Research Projects

ALMA Cycle 4 Projects

We propose an unbiased demographic study of the entire disk population (from Class I to Class III) identified by Spitzer in Ophiuchus, the closest of the major star-forming regions near the Sun. We will simultaneously observe 1.3 mm continuum, and the 12CO, 13CO, and C180 J=2-1 lines. We will observe each of the 289 targets for ~1 min and vary the spatial resolution (from 0.25" to 0.6") based on the known or expected 1.3 mm flux. The resulting wealth of data will have a long-lasting legacy value and will be invaluable for both demographic studies and the identification of interesting sources for detailed followup studies. We have designed the observations based on 3 main scientific goals: 1) derive stellar dynamical masses and assess the dust and gas content of the disks, 2) investigate the diversity of structures in protoplanetary disks to place previous and future results into context, and 3) study the dependence of disk structures on SED Class and stellar properties. The underlying motivation is to perform a complete and unbiased study of a major star-forming region to start making direct connections between disk populations and the populations of planets they may form.
In ALMA Cycle 3, our group serendipitously imaged the water snow-line in the protoplanetary disk around V883 Ori, a 1.3 M_sun protostar undergoing a massive FU Ori accretion outburst (Cieza et al. submitted to Nature). Our Band-6 observations revealed a sharp break in optical depth at a location where the disk temperature reaches the sublimation point of water. At this discontinuity, the disk shows a clear change in the spectral index, as predicted by recent water snow-line models (Banzatti et al. 2015). Due to the enormous accretion luminosity, this water snow-line lies at > 40 au as opposed to < 5 au as typically expected for solar-type objects. The V883 Ori disk thus represents a unique opportunity to characterize the water snow-line using an ample frequency leverage at the highest resolution available for Cycle 4. Our immediate objectives are 1) Constrain the temperature of the observed water snow-line, 2) Investigate the role of viscous heating in the thermal structure of the V883 Ori disk, and 3) and test dust evolution models. The V883 Ori disk is so bright that our observations, including overheads, require just 1 h per band.
FU Oris are low-mass pre-main sequence stars characterized by dramatic outbursts during which stars may gain a significant portion of their mass. Proposed mechanisms which could drive these episodic accretion events include gravitational and thermal instabilities, disk fragmentation, forming planets and stellar encounters. Most interestingly, the archetypical FU Ori system, FU Oriorins itself, is a known binary composed of two disks separated by only 0.5" that have been detected in gas and dust during Cycle 0. These data, although reaveling clear signs of interaction in the system, suffered from low sensitivity and poor resolution and the disks are only partially resolved. How does this binarity and mass exchange/interaction affect the scenarios which aim at explaining episodic accretion? We propose to map the gas and dust in FU Ori at 50 mas resolution in band 6 to build a comprehensive picture of the dynamics of the system. We aim to capture dynamic interaction through CO isotopologue kinematics and compare these with dedicated hydrodynamic simulations. Mapping both disks in continuum is also a pressing need to distinguish between outburst mechanisms.
The gas giant planets of the Solar System are surrounded by moons in large quantities, with at least 50 prograde moons thought to have formed in situ. Just like the Sun is not the only star surrounded by planets, it is very likely that extrasolar gas giant planets are also surrounded by lunar systems. Building on the properties of the solar system regular satellites, we derive models for minimum-mass protolunar disks. Using 3D radiative transfer, we show that such late circumplanetary debris disks would be readily observable given the incident flux from the central host star, and are even brighter when heated from the still-warm giant companion they are surrounding. We propose to test our predictions by observing three confirmed companions around nearby (50 pc) young (<30 Myr) stars with ALMA in Band 6 continuum and 12CO J=2-1 at 0.1 resolution. We are resubmitting our ongoing Cycle 3 program for PZ Tel and AB Pic (cat. B), and adding 51 Eri. Given our simulations, protolunar disks around these companions should be detectable at 10 sigma with just a few minutes (7-17 min) on source. Our science requires only 1 hr per target including calibrations.
Witnessing the formation process of a planet is a foremost ALMA Science Driver and we propose to accomplish it by detecting the distinc kinematical signatures of the circumplanetary disks (CPD) feeding accretion onto the two most promising protoplanet candidates: HD100546b and HD169142b. These protoplanets/CPDs have eluded detection in continuum, and their identification demands predictions on CPD gas tracers. With breakthrough angular resolutions, and informed by state-of-the-art 3D numerical simulations, our predictions show that deviations in the Keplerian pattern provoked by CPD kinematics are detectable with ALMA Cycle-4 in Band-6 with CO isotopologues at the highest resolution offered (30mas). We assessed the feasibility of these observations by corrupting our CPD simulations with realistic phase noise and uv coverage extracted from the HL Tau long baseline campaign. The detection (or lack thereof) of the most prominent protoplanet candidates will constrain CPD hydrodynamics and planet formation theories by measuring the size of the CPDs and provide
HR 8799 is the only system where multiple planets have been directly imaged. These exoplanets are at large stellar separations, and are surrounded by a massive debris belt. We wish to obtain constraints on the dynamics of this system by resolving the outer belt. We propose to achieve ALMA band 7 continuum precise mapping of this debris ring. Our Band 6 observations suggest that the inner edge of the outer belt is located further out than can be inferred from Herschel data. This seems incompatible with the outermost planet HR 8799 b sculpting this inner edge, and hints at the presence of an additional distant companion. A discrepancy with the ring inclination and tentative observations of clumps likewise suggest that this debris ring bears imprints of complex planet-disk interactions. Deeper ALMA observations will constrain the geometry of this outer belt, assess the presence of clumps and their location, and open a path towards a better knowledge of the dynamical history of this system. Our findings can test current theories of planet-disk interactions. It will help make it easier to use debris disks to inform us about planetary systems, even when the planets are unseen.
Opportunities to investigate the diversity of mature analogs to our own Solar System are rare. Therefore, we propose ALMA continuum mapping in band 6 to observe the sub-mm excess from the Gyr-old debris ring surrounding the Sun-like star HD 202628. As revealed in scattered light by HST, HD 202628 is surrounded by a large and eccentric debris disk, as confirmed by Herschel/PACS low resolution images, showing a "pericenter-glow" at the inferred position angle of periastron. The debris ring also possesses a sharp inner egde, and its properties are strikingly similar to the Fomalhaut debris ring. This likewise suggests ring sculpting by a distant eccentric planetary companion. ALMA observations will constrain the detailed architecture of this debris disk and map the distribution of the large parent dust grains with respect to the scattered light. In conjunction with the HST and Herschel data, the ALMA results will be used to build a comprehensive model of the ring dust population. They will provide a key dynamical constraint on the properties of the ring perturber, the most distant one ever inferred around a solar-type star.
The census of exoplanets only shows a few with 10s of AUseparations. On the other hand, observations of debris disks have shown that planetesimals can form at large separations, but it is not yet clear how far planets can form. Debris disks provide a unique tool as they can reveal the presence of planets at tens of AU. HD92945 and HD107146 are uniquely placed to pursue this question because they have broad disks with double rings at 10s of AU that suggest the presence of planets at large separation. However, 3 different scenarios can explain this and differ in where the planets formed: 1) a planet formed in situ opens a gap in a broad disk by direct scattering and resonance overlap; 2) a planet that formed closer in and migrated outwards traps planetesimals in resonances; 3) an eccentric planet formed closer in, but was scattered out then opens a gap through secular interactions with the disk. These scenarios predict significant differences in the disks' high resolution structure that are detectable by ALMA. We propose to observe these systems at a resolution of 0.6" to disentangle the origin of the double rings and planet formation history at large separations in these systems.
FUor objects are pre-main sequence stars that are undergoing a period of intense mass accretion from their circumstellar disks. During these outbursts, their luminosity can increase up to 100 times L_Sun for Solar-type stars and their accretion rates can reach as high as 10^-4 M_Sun/yr. While the mechanism producing these outbursts is still unclear, several leading theories have been proposed: 1) Tidal interaction of a massive disk with an eccentric stellar or giant planet companion, 2) Magnetorotational instability activated by gravitational instabilities and 3) Disk Fragmentation developing spiral structures. ALMA Cycle-2 observations of the FUor object HBC 494 show an elongated structure in the continuum, which can be caused either by a gravitational clump or by an interacting disk. So far, no evidence of binary has been claimed for this object. A gravitational clump has been identified around the star beta Pictoris, which host a debris disk, but it has not been identified in a protoplanetary system so far. We aim to resolve this FUor object to disentangle which of three scenarios above may cause its outbursts.

