168 research outputs found
Molecular Line Emission from Planet-Forming Disks with ALMA
The physical and chemical conditions in the protoplanetary disk set the initial conditions for planet formation. Constraining the properties of disks is of key importance for understanding how planets assemble.
Observations of molecular lines in disks provide valuable information on disk properties. This thesis presents ALMA observations, analysis and modelling of molecular line emission from four disks that all exhibit evidence for forming planets. Using the first-ever observations of 13C17O in protoplanetary disks, the CO gas mass of the HD 163296 and HL Tau disks are robustly constrained. The new masses are a factor of 2-10 times higher than existing estimates using C18O, and highlight the potential gravitational instability of the HL Tau disk. Analysis of the radial distribution of HCO+ and H13CO+ in the HD 97048 disk reveals a low ratio that can be explained via chemical fractionation. This indicates that the gas temperature in the outer disk is low (approx. 10 K) despite this disk being hosted by an A-type star. Both silicon and sulphur bearing volatiles are observed to be significantly depleted in disks, similar to dark clouds. Multiple lines of SO and SiO are targeted towards HD 100546 and HD 97048. The detection of the shock tracer SO in the HD 100546 disk is attributed to either a disk wind or a circumplanetary disk. Complementary chemical modelling reveals the molecular carriers of S and Si in the two sources, and predicts SiS as tracer of S and Si in disks. This thesis shows that 13C17O is a robust tracer of disk gas mass, HCO+ isotopologue emission may trace reservoirs of cold gas in typically warm disks, and Si and S bearing molecules are useful probes of shock induced structures such as circumplanetary disks
SO and SiS Emission Tracing an Embedded Planet and Compact CO and CO Counterparts in the HD 169142 Disk
Planets form in dusty, gas-rich disks around young stars, while at the same
time, the planet formation process alters the physical and chemical structure
of the disk itself. Embedded planets will locally heat the disk and sublimate
volatile-rich ices, or in extreme cases, result in shocks that sputter heavy
atoms such as Si from dust grains. This should cause chemical asymmetries
detectable in molecular gas observations. Using high-angular-resolution ALMA
archival data of the HD 169142 disk, we identify compact SO J=8-7 and
SiS J=19-18 emission coincident with the position of a 2 M
planet seen as a localized, Keplerian NIR feature within a gas-depleted,
annular dust gap at 38 au. The SiS emission is located along an
azimuthal arc and has a similar morphology as a known CO kinematic
excess. This is the first tentative detection of SiS emission in a
protoplanetary disk and suggests that the planet is driving sufficiently strong
shocks to produce gas-phase SiS. We also report the discovery of compact
CO and CO J=3-2 emission coincident with the planet location.
Taken together, a planet-driven outflow provides the best explanation for the
properties of the observed chemical asymmetries. We also resolve a bright,
azimuthally-asymmetric SO ring at 24 au. While most of this SO
emission originates from ice sublimation, its asymmetric distribution implies
azimuthal temperature variations driven by a misaligned inner disk or
planet-disk interactions. Overall, the HD 169142 disk shows several distinct
chemical signatures related to giant planet formation and presents a powerful
template for future searches of planet-related chemical asymmetries in
protoplanetary disks.Comment: 22 pages, 12 figures, accepted for publication in ApJ
Tracing snowlines and C/O ratio in a planet-hosting disk: ALMA molecular line observations towards the HD169142 disk
The composition of a forming planet is set by the material it accretes from
its parent protoplanetary disk. Therefore, it is crucial to map the chemical
make-up of the gas in disks to understand the chemical environment of planet
formation. This paper presents molecular line observations taken with the
Atacama Large Millimeter/submillimeter Array of the planet-hosting disk around
the young star HD 169142. We detect N2H+, CH3OH, [CI], DCN, CS, C34S, 13CS,
H2CS, H2CO, HC3N and c-C3H2 in this system for the first time. Combining these
data with the recent detection of SO and previously published DCO+ data, we
estimate the location of H2O and CO snowlines and investigate radial variations
in the gas phase C/O ratio. We find that the HD 169142 disk has a relatively
low N2H+ flux compared to the disks around Herbig stars HD 163296 and MWC 480
indicating less CO freeze-out and place the CO snowline beyond the millimetre
disk at ~150 au. The detection of CH3OH from the inner disk is consistent with
the H2O snowline being located at the edge of the central dust cavity at ~20
au. The radially varying CS/SO ratio across the proposed H2O snowline location
is consistent with this interpretation. Additionally, the detection of CH3OH in
such a warm disk adds to the growing evidence supporting the inheritance of
complex ices in disks from the earlier, colder stages of star formation.
