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1. Measuring the ratio of the gas and dust emission radii of protoplanetary disks in the Lupus star-forming region

   https://doi.org/10.1051/0004-6361/202039733  

   Sanchis, E. ; Testi, L. ; Natta, A. ; Facchini, S. ; Manara, C. F. ; Miotello, A. ; Ercolano, B. ; Henning, Th. ; Preibisch, T. ; Carpenter, J. M. ; et al ( May 2021 , Astronomy & Astrophysics)

   null (Ed.)

   We performed a comprehensive demographic study of the CO extent relative to dust of the disk population in the Lupus clouds in order to find indications of dust evolution and possible correlations with other disk properties. We increased the number of disks of the region with measured R CO and R dust from observations with the Atacama Large Millimeter/submillimeter Array to 42, based on the gas emission in the 12 CO J = 2−1 rotational transition and large dust grains emission at ~0.89 mm. The CO integrated emission map is modeled with an elliptical Gaussian or Nuker function, depending on the quantified residuals; the continuum is fit to a Nuker profile from interferometric modeling. The CO and dust sizes, namely the radii enclosing a certain fraction of the respective total flux (e.g., R 68% ), are inferred from the modeling. The CO emission is more extended than the dust continuum, with a R 68% CO / R 68% dust median value of 2.5, for the entire population and for a subsample with high completeness. Six disks, around 15% of the Lupus disk population, have a size ratio above 4. Based on thermo-chemical modeling, this value can only be explained if the disk has undergone grain growth and radial drift. These disks do not have unusual properties, and their properties spread across the population’s ranges of stellar mass ( M ⋆ ), disk mass ( M disk ), CO and dust sizes ( R CO , R dust ), and mass accretion of the entire population. We searched for correlations between the size ratio and M ⋆ , M disk , R CO , and R dust : only a weak monotonic anticorrelation with the R dust is found, which would imply that dust evolution is more prominent in more compact dusty disks. The lack of strong correlations is remarkable: the sample covers a wide range of stellar and disk properties, and the majority of the disks have very similar size ratios. This result suggests that the bulk of the disk population may behave alike and be in a similar evolutionary stage, independent of the stellar and disk properties. These results should be further investigated, since the optical depth difference between CO and dust continuum might play a major role in the observed size ratios of the population. Lastly, we find a monotonic correlation between the CO flux and the CO size. The results for the majority of the disks are consistent with optically thick emission and an average CO temperature of around 30 K; however, the exact value of the temperature is difficult to constrain. 

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2. ALMA Discovery of a Disk around the Planetary-mass Companion SR 12 c

   https://doi.org/10.3847/2041-8213/ac6420  

   Wu, Ya-Lin ; Bowler, Brendan P. ; Sheehan, Patrick D. ; Close, Laird M. ; Eisner, Joshua A. ; Best, William M. ; Ward-Duong, Kimberly ; Zhu, Zhaohuan ; Kraus, Adam L. ( April 2022 , The Astrophysical Journal Letters)

   Abstract We report an Atacama Large Millimeter/submillimeter Array 0.88 mm (Band 7) continuum detection of the accretion disk around SR 12 c, an ∼11 M Jup planetary-mass companion (PMC) orbiting its host binary at 980 au. This is the first submillimeter detection of a circumplanetary disk around a wide PMC. The disk has a flux density of 127 ± 14 μ Jy and is not resolved by the ∼0.″1 beam, so the dust disk radius is likely less than 5 au and can be much smaller if the dust continuum is optically thick. If, however, the dust emission is optically thin, then the SR 12 c disk has a comparable dust mass to the circumplanetary disk around PDS 70 c but is about five times lower than that of the ∼12 M Jup free-floating OTS 44. This suggests that disks around bound and unbound planetary-mass objects can span a wide range of masses. The gas mass estimated with an accretion rate of 10 −11 M ☉ yr −1 implies a gas-to-dust ratio higher than 100. If cloud absorption is not significant, a nondetection of 12 CO(3–2) implies a compact gas disk around SR 12 c. Future sensitive observations may detect more PMC disks at 0.88 mm flux densities of ≲100 μ Jy. 

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3. New Constraints on Protoplanetary Disk Gas Masses in Lupus

   https://doi.org/10.3847/1538-4357/ac517e  

   Anderson, Dana E. ; Cleeves, L. Ilsedore ; Blake, Geoffrey A. ; Bergin, Edwin A. ; Zhang, Ke ; Carpenter, John M. ; Schwarz, Kamber R. ( March 2022 , The Astrophysical Journal)

   Gas mass is a fundamental quantity of protoplanetary disks that directly relates to their ability to form planets. Because we are unable to observe the bulk H₂content of disks directly, we rely on indirect tracers to provide quantitative mass estimates. Current estimates for the gas masses of the observed disk population in the Lupus star-forming region are based on measurements of isotopologues of CO. However, without additional constraints, the degeneracy between H₂mass and the elemental composition of the gas leads to large uncertainties in such estimates. Here, we explore the gas compositions of seven disks from the Lupus sample representing a range of CO-to-dust ratios. With Band 6 and 7 ALMA observations, we measure line emission for HCO⁺, HCN, and N₂H⁺. We find a tentative correlation among the line fluxes for these three molecular species across the sample, but no correlation with¹³CO or submillimeter continuum fluxes. For the three disks where N₂H⁺is detected, we find that a combination of high disk gas masses and subinterstellar C/H and O/H are needed to reproduce the observed values. We find increases of ∼10–100× previous mass estimates are required to match the observed line fluxes. This work highlights how multimolecular studies are essential for constraining the physical and chemical properties of the gas in populations of protoplanetary disks, and that CO isotopologues alone are not sufficient for determining the mass of many observed disks.

