Current models of (exo)planet formation often rely on a large influx of so-called "pebbles" from the outer disk into the planet formation region. In this paper, we investigate how the ...formation/coagulation of pebbles in the cold outer regions of protoplanetary disks and their subsequent migration to the inner disk can alter the gas-phase CO distribution both interior and exterior to the midplane CO snowline. By simulating the resulting CO abundances in the midplane as well as the warm surface layer, we identify observable signatures of large-scale pebble formation and migration that can be used as "smoking guns" for this important process. Specifically, we find that after 1 Myr, the formation and settling of icy pebbles results in the removal of up to 80% of the CO vapor in the warm ( ) disk layers outside the CO snowline, while the radial migration of pebbles results in the generation of a plume of CO vapor inside the snowline, increasing the CO abundance by a factor ∼2-6 depending on the strength of the turbulence and the sizes of the individual pebbles. The absence of this plume of CO vapor in young nearby disks could indicate efficient conversion of CO into a more refractory species, or to the radial mass flux of pebbles being drastically reduced by, for example, disk inhomogeneities or early planetesimal formation.
Recent observations show that the CO gas abundance, relative to H2, in many 1-10 Myr old protoplanetary disks may be heavily depleted by a factor of 10-100 compared to the canonical interstellar ...medium (ISM) value of 10−4. When and how this depletion happens can significantly affect compositions of planetesimals and atmospheres of giant planets. It is therefore important to constrain whether the depletion occurs already at the earliest protostellar disk stage. Here we present spatially resolved observations of C18O, C17O, and 13C18O J = 2−1 lines in three protostellar disks. We show that the C18O line emits from both the disk and the inner envelope, while C17O and 13C18O lines are consistent with a disk origin. The line ratios indicate that both C18O and C17O lines are optically thick in the disk region, and only the 13C18O line is optically thin. The line profiles of the 13C18O emissions are best reproduced by Keplerian gaseous disks at similar sizes as their mm-continuum emissions, suggesting small radial separations between the gas and mm-sized grains in these disks, in contrast to the large separation commonly seen in protoplanetary disks. Assuming a gas-to-dust ratio of 100, we find that the CO gas abundances in these protostellar disks are consistent with the ISM abundance within a factor of 2, nearly one order of magnitude higher than the average value of 1-10 Myr old disks. These results suggest that there is a fast, ∼1 Myr, evolution of the abundance of CO gas from the protostellar disk stage to the protoplanetary disk stage.
The gas-phase CO abundance (relative to hydrogen) in protoplanetary disks decreases by up to two orders of magnitude from its interstellar medium value of ∼10−4, even after accounting for freeze-out ...and photodissociation. Previous studies have shown that while local chemical processing of CO and the sequestration of CO ice on solids in the midplane can both contribute, neither of these processes appears capable of consistently reaching the observed depletion factors on the relevant timescale of 1-3 Myr. In this study, we model these processes simultaneously by including a compact chemical network (centered on carbon and oxygen) to 2D (r + z) simulations of the outer (r > 20 au) disk regions that include turbulent diffusion, pebble formation, and pebble dynamics. In general, we find that the CO/H2 abundance is a complex function of time and location. Focusing on CO in the warm molecular layer, we find that only the most complete model (with chemistry and pebble evolution included) can reach depletion factors consistent with observations. In the absence of pressure traps, highly efficient planetesimal formation, or high cosmic-ray ionization rates, this model also predicts a resurgence of CO vapor interior to the CO ice-line. We show the impact of physical and chemical processes on the elemental (C/O) and (C/H) ratios (in the gas and ice phases), discuss the use of CO as a disk mass tracer, and, finally, connect our predicted pebble ice compositions to those of pristine planetesimals as found in the Cold Classical Kuiper Belt and debris disks.
Abstract
Constraining planet formation based on the atmospheric composition of exoplanets is a fundamental goal of the exoplanet community. Existing studies commonly try to constrain atmospheric ...abundances, or to analyze what abundance patterns a given description of planet formation predicts. However, there is also a pressing need to develop methodologies that investigate how to transform atmospheric compositions into planetary formation inferences. In this study we summarize the complexities and uncertainties of state-of-the-art planet formation models and how they influence planetary atmospheric compositions. We introduce a methodology that explores the effect of different formation model assumptions when interpreting atmospheric compositions. We apply this framework to the directly imaged planet HR 8799e. Based on its atmospheric composition, this planet may have migrated significantly during its formation. We show that including the chemical evolution of the protoplanetary disk leads to a reduced need for migration. Moreover, we find that pebble accretion can reproduce the planet’s composition, but some of our tested setups lead to too low atmospheric metallicities, even when considering that evaporating pebbles may enrich the disk gas. We conclude that the definitive inversion from atmospheric abundances to planet formation for a given planet may be challenging, but a qualitative understanding of the effects of different formation models is possible, opening up pathways for new investigations.
