73 research outputs found
Dust evolution in protoplanetary disks
Planet formation models rely on knowledge of the physical conditions and
evolutionary processes in protoplanetary disks, in particular the grain size
distribution and dust growth timescales. In theoretical models, several
barriers exist that prevent grain growth to pebble sizes and beyond, such as
the radial drift and fragmentation. Pressure bumps have been proposed to
overcome such barriers. In the past decade ALMA has revealed observational
evidence for the existence of such pressure bumps in the form of dust traps,
such as dust rings, gaps, cavities and crescents through high-resolution
millimeter continuum data originating from thermal dust emission of
pebble-sized dust grains. These substructures may be related to young
protoplanets, either as the starting point or the consequence of early planet
formation. Furthermore, disk dust masses have been measured for complete
samples of young stars in clusters, which provide initial conditions for the
solid mass budget available for planet formation. However, observational biases
exist in the selection of high-resolution ALMA observations and uncertainties
exist in the derivation of the disk dust mass, which both may affect the
observed trends. This chapter describes the latest insights in dust evolution
and disk continuum observations. Specifically, disk populations and
evolutionary trends are described, as well as the uncertainties therein, and
compared with exoplanet demographics.Comment: submitted, invited chapter for the "Handbook of Exoplanets". Comments
welcom
Disc population synthesis: decrease of the solid mass reservoir through pebble drift
Surveys of star-forming regions reveal that the dust mass of protoplanetary
discs decreases by several orders of magnitude on a timescale of a few million
years. This decrease in the mass budget of solids is likely due to the
gas-drag-induced radial drift of mm-sized solids, called pebbles. However,
quantifying the evolution of this dust component in young stellar clusters is
difficult due to the inherent large spread in stellar masses and formation
times. Therefore, we aim to model the collective evolution of a cluster to
investigate the effectiveness of radial drift in clearing the discs of mm-sized
particles. We use a protoplanetary disc model that numerically solves for disc
formation, and the viscous evolution and photoevaporative clearing of the gas
component, while also including the drift of particles limited in size by
fragmentation. We find that discs are born with dust masses between 50 Earth
masses and 1000 Earth masses, for stars with, respectively, masses between 0.1
solar masses and 1 solar masses. The majority of this initial dust reservoir is
typically lost through drift before photoevaporation opens a gap in the gas
disc for models both with and without strong X-ray-driven mass loss rates. We
conclude that the decrease in time of the mass locked in fragmentation-limited
pebbles is consistent with the evolution of dust masses and ages inferred from
nearby star-forming regions when assuming viscous evolution rates corresponding
to mean gas disc lifetimes between 3 Myr and 8 Myr.Comment: 16 pages, 11 figures, accepted for publication in A&A. Addressed
additional language comment
A concentration of centimeter-sized grains in the Oph IRS 48 dust trap
Azimuthally asymmetric dust distributions observed with ALMA in transition
disks have been interpreted as dust traps. We present VLA Ka band (34 GHz or
0.9 cm) and ALMA Cycle 2 Band 9 (680 GHz or 0.45 mm) observations at 0.2"
resolution of the Oph IRS 48 disk, which suggest that larger particles could be
more azimuthally concentrated than smaller dust grains, assuming an
axisymmetric temperature field or optically thin 680 GHz emission. Fitting an
intensity model to both data demonstrates that the azimuthal extent of the
millimeter emission is 2.3 times as wide as the centimeter emission,
marginally consistent with the particle trapping mechanism under the above
assumptions. The 34 GHz continuum image also reveals evidence for ionized gas
emission from the star. Both the morphology and the spectral index variations
are consistent with an increase of large particles in the center of the trap,
but uncertainties remain due to the continuum optical depth at 680 GHz.
Particle trapping has been proposed in planet formation models to allow dust
particles to grow beyond millimeter sizes in the outer regions of
protoplanetary disks. The new observations in the Oph IRS 48 disk provide
support for the dust trapping mechanism for centimeter-sized grains, although
additional data is required for definitive confirmation.Comment: Language editing and addition reference ALMA dat
Investigating the asymmetric chemistry in the disk around the young star HD 142527
The atmospheric composition of planets is determined by the chemistry of the
disks in which they form. Studying the gas-phase molecular composition of disks
thus allows us to infer what the atmospheric composition of forming planets
might be. Recent observations of the IRS 48 disk have shown that (asymmetric)
dust traps can directly impact the observable chemistry, through radial and
vertical transport, and the sublimation of ices. The asymmetric HD 142527 disk
provides another good opportunity to investigate the role of dust traps in
setting the disk's chemical composition. In this work, we use archival ALMA
observations of the HD 142527 disk to obtain an as large as possible molecular
inventory, which allows us to investigate the possible influence of the
asymmetric dust trap on the disk's chemistry. We present the first ALMA
detections of [C I], 13C18O, DCO+, H2CO and additional transition of HCO+ and
CS in this disk. In addition, we have acquired upper limits for non-detected
species such as SO and CH3OH. For the majority of the observed molecules, a
decrement in the emission at the location of the dust trap is found. For the
main CO isotopologues continuum over-subtraction likely causes the observed
asymmetry, while for CS and HCN we propose that the observed asymmetries are
likely due to shadows cast by the misaligned inner disk. As the emission of the
observed molecules is not co-spatial with the dust trap and no SO or CH3OH are
found, thermal sublimation of icy mantles does not appear to play a major role
in changing the gas-phase composition of the outer disk in HD 142527 disk.
Using our observations of 13C18O and DCO+ and a RADMC-3D model, we determine
the CO snowline to be located beyond the dust traps, favouring cold gas-phase
formation of H2CO, rather than the hydrogenation of CO-ice and subsequent
sublimation.Comment: Accepted for publication in A&A on 12/04/202
Outflow forces of low mass embedded objects in Ophiuchus: a quantitative comparison of analysis methods
The outflow force of molecular bipolar outflows is a key parameter in
theories of young stellar feedback on their surroundings. The focus of many
outflow studies is the correlation between the outflow force, bolometric
luminosity and envelope mass. However, it is difficult to combine the results
of different studies in large evolutionary plots over many orders of magnitude
due to the range of data quality, analysis methods and corrections for
observational effects such as opacity and inclination. We aim to determine the
outflow force for a sample of low luminosity embedded sources. We will quantify
the influence of the analysis method and the assumptions entering the
calculation of the outflow force. We use the James Clerk Maxwell Telescope to
map 12CO J=3-2 over 2'x2' regions around 16 Class I sources of a well-defined
sample in Ophiuchus at 15" resolution. The outflow force is then calculated
using seven different methods differing e.g. in the use of intensity-weighted
emission and correction factors for inclination. The results from the analysis
methods differ from each other by up to a factor of 6, whereas observational
properties and choices in the analysis procedure affect the outflow force by up
to a factor of 4. For the sample of Class I objects, bipolar outflows are
detected around 13 sources including 5 new detections, where the three
non-detections are confused by nearby outflows from other sources. When
combining outflow forces from different studies, a scatter by up to a factor of
5 can be expected. Although the true outflow force remains unknown, the
separation method (separate calculation of dynamical time and momentum) is
least affected by the uncertain observational parameters. The correlations
between outflow force, bolometric luminosity and envelope mass are further
confirmed down to low luminosity sources.Comment: 24 pages, 13 figures, Accepted by A&
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