Lillian Yushu Jiang

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.

Accretion governs the growth of young stars and the early evolution of their circumstellar disks, yet population-level measurements of accretion are often hampered by heterogeneous diagnostics and by samples preferentially selected toward disk-bearing or accreting objects. This can impact the mass accretion rate–stellar mass ( M ̇ – M _⋆ ) relation, particularly at substellar masses. We present a uniform analysis of accretion in the ∼2 Myr old star-forming cluster IC 348 based on deep Hubble Space Telescope (HST) F656N imaging. Using H α excess as a single, homogeneous accretion tracer, we derive accretion rates and robust upper limits for 200 cluster members spanning the stellar-to-substellar mass regime (3 M _⊙ to 4 M _Jup ). Accretion is detected in 37% ± 3% of the sample, with fractions of 34% ± 4% among stellar members and 46% ± 6% among substellar objects. For accretors alone, the inferred M ̇ – M _⋆ relation is consistent with those measured in similarly aged regions such as Lupus. In contrast, including weak accretors and nondetections increases the scatter and flattens the slope while lowering the intercept, demonstrating the strong influence of the low-accretion tail on population-level accretion relations. For free-floating planetary-mass objects in IC 348, extrapolating the accretors-only fit overpredicts M ̇ by approximately an order of magnitude compared to the fit that includes upper limits, which instead implies mass accretion rates of −12 M _⊙ yr ⁻¹ . These results show that sample selection—specifically whether the sample is restricted to disk-bearing or accreting targets or instead is drawn from a complete membership census—is a dominant factor shaping population-level accretion relations across a wide dynamic range in stellar mass.

The γ Cephei system hosts one of the first exoplanets discovered and is orbited by one of the closest known stellar companions to a planet-hosting star. In this work, we derive updated orbital fits for γ Cep AB, the stellar binary, and Ab, the planet, by combining literature data with Hipparcos–Gaia astrometry, new radial velocities (RVs), and adaptive optics imaging. We acquired 328 RVs of γ Cep A with Keck/HIRES, AFP/Levy, McDonald/Tull, and Whipple/TRES, and eight adaptive optics imaging epochs with Keck/NIRC2, including the earliest spatially resolved image of γ Cep B in 2003. These observations extend the precision RV baseline of γ Cep to 45 yr and the direct imaging baseline to 23 yr, improving the inferred orbital parameter precision by a factor of 2–10 compared to previous work. For γ Cep B, we derive a semimajor axis of a _B = 20.07 ± 0.06 au, a mass of M _B = 415 ± 2 M _Jup (0.396 ± 0.002 M _⊙ ), an eccentricity of e _B = 0.422 ± 0.002, and an inclination of i _B = 119 . ° 8 ± 0 . ° 1. For γ Cep Ab, we find a semimajor axis of a _Ab = 1.978 ± 0.007 au, a minimum mass of M A b sin i = 1.6 2 − 0.04 + 0.04 M Jup , and an eccentricity of e A b = 0.0 7 − 0.03 + 0.03 . The system is dynamically compact, and its orbital architecture is consistent with von Zeipel–Lidov–Kozai (vZLK) oscillations, although the planet’s orbital inclination remains insufficiently constrained to determine whether the vZLK mechanism is active. Using the RV residuals and dynamical constraints, we rule out additional Jovian companions between 2.5 and 20 au, and companions more massive than Neptune for a < 1 au, both at >90% confidence. The absence of additional giant planets over a broad range of orbital separations is consistent with a dynamically sculpted system in which the close stellar companion has strongly limited the formation or long-term survival of other long-period giant companions.

