Protoplanetary disk demographics

Disks and Proplyds in the Orion Nebula Cluster with ALMA at 3 mm

Nick Ballering (University of Virginia)

Rich clusters are the typical sites of star and planet formation in the Galaxy, and the Orion Nebula Cluster (ONC) is the most readily-observable rich cluster. We present a recent ALMA Band 3 (3 mm) observation of the inner 1.5 x 1.5 arcminutes of the ONC at high sensitivity (~30 uJy) and spatial resolution (60 mas). At 3 mm, we are sensitive to both dust thermal emission and free-free emission from ionized gas. We detect many sources, including compact protoplanetary disks and extended emission from proplyds—disks in the process of being photoevaporated by the external radiation field. We analyze these sources in conjunction with previous measurements at 850 um (from ALMA) and cm wavelengths (from the VLA) to spectrally and spatially decompose the dust versus free-free emission. We measure the disk sizes, dust masses, and dust properties (via the spectral index) and compare the results with disks in low-mass star forming regions to further our understanding of planet formation in a high-radiation cluster environment.

VLA cm-wave survey of young stellar objects in the Oph A cluster

Audrey Coutens (LAB, Univ. of Bordeaux)

We report the first results from a pilot study of young stellar objects in the Oph A cluster as a prelude to a deep SKA1-Mid Band 5 pointing Key Science Project to map the grain growth in proto-planetary discs. Cm-wave studies of the dust emission are the key to finding out where and how in discs the early stages of planet formation manage to overcome the cm grain size barrier. Investigating the radio emission of young stellar objects is also important to understand the other contributions to this emission and their associated phenomena (e.g., jets, disk photo-evaporation, magnetosphere activity) since they can affect the lifetime of disks and the formation of planets. We have carried out high-sensitivity observations of the continuum emission toward the Oph A cluster using the Jansky Very Large Array (VLA) at 10 GHz over a field-of-view of 6 arcmin with 0.2'' x 0.4" resolution. The reached sensitivity goes down to 5 μJy/beam in the center of the field. Among the eighteen sources detected, sixteen are young stellar objects (3 Class 0, 5 Class I, 6 Class II and 2 Class III) and two are extragalactic candidates. We analyzed the nature of the emission of the young stellar objects at 10 GHz and estimated the dust contribution to be less than or about 30% in most cases. The radio emission is dominated by other types of emission (gyro-synchrotron radiation from active magnetospheres, free–free emission from thermal jets or from the outflowing photoevaporated disk material, etc), which could not be clearly disentangled. Our non-detections towards Class II/III disks suggest that extreme UV-driven photoevaporation is insufficient to explain disk dispersal,  assuming that the contribution of UV photoevaporating stellar winds to radio flux does not evolve overtime. With the sensitivity of our data, we cannot exclude that photoevaporation due to the role of X-ray photons is an efficient mechanism for disk dispersal. Deeper surveys with the SKA will have the capacity to provide significant constraints to disk photoevaporation as well as to other aspects related to star and planet formation.

Five years of VLT/SPHERE: a new view of planet-forming disks in scattered light

Antonio Garufi (Osservatorio Astrofisico di Arcetri, INAF)

Sixty planet-forming disks have been imaged in the five years of VLT/SPHERE guaranteed time of observations. The high-contrast scattered-light maps reveal the high occurrence of well-known disk sub-structures such as spirals and rings but also of features of increasing interest like shadows and filaments. On behalf of the SPHERE consortium, I review the main results of the survey on the demographics and morphological characterization of disks in the near-IR.

ALMA Survey of Protoplanetary Disks in Lynds 1641

Sierra Grant (Boston University)

I present new ALMA observations of 99 protoplanetary disks in the region Lynds 1641 in the Orion Molecular Cloud A. Our ALMA observations include 1.33 mm continuum emission as well as spectral windows covering the J=2-1 transition of 12CO, 13CO, and C18O. Of the 99 protoplanetary disks surveyed, we detect 86 in the dust continuum at the 4σ level (~87% detection rate) and 32 in 12CO, 15 in 13CO, and 4 in C18O. The sample observed here was selected based on being infrared-bright meaning this sample contains many of the brightest Class II objects in Lynds 1641 and is biased towards the most massive disks. We find that Lynds 1641 has a relatively massive dust disk population compared to regions of similar and older ages, with a median dust mass of 12.0 Earth masses. A large fraction (27%) of our sample has a dust mass equal to or greater than the minimum solar nebula value of ~30 Earth masses. Our sample contains 23 transitional disks, which have a median dust mass of 16.0 Earth masses, agreeing with previous results that transitional disks are often massive. One object, [MGM2012] 512, shows large-scale structure in both the dust continuum and the three gas lines, potentially evidence of an in-falling envelope. This survey highlights the potential uses of Lynds 1641 as a large population of Class II disks that coincides with a well-characterized sample of younger, Class 0/I, protostellar systems.

Characterizing Young, Cool M-Stars and their Planet-Forming Disks

Jamila Pegues (Center for Astrophysics | Harvard & Smithsonian)

M-stars are the most common hosts of planetary systems in the local Galaxy.  Observations of protoplanetary disks around these cool stars are remarkable tools for understanding the environment within which their planets form.  We present a small sample of protoplanetary disks around M-stars (spectral types M4-M5).  Using spectrally and spatially resolved ALMA observations of a range of molecular lines, we measure the dynamical masses of these stars and characterize the chemistry in their disks.  We find that dynamical masses for a combined sample of M-stars exceed fiducial stellar evolutionary model predictions, and we use this discrepancy to constrain the nature of young, cool M-stars. We then find that similar patterns of chemistry exist between our M-star disk sample and solar-type disks, and we investigate hydrocarbons as one important possible exception.  Finally, we discuss future observations, which are crucial for obtaining a holistic view of the chemistry of planet formation around the "coolest" stars.

