42 research outputs found
A dearth of small particles in debris disks: An energy-constrained smallest fragment size
A prescription for the fragment size distribution resulting from dust grain
collisions is essential when modelling a range of astrophysical systems, such
as debris disks and planetary rings. While the slope of the fragment size
distribution and the size of the largest fragment are well known, the behaviour
of the distribution at the small size end is theoretically and experimentally
poorly understood. This leads debris disk codes to generally assume a limit
equal to, or below, the radiation blow-out size. We use energy conservation to
analytically derive a lower boundary of the fragment size distribution for a
range of collider mass ratios. Focussing on collisions between equal-sized
bodies, we apply the method to debris disks. For a given collider mass, the
size of the smallest fragments is found to depend on collision velocity,
material parameters, and the size of the largest fragment. We provide a
physically motivated recipe for the calculation of the smallest fragment, which
can be easily implemented in codes for modelling collisional systems. For
plausible parameters, our results are consistent with the observed predominance
of grains much larger than the blow-out size in Fomalhaut's main belt and in
the Herschel cold debris disks.Comment: 5 pages, 3 figures, Accepted for publication as a Letter in Astronomy
& Astrophysic
Fingerprints of giant planets in the photospheres of Herbig stars
Around 2% of all A stars have photospheres depleted in refractory elements.
This is hypothesized to arise from a preferential accretion of gas rather than
dust, but the specific processes and the origin of the material -- circum- or
interstellar -- are not known. The same depletion is seen in 30% of young,
disk-hosting Herbig Ae/Be stars. We investigate whether the chemical
peculiarity originates in a circumstellar disk. Using a sample of systems for
which both the stellar abundances and the protoplanetary disk structure are
known, we find that stars hosting warm, flaring group I disks typically have
Fe, Mg and Si depletions of 0.5 dex compared to the solar-like abundances of
stars hosting cold, flat group II disks. The volatile, C and O, abundances in
both sets are identical. Group I disks are generally transitional, having
radial cavities depleted in millimetre-sized dust grains, while those of group
II are usually not. Thus we propose that the depletion of heavy elements
emerges as Jupiter-like planets block the accretion of part of the dust, while
gas continues to flow towards the central star. We calculate gas to dust ratios
for the accreted material and find values consistent with models of disk
clearing by planets. Our results suggest that giant planets of ~0.1 to 10 M_Jup
are hiding in at least 30% of Herbig Ae/Be disks.Comment: 5 pages, 3 figures, accepted for publication in A&A Letter
First measurement of the 14N/15N ratio in the analogue of the Sun progenitor OMC-2 FIR4
We present a complete census of the 14N/15N isotopic ratio in the most
abundant N-bearing molecules towards the cold envelope of the protocluster
OMC-2 FIR4, the best known Sun progenitor. To this scope, we analysed the
unbiased spectral survey obtained with the IRAM-30m telescope at 3mm, 2mm and
1mm. We detected several lines of CN, HCN, HNC, HC3N, N2H+, and their
respective 13C and 15N isotopologues. The lines relative fluxes are compatible
with LTE conditions and moderate line opacities have been corrected via a
Population Diagram method or theoretical relative intensity ratios of the
hyperfine structures. The five species lead to very similar 14N/15N isotopic
ratios, without any systematic difference between amine and nitrile bearing
species as previously found in other protostellar sources. The weighted average
of the 14N/15N isotopic ratio is 270 +/- 30. This 14N/15N value is remarkably
consistent with the [250-350] range measured for the local galactic ratio but
significantly differs from the ratio measured in comets (around 140).
High-angular resolution observations are needed to examine whether this
discrepancy is maintained at smaller scales. In addition, using the CN, HCN and
HC3N lines, we derived a 12C/13C isotopic ratio of 50 +/- 5.Comment: Accepted for publication in ApJ ; 19 pages, 5 tables, 12 figure
KELT-9 and its ultra-hot Jupiter: stellar parameters, composition, and planetary pollution
KELT-9b is an ultra-hot Jupiter observed to be undergoing extreme mass loss.
