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1. The Giant Accreting Protoplanet Survey (GAPlanetS)—Results from a 6 yr Campaign to Image Accreting Protoplanets

   https://doi.org/10.3847/1538-3881/acc183  

   Follette, Katherine B. ; Close, Laird M. ; Males, Jared R. ; Ward-Duong, Kimberly ; Balmer, William O. ; Redai, Jéa Adams ; Morales, Julio ; Sarosi, Catherine ; Dacus, Beck ; De Rosa, Robert J. ; et al ( May 2023 , The Astronomical Journal)

   Abstract Accreting protoplanets are windows into planet formation processes, and high-contrast differential imaging is an effective way to identify them. We report results from the Giant Accreting Protoplanet Survey (GAPlanetS), which collected H α differential imagery of 14 transitional disk host stars with the Magellan Adaptive Optics System. To address the twin challenges of morphological complexity and point-spread function instability, GAPlanetS required novel approaches for frame selection and optimization of the Karhounen–Loéve Image Processing algorithm pyKLIP . We detect one new candidate, CS Cha “c,” at a separation of 68 mas and a modest Δmag of 2.3. We recover the HD 142527 B and HD 100453 B accreting stellar companions in several epochs, and the protoplanet PDS 70 c in 2017 imagery, extending its astrometric record by nine months. Though we cannot rule out scattered light structure, we also recover LkCa 15 “b,” at H α ; its presence inside the disk cavity, absence in Continuum imagery, and consistency with a forward-modeled point source suggest that it remains a viable protoplanet candidate. Through targeted optimization, we tentatively recover PDS 70 c at two additional epochs and PDS 70 b in one epoch. Despite numerous previously reported companion candidates around GAplanetS targets, we recover no additional point sources. Our moderate H α contrasts do not preclude most protoplanets, and we report limiting H α contrasts at unrecovered candidate locations. We find an overall detection rate of ∼36 − 22 + 26 % , considerably higher than most direct imaging surveys, speaking to both GAPlanetS’s highly targeted nature and the promise of H α differential imaging for protoplanet identification. 

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2. Improved Orbital Constraints and Hα Photometric Monitoring of the Directly Imaged Protoplanet Analog HD 142527 B

   https://doi.org/10.3847/1538-3881/ac73f4  

   Balmer, William O. ; Follette, Katherine B. ; Close, Laird M. ; Males, Jared R. ; De Rosa, Robert J. ; Adams Redai, Jéa I. ; Watson, Alex ; Weinberger, Alycia J. ; Morzinski, Katie M. ; Morales, Julio ; et al ( July 2022 , The Astronomical Journal)

   Companions embedded in the cavities of transitional circumstellar disks have been observed to exhibit excess luminosity at Hα, an indication that they are actively accreting. We report 5 yr (2013–2018) of monitoring of the position and Hαexcess luminosity of the embedded, accreting low-mass stellar companion HD 142527 B from the MagAO/VisAO instrument. We usepyklip, a Python implementation of the Karhunen–Loeve Image Processing algorithm, to detect the companion. Usingpyklipforward modeling, we constrain the relative astrometry to 1–2 mas precision and achieve sufficient photometric precision (±0.2 mag, 3% error) to detect changes in the Hαcontrast of the companion over time. In order to accurately determine the relative astrometry of the companion, we conduct an astrometric calibration of the MagAO/VisAO camera against 20 yr of Keck/NIRC2 images of the Trapezium cluster. We demonstrate agreement of our VisAO astrometry with other published positions for HD 142527 B, and useorbitize!to generate a posterior distribution of orbits fit to the relative astrometry of HD 142527 B. Our data suggest that the companion is close to periastron passage, on an orbit significantly misaligned with respect to both the wide circumbinary disk and the recently observed inner disk encircling HD 142527 A. We translate observed Hαcontrasts for HD 142527 B into mass accretion rate estimates on the order of 4–9 × 10⁻¹⁰M_⊙yr⁻¹. Photometric variation in the Hαexcess of the companion suggests that the accretion rate onto the companion is variable. This work represents a significant step toward observing accretion-driven variability onto protoplanets, such as PDS 70 b&c.

