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Register a new userThe Taurus Boundary of Stellar/Substellar (TBOSS) Survey II. Disk Masses from ALMA Continuum Observations
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Kimberly Ward-Duong
Publication date
2017
fields
Physics
and research's language is
English
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We report 885$mu$m ALMA continuum flux densities for 24 Taurus members spanning the stellar/substellar boundary, with spectral types from M4 to M7.75. Of the 24 systems, 22 are detected at levels ranging from 1.0-55.6 mJy. The two non-detections are transition disks, though other transition disks in the sample are detected. Converting ALMA continuum measurements to masses using standard scaling laws and radiative transfer modeling yields dust mass estimates ranging from $sim$0.3-20M$_{oplus}$. The dust mass shows a declining trend with central object mass when combined with results from submillimeter surveys of more massive Taurus members. The substellar disks appear as part of a continuous sequence and not a distinct population. Compared to older Upper Sco members with similar masses across the substellar limit, the Taurus disks are brighter and more massive. Both Taurus and Upper Sco populations are consistent with an approximately linear relationship in $M_{dust}$ to $M_{star}$, although derived power-law slopes depend strongly upon choices of stellar evolutionary model and dust temperature relation. The median disk around early M-stars in Taurus contains a comparable amount of mass in small solids as the average amount of heavy elements in Kepler planetary systems on short-period orbits around M-dwarf stars, with an order of magnitude spread in disk dust mass about the median value. Assuming a gas:dust ratio of 100:1, only a small number of low-mass stars and brown dwarfs have a total disk mass amenable to giant planet formation, consistent with the low frequency of giant planets orbiting M-dwarfs.
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With Herschel/PACS 134 low mass members of the Taurus star-forming region spanning the M4-L0 spectral type range and covering the transition from low mass stars to brown dwarfs were observed. Combining the new Herschel results with other programs, a total of 150 of the 154 M4-L0 Taurus members members have observations with Herschel. Among the 150 targets, 70um flux densities were measured for 7 of the 7 ClassI objects, 48 of the 67 ClassII members, and 3 of the 76 ClassIII targets. For the detected ClassII objects, the median 70um flux density level declines with spectral type, however, the distribution of excess relative to central object flux density does not change across the stellar/substellar boundary in the M4-L0 range. Connecting the 70um TBOSS values with the results from K0-M3 ClassII members results in the first comprehensive census of far-IR emission across the full mass spectrum of the stellar and substellar population of a star-forming region, and the median flux density declines with spectral type in a trend analogous to the flux density decline expected for the central objects. SEDs were constructed for all TBOSS targets covering the optical to far-IR range and extending to the submm/mm for a subset of sources. Based on an initial exploration of the impact of different physical parameters; inclination, scale height and flaring have the largest influence on the PACS flux densities. From the 24um to 70um spectral index of the SEDs, 5 new candidate transition disks were identified. The steep 24um to 70um slope for a subset of 8 TBOSS targets may be an indication of truncated disks in these systems.Two examples of mixed pair systems that include secondaries with disks were measured. Finally, comparing the TBOSS results with a Herschel study of Ophiuchus brown dwarfs reveals a lower fraction of disks around the Taurus substellar population.
In recent years, the disk populations in a number of young star-forming regions have been surveyed with ALMA. Understanding the disk properties and their correlation with those of the central star is critical to understand planet formation. In particular, a decrease of the average measured disk dust mass with the age of the region has been observed. We conducted high-sensitivity continuum ALMA observations of 43 Class II young stellar objects in CrA at 1.3 mm (230 GHz). The typical spatial resolution is 0.3. The continuum fluxes are used to estimate the dust masses of the disks, and a survival analysis is performed to estimate the average dust mass. We also obtained new VLT/X-Shooter spectra for 12 of the objects in our sample. 24 disks are detected, and stringent limits have been put on the average dust mass of the non-detections. Accounting for the upper limits, the average disk mass in CrA is $6pm3,rm M_oplus$, significantly lower than that of disks in other young (1-3 Myr) star forming regions (e.g. Lupus) and appears consistent with the 5-10 Myr old Upper Sco. The position of the stars in our sample on the HR diagram, however, seems to confirm that that CrA has age similar to Lupus. Neither external photoevaporation nor a lower than usual stellar mass distribution can explain the low disk masses. On the other hand, a low-mass disk population could be explained if the disks are small, which could happen if the parent cloud has a low temperature or intrinsic angular momentum, or if the the angular momentum of the cloud is removed by some physical mechanism such as magnetic braking. In order to fully explain and understand the dust mass distribution of protoplanetary disks and their evolution, it may also be necessary to take into consideration the initial conditions of star and disk formation process, which may vary from region to region, and affect planet formation.
