63 research outputs found
Constraining protoplanetary disc evolution using accretion rate and disc mass measurements: the usefulness of the dimensionless accretion parameter
We explore how measurements of protoplanetary disc masses and accretion rates provided by surveys of star-forming regions can be analysed via the , which we define as the product of the accretion rate and stellar age divided by the disc mass. By extending and generalizing the study of Jones et al., we demonstrate that this parameter should be less than or of order unity for a wide range of evolutionary scenarios, rising above unity only during the final stages of outside-in clearing by external photoevaporation. We use this result to assess the reliability of disc mass estimates derived from CO isotopologues and sub-mm continuum emission by examining the distribution of accretion efficiencies in regions that are not subject to external photoevaporation. We find that while dust-based mass estimates produce results compatible with theoretical expectations assuming a canonical dust-to-gas ratio, the systematically lower CO-based estimates yield accretion efficiencies significantly above unity in contrast with the theory. This finding provides additional evidence that CO-based disc masses are an underestimate, in line with arguments that have been made on the basis of chemical modelling of relatively small samples. On the other hand, we demonstrate that dust-based mass estimates are sufficiently accurate to reveal distinctly higher accretion efficiencies in the Trapezium cluster, where this result is expected, given the evident importance of external photoevaporation. We therefore propose the dimensionless accretion parameter as a new diagnostic of external photoevaporation in other star-forming regions.This work has been supported by the DISCSIM project, grant agreement 341137, funded by the European Research Council under ERC-2013-ADG. CFM gratefully acknowledges an ESA Research Fellowship
An initial overview of the extent and structure of recent star formation within the Serpens Molecular Cloud using Gaia Data Release 2
This is the final version. Available from American Astronomical Society / IOP Publishing via the DOI in this record.The dense clusters within the Serpens Molecular Cloud are among the most active regions of nearby
star formation. In this paper, we use Gaia DR2 parallaxes and proper motions to statistically measure
∼ 1167 kinematic members of Serpens, few of which were previously identified, to evaluate the star
formation history of the complex. The optical members of Serpens are concentrated in three distinct
groups located at 380–480 pc; the densest clusters are still highly obscured by optically-thick dust
and have few optical members. The total population of young stars and protostars in Serpens is at
least 2000 stars, including past surveys that were most sensitive to protostars and disks, and may
be far higher. Distances to dark clouds measured from deficits in star counts are consistent with
the distances to the optical star clusters. The Serpens Molecular Cloud is seen in the foreground of
the Aquila Rift, dark clouds located at 600–700 pc, and behind patchy extinction, here called the
Serpens Cirrus, located at ∼ 250 pc. Based on the lack of a distributed population of older stars,
the star formation rate throughout the Serpens Molecular Cloud increased by at least a factor of
20 within the past ∼ 5 Myr. The optically bright stars in Serpens Northeast are visible because
their natal molecular cloud has been eroded and not because they were flung outwards from a central
factory of star formation. The separation between subclusters of 20–100 pc and the absence of an
older population leads to speculation that an external forcing was needed to trigger the active star
formationNational Science Foundation of ChinaESO - European Southern ObservatoryNational Research Council CanadaNatural Sciences and Engineering Research Council of Canada (NSERC)National Natural Science Foundation of Chin
New Insights into the Nature of Transition Disks from a Complete Disk Survey of the Lupus Star-forming Region
Transition disks with large dust cavities around young stars are promising
targets for studying planet formation. Previous studies have revealed the
presence of gas cavities inside the dust cavities hinting at recently formed,
giant planets. However, many of these studies are biased towards the brightest
disks in the nearby star forming regions, and it is not possible to derive
reliable statistics that can be compared with exoplanet populations. We present
the analysis of 11 transition disks with large cavities (>20 AU radius) from a
complete disk survey of the Lupus star forming region, using ALMA Band 7
observations at 0.3" (22-30 AU radius) resolution of the 345 GHz continuum,
13CO and C18O 3-2 observations and the Spectral Energy Distribution of each
source. Gas and dust surface density profiles are derived using the
physical-chemical modeling code DALI. This is the first study of transition
disks of large cavities within a complete disk survey within a star forming
region. The dust cavity sizes range from 20-90 AU radius and in three cases, a
gas cavity is resolved as well. The deep drops in gas density and large dust
cavity sizes are consistent with clearing by giant planets. The fraction of
transition disks with large cavities in Lupus is ~11%, which is inconsistent
with exoplanet population studies of giant planets at wide orbits. Furthermore,
we present a hypothesis of an evolutionary path for large massive disks
evolving into transition disks with large cavities
ALMA Survey of Lupus Class III Stars: Early Planetesimal Belt Formation and Rapid Disk Dispersal
Class III stars are those in star forming regions without large non-photospheric infrared emission, suggesting recent dispersal of their protoplanetary disks. We observed 30 class III stars in the 1-3 Myr Lupus region with ALMA at ∼856μm, resulting in 4 detections that we attribute to circumstellar dust. Inferred dust masses are 0.036 − 0.093M⊕, ∼1 order of magnitude lower than any previous measurements; one disk is resolved with radius ∼80 au. Two class II sources in the field of view were also detected, and 11 other sources, consistent with sub-mm galaxy number counts. Stacking non-detections yields a marginal detection with mean dust mass ∼0.0048M⊕. We searched for gas emission from the CO J=3-2 line, and present its detection to NO Lup inferring a gas mass (4.9 ± 1.1) × 10−5 M⊕ and gas-to-dust ratio 1.0 ± 0.4. Combining our survey with class II sources shows a gap in the disk mass distribution from 0.09 − 2M⊕ for >0.7M⊙ Lupus stars, evidence of rapid dispersal of mm-sized dust from protoplanetary disks. The class III disk mass distribution is consistent with a population model of planetesimal belts that go on to replenish the debris disks seen around main sequence stars. This suggests that planetesimal belt formation does not require long-lived protoplanetary disks, i.e., planetesimals form within ∼2 Myr. While all 4 class III disks are consistent with collisional replenishment, for two the gas and/or mid-IR emission could indicate primordial circumstellar material in the final stages of protoplanetary disk dispersal. Two class III stars without sub-mm detections exhibit hot emission that could arise from ongoing planet formation processes inside ∼1 au
