with its neon-nova classification. Given its low-mass nature inferred by the slow light-curve evolution and relatively narrow emission lines, it provides a challenge to the stellar evolution theory that predicts the lower limit of the ONeMg WD mass being $\sim$ 1.1 $M_\odot$.

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arXiv2023

SN 2020uem: A Possible Thermonuclear Explosion within A Dense Circumstellar Medium (II) The Properties of The CSM from Polarimetry and Light Curve Modeling

Type IIn/Ia-CSM supernovae (SNe IIn/Ia-CSM) are classified by their characteristic spectra, which exhibit narrow hydrogen emission lines originating from a strong interaction with a circumstellar medium (CSM) together with broad lines of intermediate-mass elements. We performed intensive follow-up observations of SN IIn/Ia-CSM 2020uem, including photometry, spectroscopy, and polarimetry. In this paper, we focus on the results of polarimetry. We performed imaging polarimetry at $66$ days and spectropolarimetry at $103$ days after the discovery. SN 2020uem shows a high continuum polarization of $1.0-1.5\%$ without wavelength dependence. Besides, the polarization degree and position angle keep roughly constant. These results suggest that SN 2020uem is powered by a strong interaction with a confined and aspherical CSM. We performed a simple polarization modeling, based on which we suggest that SN 2020uem has an equatorial-disk/torus CSM. Besides, we performed semi-analytic light-curve modeling and estimated the CSM mass. We revealed that the mass-loss rate in the final few hundred years immediately before the explosion of SN 2020uem is in the range of $0.01 - 0.05 {\rm ~M_{\odot}~yr^{-1}}$, and that the total CSM mass is $0.5-4 {\rm ~M_{\odot}}$. The CSM mass can be accommodated by not only a red supergiant (RSG) but a red giant (RG) or an asymptotic-giant-branch (AGB) star. As a possible progenitor scenario of SN 2020uem, we propose a white-dwarf binary system including an RG, RSG or AGB star, especially a merger scenario via common envelope evolution, i.e., the core-degenerate scenario or its variant.

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arXiv2023

SN 2020uem: A Possible Thermonuclear Explosion within A Dense Circumstellar Medium (I) The Nature of Type IIn/Ia-CSM SNe from Photometry and Spectroscopy

We have performed intensive follow-up observations of a Type IIn/Ia-CSM SN (SN IIn/Ia-CSM), 2020uem, with photometry, spectroscopy, and polarimetry. In this paper, we report on the results of our observations focusing on optical/near-infrared (NIR) photometry and spectroscopy. The maximum V-band magnitude of SN 2020uem is over $-19.5$ mag. The light curves decline slowly with a rate of $\sim 0.75 {\rm ~mag}/100 {\rm ~days}$. In the late phase ($\gtrsim 300$ days), the light curves show accelerated decay ($\sim 1.2 {\rm ~mag}/100 {\rm ~days}$). The optical spectra show prominent hydrogen emission lines and broad features possibly associated with Fe-peak elements. In addition, the $\rm Hα$ profile exhibits a narrow P-Cygni profile with the absorption minimum of $\sim 100 {\rm ~km~s^{-1}}$. SN 2020uem shows a higher $\rm Hα/Hβ$ ratio ($\sim 7$) than those of SNe IIn, which suggests a denser CSM. The NIR spectrum shows the Paschen and Brackett series with continuum excess in the H and Ks bands. We conclude that the NIR excess emission originates from newly-formed carbon dust. The dust mass ($M_{\rm d}$) and temperature ($T_{\rm d}$) are derived to be $(M_{\rm d}, T_{\rm d}) \sim (4-7 \times 10^{-5} {\rm ~M_{\odot}}, 1500-1600 {\rm ~K})$. We discuss the differences and similarities between the observational properties of SNe IIn/Ia-CSM and those of other SNe Ia and interacting SNe. In particular, spectral features around $\sim 4650$ {\text Å} and $\sim 5900$ {\text Å} of SNe IIn/Ia-CSM are more suppressed than those of SNe Ia; these lines are possibly contributed, at least partly, by \ion{Mg}{1}] and \ion{Na}{1}, and may be suppressed by high ionization behind the reverse shock caused by the massive CSM.

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arXiv2022

TESS Hunt for Young and Maturing Exoplanets (THYME) VII : Membership, rotation, and lithium in the young cluster Group-X and a new young exoplanet

The public, all-sky surveys Gaia and TESS provide the ability to identify new young associations and determine their ages. These associations enable study of planetary evolution by providing new opportunities to discover young exoplanets. A young association was recently identified by Tang et al. and F{ü}rnkranz et al. using astrometry from Gaia (called "Group-X" by the former). In this work, we investigate the age and membership of this association; and we validate the exoplanet TOI 2048 b, which was identified to transit a young, late G dwarf in Group-X using photometry from TESS. We first identified new candidate members of Group-X using Gaia EDR3 data. To infer the age of the association, we measured rotation periods for candidate members using TESS data. The clear color--period sequence indicates that the association is the same age as the $300\pm50$ Myr-old NGC 3532. We obtained optical spectra for candidate members that show lithium absorption consistent with this young age. Further, we serendipitously identify a new, small association nearby Group-X, which we call MELANGE-2. Lastly, we statistically validate TOI 2048 b, which is $2.6\pm0.2$ \rearth\ radius planet on a 13.8-day orbit around its 300 Myr-old host star.