Modeling brown dwarf protoplanetary disks

Anneliese Rilinger (Boston University)

We have modeled the spectral energy distributions (SEDs) for the largest sample of brown dwarfs with protoplanetary disks to date. We compile 48 objects from 4 star-forming regions: Ophiuchus, Taurus, Lupus, and Upper Scorpius. Radiative transfer modeling of disk SEDs allows us to constrain disk masses, which in turn inform the ability of these objects to form planetary companions. These models also allow us to look for evidence of disk substructure in the form of gaps in transitional and pre-transitional disks. We find that the majority of the objects in our sample do not have sufficiently massive disks to form planetary companions, though we cannot rule out that planet formation has already occurred. About 30% of the objects in our sample are best-fit as transitional or pre-transitional disks. Furthermore, by studying multiple star-forming regions with varying ages, we are able to probe how disk mass evolves over time. We find that disk mass generally decreases with age as expected, though the youngest region, Ophiuchus, has the lowest disk masses.

The early stages at substellar formation in Lupus 1 and 3

Alejandro Santamaría-Miranda (European Southern Observatory)

The formation of brown dwarfs is still under debate. While the latest discoveries point towards a scaled-down version of the star formation process, other models, such as embryo ejection or stellar disk fragmentation,  may not be discarded. Here we present our latest ALMA cycle 3 (band 6) continuum observations of Lupus 1 and 3 star formation regions based on previous ASTE/AzTEC observations and a set of previously known class II substellar objects from the literature. We classify these sources using the spectral energy distribution obtained from archival data. We report nine new sources that could be classified as either prestellar cores or deeply embedded protostar candidates, three new class I objects, and  one new class II.  Additionally we also detected six previously known class II systems, some of them in the boundary between brown dwars and very low mass stars.
We probe the turbulent fragmentation and core collapse formation scenarios for the prestellar cores or deeply embedded protostar candidates and we compare the dust masses of the disk for the class II objects with previous studies. We also present ALMA cycle 5 band 7 data of Par-Lup3-4 where we witness for first time the presence of the base of a compact bipolar molecular cavity in radio and an optical jet in a very low mass star, close to the boundary to brown dwarfs, suggesting a scale down version of low mass star formation.

A direct link between disk structure, disk evolution and exoplanet demographics

Nienke van der Marel (University of Victoria)

Structures such as gaps and rings in observations of protoplanetary disks have long been hailed as signposts of planet formation. However, a direct link between exoplanets and protoplanetary disks remains hard to identify. We present a large sample study of ALMA dust disk surveys of nearby star-forming regions in order to disentangle this connection at a statistical level. All disks are classified as either structured (transition, ring, extended) or non-structured (compact) disks. A comparison across ages reveals that structured disks retain high dust masses up to at least 10 Myr, whereas the dust mass of non-structured disks decreases rapidly over time. This decrease can be understood if the dust mass evolves primarily by radial drift, unless drift is prevented by pressure bumps in structured disks. Furthermore, we find that massive stars are more likely to host structured disks, providing a link with giant exoplanets that also occur more frequently around more massive stars. We show that the observed disk structures can be accounted for if transitional disks are created by exoplanets more massive than Jupiter, and ring disk structures by exoplanets more massive than Neptune, under the assumption that most of those planets eventually migrate inwards. On the other hand, the occurrence of close-in super-Earths is anti-correlated with the fractions of structured disks at different stellar masses, consistent with those exoplanets forming through pebble accretion in drift-dominated disks. These findings support an evolutionary scenario where the early formation of giant planets determines the dust disk evolution and its observational appearance.

Constraining Planet Formation Around 6-8 Solar Mass Stars

Dimitri Veras (University of Warwick (UK))

Identifying planets around O-type and B-type stars is inherently difficult; the most massive known planet host has a mass of only about 3 M⊙. However, planetary systems which survive the transformation of their host stars into white dwarfs can be detected via photospheric trace metals, circumstellar dusty and gaseous discs, and transits of planetary debris crossing our line of sight. These signatures offer the potential to explore planet formation efficiency and chemical composition for host stars with masses up to the core-collapse boundary at ≈ 8 M⊙, a mass regime rarely investigated in planet formation theory. Here, we establish limits on where both major and minor planets must reside around ≈ 6–8 M⊙ stars in order to survive into the white dwarf phase. For this mass range, we find that intact terrestrial or giant planets need to leave the main sequence beyond approximate minimum star–planet separations of, respectively, about 3 and 6 au. Further, in these systems, rubble pile minor planets of radii 10, 1.0, and 0.1 km would have been shorn apart by giant branch radiative YORP spin-up if they formed and remained within, respectively, tens, hundreds, and thousands of au. Overall, we find that planet formation around 6 M⊙-8 M⊙ stars may be feasible, and hence we encourage dedicated planet formation investigations for these systems.

Disk Population Synthesis

Apostolos Zormpas (University Observatory / LMU Munich)

Recently, a sub-arcsecond resolution survey of the dust continuum emission from nearby protoplanetary disks, showed a strong correlation between the sizes and luminosities of the disks. By performing models of gas and dust disk evolution, we recreate this relation using a large grid of models that varies the initial conditions. We calculate the disk continuum emission and the effective radius for all models as a function of time. We simulate two different cases: a smooth disk surface density profile and one that include planets. By selecting only the disks that lie on the observed size-luminosity relation, we constrain the parameter range and search for trends between the initial conditions and the survival frequency of every disk. By applying neural networks, we determine the influence of every parameter on the final outcome, showing significant results for the initial disk mass, the turbulence-parameter alpha, the planet mass, the disk size and the existence of substructure in the observed disk population.