62 research outputs found
Investigating the inner discs of Herbig Ae/Be stars with CO bandhead and Br Gamma emission
Herbig Ae/Be stars lie in the mass range between low and high mass young
stars, and therefore offer a unique opportunity to observe any changes in the
formation processes that may occur across this boundary. This paper presents
medium resolution VLT/X-Shooter spectra of six Herbig Ae/Be stars, drawn from a
sample of 91 targets, and high resolution VLT/CRIRES spectra of five Herbig
Ae/Be stars, chosen based on the presence of CO first overtone bandhead
emission in their spectra. The X-Shooter survey reveals a low detection rate of
CO first overtone emission (7 per cent), consisting of objects mainly of
spectral type B. A positive correlation is found between the strength of the CO
v=2-0 and Br {\gamma} emission lines, despite their intrinsic linewidths
suggesting a separate kinematic origin. The high resolution CRIRES spectra are
modelled, and are well fitted under the assumption that the emission originates
from small scale Keplerian discs, interior to the dust sublimation radius, but
outside the co-rotation radius of the central stars. In addition, our findings
are in very good agreement for the one object where spatially resolved
near-infrared interferometric studies have also been performed. These results
suggest that the Herbig Ae/Be stars in question are in the process of gaining
mass via disc accretion, and that modelling of high spectral resolution spectra
is able to provide a reliable probe into the process of stellar accretion in
young stars of intermediate to high masses.Comment: Accepted for publication in MNRAS. 14 pages, 5 figure
Observing protoplanetary discs with the Square Kilometre Array -- I. Characterising pebble substructure caused by forming planets
High angular resolution observations of discs at mm wavelengths (on scales of
a few au) are now commonplace, but there is a current lack of a comparable
angular resolution for observations at cm wavelengths. This presents a
significant barrier to improving our understanding of planet formation, in
particular how dust grains grow from mm to cm sizes. In this paper, we examine
the ability of the Square Kilometre Array (SKA) to observe dust substructure in
a young, planet-forming disc at cm wavelengths. We use dusty hydrodynamics and
continuum radiative transfer to predict the distribution and emission of 1 cm
dust grains (or pebbles) within the disc, and simulate continuum observations
with the current SKA1-MID design baseline at frequencies of 12.5 GHz (Band 5b,
~2.4 cm) on 5-10 au scales. The SKA will provide high-fidelity observations of
the cm dust emission substructure in discs for integration times totalling
100's of hours. Radial structure can be obtained at a sufficient resolution and
S/N from shorter (10's of hours) integration times by azimuthal averaging in
the image plane. By modelling the intensity distribution directly in the
visibility plane, it is possible to recover a similar level of (axisymmetric)
structural detail from observations with integration times 1-2 orders of
magnitude lower than required for high-fidelity imaging. Our results
demonstrate that SKA1-MID will provide crucial constraints on the distribution
and morphology of the raw material for building planets, the pebbles in
protoplanetary discs.Comment: 12 pages, 8 figures, accepted for publication in MNRA
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
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
UV-driven Chemistry as a Signpost for Late-stage Planet Formation
The chemical reservoir within protoplanetary disks has a direct impact on
planetary compositions and the potential for life. A long-lived carbon-and
nitrogen-rich chemistry at cold temperatures (<=50K) is observed within cold
and evolved planet-forming disks. This is evidenced by bright emission from
small organic radicals in 1-10 Myr aged systems that would otherwise have
frozen out onto grains within 1 Myr. We explain how the chemistry of a
planet-forming disk evolves from a cosmic-ray/X-ray-dominated regime to an
ultraviolet-dominated chemical equilibrium. This, in turn, will bring about a
temporal transition in the chemical reservoir from which planets will accrete.
This photochemical dominated gas phase chemistry develops as dust evolves via
growth, settling and drift, and the small grain population is depleted from the
disk atmosphere. A higher gas-to-dust mass ratio allows for deeper penetration
of ultraviolet photons is coupled with a carbon-rich gas (C/O > 1) to form
carbon-bearing radicals and ions. This further results in gas phase formation
of organic molecules, which then would be accreted by any actively forming
planets present in the evolved disk.Comment: Accepted to Nature Astronomy, Published Dec 8th 202
Grand Challenges in Protoplanetary Disc Modelling
The Protoplanetary Discussions conference --- held in Edinburgh, UK, from 7th
--11th March 2016 --- included several open sessions led by participants. This
paper reports on the discussions collectively concerned with the multiphysics
modelling of protoplanetary discs, including the self-consistent calculation of
gas and dust dynamics, radiative transfer and chemistry. After a short
introduction to each of these disciplines in isolation, we identify a series of
burning questions and grand challenges associated with their continuing
development and integration. We then discuss potential pathways towards solving
these challenges, grouped by strategical, technical and collaborative
developments. This paper is not intended to be a review, but rather to motivate
and direct future research and collaboration across typically distinct fields
based on \textit{community driven input}, to encourage further progress in our
understanding of circumstellar and protoplanetary discs
Molecules with ALMA at Planet-forming Scales (MAPS). VIII. CO gap in AS 209-gas depletion or chemical processing?
