arXiv subjects

Stefano Facchini

Publications and source records attributed to Stefano Facchini.

At least 73 records · Page 4Linked to original sources

arXiv2025

exoALMA IV: Substructures, Asymmetries, and the Faint Outer Disk in Continuum Emission

The exoALMA Large Program targeted a sample of 15 disks to study gas dynamics within these systems, and these observations simultaneously produced continuum data at 0.9 mm (331.6 GHz) with exceptional surface brightness sensitivity at high angular resolution. To provide a robust characterization of the observed substructures, we performed a visibility space analysis of the continuum emission from the exoALMA data, characterizing axisymmetric substructures and nonaxisymmetric residuals obtained by subtracting an axisymmetric model from the observed data. We defined a nonaxisymmetry index and found that the most asymmetric disks predominantly show an inner cavity and consistently present higher values of mass accretion rate and near-infrared excess. This suggests a connection between outer disk dust substructures and inner disk properties. The depth of the data allowed us to describe the azimuthally averaged continuum emission in the outer disk, revealing that larger disks (both in dust and gas) in our sample tend to be gradually tapered compared to the sharper outer edge of more compact sources. Additionally, the data quality revealed peculiar features in various sources, such as shadows, inner disk offsets, tentative external substructures, and a possible dust cavity wall.

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arXiv2025

The centimeter emission from planet-forming disks in Taurus

The last decade has witnessed remarkable advances in the characterization of the (sub-)millimeter emission from planet-forming disks. Instead, the study of the (sub-)centimeter emission has made more limited progress, to the point that only a few exceptional disk-bearing objects have been characterized in the centimeter regime. This work takes a broad view of the centimeter emission from a large sample with VLA observations that is selected from previous ALMA surveys of more representative disks in brightness and extent. We report on the detection and characterization of flux at centimeter wavelengths from 21 sources in the Taurus star-forming region. Complemented by literature and archival data, the entire photometry from 0.85 mm to 6 cm is fitted by a two-component model that determines the ubiquitous presence of free-free emission entangled with the dust emission. The flux density of the free-free emission is found to scale with the accretion rate but is independent of the outer disk morphology depicted by ALMA. The dust emission at 2 cm is still appreciable, and offers the possibility to extract an unprecedented large set of dust spectral indices in the centimeter regime. A pronounced change between the median millimeter indices (2.3) and centimeter indices (2.8) suggests that a large portion of the disk emission is optically thick up to 3 mm. The comparison of both indices and fluxes with the ALMA disk extent indicates that this portion can be as large as 40 au, and suggests that the grain population within this disk region that emits the observed centimeter emission is similar in disks with different size and morphology. All these results await confirmation and dedicated dust modeling once facilities like ngVLA or SKA-mid are able to resolve the centimeter emission from planet-forming disks and disentangle the various components.

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arXiv2024

Disk Evolution Study Through Imaging of Nearby Young Stars (DESTINYS): Dynamical Evidence of a Spiral-Arm-Driving and Gap-Opening Protoplanet from SAO 206462 Spiral Motion

In the early stages of planetary system formation, young exoplanets gravitationally interact with their surrounding environments and leave observable signatures on protoplanetary disks. Among these structures, a pair of nearly symmetric spiral arms can be driven by a giant protoplanet. For the double-spiraled SAO 206462 protoplanetary disk, we obtained three epochs of observations spanning 7 yr using the Very Large Telescope's SPHERE instrument in near-infrared $J$-band polarized light. By jointly measuring the motion of the two spirals at three epochs, we obtained a rotation rate of $-0.85^\circ\pm0.05^\circ~{\rm yr}^{-1}$. This rate corresponds to a protoplanet at $66\pm3$ au on a circular orbit dynamically driving both spirals. The derived location agrees with the gap in ALMA dust-continuum observations, indicating that the spiral driver may also carve the observed gap. What is more, a dust filament at $\sim$63 au observed by ALMA coincides with the predicted orbit of the spiral-arm-driving protoplanet. This double-spiraled system is an ideal target for protoplanet imaging.

