arXiv subjects

Luca Ricci

Publications and source records attributed to Luca Ricci.

At least 37 records · Page 2Linked to original sources

arXiv2022

Probing Inner-Edge of Dead Zones in Protoplanetary Disks with ALMA and Next Generation Very Large Array

The discovery of substructures in protoplanetary disks with ALMA has provided us key insights on the formation of planets. However, observational constraints on the formation of rocky planets have been still sparse, especially because of the limited spatial resolution. The inner edge of so-called dead zone is one of the preferential sites of rocky planet formation. We investigate the capabilities of ALMA and ngVLA for observing a dust concentration expected at the inner edge of the dead-zone around a Herbig star. Herbig Ae/Be stars are useful laboratories for exploring the evolution of rocky grains in protoplanetary disks because of their high luminosity which pushes the dead-zone inner edge outward. We find that, thanks to its unprecedented angular resolution and sensitivity, ngVLA can detect the dust concentration at the dead-zone inner edge, with a reasonable integration time of 10 hrs at $λ=3,7$ mm and 1 cm. The dust concentration is expected to be optically thick at the ALMA wavelengths and cannot be spatially resolved due to its limited resolution. On the other hand, the flux density from the inner disk regions ($\sim$3--4 au) observed with current VLA is higher for disks with a dust ring, and hence would be a useful indicator that help us choose potential candidates of disks having a dust concentration at the inner most region. With these observations we can characterize the process of dust concentration in the innermost disk regions, where rocky planets can form.

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arXiv2022

Investigating the Future Potential of an Upgraded ALMA to Image Planet Forming Disks at Sub-au Scales

In recent years, ALMA has been able to observe large-scale substructures within protoplanetary disks. Comparison with the predictions from models of planet-disk interaction has indicated that most of these disk substructures can be explained by the presence of planets with the mass of Neptune or larger at orbital radii of $\approx 5 - 100$ au. Better resolution is needed to observe structures closer to the star, where terrestrial planets are expected to form, as well as structures opened by planets with masses lower than Neptune. We investigate the capabilities of a possible extension to ALMA that would double the longest baseline lengths in the array to detect and resolve disk substructures opened by Earth-mass and Super Earth planets at orbital radii of $1-5$ au. By simulating observations of a family of disk models using this extended configuration in ALMA Bands 6 and 7, we show that an upgraded ALMA would detect gaps in disks formed by super-Earths as close as 1 au, as well as Earth-mass planets down to $2-3$ au from the young host stars in nearby star forming regions.

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arXiv2021

The Substructures in Disks undergoing Vertical Shear Instability: II. Observational Predictions for the Dust Continuum

High-angular resolution observations at sub-millimeter/millimeter wavelengths of disks surrounding young stars have shown that their morphology is made of azimuthally-symmetric or point-symmetric substructures, in some cases with spiral arms, localized spur- or crescent-shaped features. The majority of theoretical studies with the aim of interpreting the observational results have focused on disk models with planets, under the assumption that the disk substructures are due to the disk-planet interaction. However, so far only in very few cases exoplanets have been detected in these systems. Furthermore, some substructures are expected to appear \textit{before} planets form, as they are necessary to drive the concentration of small solids which can lead to the formation of planetesimals. In this work we present observational predictions from high-resolution 3D radiative hydrodynamical models which follow the evolution of gas and solids in a protoplanetary disk. We focus on substructures in the distribution of millimeter-sized and smaller solid particles produced by the vertical shear instability. We show that their characteristics are compatible with some of the shallow gaps detected in recent observations at sub-mm/mm wavelengths, and present predictions for future observations with better sensitivity and angular resolution with ALMA and a Next Generation Very Large Array.

