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The time evolution of the dependence of the mass accretion rate with the stellar mass and the disk mass represents a fundamental way to understand the evolution of protoplanetary disks and the formation of planets. In this work, we present observations with X-shooter of 26 Class II very low-mass stars (< 0.2 M⊙) and brown dwarfs in the Ophiuchus, Chamaeleon-I, and Upper Scorpius star-forming regions. These new observations extend the measurement of the mass accretion rate down to spectral type (SpT) M9 (∼0.02 M⊙) in Ophiuchus and Chamaeleon-I and add 11 very-low-mass stars to the sample of objects previously studied with broadband spectroscopy in Upper Scorpius. We obtained the spectral type and extinction, as well as the physical parameters of the sources. We used the intensity of various emission lines in the spectra of these sources to derive the accretion luminosity and mass accretion rates for the entire sample. Combining these new observations with data from the literature, we compare relations between accretion and stellar and disk properties of four different star-forming regions with different ages: Ophiuchus (∼1 Myr), Lupus (∼2 Myr), Chamaeleon-I (∼3 Myr), and Upper Scorpius (5−12 Myr). We find the slopes of the accretion relationships (L* − Lacc, M∗ − Ṁacc) to steepen in the 1−3 Myr age range (i.e., between Ophiuchus, Lupus, and Chamaeleon-I) and that both relationships may be better described with a single power law. We find that previous claims for a double power-law behavior of the M∗ − Ṁacc relationship may have been triggered by the use of a different SpT–Teff scale. We also find the relationship between the protoplanetary disk mass and the mass accretion rate of the stellar population to steepen with time down to the age of Upper Scorpius. Overall, we observe hints of a faster evolution into low accretion rates of low-mass stars and brown dwarfs. At the same time, we also find that brown dwarfs present higher Mdisk/Ṁacc ratios (i.e., longer accretion depletion timescales) than stars in Ophiuchus, Lupus, and Cha-I. This apparently contradictory result may imply that the evolution of protoplanetary disks around brown dwarfs may be different than what is seen in the stellar regime. [ABSTRACT FROM AUTHOR]

Context. Observations of interstellar material infalling onto star- and planet-forming systems have become increasingly common thanks to recent advancements in radio interferometry. These structures have the potential to alter the dynamics of protoplanetary disks significantly by triggering the formation of substructures, inducing shocks, and modifying their physical and chemical properties. Moreover, the protoplanetary disks are replenished with new material, which increases the overall mass budget for planet formation. Aims. In this study, we combine new ALMA band 3 and archival band 6 observations to characterize the dust content and infall rate of a 4000 au arc-like structure that is infalling onto [MGM2012] 512 (hereafter M512), a class I young stellar object located in the Lynds 1641 region of the Orion A molecular cloud. Methods. We detected the extended dust emission from this structure in both ALMA bands. We tested whether the velocity pattern of the streamer is consistent with infalling trajectories by means of analytical streamline models. We measured spectral index maps for the first time and derived a dust opacity-index profile along a streamer. We constrained its grain properties and mass. Results. We find that the arc structure is consistent with infalling motions. We measure a spectral index α ~ 3.2 across the entire structure and a dust opacity index β ~ 1.6. Considering grain properties consistent with the measured β, the structure can host up to 245 M⊕ of dust, which exceeds or is comparable to the mass of the inner unresolved 600 au, which contains the protoplanetary disk of M512. Assuming a typical dust-to-gas ratio of 1% for the streamer, the free-fall timescales (50 kyr) imply total mass-infall rates up to 1.5 × 10−6M⊙ yr−1. M512 has been classified as an outbursting source with multi-epoch WISE photometry. It is thus an interesting case study for exploring the possible connection between infalling streamers and accretion outbursts. Conclusions. M512 is a unique source for which dust continuum emission of an arc-like streamer extending out to 4000 au can be characterized in a dual-band analysis. The dust properties are similar to those in the interstellar medium and imply a high dust mass. A massive streamer like this can affect the evolution of the star- and planet-forming inner system strongly. [ABSTRACT FROM AUTHOR]

