We present new 890 μ m continuum ALMA observations of five brown dwarfs (BDs) with infrared excess in Lupus I and III, which in combination with four previously observed BDs allowed us to study the millimeter properties of the full known BD disk population of one star-forming region. Emission is detected in five out of the nine BD disks. Dust disk mass, brightness profiles, and characteristic sizes of the BD population are inferred from continuum flux and modeling of the observations. Only one source is marginally resolved, allowing for the determination of its disk characteristic size. We conduct a demographic comparison between the properties of disks around BDs and stars in Lupus. Due to the small sample size, we cannot confirm or disprove a drop in the disk mass over stellar mass ratio for BDs, as suggested for Ophiuchus. Nevertheless, we find that all detected BD disks have an estimated dust mass between 0.2 and 3.2 M ⊙ ; these results suggest that the measured solid masses in BD disks cannot explain the observed exoplanet population, analogous to earlier findings on disks around more massive stars. Combined with the low estimated accretion rates, and assuming that the mm-continuum emission is a reliable proxy for the total disk mass, we derive ratios of Ṁ acc ∕ M disk that are significantly lower than in disks around more massive stars. If confirmed with more accurate measurements of disk gas masses, this result could imply a qualitatively different relationship between disk masses and inward gas transport in BD disks.
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Measuring the ratio of the gas and dust emission radii of protoplanetary disks in the Lupus star-forming region
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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- Award ID(s):
- 1907486
- NSF-PAR ID:
- 10235110
- Date Published:
- Journal Name:
- Astronomy & Astrophysics
- Volume:
- 649
- ISSN:
- 0004-6361
- Page Range / eLocation ID:
- A19
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract 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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Abstract The stellar cluster environment is expected to play a central role in the evolution of circumstellar disks. We use thermochemical modeling to constrain the dust and gas masses, disk sizes, UV and X-ray radiation fields, viewing geometries, and central stellar masses of 20 class II disks in the Orion Nebula Cluster (ONC). We fit a large grid of disk models to 350 GHz continuum, CO J = 3 − 2, and HCO + J = 4 − 3 Atacama Large Millimeter/submillimeter Array observations of each target, and we introduce a procedure for modeling interferometric observations of gas disks detected in absorption against a bright molecular cloud background. We find that the ONC disks are massive and compact, with typical radii <100 au, gas masses ≥10 −3 M ⊙ , and gas-to-dust ratios ≥100. The interstellar‐medium‐like gas-to-dust ratios derived from our modeling suggest that compact, externally irradiated disks in the ONC are less prone to gas-phase CO depletion than the massive and extended gas disks that are commonly found in nearby low-mass star-forming regions. The presence of massive gas disks indicates that external photoevaporation may have only recently begun operating in the ONC; though it remains unclear whether other cluster members are older and more evaporated than the ones in our sample. Finally, we compare our dynamically derived stellar masses with the stellar masses predicted from evolutionary models and find excellent agreement. Our study has significantly increased the number of dynamical mass measurements in the mass range ≤0.5 M ⊙ , demonstrating that the ONC is an ideal region for obtaining large samples of dynamical mass measurements toward low-mass M-dwarfs.more » « less
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Abstract It remains unclear what mechanism is driving the evolution of protoplanetary disks. Direct detection of the main candidates, either turbulence driven by magnetorotational instabilities or magnetohydrodynamical disk winds, has proven difficult, leaving the time evolution of the disk size as one of the most promising observables able to differentiate between these two mechanisms. But to do so successfully, we need to understand what the observed gas disk size actually traces. We studied the relation between
R CO,90%, the radius that encloses 90% of the12CO flux, andR c , the radius that encodes the physical disk size, in order to provide simple prescriptions for conversions between these two sizes. For an extensive grid of thermochemical models, we calculateR CO,90%from synthetic observations and relate properties measured at this radius, such as the gas column density, to bulk disk properties, such asR c and the disk massM disk. We found an empirical correlation between the gas column density atR CO,90%and disk mass:R CO,90%that only depends onR c andM disk. We deriveR c for disks in Lupus, Upper Sco, Taurus, and the DSHARP sample, finding that disks in the older Upper Sco region are significantly smaller (〈R c 〉 = 4.8 au) than disks in the younger Lupus and Taurus regions (〈R c 〉 = 19.8 and 20.9 au, respectively). This temporal decrease inR c goes against predictions of both viscous and wind-driven evolution, but could be a sign of significant external photoevaporation truncating disks in Upper Sco. -
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.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1051/0004-6361/202039733