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arXiv2021

Limits on Millimeter Continuum Emission from Circumplanetary Material in the DSHARP Disks

We present a detailed analysis for a subset of the high resolution (~35 mas, or 5 au) ALMA observations from the Disk Substructures at High Angular Resolution Project (DSHARP) to search for faint 1.3 mm continuum emission associated with dusty circumplanetary material located within the narrow annuli of depleted emission (gaps) in circumstellar disks. This search used the Jennings et al. (2020) $\tt{frank}$ modeling methodology to mitigate contamination from the local disk emission, and then deployed a suite of injection-recovery experiments to statistically characterize point-like circumplanetary disks in residual images. While there are a few putative candidates in this sample, they have only marginal local signal-to-noise ratios and would require deeper measurements to confirm. Associating a 50% recovery fraction with an upper limit, we find these data are sensitive to circumplanetary disks with flux densities $\gtrsim 50-70$ $μ$Jy in most cases. There are a few examples where those limits are inflated ($\gtrsim 110$ $μ$Jy) due to lingering non-axisymmetric structures in their host circumstellar disks, most notably for a newly identified faint spiral in the HD 143006 disk. For standard assumptions, this analysis suggests that these data should be sensitive to circumplanetary disks with dust masses $\gtrsim 0.001-0.2$ M$_\oplus$. While those bounds are comparable to some theoretical expectations for young giant planets, we discuss how plausible system properties (e.g., relatively low host planet masses or the efficient radial drift of solids) could require much deeper observations to achieve robust detections.

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arXiv2021

The Architecture of the V892 Tau System: the Binary and its Circumbinary Disk

We present high resolution millimeter continuum and CO line observations for the circumbinary disk around V892 Tau to constrain the stellar and disk properties. The total mass of the two near-equal-mass A stars is estimated to be $6.0\pm0.2\,M_{\odot}$ based on our models of the Keplerian-dominated gas disk rotation. The detection of strong ionized gas emission associated with the two stars at 8 mm, when combined with previous astrometric measurements in the near-infrared, provides an updated view of the binary orbit with $a=7.1\pm0.1$ au, $e=0.27\pm0.1$, and $P=7.7\pm0.2$ yr, which is about half of a previously reported orbital period. The binary orbital plane is proposed to be near coplanar to the circumbinary disk plane (with a mutual inclination of only $Δ=8\pm4.2$ deg; another solution with $Δ=113$ deg is less likely given the short re-alignment timescale). An asymmetric dust disk ring peaking at a radius of 0.''2 is detected at 1.3 mm and its fainter counterparts are also detected at the longer 8 and 9.8 mm. The CO gas disk, though dominated by Keplerian rotation, presents a mild inner and outer disk misalignment, such that the inner disk to the SW and outer disk to the NE appear brighter than their counterparts at the opposite disk sides. The radial extension of the disk, its asymmetric dust ring, and the presence of a disk warp could all be explained by the interaction between the eccentric binary and the circumbinary disk, which we assume were formed with non-zero mutual inclination. Some tentatively detected gas spirals in the outer disk are likely produced by interactions with the low mass tertiary component located 4'' to the northeast. Our analyses demonstrate the promising usage of V892 Tau as an excellent benchmark system to study the details of binary--disk interactions.

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arXiv2021

Testing photoevaporation and MHD disk wind models through future high-angular resolution radio observations: the case of TW Hydrae

We present theoretical predictions for the free-free emission at cm wavelengths obtained from photoevaporation and MHD wind disk models adjusted to the case of the TW Hydrae young stellar object. For this system, disk photoevaporation with heating due to the high-energy photons from the star has been proposed as a possible mechanism to open the gap observed in the dust emission with ALMA. We show that the photoevaporation disk model predicts a radial profile for the free-free emission that is made of two main spatial components, one originated from the bound disk atmosphere at 0.5-1 au from the star, and another more extended component from the photoevaporative wind at larger disk radii. We also show that the stellar X-ray luminosity has a significant impact on both these components. The predicted radio emission from the MHD wind model has a smoother radial distribution which extends to closer distances to the star than the photoevaporation case. We also show that a future radio telescope such as the \textit{Next Generation Very Large Array} (ngVLA) would have enough sensitivity and angular resolution to spatially resolve the main structures predicted by these models.