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arXiv2024

A tell-tale tracer for externally irradiated protoplanetary disks: comparing the [CI] 8727 A line and ALMA observations in proplyds

The evolution of protoplanetary disks in regions with massive OB stars is influenced by externally driven winds that deplete the outer parts of disks. These winds have previously been studied via forbidden oxygen emission lines, which also arise in isolated disks in low-mass star forming-regions (SFRs) with weak external UV fields in photoevaporative or magnetic (internal) disk winds. It is crucial to determine how to disentangle external winds from internal ones. Here, we report a proxy for unambiguously identifying externally driven winds with a forbidden line of neutral atomic carbon, [C i] 8727 A. We compare for the first time the spatial location of the emission in the [O i] 5577 A, [O i] 6300 A, and [C i] 8727 A lines traced by VLT/MUSE-NFM, with the ALMA Band 7 continuum disk emission in a sample of 12 proplyds in the Orion Nebula Cluster (ONC). We confirm that the [O i] 5577 A emission is co-spatial with the disk emission, whereas the [O i] 6300 A is emitted both on the disk surface and on the ionization front of the proplyds. We show for the first time that the [C i] 8727 A line is also co-spatial with the disk surface in proplyds, as seen in the MUSE and ALMA data comparison. To verify whether the [C i] 8727 A line is detected in regions where external photoevaporation is not expected, we examine VLT/X-Shooter spectra for young stars in low-mass SFRs. Although the [O i] lines are well detected in all these targets, there is <<10% detection rate in the case of the [C i] 8727 A line. This number increases substantially to a ~40% detection rate in sigma-Orionis, a region with intermediate UV radiation. The spatial location of the [C i] 8727 A line emission and the lack of its detection in isolated disks in low-mass SFRs strongly suggest that this line is a tell-tale tracer of externally driven photoevaporative winds, which agrees with recent excitation models.

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arXiv2024

Resolved ALMA observations of water in the inner astronomical units of the HL Tau disk

The water molecule is a key ingredient in the formation of planetary systems, with the water snowline being a favourable location for the growth of massive planetary cores. Here we present Atacama Large Millimeter/ submillimeter Array data of the ringed protoplanetary disk orbiting the young star HL Tauri that show centrally peaked, bright emission arising from three distinct transitions of the main water isotopologue. The spatially and spectrally resolved water content probes gas in a thermal range down to the water sublimation temperature. Our analysis implies a stringent lower limit of 3.7 Earth oceans of water vapour available within the inner 17 astronomical units of the system. We show that our observations are limited to probing the water content in the atmosphere of the disk, due to the high dust column density and absorption, and indicate that the main water isotopologue is the best tracer to spatially resolve water vapour in protoplanetary disks.

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arXiv2024

Weighing protoplanetary discs with kinematics: physical model, method and benchmark

The mass of protoplanetary discs sets the amount of material available for planet formation, determines the level of coupling between gas and dust, and possibly sets gravitational instabilities. Measuring mass of discs is challenging, since it is not possible to directly detect H$_2$, and CO-based estimates remain poorly constrained. An alternative method that does not rely on tracers-to-H$_2$ ratios has recently been proposed to dynamically measure the disc mass altogether with the star mass and the disc critical radius by looking at deviations from Keplerian rotation induced by the self-gravity of the disc. So far, this method has been applied to weigh three protoplanetary discs: Elias 2-27, IM Lup and GM Aurigae. We provide here a numerical benchmark of the method by simulating isothermal self-gravitating discs with a range of masses from 0.01 to $0.2 \,M_{\odot}$ with the phantom code and post-process them with radiative transfer (mcfost) to obtain synthetic observations. We find that dynamical weighing allows to retrieve the expected value of disc masses as long as the disc-to-star mass ratio is larger than $M_d/M_\star=0.05$. The estimated uncertainty for the disc mass measurement is $\sim 25\%$.