Funding: I.C. was supported by NASA through the NASA Hubble Fellowship grant HST-HF2-51405.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555. J.D.I. acknowledges support from the Science and Technology Facilities Council of the United Kingdom (STFC) under ST/T000287/1. C.W. acknowledges financial support from the University of Leeds, SFTC, and UKRI (grant Nos. ST/R000549/1, ST/T000287/1, and MR/T040726/1).Emission substructures in gas and dust are common in protoplanetary disks. Such substructures can be linked to planet formation or planets themselves. We explore the observed gas substructures in AS 209 using thermochemical modeling with RAC2D and high-spatial-resolution data from the Molecules with ALMA at Planet-forming Scales (MAPS) program. The observations of C18O J = 2-1 emission exhibit a strong depression at 88 au overlapping with the positions of multiple gaps in millimeter dust continuum emission. We find that the observed CO column density is consistent with either gas surface-density perturbations or chemical processing, while C2H column density traces changes in the C/O ratio rather than the H2 gas surface density. However, the presence of a massive planet (>0.2 MJup) would be required to account for this level of gas depression, which conflicts with constraints set by the dust emission and the pressure profile measured by gas kinematics. Based on our models, we infer that a local decrease of CO abundance is required to explain the observed structure in CO, dominating over a possible gap-carving planet present and its effect on the H2 surface density. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.Publisher PDFPeer reviewe
Molecules with ALMA at Planet-forming Scales (MAPS). VI. Distribution of the small organics HCN, C2H, and H2CO
Funding: I.C. was supported by NASA through the NASA Hubble Fellowship grant HST-HF2-51405.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555. C.W. acknowledges financial support from the University of Leeds, STFC, and UKRI (grant Nos. ST/R000549/1, ST/T000287/1, and MR/T040726/1). J.D.I. acknowledges support from the Science and Technology Facilities Council of the United Kingdom (STFC) under ST/T000287/1.Small organic molecules, such as C2H, HCN, and H2CO, are tracers of the C, N, and O budget in protoplanetary disks. We present high-angular-resolution (10-50 au) observations of C2H, HCN, and H2CO lines in five protoplanetary disks from the Molecules with ALMA at Planet-forming Scales (MAPS) ALMA Large Program. We derive column density and excitation temperature profiles for HCN and C2H, and find that the HCN emission arises in a temperate (20-30 K) layer in the disk, while C2H is present in relatively warmer (20-60 K) layers. In the case of HD 163296, we find a decrease in column density for HCN and C2H inside one of the dust gaps near ~83 au, where a planet has been proposed to be located. We derive H2CO column density profiles assuming temperatures between 20 and 50 K, and find slightly higher column densities in the colder disks around T Tauri stars than around Herbig Ae stars. The H2CO column densities rise near the location of the CO snowline and/or millimeter dust edge, suggesting an efficient release of H2CO ices in the outer disk. Finally, we find that the inner 50 au of these disks are rich in organic species, with abundances relative to water that are similar to cometary values. Comets could therefore deliver water and key organics to future planets in these disks, similar to what might have happened here on Earth. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.Publisher PDFPeer reviewe
Molecules with ALMA at Planet-forming Scales (MAPS). XIV. Revealing disk substructures in multiwavelength continuum emission
Funding: I.C. was supported by NASA through the NASA Hubble Fellowship grant HST-HF2-51405.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555. A.S.B acknowledges the studentship funded by the Science and Technology Facilities Council of the United Kingdom (STFC). C.W. acknowledges financial support from the University of Leeds, STFC and UKRI (grant Nos. ST/R000549/1, ST/T000287/1, MR/T040726/1).Constraining dust properties of planet-forming disks via high-angular-resolution observations is fundamental to understanding how solids are trapped in substructures and how dust growth may be favored or accelerated therein. We use ALMA dust continuum observations of the Molecules with ALMA at Planet-forming Scales (MAPS) disks and explore a large parameter space to constrain the radial distribution of solid mass and maximum grain size in each disk, including or excluding dust scattering. In the nonscattering model, the dust surface density and maximum grain size profiles decrease from the inner disks to the outer disks, with local maxima at the bright ring locations, as expected from dust trapping models. The inferred maximum grain sizes from the inner to outer disks decrease from 1 cm to 1 mm. For IM Lup, HD 163296, and MWC 480 in the scattering model, two solutions are compatible with their observed inner disk emission: one solution corresponding to a maximum grain size of a few millimeters (similar to the nonscattering model), and the other corresponding to a size of a few hundred micrometers. Based on the estimated Toomre parameter, only IM Lup-which shows a prominent spiral morphology in millimeter dust-is found to be gravitationally unstable. The estimated maximum Stokes number in all the disks lies between 0.01 and 0.3, and the estimated turbulence parameters in the rings of AS 209 and HD 163296 are close to the threshold where dust growth is limited by turbulent fragmentation. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.Publisher PDFPeer reviewe
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