ALMA Cycle 3 Projects

One of the surprising findings of ALMA's HL Tau observations is the lack of spiral arms and/or clumps, which are the ubiquitous features of gravitational instability (GI). Considering HL Tau is one of the most massive disks, it cast doubts on whether GI ever operates in protoplanetary disks. We propose to look for the signatures of GI in disks which are even more massive than HL Tau. FU Orionis/Exor objects are great candidates to search for GI as most theoretical models imply thay their disks should be unstable. Our ALMA Cycle-2 program has targeted several FU Orionis/EXor objects and we choose the most massive disk, V883 Ori (~0.6 Msun), to obtain high angular resolution data to search for the spiral arms and/or clumps caused by GI. Imaging GI would be a ground-breaking discovery. However, if we don't find any signatures of GI in this extremely massive disk, it will put stringent constraints on all current disk outburst models. Whatever the case, our program will be a major contribution to the understanding of the FU Ori phenomenon. At 450 pc, V883 Ori is so bright (360 mJy in band-6) that it can be imaged at 0.034" (15 AU) resolution in only 1.2 hs.
Investigating the evolution of disk masses as a function of time is of critical importance to planet formation theory as it indicates the amount of raw material that is still available for planet-building at any given time. Compact young stellar clusters are ideal targets for such studies as they provide large population of disks with relatively narrow age distributions. We propose an ALMA band-6 survey of all ~140 Class II disks in the benchmark cluster IC 348. This cluster has an IR disk fraction of 50% and an estimated age of 2-3 Myr, and thus is particularly useful to establish the distribution of disk masses at the time half the disk population has already dissipated. Our scientific objectives are 1) to construct the disk luminosity function of this benchmark cluster to allow direct comparisons to younger and older regions, and 2) to measure the gas content and the gas to dust mass ratios in 2-3 Myr old disks in order to constrain the time available for the formation of different types of planetary systems.
In Cycle 0, we discovered CO spiral arms extending out to 500 au in the outer disk of HD 142527. Surprisingly these spirals are very faint in CO (3-2), but brighter in CO (2-1), with temperatures as low as 10 to 15 K. We propose to confirm such low temperatures in HD 142527, and put deep limits on the continuum so as to test for low dust-to-gas mass ratios, that may explain the detection of CO gas colder than the freeze-out temperature (18 K). The new measured dynamics of the gas in the spirals will allow us to put conclusive constraints on their origin. In particular, the new observations of the spirals will also be confronted to our novel hypothesis of spirals launched by the shadows cast by the inclined inner disk.

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