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We conduct 3D hydrodynamic simulations to investigate the nonlinear outcomes and observability of vertical shear instability (VSI) in protoplanetary disks. Our models include both vertically isothermal and thermally stratified disks, with the latter representing realistic conditions featuring a hotter atmosphere above the midplane. We find that the VSI grows more rapidly and becomes stronger in thermally stratified disks due to enhanced shear, resulting in higher levels of turbulence. At saturation, the turbulence stress reaches αRϕ ≳ 10−3, more than 1 order of magnitude stronger than the isothermal case. The saturated turbulence is more pronounced near the disk surfaces than at the midplane. On synthetic velocity residual maps, obtained by subtracting the Keplerian rotational velocity, perturbations driven by the VSI manifest as axisymmetric rings in isothermal disks and as ring segments in thermally stratified disks. The latter are visible at disk inclinations as high as 45° in thermally stratified disks. The amplitudes of these residual velocities range from ∼50 to ∼100 m s−1 at a 20° inclination, with larger values corresponding to greater thermal stratification. The magnitude of the observed velocity residual increases with the optical depth of the tracer used, as optically thick lines probe the regions near the disk surfaces.

Vertical shear instability (VSI), driven by a vertical gradient of rotational angular velocity, is a promising source of turbulence in protoplanetary disks. We examine the semiglobal stability of thermally stratified disks and find that the VSI consists of surface and body modes: surface modes are confined to regions of strong shear, while body modes extend perturbations across the disk, consistent with the previous findings. In thermally stratified disks, surface modes bifurcate into two branches. The branch associated with the strongest shear at mid-height exhibits a higher growth rate compared to the branch near the surfaces. Surface modes generally grow rapidly and require a high radial wavenumber kR, whereas body mode growth rates increase as kR decreases. Thermal stratification enhances the growth rates of both surface and body modes and boosts VSI-driven radial kinetic energy relative to vertical energy. Our results suggest that simulations will initially favor surface modes with large kR, followed by an increase in body modes with smaller kR, with faster progression in more thermal stratified disks.

New BVR photometric and high-resolution spectroscopic observations of V505 Lac are presented with Transiting Exoplanet Survey Satellite (TESS) photometric data. The orbital period has experienced a secular decrease during the past 16 yr. A clear anticorrelation in the primary and secondary eclipse timing variation (PSETV) obtained from the TESS data is also identified. A double-lined radial velocity (RV) curve is secured, and the effective temperatures of the less- and more-massive stars (Stars 1 and 2, respectively) are measured. Using a spectral subtraction technique, excess emissions are detected in the time-series Ca ii H and K and Hα lines for Star 2. Simultaneous analysis of the light and RV curves using the Wilson–Devinney (WD) code reveals that V505 Lac is a photospherically and chromospherically active W-subtype contact binary system. The component-star masses and radii are determined to an accuracy of approximately 1%. The WD spot model is individually applied to 221 light curves segmented from the TESS data so as to derive the spot parameters of a cool spot on Star 2. The combined variations in both longitude and colatitude among the spot parameters appear to be strongly associated with those of both the anticorrelation in the PSETV and the O’Connell effect in the TESS light curves. Robust negative linear relationships between the PSETV anticorrelation size and the O’Connell effect magnitude are found for the first time. Mass–radius, mass–luminosity, and mass ratio–mass diagrams of contact binaries, along with the mass ratio frequency distribution, are presented in an attempt to elucidate the evolutionary characteristics of these systems.

We report a free-floating planet (FFP) candidate identified from the analysis of the microlensing event KMT-2023-BLG-2669. The lensing light curve is characterized by a short duration (≲3 days) and a small amplitude (≲0.7 mag). From the analysis, we find an Einstein timescale of t E ⋍ 0.33 days and an Einstein radius of θ E ⋍ 4.41 μas. These measurements enable us to infer the lens mass as M=8M⊕πrel/0.1mas−1 , where π rel is the relative lens–source parallax. The inference implies that the lens is a sub-Neptune- to Saturn-mass object, depending on its unknown distance. This is the ninth isolated planetary mass microlens with θ E < 10 μas, which is a useful threshold for an FFP candidate. We conduct extensive searches for possible signals of a host star in the light curve, but find no strong evidence for the host. We investigate the possibility of using late-time high-resolution imaging to probe for possible hosts. In particular, we discuss the case of finite-source point-lens FFP candidates, for which it would be possible to search for very-wide-separation hosts immediately, although such searches are “high risk, high reward.”