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arXiv2024

Kaleidoscope of irradiated disks: MUSE observations of proplyds in the Orion Nebula Cluster. I. Sample presentation and ionization front sizes

In the Orion Nebula Cluster (ONC), protoplanetary disks exhibit ionized gas clouds in the form of a striking teardrop shape as massive stars irradiate the disk material. We present the first spatially and spectrally resolved observations of 12 proplyds, using Integral Field Spectroscopy observations performed with the MUSE instrument in Narrow Field Mode (NFM) on the VLT. We present the morphology of the proplyds in seven emission lines and measure the radius of the ionization front (I-front) of the targets in four tracers, covering transitions of different ionization states for the same element. We also derive stellar masses for the targets. The measurements follow a consistent trend of increasing I-front radius for a decreasing strength of the far-UV radiation as expected from photoevaporation models. By analyzing the ratios of the I-front radii as measured in the emission lines of Ha, [OI] 6300, [OII] 7330, and [OIII] 5007, we observe the ionization stratification, that is, the most ionized part of the flow being the furthest from the disk (and closest to the UV source). The ratios of I-front radii scale in the same way for all proplyds in our sample regardless of the incident radiation. We show that the stratification can help constrain the densities near the I-front by using a 1D photoionization model. We derive the upper limits of photoevaporative mass-loss rates by assuming ionization equilibrium, and estimate values decreasing towards lower impinging radiation. We do not find a correlation between Mloss and stellar mass. The highest mass-loss rate is for the proplyd 244-440. These values of Mloss, combined with estimates of the disk mass with ALMA, confirm previous estimates of the short lifetime of these proplyds. This work demonstrates the potential of this dataset and offers a new set of observables to be used to test current and future models of external photoevaporation.

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arXiv2024

Survival of the long-lived inner disk of PDS 70

The K7 T Tauri star PDS 70 remains the best laboratory for investigating the influence of giant planet formation on the structure of the parental disk. One of the most intriguing discoveries is the detection of a resolved inner disk from ALMA observations that extends up to the orbit of PDS 70b. It is challenging to explain this inner disk because most of the dust particles are expected to be trapped at the outer edge of the gap opened by PDS 70b and PDS 70c. By performing dust evolution models in combination with radiative transfer simulations that match the gas disk masses obtained from recent thermo-chemical models of PDS 70, we find that when the minimum grain size in the models is larger than 0.1$μ$m, there is an efficient filtration of dust particles, and the inner disk is depleted during the first million year of dust evolution. To maintain an inner disk, the minimum grain size in the models therefore needs to be smaller than 0.1$μ$m. Only when grains are that small are they diffused and dragged along with the gas throughout the gap opened by the planets. The small grains transported in the inner disk grow and drift into it, but the constant reservoir of dust particles that are trapped at the outer edge of the gap and that continuously fragment allows the inner disk to refill on million-year timescales. Our flux predictions at millimeter wavelength of these models agree with ALMA observations. These models predict a spectral index of 3.2 in the outer and 3.6 in the inner disk. Our simple analytical calculations show that the water emission in the inner disk that was recently observed with the James Webb Space Telescope may originate from these ice-coated small grains that flow through the gap, grow, and drift toward the innermost disk regions to reach the water snowline.

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arXiv2024

Rotation curves in protoplanetary disks with thermal stratification

In recent years the gas kinematics probed by molecular lines detected with ALMA has opened a new window to study protoplanetary disks. High spatial and spectral resolution observations have revealed the complexity of protoplanetary disk structure and correctly interpreting these data allow us to gain a better comprehension of the planet formation process. We investigate the impact of thermal stratification on the azimuthal velocity of protoplanetary disks. High resolution gas observations are showing velocity differences between CO isotopologues, which cannot be adequately explained with vertically isothermal models. The aim of this work is to determine whether a stratified model can explain this discrepancy. We analytically solve the hydrostatic equilibrium for a stratified disk and we derive the azimuthal velocity. We test the model with SPH numerical simulations and then we use it to fit for star mass, disk mass and scale radius of the sources in the MAPS sample. In particular, we use 12CO and 13CO datacubes.