Context. Protostellar disks are the product of angular momentum conservation during protostellar collapse. Understanding their formation is crucial because they are the birthplace of planets and their formation is also tightly related to star formation. Unfortunately, the initial properties of Class 0 disks and their evolution are still poorly constrained both theoretically and observationally. Aims. We aim to better understand the mechanisms that set the statistics of disk properties as well as to study their formation in massive protostellar clumps. We also want to provide the community with synthetic disk populations to better interpret young disk observations. Methods. We used the ramses code to model star and disk formation in massive protostellar clumps with magnetohydrodynamics, including the effect of ambipolar diffusion and radiative transfer as well as stellar radiative feedback. Those simulations, resolved up to the astronomical unit scale, have allowed us to investigate the formation of disk populations. Results. Magnetic fields play a crucial role in disk formation. A weaker initial field leads to larger and massive disks and weakens the stellar radiative feedback by increasing fragmentation. We find that ambipolar diffusion impacts disk and star formation and leads to very different disk magnetic properties. The stellar radiative feedback also have a strong influence, increasing the temperature and reducing fragmentation. Comparing our disk populations with observations reveals that our models with a mass-to-flux ratio of 10 seems to better reproduce observed disk sizes. This also sheds light on a tension between models and observations for the disk masses. Conclusions. The clump properties and physical modeling significantly impact disk populations. It is critical to for the tension, with respect to disk mass estimates, between observations and models to be solved with synthetic observations. This is particularly important in the context of understanding planet formation. [ABSTRACT FROM AUTHOR]

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 submillimeter 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 ~0.4 M⊙ star with an extended gas emission and a compact and apparently structureless dust disk, with an average millimeter flux compared to Class II sources in Taurus. We present observations from the Karl G. Jansky Very Large Array across four bands (between 9.0 mm and 6.0 cm) and combine them with archival data from the Atacama Large Millimeter/submillimeter Array, the Submillimeter Array, and the Plateau de Bure Interferometer. 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. The 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. [ABSTRACT FROM AUTHOR]

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  |$\mu$| Jy beam−1) at ∼0.35 arcsec 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 |$\alpha _\mathrm{1-3\, mm}=2.23\pm 0.06$|⁠ , in all cases |$\alpha _\mathrm{1-3\, mm}\lt 3.0$|⁠ , 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 |$\alpha _\mathrm{1-3\, mm}\le 2.5$| 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 millimetre-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 substructures may be a universal explanation for the empirical millimetre size-luminosity correlation observed at 0.89 mm. [ABSTRACT FROM AUTHOR]

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 characterize the discs multiwavelength 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 millimetre 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 per cent smaller than when observed at 0.9 mm, in tension with models of dust evolution that 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 (⁠|$a_\mathrm{max}\gt 1\,$| 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. [ABSTRACT FROM AUTHOR]

The chemical composition of planets is inherited from that of the natal protoplanetary disk at the time of planet formation. Increasing observational evidence suggests that planet formation occurs in less than 1−2 Myr. This motivates the need for spatially resolved spectral observations of young Class I disks, as carried out by the ALMA chemical survey of Disk-Outflow sources in Taurus (ALMA-DOT). In the context of ALMA-DOT, we observe the edge-on disk around the Class I source IRAS 04302+2247 (the butterfly star) in the 1.3 mm continuum and five molecular lines. We report the first tentative detection of methanol (CH3OH) in a Class I disk and resolve, for the first time, the vertical structure of a disk with multiple molecular tracers. The bulk of the emission in the CO 2−1, CS 5−4, and o–H2CO 31, 2 − 21, 1 lines originates from the warm molecular layer, with the line intensity peaking at increasing disk heights, z, for increasing radial distances, r. Molecular emission is vertically stratified, with CO observed at larger disk heights (aperture z/r ∼ 0.41−0.45) compared to both CS and H2CO, which are nearly cospatial (z/r ∼ 0.21−0.28). In the outer midplane, the line emission decreases due to molecular freeze-out onto dust grains (freeze-out layer) by a factor of > 100 (CO) and 15 (CS). The H2CO emission decreases by a factor of only about 2, which is possibly due to H2CO formation on icy grains, followed by a nonthermal release into the gas phase. The inferred [CH3OH]/[H2CO] abundance ratio is 0.5−0.6, which is 1−2 orders of magnitude lower than for Class 0 hot corinos, and a factor ∼2.5 lower than the only other value inferred for a protoplanetary disk (in TW Hya, 1.3−1.7). Additionally, it is at the lower edge but still consistent with the values in comets. This may indicate that some chemical reprocessing occurs in disks before the formation of planets and comets. [ABSTRACT FROM AUTHOR]