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arXiv2021

A search for companions via direct imaging in the DSHARP planet-forming disks

The "Disk Substructures at High Angular Resolution Project" (DSHARP) has revealed an abundance and ubiquity of rings and gaps over a large sample of young planet-forming disks, which are hypothesised to be induced by the presence of forming planets. In this context, we present the first attempt to directly image these young companions for 10 of the DSHARP disks, by using NaCo/VLT high contrast observations in L'-band instrument and angular differential imaging techniques. We report the detection of a point-like source candidate at 1.1" (174.9 au) for RU Lup, and at 0.42" (55 AU) for Elias 24. In the case of RU Lup, the proper motion of the candidate is consistent with a stationary background contaminant, based on the astrometry derived from our observations and available archival data. For Elias 24 the point-like source candidate is located in one of the disk gaps at 55 AU. Assuming it is a planetary companion, our analysis suggest a mass ranging from $0.5 M_J$ up to $5 M_J$, depending on the presence of a circumplanetary disk and its contribution to the luminosity of the system. However, no clear confirmation is obtained at this stage, and follow-up observations are mandatory to verify if the proposed source is physical, comoving with the stellar host, and associated with a young massive planet sculpting the gap observed at 55\,AU. For all the remaining systems, the lack of detections suggests the presence of planetary companions with masses lower than $5M_J$, based on our derived mass detection limits. This is consistent with predictions of both hydrodynamical simulations and kinematical signatures on the disk, and allows us to set upper limits on the presence of massive planets in these young disks.

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arXiv2020

Size and structures of disks around very low mass stars in the Taurus star-forming region

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. We studied six disks around VLMS in the Taurus star-forming region using ALMA Band 7 ($\sim 340\,$GHz) at a resolution of $\sim0.1''$. The targets were selected because of their high disk dust content in their stellar mass regime. 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 (M5.0) and CIDA\,1 (M4.5), while we have a tentative detection of a multi-ringed disk around J0433. Single planets of masses $0.1\sim0.4\,M_{\rm{Jup}}$ would be required. The other three disks with no observed structures are the most compact and faintest in our sample. 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 $R_{\rm{gas}}$, the ratio of $R_{\rm{gas}}/R_{\rm{dust}}$ in our sample varies from 2.3 to 6.0, which is consistent with models of radial drift being very efficient around VLMS in the absence of substructures. 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.

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arXiv2020

Imaging the Dusty Substructures due to Terrestrial Planets in Planet-forming Disks with ALMA and the Next Generation Very Large Array

We present simulations of the capabilities of the Atacama Large Millimeter/submillimeter Array (ALMA) and of a Next Generation Very Large Array (ngVLA) to detect and resolve substructures due to terrestrial planets and Super-Earths in nearby planet-forming disks. We adopt the results of global 2-D hydrodynamical planet-disk simulations that account for the dynamics of gas and dust in a disk with an embedded planet. Our simulations follow the combined evolution of gas and dust for several thousand planetary orbits. We show that long integrations (several tens of hours) with the ngVLA can detect and spatially resolve dust structures due to low-mass rocky planets in the terrestrial planet formation regions of nearby disks (stellocentric radii $r = 1 - 3$ au), under the assumption that the disk viscosity in those regions is low ($α\le 10^{-5}$). ALMA is instead unable to resolve these structures in these disk regions. We also show that high-resolution ngVLA observations separated by several days to few weeks would allow to detect the proper motion of the azimuthally asymmetric structures expected in the disk regions of terrestrial planet formation.