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arXiv2024

Mapping the Vertical Gas Structure of the Planet-hosting PDS 70 Disk

PDS 70 hosts two massive, still-accreting planets and the inclined orientation of its protoplanetary disk presents a unique opportunity to directly probe the vertical gas structure of a planet-hosting disk. Here, we use high-spatial-resolution (${\approx}$0."1;10 au) observations in a set of CO isotopologue lines and HCO$^+$ J=4-3 to map the full 2D $(r,z)$ disk structure from the disk atmosphere, as traced by $^{12}$CO, to closer to the midplane, as probed by less abundant isotopologues and HCO$^+$. In the PDS 70 disk, $^{12}$CO traces a height of $z/r\approx0.3$, $^{13}$CO is found at $z/r\approx0.1$, and C$^{18}$O originates at, or near, the midplane. The HCO$^+$ surface arises from $z/r\approx0.2$ and is one of the few non-CO emission surfaces constrained with high fidelity in disks to date. In the $^{12}$CO J=3-2 line, we resolve a vertical dip and steep rise in height at the cavity wall, making PDS 70 the first transition disk where this effect is directly seen in line emitting heights. In the outer disk, the CO emission heights of PDS 70 appear typical for its stellar mass and disk size and are not substantially altered by the two inner embedded planets. By combining CO isotopologue and HCO$^+$ lines, we derive the 2D gas temperature structure and estimate a midplane CO snowline of ${\approx}$56-85 au. This implies that both PDS 70b and 70c are located interior to the CO snowline and are likely accreting gas with a high C/O ratio of ${\approx}$1.0, which provides context for future planetary atmospheric measurements from, e.g., JWST, and for properly modeling their formation histories.

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arXiv2023

Observing planetesimal formation under streaming instability in the rings of HD 163296

We introduce a new technique to determine the gas turbulence and surface density in bright disc rings, under the assumption that dust growth is limited by turbulent fragmentation at the ring centre. We benchmark this prescription in HD 163296, showing that our measurements are consistent with available turbulence upper limits and agree with independent estimates of the gas surface density within a factor of two. We combine our results with literature measurements of the dust surface density and grain size to determine the dust-to-gas ratio and Stokes number in the 67 au and 100 au rings. Our estimates suggest that particle clumping is taking place under the effect of streaming instability (SI) in the 100 au ring. Even though in the presence of external isotropic turbulence this process might be hindered, we provide evidence that turbulence is non-isotropic in both rings and likely originating from mechanisms (such as ambipolar diffusion) that could ease particle clumping under SI. Finally, we determine the mass accretion rate under the assumption that the disc is in steady state and turbulence regulates angular momentum transport. Our results are in tension with spectroscopic measurements and suggest that other mechanisms might be responsible for accretion, in qualitative agreement with the detection of a magneto-centrifugal wind in this system. Applying our method to larger samples can be used to statistically assess if SI is a viable mechanism to form planetesimals in bright rings.

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arXiv2023

The external photoevaporation of structured protoplanetary disks

The dust in planet-forming disks evolve rapidly through growth and radial drift, and external photoevaporation also contributes to this evolution in massive star-forming regions. We test whether the presence of substructures can explain the survival of the dust component and observed millimeter continuum emission in protoplanetary disks located within massive star-forming regions. We also characterize the dust content removed by the photoevaporative winds. For this, we performed hydrodynamical simulations of protoplanetary disks subject to irradiation fields of $F_{UV} = 10^2$, $10^3$, and $10^4\, G_0$, with different dust trap locations. We used the FRIED grid to derive the mass loss rate for each irradiation field and disk properties, and then measure the evolution of the dust mass over time. For each simulation we estimate continuum emission at $λ= 1.3\, \textrm{mm}$ along with the radii encompassing $90\%$ of the continuum flux, and characterize the dust size distribution entrained in the photoevaporative winds, along with the resulting far-ultraviolet (FUV) cross section. Our simulations show that the presence of dust traps can extend the lifetime of the dust component of the disk to a few millionyears if the FUV irradiation is $F_{UV} \lesssim 10^3 G_0$, but only if the dust traps are located inside the photoevaporative truncation radius. The dust component of a disk quickly disperse if the FUV irradiation is strong ($10^4\, G_0$) or if the substructures are located outside the photoevaporation radius. We do find however, that the dust grains entrained with the photoevaporative winds may result in an absorption FUV cross section of $σ\approx 10^{-22}\, \textrm{cm}^2$ at early times of evolution ($<$0.1 Myr), which is enough to trigger a self-shielding effect that reduces the total mass loss rate, and slow down the disk dispersal in a negative feedback loop process.