While planets are commonly discovered around main-sequence stars, the processes leading to their formation are still far from being understood. Current planet population synthesis models, which aim to describe the planet formation process from the protoplanetary disk phase to the time exoplanets are observed, rely on prescriptions for the underlying properties of protoplanetary disks where planets form and evolve. The recent development in measuring disk masses and disk-star interaction properties, i.e., mass accretion rates, in large samples of young stellar objects demand a more careful comparison between the models and the data. We performed an initial critical assessment of the assumptions made by planet synthesis population models by looking at the relation between mass accretion rates and disk masses in the models and in the currently available data. We find that the currently used disk models predict mass accretion rate in line with what is measured, but with a much lower spread of values than observed. This difference is mainly because the models have a smaller spread of viscous timescales than what is needed to reproduce the observations. We also find an overabundance of weakly accreting disks in the models where giant planets have formed with respect to observations of typical disks. We suggest that either fewer giant planets have formed in reality or that the prescription for planet accretion predicts accretion on the planets that is too high. Finally, the comparison of the properties of transition disks with large cavities confirms that in many of these objects the observed accretion rates are higher than those predicted by the models. On the other hand, PDS70, a transition disk with two detected giant planets in the cavity, shows mass accretion rates well in line with model predictions. [ABSTRACT FROM AUTHOR]

Observations of young stellar objects (YSOs) in centimeter bands can probe the continuum emission from growing dust grains, ionized winds, and magnetospheric activity that are intimately connected to the evolution of protoplanetary disks and the formation of planets. We carried out sensitive continuum observations toward the Ophiuchus A star-forming region, using the Karl G. Jansky Very Large Array (VLA) at 10 GHz over a field-of-view of 6′ and with a spatial resolution of θmaj ×θmin ~ 0.′′4 × 0.′′2. We achieved a 5 μJy beam−1 rms noise level at the center of our mosaic field of view. Among the 18 sources we detected, 16 were YSOs (three Class 0, five Class I, six Class II, and two Class III) and two were extragalactic candidates. We find that thermal dust emission generally contributed less than 30% of the emission at 10 GHz. The radio emission is dominated by other types of emission, such as gyro-synchrotron radiation from active magnetospheres, free–free emission from thermal jets, free–free emission from the outflowing photoevaporated disk material, and synchrotron emission from accelerated cosmic-rays in jet or protostellar surface shocks. These different types of emission could not be clearly disentangled. Our non-detections for Class II/III disks suggest that extreme UV-driven photoevaporation is insufficient to explain disk dispersal, assuming that the contribution of UV photoevaporating stellar winds to radio flux does not evolve over time. The sensitivity of our data cannot exclude photoevaporation due to the role of X-ray photons as an efficient mechanism for disk dispersal. Deeper surveys using the Square Kilometre Array (SKA) will have the capacity to provide significant constraints to disk photoevaporation. [ABSTRACT FROM AUTHOR]

Context. In recent years, the disk populations in a number of young star-forming regions have been surveyed with the Atacama Large Millimeter/submillimeter Array (ALMA). Understanding the disk properties and their correlation with the properties of the central star is critical to understanding planet formation. In particular, a decrease of the average measured disk dust mass with the age of the region has been observed, consistent with grain growth and disk dissipation. Aims. We aim to compare the general properties of disks and their host stars in the nearby (d = 160 pc) Corona Australis (CrA) star forming region to those of the disks and stars in other regions. Methods. We conducted high-sensitivity continuum ALMA observations of 43 Class II young stellar objects in CrA at 1.3 mm (230 GHz). The typical spatial resolution is ~0.3′′. The continuum fluxes are used to estimate the dust masses of the disks, and a survival analysis is performed to estimate the average dust mass. We also obtained new VLT/X-shooter spectra for 12 of the objects in our sample for which spectral type (SpT) information was missing. Results. Twenty-four disks were detected, and stringent limits have been put on the average dust mass of the nondetections. Taking into account the upper limits, the average disk mass in CrA is 6 ± 3 M⊕. This value is significantly lower than that of disks in other young (1–3 Myr) star forming regions (Lupus, Taurus, Chamaeleon I, and Ophiuchus) and appears to be consistent with the average disk mass of the 5–10 Myr-old Upper Sco. The position of the stars in our sample on the Herzsprung-Russel diagram however seems to confirm that CrA has an age similar to Lupus. Neither external photoevaporation nor a lower-than-usual stellar mass distribution can explain the low disk masses. On the other hand, a low-mass disk population could be explained if the disks were small, which could happen if the parent cloud had a low temperature or intrinsic angular momentum, or if the angular momentum of the cloud were removed by some physical mechanism such as magnetic braking. Even in detected disks, none show clear substructures or cavities. Conclusions. Our results suggest that in order to fully explain and understand the dust mass distribution of protoplanetary disks and their evolution, it may also be necessary to take into consideration the initial conditions of star- and disk-formation process. These conditions at the very beginning may potentially vary from region to region, and could play a crucial role in planet formation and evolution. [ABSTRACT FROM AUTHOR]