CO is commonly used as a tracer of the total gas mass in both the interstellar medium and in protoplanetary disks. Recently, there has been much debate about the utility of CO as a mass tracer in ...disks. Observations of CO in protoplanetary disks reveal a range of CO abundances, with measurements of low CO to dust mass ratios in numerous systems. One possibility is that carbon is removed from CO via chemistry. However, the full range of physical conditions conducive to this chemical reprocessing is not well understood. We perform a systematic survey of the time dependent chemistry in protoplanetary disks for 198 models with a range of physical conditions. We vary dust grain size distribution, temperature, comic-ray and X-ray ionization rates, disk mass, and initial water abundance, detailing what physical conditions are necessary to activate the various CO depletion mechanisms in the warm molecular layer. We focus our analysis on the warm molecular layer in two regions: the outer disk (100 au) well outside the CO snowline and the inner disk (19 au) just inside the midplane CO snowline. After 1 Myr, we find that the majority of models have a CO abundance relative to H2 less than 10−4 in the outer disk, while an abundance less than 10−5 requires the presence of cosmic-rays. Inside the CO snowline, significant depletion of CO only occurs in models with a high cosmic-ray rate. If cosmic-rays are not present in young disks, it is difficult to chemically remove carbon from CO. Additionally, removing water prior to CO depletion impedes the chemical processing of CO. Chemical processing alone cannot explain current observations of low CO abundances. Other mechanisms must also be involved.
Abstract
The formation of asteroids, comets, and planets occurs in the interior of protoplanetary disks during the early phase of star formation. Consequently, the chemical composition of the disk ...might shape the properties of the emerging planetary system. In this context, it is crucial to understand whether and what organic molecules are synthesized in the disk. In this Letter, we report the first detection of formic acid (HCOOH) toward the TW Hydrae protoplanetary disk. The observations of the trans-HCOOH 6
(1,6)–5(1,5)
transition were carried out at 129 GHz with Atacama Large Millimeter/Submillimeter Array (ALMA). We measured a disk-averaged gas-phase t-HCOOH column density of ∼(2–4) × 10
12
cm
−2
, namely as large as that of methanol. HCOOH is the first organic molecule containing two oxygen atoms detected in a protoplanetary disk, a proof that organic chemistry is very active, albeit difficult to observe, in these objects. Specifically, this simplest acid stands as the basis for synthesis of more complex carboxylic acids used by life on Earth.
Our understanding of protoplanetary disks is rapidly departing from the classical view of a smooth, axisymmetric disk. This is in part thanks to the high angular resolution that (sub)millimeter ...observations can provide. Here, we present the combined results of Atacama Large Millimeter/submillimeter Array (ALMA) (0.9 mm) and Very Large Array (VLA) (7 mm) dust continuum observations toward the protoplanetary disk around the solar analog GM Aur. Both images clearly resolve the ∼35 au inner cavity. The ALMA observations also reveal a fainter disk that extends up to ∼250 au. We model our observations using two approaches: an analytical fit to the observed deprojected visibilities, and a physical disk model that fits the spectral energy distribution as well as the VLA and ALMA observations. Despite not being evident in the deconvolved images, the VLA and ALMA visibilities can only be fitted with two bright rings of radii ∼40 and ∼80 au. Our physical model indicates that this morphology is the result of an accumulation or trapping of large dust grains, probably due to the presence of two pressure bumps in the disk. Even though alternative mechanisms cannot be discarded, the multiple rings suggest that forming planets may have cleared at least two gaps in the disk. Finally, our analysis suggests that the inner cavity might display different sizes at 0.9 and 7 mm. This discrepancy could be caused by the presence of free-free emission close to the star at 7 mm, or by a more compact accumulation of the large dust grains at the edge of the cavity.
