Rings are the most frequently revealed substructure in Atacama Large Millimeter/submillimeter Array (ALMA) dust observations of protoplanetary disks, but their origin is still hotly debated. In this ...paper, we identify dust substructures in 12 disks and measure their properties to investigate how they form. This subsample of disks is selected from a high-resolution (∼0 12) ALMA 1.33 mm survey of 32 disks in the Taurus star-forming region, which was designed to cover a wide range of brightness and to be unbiased to previously known substructures. While axisymmetric rings and gaps are common within our sample, spiral patterns and high-contrast azimuthal asymmetries are not detected. Fits of disk models to the visibilities lead to estimates of the location and shape of gaps and rings, the flux in each disk component, and the size of the disk. The dust substructures occur across a wide range of stellar mass and disk brightness. Disks with multiple rings tend to be more massive and more extended. The correlation between gap locations and widths, the intensity contrast between rings and gaps, and the separations of rings and gaps could all be explained if most gaps are opened by low-mass planets (super-Earths and Neptunes) in the condition of low disk turbulence ( = 10−4). The gap locations are not well correlated with the expected locations of CO and N2 ice lines, so condensation fronts are unlikely to be a universal mechanism to create gaps and rings, though they may play a role in some cases.
We present a synergic study of protoplanetary disks to investigate links between inner-disk gas molecules and the large-scale migration of solid pebbles. The sample includes 63 disks where two types ...of measurements are available: (1) spatially resolved disk images revealing the radial distribution of disk pebbles (millimeter to centimeter dust grains), from millimeter observations with the Atacama Large Millimeter/Submillimeter Array or the Submillimeter Array, and (2) infrared molecular emission spectra as observed with Spitzer. The line flux ratios of H2O with HCN, C2H2, and CO2 all anticorrelate with the dust disk radius Rdust, expanding previous results found by Najita et al. for HCN/H2O and the dust disk mass. By normalization with the dependence on accretion luminosity common to all molecules, only the H2O luminosity maintains a detectable anticorrelation with disk radius, suggesting that the strongest underlying relation is between H2O and Rdust. If Rdust is set by large-scale pebble drift, and if molecular luminosities trace the elemental budgets of inner-disk warm gas, these results can be naturally explained with scenarios where the inner disk chemistry is fed by sublimation of oxygen-rich icy pebbles migrating inward from the outer disk. Anticorrelations are also detected between all molecular luminosities and the infrared index n13-30, which is sensitive to the presence and size of an inner-disk dust cavity. Overall, these relations suggest a physical interconnection between dust and gas evolution, both locally and across disk scales. We discuss fundamental predictions to test this interpretation and study the interplay between pebble drift, inner disk depletion, and the chemistry of planet-forming material.
The mass of a protoplanetary disk limits the formation and future growth of any planet. Masses of protoplanetary disks are usually calculated from measurements of the dust continuum emission by ...assuming an interstellar gas-to-dust ratio. To investigate the utility of CO as an alternate probe of disk mass, we use ALMA to survey 13CO and C18O J = 3-2 line emission from a sample of 93 protoplanetary disks around stars and brown dwarfs with masses from in the nearby Chamaeleon I star-forming region. We detect 13CO emission from 17 sources and C18O from only one source. Gas masses for disks are then estimated by comparing the CO line luminosities to results from published disk models that include CO freeze-out and isotope-selective photodissociation. Under the assumption of a typical interstellar medium CO-to-H2 ratio of 10−4, the resulting gas masses are implausibly low, with an average gas mass of ∼0.05 MJup as inferred from the average flux of stacked 13CO lines. The low gas masses and gas-to-dust ratios for Cha I disks are both consistent with similar results from disks in the Lupus star-forming region. The faint CO line emission may instead be explained if disks have much higher gas masses, but freeze-out of CO or complex C-bearing molecules is underestimated in disk models. The conversion of CO flux to CO gas mass also suffers from uncertainties in disk structures, which could affect gas temperatures. CO emission lines will only be a good tracer of the disk mass when models for C and CO depletion are confirmed to be accurate.