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arXiv2020

The Disk Substructures at High Angular Resolution Project (DSHARP): II. Characteristics of Annular Substructures

The Disk Substructures at High Angular Resolution Project used ALMA to map the 1.25 millimeter continuum of protoplanetary disks at a spatial resolution of ~5 au. We present a systematic analysis of annular substructures in the 18 single-disk systems targeted in this survey. No dominant architecture emerges from this sample; instead, remarkably diverse morphologies are observed. Annular substructures can occur at virtually any radius where millimeter continuum emission is detected and range in widths from a few au to tens of au. Intensity ratios between gaps and adjacent rings range from near-unity to just a few percent. In a minority of cases, annular substructures co-exist with other types of substructures, including spiral arms (3/18) and crescent-like azimuthal asymmetries (2/18). No clear trend is observed between the positions of the substructures and stellar host properties. In particular, the absence of an obvious association with stellar host luminosity (and hence the disk thermal structure) suggests that substructures do not occur preferentially near major molecular snowlines. Annular substructures like those observed in DSHARP have long been hypothesized to be due to planet-disk interactions. A few disks exhibit characteristics particularly suggestive of this scenario, including substructures in possible mean-motion resonance and "double gap" features reminiscent of hydrodynamical simulations of multiple gaps opened by a planet in a low-viscosity disk.

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arXiv2020

Large-scale CO spiral arms and complex kinematics associated with the T Tauri star RU Lup

While protoplanetary disks often appear to be compact and well-organized in millimeter continuum emission, CO spectral line observations are increasingly revealing complex behavior at large distances from the host star. We present deep ALMA maps of the $J=2-1$ transition of $^{12}$CO, $^{13}$CO, and C$^{18}$O, as well as the $J=3-2$ transition of DCO$^+$, toward the T Tauri star RU Lup at a resolution of $\sim0.3''$ ($\sim50$ au). The CO isotopologue emission traces four major components of the RU Lup system: a compact Keplerian disk with a radius of $\sim120$ au, a non-Keplerian ``envelope-like'' structure surrounding the disk and extending to $\sim260$ au from the star, at least five blueshifted spiral arms stretching up to 1000 au, and clumps outside the spiral arms located up to 1500 au in projection from RU Lup. We comment on potential explanations for RU Lup's peculiar gas morphology, including gravitational instability, accretion of material onto the disk, or perturbation by another star. RU Lup's extended non-Keplerian CO emission, elevated stellar accretion rate, and unusual photometric variability suggest that it could be a scaled-down Class II analog of the outbursting FU Ori systems.

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arXiv2020

ALMA 0.88 mm Survey of Disks around Planetary-mass Companions

Characterizing the physical properties and compositions of circumplanetary disks can provide important insights into the formation of giant planets and satellites. We report ALMA 0.88 mm (Band 7) continuum observations of six planetary-mass (10-20 $M_{\rm Jup}$) companions: CT Cha b, 1RXS 1609 b, ROXs 12 b, ROXs 42B b, DH Tau b, and FU Tau b. No continuum sources are detected at the locations of the companions down to 3$σ$ limits of 120-210 $μ$Jy. Given these non-detections, it is not clear whether disks around planetary-mass companions indeed follow the disk flux-host mass trend in the stellar regime. The faint radio brightness of these companion disks may result from a combination of fast radial drift and a lack of dust traps. Alternatively, as disks in binary systems are known to have significantly lower millimeter fluxes due to tidal interactions, these companion disks may instead follow the relationship of moderate-separation binary stars. This scenario can be tested with sensitive continuum imaging at rms levels of $\lesssim$10 $μ$Jy.