    

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4. Dust masses of young disks: constraining the initial solid reservoir for planet formation

   https://doi.org/10.1051/0004-6361/202037851  

   Tychoniec, Łukasz ; Manara, Carlo F. ; Rosotti, Giovanni P. ; van Dishoeck, Ewine F. ; Cridland, Alexander J. ; Hsieh, Tien-Hao ; Murillo, Nadia M. ; Segura-Cox, Dominique ; van Terwisga, Sierk E. ; Tobin, John J. ( August 2020 , Astronomy & Astrophysics)

   null (Ed.)

   Context. Recent years have seen building evidence that planet formation starts early, in the first ~0.5 Myr. Studying the dust masses available in young disks enables us to understand the origin of planetary systems given that mature disks are lacking the solid material necessary to reproduce the observed exoplanetary systems, especially the massive ones. Aims. We aim to determine if disks in the embedded stage of star formation contain enough dust to explain the solid content of the most massive exoplanets. Methods. We use Atacama Large Millimeter/submillimeter Array (ALMA) Band 6 (1.1–1.3 mm) continuum observations of embedded disks in the Perseus star-forming region together with Very Large Array (VLA) Ka -band (9 mm) data to provide a robust estimate of dust disk masses from the flux densities measured in the image plane. Results. We find a strong linear correlation between the ALMA and VLA fluxes, demonstrating that emission at both wavelengths is dominated by dust emission. For a subsample of optically thin sources, we find a median spectral index of 2.5 from which we derive the dust opacity index β = 0.5, suggesting significant dust growth. Comparison with ALMA surveys of Orion shows that the Class I dust disk mass distribution between the two regions is similar, but that the Class 0 disks are more massive in Perseus than those in Orion. Using the DIANA opacity model including large grains, with a dust opacity value of κ 9 mm = 0.28 cm 2 g −1 , the median dust masses of the embedded disks in Perseus are 158 M ⊕ for Class 0 and 52 M ⊕ for Class I from the VLA fluxes. The lower limits on the median masses from ALMA fluxes are 47 M ⊕ and 12 M ⊕ for Class 0 and Class I, respectively, obtained using the maximum dust opacity value κ 1.3 mm = 2.3 cm 2 g −1 . The dust masses of young Class 0 and I disks are larger by at least a factor of ten and three, respectively, compared with dust masses inferred for Class II disks in Lupus and other regions. Conclusions. The dust masses of Class 0 and I disks in Perseus derived from the VLA data are high enough to produce the observed exoplanet systems with efficiencies acceptable by planet formation models: the solid content in observed giant exoplanets can be explained if planet formation starts in Class 0 phase with an efficiency of ~15%. A higher efficiency of ~30% is necessary if the planet formation is set to start in Class I disks. 

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5. Annular substructures in the transition disks around LkCa 15 and J1610

   https://doi.org/10.1051/0004-6361/202038027  

   Facchini, S. ; Benisty, M. ; Bae, J. ; Loomis, R. ; Perez, L. ; Ansdell, M. ; Mayama, S. ; Pinilla, P. ; Teague, R. ; Isella, A. ; et al ( July 2020 , Astronomy & Astrophysics)

   We present high-resolution millimeter continuum ALMA observations of the disks around the T Tauri stars LkCa 15 and 2MASS J16100501-2132318 (hereafter, J1610). These transition disks host dust-depleted inner regions, which have possibly been carved by massive planets, and they are of prime interest to the study of the imprints of planet-disk interactions. While at moderate angular resolution, they appear as a broad ring surrounding a cavity, the continuum emission resolves into multiple rings at a resolution of ~60 × 40 mas (~7.5 au for LkCa 15, ~6 au for J1610) and ~7 μ Jy beam −1 rms at 1.3 mm. In addition to a broad extended component, LkCa 15 and J1610 host three and two narrow rings, respectively, with two bright rings in LkCa 15 being radially resolved. LkCa 15 possibly hosts another faint ring close to the outer edge of the mm emission. The rings look marginally optically thick, with peak optical depths of ~0.5 (neglecting scattering), in agreement with high angular resolution observations of full disks. We performed hydrodynamical simulations with an embedded, sub-Jovian-mass planet and show that the observed multi-ringed substructure can be qualitatively explained as the outcome of the planet-disk interaction. We note, however, that the choice of the disk cooling timescale alone can significantly impact the resulting gas and dust distributions around the planet, leading to different numbers of rings and gaps and different spacings between them. We propose that the massive outer disk regions of transition disks are favorable places for planetesimals, and possibly second-generation planet formation of objects with a lower mass than the planets carving the inner cavity (typically few M Jup ), and that the annular substructures observed in LkCa 15 and J1610 may be indicative of planetary core formation within dust-rich pressure traps. Current observations are compatible with other mechanisms contributing to the origin of the observed substructures, in particular with regard to narrow rings generated (or facilitated) at the edge of the CO and N 2 snowlines. 

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