We present detailed analyses of updated eclipse timing diagrams for 32 contact binary merger candidates in the Galactic bulge. The photometric data was obtained from 2016 to 2021 using the Korea Microlensing Telescope Network with the 1.6 m telescopes located at three southern sites (CTIO, SAAO, and SSO). The times of minimum lights were determined by applying the binary-star model to full light curves created at half-year intervals from the observations. The orbital period variations of the binary systems were analyzed using the O − C diagrams from our new timings with the others published in the literature, which are based on the OGLE observations from 2001 to 2015. As results, the orbital periods and period-decreasing rates of 32 binary systems were located to be in the ranges of 0.370 to 1.238 days and from −3.0 to −13.1 × 10−6 day yr−1, respectively. Out of these stars, 24 systems show a combination effect of a parabola and a light travel time caused by a third body, and their outer orbital periods are in the range of 9.1–26.5 yr. We propose that all of our merger candidates need additional monitoring observations to study a luminous-red nova progenitor.

Planet-formation theories suggest the presence of free-floating planets (FFPs) that are ejected from their formation sites. While these planets emit very little light, they can be identified through gravitational microlensing. Here, we report the discovery of a FFP candidate in the microlensing event KMT-2024-BLG-3237. The observed light curve exhibits strong finite-source effects characterized by a small amplitude (≲0.9 mag) and a short timescale (≲3 days). The analysis yields an Einstein timescale of tE = 0.54 ± 0.02 days and an angular Einstein radius of θE = 6.30 ± 0.48 μas. The measurements make it possible to estimate the lens mass as M≃102M⊕(πrel/16μas)−1 , where πrel is the relative lens-source parallax. Depending on the unknown πrel, the lens could be a Neptune-mass planet (πrel ≃ 0.1 mas) or a Saturn-mass planet (πrel ≃ 16 μas). A Bayesian analysis yields the lens mass M=67.3−42.5+103.2M⊕ and the lens distance DL=7.34−2.11+0.96kpc . This lens is thirteenth isolated microlens with a measurement of θE < 10 μas. We find that additional searches for possible signatures of a lens host do not show significant evidence for the host.

We present the analysis of seven microlensing planetary events with planet/host mass ratios q < 10−4: KMT-2017-BLG-1194, KMT-2017-BLG-0428, KMT-2019-BLG-1806, KMT-2017-BLG-1003, KMT-2019-BLG-1367, OGLE-2017-BLG-1806, and KMT-2016-BLG-1105. They were identified by applying the Korea Microlensing Telescope Network (KMTNet) AnomalyFinder algorithm to 2016–2019 KMTNet events. A Bayesian analysis indicates that all the lens systems consist of a cold super-Earth orbiting an M or K dwarf. Together with 17 previously published and three that will be published elsewhere, AnomalyFinder has found a total of 27 planets that have solutions with q < 10−4 from 2016–2019 KMTNet events, which lays the foundation for the first statistical analysis of the planetary mass-ratio function based on KMTNet data. By reviewing the 27 planets, we find that the missing planetary caustics problem in the KMTNet planetary sample has been solved by AnomalyFinder. We also find a desert of high-magnification planetary signals (A ≳ 65), and a follow-up project for KMTNet high-magnification events could detect at least two more q < 10−4 planets per year and form an independent statistical sample.

