Ya-Lin Wu

The young intermediate-mass star AB Aurigae is surrounded by a transition disk whose cavity contains several proposed protoplanets, including the debated source “AB Aur b” at a projected separation of ≈ 0 . ″ 6 (≈93 au). To search for circumplanetary material associated with these candidates, we have obtained the most sensitive (∼20 μ Jy beam ⁻¹ rms) and highest angular resolution (∼30 mas) Atacama Large Millimeter/submillimeter Array (ALMA) Band 7 (0.88 mm) observations of AB Aur to date. We recover the asymmetric dust ring in the continuum emission peaking at ∼ 0 . ″ 9 (∼140 au) and compact emission at the stellar position, but detect no point source at the location of AB Aur b or any of the other proposed protoplanet candidates down to a 3 σ limit of 60 μ Jy. For optically thin emission, this corresponds to a dust mass of ≲0.0059 M _⊕ (for a dust temperature of 40 K), comparable to the circumplanetary disk around PDS 70 c. The ¹² CO emission fills the continuum cavity, revealing an inner spiral structure and a clear rotation pattern. The gas at the position of AB Aur b is smoothly connected to the ambient disk with no localized excess or obvious kinematic deviation. If any proposed candidates are accreting, their circumplanetary material must have a low dust mass, a small physical size, dust that is optically thick, or a dearth of large millimeter-emitting grains.

The physical properties of circumplanetary disks offer insights into the formation of giant planets and their satellites. The circumbinary planetary-mass companion SR 12 c (1200 au from SR 12 AB) hosts an isolated dust disk directly detected with ALMA. We present JWST/MIRI imaging of SR 12 c from 5.6 to 21 μ m and analyze its spectral energy distribution from infrared to submillimeter wavelengths. Phenomenologically, the dust disk can be described by a two-component blackbody with large contrasts in characteristic temperatures and radii. Physically motivated models with varied geometries and heating mechanisms suggest a predominantly flat, irradiation-heated disk with a smaller contribution from viscous dissipation. The disk is inferred to be compact, with an inner radius of ∼3 R _Jup . The outer radius is only loosely constrained, with a characteristic value of ∼3 au. The lack of strong 10 μ m excess emission suggests significant grain growth and dust settling. We also find that SR 12 c is broadly consistent with the empirical star–disk scaling relations, providing context for its possible formation pathway. These results demonstrate the power of combining mid-infrared and submillimeter observations with disk modeling to better understand the physical properties of circumplanetary disks.

Many details of the gas accretion phase during giant planet formation remain untested. We present new 0.2–0.7 μ m UV-through-red optical imaging of the young, wide-orbit planetary-mass companion SR 12 c from the Wide Field Camera 3 (WFC3) instrument on board the Hubble Space Telescope. SR 12 c exhibits strong accretion-related continuum excess blueward of ∼5000 Å and clear signs of the Balmer jump at 3646 Å. We derive a total accretion luminosity of 1.65 ± 0.19 × 10 ⁻⁵ L _⊙ and a mass accretion rate of 8 ± 2 × 10 ⁻¹² M _⊙ yr ⁻¹ . Based on its mass and age, SR 12 c will not grow by an appreciable amount at its current accretion rate; it is at the end stages of assembly. No accretion variability is evident between the two epochs of the WFC3 observations spanning a month-long baseline, but the H α emission line strength decreases by 90% compared to the reported flux from five years earlier. Combined with previous observations of SR 12 c, we assemble one of the most complete spectral energy distributions of a young giant planet to date, spanning the UV through sub-mm wavelengths (0.2–880 μ m). This adds SR 12 c to the small yet growing sample of planets with detailed accretion and disk constraints, which together are beginning to establish the diversity of timescales and physical processes governing the formation of giant planets.

We present revised stellar ages for 23 pre-main-sequence (PMS) K- and M-type stars in the Upper Scorpius (Upper Sco) star-forming region, derived using stellar dynamical masses to constrain isochronal ages from five PMS stellar evolutionary models. We find that mass-constrained stellar ages for all model sets are more consistent with the older, ∼8–11 Myr age for Upper Sco derived using earlier-type stars. Additionally, applying the independent mass constraint to isochronal ages tends to (1) increase stellar ages for most model sets, and (2) decrease age scatter for individual sources between model sets. Models that account for global magnetic fields consistently provide the best match to our observations: they change comparatively little when the mass constraint is applied, and produce 9–10 Myr ages under both unconstrained and mass-constrained conditions. Most standard (nonmagnetic) models produce younger ages (3–5 Myr) when unconstrained, but older ages (6–9 Myr) when constrained by dynamical mass. Our results are consistent with recent literature findings that suggest median disk lifetimes may be ≳2× longer than previously thought.