Its A0-type host star has a radiative envelope, which makes its surface layers
prone to retaining recently accreted material. To search for potential signs of
planetary material polluting the stellar surface, we carry out the most
comprehensive chemical characterisation of KELT-9 to-date. New element
detections include Na and Y, which had previously been detected in the
ultra-hot Jupiter but not studied in the star; these detections complete the
set of nine elements measured in both star and planet. In comparing KELT-9 with
similar open cluster stars we find no strong anomalies. This finding is
consistent with calculations of photospheric pollution accounting for stellar
mixing and using observationally estimated KELT-9b mass loss rates. We also
rule out recent, short-lived intensive mass transfer such as the stellar
ingestion of an Earth-mass exomoon.Comment: 7 pages, 7 figures, accepted for publication in MNRA
The First Interferometric Measurements of NH₂D/NH₃ Ratio in Hot Corinos
The chemical evolution of nitrogen during star and planet formation is still not fully understood. Ammonia (NH_{3}) is a key specie in the understanding of the molecular evolution in star-forming clouds and nitrogen isotope fractionation. In this paper, we present high-spatial-resolution observations of multiple emission lines of NH_{3} toward the protobinary system NGC1333 IRAS4A with the Karl G. Jansky Very Large Array. We spatially resolved the binary (hereafter, 4A1 and 4A2) and detected compact emission of NH3 transitions with high excitation energies (≳100 K) from the vicinity of the protostars, indicating the NH_{3} ice has sublimated at the inner hot region. The NH3 column density is estimated to be ∼10^{17}–10^{18} cm^{−2}. We also detected two NH_{2}D transitions, allowing us to constrain the deuterium fractionation of ammonia. The NH_{2}D/NH_{3} ratios are as high as ∼0.3–1 in both 4A1 and 4A2. From comparisons with the astrochemical models in the literature, the high NH_{2}D/NH_{3} ratios suggest that the formation of NH3 ices mainly started in the prestellar phase after the formation of bulk water ice finished, and that the primary nitrogen reservoir in the star-forming cloud could be atomic nitrogen (or N atoms) rather than nitrogen-bearing species such as N_{2} and NH_{3}. The implications on the physical properties of IRAS4A's cores are discussed as well
Evolution of protoplanetary disks from their taxonomy in scattered light: Group I vs. Group II
High-resolution imaging reveals a large morphological variety of
protoplanetary disks. To date, no constraints on their global evolution have
been found from this census. An evolutionary classification of disks was
proposed based on their IR spectral energy distribution, with the Group I
sources showing a prominent cold component ascribed to an earlier stage of
evolution than Group II. Disk evolution can be constrained from the comparison
of disks with different properties. A first attempt of disk taxonomy is now
possible thanks to the increasing number of high-resolution images of Herbig
Ae/Be stars becoming available. Near-IR images of six Group II disks in
scattered light were obtained with VLT/NACO in Polarimetric Differential
Imaging, which is the most efficient technique to image the light scattered by
the disk material close to the stars. We compare the stellar/disk properties of
this sample with those of well-studied Group I sources available from the
literature. Three Group II disks are detected. The brightness distribution in
the disk of HD163296 indicates the presence of a persistent ring-like structure
with a possible connection with the CO snowline. A rather compact (less than
100 AU) disk is detected around HD142666 and AK Sco. A taxonomic analysis of 17
Herbig Ae/Be sources reveals that the difference between Group I and Group II
is due to the presence or absence of a large disk cavity (larger than 5 AU).
There is no evidence supporting the evolution from Group I to Group II. Group
II are not evolved version of the Group I. Within the Group II disks, very
different geometries (both self-shadowed and compact) exist. HD163296 could be
the primordial version of a typical Group I. Other Group II, like AK Sco and
HD142666, could be smaller counterpart of Group I unable to open cavities as
large as those of Group I.Comment: 16 pages, 7 figures, published by A&
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
The Demographics and Atmospheres of Giant Planets with the ELTs
Gas giants are the most readily detectable exoplanets but fundamental
questions about their system architectures, formation, migration, and
atmospheres have been unanswerable with the current generation of ground- and
space-based facilities. The dominant techniques to detect and characterize
giant planets radial velocities, transits, direct imaging, microlensing,
and astrometry are each isolated to a limited range of planet masses,
separations, ages, and temperatures. These windows into the arrangement and
physical properties of giant planets have spawned new questions about the
timescale and location of their assembly; the distributions of planet mass and
orbital separation at young and old ages; the composition and structure of
their atmospheres; and their orbital and rotational angular momentum
architectures. The ELTs will address these questions by building bridges
between these islands of mass, orbital distance, and age. The angular
resolution, collecting area, all-sky coverage, and novel instrumentation suite
of these facilities are needed to provide a complete map of the orbits and
atmospheric evolution of gas giant planets (0.310 ) across
space (0.1100 AU) and time (1 Myr to 10 Gyr). This white paper highlights
the scientific potential of the GMT and TMT to address these outstanding
questions, with a particular focus on the role of direct imaging and
spectroscopy of large samples of giant planets that will soon be made available
with .Comment: White paper for the Astro2020 decadal surve
The VLA/ALMA Nascent Disk and Multiplicity (VANDAM) Survey of Orion Protostars IV. Unveiling the Embedded Intermediate-Mass Protostar and Disk within OMC2-FIR3/HOPS-370
We present ALMA (0.87~mm and 1.3~mm) and VLA (9~mm) observations toward the
candidate intermediate-mass protostar OMC2-FIR3 (HOPS-370;
L~314~L) at 0.1" (40~au) resolution for the continuum
emission and ~0.25" (100 au) resolution of nine molecular lines. The dust
continuum observed with ALMA at 0.87~mm and 1.3~mm resolve a near edge-on disk
toward HOPS-370 with an apparent radius of ~100 au. The VLA observations detect
both the disk in dust continuum and free-free emission extended along the jet
direction. The ALMA observations of molecular lines (HCO, SO, CHOH,
CO, CO, NS, and HCN) reveal rotation of the apparent disk
surrounding HOPS-370 orthogonal to the jet/outflow direction. We fit radiative
transfer models to both the dust continuum structure of the disk and molecular
line kinematics of the inner envelope and disk for the HCO, CHOH, NS,
and SO lines. The central protostar mass is determined to be 2.5 M_sun
with a disk radius of 94~au, when fit using combinations of the HCO,
CHOH, NS, and SO lines, consistent with an intermediate-mass protostar.
Modeling of the dust continuum and spectral energy distribution (SED) yields a
disk mass of 0.035~M (inferred dust+gas) and a dust disk radius of
62~au, thus the dust disk may have a smaller radius than the gas disk, similar
to Class II disks. In order to explain the observed luminosity with the
measured protostar mass, HOPS-370 must be accreting at a rate between 1.7 and
3.210~M~yr.Comment: Accepted to ApJ; 51 pages, 12 Figures, 7 Table