    

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3. Deep Paβ Imaging of the Candidate Accreting Protoplanet AB Aur b

   https://doi.org/10.3847/1538-3881/ad2a52  

   Biddle, Lauren I. ; Bowler, Brendan P. ; Zhou, Yifan ; Franson, Kyle ; Zhang, Zhoujian ( March 2024 , The Astronomical Journal)

   Giant planets grow by accreting gas through circumplanetary disks, but little is known about the timescale and mechanisms involved in the planet-assembly process because few accreting protoplanets have been discovered. Recent visible and infrared imaging revealed a potential accreting protoplanet within the transition disk around the young intermediate-mass Herbig Ae star, AB Aurigae (AB Aur). Additional imaging in Hαprobed for accretion and found agreement between the line-to-continuum flux ratio of the star and companion, raising the possibility that the emission source could be a compact disk feature seen in scattered starlight. We present new deep Keck/NIRC2 high-contrast imaging of AB Aur to characterize emission in Paβ, another accretion tracer less subject to extinction. Our narrow band observations reach a 5σcontrast of 9.6 mag at 0.″6, but we do not detect significant emission at the expected location of the companion, nor from other any other source in the system. Our upper limit on Paβemission suggests that if AB Aur b is a protoplanet, it is not heavily accreting or accretion is stochastic and was weak during the observations.

    

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4. Resolving the Peak of the Black Hole Mass Spectrum

   https://doi.org/10.3847/1538-4357/ac8b83  

   Farag, Ebraheem ; Renzo, Mathieu ; Farmer, Robert ; Chidester, Morgan T. ; Timmes, F. X. ( October 2022 , The Astrophysical Journal)

   Gravitational-wave (GW) detections of binary black hole (BH) mergers have begun to sample the cosmic BH mass distribution. The evolution of single stellar cores predicts a gap in the BH mass distribution due to pair-instability supernovae (PISNe). Determining the upper and lower edges of the BH mass gap can be useful for interpreting GW detections of merging BHs. We useMESAto evolve single, nonrotating, massive helium cores with a metallicity ofZ= 10⁻⁵, until they either collapse to form a BH or explode as a PISN, without leaving a compact remnant. We calculate the boundaries of the lower BH mass gap for S-factors in the range S(300 keV) = (77,203) keV b, corresponding to the ±3σuncertainty in our high-resolution tabulated¹²C(α,γ)¹⁶O reaction rate probability distribution function. We extensively test temporal and spatial resolutions for resolving the theoretical peak of the BH mass spectrum across the BH mass gap. We explore the convergence with respect to convective mixing and nuclear burning, finding that significant time resolution is needed to achieve convergence. We also test adopting a minimum diffusion coefficient to help lower-resolution models reach convergence. We establish a new lower edge of the upper mass gap asM_lower≃${60}_{-14}^{+32}$M_⊙from the ±3σuncertainty in the¹²C(α,γ)¹⁶O rate. We explore the effect of a larger 3αrate on the lower edge of the upper mass gap, findingM_lower≃${69}_{-18}^{+34}$M_⊙. We compare our results with BHs reported in the Gravitational-Wave Transient Catalog.

    

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5. A Gap in the Densities of Small Planets Orbiting M Dwarfs: Rigorous Statistical Confirmation Using the Open-source Code RhoPop

   https://doi.org/10.3847/PSJ/ad26f5  

   Schulze, J. G. ; Wang 王, Ji 吉. ; Johnson, J. A. ; Gaudi, B. S. ; Rodriguez Martinez, R. ; Unterborn, C. T. ; Panero, W. R. ( March 2024 , The Planetary Science Journal)

   Using mass–radius composition models, small planets (R≲ 2R_⊕) are typically classified into three types: iron-rich, nominally Earth-like, and those with solid/liquid water and/or atmosphere. These classes are generally expected to be variations within a compositional continuum. Recently, however, Luque & Pallé observed that potentially Earth-like planets around M dwarfs are separated from a lower-density population by a density gap. Meanwhile, the results of Adibekyan et al. hint that iron-rich planets around FGK stars are also a distinct population. It therefore remains unclear whether small planets represent a continuum or multiple distinct populations. Differentiating the nature of these populations will help constrain potential formation mechanisms. We present theRhoPopsoftware for identifying small-planet populations.RhoPopemploys mixture models in a hierarchical framework and a nested sampler for parameter and evidence estimates. UsingRhoPop, we confirm the two populations of Luque & Pallé with >4σsignificance. The intrinsic scatter in the Earth-like subpopulation is roughly half that expected based on stellar abundance variations in local FGK stars, perhaps implying M dwarfs have a smaller spread in the major rock-building elements (Fe, Mg, Si) than FGK stars. We applyRhoPopto the Adibekyan et al. sample and find no evidence of more than one population. We estimate the sample size required to resolve a population of planets with Mercury-like compositions from those with Earth-like compositions for various mass–radius precisions. Only 16 planets are needed when${\sigma }_{M_p}=5\%$and${\sigma }_{R_p}=1\%$. At${\sigma }_{M_p}=10\%$and${\sigma }_{R_p}=2.5\%$, however, over 154 planets are needed, an order of magnitude increase.

    

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