Magnetic braking can drive angular momentum loss in star formation and influence disk evolution. A previous study of HH 111 VLA 1 suggested a decrease in rotation velocity in a region between the infalling envelope and rotating disk. Using Atacama Large Millimeter/submillimeter Array C ¹⁸ O ( J = 2–1) data, we analyzed the gas motion within 6000 au and found clear deviations from the simplest expectations of freefall toward the central star with conserved angular momentum in the transition region between the envelope and disk (from ∼5200 to 160 au). The region can be further divided into three zones: (1) outer region with a significant decrease in infall velocity, dropping to approximately 60% of the freefall velocity and 70% of conservation of angular momentum and energy; (2) middle region with a sharp drop in angular momentum and thus rotation velocity and an increase in infall velocity; and (3) inner region with rotation velocity increasing inward to connect to that of the Keplerian disk and infall velocity decreasing to zero. Comparison with nonideal MHD simulations suggests that the reduced infall velocity in the outer region can be due to magnetic tension by the pinched magnetic field lines, the sharp drop of angular momentum in the middle region can be due to magnetic braking as the field lines pileup, and the rapid increase in rotation velocity in the inner region might result from weaker magnetic braking due to ambipolar diffusion of the field lines. The resulting dip in the rotation profile supports magnetic braking.

We present dynamical masses for 23 pre-main-sequence (PMS) K- and M-type stars in the Upper Scorpius star-forming region. These masses were derived from the Keplerian rotation of CO disk gas using the Markov Chain Monte Carlo radiative-transfer package pdspy and a flared-disk model with 15 free parameters. We compare our dynamical masses to those derived from five PMS evolutionary models, and find that most models consistently underestimate stellar mass by ≳25%. Models with updated treatment of stellar magnetic fields are a notable exception—they consistently return stellar masses in good agreement with the dynamical results. We find that the magnetic models’ performance is valid even at low masses, in contrast with some literature results suggesting they may overestimate stellar mass for M _⋆ ≲ 0.6 M _⊙ . Our results are consistent with dynamical versus isochronal evaluations for younger samples (e.g., Taurus, 1–3 Myr), and extend the systematic evaluation of stellar evolutionary models up to stars ∼11 Myr in age. Finally, we derive disk dust masses to evaluate whether using dynamical masses versus isochronal masses changes the slope of the log( M _dust )−log( M _⋆ ) relation. We derive a slightly shallower relation using dynamical masses versus isochronal masses, but the slopes of these relations agree within uncertainties. In all cases, we derive a steeper than linear relation for log( M _dust )−log( M _⋆ ), consistent with previous literature results for Upper Sco.

Magnetic braking can drive angular momentum loss in star formation and influence disk evolution. A previous study of HH 111 VLA1 suggested a decrease in rotation velocity in a region between the infalling envelope and rotating disk. Using ALMA C$^{18}$O ($J = 2-1$) data, we analyzed the gas motion within 6000 au and found clear deviations from the simplest expectations of free-fall towards the central star with conserved angular momentum in the transition region between the envelope and disk (from $\sim$5200 to 160 au). The region can be further divided into three zones: (1) outer region with a significant decrease in infall velocity, dropping to approximately 60\% of the free-fall velocity and 70\% of conservation of angular momentum and energy; (2) middle region with a sharp drop in angular momentum and thus rotation velocity and an increase in infall velocity; and (3) inner region with rotation velocity increasing inward to connect to that of the Keplerian disk and infall velocity decreasing to zero. Comparison with non-ideal MHD simulations suggests that the reduced infall velocity in the outer region can be due to magnetic tension by the pinched magnetic field lines, the sharp drop of angular momentum in the middle region can be due to magnetic braking as the field lines pile up, and the rapid increase in rotation velocity in the inner region might result from weaker magnetic braking due to ambipolar diffusion of the field lines. The resulting dip in the rotation profile supports magnetic braking.

