arXiv subjects

Carlo F. Manara

Publications and source records attributed to Carlo F. Manara.

At least 55 records · Page 3Linked to original sources

arXiv2023

Testing protoplanetary disc evolution with CO fluxes. A proof of concept in Lupus and Upper Sco

The Atacama Large Millimeter/submillimeter Array (ALMA) revolutionised our understanding of protoplanetary discs. However, the available data have not given conclusive answers yet on the underlying disc evolution mechanisms (viscosity or MHD winds). Improving upon the current results, mostly based on the analysis of disc sizes, is difficult because larger, deeper and higher angular resolution surveys would be required, which could be prohibitive even for ALMA. In this Letter, we introduce an alternative method to study disc evolution based on $^{12}$CO fluxes. In fact, fluxes can be readily collected using less time-consuming, lower resolution observations, while tracing the same disc physico-chemical processes as sizes: assuming that $^{12}$CO is optically thick, fluxes scale with the disc surface area. We developed a semi-analytical model to compute $^{12}$CO fluxes and benchmarked it against the results of DALI thermochemical models, recovering an agreement within a factor of three. As a proof of concept we compared our models with Lupus and Upper Sco data, taking advantage of the increased samples, by a factor 1.3 (Lupus) and 3.6 (Upper Sco), when studying fluxes instead of sizes. Models and data agree well only if CO depletion is considered. However, the uncertainties on the initial conditions limited our interpretation of the observations. Our new method can be used to design future ad hoc observational strategies to collect better data and give conclusive answers on disc evolution.

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arXiv2023

A Large Double-ring Disk around the Taurus M Dwarf J04124068+2438157

Planet formation imprints signatures on the physical structures of disks. In this paper, we present high-resolution ($\sim$50 mas, 8 au) Atacama Large Millimeter/submillimeter Array (ALMA) observations of 1.3 mm dust continuum and CO line emission toward the disk around the M3.5 star 2MASS J04124068+2438157. The dust disk consists only of two narrow rings at radial distances of 0.47 and 0.78 arcsec ($\sim$70 and 116 au), with Gaussian $σ$ widths of 5.6 and 8.5 au, respectively. The width of the outer ring is smaller than the estimated pressure scale height by $\sim25\%$, suggesting dust trapping in a radial pressure bump. The dust disk size, set by the location of the outermost ring, is significantly larger (by $3σ$) than other disks with similar millimeter luminosity, which can be explained by an early formation of local pressure bump to stop radial drift of millimeter dust grains. After considering the disk's physical structure and accretion properties, we prefer planet--disk interaction over dead zone or photoevaporation models to explain the observed dust disk morphology. We carry out high-contrast imaging at $L'$ band using Keck/NIRC2 to search for potential young planets, but do not identify any source above $5σ$. Within the dust gap between the two rings, we reach a contrast level of $\sim$7 mag, constraining the possible planet below $\sim$2--4 $M_{\rm Jup}$. Analyses of the gap/ring properties suggest a $\sim$Saturn mass planet at $\sim$90 au is likely responsible for the formation of the outer ring, which can be potentially revealed with JWST.

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arXiv2023

Empirical Determination of the Lithium 6707.856 Å Wavelength in Young Stars

Absorption features in stellar atmospheres are often used to calibrate photocentric velocities for kinematic analysis of further spectral lines. The Li feature at $\sim$ 6708 Å is commonly used, especially in the case of young stellar objects for which it is one of the strongest absorption lines. However, this is a complex line comprising two isotope fine-structure doublets. We empirically measure the wavelength of this Li feature in a sample of young stars from the PENELLOPE/VLT programme (using X-Shooter, UVES and ESPRESSO data) as well as HARPS data. For 51 targets, we fit 314 individual spectra using the STAR-MELT package, resulting in 241 accurately fitted Li features, given the automated goodness-of-fit threshold. We find the mean air wavelength to be 6707.856 Å, with a standard error of 0.002 Å (0.09 km/s) and a weighted standard deviation of 0.026 Å (1.16 km/s). The observed spread in measured positions spans 0.145 Å, or 6.5 km/s, which is up to a factor of six higher than typically reported velocity errors for high-resolution studies. We also find a correlation between the effective temperature of the star and the wavelength of the central absorption. We discuss how exclusively using this Li feature as a reference for photocentric velocity in young stars could potentially be introducing a systematic positive offset in wavelength to measurements of further spectral lines. If outflow tracing forbidden lines, such as [O i] 6300 Å, are actually more blueshifted than previously thought, this then favours a disk wind as the origin for such emission in young stars.