The paper presents new high angular resolution ALMA 1.3 mm dust continuum observations of the protoplanetary system AS 209 in the Ophiuchus star forming region. The dust continuum emission is characterized by a main central core and two prominent rings at $r = 75,$au and $r = 130,$au intervaled by two gaps at at $r = 62,$au and $r = 103,$au. The two gaps have different widths and depths, with the inner one being narrower and shallower. We determined the surface density of the millimeter dust grains using the 3D radiative transfer disk code textsc{dali}. According to our fiducial model the inner gap is partially filled with millimeter grains while the outer gap is largely devoid of dust. The inferred surface density is compared to 3D hydrodynamical simulations (FARGO-3D) of planet-disk interaction. The outer dust gap is consistent with the presence of a giant planet ($M_{rm planet} sim 0.8,M_{rm Staturn}$); the planet is responsible for the gap opening and for the pile-up of dust at the outer edge of the planet orbit. The simulations also show that the same planet can give origin to the inner gap at $r = 62,$au. The relative position of the two dust gaps is close to the 2:1 resonance and we have investigated the possibility of a second planet inside the inner gap. The resulting surface density (including location, width and depth of the two dust gaps) are in agreement with the observations. The properties of the inner gap pose a strong constraint to the mass of the inner planet ($M_{rm planet} < 0.1,M_{rm J}$). In both scenarios (single or pair of planets), the hydrodynamical simulations suggest a very low disk viscosity ($alpha < 10^{-4}$). Given the young age of the system (0.5 - 1 Myr), this result implies that the formation of giant planets occurs on a timescale of $lesssim$ 1,Myr.
We aim at estimating the dust scale height of protoplanetary disks from millimeter continuum observations. First, we present a general expression of intensity of a ring in a protoplanetary disk, and show that we can constrain the dust scale height by the azimuthal intensity variation. Then, we apply the presented methodology to the two distinct rings at 68 au and at 100 au of the protoplanetary disk around HD 163296. We constrain the dust scale height by comparing the DSHARP high-resolution millimeter dust continuum image with radiative transfer simulations using RADMC-3D. We find that h_d/h_g > 0.84 at the inner ring and h_d/h_g < 0.11 at the outer ring with the 3 sigma uncertainties, where h_d is the dust scale height and h_g is the gas scale height. This indicates that the dust is flared at the inner ring and settled at the outer ring. We further constrain the ratio of turbulence parameter alpha to gas-to-dust-coupling parameter St from the derived dust scale height; alpha/St > 2.4 at the inner ring, and alpha/St < 1.1*10^{-2} at the outer ring. This result shows that the turbulence is stronger or the dust is smaller at the inner ring than at the outer ring.
Using the Atacama Large Millimeter/submillimeter Array (ALMA), we observed the young Herbig star HD 100546, host to a prominent disk with a deep, wide gap in the dust. The high-resolution 1.3 mm continuum observation reveals fine radial and azimuthal substructures in the form of a complex maze of ridges and trenches sculpting a dust ring. The $^{12}$CO(2-1) channel maps are modulated by wiggles or kinks that deviate from Keplerian kinematics particularly over the continuum ring, where deviations span 90$^circ$ in azimuth, covering 5 km s$^{-1}$. The most pronounced wiggle resembles the imprint of an embedded massive planet of at least 5 M$_{rm Jup}$ predicted from previous hydrodynamical simulations (Perez, Casassus, & Benitez-Llambay 2018). Such planet is expected to open a deep gap in both gas and dust density fields within a few orbital timescales, yet the kinematic wiggles lie near ridges in the continuum. The lesser strength of the wiggles in the $^{13}$CO and C$^{18}$O isotopologues show that the kinematic signature weakens at lower disk heights, and suggests qualitatively that it is due to vertical flows in the disk surface. Within the gap, the velocity field transitions from Keplerian to strongly non-Keplerian via a twist in position angle, suggesting the presence of another perturber and/or an inner warp. We also present VLT/SPHERE sparse aperture masking data which recovers scattered light emission from the gaps edges but shows no evidence for signal within the gap, discarding a stellar binary origin for its opening.
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