We present Atacama Large Millimeter/Sub-Millimeter Array (ALMA) Band 6 observations of a complete sample of protoplanetary disks in the young (∼1-3 Myr) Lupus star-forming region, covering the 1.33 ...mm continuum and the 12CO, 13CO, and C18O J = 2-1 lines. The spatial resolution is ∼0 25 with a medium 3 continuum sensitivity of 0.30 mJy, corresponding to Mdust ∼ 0.2 M⊕. We apply Keplerian masking to enhance the signal-to-noise ratios of our 12CO zero-moment maps, enabling measurements of gas disk radii for 22 Lupus disks; we find that gas disks are universally larger than millimeter dust disks by a factor of two on average, likely due to a combination of the optically thick gas emission and the growth and inward drift of the dust. Using the gas disk radii, we calculate the dimensionless viscosity parameter, visc, finding a broad distribution and no correlations with other disk or stellar parameters, suggesting that viscous processes have not yet established quasi-steady states in Lupus disks. By combining our 1.33 mm continuum fluxes with our previous 890 m continuum observations, we also calculate the millimeter spectral index, mm, for 70 Lupus disks; we find an anticorrelation between mm and millimeter flux for low-mass disks (Mdust 5), followed by a flattening as disks approach mm 2, which could indicate faster grain growth in higher-mass disks, but may also reflect their larger optically thick components. In sum, this work demonstrates the continuous stream of new insights into disk evolution and planet formation that can be gleaned from unbiased ALMA disk surveys.
ABSTRACT We present the first high-resolution sub-millimeter survey of both dust and gas for a large population of protoplanetary disks. Characterizing fundamental properties of protoplanetary disks ...on a statistical level is critical to understanding how disks evolve into the diverse exoplanet population. We use the Atacama Large Millimeter/Submillimeter Array (ALMA) to survey 89 protoplanetary disks around stars with in the young (1-3 Myr), nearby (150-200 pc) Lupus complex. Our observations cover the 890 m continuum and the 13CO and C18O 3-2 lines. We use the sub-millimeter continuum to constrain to a few Martian masses (0.2-0.4 M⊕) and the CO isotopologue lines to constrain to roughly a Jupiter mass (assuming an interstellar medium (ISM)-like abundance). Of 89 sources, we detect 62 in continuum, 36 in 13CO, and 11 in C18O at significance. Stacking individually undetected sources limits their average dust mass to Lunar masses (0.03 M⊕), indicating rapid evolution once disk clearing begins. We find a positive correlation between and M*, and present the first evidence for a positive correlation between and M*, which may explain the dependence of giant planet frequency on host star mass. The mean dust mass in Lupus is 3× higher than in Upper Sco, while the dust mass distributions in Lupus and Taurus are statistically indistinguishable. Most detected disks have and gas-to-dust ratios , assuming an ISM-like abundance; unless CO is very depleted, the inferred gas depletion indicates that planet formation is well underway by a few Myr and may explain the unexpected prevalence of super-Earths in the exoplanet population.
Context. The formation of planets strongly depends on the total amount as well as on the spatial distribution of solids in protoplanetary disks. Thanks to the improvements in resolution and ...sensitivity provided by ALMA, measurements of the surface density of mm-sized grains are now possible on large samples of disks. Such measurements provide statistical constraints that can be used to inform our understanding of the initial conditions of planet formation. Aims. We aim to analyze spatially resolved observations of 36 protoplanetary disks in the Lupus star forming complex from our ALMA survey at 890 μm, aiming to determine physical properties such as the dust surface density, the disk mass and size, and to provide a constraint on the temperature profile. Methods. We fit the observations directly in the uv-plane using a two-layer disk model that computes the 890 μm emission by solving the energy balance at each disk radius. Results. For 22 out of 36 protoplanetary disks we derive robust estimates of their physical properties. The sample covers stellar masses between ~0.1 and ~ 2 M⊙, and we find no trend in the relationship between the average disk temperatures and the stellar parameters. We find, instead, a correlation between the integrated sub-mm flux (a proxy for the disk mass) and the exponential cut-off radii (a proxy of the disk size) of the Lupus disks. Comparing these results with observations at similar angular resolution of Taurus-Auriga and Ophiuchus disks found in literature and scaling them to the same distance, we observe that the Lupus disks are generally fainter and larger at a high level of statistical significance. Considering the 1–2 Myr age difference between these regions, it is possible to tentatively explain the offset in the disk mass-size relation with viscous spreading, however with the current measurements other mechanisms cannot be ruled out.
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.