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arXiv2023

Disk Evolution Study Through Imaging of Nearby Young Stars (DESTINYS): Diverse outcomes of binary-disk interactions

Circumstellar disks do not evolve in isolation, as about half of solar-type stars were born in binary or multiple systems. Resolving disks in binary systems provides the opportunity to examine the influence of stellar companions on the outcomes of planet formation. We aim to investigate and compare disks in stellar multiple systems with near-infrared scattered-light imaging as part of the Disk Evolution Study Through Imaging of Nearby Young Stars (DESTINYS) program. We used polarimetric differential imaging with SPHERE/IRDIS at the VLT to search for scattered light from the circumstellar disks in three multiple systems, CHX 22, S CrA, and HP Cha. We performed astrometric and orbit analyses for the stellar companions using archival HST, VLT/NACO, and SPHERE data. Combined with the age and orbital constraints, the observed disk structures provide insights into the evolutionary history and the impact of the stellar companions. The small grains in CHX 22 form a tail-like structure surrounding the close binary, which likely results from a close encounter and capture of a cloudlet. S CrA shows intricate structures (tentative ringed and spiral features) in the circumprimary disk as a possible consequence of perturbations by companions. The circumsecondary disk is truncated and connected to the primary disk via a streamer, suggesting tidal interactions. In HP Cha, the primary disk is less disturbed and features a tenuous streamer, through which the material flows towards the companions. The comparison of the three systems spans a wide range of binary separation (50 - 500 au) and illustrates the decreasing influence on disk structures with the distance of companions. This agrees with the statistical analysis of exoplanet population in binaries, that planet formation is likely obstructed around close binary systems, while it is not suppressed in wide binaries.

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arXiv2023

Reflections on nebulae around young stars: A systematic search for late-stage infall of material onto Class II disks

Context. While it is generally assumed that Class II sources evolve largely in isolation from their environment, many still lie close to molecular clouds and may continue to interact with them. This may result in late accretion of material onto the disk that can significantly influence disk processes and planet formation. Aims. In order to systematically study late infall of gas onto disks, we identify candidate Class II sources in close vicinity to a reflection nebula (RN) that may be undergoing this process. Methods. First we targeted Class II sources with known kilo-au scale gas structures - possibly due to late infall of material - and we searched for RNe in their vicinity in optical and near-infrared images. Second, we compiled a catalogue of Class II sources associated with RNe and looked for the large-scale CO structures in archival ALMA data. Using the catalogues of protostars and RNe, we also estimated the probability of Class II sources interacting with surrounding material. Results. All of the sources with large-scale gas structures also exhibit some reflection nebulosity in their vicinity. Similarly, at least five Class II objects associated with a prominent RNe, and for which adequate ALMA observations are available, were found to have spirals or stream-like structures which may be due to late infall. We report the first detection of these structures around S CrA. Conclusions. Our results suggest that a non-negligible fraction of Class II disks in nearby star-forming regions may be associated with RNe and could therefore be undergoing late accretion of gas. Surveys of RNe and kilo-au scale gas structures around Class II sources will allow us to better understand the frequency and impact of late-infall phenomena.

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arXiv2023

Lyman-alpha Scattering Models Trace Accretion and Outflow Kinematics in T Tauri Systems

T Tauri stars produce broad Lyman-alpha emission lines that contribute $\sim$88% of the total UV flux incident on the inner circumstellar disks. Lyman-alpha photons are generated at the accretion shocks and in the protostellar chromospheres and must travel through accretion flows, winds and jets, the protoplanetary disks, and the interstellar medium before reaching the observer. This trajectory produces asymmetric, double-peaked features that carry kinematic and opacity signatures of the disk environments. To understand the link between the evolution of Lyman-alpha emission lines and the disks themselves, we model HST-COS spectra from targets included in Data Release 3 of the Hubble UV Legacy Library of Young Stars as Essential Standards (ULLYSES) program. We find that resonant scattering in a simple spherical expanding shell is able to reproduce the high velocity emission line wings, providing estimates of the average velocities within the bulk intervening H I. The model velocities are significantly correlated with the K band veiling, indicating a turnover from Lyman-alpha profiles absorbed by outflowing winds to emission lines suppressed by accretion flows as the hot inner disk is depleted. Just 30% of targets in our sample have profiles with red-shifted absorption from accretion flows, many of which have resolved dust gaps. At this stage, Lyman-alpha photons may no longer intersect with disk winds along the path to the observer. Our results point to a significant evolution of Lyman-alpha irradiation within the gas disks over time, which may lead to chemical differences that are observable with ALMA and JWST.