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arXiv2020

Near-Infrared Imaging of a Spiral in the CQ Tau Disk

We present $L^\prime$-band Keck/NIRC2 imaging and $H$-band Subaru/AO188+HiCIAO polarimetric observations of CQ Tau disk with a new spiral arm. Apart from the spiral feature our observations could not detect any companion candidates. We traced the spiral feature from the $r^2$-scaled HiCIAO polarimetric intensity image and the fitted result is used for forward modeling to reproduce the ADI-reduced NIRC2 image. We estimated the original surface brightness after throughput correction in $L^\prime$-band to be $\sim126$ mJy/arcsec$^2$ at most. We suggest that the grain temperature of the spiral may be heated up to $\sim$200 K in order to explain both of the $H$- and $L^{\prime}$-bands results. The $H$-band emission at the location of the spiral originates from the scattering from the disk surface while both scattering and thermal emission may contribute to the $L^{\prime}$-band emission. If the central star is only the light source of scattered light, the spiral emission at $L^\prime$-band should be thermal emission. If an inner disk also acts as the light source, the scattered light and the thermal emission may equally contribute to the $L^\prime$-band spiral structure.

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arXiv2019

One Solution to the Mass Budget Problem for Planet Formation: Optically Thick Disks with Dust Scattering

ALMA surveys have suggested that the dust in Class II disks may not be enough to explain the averaged solid mass in exoplanets, under the assumption that the mm disk continuum emission is optically thin. This optically thin assumption seems to be supported by recent DSHARP observations where the measured optical depths of spatially resolved disks are mostly less than one. However, we point out that dust scattering can considerably reduce the emission from an optically thick region. If that scattering is ignored, the optical depth will be considerably underestimated. An optically thick disk with scattering can be misidentified as an optically thin disk. Dust scattering in more inclined disks can reduce the intensity even further, making the disk look even fainter. The measured optical depth of $\sim$0.6 in several DSHARP disks can be naturally explained by optically thick dust with an albedo of $\sim$0.9 at 1.25 mm. Using the DSHARP opacity, this albedo corresponds to a dust population with the maximum grain size ($s_{max}$) of 0.1-1 mm. For optically thick scattering disks, the measured spectral index $α$ can be either larger or smaller than 2 depending on if the dust albedo increases or decreases with wavelength. Using the DSHARP opacity, $α<2$ corresponds to $s_{max}$ of 0.03-0.3 mm. We describe how this optically thick scattering scenario could explain the observed scaling between submm continuum sizes and luminosities, and might help ease the tension between the dust size constraints from polarization and dust continuum measurements. We suggest that a significant amount of disk mass can be hidden from ALMA observations at short millimeter wavelengths. For compact disks smaller than 30 au, we can easily underestimate the dust mass by more than a factor of 10. Longer wavelength observations (e.g. VLA or SKA) are desired to probe the dust mass in disks.

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arXiv2019

Effects of Ringed Structures and Dust Size Growth on Millimeter Observations of Protoplanetary Disks

The growth of solids from sub-micron to millimeter and centimeter sizes is the early step toward the formation of planets inside protoplanetary disks (PPDs). However, such processes and their potential impact on the later stages of solid growth are still poorly understood. In this work, we test the hypothesis that most disks contain at least one ringed structure with a relative small radius. We have carried out a large family of 1D two-fluid (gas+dust) hydrodynamical simulations by evolving the gas and dust motion self-consistently while allowing dust size to evolve via coagulation and fragmentation. We investigate the joint effects of ringed structures and dust size growth on the overall sub-millimeter and millimeter (mm) flux and spectral index of PPDs. Ringed structures slow down the dust radial drift and speed up the dust growth. In particular, we find that those unresolved disks with a high fragmentation velocity ($\sim10\ {\rm m~s^{-1}}$) and a high dust surface density ($\sim10\ {\rm g\ cm^{-2}}$ in the ring) can have mm spectral indices as low as $\sim2.0$, consistent with mm observations of faint disks in nearby star forming regions. Furthermore, disks with more than one ringed structure can potentially reproduce brighter disks with spectral indices lower than $\sim2.5$. Future multi-wavelength high-resolution observations of these low spectral index sources can be used to test the existence of the ringed structures in the unresolved disks and differentiate the effects of dust size growth from optical depth.