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arXiv2023

Protoplanetary disks in $K_s$-band total intensity and polarized light

Diverse protoplanetary disk morphology can result from planet-disk interaction, suggesting planetary presence. To date, most scattered light imaging campaigns have probed polarized light, which is only a fraction of the total light and not very sensitive to planets. To observe and characterize protoplanetary disk systems in the near-infrared in both polarized and total intensity light, we carried out an unprecedented study of scattering properties of disks, as well as of any planetary companions. Using SPHERE with star-hopping at the Very Large Telescope, we observed 29 disk hosts and their reference stars in $K_s$-band polarized light. We extracted disks in total intensity by adopting the data imputation concept with sequential non-negative matrix factorization (DI-sNMF). We obtained high-quality disk images in total intensity for 15 systems and in polarized light for 23. For well-recovered disks in polarized light and total intensity, we parameterized the polarization fraction phase functions using scaled beta distribution: the peak of polarization fraction tentatively correlates with the peak scattering angle, which could be reproduced using certain compact dust, yet more detailed modeling studies are needed. We investigated the empirical DI-sNMF detectability of disks using logistic regression: total intensity detectability of disks primarily depends on host star brightness. For disks with SPHERE data in $Y$-/$J$-/$H$-band, we summarized their polarized color at ~90 deg scattering angle: most of disks are blue in polarized $J-K_s$ color, and they are relatively redder as stellar luminosity increases, indicating larger scatterers. High-quality disk imagery in both total intensity and polarized light thus allows for disk characterization in polarization fraction, and reduces the confusion between disk and planetary signals.

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arXiv2023

Protoplanetary Disk Chemistry

Planets form in disks of gas and dust around young stars. The disk molecular reservoirs and their chemical evolution affect all aspects of planet formation, from the coagulation of dust grains into pebbles, to the elemental and molecular compositions of the mature planet. Disk chemistry also enables unique probes of disk structures and dynamics, including those directly linked to ongoing planet formation. Here we review the protoplanetary disk chemistry of the volatile elements HOCNSP, the associated observational and theoretical methods, and the links between disk and planet chemical compositions. Three takeaways from this review are: (1) The disk chemical composition, including the organic reservoirs, is set by both inheritance and in situ chemistry. (2) Disk gas and solid O/C/N/H elemental ratios often deviate from stellar values due to a combination of condensation of molecular carriers, chemistry, and dynamics. (3) Chemical, physical, and dynamical processes in disks are closely linked, which complicates disk chemistry modeling, but these links also present an opportunity to develop chemical probes of different aspects of disk evolution and planet formation.

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arXiv2023

MagAO-X and HST high-contrast imaging of the AS209 disk at H$α$

The detection of emission lines associated with accretion processes is a direct method for studying how and where gas giant planets form, how young planets interact with their natal protoplanetary disk and how volatile delivery to their atmosphere takes place. H$α$ ($λ=0.656\,μ$m) is expected to be the strongest accretion line observable from the ground with adaptive optics systems, and is therefore the target of specific high-contrast imaging campaigns. We present MagAO-X and HST data obtained to search for H$α$ emission from the previously detected protoplanet candidate orbiting AS209, identified through ALMA observations. No signal was detected at the location of the candidate, and we provide limits on its accretion. Our data would have detected an H$α$ emission with $F_\mathrm{Hα}>2.5\pm0.3 \times10^{-16}$ erg s$^{-1}$ cm$^{-2}$, a factor 6.5 lower than the HST flux measured for PDS70b (Zhou et al., 2021). The flux limit indicates that if the protoplanet is currently accreting it is likely that local extinction from circumstellar and circumplanetary material strongly attenuates its emission at optical wavelengths. In addition, the data reveal the first image of the jet north of the star as expected from previous detections of forbidden lines. Finally, this work demonstrates that current ground-based observations with extreme adaptive optics systems can be more sensitive than space-based observations, paving the way to the hunt for small planets in reflected light with extremely large telescopes.