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arXiv2022

The Mass Accretion Rate and Stellar Properties in Class I Protostars

Stars collect most of their mass during the protostellar stage, yet the accretion luminosity and stellar parameters, which are needed to compute the mass accretion rate, are poorly constrained for the youngest sources. The aim of this work is to fill this gap, computing the stellar properties and the accretion rates for a large sample of Class I protostars located in nearby (< 500 pc) star-forming regions and analysing their interplay. We used a self-consistent method to provide accretion and stellar parameters using SED modeling and veiling information from near-IR observations, when possible. We calculated accretion and stellar properties for the first time for 50 young stars. We focused our analysis on the 39 confirmed protostars, finding that their mass accretion rate varies between about 10^(-8) and about 10^(-4) Msun/yr in a stellar mass range between about 0.1 and 3 Msun. We find systematically larger mass accretion rates for our Class I sample than in Class II objects. Although the mass accretion rate we found is high, it still suggests that either stars collect most of its mass before Class I stage, or eruptive accretion is needed during the overall protostellar phase. Indeed, our results suggest that for a large number of protostars the disk can be unstable, which can result in accretion bursts and disk fragmentation in the past or in the future.

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arXiv2022

Accretion and extinction variations in the low-mass pre-main sequence binary system WX Cha

Light curves of young star systems show photometric variability due to different kinematic, and physical processes. One of the main contributors to the photometric variability is the changing mass accretion rate, which regulates the interplay between the forming young star and the protoplanetary disk. We collected high-resolution spectroscopy in eight different epochs, as well as ground-based and space-borne multi-epoch optical and infrared photometry of WX Cha, an M0 binary system, with an almost edge-on disk (i = 87degrees) in the Chamaeleon I star-forming region. Spectroscopic observations cover 72 days, the ground-based optical monitoring covers 42 days while space-borne TESS photometry extends for 56 days. The multi-wavelength light curves exhibit quasi-periodic variability of 0.35 - 0.53 mag in the near-infrared, and of 1.3 mag in g band. We studied the variability of selected emission lines that trace the accretion, computed the accretion luminosity and the mass accretion rate using empirical relations and obtained values of the accretion luminosity between 1.6 and 3.2 Lsun and mass accretion rate between 3.31x10{-7} Msun/yr and 7.76x10^{-7} Msun/yr. Our results show that WX Cha is accreting at a rate larger than what is typical for T Tauri stars in the same star-forming region with the same stellar parameters. We theorize that this is due to the higher disk mass of WX Cha than what is usual for stars with similar stellar mass, and to the binary nature of the system. Daily changes in the accretion luminosity and in the extinction can explain the photometric variability.

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arXiv2022

The relation between the Mass Accretion Rate and the Disk Mass in Class I Protostars

The evidence of a relation between the mass accretion rate and the disk mass is established for young, Class II pre-main sequence stars. This observational result opened an avenue to test theoretical models and constrain the initial conditions of the disk formation, fundamental in the understanding of the emergence of planetary systems. However, it is becoming clear that the planet formation starts even before the Class II stage, in disks around Class 0 and I protostars. We show for the first time evidence for a correlation between the mass accretion rate and the disk mass for a large sample of Class I young stars located in nearby (< 500 pc) star-forming regions. We fit our sample, finding that the Class I objects relation has a slope flatter than Class II stars, and have higher mass accretion rates and disk masses. The results are put in context of the disk evolution models.