Multiwavelength 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 (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
The ALMA Lupus protoplanetary disk survey: Evidence for compact gas disks and molecular rings from CN
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 = 32 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 = 32 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 Ophiuchus disks may be fainter on average. We find ring-shaped CN emission with peak radii of 50AU in two resolved disks. Conclusions. A large fraction of sources are faint in CN; only exponential gas surface density cutoffs at Rc 15AU can reconcile observations with models. This is the first observational evidence of such a compact gas disk population in Lupus. bsolute 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
The ALMA Lupus protoplanetary disk survey: Evidence for compact gas disks and molecular rings from CN
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 = 32 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 = 32 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 Ophiuchus disks may be fainter on average. We find ring-shaped CN emission with peak radii of 50AU in two resolved disks. Conclusions. A large fraction of sources are faint in CN; only exponential gas surface density cutoffs at Rc 15AU can reconcile observations with models. This is the first observational evidence of such a compact gas disk population in Lupus. bsolute 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
Homogeneous Analysis of the Dust Morphology of Transition Disks Observed with ALMA: Investigating Dust Trapping and the Origin of the Cavities
We analyze the dust morphology of 29 transition disks (TDs) observed with
ALMA at (sub-) millimeter-emission. We perform the analysis in the visibility
plane to characterize the total flux, cavity size, and shape of the ring-like
structure. First, we found that the relation is much
flatter for TDs than the observed trends from samples of class II sources in
different star forming regions. This relation demonstrates that cavities open
in high (dust) mass disks, independent of the stellar mass. The flatness of
this relation contradicts the idea that TDs are a more evolved set of disks.
Two potential reasons (not mutually exclusive) may explain this flat relation:
the emission is optically thick or/and millimeter-sized particles are trapped
in a pressure bump. Second, we discuss our results of the cavity size and ring
width in the context of different physical processes for cavity formation.
Photoevaporation is an unlikely leading mechanism for the origin of the cavity
of any of the targets in the sample. Embedded giant planets or dead zones
remain as potential explanations. Although both models predict correlations
between the cavity size and the ring shape for different stellar and disk
properties, we demonstrate that with the current resolution of the
observations, it is difficult to obtain these correlations. Future observations
with higher angular resolution observations of TDs with ALMA will help to
discern between different potential origins of cavities in TDs
Disk Evolution Study Through Imaging of Nearby Young Stars (DESTINYS): Diverse outcomes of binary-disk interactions
CONTEXT: Circumstellar disks play an essential role in the outcomes of planet formation. Disks do not evolve in isolation, as about half of solar-type stars were born in binary or multiple systems. The presence of stellar companions modifies the morphology and evolution of disks, potentially resulting in a different planet population. Resolving disks in binary systems provides the opportunity to examine the influence of stellar companions on the outcomes of planet formation. AIMS: 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) large program. In particular, we present observations of circumstellar disks in three systems, namely, CHX 22, S CrA, and HP Cha. METHODS: We used polarimetric differential imaging with SPHERE/IRDIS at the VLT to search for scattered light from the circum-stellar disks in these multiple systems. We performed astrometric and orbit analyses for the stellar companions using archival HST, VLT/NACO, and SPHERE data to better understand the interplay between disks and companions. RESULTS: Combined with the age and orbital constraints, the observed disk structures in scattered light 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 toward the companions. CONCLUSIONS: 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 the exoplanet population in binaries, that planet formation is likely obstructed around close binary systems, while it is not suppressed in wide binaries
High gas-to-dust size ratio indicating efficient radial drift in the mm-faint CX Tauri disk
The large majority of protoplanetary disks have very compact (AU) continuum emission at mm wavelengths. However, high angular resolution observations that resolve these small disks are still lacking, due to their intrinsically fainter emission compared with large bright disks. In this letter, we present mm ALMA data of the faint (mJy) disk orbiting the TTauri star CX Tau at a resolution of mas, AU in diameter. The mm-dust disk is compact, with a 68 enclosing flux radius of 14AU, and the intensity profile exhibits a sharp drop between 10-20AU, and a shallow tail between 20-40AU. No clear signatures of substructure in the dust continuum are observed, down to the same sensitivity level of the DSHARP large program. However, the angular resolution does not allow to detect substructures at a scale of the disk aspect ratio in the inner regions. The radial intensity profile resembles well the inner regions of more extended disks imaged at the same resolution in DSHARP, but with no rings present in the outer disk. No inner cavity is detected, even though the disk has been classified as a transition disk from the spectral energy distribution in the near infrared. The emission of CO is much more extended, with a 68 enclosing flux radius of 75AU. The large difference of the mm dust and gas extents () strongly points to the occurrence of radial drift, and matches well the predictions of theoretical models
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