2MASS J16120668–3010270 (hereafter 2MJ1612) is a young M0 star that hosts a protoplanetary disk in the Upper Scorpius star-forming region. Recent Atacama Large Millimeter/submillimeter Array (ALMA) observations of 2MJ1612 show a mildly inclined disk (i = 37°) with a large dust-depleted gap (R _cav ≈ 0 . ″ 4 or 53 au). We present high-contrast Hα observations from MagAO-X on the 6.5 m Magellan telescope and new high-resolution submillimeter dust continuum observations with ALMA of 2MJ1612. On both 2025 April 13 and 16, we recovered a point source with Hα excess with a signal-to-noise ratio ≳5 within the disk gap in our MagAO-X angular and spectral differential images at a separation of 141.96 ± 2.10 mas (23.45 ± 0.29 au deprojected) from the star and a position angle ​​​​​of 159 . ° 00 ± 0 . ° 55. Furthermore, this Hα source is within close proximity to a K-band point source in the SPHERE/IRDIS observation taken on 2023 July 21. The astrometric offset between the K band and Hα source can be explained by orbital motion of a bound companion. Thus, our observations can be best explained by the discovery of an accreting protoplanet, 2MJ1612 b, with an estimated mass of 4 M _Jup and a Hα line flux ranging from (29.7 ± 7.5) × 10⁻¹⁶ erg s cm² to (8.2 ± 3.4) × 10⁻¹⁶ erg s cm². 2MJ1612 b is likely the third example of an accreting Hα protoplanet responsible for carving the gap in its host disk, joining PDS 70 b and c. Further study is necessary to confirm and characterize this protoplanet candidate and to identify any additional protoplanets that may also play a role in shaping the gap.

Excellent (α images of the star TYC 5709-354-1 led to the discovery of a rare Hα protoplanet. This star was discovered by the WISPIT survey to have a large multi-ring transitional disk, and is hereafter WISPIT 2. Our Hα images of 2025 April 13 and 16 discovered an accreting (Hα in emission) protoplanet: WISPIT 2b (r = 309.43 ± 1.56 mas; (∼54 au deprojected), PA = 242 . ° 21 ± 0 . ° 41) likely clearing a dust-free gap between the two brightest dust rings in the transitional disk. Our signal-to-noise ratio of 12.5 detection gave an Hα ASDI contrast of (6.5 ± 0.5) × 10⁻⁴ and an Hα line flux of (1.29 ± 0.28) × 10⁻¹⁵ erg s⁻¹ cm⁻². We also present L′ photometry from LBT/LMIRcam of the planet (L′ = 15.30 ± 0.05 mag), which, when coupled with an age of 5 . 1 − 1 . 3 + 2 . 4 Myr, yields a planet mass estimate of 5.3 ± 1.0 M _jup from the DUSTY evolutionary models. WISPIT 2b is accreting at 2 . 25 − 0 . 17 + 3 . 75 × 10⁻¹² M _Sun yr⁻¹. WISPIT 2b is very similar to the other Hα protoplanets in terms of mass, age, flux, and accretion rate. The inclination of the system (i = 44°) is also, surprisingly, very similar to the other known Hα protoplanet systems, which all cluster from 37° ≤ i ≤ 52°. We argue this clustering has only a ∼1.0% (2.6σ) probability of occurring randomly, and so we speculate that magnetospherical accretion might have a preferred inclination range (∼37°–52°) for the direct (cloud free, low extinction) line of sight to the Hα line formation/shock region. We also find at 110 mas (∼15 au deprojected) a close companion candidate (CC1) that may be consistent with an inner dusty 9 ± 4 M _jup planet.

Giant planets generate accretion luminosity as they form. Much of this energy is radiated in strong Hα line emission, which has motivated direct imaging surveys at optical wavelengths to search for accreting protoplanets. However, compact disk structures can mimic accreting planets by scattering emission from the host star. This can complicate the interpretation of Hα point sources, especially if the host star itself is accreting. We describe an approach to distinguish accreting protoplanets from scattered-light disk features using “accretion light echoes.” This method relies on variable Hα emission from a stochastically accreting host star to search for a delayed brightness correlation with a candidate protoplanet. We apply this method to the candidate protoplanet AB Aur b with a dedicated Hubble Space Telescope Wide Field Camera 3 program designed to sequentially sample the host star and the candidate planet in Hα while accounting for the light travel time delay and orbital geometry of the source within the protoplanetary disk. Across five epochs spanning 14 months, AB Aur b is over 20 times more variable than its host star; AB Aur’s Hα emission changes by 15% while AB Aur b varies by 330%. These brightness changes are not correlated, which rules out unobstructed scattered starlight from the host star as the only source of AB Aur b’s Hα emission and is consistent with tracing emission from an independently accreting protoplanet, inner disk shadowing effects, or a physically evolving compact disk structure. More broadly, accretion light echoes offer a novel tool to explore the nature of protoplanet candidates with well-timed observations of the host star prior to deep imaging in Hα.