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arXiv2022

Towards a comprehensive view of accretion, inner disks, and extinction in classical T Tauri stars: an ODYSSEUS study of the Orion OB1b association

The coevolution of T Tauri stars and their surrounding protoplanetary disks dictates the timescales of planet formation. In this paper, we present magnetospheric accretion and inner disk wall model fits to NUV-NIR spectra of nine classical T Tauri stars in Orion OB1b as part of the Outflows and Disks around Young Stars: Synergies for the Exploration of ULLYSES Spectra (ODYSSEUS) Survey. Using NUV-optical spectra from the Hubble UV Legacy Library of Young Stars as Essential Standards (ULLYSES) Director's Discretionary Program and optical-NIR spectra from the PENELLOPE VLT Large Programme, we find that the accretion rates of these targets are relatively high for the region's intermediate age of 5.0 Myr; rates range from $0.5-17.2 \times 10^{-8}$ M$_{\odot}$/yr, with a median value of $1.2\times 10^{-8}$ M$_{\odot}$/yr. The NIR excesses can be fit with 1200-1800 K inner disk walls located at 0.05-0.10 AU from the host stars. We discuss the significance of the choice in extinction law, as the measured accretion rate depends strongly on the adopted extinction value. This analysis will be extended to the complete sample of T Tauri stars being observed through ULLYSES to characterize accretion and inner disks in star-forming regions of different ages and stellar populations.

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arXiv2022

Accretion variability of the multiple T Tauri system VW Cha

Classical T Tauri stars are low-mass objects, which are still accreting material from the surrounding circumstellar disk. The accretion process is essential in the formation of Sun-like stars and in setting the properties of the disk at the time when planet formation occurs. We constructed a complex dataset in order to examine the accretion process of VW Cha, a classical T Tauri multiple system with the aim of studying the physical origin of the photometric and spectroscopic variability of the system. The TESS Space Telescope observed VW Cha between 2019 April 22 and June 19, and we complemented these data with contemporaneous ground-based $I_CJHK$ band photometric measurements. In addition, we obtained high-resolution optical spectra with the VLT/ESPRESSO and the 2.2\,m/FEROS instruments. Analyzing these data, we found that the TESS light curve shows photometric variations on timescales from minutes to weeks with a peak-to-peak amplitude of $\sim$0.8 mag. The near-infrared light curves follow the shape of the optical measurements, however, the peak-to-peak amplitudes are slightly increasing towards the longer wavelengths. We took spectra in both fainter and brighter photometric states of the system, allowing us to examine the origin of a photometric brightening event. Our results show that this brightening event can be explained by increased accretion. In addition, our spectroscopic data also suggest that the primary component of VW Cha is a spectroscopic binary, as it was proposed in earlier works.

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arXiv2022

Disk Evolution Study Through Imaging of Nearby Young Stars (DESTINYS): A Panchromatic View of DO Tau's Complex Kilo-au Environment

While protoplanetary disks are often treated as isolated systems in planet formation models, observations increasingly suggest that vigorous interactions between Class II disks and their environments are not rare. DO Tau is a T Tauri star that has previously been hypothesized to have undergone a close encounter with the HV Tau system. As part of the DESTINYS ESO Large Programme, we present new VLT/SPHERE polarimetric observations of DO Tau and combine them with archival HST scattered light images and ALMA observations of CO isotopologues and CS to map a network of complex structures. The SPHERE and ALMA observations show that the circumstellar disk is connected to arms extending out to several hundred au. HST and ALMA also reveal stream-like structures northeast of DO Tau, some of which are at least several thousand au long. These streams appear not to be gravitationally bound to DO Tau, and comparisons with previous Herschel far-IR observations suggest that the streams are part of a bridge-like structure connecting DO Tau and HV Tau. We also detect a fainter redshifted counterpart to a previously known blueshifted CO outflow. While some of DO Tau's complex structures could be attributed to a recent disk-disk encounter, they might be explained alternatively by interactions with remnant material from the star formation process. These panchromatic observations of DO Tau highlight the need to contextualize the evolution of Class II disks by examining processes occurring over a wide range of size scales.