Context. A good constraint of when the growth of dust grains from sub-micrometer to millimeter sizes occurs, is crucial for planet formation models. This provides the first step towards the production of pebbles and planetesimals in protoplanetary disks. Currently, it is well established that Class II objects have large dust grains. However, it is not clear when in the star formation process this grain growth occurs. Aims. We use multi-wavelength millimeter observations of a Class I protostar to obtain the spectral index of the observed flux densities αmm of the unresolved disk and the surrounding envelope. Our goal is to compare our observational results with visibility modeling at both, 1.3 and 2.7 mm simultaneously. Methods. We present data from NOEMA at 2.7 mm and SMA at 1.3 mm of the Class I protostar, Per-emb-50. We model the dust emission with a variety of parametric and radiative-transfer models to deduce the grain size from the observed emission spectral index. Results. We find a spectral index in the envelope of Per-emb-50 of αenv = 3.3 ± 0.3, similar to the typical ISM values. The radiative-transfer modeling of the source confirms this value of αenv with the presence of dust with a amax ≤ 100 μm. Additionally, we explore the backwarming effect, where we find that the envelope structure affects the millimeter emission of the disk. Conclusions. Our results reveal grains with a maximum size no larger than 100 μm in the inner envelope of the Class I protostar Per-emb-50, providing an interesting case to test the universality of millimeter grain growth expected in these sources. [ABSTRACT FROM AUTHOR]

Context. The cyanide radical CN is abundant in protoplanetary disks, with line fluxes often comparable to those of 13CO. It is known to be sensitive to UV irradiation of the upper disk atmosphere, with models predicting ring-shaped emission. Aims. We seek to characterize the CN emission from 94 Class-II disks in the Lupus star-forming region, compare it to observations in other regions, and interpret our observations with a grid of models. The CN emission morphology is discussed for two primordial disks, Sz 71 and Sz 98, and is modeled in more detail. Methods. ALMA observed CN N = 3−2 in Lupus disks down to sensitivities better than previous surveys. Models constructed with the physico-chemical code DALI are used to study the integrated fluxes of the disks and resolved emission of CN in disks without (dust) substructures. Results. CN N = 3−2 is bright, and detected in 38% of sources, but its disk-integrated flux is not strongly correlated to either 13CO or continuum flux. Compared to pre-ALMA single-dish surveys, no significant difference in the CN flux distributions in Lupus and Taurus–Auriga is found, although ρ Ophiuchus disks may be fainter on average. We find ring-shaped CN emission with peak radii of ~50 AU in two resolved disks. Conclusions. A large fraction of sources are faint in CN; only exponential gas surface density cutoffs at Rc ≤ 15 AU can reconcile observations with models. This is the first observational evidence of such a compact gas disk population in Lupus. Absolute intensities and the emission morphology of CN are reproduced by DALI models without the need for any continuum substructure; they are unrelated to the CO snowline location. The observations presented here, together with the modeling of these rings, provide a new probe of the structure and conditions in disks, and particularly their incident UV radiation field, if disk size is determined from the data. [ABSTRACT FROM AUTHOR]

Context. Understanding the formation mechanisms of protoplanetary disks and multiple systems and also their pristine properties are key questions for modern astrophysics. The properties of the youngest disks, embedded in rotating infalling protostellar envelopes, have largely remained unconstrained up to now. Aims. We aim to observe the youngest protostars with a spatial resolution that is high enough to resolve and characterize the progenitors of protoplanetary disks. This can only be achieved using submillimeter and millimeter interferometric facilities. In the framework of the IRAM Plateau de Bure Interferometer survey CALYPSO, we have obtained subarcsecond observations of the dust continuum emission at 231 and 94 GHz for a sample of 16 solar-type Class 0 protostars. Methods. In an attempt to identify disk-like structures embedded at small scales in the protostellar envelopes, we modeled the dust continuum emission visibility profiles using Plummer-like envelope models and envelope models that include additional Gaussian disk-like components. Results. Our analysis shows that in the CALYPSO sample, 11 of the 16 Class 0 protostars are better reproduced by models including a disk-like dust continuum component contributing to the flux at small scales, but less than 25% of these candidate protostellar disks are resolved at radii >60 au. Including all available literature constraints on Class 0 disks at subarcsecond scales, we show that our results are representative: most (>72% in a sample of 26 protostars) Class 0 protostellar disks are small and emerge only at radii <60 au. We find a multiplicity fraction of the CALYPSO protostars ≲57% ± 10% at the scales 100–5000 au, which generally agrees with the multiplicity properties of Class I protostars at similar scales. Conclusions. We compare our observational constraints on the disk size distribution in Class 0 protostars to the typical disk properties from protostellar formation models. If Class 0 protostars contain similar rotational energy as is currently estimated for prestellar cores, then hydrodynamical models of protostellar collapse systematically predict a high occurrence of large disks. Our observations suggest that these are rarely observed, however. Because they reduce the centrifugal radius and produce a disk size distribution that peaks at radii <100 au during the main accretion phase, magnetized models of rotating protostellar collapse are favored by our observations. [ABSTRACT FROM AUTHOR]