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arXiv2023

Radio multiwavelength analysis of the compact disk CX Tau: Presence of strong free-free variability or anomalous microwave emission

Protoplanetary disks emit radiation across a broad range of wavelengths, requiring a multiwavelength approach to fully understand their physical mechanisms and how they form planets. Observations at sub-millimeter to centimeter wavelengths can provide insights into the thermal emission from dust, free-free emission from ionized gas, and possible gyro-synchrotron emission from the stellar magnetosphere. This work is focused on CX Tau, a ${\sim}0.4\,M_\odot$ star with an extended gas emission and a compact and apparently structureless dust disk, with an average millimeter flux when compared to Class II sources in Taurus. We present observations from the Karl G. Jansky Very Large Array (VLA) across four bands (between 9.0 mm and 6.0 cm) and combine them with archival data from the Atacama Large Millimeter/submillimeter Array (ALMA), the Submillimeter Array (SMA) and the Plateau de Bure Interferometer (PdBI). This multiwavelength approach allows us to separate the dust continuum from other emissions. After isolating the dust thermal emission, we derived an upper limit of the dust disk extent at 1.3 cm which is consistent with theoretical predictions of a radial drift-dominated disk. Centimeter data show a peculiar behavior: deep observations at 6.0 cm did not detect the source, while at 1.3 cm the flux density is anomalously higher than adjacent bands. Intraband spectral indices suggest a dominant contribution from free-free emission, whereas gyro-synchrotron emission is excluded. To explain these observations, we propose a strong variability among the free-free emission with timescales shorter than a month. Another possible interpretation is the presence of anomalous microwave emission from spinning dust grains.

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arXiv2023

A magnetically driven disc wind in the inner disc of PDS 70

PDS 70 is so far the only young disc where multiple planets have been detected by direct imaging. The disc has a large cavity when seen at sub-mm and NIR wavelengths, which hosts two massive planets. This makes PDS 70 the ideal target to study the physical conditions in a strongly depleted inner disc shaped by two giant planets, and in particular to test whether disc winds can play a significant role in its evolution. Using X-Shooter and HARPS spectra, we detected for the first time the wind-tracing [O I] 6300AA line, and confirm the low-moderate value of mass-accretion rate in the literature. The [O I] line luminosity is high with respect to the accretion luminosity when compared to a large sample of discs with cavities in nearby star-forming regions. The FWHM and blue-shifted peak of the [O I] line suggest an emission in a region very close to the star, favouring a magnetically driven wind as the origin. We also detect wind emission and high variability in the He I 10830AA line, which is unusual for low-accretors. We discuss that, although the cavity of PDS 70 was clearly carved out by the giant planets, the substantial inner disc wind could also have had a significant contribution to clearing the inner-disc.

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arXiv2023

Tentative co-orbital submillimeter emission within the Lagrangian region L5 of the protoplanet PDS 70 b

Context: High-spatial resolution Atacama Large Millimeter/submillimeter Array (ALMA) data have revealed a plethora of substructures in protoplanetary disks. Some of those features are thought to trace the formation of embedded planets. One example is the gas and dust that accumulated in the co-orbital Lagrangian regions $L_4$/$L_5$, which were tentatively detected in recent years and might be the pristine material for the formation of Trojan bodies. Aims: This work is part of the TROY project, whose ultimate goal is to find robust evidence of exotrojan bodies and study their implications in the exoplanet field. Here, we focus on the early stages of the formation of these bodies by inspecting the iconic system PDS 70, the only confirmed planetary system in formation. Methods: We reanalyzed archival high-angular resolution Band 7 ALMA observations from PDS 70 by doing an independent imaging process to look for emission in the Lagrangian regions of the two detected gas giant protoplanets, PDS 70 b and c. We then projected the orbital paths and visually inspected emission features at the regions around the $L_4$/$L_5$ locations as defined by $\pm$ 60$^{\circ}$ in azimuth from the planet position. Results: We found emission at a $\sim$4-$σ$ level ($\sim$6-$σ$ when correcting from a cleaning effect) at the position of the $L_{5}$ region of PDS 70 b. This emission corresponds to a dust mass in a range of 0.03- 2 M$_{Moon}$, which potentially accumulated in this gravitational well. Conclusions: The tentative detection of the co-orbital dust trap that we report requires additional observations to be confirmed. We predict that we could detect the co-orbital motion of PDS 70 b and the dust presumably associated with $L_5$ by observing again with the same sensitivity and angular resolution as early as February 2026.

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