The dominant form of nitrogen provided to most solar system bodies is currently unknown, though available measurements show that the detected nitrogen in solar system rocks and ices is depleted with ...respect to solar abundances and the interstellar medium. We use a detailed chemical/physical model of the chemical evolution of a protoplanetary disk to explore the evolution and abundance of nitrogen-bearing molecules. Based on this model, we analyze how initial chemical abundances provided as either gas or ice during the early stages of disk formation influence which species become the dominant nitrogen bearers at later stages. We find that a disk with the majority of its initial nitrogen in either atomic or molecular nitrogen is later dominated by atomic and molecular nitrogen as well as NH sub(3) and HCN ices, where the dominant species varies with disk radius. When nitrogen is initially in gaseous ammonia, it later becomes trapped in ammonia ice except in the outer disk where atomic nitrogen dominates. For a disk with the initial nitrogen in the form of ammonia ice, the nitrogen remains trapped in the ice as NH sub(3) at later stages. The model in which most of the initial nitrogen is placed in atomic N best matches the ammonia abundances observed in comets. Furthermore, the initial state of nitrogen influences the abundance of N sub(2)H super(+), which has been detected in protoplanetary disks. Strong N sub(2)H super(+) emission is found to be indicative of an N sub(2) abundance greater than n sub(N2)/n sub(H2) > 10 super(-6) in addition to tracing the CO snow line. Our models also indicate that NO is potentially detectable, with lower N gas abundances leading to higher NO abundances.
CO is thought to be the main reservoir of volatile carbon in protoplanetary disks, and thus the primary initial source of carbon in the atmospheres of forming giant planets. However, recent ...observations of protoplanetary disks point toward low volatile carbon abundances in many systems, including at radii interior to the CO snowline. One potential explanation is that gas phase carbon is chemically reprocessed into less volatile species, which are frozen on dust grain surfaces as ice. This mechanism has the potential to change the primordial C/H ratio in the gas. However, current observations primarily probe the upper layers of the disk. It is not clear if the low volatile carbon abundances extend to the midplane, where planets form. We have run a grid of 198 chemical models, exploring how the chemical reprocessing of CO depends on disk mass, dust grain size distribution, temperature, cosmic-ray and X-ray ionization rate, and initial water abundance. Building on our previous work focusing on the warm molecular layer, here we analyze the results for our grid of models in the disk midplane at 12 au. We find that either an ISM level cosmic-ray ionization rate or the presence of UV photons due to a low dust surface density are needed to chemically reduce the midplane CO gas abundance by at least an order of magnitude within 1 Myr. In the majority of our models CO does not undergo substantial reprocessing by in situ chemistry and there is little change in the gas phase C/H and C/O ratios over the lifetime of the typical disk. However, in the small subset of disks where the disk midplane is subject to a source of ionization or photolysis, the gas phase C/O ratio increases by up to nearly 9 orders of magnitude due to conversion of CO into volatile hydrocarbons.
Abstract
High-spatial-resolution observations of CO isotopologue line emission in protoplanetary disks at mid-inclinations (≈30°–75°) allow us to characterize the gas structure in detail, including ...radial and vertical substructures, emission surface heights and their dependencies on source characteristics, and disk temperature profiles. By combining observations of a suite of CO isotopologues, we can map the two-dimensional (
r
,
z
) disk structure from the disk upper atmosphere, as traced by CO, to near the midplane, as probed by less abundant isotopologues. Here, we present high-angular-resolution (≲0.″1 to ≈0.″2; ≈15–30 au) observations of CO,
13
CO, and C
18
O in either or both
J
= 2–1 and
J
= 3–2 lines in the transition disks around DM Tau, Sz 91, LkCa 15, and HD 34282. We derived line emission surfaces in CO for all disks and in
13
CO for the DM Tau and LkCa 15 disks. With these observations, we do not resolve the vertical structure of C
18
O in any disk, which is instead consistent with C
18
O emission originating from the midplane. Both the
J
= 2–1 and
J
= 3–2 lines show similar heights. Using the derived emission surfaces, we computed radial and vertical gas temperature distributions for each disk, including empirical temperature models for the DM Tau and LkCa 15 disks. After combining our sample with literature sources, we find that
13
CO line emitting heights are also tentatively linked with source characteristics, e.g., stellar host mass, gas temperature, disk size, and show steeper trends than seen in CO emission surfaces.
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