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arXiv2022

High-resolution ALMA observations of transition disk candidates in Lupus

Transition disks with small inner dust cavities are interesting targets for the study of disk clearing mechanisms. Such disks have been identified through a deficit in the infrared part of their SED, but spatially resolved millimeter imaging is required to confirm the presence of an inner dust cavity. We use high-resolution ALMA observations of 30 mas resolution in Band 6 continuum and $^{12}$CO 2--1 emission of 10 transition disk candidates in the Lupus star forming region, in order to confirm the presence of inner dust cavities and infer the responsible mechanism. The continuum data are analyzed using visibility modeling and the SEDs are compared with radiative transfer models. Out of the six transition disk candidates selected from their SED, only one disk revealed an inner dust cavity of 4 au in radius. Three of the other disks are highly inclined, which limits the detectability of an inner dust cavity but it is also demonstrated to be the possible cause for the infrared deficit in their SED. The two remaining SED-selected disks are very compact, with dust radii of only $\sim$3 au. From the four candidates selected from low-resolution images, three new transition disks with large inner cavities $>$20 au are identified, bringing the total number of transition disks with large cavities in Lupus to 13. SED-selected transition disks with small cavities are biased towards highly inclined and compact disks, which casts doubt on the use of their occurrence rates in estimating dispersal timescales of photoevaporation. Using newly derived disk dust masses and radii, we re-evaluate the size-luminosity and $M_{\rm dust}-M_{\rm star}$ relations. These relations can be understood if the bright disks are dominated by disks with substructure whereas faint disks are dominated by drift-dominated disks. (Abridged)

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arXiv2022

Stellar multiplicity affects the correlation between proto-planetary disc masses and accretion rates: binaries explain high-accretors in Upper Sco

In recent years a correlation between mass accretion rates onto new-born stars and their proto-planetary disc masses was detected in nearby young star-forming regions. Although such a correlation can be interpreted as due to viscous-diffusion processes in the disc, highly-accreting sources with low disc masses in more evolved regions remain puzzling. In this paper, we hypothesise that the presence of a stellar companion truncating the disc can explain these outliers. Firstly, we searched the literature for information on stellar multiplicity in Lupus, Chamaeleon~I and Upper Sco, finding that roughly 20 per cent of the discs involved in the correlation are in binaries or higher-order multiple stellar systems. We prove with high statistical significance that at any disc mass these sources have systematically higher accretion rates than those in single-stars, with the bulk of the binary population being clustered around $M_\mathrm{disc}/\dot{M}_\mathrm{acc}\approx0.1\,\mathrm{Myr}$. We then run coupled gas and dust one-dimensional evolutionary models of tidally truncated discs to be compared with the data. We find that these models are able to reproduce well most of the population of observed discs in Lupus and Upper Sco, even though the unknown eccentricity of each binary prevents an object by object comparison. In the latter region, the agreement improves if the grain coagulation efficiency is reduced, as may be expected in discs around close binaries. Finally, we mention that thermal winds and sub-structures can be important in explaining few outlying sources.

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arXiv2021

The STAR-MELT Python package for emission line analysis of YSOs

We introduce the STAR-MELT Python package that we developed to facilitate the analysis of time-resolved emission line spectroscopy of young stellar objects. STAR-MELT automatically extracts, identifies and fits emission lines. We summarise our analysis methods that utilises the time domain of high-resolution stellar spectra to investigate variability in the line profiles and corresponding emitting regions. This allows us to probe the innermost disc and accretion structures of YSOs. Local temperatures and densities can be determined using Boltzmann statistics, the Saha equation, and the Sobolev large velocity gradient approximation. STAR-MELT allows for new results to be obtained from archival data, as well as facilitating timely analysis of new data as it is obtained. We present the results of applying STAR-MELT to three YSOs, using spectra from UVES, XSHOOTER, FEROS, HARPS, and ESPaDOnS. We demonstrate what can be achieved for data with disparate time sampling, for stars with different inclinations and variability types. For EX Lupi, we confirm the presence of a localised and stable stellar-surface hot spot associated with the footprint of the accretion column. For GQ Lupi A, we find that the maximum infall rate from an accretion column is correlated with lines produced in the lowest temperatures. For CVSO109 we investigate the rapid temporal variability of a redshifted emission wing, indicative of rotating and infalling material in the inner disc. Our results show that STAR-MELT is a useful tool for such analysis, as well as other applications for emission lines.