We present Atacama Large Millimeter/sub-millimeter Array (ALMA) Cycle 2 observations of the 1.3-mm dust continuum emission of the protoplanetary disc surrounding the T Tauri star Elias 24 with an angular resolution of ~0.2 arcsec (~28 au). The dust continuum emission map reveals a dark ring at a radial distance of 0.47 arcsec (~65 au) from the central star, surrounded by a bright ring at 0.58 arcsec (~81 au). In the outer disc, the radial intensity profile shows two inflection points at 0.71 and 0.87 arcsec (~99 and 121 au, respectively). We perform global three-dimensional smoothed particle hydrodynamic gas/dust simulations of discs hosting a migrating and accreting planet. Combining the dust density maps of small and large grains with three-dimensional radiative transfer calculations, we produce synthetic ALMA observations of a variety of disc models in order to reproduce the gap- and ring-like features observed in Elias 24. We find that the dust emission across the disc is consistent with the presence of an embedded planet with a mass of ~0.7MJ at an orbital radius of ~ 60 au. Our model suggests that the two inflection points in the radial intensity profile are due to the inward radial motion of large dust grains from the outer disc. The surface brightness map of our disc model provides a reasonable match to the gap- and ring-like structures observed in Elias 24, with an average discrepancy of ~5 per cent of the observed fluxes around the gap region. [ABSTRACT FROM AUTHOR]

We present the results of formaldehyde and methanol deuteration measurements towards the Class I low-mass protostar SVS13-A in the framework of the IRAM 30-m ASAI (Astrochemical Surveys at IRAM) project. We detected emission lines of formaldehyde, methanol and their deuterated forms (HDCO, D2CO, CHD2OH, CH3OD) with Eup up to 276 K. The formaldehyde analysis indicates Tkin ~ 15-30 K, ... ≥ 106 cm-3 and a size of about 1200 au suggesting an origin in the protostellar envelope. For methanol, we find two components: (i) a high temperature (Tkin ~ 80 K) and very dense (>108 cm-3) gas from a hot corino (radius ≃ 35 au), and (ii) a colder (Tkin ≤ 70 K) and more extended (radius ≃ 350 au) region. The deuterium fractionation is 9 x 10-2 for HDCO, 4 x 10-3 for D2CO and 2-7 x 10-3 for CH2DOH, up to two orders of magnitude lower than the values measured in Class 0 sources. We also derive formaldehyde deuteration in the outflow: 4 x 10-3, in agreement with what found in the L1157-B1 protostellar shock. Finally, we estimate [CH2DOH]/[CH3OD] ≃ 2. The decrease of deuteration in the Class I source SVS13-A with respect to Class 0 sources can be explained by gas-phase processes. Alternatively, a lower deuteration could be the effect of a gradual collapse of less deuterated external shells of the protostellar envelope. The present measurements fill in the gap between pre-stellar cores and protoplanetary discs in the context of organic deuteration measurements. [ABSTRACT FROM AUTHOR]