To conduct a comprehensive demographic study of microlensing planets, it is essential to detect all planetary signals that exceed a predefined threshold through a detailed analysis of survey data. We reanalyzed previous data from the Korea Microlensing Telescope Network survey to search for weak planetary signals in lensing events involving faint source stars. For events with potential short-term anomalies identified in the initial search, we validated the signals using rereduced data and conducted detailed modeling of the anomalous events. This process led to the discovery of four planetary events: KMT-2017-BLG-2197, KMT-2022-BLG-1790, KMT-2022-BLG-2076, and KMT-2023-BLG-2209. For all these events, the modeling resulted in two solutions due to the well-known inner–outer degeneracy. The estimated masses of the planets and their hosts are approximately (Mp/MJ, Mh/M⊙) ∼ (0.36, 7.9) for KMT-2017-BLG-2197L, ∼(0.6, 1.7) for KMT-2022-BLG-1790L, ∼(0.67, 0.9) for KMT-2022-BLG-2076L, and ∼(0.73, 0.8) for KMT-2023-BLG-2209L. The planetary systems KMT-2017-BLG-2197L and KMT-2022-BLG-1790L are likely located in the Galactic bulge, while KMT-2022-BLG-2076L and KMT-2023-BLG-2209L are more likely situated in the disk.

We inaugurate a program of “mass production” of microlensing planets discovered in 2021 KMTNet data, with the aim of laying the basis for future statistical studies. While we ultimately plan to quickly publish all 2021 planets meeting some minimal criteria, the current sample of four was chosen simply on the basis of having low initial estimates of the planet–host mass ratio, q. It is therefore notable that two members of this sample suffer from a degeneracy in the normalized source radius ρ that arises from different morphologies of closely spaced caustics. All four planets (KMT-2021-BLG-1391, KMT-2021-BLG-1253, KMT-2021-BLG-1372, KMT-2021-BLG-0748) have well-characterized mass ratios, q, and therefore are suitable for mass-ratio frequency studies. Both of the ρ degeneracies can be resolved by future adaptive optics (AO) observations on 30 m class telescopes. We provide general guidance for such AO observations for all events in anticipation of the prospect that they will revolutionize the field of microlensing planets.

We complete the analysis of all 2018 sub-prime-field microlensing planets identified by the KMTNet AnomalyFinder. Among the 9 previously unpublished events with clear planetary solutions, 6 are clearly planetary (OGLE-2018-BLG-0298, KMT-2018-BLG-0087, KMT-2018-BLG-0247, KMT-2018-BLG-0030, OGLE-2018-BLG-1119, and KMT-2018-BLG-2602), while the remaining 3 are ambiguous in nature. The above ordering of these events is made to facilitate grouping of their Bayesian estimates: the first two are lower-mass gas giants while the last four are Jovian-class planets; the first three most likely lie in the bulge, the last in the disk, and the remaining two are equally likely to be in either population. More specifically, these six planets have host masses Mhost=(0.69−0.30+0.34,0.10−0.05+0.14,0.29−0.14+0.28,0.51−0.31+0.43,0.48−0.28+0.35,0.66−0.36+0.42)M⊙ , planet masses Mplanet=(0.14−0.06+0.07,0.23−0.12+0.32,2.11−1.04+2.09,1.45−0.88+1.23,0.91−0.52+0.66,1.15−0.63+0.73)MJup , and distances DL=(6.54−1.23+0.95,7.02−1.15+1.03,6.76−1.24+0.99,6.48−1.96+1.28,5.76−2.48+1.43,4.31−1.84+1.97)kpc . In addition, there are 8 previously published sub-prime-field planets that were selected by the AnomalyFinder algorithm. Together with a companion paper on 2018 prime-field planets, this work lays the basis for comprehensive statistical studies. We carry out two such studies, one on caustic topologies and the other on the role of Gaia data. From the first, as expected, half (17/33) of the 2018 planets likely to enter the mass-ratio analysis have non-caustic-crossing anomalies. However, only 1 of the 5 noncaustic anomalies with planet-host mass ratio q < 10−3 was discovered by eye (compared to 7 of the 12 with q > 10−3), showing the importance of the semiautomated AnomalyFinder search. From the second, we find that Gaia has played a major role in the interpretation of 16% of the sample and a supplementary role in 6%.