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arXiv2019

A dust and gas cavity in the disc around CQ Tau revealed by ALMA

The combination of high resolution and sensitivity offered by ALMA is revolutionizing our understanding of protoplanetary discs, as their bulk gas and dust distributions can be studied independently. In this paper we present resolved ALMA observations of the continuum emission ($λ=1.3$ mm) and CO isotopologues ($^{12}$CO, $^{13}$CO, C$^{18}$O $J=2-1$) integrated intensity from the disc around the nearby ($d = 162$ pc), intermediate mass ($M_{\star}=1.67\,M_{\odot}$) pre-main-sequence star CQ Tau. The data show an inner depression in continuum, and in both $^{13}$CO and C$^{18}$O emission. We employ a thermo-chemical model of the disc reproducing both continuum and gas radial intensity profiles, together with the disc SED. The models show that a gas inner cavity with size between 15 and 25 au is needed to reproduce the data with a density depletion factor between $\sim 10^{-1}$ and $\sim 10^{-3}$. The radial profile of the distinct cavity in the dust continuum is described by a Gaussian ring centered at $R_{\rm dust}=53\,$au and with a width of $σ=13\,$au. Three dimensional gas and dust numerical simulations of a disc with an embedded planet at a separation from the central star of $\sim20\,$au and with a mass of $\sim 6\textrm{-} 9\,M_{\rm Jup}$ reproduce qualitatively the gas and dust profiles of the CQ Tau disc. However, a one planet model appears not to be able to reproduce the dust Gaussian density profile predicted using the thermo-chemical modeling.

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arXiv2019

Realizing the Unique Potential of ALMA to Probe the Gas Reservoir of Planet Formation

Understanding the origin of the astonishing diversity of exoplanets is a key question for the coming decades. ALMA has revolutionized our view of the dust emission from protoplanetary disks, demonstrating the prevalence of ring and spiral structures that are likely sculpted by young planets in formation. To detect kinematic signatures of these protoplanets and to probe the chemistry of their gas accretion reservoir will require the imaging of molecular spectral line emission at high angular and spectral resolution. However, the current sensitivity of ALMA limits these important spectral studies to only the nearest protoplanetary disks. Although some promising results are emerging, including the identification of the snowlines of a few key molecules and the first attempt at detecting a protoplanet's spiral wake, it is not yet possible to search for these important signatures in a population of disks in diverse environments and ages. Harnessing the tremendous power of (sub)mm observations to pinpoint and characterize the chemistry of planets in formation will require a major increase of ALMA's spectral sensitivity (5-10x), increase in instantaneous bandwidth (2x) at high spectral resolution, and improved angular resolution (2x) in the 2030 era.

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arXiv2019

Planet formation: The case for large efforts on the computational side

Modern astronomy has finally been able to observe protoplanetary disks in reasonable resolution and detail, unveiling the processes happening during planet formation. These observed processes are understood under the framework of disk-planet interaction, a process studied analytically and modeled numerically for over 40 years. Long a theoreticians' game, the wealth of observational data has been allowing for increasingly stringent tests of the theoretical models. Modeling efforts are crucial to support the interpretation of direct imaging analyses, not just for potential detections but also to put meaningful upper limits on mass accretion rates and other physical quantities in current and future large-scale surveys. This white paper addresses the questions of what efforts on the computational side are required in the next decade to advance our theoretical understanding, explain the observational data, and guide new observations. We identified the nature of accretion, ab initio planet formation, early evolution, and circumplanetary disks as major fields of interest in computational planet formation. We recommend that modelers relax the approximations of alpha-viscosity and isothermal equations of state, on the grounds that these models use flawed assumptions, even if they give good visual qualitative agreement with observations. We similarly recommend that population synthesis move away from 1D hydrodynamics. The computational resources to reach these goals should be developed during the next decade, through improvements in algorithms and the hardware for hybrid CPU/GPU clusters. Coupled with high angular resolution and great line sensitivity in ground based interferometers, ELTs and JWST, these advances in computational efforts should allow for large strides in the field in the next decade.

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