We present new VLT/X-Shooter optical and near-infrared spectra of a sample of 17 candidate young low-mass stars and brown dwarfs located in the ρ-Ophiucus cluster.We derived the spectral type and extinction for all the targets, and then we determined their physical parameters. All the objects but one have M* ≲ 0.6 M☉, and eight have mass below or close to the hydrogen-burning limit. Using the intensity of various permitted emission lines present in their spectra, we determined the accretion luminosity and mass accretion rates (Macc) for all the objects. When compared with previous works targeting the same sample, we find that, in general, these objects are not as strongly accreting as previously reported, and we suggest that the reason is our more accurate estimate of the photospheric parameters.We also compare our findings with recent works in other slightly older star-forming regions, such as Lupus, to investigate possible di erences in the accretion properties, but we find that the accretion properties for our targets have the same dependence on the stellar and substellar parameters as in the other regions. This leads us to conclude that we do not find evidence for a di erent dependence of Macc with M* when comparing low-mass stars and brown dwarfs. Moreover, we find a similar small (≲1 dex) scatter in the Macc-M* relation as in some of our recent works in other star-forming regions, and no significant di erences in Macc due to di erent ages or properties of the regions. The latter result su ers, however, from low statistics and sample selection biases in the current studies. The small scatter in the Macc-M* correlation confirms that mass accretion rate measurements in the literature based on uncertain photospheric parameters and single accretion indicators, such as the Hα width, can lead to a scatter that is unphysically large. Our studies show that only broadband spectroscopic surveys coupled with a detailed analysis of the photospheric and accretion properties allows us to properly study the evolution of disk accretion rates in star-forming regions. [ABSTRACT FROM AUTHOR]

Context. Transitional disks are thought to be a late evolutionary stage of protoplanetary disks whose inner regions have been depleted of dust. The mechanism responsible for this depletion is still under debate. To constrain the various models it is mandatory to have a good understanding of the properties of the gas content in the inner part of the disk. Aims. Using X-Shooter broad band - UV to near-infrared - medium-resolution spectroscopy, we derive the stellar, accretion, and wind properties of a sample of 22 transitional disks. The analysis of these properties allows us to place strong constraints on the gas content in a region very close to the star (≲0.2 AU) that is not accessible with any other observational technique. Methods. We fitted the spectra with a self-consistent procedure to simultaneously derive spectral type, extinction, and accretion properties of the targets. From the continuum excess at near-infrared wavelength we distinguished whether our targets have dust free inner holes. By analyzing forbidden emission lines, we derived the wind properties of the targets.We then compared our findings with results for classical T Tauri stars. Results. The accretion rates and wind properties of 80% of the transitional disks in our sample, which is strongly biased toward stongly accreting objects, are comparable to those of classical T Tauri stars. Thus, there are (at least) some transitional disks with accretion properties compatible with those of classical T Tauri stars, irrespective of the size of the dust inner hole. Only in two cases are the mass accretion rates much lower, while the wind properties remain similar. We detected no strong trend of the mass accretion rates with the size of the dust-depleted cavity or with the presence of a dusty optically thick disk very close to the star. These results suggest that, close to the central star, there is a gas-rich inner disk with a density similar to that of classical T Tauri star disks. Conclusions. The sample analyzed here suggests that, at least for some objects, the process responsible of the inner disk clearing allows for a transfer of gas from the outer disk to the inner region. This should proceed at a rate that does not depend on the physical mechanisms that produces the gap seen in the dust emission and results in a gas density in the inner disk similar to that of unperturbed disks around stars of similar mass. [ABSTRACT FROM AUTHOR]

We present new 3 mm ATCA data of two Class I young stellar objects (YSOs) in the Ophiucus star forming region: Elias29 and WL12. For our analysis we compare them with archival 1.1 mm SMA data. In the (u, v) plane the two sources present a similar behavior: a nearly constant non-zero emission at long baselines, which suggests the presence of an unresolved component and an increase of the fluxes at short baselines, related to the presence of an extended envelope. Our data analysis leads to unusually low values of the spectral index α1.1-3 mm, which may indicate that mm-sized dust grains have already formed both in the envelopes and in the disk-like structures at such early stages. To explore the possible scenarios for the interpretation of the sources we perform a radiative transfer modeling using a Monte Carlo code, in order to take into account possible deviations from the Rayleigh-Jeans and optically thin regimes. Comparison between the model outputs and the observations indicates that dust grains may form aggregates up to millimeter size already in the inner regions of the envelopes of Class I YSOs. Moreover, we conclude that the embedded disk-like structures in our two Class I YSOs are probably very compact, in particular in the case of WL12, with outer radii down to tens of AU. [ABSTRACT FROM AUTHOR]