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arXiv2021

Multi-wavelength continuum sizes of protoplanetary discs: scaling relations and implications for grain growth and radial drift

We analyse spatially resolved ALMA observations at 0.9, 1.3, and 3.1 mm for the 26 brightest protoplanetary discs in the Lupus star-forming region. We characterise the discs multi-wavelength brightness profiles by fitting the interferometric visibilities in a homogeneous way, obtaining effective disc sizes at the three wavelengths, spectral index profiles and optical depth estimates. We report three fundamental discoveries: first, the millimeter continuum size - luminosity relation already observed at 0.9 mm is also present at 1.3 mm with an identical slope, and at 3.1 mm with a steeper slope, confirming that emission at longer wavelengths becomes increasingly optically thin. Second, when observed at 3.1 mm the discs appear to be only 9% smaller than when observed at 0.9 mm, in tension with models of dust evolution which predict a starker difference. Third, by forward modelling the sample of measurements with a simple parametric disc model, we find that the presence of large grains ($a_\mathrm{max}>1 $mm) throughout the discs is the most favoured explanation for all discs as it reproduces simultaneously their spectral indices, optical depth, luminosity, and radial extent in the 0.9-1.3 mm wavelength range. We also find that the observations can be alternatively interpreted with the discs being dominated by optically thick, unresolved, substructures made of mm-sized grains with a high scattering albedo.

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arXiv2021

The first ALMA survey of protoplanetary discs at 3 mm: demographics of grain growth in the Lupus region

We present the first ALMA survey of protoplanetary discs at 3 mm, targeting 36 young stellar objects in the Lupus star-forming region with deep observations (sensitivity 20-50 microJy/beam) at ~0.35" resolution (~50 au). Building on previous ALMA surveys at 0.89 and 1.3 mm that observed the complete sample of Class II discs in Lupus at a comparable resolution, we aim to assess the level of grain growth in the relatively young Lupus region. We measure 3 mm integrated fluxes, from which we derive disc-averaged 1-3 mm spectral indices. We find that the mean spectral index of the observed Lupus discs is $α_\mathrm{1-3 mm}=2.23\pm0.06$, in all cases $α_\mathrm{1-3 mm}<3.0$, with a tendency for larger spectral indices in the brightest discs and in transition discs. Furthermore, we find that the distribution of spectral indices in Lupus discs is statistically indistinguishable from that of the Taurus and Ophiuchus star-forming regions. Assuming the emission is optically thin, the low values $α_\mathrm{1-3 mm}\leq 2.5$ measured for most discs can be interpreted with the presence of grains larger than 1 mm. The observations of the faint discs in the sample can be explained without invoking the presence of large grains, namely through a mixture of optically thin and optically thick emission from small grains. However, the bright (and typically large) discs do inescapably require the presence of millimeter-sized grains in order to have realistic masses. Based on a disc mass argument, our results challenge previous claims that the presence of optically thick sub-structures may be a universal explanation for the empirical millimeter size-luminosity correlation observed at 0.89 mm.

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arXiv2021

Compact Disks: an explanation to faint CO emission in Lupus disks

A large fraction of observed protoplanetary disks in nearby Star-Forming Regions (SFRs) are fainter than expected in CO isotopologue emission. Disks not detected in 13CO line emission are also faint and often unresolved in the continuum emission at an angular resolution of around 0.2 arcseconds. Focusing on the Lupus SFR, the aim of this work is to investigate whether the population of CO-faint disks comprises radially extended and low mass disks - as commonly assumed so far - or if it is of intrinsically radially compact disks, an interpretation that we propose in this paper. The latter scenario was already proposed for individual sources or small samples of disks, while this work targets a large population of disks in a single SFR for which statistical arguments can be made. A new grid of physical-chemical models of compact disks has been run with DALI in order to cover a region of the parameter space that had not been explored before with this code. Such models have been compared with 12CO and 13CO ALMA observations of faint disks in Lupus. Disks that are not detected in 13CO emission and with faint or undetected 12CO emission are consistent with compact disk models. For radially compact disk, the emission of CO isotopologues is mostly optically thick and it scales with the surface area: i.e., it is fainter for smaller objects. The fraction of compact disks is potentially between roughly 50% and 60% of the entire Lupus sample. Deeper observations of 12CO and 13CO at a moderate angular resolution will allow us to distinguish whether faint disks are intrinsically compact, or if they are extended but faint, without the need of resolving them. If the fainter end of the disk population observed by ALMA disk surveys is consistent with such objects being very compact, this will either create a tension with viscous spreading or require MHD winds or external processes to truncate the disks.

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