ABSTRACT We present ten young ( 10 Myr) late-K and M dwarf stars observed in K2 Campaign 2 that host protoplanetary disks and exhibit quasi-periodic or aperiodic dimming events. Their optical light ...curves show ∼10-20 dips in flux over the 80-day observing campaign with durations of ∼0.5-2 days and depths of up to ∼40%. These stars are all members of the Ophiuchus (∼1 Myr) or Upper Scorpius (∼10 Myr) star-forming regions. To investigate the nature of these "dippers" we obtained: optical and near-infrared spectra to determine stellar properties and identify accretion signatures; adaptive optics imaging to search for close companions that could cause optical variations and/or influence disk evolution; and millimeter-wavelength observations to constrain disk dust and gas masses. The spectra reveal Li i absorption and H emission consistent with stellar youth (<50 Myr), but also accretion rates spanning those of classical and weak-line T Tauri stars. Infrared excesses are consistent with protoplanetary disks extending to within ∼10 stellar radii in most cases; however, the sub-millimeter observations imply disk masses that are an order of magnitude below those of typical protoplanetary disks. We find a positive correlation between dip depth and WISE-2 (Wide-field Infrared Survey Explorer-2) excess, which we interpret as evidence that the dipper phenomenon is related to occulting structures in the inner disk, although this is difficult to reconcile with the weakly accreting aperiodic dippers. We consider three mechanisms to explain the dipper phenomenon: inner disk warps near the co-rotation radius related to accretion; vortices at the inner disk edge produced by the Rossby Wave Instability; and clumps of circumstellar material related to planetesimal formation.
Context. An era has started in which gas and dust can be observed independently in protoplanetary disks, thanks to the recent surveys with the Atacama Large Millimeter/sub-millimeter Array (ALMA). ...The first near-complete high-resolution disk survey in both dust and gas in a single star-forming region has been carried out in Lupus, finding surprisingly low gas-to-dust ratios. Aims. The goal of this work is to fully exploit CO isotopologue observations in Lupus, comparing them with physical-chemical model results, in order to obtain gas masses for a large number of disks and compare gas and dust properties. Methods. We have employed the grid of physical-chemical models presented previously to analyze continuum and CO isotopologue (13CO J = 3−2 and C18O J = 3−2) observations of Lupus disks, including isotope-selective processes and freeze-out. We also employed the ALMA 13CO-only detections to calculate disk gas masses for a total of 34 sources, which expands the sample of 10 disks reported earlier, where C18O was also detected. Results. We confirm that overall gas-masses are very low, often lower than 1MJ, when volatile carbon is not depleted. Accordingly, global gas-to-dust ratios are much lower than the expected interstellar-medium value of 100, which is predominantly between 1 and 10. Low CO-based gas masses and gas-to-dust ratios may indicate rapid loss of gas, or alternatively chemical evolution, for example, through sequestering of carbon from CO to more complex molecules, or carbon locked up in larger bodies. Conclusions. Current ALMA observations of 13CO and continuum emission cannot distinguish between these two hypotheses. We have simulated both scenarios, but chemical model results do not allow us to rule out one of the two, pointing to the need to calibrate CO-based masses with other tracers. Assuming that all Lupus disks have evolved mainly as a result of viscous processes over the past few Myr, the previously observed correlation between the current mass accretion rate and dust mass implies a constant gas-to-dust ratio, which is close to 100 based on the observed Mdisk/Ṁacc ratio. This in turn points to a scenario in which carbon depletion is responsible for the low luminosities of the CO isotopologue line.
Context.
Recent ALMA surveys of protoplanetary disks have shown that for most disks the extent of the gas emission is greater than the extent of the thermal emission of millimeter-sized dust. Both ...line optical depth and the combined effect of radially dependent grain growth and radial drift may contribute to this observed effect. To determine whether or not radial drift is common across the disk population, quantitative estimates of the effect of line optical depth are required.
Aims.
For a sample of ten disks from the Lupus survey we investigate how well dust-based models without radial dust evolution reproduce the observed
12
CO outer radius, and determine whether radial dust evolution is required to match the observed gas–dust size difference.
Methods.
Based on surface density profiles derived from continuum observations we used the thermochemical code DALI to obtain
12
CO synthetic emission maps. Gas and dust outer radii of the models were calculated using the same methods as applied to the observations. The gas and dust outer radii (
R
CO
,
R
mm
) calculated using only line optical depth were compared to observations on a source-by-source basis.
Results.
For five disks, we find
R
CO, obs
∕
R
mm, obs
>
R
CO, mdl
∕
R
mm, mdl
. For these disks we need both dust evolution and optical depth effects to explain the observed gas–dust size difference. For the other five disks, the observed
R
CO
∕
R
mm
lies within the uncertainties on
R
CO, mdl
∕
R
mm, mdl
due to noise. For these disks the observed gas–dust size difference can be explained using only line optical depth effects. We also identify six disks not included in our initial sample but part of a survey of the same star-forming region that show significant signal-to-noise ratio (
S
∕
N
≥ 3)
12
CO
J
= 2−1 emission beyond 4 ×
R
mm
. These disks, for which no
R
CO
is available, likely have
R
CO
∕
R
mm
≫ 4 and are difficult to explain without substantial dust evolution.