Young stars show evidence of accretion discs which evolve quickly and disperse with an e-folding time of ~3 Myr. This is in striking contrast with recent observations that suggest evidence of numerous >30 Myr old stars with an accretion disc in large star-forming complexes. We consider whether these observations of apparently old accretors could be explained by invoking Bondi-Hoyle accretion to rebuild a new disc around these stars during passage through a clumpy molecular cloud. We combine a simple Monte Carlo model to explore the capture of mass by such systems with a viscous evolution model to infer the levels of accretion that would be observed. We find that a significant fraction of stars may capture enough material via the Bondi-Hoyle mechanism to rebuild a disc of mass ?1 minimum-mass solar nebula, and ?10% accrete at observable levels at any given time. A significant fraction of the observed old accretors may be explained with our proposed mechanism. Such accretion may provide a chance for a second epoch of planet formation, and have unpredictable consequences for planetary evolution. [ABSTRACT FROM AUTHOR]

Context. Transition disks are protoplanetary disks with inner depleted dust cavities that are excellent candidates for investigating the dust evolution when there is a pressure bump. A pressure bump at the outer edge of the cavity allows dust grains from the outer regions to stop their rapid inward migration towards the star and to efficiently grow to millimetre sizes. Dynamical interactions with planet(s) have been one of the most exciting theories to explain the clearing of the inner disk. Aims. We look for evidence of millimetre dust particles in transition disks by measuring their spectral index mm with new and available photometric data. We investigate the influence of the size of the dust depleted cavity on the disk integrated millimetre spectral index. Methods. We present the 3-mm (100 GHz) photometric observations carried out with the Plateau de Bure Interferometer of four transition disks: LkHα 330, UX Tau A, LRLL 31, and LRLL 67. We used the available values of their fluxes at 345 GHz to calculate their spectral index, as well as the spectral index for a sample of twenty transition disks. We compared the observations with two kinds of models. In the first set of models, we considered coagulation and fragmentation of dust in a disk in which a cavity is formed by a massive planet located at different positions. The second set of models assumes disks with truncated inner parts at different radii and with power-law dust-size distributions, where the maximum size of grains is calculated considering turbulence as the source of destructive collisions. Results. We show that the integrated spectral index is higher for transition disks (TD) than for regular protoplanetary disks (PD) with mean values of αTD mm = 2:70 ± 0:13 and αPD mm = 2:20 ± 0:07 respectively. For transition disks, the probability that the measured spectral index is positively correlated with the cavity radius is 95%. High angular resolution imaging of transition disks is needed to distinguish between the dust trapping scenario and the truncated disk case. [ABSTRACT FROM AUTHOR]

Context. Grain growth in protoplanetary disks is the first step towards the formation of the rocky cores of planets. Models predict that grains grow, migrate, and fragment in the disk and predict varying dust properties as a function of radius, age, and physical properties. High-angular resolution observations at more than one (sub-)mm wavelength are the essential tool for constraining grain growth and migration on the disk midplane. Aims. We developed a procedure to analyze self-consistently multiwavelength (sub-)mm continuum interferometric observations of protoplanetary disks to constrain the radial distribution of dust properties. Methods. We apply this technique to existing multifrequency continuum mm observations of the disk around CQ Tau, a A8 pre-main sequence star with a well-studied disk. Results. We demonstrate that our models can be used to simultaneously constrain the disk and dust structure. In CQ Tau, the bestfitting model has a radial dependence of the maximum grain size, which decreases from a few cm in the inner disk (<40 AU) to a few mm at 80 AU. Nevertheless, the currently available dataset does not allow us to exclude the possibility of a uniform grain size distribution at a 3σ level. [ABSTRACT FROM AUTHOR]

Aims. The aim of this work is to study the structure of the protoplanetary disk surrounding the Herbig Ae star HD 163296. Methods. We used high-resolution and high-sensitivity ALMA observations of the CO(3-2) emission line and the continuum at 850 μm, as well as the three-dimensional Monte Carlo radiative transfer code, MCFOST, to model the data presented in this work. Results. The CO(3-2) emission unveils for the first time at submillimeter frequencies the vertical structure details of a gaseous disk in Keplerian rotation, showing the back and front sides of a flared disk. Continuum emission at 850 μm reveals a compact dust disk with a 240 AU outer radius and a surface brightness profile that shows a very steep decline at radius larger than 125 AU. The gaseous disk is more than two times larger than the dust disk, with a similar critical radius but with a shallower radial profile. Radiative transfer models of the continuum data confirm the need for a sharp outer edge to the dust disk. The models for the CO(3-2) channel map require the disk to be slightly more geometrically thick than previous models suggested, and that the temperature at which CO gas becomes depleted (i.e., frozen out) from the outer regions of the disk midplane is T < 20 K, in agreement with previous studies. [ABSTRACT FROM AUTHOR]