We present new Atacama Large Millimeter/submillimeter Array observations for three protoplanetary disks in Taurus at 2.9 mm and comparisons with previous 1.3 mm data both at an angular resolution of ...∼0 1 (15 au for the distance of Taurus). In the single-ring disk DS Tau, double-ring disk GO Tau, and multiring disk DL Tau, the same rings are detected at both wavelengths, with radial locations spanning from 50 to 120 au. To quantify the dust emission morphology, the observed visibilities are modeled with a parametric prescription for the radial intensity profile. The disk outer radii, taken as 95% of the total flux encircled in the model intensity profiles, are consistent at both wavelengths for the three disks. Dust evolution models show that dust trapping in local pressure maxima in the outer disk could explain the observed patterns. Dust rings are mostly unresolved. The marginally resolved ring in DS Tau shows a tentatively narrower ring at the longer wavelength, an observational feature expected from efficient dust trapping. The spectral index ( mm) increases outward and exhibits local minima that correspond to the peaks of dust rings, indicative of the changes in grain properties across the disks. The low optical depths (τ ∼ 0.1-0.2 at 2.9 mm and 0.2-0.4 at 1.3 mm) in the dust rings suggest that grains in the rings may have grown to millimeter sizes. The ubiquitous dust rings in protoplanetary disks modify the overall dynamics and evolution of dust grains, likely paving the way toward the new generation of planet formation.
Gap-like structures in protoplanetary disks are likely related to planet formation processes. In this paper, we present and analyze high-resolution (0.17′′× 0.11′′) 1.3 mm ALMA continuum observations ...of the protoplanetary disk around the Herbig Ae star MWC 480. Our observations show for the first time a gap centered at ~74 au with a width of ~23 au, surrounded by a bright ring centered at ~98 au from the central star. Detailed radiative transfer modeling of the ALMA image and the broadband spectral energy distribution is used to constrain the surface density profile and structural parameters of the disk. If the width of the gap corresponds to 4–8 times the Hill radius of a single forming planet, then the putative planet would have a mass of 0.4–3 MJ. We test this prediction by performing global three-dimensional smoothed particle hydrodynamic gas/dust simulations of disks hosting a migrating and accreting planet. We find that the dust emission across the disk is consistent with the presence of an embedded planet with a mass of ~2.3 MJ at an orbital radius of ~78 au. Given the surface density of the best-fit radiative transfer model, the amount of depleted mass in the gap is higher than the mass of the putative planet, which satisfies the basic condition for the formation of such a planet.
Abstract
ALMA has observed a plethora of ring-like structures in planet-forming discs at distances of 10–100 au from their host star. Although several mechanisms have been invoked to explain the ...origin of such rings, a common explanation is that they trace new-born planets. Under the planetary hypothesis, a natural question is how to reconcile the apparently high frequency of gap-carving planets at 10–100 au with the paucity of Jupiter-mass planets observed around main-sequence stars at those separations. Here, we provide an analysis of the new-born planet population emerging from observations of gaps in discs, under the assumption that the observed gaps are due to planets. We use a simple estimate of the planet mass based on the gap morphology, and apply it to a sample of gaps recently obtained by us in a survey of Taurus with ALMA. We also include additional data from recent published surveys, thus analysing the largest gap sample to date, for a total of 48 gaps. The properties of the purported planets occupy a distinctively different region of parameter space with respect to the known exo-planet population, currently not accessible through planet finding methods. Thus, no discrepancy in the mass and radius distribution of the two populations can be claimed at this stage. We show that the mass of the inferred planets conforms to the theoretically expected trend for the minimum planet mass needed to carve a dust gap. Finally, we estimate the separation and mass of the putative planets after accounting for migration and accretion, for a range of evolutionary times, finding a good match with the distribution of cold Jupiters.
We present a synergic study of protoplanetary disks to investigate links between inner disk gas molecules and the large-scale migration of solid pebbles. The sample includes 63 disks where two types ...of measurements are available: i) spatially-resolved disk images revealing the radial distribution of disk pebbles (mm-cm dust grains), from millimeter observations with ALMA or the SMA, and ii) infrared molecular emission spectra as observed with Spitzer. The line flux ratios of H2O with HCN, C2H2, and CO2 all anti-correlate with the dust disk radius R\(_{dust}\), expanding previous results found by Najita et al. (2013) for HCN/H2O and the dust disk mass. By normalization with the dependence on accretion luminosity common to all molecules, only the H2O luminosity maintains a detectable anti-correlation with disk radius, suggesting that the strongest underlying relation is between H2O and R\(_{dust}\). If R\(_{dust}\) is set by large-scale pebble drift, and if molecular luminosities trace the elemental budgets of inner disk warm gas, these results can be naturally explained with scenarios where the inner disk chemistry is fed by sublimation of oxygen-rich icy pebbles migrating inward from the outer disk. Anti-correlations are also detected between all molecular luminosities and the infrared index n\(_{13-30}\), which is sensitive to the presence and size of an inner disk dust cavity. Overall, these relations suggest a physical interconnection between dust and gas evolution both locally and across disk scales. We discuss fundamental predictions to test this interpretation and study the interplay between pebble drift, inner disk depletion, and the chemistry of planet-forming material.
