ABSTRACT The disk mass is among the most important input parameter for every planet formation model to determine the number and masses of the planets that can form. We present an ALMA 887 m survey of ...the disk population around objects from ∼2 to 0.03 M in the nearby ∼2 Myr old Chamaeleon I star-forming region. We detect thermal dust emission from 66 out of 93 disks, spatially resolve 34 of them, and identify two disks with large dust cavities of about 45 au in radius. Assuming isothermal and optically thin emission, we convert the 887 m flux densities into dust disk masses, hereafter Mdust. We find that the relation is steeper than linear and of the form Mdust ∝ (M*)1.3-1.9, where the range in the power-law index reflects two extremes of the possible relation between the average dust temperature and stellar luminosity. By reanalyzing all millimeter data available for nearby regions in a self-consistent way, we show that the 1-3 Myr old regions of Taurus, Lupus, and Chamaeleon I share the same relation, while the 10 Myr old Upper Sco association has a steeper relation. Theoretical models of grain growth, drift, and fragmentation reproduce this trend and suggest that disks are in the fragmentation-limited regime. In this regime millimeter grains will be located closer in around lower-mass stars, a prediction that can be tested with deeper and higher spatial resolution ALMA observations.
ABSTRACT
We present models of giant planet migration in evolving protoplanetary discs. We show that disc clearing by extreme‐ultraviolet photoevaporation can have a strong effect on the distribution ...of giant planet semimajor axes. During disc clearing, planet migration is slowed or accelerated in the region where photoevaporation opens a gap in the disc, resulting in ‘deserts’ where few giant planets are found and corresponding ‘pile‐ups’ at smaller and larger radii. However, the precise locations and sizes of these features are strong functions of the efficiency of planetary accretion, and therefore also strongly dependent on planet mass. We suggest that photoevaporative disc clearing may be responsible for the pile‐up of ∼Jupiter‐mass planets at ∼1 au seen in exoplanet surveys, and show that observations of the distribution of exoplanet semimajor axes can be used to test models of both planet migration and disc clearing.
Context.
The discovery of giant planets orbiting very low mass stars (VLMS) and the recent observed substructures in disks around VLMS is challenging planet formation models. Specifically, radial ...drift of dust particles is a catastrophic barrier in these disks, which prevents the formation of planetesimals and therefore planets.
Aims.
We aim to estimate if structures, such as cavities, rings, and gaps, are common in disks around VLMS and to test models of structure formation in these disks. We also aim to compare the radial extent of the gas and dust emission in disks around VLMS, which can give us insight about radial drift.
Methods.
We studied six disks around VLMS in the Taurus star-forming region using ALMA Band 7 (~340 GHz) at a resolution of ~0.1″. The targets were selected because of their high disk dust content in their stellar mass regime.
Results.
Our observations resolve the disk dust continuum in all disks. In addition, we detect the
12
CO (
J
= 3−2) emission line in all targets and
13
CO (
J
= 3−2) in five of the six sources. The angular resolution allows the detection of dust substructures in three out of the six disks, which we studied by using UV-modeling. Central cavities are observed in the disks around stars MHO 6 (M 5.0) and CIDA 1 (M 4.5), while we have a tentative detection of a multi-ringed disk around J0433. We estimate that a planet mass of ~0.1
M
Jup
or ~0.4
M
Saturn
is required for a single planet to create the first gap in J0433. For the cavities of MHO 6 and CIDA 1, a Saturn-mass planet (~0.3
M
Jup
) is required. The other three disks with no observed structures are the most compact and faintest in our sample, with the radius enclosing 90% of the continuum emission varying between ~13 and 21 au. The emission of
12
CO and
13
CO is more extended than the dust continuum emission in all disks of our sample. When using the
12
CO emission to determine the gas disk extension
R
gas
, the ratio of
R
gas
∕
R
dust
in our sample varies from 2.3 to 6.0. One of the disks in our sample, CIDA 7, has the largest
R
gas
∕
R
dust
ratio observed so far, which is consistent with models of radial drift being very efficient around VLMS in the absence of substructures.
Conclusions.
Given our limited angular resolution, substructures were only directly detected in the most extended disks, which represent 50% of our sample, and there are hints of unresolved structured emission in one of the bright smooth sources. Our observations do not exclude giant planet formation on the substructures observed. A comparison of the size and luminosity of VLMS disks with their counterparts around higher mass stars shows that they follow a similar relation.
