Research Projects

ALMA Cycle 2 Projects

According to recent theoretical progress, the clearing of the protoplanetary cavity by an accreting gaseous giant should lead to apressure enhancement in the outer rim of the gap, shaped into a horseshoe, where dust grains are trapped, and where temperatures drop. Core-accretion may occur efficiently in these dust traps, leading to second generation planet formation. In this scenario the origin of the outer disk pressure enhancement is due to on-going dynamical clearing. Inside the gap, the first generation giant(s) channel outer disk material across the gap, thus feeding stellar accretion. The HD142527 disk is an ideal laboratory to test the scenario of second generation core accretion at large stellocentric radii. It hosts the best-studied horse-shoe and the largest cavity in a face-on orientation, in which gas kinematics have been resolved in Cycle0. We aim to 1- measure grain growth inside the dust trap of HD142527, 2- measure physical conditions and kinematics in the dust trap, and 3- understand the cavity dynamics to ascertain the physical origin of the gap-crossing flows.
The recent frequent detection of giant planets around detached post-common envelope binaries (PCEBs) may significantly influence our understanding of planet formation. These circumbinary planets have likely formed from the expelled material following the common envelope evolution of the host binary system. Such a second generation scenario, if confirmed, could provide crucial constraints on planet formation theories. An ideal test-bed for the hypothesis of second generation planet formation is the PCEB NNSer with its two circumbinary giant planets. The common envelope evolution of NNSer occurred only a million years ago thus any second generation protoplanetary disk will still be detectable with ALMA. We therefore here request 1.16 hrs in band-6 to ultimately test the second generation planet formation scenario.
Gaps in disks are a signpost of disk dispersion and planet formation, but finding disk gaps is challenging. Spectral Energy Distribution (SED) studies do not provide unequivocal evidence for gap, especially in disks around higher mass Herbig AeBe stars. Recently it has been shown that many of these Herbig stars classified as flaring are in fact disks with developing or fully cleared large gaps. We propose to image three flaring HAEBE disks for which strong indirect evidence of such gaps exists. Our proposal will increase the number of Herbig disks with proven gaps by 3 and deliver constraints on the evolution planet-forming potential of these disks.
The η Chamaeleontis association and the β Pictoris moving group of young low-luminosity nearby stars, represents an ideal laboratory to study planet formation at a critical age range: between 8 and 20 Myr. This stage is regarded as post protoplanetary/primordial disk, hence they are assumed to be young debris disks. We propose to conduct ALMA band 6 observations of 10 systems with clear excesses of emission at long wavelengths, evidencing the presence of circumstellar disks. Some of these systems may also contain disks with inner cavities (based on SED studies). The aims of this proposal concern mainly the determination of disk masses, sizes and detect primordial gas. Recent studies have shown evidence that some of these ’debris’ disks might still contain appreciable quantities of gas, challenging the idea that most disks lose all their gas at a rather early age (2-10 Myr).
Despite the large number of extrasolar planets that have been identified in the last decade, we still struggle to fully understand the planet formation process. The identification and characterization of protoplanetary disks that are currently forming giant planets can provide the most direct observational constraints on the theories of planet formation. From our large survey of transition disk systems in Lupus, ALMA Cycle 0 data, and VLT/NaCo sparse aperture masking observations, we identified two transition disks that are excellent candidates for harboring forming planets. We here request 0.5 hours of ALMA observations of Sz 91 and MY Lup in band-6 to measure the gas mass of these disks and 1.8 hours in band-7 to resolve the structure of the inner disks where planet formation could take place.
Dust in Protoplanetary disks dissipates with typical timescales of ~3 Myr. By 10Myr, very few disks are detected and little primordial material is left. The formation of gaseous planets must be complete by then. However, this is based almost entirely on measurements of the continuum, of dust, and it assumes that the gas dissipates on the same timescale. Very little information is available regarding the gas dispersal timescale. The Cha II association has an estimated age of about 3-4Myr. The disk detection rate in Cha II, as seen by continuum, is down to ~50% for KM stars making Cha II a perfect place to study disk evolution at the critical time when they evolve rapidly. For comparison the same fraction is ~80% in the 1-2Myr Taurus and Rho Oph. We propose to survey all known K&M; stars with disks of the Chamaeleon II association to measure the band 6 continuum and resolve the disk sizes. We also propose to observe three CO isotopologues to search for the presence cold gas. The sample is carefully selected in stellar mass and will be readily comparable, including gas for the first time,with younger associations with minimum biases.

ALMA Cycle 1 Projects

Abstract: Understanding the evolution of circumstellar disks around young stellar objects is crucial for theories of star and planet formation. Most young stellar objects are either accreting classical T\,Tauri stars with “full” disks or non-accreting weak-line T\,Tauri stars (WTTSs) with bare stellar photospheres, which implies that the transition phase between the two states must be very short. Currently, the only mechanism able to consistently explain the rapidly vanishing disks is photoevaporation by EUV/FUV or X-ray radiation. The photoevaporation of circumstellar gas is expected to mark the transition from the primordial to the debris disk stage. The objects most likely to be caught in this crucial disk evolution phase are WTTSs with weak levels of IR excesses. Such objects represent 20% of the WTTSs population, but the dust and gas content in their disks remain unknown. Here we propose to obtain deep continuum and CO line observations of 22 WTTS in 3 nearby (within 160 pc) star-forming regions (Taurus, Lupus-Scorpius, and Chameleonis) in order to: 1) Conclusively distinguish between primordial photoevaporating disks from young debris disks for the first time in a sample of WTTSs, 2) Constrain disk photoevaporation rates by measuring the mass of primordial disks with photoevaporation-induced inner holes, which is key to test the predictions of different photoevaporation models (e.g., EUV, FUV, and X-ray photoevaporation), and 3) Constrain models of the early evolution of debris disks and the onset of the debris disk phenomenon.
Abstract: With the powerful combination of the sensitivity of ALMA and an ideal sample of 32 young, low-mass objects, we propose to measure the population statistics of protoplanetary disks over the stellar/substellar transition. By targeting objects detected in our 70um and 160um Herschel surveys, we have maximized both the probability of detection and the coverage of the SEDs, important for the characterization of disk properties. We will model the well-sampled SEDs with the state-of-the-art radiative transfer code MCFOST. ALMA is essential to enable the transformation from small number detections of the brightest few objects to a comprehensive understanding of disk properties such as mass and their viability for planet formation. The targets were selected to belong to two distinct, well known, Star Forming Regions. They are located in Taurus and Upper Sco and cover two ages (~2Myr and ~10Myr), enabling an investigation into the evolution of disks over the critical age range during which planets may form and interact dynamically with disks. By targeting the Taurus region previously surveyed with single dish surveys, we will efficiently complete a comprehensive survey across the full mass spectrum by combining the proposed ALMA study with existing measurements.

ALMA Cycle 0 Projects

Abstract: In recent years it has been established that the evolution of circumstellar disks is fundamentally driven by the following processes: viscous accretion, grain growth, planet formation, and photoevaporation. Despite this progress, we are still far from developing a comprehensive disk evolution theory and additional observational constraints are highly required. Transition disk objects are crucial in this context as they show clear signs of disk evolution: inner opacity holes. We have performed a detailed follow-up project of Spitzer-selected transition disk systems with the aim to distinguish between different types of transition disks (Cieza et al. 2010, Romero et al. 2011). Here we propose to observe 26 transition disks characterized by Romero et al. (2011) with ALMA’s unprecedented sensitivity to address fundamental issues related to grain growth, planet formation, and photoveporation in circumstellar disks