We present new Atacama Large Millimeter/submillimeter Array (ALMA) observations for three protoplanetary disks in Taurus at 2.9\,mm and comparisons with previous 1.3\,mm data both at an angular ...resolution of \(\sim0.''1\) (15\,au for the distance of Taurus). In the single-ring disk DS Tau, double-ring disk GO Tau, and multiple-ring disk DL Tau, the same rings are detected at both wavelengths, with radial locations spanning from 50 to 120\,au. To quantify the dust emission morphology, the observed visibilities are modeled with a parametric prescription for the radial intensity profile. The disk outer radii, taken as 95\% of the total flux encircled in the model intensity profiles, are consistent at both wavelengths for the three disks. Dust evolution models show that dust trapping in local pressure maxima in the outer disk could explain the observed patterns. Dust rings are mostly unresolved. The marginally resolved ring in DS Tau shows a tentatively narrower ring at the longer wavelength, an observational feature expected from efficient dust trapping. The spectral index (\(\alpha_{\rm mm}\)) increases outward and exhibits local minima that correspond to the peaks of dust rings, indicative of the changes in grain properties across the disks. The low optical depths (\(\tau\sim\)0.1--0.2 at 2.9\,mm and 0.2--0.4 at 1.3\,mm) in the dust rings suggest that grains in the rings may have grown to millimeter sizes. The ubiquitous dust rings in protoplanetary disks modify the overall dynamics and evolution of dust grains, likely paving the way towards the new generation of planet formation.
The mass of a protoplanetary disk limits the formation and future growth of any planet. Masses of protoplanetary disks are usually calculated from measurements of the dust continuum emission by ...assuming an interstellar gas-to-dust ratio. To investigate the utility of CO as an alternate probe of disk mass, we use ALMA to survey \(^{13}\)CO and C\(^{18}\)O J = \(3-2\) line emission from a sample of 93 protoplanetary disks around stars and brown dwarfs with masses from 0.03 -- 2 M\(_{\odot}\) in the nearby Chamaeleon I star-forming region. We detect \(^{13}\)CO emission from 17 sources and C\(^{18}\)O from only one source. Gas masses for disks are then estimated by comparing the CO line luminosities to results from published disk models that include CO freeze-out and isotope-selective photodissociation. Under the assumption of a typical ISM CO-to-H\(_2\) ratios of \(10^{-4}\), the resulting gas masses are implausibly low, with an average gas mass of \(\sim\) 0.05 M\(_{Jup}\) as inferred from the average flux of stacked \(^{13}\)CO lines. The low gas masses and gas-to-dust ratios for Cha I disks are both consistent with similar results from disks in the Lupus star-forming region. The faint CO line emission may instead be explained if disks have much higher gas masses, but freeze-out of CO or complex C-bearing molecules is underestimated in disk models. The conversion of CO flux to CO gas mass also suffers from uncertainties in disk structures, which could affect gas temperatures. CO emission lines will only be a good tracer of the disk mass when models for C and CO depletion are confirmed to be accurate.
Gap-like structures in protoplanetary disks are likely related to planet formation processes. In this paper, we present and analyze high resolution (0.17*0.11 arcsec) 1.3 mm ALMA continuum ...observations of the protoplanetary disk around the Herbig Ae star MWC 480. Our observations for the first time show a gap centered at ~74au with a width of ~23au, surrounded by a bright ring centered at ~98au from the central star. Detailed radiative transfer modeling of both the ALMA image and the broadband spectral energy distribution is used to constrain the surface density profile and structural parameters of the disk. If the width of the gap corresponds to 4~8 times the Hill radius of a single forming planet, then the putative planet would have a mass of 0.4~3 M_Jup. We test this prediction by performing global three-dimensional smoothed particle hydrodynamic gas/dust simulations of disks hosting a migrating and accreting planet. We find that the dust emission across the disk is consistent with the presence of an embedded planet with a mass of ~2.3 M_Jup at an orbital radius of ~78au. Given the surface density of the best-fit radiative transfer model, the amount of depleted mass in the gap is higher than the mass of the putative planet, which satisfies the basic condition for the formation of such a planet.
PDF