The lifetime of isolated protoplanetary disks is thought to be set by the combination of viscous accretion and photoevaporation driven by stellar high-energy photons. Observational evidence for ...magnetospheric accretion in young Sun-like stars is robust. Here we report the first observational evidence for disk photoevaporation driven by the central star. We acquired high-resolution (R~ 30,000) spectra of the Ne II 12.81 Delta *mm line from seven circumstellar disks using VISIR on Melipal/VLT. We show that the three transition disks in the sample all have Ne II line profiles consistent with those predicted by a photoevaporative flow driven by stellar extreme-ultraviolet (EUV) photons. The ~6 km s-1 blueshift of the line from the almost face-on disk of TW Hya is clearly inconsistent with emission from a static disk atmosphere and convincingly points to the presence of a photoevaporative wind. We do not detect any Ne II line close to the stellar velocity from the sample of classical optically thick (nontransition) disks. We conclude that most of the spectrally unresolved Ne II emission in these less-evolved systems arises from jets/outflows rather than from the disk. The pattern of the Ne II detections and nondetections suggests that EUV-driven photoevaporation starts only at a later stage in the disk evolution.
We present Atacama Large Millimeter Array 850 m continuum observations of the Orion Nebula Cluster that provide the highest angular resolution (∼0 1 40 au) and deepest sensitivity (∼0.1 mJy) of the ...region to date. We mosaicked a field containing ∼225 optical or near-IR-identified young stars, ∼60 of which are also optically identified "proplyds." We detect continuum emission at 850 m toward ∼80% of the proplyd sample, and ∼50% of the larger sample of previously identified cluster members. Detected objects have fluxes of ∼0.5-80 mJy. We remove submillimeter flux due to free-free emission in some objects, leaving a sample of sources detected in dust emission. Under standard assumptions of isothermal, optically thin disks, submillimeter fluxes correspond to dust masses of ∼0.5-80 Earth masses. We measure the distribution of disk sizes, and find that disks in this region are particularly compact. Such compact disks are likely to be significantly optically thick. The distributions of submillimeter flux and inferred disk size indicate smaller, lower-flux disks than in lower-density star-forming regions of similar age. Measured disk flux is correlated weakly with stellar mass, contrary to studies in other star-forming regions that found steeper correlations. We find a correlation between disk flux and distance from the massive star θ1 Ori C, suggesting that disk properties in this region are influenced strongly by the rich cluster environment.
Abstract
Spectroastrometry is used to investigate the low-velocity component (LVC) of the optical forbidden emission from the T Tauri stars RU Lupi and AS 205 N. Both stars also have high-velocity ...forbidden emission, which is tracing a jet. For AS 205 N, analysis reveals a complicated outflow system. For RU Lupi, the O
i
λ
6300 and S
ii
λλ
6716,6731 LV narrow component (NC) is offset along the same position angle (PA) as the high-velocity component but with a different velocity gradient than the jet, in that displacement from the stellar position along the rotation axis is decreasing with increasing velocity. From the LVC, NC, PA, and velocity gradient, it is inferred that the NC is tracing a wide-angled magnetohydrodynamic (MHD) disk wind. A photoevaporative wind is ruled out. This is supported by a comparison with a previous spectroastrometric study of the CO fundamental line. The decrease in offset with increasing velocity is interpreted as tracing an increase in the height of the wind with increasing disk radius. This is one of the first measurements of the spatial extent of the forbidden emission line LVC NC (∼40 au, 8 au for RU Lupi in the S
ii
λ
6731 and O
i
λ
6300 lines) and the first direct confirmation that the LVC narrow component can trace an MHD disk wind.
ABSTRACT Using Keck/HIRES spectra (Δ v ∼ 7 km s−1) we analyze forbidden lines of O i 6300 , O i 5577 and S ii 6731 from 33 T Tauri stars covering a range of disk evolutionary stages. After removing a ...high-velocity component (HVC) associated with microjets, we study the properties of the low-velocity component (LVC). The LVC can be attributed to slow disk winds that could be magnetically (magnetohydrodynamic) or thermally (photoevaporative) driven. Both of these winds play an important role in the evolution and dispersal of protoplanetary material. LVC emission is seen in all 30 stars with detected O i but only in two out of eight with detected S ii, so our analysis is largely based on the properties of the O i LVC. The LVC itself is resolved into broad (BC) and narrow (NC) kinematic components. Both components are found over a wide range of accretion rates and their luminosity is correlated with the accretion luminosity, but the NC is proportionately stronger than the BC in transition disks. The full width at half maximum of both the BC and NC correlates with disk inclination, consistent with Keplerian broadening from radii of 0.05 to 0.5 au and 0.5 to 5 au, respectively. The velocity centroids of the BC suggest formation in an MHD disk wind, with the largest blueshifts found in sources with closer to face-on orientations. The velocity centroids of the NC, however, show no dependence on disk inclination. The origin of this component is less clear and the evidence for photoevaporation is not conclusive.