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arXiv2022

PNV J00444033+4113068: early superhumps with 0.7 mag amplitude and non-red color

In the first days of WZ Sge-type dwarf nova (DN) outbursts, the 2:1 resonance induces a spiral arm structure in the accretion disk, which is observed as early superhumps in optical light curves. This paper reports our optical observations of an eclipsing WZ Sge-type DN PNV J00444033+4113068 during its 2021 superoutburst with the 3.8m Seimei telescope and through VSNET collaboration. The eclipse analysis gave its orbital period as 0.055425534(1) d. Our observations confirmed early superhumps with an amplitude of 0.7 mag, the largest amplitude among known WZ Sge-type DNe. More interestingly, its early superhumps became the reddest around their secondary minimum, whereas other WZ Sge-type DNe show the reddest color around the early superhump maximum. The spectrum around the peak of the outburst showed the double-peaked emission lines of He II 4686Å~ and H$α$ with a peak separation of $\ge 700$ km/s, supporting a very high-inclination system. With the early superhump mapping, the unique profile and color of the early superhump of PNV J00444033+4113068 are successfully reproduced by the accretion disk with vertically extended double arm structure. Therefore, the large amplitude and unique color behavior of the early superhumps in PNV J00444033+4113068 can be explained by the 2:1 resonance model along with other WZ Sge-type DNe.

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arXiv2022

The TESS-Keck Survey. XIII. An Eccentric Hot Neptune with a Similar-Mass Outer Companion around TOI-1272

We report the discovery of an eccentric hot Neptune and a non-transiting outer planet around TOI-1272. We identified the eccentricity of the inner planet, with an orbital period of 3.3 d and $R_{\rm p,b} = 4.1 \pm 0.2$ $R_\oplus$, based on a mismatch between the observed transit duration and the expected duration for a circular orbit. Using ground-based radial velocity measurements from the HIRES instrument at the Keck Observatory, we measured the mass of TOI-1272b to be $M_{\rm p,b} = 25 \pm 2$ $M_\oplus$. We also confirmed a high eccentricity of $e_b = 0.34 \pm 0.06$, placing TOI-1272b among the most eccentric well-characterized sub-Jovians. We used these RV measurements to also identify a non-transiting outer companion on an 8.7-d orbit with a similar mass of $M_{\rm p,c}$ sin$i= 27 \pm 3$ $M_\oplus$ and $e_c \lesssim 0.35$. Dynamically stable planet-planet interactions have likely allowed TOI-1272b to avoid tidal eccentricity decay despite the short circularization timescale expected for a close-in eccentric Neptune. TOI-1272b also maintains an envelope mass fraction of $f_{\rm env} \approx 11\%$ despite its high equilibrium temperature, implying that it may currently be undergoing photoevaporation. This planet joins a small population of short-period Neptune-like planets within the "Hot Neptune Desert" with a poorly understood formation pathway.

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arXiv2022

TOI-1696: a nearby M4 dwarf with a $3R_\oplus$ planet in the Neptunian desert

We present the discovery and validation of a temperate sub-Neptune around the nearby mid-M dwarf TIC 470381900 (TOI-1696), with a radius of $3.09 \pm 0.11 \,R_\oplus$ and an orbital period of $2.5 \,\rm{days}$, using a combination of TESS and follow-up observations using ground-based telescopes. Joint analysis of multi-band photometry from TESS, MuSCAT, MuSCAT3, Sinistro, and KeplerCam confirmed the transit signal to be achromatic as well as refined the orbital ephemeris. High-resolution imaging with Gemini/'Alopeke and high-resolution spectroscopy with the Subaru/IRD confirmed that there are no stellar companions or background sources to the star. The spectroscopic observations with IRD and IRTF/SpeX were used to determine the stellar parameters, and found the host star is an M4 dwarf with an effective temperature of $T_{eff} = 3185 \pm 76\,\rm{K}$ and a metallicity of [Fe/H] $=0.336 \pm 0.060 \,\rm{dex}$. The radial velocities measured from IRD set a $2$-$σ$ upper limit on the planetary mass to be $48.8 \,M_\oplus$. The large radius ratio ($R_p/R_\star \sim 0.1$) and the relatively bright NIR magnitude ($J=12.2 \,\rm{mag}$) make this planet an attractive target for further followup observations. TOI-1696b is one of the planets belonging to the Neptunian desert with the highest transmission spectroscopy metric discovered to date, making it an interesting candidate for atmospheric characterizations with JWST.

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