51 Pegasi harbors the first confirmed extrasolar planet orbiting a Sun-like star. Decades of continued radial velocity (RV) observations have since uncovered signatures of an additional distant companion in the system from a shallow radial acceleration. We present new constraints on the mass and separation of a potential outer companion based on a synthesis of RVs, absolute astrometry, and new high-contrast imaging. Our analysis combines 31 yr of new and previously published RV measurements from the OHP/ELODIE, Lick/Hamilton, Keck/HIRES, and Automated Planet Finder (APF)/Levy spectrographs; a ∼25 yr baseline of absolute astrometry from Hipparcos and Gaia; and deep imaging from Keck/NIRC2 and HST/WFPC2. We find evidence for curvature in the RVs, which, when combined with nondetections from imaging and astrometry, point to a super-Jupiter at ≃15–100 au or a brown dwarf companion at ≈20–170 au. However, the inferred radial acceleration of the host star is driven primarily by the Lick/Hamilton dataset, and its slope is consistent with long-term instrument drift, calling into question the nature of the long-period signal. If an outer companion is present, it could explain the origin of the inner hot Jupiter if 51 Peg b arrived at its current location through high-eccentricity migration. On the other hand, if the signal is spurious, the exceptional baseline rules out Jovian planets within ∼10 au and most brown dwarfs within several tens of astronomical units, implying that the system is devoid of massive companions. Continued RV and astrometric monitoring together with high-contrast imaging can be used to distinguish these scenarios.

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.

Giant planets generate accretion luminosity as they form. Much of this energy is radiated in strong H$α$ 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 H$α$ 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 H$α$ 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 H$α$ 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 H$α$ 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 H$α$ 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 H$α$.

The proposed protoplanet AB Aur b is a spatially concentrated emission source imaged in the millimeter-wavelength disk gap of the Herbig Ae/Be star AB Aur. Its near-infrared spectrum and absence of strong polarized light have been interpreted as evidence supporting the protoplanet interpretation. However, the complex scattered-light structures in the AB Aur disk pose challenges in resolving the emission source and interpreting the true nature of AB Aur b. We present new images of the AB Aur system obtained using the Hubble Space Telescope Wide Field Camera 3 in the ultraviolet (UV) and optical bands. AB Aur b and the known disk spirals are recovered in the F336W, F410M, and F645N bands. The spectral energy distribution of AB Aur b shows absorption in the Balmer jump, mimicking that of early-type stars. By comparing the colors of AB Aur b to those of the host star, the disk spirals, and predictions from scattered light and self-luminous models, we find that the emission from AB Aur b is inconsistent with planetary photospheric or accretion shock models. Instead, it is consistent with those measured in the circumstellar disks that trace scattered light. We conclude that the UV and visible emission from AB Aur b does not necessitate the presence of a protoplanet. We synthesize observational constraints on AB Aur b and discuss inconsistent interpretations among different data sets. Considering the significance of the AB Aur b discovery, we advocate for further observational evidence to verify its planetary nature.

The proposed protoplanet AB Aur b is a spatially concentrated emission source imaged in the mm-wavelength disk gap of the Herbig Ae/Be star AB Aur. Its near-infrared spectrum and absence of strong polarized light have been interpreted as evidence supporting the protoplanet interpretation. However, the complex scattered light structures in the AB Aur disk pose challenges in resolving the emission source and interpreting the true nature of AB Aur b. We present new images of the AB Aur system obtained using the Hubble Space Telescope Wide Field Camera 3 in the ultraviolet (UV) and optical bands. AB Aur b and the known disk spirals are recovered in the F336W, F410M, and F645N bands. The spectral energy distribution of AB Aur b shows absorption in the Balmer jump, mimicking those of early-type stars. By comparing the colors of AB Aur b to those of the host star, the disk spirals, and predictions from scattered light and self-luminous models, we find that the emission from AB Aur b is inconsistent with planetary photospheric or accretion shock models. Instead, it is consistent with those measured in the circumstellar disks that trace scattered light. We conclude that the UV and visible emission from AB Aur b does not necessitate the presence of a protoplanet. We synthesize observational constraints on AB Aur b and discuss inconsistent interpretations of AB Aur b among different datasets. Considering the significance of the AB Aur b discovery, we advocate for further observational evidence to verify its planetary nature.