We compare line emission calculated from theoretical disk models with optical to submillimeter wavelength observational data of the gas disk surrounding TW Hya and infer the spatial distribution of ...mass in the gas disk. The model disk that best matches observations has a gas mass ranging from ~10--4 to 10--5 M for 0.06 AU < r < 3.5 AU and ~0.06 M for 3.5 AU < r < 200 AU. We find that the inner dust hole (r < 3.5 AU) in the disk must be depleted of gas by ~1-2 orders of magnitude compared with the extrapolated surface density distribution of the outer disk. Grain growth alone is therefore not a viable explanation for the dust hole. CO vibrational emission arises within r ~ 0.5 AU from thermal excitation of gas. O I 6300 A and 5577 A forbidden lines and OH mid-infrared emission are mainly due to prompt emission following UV photodissociation of OH and water at r 0.1 AU and at r ~ 4 AU. Ne II emission is consistent with an origin in X-ray heated neutral gas at r 10 AU, and may not require the presence of a significant extreme-ultraviolet (h Delta *n > 13.6 eV) flux from TW Hya. H2 pure rotational line emission comes primarily from r ~ 1 to 30 AU. O I 63 Delta *mm, HCO+, and CO pure rotational lines all arise from the outer disk at r ~ 30-120 AU. We discuss planet formation and photoevaporation as causes for the decrease in surface density of gas and dust inside 4 AU. If a planet is present, our results suggest a planet mass ~4-7 MJ situated at ~3 AU. Using our photoevaporation models and the best surface density profile match to observations, we estimate a current photoevaporative mass loss rate of 4 X 10--9 M yr--1 and a remaining disk lifetime of ~5 million years.
The formation time, masses, and location of planets are strongly impacted by the physical mechanisms that disperse protoplanetary disks and the timescale over which protoplanetary material is cleared ...out. Accretion of matter onto the central star, protostellar winds/jets, magnetic disk winds, and photoevaporative winds operate concurrently. Here, we analyze the low-velocity component (LVC) of the oxygen optical forbidden lines, which is found to be blueshifted by a few km s super(-1) with respect to the stellar velocity. We find that the OI LVC profiles are different from those of NeII at 12.81 mu m and CO at 4.7 mu m lines pointing to different origins for these gas lines. We report a correlation between the luminosity of the OI LVC and the accretion luminosity L sub(acc). These findings favor an origin of the OI LVC in a region where OH is photodissociated by stellar FUV photons and argue against thermal emission from an X-ray-heated layer.
There is growing observational evidence that disk evolution is stellar-mass-dependent. Here, we show that these dependencies extend to the atomic and molecular content of disk atmospheres. We analyze ...a unique dataset of high-resolution Spitzer/IRS spectra from eight very low mass star and brown dwarf disks. We report the first detections of Ne super(+), H sub(2), CO sub(2), and tentative detections of H sub(2)O toward these faint and low-mass disks. Two of our NeII 12.81 mu m emission lines likely trace the hot (> or =, slanted5000 K) disk surface irradiated by X-ray photons from the central stellar/sub-stellar object. The H sub(2) S(2) and S(1) fluxes are consistent with arising below the fully or partially ionized surface traced by the NeII emission in gas at ~600 K. We confirm the higher C sub(2)H sub(2)/HCN flux and column density ratio in brown dwarf disks previously noted from low-resolution IRS spectra. Our high-resolution spectra also show that the HCN/H sub(2)O fluxes of brown dwarf disks are on average higher than those of T Tauri disks. Our LTE modeling hints that this difference extends to column density ratios if H sub(2)O lines trace warm > or =, slanted600 K disk gas. These trends suggest that the inner regions of brown dwarf disks have a lower O/C ratio than those of T Tauri disks, which may result from a more efficient formation of non-migrating icy planetesimals. An O/C = 1, as inferred from our analysis, would have profound implications on the bulk composition of rocky planets that can form around very low mass stars and brown dwarfs.
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