Finally, we propose that the giant HD 169142 b located at 37 au is forming
between the CO2 and H2O snowlines where the local elemental make of the gas is
expected to have C/O=1.0.Comment: Accepted A&A 13th August 202
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
Resolved Debris Discs Around A Stars in the Herschel DEBRIS Survey
The majority of debris discs discovered so far have only been detected
through infrared excess emission above stellar photospheres. While disc
properties can be inferred from unresolved photometry alone under various
assumptions for the physical properties of dust grains, there is a degeneracy
between disc radius and dust temperature that depends on the grain size
distribution and optical properties. By resolving the disc we can measure the
actual location of the dust. The launch of Herschel, with an angular resolution
superior to previous far-infrared telescopes, allows us to spatially resolve
more discs and locate the dust directly. Here we present the nine resolved
discs around A stars between 20 and 40 pc observed by the DEBRIS survey. We use
these data to investigate the disc radii by fitting narrow ring models to
images at 70, 100 and 160 {\mu}m and by fitting blackbodies to full spectral
energy distributions. We do this with the aim of finding an improved way of
estimating disc radii for unresolved systems. The ratio between the resolved
and blackbody radii varies between 1 and 2.5. This ratio is inversely
correlated with luminosity and any remaining discrepancies are most likely
explained by differences to the minimum size of grain in the size distribution
or differences in composition. We find that three of the systems are well fit
by a narrow ring, two systems are borderline cases and the other four likely
require wider or multiple rings to fully explain the observations, reflecting
the diversity of planetary systems.Comment: 19 pages, 13 figures, 6 tables. Accepted for publication in MNRA
The debris disk around gamma Doradus resolved with Herschel
We present observations of the debris disk around gamma Doradus, an F1V star,
from the Herschel Key Programme DEBRIS (Disc Emission via Bias-free
Reconnaissance in the Infrared/Submillimetre). The disk is well-resolved at 70,
100 and 160 micron, resolved along its major axis at 250 micron, detected but
not resolved at 350 micron, and confused with a background source at 500
micron. It is one of our best resolved targets and we find it to have a
radially broad dust distribution. The modelling of the resolved images cannot
distinguish between two configurations: an arrangement of a warm inner ring at
several AU (best-fit 4 AU) and a cool outer belt extending from ~55 to 400 AU
or an arrangement of two cool, narrow rings at ~70 AU and ~190 AU. This
suggests that any configuration between these two is also possible. Both models
have a total fractional luminosity of ~10^{-5} and are consistent with the disk
being aligned with the stellar equator. The inner edge of either possible
configuration suggests that the most likely region to find planets in this
system would be within ~55 AU of the star. A transient event is not needed to
explain the warm dust's fractional luminosity.Comment: 12 pages, 6 figures, accepted for publication in Ap
Azimuthal C/O variations in a planet-forming disk
The elemental carbon-to-oxygen ratio (C/O) in the atmosphere of a giant planet is a promising diagnostic of that planet’s formation history in a protoplanetary disk. Alongside efforts in the exoplanet community to measure the C/O ratio in planetary atmospheres, observational and theoretical studies of disks are increasingly focused on understanding how the gas-phase C/O ratio varies both with radial location and between disks. This is mostly tied to the icelines of major volatile carriers such as CO and H2O. Using ALMA observations of CS and SO, we have found evidence for an entirely unexpected type of C/O variation in the protoplanetary disk around HD 100546: an azimuthal variation from a typical, oxygen-dominated ratio (C/O ≈ 0.5) to a carbon-dominated ratio (C/O ≳ 1.0). We show that the spatial distribution and peculiar line kinematics of both CS and SO molecules can be well explained by azimuthal variations in the C/O ratio. We propose a shadowing mechanism that could lead to such a chemical dichotomy. Our results imply that tracing the formation history of giant exoplanets using their atmospheric C/O ratios will need to take into account time-dependent azimuthal C/O variations in a planet’s accretion zone