Conclusions.
Most of the disks in our sample of predominantly bright disks are consistent with radial drift and grain growth. We also find six faint disks where the observed gas–dust size difference hints at considerable radial drift and grain growth, suggesting that these are common features among both bright and faint disks. The effects of radial drift and grain growth can be observed in disks where the dust and gas radii are significantly different, while more detailed models and deeper observations are needed to see this effect in disks with smaller differences.
ABSTRACT
Spectroscopic transit detection of constituents in winds from ‘evaporating’ planets on close-in transiting orbits could provide desperately needed information on the composition, formation, ...and orbital evolution of such objects. We obtained high-resolution optical spectra of the host stars during a single transit of Kepler-1520b and two transits of K2-22b to search for transient, Doppler-shifted absorption in the D lines of neutral sodium. Sodium should be released in the same silicate vapour wind that lofts the dust responsible for the periodic ‘dips’ in the light curve. We do not detect any absorption lines with depths >30 per cent at the predicted Doppler-shifted wavelengths during any of the transits. Detection sensitivity is limited by instrumental resolution that dilutes the saturated lines, and blurring of the lines by Doppler acceleration due to the short orbital period of the planet and long integration times for these faint stars. A model of neutral sodium production, escape, and ionization by UV radiation suggests that clouds of partially ionized sodium that are comparable in size to the host stars and optically thick in the D lines could accompany the planets. We consider the prospects for future detections brought about by the TESS all-sky survey of brighter stars and the advent of high-resolution spectrographs on Extremely Large Telescopes.
A relation between the mass accretion rate onto the central young star and the mass of the surrounding protoplanetary disk has long been theoretically predicted and observationally sought. For the ...first time, we have accurately and homogeneously determined the photospheric parameters, mass accretion rate, and disk mass for an essentially complete sample of young stars with disks in the Lupus clouds. Our work combines the results of surveys conducted with VLT/X-Shooter and ALMA. With this dataset we are able to test a basic prediction of viscous accretion theory, the existence of a linear relation between the mass accretion rate onto the central star and the total disk mass. We find a correlation between the mass accretion rate and the disk dust mass, with a ratio that is roughly consistent with the expected viscous timescale when assuming an interstellar medium gas-to-dust ratio. This confirms that mass accretion rates are related to the properties of the outer disk. We find no correlation between mass accretion rates and the disk mass measured by CO isotopologues emission lines, possibly owing to the small number of measured disk gas masses. This suggests that the mm-sized dust mass better traces the total disk mass and that masses derived from CO may be underestimated, at least in some cases.
Context. Recent ALMA surveys in different star-forming regions have shown that CO emission in protoplanetary discs is much fainter than expected. Accordingly, CO-based gas masses and gas to dust ...ratios are orders of magnitude lower than previously thought. This may be explained either as fast gas dispersal, or as chemical evolution and locking up of volatiles in larger bodies leading to the low observed CO fluxes. The latter processes lead to enhanced C/O ratios in the gas, which may be reflected in enhanced abundances of carbon-bearing molecules like C2H. Aims. The goal of this work is to use C2H observations to understand whether low CO fluxes are caused by volatile depletion or by fast gas dissipation. Methods. We present ALMA Cycle 4 C2H (N = 3–2, J = 7∕2–5∕2, F = 4–3 and F = 3–2) observations of a subsample of nine sources in the Lupus star-forming region. The integrated C2H emission is determined and compared to previous CO isotopologue observations and physical-chemical model predictions. Results. Seven out of nine discs are detected in C2H, whose line emission is almost as bright as 13CO. All detections are significantly brighter than the typical sensitivity of the observations, hinting at a bimodal distribution of the C2H line intensities. This conclusion is strengthened when our observations are compared with additional C2H observations of other discs. When compared with physical-chemical models, the observed C2H fluxes can be reproduced only if some level of volatile carbon and oxygen depletion is allowed and C/O > 1 in the gas. Models with reduced gas-to-dust ratios near unity however fail to reproduce the observed C2H line luminosity. A steeper than linear correlation between C2H and CN emission line is found for the Lupus discs. This is linked to the fact that C2H emission lines are affected more strongly by C/O variations than CN lines. Ring-like structures are detected both in C2H and in continuum emission but, as for CN, they do not seem to be connected. The source Sz 71 shows ring-shaped emission in both C2H and CN with the location of the peak intensity coinciding, within our 30 au resolution. Conclusions. Our new ALMA C2H observations favour volatile carbon and oxygen depletion rather than fast gas dispersal to explain the faint CO observations for most of the discs. This result has implications for disc-evolution and planet-formation theories, as disc gas masses may be larger than expected if CO is considered to be the main carbon carrier in the gas phase.
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