Context. The high spatial resolution and line sensitivity of the Atacama Large Millimeter/submillimeter Array (ALMA) opens the possibility of resolving emission from molecules in large samples of circumstellar disks. With an understanding of the conditions under which these molecules can have high abundance, they can be used as direct tracers of distinct physical regions. In particular, DCO+ is expected to have an enhanced abundance within a few Kelvin of the CO freezeout temperature of 19 K, making it a useful probe of the cold disk midplane. Aims. We aim to use line emission from DCO+ to directly resolve the CO "snowline" - the region at which the gas-phase CO abundance drops due to freezeout - and determine the temperature boundaries of the region of DCO+ emission in the HD 163296 disk. This will serve as a test of deuteration models based on enhanced formation of the parent molecule H2D+ and a direct probe of midplane disk structure and ionization. Methods. We compare ALMA line observations of HD 163296 to a grid of models based on the best fit physical model of Qi et al. (2011, ApJ, 740, 84). We vary the upper- and lower-limit temperatures of the region in which DCO+ is present as well as the abundance of DCO+ in order to fit channel maps of the DCO+ J = 5-4 line. To determine the abundance enhancement compared to the general interstellar medium, we carry out similar fitting to HCO+ J = 4-3 and H13CO+ J = 4-3 observations. Results. ALMA images show centrally peaked extended emission from HCO+ and H13CO+. DCO+ emission lies in a resolved ring from ~110 to 160 AU. The outer radius approximately corresponds to the size of the CO snowline as measured by previous lower resolution observations of CO lines in this disk. The ALMA DCO+ data now resolve and image the CO snowline directly. Conclusions. In the best fitting models, HCO+ exists in a region extending from the 19 K isotherm to the photodissociation layer with an abundance of 3 × 10-10 relative to H2. DCO+ exists within the 19-21 K region of the disk with an abundance ratio [DCO+]/[HCO+] = 0.3. This represents a factor of 104 enhancement of the DCO+ abundance within this narrow region of the HD 163296 disk. Such a high enhancement has only previously been seen in prestellar cores. The inferred abundances provide a lower limit to the ionization fraction in the midplane of the cold outer disk (≳4 × 10-10), and suggest the utility of DCO+ as a tracer of its parent molecule H2D+. [ABSTRACT FROM AUTHOR]

Aims. We attempt to explain grain growth to mm sized particles and their retention in the outer regions of protoplanetary disks, as observed at sub-mm and mm wavelengths, by investigating whether strong in homogeneities in the gas density profiles can decelerate excessive radial drift and help the dust particles to grow. Methods. We use coagulation/fragmentation and disk-structure models, to simulate the evolution of dust in a bumpy surface density profile, which we mimic with a sinusoidal disturbance. For different values of the amplitude and length scale of the bumps, we investigate the ability of this model to produce and retain large particles on million-year timescales. In addition, we compare the pressure inhomogeneities considered in this work with the pressure profiles that come from magnetorotational instability. Using the Common Astronomy Software Applications ALMA simulator, we study whether there are observational signatures of these pressure inhomogeneities that can be seen with ALMA. Results. We present the conditions required to trap dust particles and the corresponding calculations predicting the spectral slope in the mm-wavelength range, to compare with current observations. Finally, we present simulated images using different antenna configurations of ALMA at different frequencies, to show that the ring structures will be detectable at the distances of either the Taurus Auriga or Ophiucus star-forming regions. [ABSTRACT FROM AUTHOR]

We present Atacama Large Millimeter/submillimeter Array continuum and spectral line data at 0.89 mm and 3.2 mm for three disks surrounding young brown dwarfs and very low mass stars in the Taurus star forming region. Dust thermal emission is detected and spatially resolved for all the three disks, while CO(J = 3-2) emission is seen in two disks. We analyze the continuum visibilities and constrain the disks' physical structure in dust. The results of our analysis show that the disks are relatively large; the smallest one has an outer radius of about 70 AU. The inferred disk radii, radial profiles of the dust surface density, and disk to central object mass ratios lie within the ranges found for disks around more massive young stars. We derive from our observations the wavelength dependence of the millimeter dust opacity. In all the three disks, data are consistent with the presence of grains with at least millimeter sizes, as also found for disks around young stars, and confirm that the early stages of the solid growth toward planetesimals occur also around very low-mass objects. We discuss the implications of our findings on models of solids evolution in protoplanetary disks, the main mechanisms proposed for the formation of brown dwarfs and very low-mass stars, as well as the potential of finding rocky and giant planets around very low-mass objects. [ABSTRACT FROM AUTHOR]