An ALMA molecular inventory of warm Herbig Ae disks: I. Molecular rings, asymmetries and complexity in the HD 100546 disk
Observations of disks with the Atacama Large Millimeter/submillimeter Array
(ALMA) allow us to map the chemical makeup of nearby protoplanetary disks with
unprecedented spatial resolution and sensitivity. The typical outer Class II
disk observed with ALMA is one with an elevated C/O ratio and a lack of
oxygen-bearing complex organic molecules, but there are now some interesting
exceptions: three transition disks around Herbig Ae stars all show oxygen-rich
gas traced via the unique detections of the molecules SO and CH3OH. We present
the first results of an ALMA line survey at 337 to 357 GHz of such disks and
focus this paper on the first Herbig Ae disk to exhibit this chemical signature
- HD 100546. In these data, we detect 19 different molecules including NO, SO
and CH3OCHO (methyl formate). We also make the first tentative detections of
H213CO and 34SO in protoplanetary disks. Multiple molecular species are
detected in rings, which are, surprisingly, all peaking just beyond the
underlying millimeter continuum ring at 200 au. This result demonstrates a
clear connection between the large dust distribution and the chemistry in this
flat outer disk. We discuss the physical and/or chemical origin of these
sub-structures in relation to ongoing planet formation in the HD 100546 disk.
We also investigate how similar and/or different the molecular make up of this
disk is to other chemically well-characterised Herbig Ae disks. The line-rich
data we present motivates the need for more ALMA line surveys to probe the
observable chemistry in Herbig Ae systems which offer unique insight into the
composition of disk ices, including complex organic molecules.Comment: Accepted to AJ, 25 pages, 11 figure
An ALMA molecular inventory of warm Herbig Ae disks: II. Abundant complex organics and volatile sulphur in the IRS 48 disk
The Atacama Large Millimeter/submillimeter Array (ALMA) can probe the
molecular content of planet-forming disks with unprecedented sensitivity. These
observations allow us to build up an inventory of the volatiles available for
forming planets and comets. Herbig Ae transition disks are fruitful targets due
to the thermal sublimation of complex organic molecule (COM) and likely
H2O-rich ices in these disks. The IRS 48 disk shows a particularly rich
chemistry that can be directly linked to its asymmetric dust trap. Here, we
present ALMA observations of the IRS 48 disk where we detect 16 different
molecules and make the first robust detections of H213CO, 34SO, 33SO and
c-H2COCH2 (ethylene oxide) in a protoplanetary disk. All of the molecular
emissions, aside from CO, are colocated with the dust trap and this includes
newly detected simple molecules such as HCO+, HCN and CS. Interestingly, there
are spatial offsets between different molecular families, including between the
COMs and sulphur-bearing species, with the latter being more azimuthally
extended and located radially further from the star. The abundances of the
newly detected COMs relative to CH3OH are higher than the expected protostellar
ratios, which implies some degree of chemical processing of the inherited ices
during the disk lifetime. These data highlight IRS 48 as a unique astrochemical
laboratory to unravel the full volatile reservoir at the epoch of planet and
comet formation and the role of the disk in (re)setting chemical complexity.Comment: Accepted to AJ, 21 pages, 7 figure
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